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Request For Comments - RFC4782

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Network Working Group                                           S. Floyd
Request for Comments: 4782                                     M. Allman
Category: Experimental                                              ICIR
                                                                 A. Jain
                                                             F5 Networks
                                                            P. Sarolahti
                                                   Nokia Research Center
                                                            January 2007


                       Quick-Start for TCP and IP

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies an optional Quick-Start mechanism for
   transport protocols, in cooperation with routers, to determine an
   allowed sending rate at the start and, at times, in the middle of a
   data transfer (e.g., after an idle period).  While Quick-Start is
   designed to be used by a range of transport protocols, in this
   document we only specify its use with TCP.  Quick-Start is designed
   to allow connections to use higher sending rates when there is
   significant unused bandwidth along the path, and the sender and all
   of the routers along the path approve the Quick-Start Request.

   This document describes many paths where Quick-Start Requests would
   not be approved.  These paths include all paths containing routers,
   IP tunnels, MPLS paths, and the like that do not support Quick-
   Start.  These paths also include paths with routers or middleboxes
   that drop packets containing IP options.  Quick-Start Requests could
   be difficult to approve over paths that include multi-access layer-
   two networks.  This document also describes environments where the
   Quick-Start process could fail with false positives, with the sender
   incorrectly assuming that the Quick-Start Request had been approved
   by all of the routers along the path.  As a result of these concerns,
   and as a result of the difficulties and seeming absence of motivation
   for routers, such as core routers to deploy Quick-Start, Quick-Start
   is being proposed as a mechanism that could be of use in controlled



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   environments, and not as a mechanism that would be intended or
   appropriate for ubiquitous deployment in the global Internet.

Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions and Terminology ................................5
   2. Assumptions and General Principles ..............................6
      2.1. Overview of Quick-Start ....................................7
   3. The Quick-Start Option in IP ...................................10
      3.1. The Quick-Start Option for IPv4 ...........................10
      3.2. The Quick-Start Option for IPv6 ...........................13
      3.3. Processing the Quick-Start Request at Routers .............14
           3.3.1. Processing the Report of Approved Rate .............15
      3.4. The QS Nonce ..............................................16
   4. The Quick-Start Mechanisms in TCP ..............................18
      4.1. Sending the Quick-Start Request ...........................19
      4.2. The Quick-Start Response Option in the TCP header .........20
      4.3. TCP: Sending the Quick-Start Response .....................21
      4.4. TCP: Receiving and Using the Quick-Start Response Packet ..22
      4.5. TCP: Controlling Acknowledgement Traffic on the
           Reverse Path ..............................................24
      4.6. TCP: Responding to a Loss of a Quick-Start Packet .........26
      4.7. TCP: A Quick-Start Request for a Larger Initial Window ....26
           4.7.1. Interactions with Path MTU Discovery ...............26
           4.7.2. Quick-Start Request Packets that are
                  Discarded by Routers or Middleboxes ................27
      4.8. TCP: A Quick-Start Request in the Middle of a Connection ..28
      4.9. An Example Quick-Start Scenario with TCP ..................29
   5. Quick-Start and IPsec AH .......................................30
   6. Quick-Start in IP Tunnels and MPLS .............................31
      6.1. Simple Tunnels that Are Compatible with Quick-Start .......33
           6.1.1. Simple Tunnels that Are Aware of Quick-Start .......33
      6.2. Simple Tunnels that Are Not Compatible with Quick-Start ...34
      6.3. Tunnels That Support Quick-Start ..........................35
      6.4. Quick-Start and MPLS ......................................35
   7. The Quick-Start Mechanism in Other Transport Protocols .........36
   8. Using Quick-Start ..............................................37
      8.1. Determining the Rate to Request ...........................37
      8.2. Deciding the Permitted Rate Request at a Router ...........37
   9. Evaluation of Quick-Start ......................................38
      9.1. Benefits of Quick-Start ...................................38
      9.2. Costs of Quick-Start ......................................39
      9.3. Quick-Start with QoS-Enabled Traffic ......................41
      9.4. Protection against Misbehaving Nodes ......................41
           9.4.1. Misbehaving Senders ................................41
           9.4.2. Receivers Lying about Whether the Request
                  was Approved .......................................43



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           9.4.3. Receivers Lying about the Approved Rate ............43
           9.4.4. Collusion between Misbehaving Routers ..............44
      9.5. Misbehaving Middleboxes and the IP TTL ....................46
      9.6. Attacks on Quick-Start ....................................46
      9.7. Simulations with Quick-Start ..............................47
   10. Implementation and Deployment Issues ..........................47
      10.1. Implementation Issues for Sending Quick-Start Requests ...47
      10.2. Implementation Issues for Processing Quick-Start
            Requests .................................................48
      10.3. Possible Deployment Scenarios ............................48
      10.4. A Comparison with the Deployment Problems of ECN .........50
   11. Security Considerations .......................................50
   12. IANA Considerations ...........................................52
      12.1. IP Option ................................................52
      12.2. TCP Option ...............................................52
   13. Conclusions ...................................................53
   14. Acknowledgements ..............................................53
   Appendix A. Related Work ..........................................54
      A.1. Fast Start-Ups without Explicit Information from Routers ..54
      A.2. Optimistic Sending without Explicit Information from
           Routers ...................................................56
      A.3. Fast Start-Ups with Other Information from Routers ........56
      A.4. Fast Start-Ups with More Fine-Grained Feedback from
           Routers ...................................................57
      A.5. Fast Start-ups with Lower-Than-Best-Effort Service ........58
   Appendix B. Design Decisions ......................................59
      B.1. Alternate Mechanisms for the Quick-Start Request:
           ICMP and RSVP .............................................59
           B.1.1. ICMP ...............................................59
           B.1.2. RSVP ...............................................60
      B.2. Alternate Encoding Functions ..............................61
      B.3. The Quick-Start Request: Packets or Bytes? ................63
      B.4. Quick-Start Semantics: Total Rate or Additional Rate? .....64
      B.5. Alternate Responses to the Loss of a Quick-Start Packet ...65
      B.6. Why Not Include More Functionality? .......................66
      B.7. Alternate Implementations for a Quick-Start Nonce .........69
           B.7.1. An Alternate Proposal for the Quick-Start Nonce ....69
           B.7.2. The Earlier Request-Approved Quick-Start Nonce .....69
   Appendix C. Quick-Start with DCCP .................................70
   Appendix D. Possible Router Algorithm .............................72
   Appendix E. Possible Additional Uses for the Quick-Start ..........74
   Normative References ..............................................75
   Informative References ............................................75








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1.  Introduction

   Each connection begins with a question: "What is the appropriate
   sending rate for the current network path?"  The question is not
   answered explicitly, but each TCP connection determines the sending
   rate by probing the network path and altering the congestion window
   (cwnd) based on perceived congestion.  Each TCP connection starts
   with a pre-configured initial congestion window (ICW).  Currently,
   TCP allows an initial window of between one and four segments of
   maximum segment size (MSS) ([RFC2581], [RFC3390]).  The TCP
   connection then probes the network for available bandwidth using the
   slow-start procedure ([Jac88], [RFC2581]), doubling cwnd during each
   congestion-free round-trip time (RTT).

   The slow-start algorithm can be time-consuming --- especially over
   networks with large bandwidth or long delays.  It may take a number
   of RTTs in slow-start before the TCP connection begins to fully use
   the available bandwidth of the network.  For instance, it takes
   log_2(N) - 2 round-trip times to build cwnd up to N segments,
   assuming an initial congestion window of 4 segments.  This time in
   slow-start is not a problem for large file transfers, where the
   slow-start stage is only a fraction of the total transfer time.
   However, in the case of moderate-sized transfers, the connection
   might carry out its entire transfer in the slow-start phase, taking
   many round-trip times, where one or two RTTs might have been
   sufficient when using the currently available bandwidth along the
   path.

   A fair amount of work has already been done to address the issue of
   choosing the initial congestion window for TCP, with RFC 3390
   allowing an initial window of up to four segments based on the MSS
   used by the connection [RFC3390].  Our underlying premise is that
   explicit feedback from all the routers along the path would be
   required, in the current architecture, for best-effort connections to
   use initial windows significantly larger than those allowed by
   [RFC3390], in the absence of other information about the path.

   In using Quick-Start, a TCP host (say, host A) would indicate its
   desired sending rate in bytes per second, using a Quick-Start Option
   in the IP header of a TCP packet.  Each router along the path could,
   in turn, either approve the requested rate, reduce the requested
   rate, or indicate that the Quick-Start Request is not approved.  (We
   note that the `routers' referred to in this document also include the
   IP-layer processing of the Quick-Start Request at the sender.)  In
   approving a Quick-Start Request, a router does not give preferential
   treatment to subsequent packets from that connection; the router is
   only asserting that it is currently underutilized and believes there
   is sufficient available bandwidth to accommodate the sender's



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RFC 4782               Quick-Start for TCP and IP           January 2007


   requested rate.  The Quick-Start mechanism can determine if there are
   routers along the path that do not understand the Quick-Start Option,
   or have not agreed to the Quick-Start rate request.  TCP host B
   communicates the final rate request to TCP host A in a transport-
   level Quick-Start Response in an answering TCP packet.

   If the Quick-Start Request is approved by all routers along the path,
   then the TCP host can send at up to the approved rate for a window of
   data.  Subsequent transmissions will be governed by the default TCP
   congestion control mechanisms of that connection.  If the Quick-Start
   Request is not approved, then the sender would use the default
   congestion control mechanisms.

   Quick-Start would not be the first mechanism for explicit
   communication from routers to transport protocols about sending
   rates.  Explicit Congestion Notification (ECN) gives explicit
   congestion control feedback from routers to transport protocols,
   based on the router detecting congestion before buffer overflow
   [RFC3168].  In contrast, routers would not use Quick-Start to give
   congestion information, but instead would use Quick-Start as an
   optional mechanism to give permission to transport protocols to use
   higher sending rates, based on the ability of all the routers along
   the path to determine if their respective output links are
   significantly underutilized.

   Section 2 gives an overview of Quick-Start.  The formal
   specifications for Quick-Start are contained in Sections 3, 4, 6.1.1,
   and 6.3.  In particular, Quick-Start is specified for IPv4 and for
   IPv6 in Section 3, and is specified for TCP in Section 4.  Section 6
   consists mostly of a non-normative discussion of interactions of
   Quick-Start with IP tunnels and MPLS; however, Section 6.1.1 and 6.3
   specify behavior for IP tunnels that are aware of Quick-Start.

   The rest of the document is non-normative, as follows.  Section 5
   shows that Quick-Start is compatible with IPsec AH (Authentication
   Header).  Section 7 gives a non-normative set of guidelines for
   specifying Quick-Start in other transport protocols, and Section 8
   discusses using Quick-Start in transport end-nodes and routers.
   Section 9 gives an evaluation of the costs and benefits of Quick-
   Start, and Section 10 discusses implementation and deployment issues.
   The appendices discuss related work, Quick-Start design decisions,
   and possible router algorithms.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



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2.  Assumptions and General Principles

   This section describes the assumptions and general principles behind
   the design of the Quick-Start mechanism.

   Assumptions:

   * The data transfer in the two directions of a connection traverses
     different queues, and possibly even different routers.  Thus, any
     mechanism for determining the allowed sending rate would have to be
     used independently for each direction.

   * The path between the two endpoints is relatively stable, such that
     the path used by the Quick-Start Request is generally the same path
     used by the Quick-Start packets one round-trip time later.
     [ZDPS01] shows this assumption should be generally valid.  However,
     [RFC3819] discusses a range of Bandwidth on Demand subnets that
     could cause the characteristics of the path to change over time.

   * Any new mechanism must be incrementally deployable and might not be
     supported by all the routers and/or end-hosts.  Thus, any new
     mechanism must be able to accommodate non-supporting routers or
     end-hosts without disturbing the current Internet semantics.  We
     note that, while Quick-Start is incrementally deployable in this
     sense, a Quick-Start Request cannot be approved for a particular
     connection unless both end-nodes and all the routers along the path
     have been configured to support Quick-Start.

   General Principles:

   * Our underlying premise is that explicit feedback from all the
     routers along the path would be required, in the current
     architecture, for best-effort connections to use initial windows
     significantly larger than those allowed by [RFC3390], in the
     absence of other information about the path.

   * A router should only approve a Quick-Start Request if the output
     link is underutilized.  Any other approach will result in either
     per-flow state at the router, or the possibility of a (possibly
     transient) queue at the router.

   * No per-flow state should be required at the router.  Note that,
     while per-flow state is not required, we also do not preclude a
     router from storing per-flow state for making Quick-Start decisions
     or for checking for misbehaving nodes.






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2.1.  Overview of Quick-Start

   In this section, we give an overview of the use of Quick-Start with
   TCP to request a higher congestion window.  The description in this
   section is non-normative; the normative description of Quick-Start
   with IP and TCP follows in Sections 3 and 4.  Quick-Start could be
   used in the middle of a connection, e.g., after an idle or
   underutilized period, as well as for the initial sending rate; these
   uses of Quick-Start are discussed later in the document.

   Quick-Start requires end-points and routers to work together, with
   end-points requesting a higher sending rate in the Quick-Start (QS)
   option in IP, and routers along the path approving, modifying,
   discarding, or ignoring (and therefore disallowing) the Quick-Start
   Request.  The receiver uses reliable, transport-level mechanisms to
   inform the sender of the status of the Quick-Start Request.  For
   example, when TCP is used, the TCP receiver sends feedback to the
   sender using a Quick-Start Response option in the TCP header.  In
   addition, Quick-Start assumes a unicast, congestion-controlled
   transport protocol; we do not consider the use of Quick-Start for
   multicast traffic.

   When sent as a request, the Quick-Start Option includes a request for
   a sending rate in bits per second, and a Quick-Start Time to Live (QS
   TTL) to be decremented by every router along the path that
   understands the option and approves the request.  The Quick-Start TTL
   is initialized by the sender to a random value.  The transport
   receiver returns the rate, information about the TTL, and the Quick-
   Start Nonce to the sender using transport-level mechanisms; for TCP,
   the receiver sends this information in the Quick-Start Response in
   the TCP header.  In particular, the receiver computes the difference
   between the Quick-Start TTL and the IP TTL (the TTL in the IP header)
   of the Quick-Start Request packet, and returns this in the Quick-
   Start Response.  The sender uses the TTL difference to determine if
   all the routers along the path decremented the Quick-Start TTL,
   approving the Quick-Start Request.

   If the request is approved by all the routers along the path, then
   the TCP sender combines this allowed rate with the measurement of the
   round-trip time, and ends up with an allowed TCP congestion window.
   This window is sent rate-paced over the next round-trip time, or
   until an ACK packet is received.

   Figure 1 shows a successful use of Quick-Start, with the sender's IP
   layer and both routers along the path approving the Quick-Start
   Request, and the TCP receiver using the Quick-Start Response to
   return information to the TCP sender.  In this example, Quick-Start
   is used by TCP to establish the initial congestion window.



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   Sender        Router 1       Router 2          Receiver
   ------        --------       --------          --------
   | <IP TTL: 63>
   | <QS TTL: 91>
   | <TTL Diff: 28>
   | Quick-Start Request
   | in SYN or SYN/ACK.
   | IP: Decrement QS TTL
   | to approve request -->
   |
   |               Decrement
   |               QS TTL
   |               to approve
   |               request -->
   |
   |                              Decrement
   |                              QS TTL
   |                              to approve
   |                              request -->
   |
   |                                           <IP TTL: 60>
   |                                           <QS TTL: 88>
   |                                           <TTL Diff: 28>
   |                                           Return Quick-Start
   |                                            info to sender in
   |                                           Quick-Start Response
   |                                          <-- in TCP ACK packet.
   |
   | <TTL Diff: 28>
   | Quick-Start approved,
   | translate to cwnd.
   | Report Approved Rate.
   V Send cwnd paced over one RTT. -->

                Figure 1: A Successful Quick-Start Request.

   Figure 2 shows an unsuccessful use of Quick-Start, with one of the
   routers along the path not approving the Quick-Start Request.  If the
   Quick-Start Request is not approved, then the sender uses the default
   congestion control mechanisms for that transport protocol, including
   the default initial congestion window, response to idle periods, etc.










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   Sender        Router 1       Router 2          Receiver
   ------        --------       --------          --------
   | <IP TTL: 63>
   | <QS TTL: 91>
   | <TTL Diff: 28>
   | Quick-Start Request
   | in SYN or SYN/ACK.
   | IP: Decrement QS TTL
   | to approve request -->
   |
   |               Decrement
   |               QS TTL
   |               to approve
   |               request -->
   |
   |                              Forward packet
   |                              without modifying
   |                              Quick-Start Option. -->
   |
   |                                           <IP TTL: 60>
   |                                           <QS TTL: 89>
   |                                           <TTL Diff: 29>
   |                                           Return Quick-Start
   |                                            info to sender in
   |                                           Quick-Start Response
   |                                          <-- in TCP ACK packet.
   |
   | <TTL Diff: 29>
   | Quick-Start not approved.
   | Report approved rate.
   V Use default initial cwnd. -->

              Figure 2: An Unsuccessful Quick-Start Request.


















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3.  The Quick-Start Option in IP

3.1.  The Quick-Start Option for IPv4

   The Quick-Start Request for IPv4 is defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Option      |  Length=8     | Func. | Rate  |   QS TTL      |
   |               |               | 0000  |Request|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        QS Nonce                           | R |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: The Quick-Start Option for IPv4.
                          A Quick-Start Request.

   The first byte contains the option field, which includes the one-bit
   copy flag, the 2-bit class field, and the 5-bit option number.

   The second byte contains the length field, indicating an option
   length of eight bytes.

   The third byte includes a four-bit Function field.  If the Function
   field is set to "0000", then the Quick-Start Option is a Rate
   Request.  In this case, the second half of the third byte is a four-
   bit Rate Request field.

   For a Rate Request, the fourth byte contains the Quick-Start TTL (QS
   TTL) field.  The sender MUST set the QS TTL field to a random value.
   Routers that approve the Quick-Start Request decrement the QS TTL
   (mod 256) by the same amount that they decrement the IP TTL.  (As
   elsewhere in this document, we use the term `router' imprecisely to
   also include the Quick-Start functionality at the IP layer at the
   sender.)  The QS TTL is used by the sender to detect if all the
   routers along the path understood and approved the Quick-Start
   option.

   For a Rate Request, the transport sender MUST calculate and store the
   TTL Diff, the difference between the IP TTL value, and the QS TTL
   value in the Quick-Start Request packet, as follows:

   TTL Diff = ( IP TTL - QS TTL ) mod 256                         (1)







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   For a Rate Request, bytes 5-8 contain a 30-bit QS Nonce, discussed in
   Section 3.4, and a two-bit Reserved field.  The sender SHOULD set the
   reserved field to zero, and routers and receivers SHOULD ignore the
   reserved field.  The sender SHOULD set the 30-bit QS Nonce to a
   random value.

   The sender initializes the Rate Request to the desired sending rate,
   including an estimate of the transport and IP header overhead.  The
   encoding function for the Rate Request sets the request rate to K*2^N
   bps (bits per second), for N the value in the Rate Request field, and
   for K set to 40,000.  For N=0, the rate request would be set to zero,
   regardless of the encoding function.  This is illustrated in Table 1
   below.  For the four-bit Rate Request field, the request range is
   from 80 Kbps to 1.3 Gbps.  Alternate encodings that were considered
   for the Rate Request are given in Appendix B.2.

    N     Rate Request (in Kbps)
   ---    ----------------------
    0            0
    1           80
    2          160
    3          320
    4          640
    5        1,280
    6        2,560
    7        5,120
    8       10,240
    9       20,480
   10       40,960
   11       81,920
   12      163,840
   13      327,680
   14      655,360
   15    1,310,720

   Table 1: Mapping from Rate Request Field to Rate Request in Kbps.

   Routers can approve the Quick-Start Request for a lower rate by
   decreasing the Rate Request in the Quick-Start Request.  Section 4.2
   discusses the Quick-Start Response from the TCP receiver to the TCP
   sender, and Section 4.4 discusses the TCP sender's mechanism for
   determining if a Quick-Start Request has been approved.









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Option      |  Length=8     | Func. | Rate  |   Not Used    |
   |               |               | 1000  | Report|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        QS Nonce                           | R |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 4: The Quick-Start Option for IPv4.
                         Report of Approved Rate.

   If the Function field in the third byte of the Quick-Start Option is
   set to "1000", then the Quick-Start Option is a Report of Approved
   Rate.  In this case, the second four bits in the third byte are the
   Rate Report field, formatted exactly as in the Rate Request field in
   a Rate Request.  For a Report of Approved Rate, the fourth byte of
   the Quick-Start Option is not used.  Bytes 5-8 contain a 30-bit QS
   Nonce and a 2-bit Reserved field.

   After an approved Rate Request, the sender MUST report the Approved
   Rate, using a Quick-Start Option configured as a Report of Approved
   Rate with the Rate Report field set to the approved rate, and the QS
   Nonce set to the QS Nonce sent in the Quick-Start Request.  The
   packet containing the Report of Approved Rate MUST be either a
   control packet sent before the first Quick-Start data packet, or a
   Quick-Start Option in the first data packet itself.  The Report of
   Approved Rate does not have to be sent reliably; for example, if the
   approved rate is reported in a separate control packet, the sender
   does not necessarily know if the control packet has been dropped in
   the network.  If the packet containing the Quick-Start Request is
   acknowledged, but the acknowledgement packet does not contain a
   Quick-Start Response, then the sender MUST assume that the Quick-
   Start Request was denied, and set a Report of Approved Rate with a
   rate of zero.  Routers may choose to ignore the Report of Approved
   Rate, or to use the Report of Approved Rate but ignore the QS Nonce.
   Alternately, some routers that use the Report of Approved Rate may
   choose to match the QS Nonce, masked by the approved rate, with the
   QS Nonce seen in the original request.

   If the Rate Request is denied, the sender MUST send a Report of
   Approved Rate with the Rate Report field set to zero.

   We note that, unlike a Quick-Start Request sent at the beginning of a
   connection, when a Quick-Start Request is sent in the middle of a
   connection, the connection could already have an established
   congestion window or sending rate.  The Rate Request is the requested
   total rate for the connection, including the current rate of the



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   connection; the Rate Request is *not* a request for an additional
   sending rate over and above the current sending rate.  If the Rate
   Request is denied, or lowered to a value below the connection's
   current sending rate, then the sender ignores the request, and
   reverts to the default congestion control mechanisms of the transport
   protocol.

   The use of the Quick-Start Option does not require the additional use
   of the Router Alert Option [RFC2113].

   We note that in IPv4, a change in IP options at routers requires
   recalculating the IP header checksum.

3.2.  The Quick-Start Option for IPv6

   The Quick-Start Option for IPv6 is placed in the Hop-by-Hop Options
   extension header that is processed at every network node along the
   communication path [RFC2460].  The option format following the
   generic Hop-by-Hop Options header is identical to the IPv4 format,
   with the exception that the Length field should exclude the common
   type and length fields in the option format and be set to 6 bytes
   instead of 8 bytes.

   For a Quick-Start Request, the transport receiver compares the
   Quick-Start TTL with the IPv6 Hop Limit field to calculate the TTL
   Diff.  (The Hop Limit in IPv6 is the equivalent of the TTL in IPv4.)
   That is, TTL Diff MUST be calculated and stored as follows:

   TTL Diff = ( IPv6 Hop Limit - QS TTL ) mod 256                  (2)

   Unlike IPv4, modifying or deleting the Quick-Start IPv6 Option does
   not require checksum re-calculation, because the IPv6 header does not
   have a checksum field, and modifying the Quick-Start Request in the
   IPv6 Hop-by-Hop options header does not affect the IPv6 pseudo-
   header checksum used in upper-layer checksum calculations.

   Appendix A of RFC 2460 requires that all packets with the same flow
   label must be originated with the same hop-by-hop header contents,
   which would be incompatible with Quick-Start.  However, a later IPv6
   flow label specification [RFC3697] updates the use of flow labels in
   IPv6 and removes this restriction.  Therefore, Quick-Start is
   compatible with the current IPv6 specifications.









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3.3.  Processing the Quick-Start Request at Routers

   The Quick-Start Request does not report the current sending rate of
   the connection sending the request; in the default case of a router
   (or IP-layer implementation at an end-node) that does not maintain
   per-flow state, a router makes the conservative assumption that the
   flow's current sending rate is zero.  Each participating router can
   either terminate or approve the Quick-Start Request.  A router MUST
   only approve a Quick-Start Request if the output link is
   underutilized, and if the router judges that the output link will
   continue to be underutilized if this and earlier approved requests
   are used by the senders.  Otherwise, the router reduces or terminates
   the Quick-Start Request.

   While the paragraph above defines the *semantics* of approving a
   Quick-Start Request, this document does not specify the specific
   algorithms to be used by routers in processing Quick-Start Requests
   or Reports.  This is similar to RFC 3168, which specifics the
   semantics of the ECN codepoints in the IP header, but does not
   specify specific algorithms for routers to use in deciding when to
   drop or mark packets before buffer overflow.

   A router that wishes to terminate the Quick-Start Request SHOULD
   either delete the Quick-Start Request from the IP header or zero the
   QS TTL, QS Nonce, and Rate Request fields.  Deleting the Quick-Start
   Request saves resources because downstream routers will have no
   option to process, but zeroing the Rate Request field may be more
   efficient for routers to implement.  As suggested in [B05], future
   additions to Quick-Start could define error codes for routers to
   insert into the QS Nonce field to report back to the sender the
   reason that the Quick-Start Request was denied, e.g., that the router
   is denying all Quick-Start Requests at this time, or that this
   router, as a matter of policy, does not grant Quick-Start requests.
   A router that doesn't understand the Quick-Start Option will simply
   forward the packet with the Quick-Start Request unchanged (ensuring
   that the TTL Diff will not match and Quick-Start will not be used).

   If the participating router has decided to approve the Quick-Start
   Request, it does the following:

   * The router MUST decrement the QS TTL by the amount the IP TTL is
     decremented (usually one).

   * If the router is only willing to approve a Rate Request less than
     that in the Quick-Start Request, then the router replaces the Rate
     Request with a smaller value.  The router MUST NOT increase the
     Rate Request in the Quick-Start Request.  If the router decreases




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     the Rate Request, the router MUST also modify the QS Nonce, as
     described in Section 3.4.

   * In IPv4, the router MUST update the IP header checksum.

   If the router approves the Quick-Start Request, this approval SHOULD
   be taken into account in the router's decision to accept or reject
   subsequent Quick-Start Requests (e.g., using a variable that tracks
   the recent aggregate of accepted Quick-Start Requests).  This
   consideration of earlier approved Quick-Start Requests is necessary
   to ensure that the router only approves a Quick-Start Request when
   the router judges that the output link will remain underutilized if
   this and earlier Quick-Start Requests are used by the senders.

   In addition, the approval of a Quick-Start Request SHOULD NOT be used
   by the router to affect the treatment of the data packets that arrive
   from this connection in the next few round-trip times.  That is, the
   approval by the router of a Quick-Start Request does not give
   differential treatment for Quick-Start data packets at that router;
   it is only a statement from the router that the router believes that
   the subsequent Quick-Start data packets from this connection will not
   change the current underutilized state of the router.

   A non-participating router forwards the Quick-Start Request
   unchanged, without decrementing the QS TTL.  The non-participating
   router still decrements the TTL field in the IP header, as is
   required for all routers [RFC1812].  As a result, the sender will be
   able to detect that the Quick-Start Request had not been understood
   or approved by all of the routers along the path.

   A router that uses multipath routing for packets within a single
   connection MUST NOT approve a Quick-Start Request.  This is discussed
   in more detail in Section 9.2.

3.3.1.  Processing the Report of Approved Rate

   If the Quick-Start Option has the Function field set to "1000", then
   this is a Report of Approved Rate, rather than a Rate Request.  The
   router MAY ignore such an option, and, in any case, it MUST NOT
   modify the contents of the option for a Report of Approved Rate.
   However, the router MAY use the Approved Rate report to check that
   the sender is not lying about the approved rate.  If the reported
   Approved Rate is higher than the rate that the router actually
   approved for this connection in the previous round-trip time, then
   the router may implement some policy for cheaters.  For instance, the
   router MAY decide to deny future Quick-Start Requests from this
   sender, including, if desired, deleting Quick-Start Requests from
   future packets from this sender.  Section 9.4.1 discusses misbehaving



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   senders in more detail.  From the Report of Approved Rate, the router
   can also learn if some of the downstream routers have approved the
   Quick-Start Request for a smaller rate or denied the use of Quick-
   Start, and adjust its bandwidth allocations accordingly.

3.4.  The QS Nonce

   The QS Nonce gives the Quick-Start sender some protection against
   receivers lying about the value of the received Rate Request.  This
   is particularly important if the receiver knows the original value of
   the Rate Request (e.g., when the sender always requests the same
   value, and the receiver has a long history of communication with that
   sender).  Without the QS Nonce, there is nothing to prevent the
   receiver from reporting back to the sender a Rate Request of K, when
   the received Rate Request was, in fact, less than K.

   Table 2 gives the format for the 30-bit QS Nonce.

   Bits         Purpose
   ---------    ------------------
   Bits 0-1:    Rate 15 -> Rate 14
   Bits 2-3:    Rate 14 -> Rate 13
   Bits 4-5:    Rate 13 -> Rate 12
   Bits 6-7:    Rate 12 -> Rate 11
   Bits 8-9:    Rate 11 -> Rate 10
   Bits 10-11:  Rate 10 -> Rate 9
   Bits 12-13:  Rate 9 -> Rate 8
   Bits 14-15:  Rate 8 -> Rate 7
   Bits 16-17:  Rate 7 -> Rate 6
   Bits 18-19:  Rate 6 -> Rate 5
   Bits 20-21:  Rate 5 -> Rate 4
   Bits 22-23:  Rate 4 -> Rate 3
   Bits 24-25:  Rate 3 -> Rate 2
   Bits 26-27:  Rate 2 -> Rate 1
   Bits 28-29:  Rate 1 -> Rate 0

   Table 2: The QS Nonce.

   The transport sender MUST initialize the QS Nonce to a random value.
   If the router reduces the Rate Request from rate K to rate K-1, then
   the router MUST set the field in the QS Nonce for "Rate K -> Rate
   K-1" to a new random value.  Similarly, if the router reduces the
   Rate Request by N steps, the router MUST set the 2N bits in the
   relevant fields in the QS Nonce to a new random value.  The receiver
   MUST report the QS Nonce back to the sender.






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   If the Rate Request was not decremented in the network, then the QS
   Nonce should have its original value.  Similarly, if the Rate Request
   was decremented by N steps in the network, and the receiver reports
   back a Rate Request of K, then the last 2K bits of the QS Nonce
   should have their original value.

   With the QS Nonce, the receiver has a 1/4 chance of cheating about
   each step change in the rate request.  Thus, if the rate request is
   reduced by two steps in the network, the receiver has a 1/16 chance
   of successfully reporting that the original request was approved, as
   this requires reporting the original value for the QS nonce.
   Similarly, if the rate request is reduced many steps in the network,
   and the receiver receives a QS Option with a rate request of K, the
   receiver has a 1/16 chance of guessing the original values for the
   fields in the QS nonce for "Rate K+2 -> Rate K+1" and "Rate K+1 ->
   Rate K".  Thus, the receiver has a 1/16 chance of successfully lying
   and saying that the received rate request was K+2 instead of K.

   We note that the protection offered by the QS Nonce is the same
   whether one router makes all the decrements in the rate request, or
   whether they are made at different routers along the path.

   The requirements for randomization for the sender and routers in
   setting `random' values in the QS Nonce are not stringent -- almost
   any form of pseudo-random numbers will do.  The requirement is that
   the original value for the QS Nonce is not easily predictable by the
   receiver, and in particular, the nonce MUST NOT be easily determined
   from inspection of the rest of the packet or from previous packets.
   In particular, the nonce MUST NOT be based only on a combination of
   specific packet header fields.  Thus, if two bits of the QS Nonce are
   changed by a router along the path, the receiver should not be able
   to guess those two bits from the other 28 bits in the QS Nonce.

   An additional requirement is that the receiver cannot be able to
   tell, from the QS Nonce itself, which numbers in the QS Nonce were
   generated by the sender, and which were generated by routers along
   the path.  This makes it harder for the receiver to infer the value
   of the original rate request, making it one step harder for the
   receiver to cheat.

   Section 9.4 also considers issues of receiver cheating in more
   detail.









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4.  The Quick-Start Mechanisms in TCP

   This section describes how the Quick-Start mechanism would be used in
   TCP.  We first sketch the procedure and then tightly define it in the
   subsequent subsections.

   If a TCP sender (say, host A) would like to use Quick-Start, the TCP
   sender puts the requested sending rate in bits per second,
   appropriately formatted, in the Quick-Start Option in the IP header
   of the TCP packet, called the Quick-Start Request packet.  (We will
   be somewhat loose in our use of "packet" vs. "segment" in this
   section.)  When used for initial start-up, the Quick-Start Request
   packet can be either the SYN or SYN/ACK packet, as illustrated in
   Figure 1.  The requested rate includes an estimate for the transport
   and IP header overhead.  The TCP receiver (say, host B) returns the
   Quick-Start Response option in the TCP header in the responding
   SYN/ACK packet or ACK packet, called the Quick-Start Response packet,
   informing host A of the results of their request.

   If the acknowledging packet does not contain a Quick-Start Response,
   or contains a Quick-Start Response with the wrong value for the TTL
   Diff or the QS Nonce, then host A MUST assume that its Quick-Start
   request failed.  In this case, host A sends a Report of Approved Rate
   with a Rate Report of zero, and uses TCP's default congestion control
   procedure.  For initial start-up, host A uses the default initial
   congestion window ([RFC2581], [RFC3390]).

   If the returning packet contains a valid Quick-Start Response, then
   host A uses the information in the response, along with its
   measurement of the round-trip time, to determine the Quick-Start
   congestion window (QS-cwnd).  Quick-Start data packets are defined as
   data packets sent as the result of a successful Quick-Start request,
   up to the time when the first Quick-Start packet is acknowledged.
   The sender also sends a Report of Approved Rate.  In order to use
   Quick-Start, the TCP host MUST use rate-based pacing [VH97] to
   transmit Quick-Start packets at the rate indicated in the Quick-Start
   Response, at the level of granularity possible by the sending host.
   We note that the limitations of interrupt timing on computers can
   limit the ability of the TCP host in rate-pacing the outgoing
   packets.

   The two TCP end-hosts can independently decide whether to request
   Quick-Start.  For example, host A could send a Quick-Start Request in
   the SYN packet, and host B could also send a Quick-Start Request in
   the SYN/ACK packet.






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4.1.  Sending the Quick-Start Request

   When sending a Quick-Start Request, the TCP sender SHOULD send the
   request on a packet that requires an acknowledgement, such as a SYN,
   SYN/ACK, or data packet.  In this case, if the packet is acknowledged
   but no Quick-Start Response is included, then the sender knows that
   the Quick-Start Request has been denied, and can send a Report of
   Approved Rate.

   In addition to the use of Quick-Start when a connection is
   established, there are several additional points in a connection when
   a transport protocol may want to issue a Rate Request.  We first
   reiterate the notion that Quick-Start is a coarse-grained mechanism.
   That is, Quick-Start's Rate Requests are not meant to be used for
   fine-grained control of the transport's sending rate.  Rather, the
   transport MAY issue a Rate Request when no information about the
   appropriate sending rate is available, and the default congestion
   control mechanisms might be significantly underestimating the
   appropriate sending rate.

   The following are potential points where Quick-Start may be useful:

   (1) At or soon after connection initiation, when the transport has no
       idea of the capacity of the network path, as discussed above.  (A
       transport that uses TCP Control Block sharing [RFC2140], the
       Congestion Manager [RFC3124], or other mechanisms for sharing
       congestion information may not need Quick-Start to determine an
       appropriate rate.)

   (2) After an idle period when the transport no longer has a validated
       estimate of the available bandwidth for this flow.  (An example
       could be a persistent-HTTP connection when a new HTTP request is
       received after an idle period.)

   (3) After a host has received explicit indications that one of the
       endpoints has moved its point of network attachment.  This can
       happen due to some underlying mobility mechanism like Mobile IP
       ([RFC3344], [RFC3775]).  Some transports, such as Steam Control
       Transmission Protocol (SCTP) [RFC2960], may associate with
       multiple IP addresses and can switch addresses (and therefore
       network paths) in mid-connection.  If the transport has concrete
       knowledge of a changing network path, then the current sending
       rate may not be appropriate, and the transport sender may use
       Quick-Start to probe the network to see if it can send at a
       higher rate.  (Alternatively, traditional slow-start should be
       used in this case when Quick-Start is not available.)





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   (4) After an application-limited period, when the sender has been
       using only a small amount of its appropriate share of the network
       capacity and has no valid estimate for its fair share.  In this
       case, Quick-Start may be an appropriate mechanism to determine if
       the sender can send at a higher rate.  For instance, consider an
       application that steadily exchanges low- rate control messages
       and suddenly needs to transmit a large amount of data.

   Of the above, this document recommends that a TCP sender MAY attempt
   to use Quick-Start in cases (1) and (2).  It is NOT RECOMMENDED that
   a TCP sender use Quick-Start for case (3) at the current time.  Case
   (3) requires external notifications not presently defined for TCP or
   other transport protocols.  Finally, a TCP SHOULD NOT use Quick-
   Start for case (4) at the current time.  Case (4) requires further
   thought and investigation with regard to how the transport protocol
   could determine it was in a situation that would warrant transmitting
   a Quick-Start Request.

   As a general guideline, a TCP sender SHOULD NOT request a sending
   rate larger than it is able to use over the next round-trip time of
   the connection (or over 100 ms, if the round-trip time is not known),
   except as required to round up the desired sending rate to the next-
   highest allowable request.

   In any circumstances, the sender MUST NOT make a QS request if it has
   made a QS request within the most recent round-trip time.

   Section 4.7 discusses some of the issues of using Quick-Start at
   connection initiation, and Section 4.8 discusses issues that arise
   when Quick-Start is used to request a larger sending rate after an
   idle period.

4.2.  The Quick-Start Response Option in the TCP header

   In order to approve the use of Quick-Start, the TCP receiver responds
   to the receipt of a Quick-Start Request with a Quick-Start Response,
   using the Quick-Start Response Option in the TCP header.  TCP's
   Quick-Start Response option is defined as follows:













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Kind      |  Length=8     | Resv. | Rate  |   TTL Diff    |
   |               |               |       |Request|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   QS Nonce                                | R |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 5: The Quick-Start Response Option in the TCP Header.

   The first byte of the Quick-Start Response option contains the option
   kind, identifying the TCP option.

   The second byte of the Quick-Start Response option contains the
   option length in bytes.  The length field MUST be set to 8 bytes.

   The third byte of the Quick-Start Response option contains a four-
   bit Reserved field, and the four-bit allowed Rate Request, formatted
   as in the Quick-Start Rate Request option.

   The fourth byte of the TCP option contains the TTL Diff.  The TTL
   Diff contains the difference between the IP TTL and QS TTL fields in
   the received Quick-Start Request packet, as calculated in equations
   (1) or (2) (depending on whether IPv4 or IPv6 is used).

   Bytes 5-8 of the TCP option contain the 30-bit QS Nonce and a two-
   bit Reserved field.

   We note that, while there are limitations on the potential size of
   the Quick-Start Response Option, a Quick-Start Response Option of
   eight bytes should not be a problem.  The TCP Options field can
   contain up to 40 bytes.  Other TCP options that might be used in a
   SYN or SYN/ACK packet include Maximum Segment Size (four bytes), Time
   Stamp (ten bytes), Window Scale (three bytes), and Selective
   Acknowledgments Permitted (two bytes).

4.3.  TCP: Sending the Quick-Start Response

   An end host (say, host B) that receives an IP packet containing a
   Quick-Start Request passes the Quick-Start Request, along with the
   value in the IP TTL field, to the receiving TCP layer.

   If the TCP host is willing to permit the Quick-Start Request, then a
   Quick-Start Response option is included in the TCP header of the
   corresponding acknowledgement packet.  The Rate Request in the
   Quick-Start Response option is set to the received value of the Rate
   Request in the Quick-Start Option, or to a lower value if the TCP



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   receiver is only willing to allow a lower Rate Request.  The TTL Diff
   in the Quick-Start Response is set to the difference between the IP
   TTL value and the QS TTL value as given in equation (1) or (2)
   (depending on whether IPv4 or IPv6 is used).  The QS Nonce in the
   Response is set to the received value of the QS Nonce in the Quick-
   Start Option.

   If an end host receives an IP packet with a Quick-Start Request with
   a rate request of zero, then that host SHOULD NOT send a Quick-Start
   Response.

   The Quick-Start Response MUST NOT be resent if it is lost in the
   network.  Packet loss could be an indication of congestion on the
   return path, in which case it is better not to approve the Quick-
   Start Request.

4.4.  TCP: Receiving and Using the Quick-Start Response Packet

   A TCP host (say, TCP host A) that sent a Quick-Start Request and
   receives a Quick-Start Response in an acknowledgement first checks
   that the Quick-Start Response is valid.  The Quick-Start Response is
   valid if it contains the correct value for the TTL Diff, and an equal
   or lesser value for the Rate Request than that transmitted in the
   Quick-Start Request.  In addition, if the received Rate Request is K,
   then the rightmost 2K bits of the QS Nonce must match those bits in
   the QS Nonce sent in the Quick-Start Request.  If these checks are
   not successful, then the Quick-Start Request failed, and the TCP host
   MUST use the default TCP congestion window that it would have used
   without Quick-Start.  If the rightmost 2K bits of the QS Nonce do not
   match those bits in the QS Nonce sent in the Quick-Start Request, for
   a received Rate Request of K, then the TCP host MUST NOT send
   additional Quick-Start Requests during the life of the connection.
   Whether or not the Quick-Start Request was successful, the host
   receiving the Quick-Start Response MUST send a Report of Approved
   Rate.  Similarly, if the packet containing the Quick-Start Request is
   acknowledged, but the acknowledgement does not include a Quick-Start
   Response, then the sender MUST send a Report of Approved Rate.

   If the checks of the TTL Diff and the Rate Request are successful,
   and the TCP host is going to use the Quick-Start Request, it MUST
   start using it within one round-trip time of receiving the Quick-
   Start Response, or within three seconds, whichever is smaller.  To
   use the Quick-Start Request, the host sets its Quick-Start congestion
   window (in terms of MSS-sized segments), QS-cwnd, as follows:

   QS-cwnd = (R * T) / (MSS + H)                                (3)





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   where R is the Rate Request in bytes per second, T is the measured
   round-trip time in seconds, and H is the estimated TCP/IP header size
   in bytes (e.g., 40 bytes).

   Derivation: the sender is allowed to transmit at R bytes per second
   including packet headers, but only R*MSS/(MSS+H) bytes per second, or
   equivalently R*T*MSS/(MSS+H) bytes per round-trip time, of
   application data.

   The TCP host SHOULD set its congestion window cwnd to QS-cwnd only if
   QS-cwnd is greater than cwnd; otherwise, QS-cwnd is ignored.  If
   QS-cwnd is used, the TCP host sets a flag that it is in Quick-Start
   mode, and while in Quick-Start mode, the TCP sender MUST use rate-
   based pacing to pace out Quick-Start packets at the approved rate.
   If, during Quick-Start mode, the TCP sender receives ACKs for packets
   sent before this Quick-Start mode was entered, these ACKs are
   processed as usual, following the default congestion control
   mechanisms.  Quick-Start mode ends when the TCP host receives an ACK
   for one of the Quick-Start packets.

   If the congestion window has not been fully used when the first ack
   arrives ending the Quick-Start mode, then the congestion window is
   decreased to the amount that has actually been used so far.  This is
   necessary because when the Quick-Start Response is received, the TCP
   sender's round-trip-time estimate might be longer than for succeeding
   round-trip times, e.g., because of delays at routers processing the
   IP Quick-Start Option, or because of delays at the receiver in
   responding to the Quick-Start Request packet.  In this case, an
   overly large round-trip-time estimate could have caused the TCP
   sender to translate the approved Quick-Start sending rate in bytes
   per second into a congestion window that is larger than needed, with
   the TCP sender receiving an ACK for the first Quick- Start packet
   before the entire congestion window has been used.  Thus, when the
   TCP sender receives the first ACK for a Quick-Start packet, the
   sender MUST reduce the congestion window to the amount that has
   actually been used.

   As an example, a TCP sender with an approved Quick-Start Request of R
   KBps, B-byte packets including headers, and an RTT estimate of T
   seconds, would translate the Rate Request of R KBps to a congestion
   window of R*T/B packets.  The TCP sender would send the Quick-Start
   packets rate-paced at R KBps.  However, if the actual current round-
   trip time was T/2 seconds instead of T seconds, then the sender would
   begin to receive acknowledgements for Quick-Start packets after T/2
   seconds.  Following the paragraph above, the TCP sender would then
   reduce its congestion window from R*T/B to approximately R*T/(B*2)
   packets, the actual number of packets that were needed to fill the
   pipe at a sending rate of R KBps.  (Note: this is just an



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   illustration; the congestion window is actually set to the amount of
   data sent before the ACK arrives and not based on equations above.)

   After Quick-Start mode is exited and the congestion window adjusted
   if necessary, the TCP sender returns to using the default congestion-
   control mechanisms, processing further incoming ACK packets as
   specified by those congestion control mechanisms.  For example, if
   the TCP sender was in slow-start prior to the Quick-Start Request,
   and no packets were lost or marked since that time, then the sender
   continues in slow-start after exiting Quick-Start mode, as allowed by
   ssthresh.

   To add robustness, the TCP sender MUST use Limited Slow-Start
   [RFC3742] along with Quick-Start.  With Limited Slow-Start, the TCP
   sender limits the number of packets by which the congestion window is
   increased for one window of data during slow-start.

   When Quick-Start is used at the beginning of a connection, before any
   packet marks or losses have been reported, the TCP host MAY use the
   reported Rate Request to set the slow-start threshold to a desired
   value, e.g., to some small multiple of the congestion window.  A
   possible future research topic is how the sender might modify the
   slow-start threshold at the beginning of a connection to avoid
   overshooting the path capacity.  (The initial value of ssthresh is
   allowed to be arbitrarily high, and some TCP implementations use the
   size of the advertised window for ssthresh [RFC2581].)

4.5.  TCP: Controlling Acknowledgement Traffic on the Reverse Path

   When a Quick-Start Request is approved for a TCP sender, the
   resulting Quick-Start data traffic can result in a sudden increase in
   traffic for pure ACK packets on the reverse path.  For example, for
   the largest Quick-Start Request of 1.3 Gbps, given a TCP sender with
   1500-byte packets and a TCP receiver with delayed acknowledgements
   acking every other packet, this could result in 17.3 Mbps of
   acknowledgement traffic on the reverse path.

   One possibility, in cases with large Quick-Start Requests, would be
   for TCP receivers to send Quick-Start Requests to request bandwidth
   for the acknowledgement traffic on the reverse path.  However, in our
   view, a better approach would be for TCP receivers to simply control
   the rate of sending acknowledgement traffic.  The optimal future
   solution would involve the explicit use of congestion control for TCP
   acknowledgement traffic, as is done now for the acknowledgement
   traffic in DCCP's CCID2 [RFC4341].






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   In the absence of congestion control for acknowledgement traffic, the
   TCP receiver could limit its sending rate for ACK packets sent in
   response to Quick-Start data packets.  The following information is
   needed by the TCP receiver:

   * The RTT: TCP naturally measures the RTT of the path and therefore
     should have a sample of the RTT.  If the TCP receiver does not have
     a measurement of the round-trip time, it can use the time between
     the receipt of the Quick-Start Request and the Report of Approved
     Rate.

   * The Approved Rate Request (R): When the TCP receiver receives the
     Quick-Start Response packet, the receiver knows the value of the
     approved Rate Request.

   * The MSS: TCP advertises the MSS during the initial three-way
     handshake; therefore, the receiver should have an understanding of
     the packet size the sender will be using.  If the receiver does not
     have such an understanding or wishes to confirm the negotiated MSS,
     the size of the first data packet can be used.

   With this set of information, the TCP receiver can restrict its
   sending rate for pure acknowledgment traffic to at most 100 pure ACK
   packets per RTT by sending at most one ACK for every K data packets,
   for the ACK Ratio K set to R*RTT/(100*8*MSS).  The receiver would
   acknowledge the first Quick-Start data packet, and every succeeding K
   data packets.  Thus, for a somewhat extreme case of R=1.3 Gbps,
   RTT=0.2 seconds, and MSS=1500 bytes, K would be set to 216, and the
   receiver would acknowledge every 216 data packets.  From [RFC2581],
   the ACK Ratio K should have a minimum value of two.  When the ACK
   Ratio is greater than two, and the TCP sender receives
   acknowledgements each acknowledging more than two data packets, the
   TCP sender may want to use rate-based pacing to control the
   burstiness of its outgoing data traffic.

   In the absence of explicit congestion control mechanisms, the TCP end
   nodes cannot determine the packet drop rate for pure acknowledgement
   traffic.  This is true with or without Quick-Start.  However, the TCP
   receiver could limit its increase in the sending rate for pure ACK
   packets by at most doubling the sending rate for pure ACK packets
   from one round-trip time to the next.  The TCP receiver would do this
   by halving the ACK Ratio each round-trip time.

   Note that the above is one particular mechanism that could be used to
   control the ACK stream.  Future work that investigates this scheme
   and others in detail is encouraged.





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4.6.  TCP: Responding to a Loss of a Quick-Start Packet

   For TCP, we have defined a "Quick-Start packet" as one of the packets
   sent in the window immediately following a successful Quick-Start
   Request.  After detecting the loss or ECN-marking of a Quick-Start
   packet, TCP MUST revert to the default congestion control procedures
   that would have been used if the Quick-Start Request had not been
   approved.  For example, if Quick-Start is used for setting the
   initial window, and a packet from the initial window is lost or
   marked, then the TCP sender MUST then slow-start with the default
   initial window that would have been used if Quick-Start had not been
   used.  In addition to reverting to the default congestion control
   mechanisms, the sender MUST take into account that the Quick-Start
   congestion window was too large.  Thus, the sender SHOULD decrease
   ssthresh to, at most, half the number of Quick-Start packets that
   were successfully transmitted.  Appendix B.5 discusses possible
   alternatives in responding to the loss of a Quick-Start packet.

   If a Quick-Start packet is lost or ECN-marked, then the sender SHOULD
   NOT make future Quick-Start Requests for this connection.

   We note that ECN [RFC3168] MAY be used with Quick-Start.  As is
   always the case with ECN, the sender's congestion control response to
   an ECN-marked Quick-Start packet is the same as the response to a
   dropped Quick-Start packet, thus reverting to slow start in the case
   of Quick-Start packets marked as experiencing congestion.

4.7.  TCP: A Quick-Start Request for a Larger Initial Window

   Some of the issues of using Quick-Start are related to the specific
   scenario in which Quick-Start is used.  This section discusses the
   following issues that arise when Quick-Start is used by TCP to
   request a larger initial window: (1) interactions with Path MTU
   Discovery (PMTUD); and (2) Quick-Start Request packets that are
   discarded by middleboxes.

4.7.1.  Interactions with Path MTU Discovery

   One issue when Quick-Start is used to request a large initial window
   concerns the interactions between the large initial window and Path
   MTU Discovery.  Some of the issues are discussed in RFC 3390:

   "When larger initial windows are implemented along with Path MTU
   Discovery [RFC1191], alternatives are to set the `Don't Fragment'
   (DF) bit in all segments in the initial window, or to set the `Don't
   Fragment' (DF) bit in one of the segments.  It is an open question as
   to which of these two alternatives is best."




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   If the sender knows the Path MTU when the initial window is sent
   (e.g., from a PMTUD cache or from some other IETF-approved method),
   then the sender SHOULD use that MTU for segments in the initial
   window.  Unfortunately, the sender doesn't necessarily know the Path
   MTU when it sends packets in the initial window.  In this case, the
   sender should be conservative in the packet size used.  Sending a
   large number of overly large packets with the DF bit set is not
   desirable, but sending a large number of packets that are fragmented
   in the network can be equally undesirable.

   If the sender doesn't know the Path MTU when the initial window is
   sent, the sender SHOULD send one large packet in the initial window
   with the DF bit set, and send the remaining packets in the initial
   window with a smaller MTU of 576 bytes (or 1280 bytes with IPv6).

   A second possibility would be for the sender to delay sending the
   Quick-Start Request for one round-trip time by sending the Quick-
   Start Request with the first window of data, while also doing Path
   MTU Discovery.

   The sender may be using an iterative approach such as Packetization
   Layer Path MTU Discovery (PLPMTUD) [MH06] for Path MTU Discovery,
   where the sender tests successively larger MTUs.  If a probe is
   successfully delivered, then the MTU can be raised to reflect the
   value used in that probe.  We would note that PLPMTUD does not allow
   the sender to determine the Path MTU before sending the initial
   window of data.

4.7.2.  Quick-Start Request Packets that are Discarded by Routers or
        Middleboxes

   It is always possible for a TCP SYN packet carrying a Quick-Start
   request to be dropped in the network due to congestion, or to be
   blocked due to interactions with routers or middleboxes, where a
   middlebox is defined as any intermediary box performing functions
   apart from normal, standard functions of an IP router on the data
   path between a source host and destination host [RFC3234].
   Measurement studies of interactions between transport protocols and
   middleboxes [MAF04] show that for 70% of the Web servers
   investigated, no connection is established if the TCP SYN packet
   contains an unknown IP option (and for 43% of the Web servers, no
   connection is established if the TCP SYN packet contains an IP
   TimeStamp Option).  In both cases, this is presumably due to routers
   or middleboxes along that path.

   If the TCP sender doesn't receive a response to the SYN or SYN/ACK
   packet containing the Quick-Start Request, then the TCP sender SHOULD
   resend the SYN or SYN/ACK packet without the Quick-Start Request.



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   Similarly, if the TCP sender receives a TCP reset in response to the
   SYN or SYN/ACK packet containing the Quick-Start Request, then the
   TCP sender SHOULD resend the SYN or SYN/ACK packet without the
   Quick-Start Request [RFC3360].

   RFCs 1122 and 2988 specify that the sender should set the initial RTO
   (retransmission timeout) to three seconds, though many TCP
   implementations set the initial RTO to one second.  For a TCP SYN
   packet sent with a Quick-Start request, the TCP sender SHOULD use an
   initial RTO of three seconds.

   We note that if the TCP SYN packet is using the IP Quick-Start Option
   for a Quick-Start Request, and it is also using bits in the TCP
   header to negotiate ECN-capability with the TCP host at the other
   end, then the drop of a TCP SYN packet could be due to congestion, a
   router or middlebox dropping the packet because of the IP Option, or
   a router or middlebox dropping the packet because of the information
   in the TCP header negotiating ECN.  In this case, the sender could
   resend the dropped packet without either the Quick-Start or the ECN
   requests.  Alternately, the sender could resend the dropped packet
   with only the ECN request in the TCP header, resending the TCP SYN
   packet without either the Quick-Start or the ECN requests if the
   second TCP SYN packet is dropped.  The second choice seems
   reasonable, given that a TCP SYN packet today is more likely to be
   blocked due to policies that discard packets with IP Options than due
   to policies that discard packets with ECN requests in the TCP header
   [MAF04].

4.8.  TCP: A Quick-Start Request in the Middle of a Connection

   This section discusses the following issues that arise when Quick-
   Start is used by TCP to request a larger window in the middle of a
   connection, such as after an idle period: (1) determining the rate to
   request; (2) when to make a request; and (3) the response if Quick-
   Start packets are dropped.

   (1) Determining the rate to request:
       For a connection that has not yet had a congestion event (that
       is, a marked or dropped packet), the TCP sender is not restricted
       in the rate that it requests.  As an example, a server might wait
       and send a Quick-Start Request after a short interaction with the
       client.

       To use a Quick-Start Request in a connection that has already
       experienced a congestion event, and that has not had a recent
       mobility event, the TCP sender can determine the largest
       congestion window that the TCP connection achieved since the last
       packet drop and translate this to a sending rate to get the



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       maximum allowed request rate.  If the request is granted, then
       the sender essentially restarts with its old congestion window
       from before it was reduced, for example, during an idle period.

       A Quick-Start Request sent in the middle of a TCP connection
       SHOULD be sent on a data packet.

   (2) When to make a request:
       A TCP connection MAY make a Quick-Start Request before the
       connection has experienced a congestion event, or after an idle
       period of at least one RTO.

   (3) Response if Quick-Start packets are dropped:
       If Quick-Start packets are dropped in the middle of connection,
       then the sender MUST revert to half the Quick-Start window, or to
       the congestion window that the sender would have used if the
       Quick-Start request had not been approved, whichever is smaller.

4.9.  An Example Quick-Start Scenario with TCP

   The following is an example scenario of when both hosts request
   Quick-Start for setting their initial windows.  This is similar to
   Figures 1 and 2 in Section 2.1, except that it illustrates a TCP
   connection with both TCP hosts sending Quick-Start Requests.

   * The TCP SYN packet from Host A contains a Quick-Start Request in
     the IP header.

   * Routers along the forward path modify the Quick-Start Request as
     appropriate.

   * Host B receives the Quick-Start Request in the SYN packet, and
     calculates the TTL Diff.  If Host B approves the Quick-Start
     Request, then Host B sends a Quick-Start Response in the TCP header
     of the SYN/ACK packet.  Host B also sends a Quick-Start Request in
     the IP header of the SYN/ACK packet.

   * Routers along the reverse path modify the Quick-Start Request as
     appropriate.

   * Host A receives the Quick-Start Response in the SYN/ACK packet, and
     checks the TTL Diff, Rate Request, and QS Nonce for validity.  If
     they are valid, then Host A sets its initial congestion window
     appropriately, and sets up rate-based pacing to be used with the
     initial window.  If the Quick-Start Response is not valid, then
     Host A uses TCP's default initial window.  In either case, Host A
     sends a Report of Approved Rate.




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     Host A also calculates the TTL Diff for the Quick-Start Request in
     the incoming SYN/ACK packet, and sends a Quick-Start Response in
     the TCP header of the ACK packet.

   * Host B receives the Quick-Start Response in an ACK packet, and
     checks the TTL Diff, Rate Request, and QS Nonce for validity.  If
     the Quick-Start Response is valid, then Host B sets its initial
     congestion window appropriately, and sets up rate-based pacing to
     be used with its initial window.  If the Quick-Start Response is
     not valid, then Host B uses TCP's default initial window.  In
     either case, Host B sends a Report of Approved Rate.

5.  Quick-Start and IPsec AH

   This section shows that Quick-Start is compatible with IPsec
   Authentication Header (AH).  AH uses an Integrity Check Value (ICV)
   in the IPsec Authentication Header to verify both message
   authentication and integrity [RFC4302].  Changes to the Quick-Start
   Option in the IP header do not affect this AH ICV.  The tunnel
   considerations in Section 6 below apply to all IPsec tunnels,
   regardless of what IPsec headers or processing are used in
   conjunction with the tunnel.

   Because the contents of the Quick-Start Option can change along the
   path, it is important that these changes not affect the IPsec
   Authentication Header Integrity Check Value (AH ICV).  For IPv4, RFC
   4302 requires that unrecognized IPv4 options be zeroed for AH ICV
   computation purposes, so Quick-Start IP Option data changing en route
   does not cause problems with existing IPsec AH implementations for
   IPv4.  If the Quick-Start Option is recognized, it MUST be treated as
   a mutable IPv4 option, and hence be completely zeroed for AH ICV
   calculation purposes.  IPv6 option numbers explicitly indicate
   whether the option is mutable; the third-highest order bit in the
   IANA-allocated option type has the value 1 to indicate that the
   Quick-Start Option data can change en route.  RFC 4302 requires that
   the option data of any such option be zeroed for AH ICV computation
   purposes.  Therefore, changes to the Quick-Start Option in the IP
   header do not affect the calculation of the AH ICV.













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6.  Quick-Start in IP Tunnels and MPLS

   This section considers interactions between Quick-Start and IP
   tunnels, including IPsec ([RFC4301]), IP in IP [RFC2003], GRE
   [RFC2784], and others.  This section also considers interactions
   between Quick-Start and MPLS [RFC3031].

   In the discussion, we use TTL Diff, defined earlier as the difference
   between the IP TTL and the Quick-Start TTL, mod 256.  Recall that the
   sender considers the Quick-Start Request approved only if the value
   of TTL Diff for the packet entering the network is the same as the
   value of TTL Diff for the packet exiting the network.

   Simple tunnels: IP tunnel modes are generally based on adding a new
   "outer" IP header that encapsulates the original or "inner" IP header
   and its associated packet.  In many cases, the new "outer" IP header
   may be added and removed at intermediate points along a path,
   enabling the network to establish a tunnel without requiring endpoint
   participation.  We denote tunnels that specify that the outer header
   be discarded at tunnel egress as "simple tunnels", and we denote
   tunnels where the egress saves and uses information from the outer
   header before discarding it as "non-simple tunnels".  An example of a
   "non-simple tunnel" would be a tunnel configured to support ECN,
   where the egress router might copy the ECN codepoint in the outer
   header to the inner header before discarding the outer header
   [RFC3168].

                       __ Tunnels Compatible with Quick-Start
                      /
   Simple Tunnels  __/
                     \
                      \__ Tunnels Not Compatible with Quick-Start
                                    (False Positives!)


                           __ Tunnels Supporting Quick-Start
                          /
                         /
   Non-Simple Tunnels __/_____ Tunnels Compatible with Quick-Start,
                        \          but Not Supporting Quick-Start
                         \
                          \__ Tunnels Not Compatible with Quick-Start?


                     Figure 6: Categories of Tunnels.






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   Tunnels that are compatible with Quick-Start: We say that an IP
   tunnel `is not compatible with Quick-Start' if the use of a Quick-
   Start Request over such a tunnel allows false positives, where the
   TCP sender incorrectly believes that the Quick-Start Request was
   approved by all routers along the path.  If the use of Quick-Start
   over the tunnel does not cause false positives, we say that the IP
   tunnel `is compatible with Quick-Start'.

   If the IP TTL of the inner header is decremented during forwarding
   before tunnel encapsulation takes place, then the simple tunnel is
   compatible with Quick-Start, with Quick-Start Requests being
   rejected.  Section 6.1 describes in more detail the ways that a
   simple tunnel can be compatible with Quick-Start.

   There are some simple tunnels that are not compatible with Quick-
   Start, allowing `false positives' where the TCP sender incorrectly
   believes that the Quick-Start Request was approved by all routers
   along the path.  This is discussed in Section 6.2 below.

   One of our tasks in the future will be to investigate the occurrence
   of tunnels that are not compatible with Quick-Start, and to track the
   extent to which such tunnels are modified over time.  The evaluation
   of the problem of false positives from tunnels that are not
   compatible with Quick-Start will affect the progression of Quick-
   Start from Experimental to Proposed Standard, and will affect the
   degree of deployment of Quick-Start while in Experimental mode.

   Tunnels that support Quick-Start: We say that an IP tunnel `supports
   Quick-Start' if it allows routers along the tunnel path to process
   the Quick-Start Request and give feedback, resulting in the
   appropriate possible acceptance of the Quick-Start Request.  Some
   tunnels that are compatible with Quick-Start support Quick-Start,
   while others do not.  We note that a simple tunnel is not able to
   support Quick-Start.

   From a security point of view, the use of Quick-Start in the outer
   header of an IP tunnel might raise security concerns because an
   adversary could tamper with the Quick-Start information that
   propagates beyond the tunnel endpoint, or because the Quick-Start
   Option exposes information to network scanners.  Our approach is to
   make supporting Quick-Start an option for IP tunnels.  That is, in
   environments or tunneling protocols where the risks of using Quick-
   Start are judged to outweigh its benefits, the tunnel can simply
   delete the Quick-Start Option or zero the Quick-Start rate request
   and QS TTL fields before encapsulation.  The result is that there are
   two viable options for IP tunnels to be compatible with Quick-Start.
   The first option is the simple tunnel described above and in Section
   6.1, where the tunnel is compatible with Quick-Start but does not



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   support Quick-Start, where all Quick-Start Requests along the path
   will be rejected.  The second approach is a Quick-Start-capable mode,
   described in Section 6.3, where the tunnel actively supports Quick-
   Start.

6.1.  Simple Tunnels that Are Compatible with Quick-Start

   This section describes the ways that a simple tunnel can be
   compatible with Quick-Start but not support Quick-Start, resulting in
   the rejection of all Quick-Start Requests that traverse the tunnel.

   If the tunnel ingress for the simple tunnel is at a router, the IP
   TTL of the inner header is generally decremented during forwarding
   before tunnel encapsulation takes place.  In this case, TTL Diff will
   be changed, correctly causing the Quick-Start Request to be rejected.
   For a simple tunnel, it is preferable if the Quick-Start Request is
   not copied to the outer header, saving the routers within the tunnel
   from unnecessarily processing the Quick-Start Request.  However, the
   Quick-Start Request will be rejected correctly in this case whether
   or not the Quick-Start Request is copied to the outer header.

6.1.1.  Simple Tunnels that Are Aware of Quick-Start

   If a tunnel ingress is aware of Quick-Start, but does not want to
   support Quick-Start, then the tunnel ingress MUST either zero the
   Quick-Start rate request, QS TTL, and QS Nonce fields, or remove the
   Quick-Start Option from the inner header before encapsulation.
   Section 6.3 describes the procedures for a tunnel that does want to
   support Quick-Start.

   Deleting the Quick-Start Option or zeroing the Quick-Start rate
   request *after decapsulation* also serves to prevent the propagation
   of Quick-Start information, and is compatible with Quick-Start.  If
   the outer header does not contain a Quick-Start Request, a Quick-
   Start-aware tunnel egress MUST reject the inner Quick-Start Request
   by zeroing the Rate Request field in the inner header, or by deleting
   the Quick-Start Option.

   If the tunnel ingress is at a sending host or router where the IP TTL
   is not decremented prior to encapsulation, and neither tunnel
   endpoint is aware of Quick-Start, then this allows false positives,
   described in the section below.









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6.2.  Simple Tunnels that Are Not Compatible with Quick-Start

   Sometimes a tunnel implementation that does not support Quick-Start
   is independent of the TCP sender or a router implementation that
   supports Quick-Start.  In these cases, it is possible that a Quick-
   Start Request gets erroneously approved without the routers in the
   tunnel having individually approved the request, causing a false
   positive.

   If a tunnel ingress is a separate component from the TCP sender or IP
   forwarding, it is possible that a packet with a Quick-Start option is
   encapsulated without the IP TTL being decremented first, or with both
   IP TTL and QS TTL being decremented before the tunnel encapsulation
   takes place.  If the tunnel ingress does not know about Quick-Start,
   a valid Quick-Start Request with unchanged TTL Diff traverses in the
   inner header, while the outer header most likely does not carry a
   Quick-Start Request.  If the tunnel egress also does not support
   Quick-Start, it remains possible that the Quick-Start Request would
   be falsely approved, because the packet is decapsulated using the
   Quick-Start Request from the inner header, and the value of TTL Diff
   echoed to the sender remains unchanged.  For example, such a scenario
   can occur with a Bump-In-The-Stack (BITS), an IPsec encryption
   implementation where the data encryption occurs between the network
   drivers and the TCP/IP protocol stack [RFC4301].

   As one example, if a remote access VPN client uses a BITS structure,
   then Quick-Start obstacles between the client and the VPN gateway
   won't be seen.  This is a particular problem because the path between
   the client and the VPN gateway is likely to contain the most
   congested part of the path.  Because most VPN clients are reported to
   use BITS [H05], we will explore this in more detail.

   A Bump-In-The-Wire (BITW) is an IPsec encryption implementation where
   the encryption occurs on an outboard processor, offloading the
   encryption processing overhead from the host or router [RFC4301].
   The BITW device is usually IP addressable, which means that the IP
   TTL is decremented before the packet is passed to the BITW.  If the
   QS TTL is not decremented, then the value of TTL Diff is changed, and
   the Quick-Start Request will be denied.  However, if the BITW
   supports a host and does not have its own IP address, then the IP TTL
   is not decremented before the packet is passed from the host to the
   BITW, and a false positive could occur.

   Other tunnels that need to be looked at are IP tunnels over non-
   network protocols, such as IP over TCP and IP over UDP [RFC3948], and
   tunnels using the Layer Two Tunneling Protocol [RFC2661].





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   Section 9.2 discusses the related issue of non-IP queues, such as
   layer-two Ethernet or ATM (Asynchronous Transfer Mode) networks, as
   another instance of possible bottlenecks that do not participate in
   the Quick-Start feedback.

6.3.  Tunnels That Support Quick-Start

   This section discusses tunnels configured to support Quick-Start.

   If the tunnel ingress node chooses to locally approve the Quick-
   Start Request, then the ingress node MUST decrement the Quick-Start
   TTL at the same time it decrements the IP TTL, and MUST copy IP TTL
   and the Quick-Start Option from the inner IP header to the outer
   header.  During encapsulation, the tunnel ingress MUST zero the
   Quick-Start rate request field in the inner header to ensure that the
   Quick-Start Request will be rejected if the tunnel egress does not
   support Quick-Start.

   If the tunnel ingress node does not choose to locally approve the
   Quick-Start Request, then it MUST either delete the Quick-Start
   option from the inner header before encapsulation, or zero the QS TTL
   and the Rate Request fields before encapsulation.

   Upon decapsulation, if the outer header contains a Quick-Start
   option, the tunnel egress MUST copy the IP TTL and the Quick-Start
   option from the outer IP header to the inner header.

   IPsec uses the IKE (Internet Key Exchange) Protocol for security
   associations.  We do not consider the interactions between Quick-
   Start and IPsec with IKEv1 [RFC2409] in this document.  Now that the
   RFC for IKEv2 [RFC4306] is published, we plan to specify a
   modification of IPsec to allow the support of Quick-Start to be
   negotiated; this modification will specify the negotiation between
   tunnel endpoints to allow or forbid support for Quick-Start within
   the tunnel.  This was done for ECN for IPsec tunnels, with IKEv1
   [RFC3168, Section 9.2].  This negotiation of Quick-Start capability
   in an IPsec tunnel will be specified in a separate IPsec document.
   This document will also include a discussion of the potential effects
   of an adversary's modifications of the Quick-Start field (as in
   Sections 18 and 19 of RFC 3168), and of the security considerations
   of exposing the Quick-Start rate request to network scanners.

6.4.  Quick-Start and MPLS

   The behavior of Quick-Start with MPLS is similar to the behavior of
   Quick-Start with IP Tunnels.  For those MPLS paths where the IP TTL
   is decremented as part of traversing the MPLS path, these paths are
   compatible with Quick-Start, but do not support Quick-Start; Quick-



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   Start Requests that are traversing these paths will be correctly
   understood by the transport sender as having been denied.  Any MPLS
   paths where the IP TTL is not decremented as part of traversing the
   MPLS path would be not compatible with Quick-Start; such paths would
   result in false positives, where the TCP sender incorrectly believes
   that the Quick-Start Request was approved by all routers along the
   path.

   For cases where the ingress node to the MPLS path is aware of Quick-
   Start, this node should either zero the Quick-Start rate request, QS
   TTL, and QS Nonce fields, or remove the Quick-Start Option from the
   IP header.

7.  The Quick-Start Mechanism in Other Transport Protocols

   The section earlier specified the use of Quick-Start in TCP.  In this
   section, we generalize this to give guidelines for the use of Quick-
   Start with other transport protocols.  We also discuss briefly how
   Quick-Start could be specified for other transport protocols.

   The general guidelines for Quick-Start in transport protocols are as
   follows:

   * Quick-Start is only specified for unicast transport protocols with
     appropriate congestion control mechanisms.  Note: Quick-Start is
     not a replacement for standard congestion control techniques, but
     meant to augment their operation.

   * A transport-level mechanism is needed for the Quick-Start Response
     from the receiver to the sender.  This response contains the Rate
     Request, TTL Diff, and QS Nonce.

   * The sender checks the validity of the Quick-Start Response.

   * The sender has an estimate of the round-trip time, and translates
     the Quick-Start Response into an allowed window or allowed sending
     rate.  The sender sends a Report of the Approved Rate.  The sender
     starts sending Quick-Start packets, rate-paced out at the approved
     sending rate.

   * After the sender receives the first acknowledgement packet for a
     Quick-Start packet, no more Quick-Start packets are sent.  The
     sender adjusts its current congestion window or sending rate to be
     consistent with the actual amount of data that was transmitted in
     that round-trip time.






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   * When the last Quick-Start packet is acknowledged, the sender
     continues using the standard congestion control mechanisms of that
     protocol.

   * If one of the Quick-Start packets is lost, then the sender reverts
     to the standard congestion control method of that protocol that
     would have been used if the Quick-Start Request had not been
     approved.  In addition, the sender takes into account the
     information that the Quick-Start congestion window was too large
     (e.g., by decreasing ssthresh in TCP).

8.  Using Quick-Start

8.1.  Determining the Rate to Request

   As discussed in [SAF06], the data sender does not necessarily have
   information about the size of the data transfer at connection
   initiation; for example, in request-response protocols such as HTTP,
   the server doesn't know the size or name of the requested object
   during connection initiation.  [SAF06] explores some of the
   performance implications of overly large Quick-Start Requests, and
   discusses heuristics that end-nodes could use to size their requests
   appropriately.  For example, the sender might have information about
   the bandwidth of the last-mile hop, the size of the local socket
   buffer, or of the TCP receive window, and could use this information
   in determining the rate to request.  Web servers that mostly have
   small objects to transfer might decide not to use Quick-Start at all,
   since Quick-Start would be of little benefit to them.

   Quick-Start will be more effective if Quick-Start Requests are not
   larger than necessary; every Quick-Start Request that is approved but
   not used (or not fully used) takes away from the bandwidth pool
   available for granting successive Quick-Start Requests.

8.2.  Deciding the Permitted Rate Request at a Router

   In this section, we briefly outline how a router might decide whether
   or not to approve a Quick-Start Request.  The router should ask the
   following questions:

   * Has the router's output link been underutilized for some time
     (e.g., several seconds)?

   * Would the output link remain underutilized if the arrival rate were
     to increase by the aggregate rate requests that the router has
     approved over the last fraction of a second?





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   In order to answer the last question, the router must have some
   knowledge of the available bandwidth on the output link and of the
   Quick-Start bandwidth that could arrive due to recently approved
   Quick-Start Requests.  In this way, if an underutilized router
   experiences a flood of Quick-Start Requests, the router can begin to
   deny Quick-Start Requests while the output link is still
   underutilized.

   A simple way for the router to keep track of the potential bandwidth
   from recently approved requests is to maintain two counters: one for
   the total aggregate Rate Requests that have been approved in the
   current time interval [T1, T2], and one for the total aggregate Rate
   Requests approved over a previous time interval [T0, T1].  However,
   this document doesn't specify router algorithms for approving Quick-
   Start Requests, or make requirements for the appropriate time
   intervals for remembering the aggregate approved Quick-Start
   bandwidth.  A possible router algorithm is given in Appendix E, and
   more discussion of these issues is available in [SAF06].

   * If the router's output link has been underutilized and the
     aggregate of the Quick-Start Request Rate options granted is low
     enough to prevent a near-term bandwidth shortage, then the router
     could approve the Quick-Start Request.

   Section 10.2 discusses some of the implementation issues in
   processing Quick-Start Requests at routers.  [SAF06] discusses the
   range of possible Quick-Start algorithms at the router for deciding
   whether to approve a Quick-Start Request.  In order to explore the
   limits of the possible functionality at routers, [SAF06] also
   discusses Extreme Quick-Start mechanisms at routers, where the router
   would keep per-flow state concerning approved Quick-Start requests.

9.  Evaluation of Quick-Start

9.1.  Benefits of Quick-Start

   The main benefit of Quick-Start is the faster start-up for the
   transport connection itself.  For a small TCP transfer of one to five
   packets, Quick-Start is probably of very little benefit;  at best, it
   might shorten the connection lifetime from three to two round-trip
   times (including the round-trip time for connection establishment).
   Similarly, for a very large transfer, where the slow-start phase
   would have been only a small fraction of the connection lifetime,
   Quick-Start would be of limited benefit.  Quick-Start would not
   significantly shorten the connection lifetime, but it might eliminate
   or at least shorten the start-up phase.  However, for moderate-sized
   connections in a well-provisioned environment, Quick-Start could
   possibly allow the entire transfer of M packets to be completed in



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   one round-trip time (after the initial round-trip time for the SYN
   exchange), instead of the log_2(M)-2 round-trip times that it would
   normally take for the data transfer, in an uncongested environments
   (assuming an initial window of four packets).

9.2.  Costs of Quick-Start

   This section discusses the costs of Quick-Start for the connection
   and for the routers along the path.

   The cost of having a Quick-Start Request packet dropped:
   Measurement studies cited earlier [MAF04] suggest that on a wide
   range of paths in the Internet, TCP SYN packets containing unknown IP
   options will be dropped.  Thus, for the sender one risk in using
   Quick-Start is that the packet carrying the Quick-Start Request could
   be dropped in the network.  It is particularly costly to the sender
   when a TCP SYN packet is dropped, because in this case the sender
   should wait for an RTO of three seconds before re-sending the SYN
   packet, as specified in Section 4.7.2.

   The cost of having a Quick-Start data packet dropped:
   Another risk for the sender in using Quick-Start lies in the
   possibility of suffering from congestion-related losses of the
   Quick-Start data packets.  This should be an unlikely situation
   because routers are expected to approve Quick-Start Requests only
   when they are significantly underutilized.  However, a transient
   increase in cross-traffic in one of the routers, a sudden decrease in
   available bandwidth on one of the links, or congestion at a non-IP
   queue could result in packet losses even when the Quick-Start Request
   was approved by all of the routers along the path.  If a Quick-Start
   packet is dropped, then the sender reverts to the congestion control
   mechanisms it would have used if the Quick-Start Request had not been
   approved, so the performance cost to the connection of having a
   Quick-Start packet dropped is small, compared to the performance
   without Quick-Start.  (On the other hand, the performance difference
   between Quick-Start with a Quick-Start packet dropped and Quick-
   Start with no Quick-Start packet dropped can be considerable.)

   Added complexity at routers:
   The main cost of Quick-Start at routers concerns the costs of added
   complexity.  The added complexity at the end-points is moderate, and
   might easily be outweighed by the benefit of Quick-Start to the end
   hosts.  The added complexity at the routers is also somewhat
   moderate; it involves estimating the unused bandwidth on the output
   link over the last several seconds, processing the Quick-Start
   request, and keeping a counter of the aggregate Quick-Start rate
   approved over the last fraction of a second.  However, this added
   complexity at routers adds to the development cycle, and could



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   prevent the addition of other competing functionality to routers.
   Thus, careful thought would have to be given to the addition of
   Quick-Start to IP.

   The slow path in routers:
   Another drawback of Quick-Start is that packets containing the
   Quick-Start Request message might not take the fast path in routers,
   particularly in the beginning of Quick-Start's deployment in the
   Internet.  This would mean some extra delay for the end hosts, and
   extra processing burden for the routers.  However, as discussed in
   Sections 4.1 and 4.7, not all packets would carry the Quick-Start
   option.  In addition, for the underutilized links where Quick-Start
   Requests could actually be approved, or in typical environments where
   most of the packets belong to large flows, the burden of the Quick-
   Start Option on routers would be considerably reduced.  Nevertheless,
   it is still conceivable, in the worst case, that many packets would
   carry Quick-Start Requests; this could slow down the processing of
   Quick-Start packets in routers considerably.  As discussed in Section
   9.6, routers can easily protect against this by enforcing a limit on
   the rate at which Quick-Start Requests will be considered.  [RW03]
   and [RW04] contain measurements of the impact of IP Option Processing
   on packet round-trip times.

   Multiple paths:

   One limitation of Quick-Start is that it presumes that the data
   packets of a connection will follow the same path as the Quick-Start
   request packet.  If this is not the case, then the connection could
   be sending the Quick-Start packets, at the approved rate, along a
   path that was already congested, or that became congested as a result
   of this connection.  Thus, Quick-Start could give poor performance
   when there is a routing change immediately after the Quick-Start
   Request is approved, and the Quick-Start data packets follow a
   different path from that of the original Quick-Start Request.  This
   is, however, similar to what would happen for a connection with
   sufficient data, if the connection's path was changed in the middle
   of the connection, which had already established the allowed initial
   rate.

   As specified in Section 3.3, a router that uses multipath routing for
   packets within a single connection must not approve a Quick-Start
   Request.  Quick-Start would not perform robustly in an environment
   with multipath routing, where different packets in a connection
   routinely follow different paths.  In such an environment, the
   Quick-Start Request and some fraction of the packets in the
   connection might take an underutilized path, while the rest of the
   packets take an alternate, congested path.




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   Non-IP queues:
   A problem of any mechanism for feedback from routers at the IP level
   is that there can be queues and bottlenecks in the end-to-end path
   that are not in IP-level routers.  As an example, these include
   queues in layer-two Ethernet or ATM networks.  One possibility would
   be that an IP-level router adjacent to such a non-IP queue or
   bottleneck would be configured to reject Quick-Start Requests if that
   was appropriate.  One would hope that, in general, IP networks are
   configured so that non-IP queues between IP routers do not end up
   being the congested bottlenecks.

9.3.  Quick-Start with QoS-Enabled Traffic

   The discussion in this document has largely been of Quick-Start with
   default, best-effort traffic.  However, Quick-Start could also be
   used by traffic using some form of differentiated services, and
   routers could take the traffic class into account when deciding
   whether or not to grant the Quick-Start Request.  We don't address
   this context further in this paper, since it is orthogonal to the
   specification of Quick-Start.

   Routers are also free to take into account their own priority
   classifications in processing Quick-Start Requests.

9.4.  Protection against Misbehaving Nodes

   In this section, we discuss the protection against senders,
   receivers, or colluding routers or middleboxes lying about the
   Quick-Start Request.

9.4.1.  Misbehaving Senders

   A transport sender could try to transmit data at a higher rate than
   that approved in the Quick-Start Request.  The network could use a
   traffic policer to protect against misbehaving senders that exceed
   the approved rate, for example, by dropping packets that exceed the
   allowed transmission rate.  The required Report of Approved Rate
   allows traffic policers to check that the Report of Approved Rate
   does not exceed the Rate Request actually approved at that point in
   the network in the previous Quick-Start Request from that connection.
   The required Approved Rate report also allows traffic policers to
   check that the sender's sending rate does not exceed the rate in the
   Report of Approved Rate.

   If a router or receiver receives an Approved Rate report that is
   larger than the Rate Request in the Quick-Start Request approved for
   that sender for that connection in the previous round-trip time, then
   the router or receiver could deny future Quick-Start Requests from



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   that sender, e.g., by deleting the Quick-Start Request from future
   packets from that sender.  We note that routers are not required to
   use Approved Rate reports to check if senders are cheating; this is
   at the discretion of the router.

   If a router sees a Report of Approved Rate, and did not see an
   earlier Quick-Start Request, then either the sender could be
   cheating, or the connection's path could have changed since the
   Quick-Start Request was sent.  In either case, the router could
   decide to deny future Quick-Start Requests for this connection.  In
   particular, it is reasonable for the router to deny a Quick-Start
   request if either the sender is cheating, or if the connection path
   suffers from path changes or multipathing.

   If a router approved a Quick-Start Request, but does not see a
   subsequent Approved Rate report, then there are several
   possibilities: (1) the request was denied and/or dropped downstream,
   and the sender did not send a Report of Approved Rate; (2) the
   request was approved, but the sender did not send a Report of
   Approved Rate; (3) the Approved Rate report was dropped in the
   network; or (4) the Approved Rate report took a different path from
   the Quick-Start Request.  In any of these cases, the router would be
   justified in denying future Quick-Start Requests for this connection.

   In any of the cases mentioned in the three paragraphs above (i.e., an
   Approved Rate report that is larger than the Rate Request in the
   earlier Quick-Start Request, a Report of Approved Rate with no
   preceding Rate Request, or a Rate Request with no Report of Approved
   Rate), a traffic policer may assume that Quick-Start is not being
   used appropriately, or is being used in an unsuitable environment
   (e.g., with multiple paths), and take some corresponding action.

   What are the incentives for a sender to cheat by over-sending after a
   Quick-Start Request?  Assuming that the sender's interests are
   measured by a performance metric such as the completion time for its
   connections, sometimes it might be in the sender's interests to
   cheat, and sometimes it might not;  in some cases, it could be
   difficult for the sender to judge whether it would be in its
   interests to cheat.  The incentives for a sender to cheat by over-
   sending after a Quick-Start Request are not that different from the
   incentives for a sender to cheat by over-sending even in the absence
   of Quick-Start, with one difference: the use of Quick-Start could
   help a sender evade policing actions from policers in the network.
   The Report of Approved Rate is designed to address this and to make
   it harder for senders to use Quick-Start to `cover' their cheating.






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9.4.2.  Receivers Lying about Whether the Request was Approved

   One form of misbehavior would be for the receiver to lie to the
   sender about whether the Quick-Start Request was approved, by falsely
   reporting the TTL Diff and QS Nonce.  If a router that understands
   the Quick-Start Request denies the request by deleting the request or
   by zeroing the QS TTL and QS Nonce, then the receiver can "lie" about
   whether the request was approved only by successfully guessing the
   value of the TTL Diff and QS Nonce to report.  The chance of the
   receiver successfully guessing the correct value for the TTL Diff is
   1/256, and the chance of the receiver successfully guessing the QS
   nonce for a reported rate request of K is 1/(2K).

   However, if the Quick-Start Request is denied only by a non-Quick-
   Start-capable router, or by a router that is unable to zero the QS
   TTL and QS Nonce fields, then the receiver could lie about whether
   the Quick-Start Requests were approved by modifying the QS TTL in
   successive requests received from the same host.  In particular, if
   the sender does not act on a Quick-Start Request, then the receiver
   could decrement the QS TTL by one in the next request received from
   that host before calculating the TTL Diff, and decrement the QS TTL
   by two in the following received request, until the sender acts on
   one of the Quick-Start Requests.

   Unfortunately, if a router doesn't understand Quick-Start, then it is
   not possible for that router to take an active step such as zeroing
   the QS TTL and QS Nonce to deny a request.  As a result, the QS TTL
   is not a fail-safe mechanism for preventing lying by receivers in the
   case of non-Quick-Start-capable routers.

   What would be the incentives for a receiver to cheat in reporting on
   a Quick-Start Request, in the absence of a mechanism such as the QS
   Nonce?  In some cases, cheating would be of clear benefit to the
   receiver, resulting in a faster completion time for the transfer.  In
   other cases, where cheating would result in Quick-Start packets being
   dropped in the network, cheating might or might not improve the
   receiver's performance metric, depending on the details of that
   particular scenario.

9.4.3.  Receivers Lying about the Approved Rate

   A second form of receiver misbehavior would be for the receiver to
   lie to the sender about the Rate Request for an approved Quick-Start
   Request, by increasing the value of the Rate Request field.  However,
   the receiver doesn't necessarily know the Rate Request in the
   original Quick-Start Request sent by the sender, and a higher Rate
   Request reported by the receiver will only be considered valid by the
   sender if it is no higher than the Rate Request originally requested



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   by the sender.  For example, if the sender sends a Quick-Start
   Request with a Rate Request of X, and the receiver reports receiving
   a Quick-Start Request with a Rate Request of Y > X, then the sender
   knows that either some router along the path malfunctioned
   (increasing the Rate Request inappropriately), or the receiver is
   lying about the Rate Request in the received packet.

   If the sender sends a Quick-Start Request with a Rate Request of Z,
   the receiver receives the Quick-Start Request with an approved Rate
   Request of X, and reports a Rate Request of Y, for X < Y <= Z, then
   the receiver only succeeds in lying to the sender about the approved
   rate if the receiver successfully reports the rightmost 2Y bits in
   the QS nonce.

   If senders often use a configured default value for the Rate Request,
   then receivers would often be able to guess the original Rate
   Request, and this would make it easier for the receiver to lie about
   the value of the Rate Request field.  Similarly, if the receiver
   often communicates with a particular sender, and the sender always
   uses the same Rate Request for that receiver, then the receiver might
   over time be able to infer the original Rate Request used by the
   sender.

   There are several possible additional forms of protection against
   receivers lying about the value of the Rate Request.  One possible
   additional protection would be for a router that decreases a Rate
   Request in a Quick-Start Request to report the decrease directly to
   the sender.  However, this could lead to many reports back to the
   sender for a single request, and could also be used in address-
   spoofing attacks.

   A second limited form of protection would be for senders to use some
   degree of randomization in the requested Rate Request, so that it is
   difficult for receivers to guess the original value for the Rate
   Request.  However, this is difficult because there is a fairly coarse
   granularity in the set of rate requests available to the sender, and
   randomizing the initial request only offers limited protection, in
   any case.

9.4.4.  Collusion between Misbehaving Routers

   In addition to protecting against misbehaving receivers, it is
   necessary to protect against misbehaving routers.  Consider collusion
   between an ingress router and an egress router belonging to the same
   intranet.  The ingress router could decrement the Rate Request at the
   ingress, with the egress router increasing it again at the egress.
   The routers between the ingress and egress that approved the




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   decremented rate request might not have been willing to approve the
   larger, original request.

   Another form of collusion would be for the ingress router to inform
   the egress router out-of-band of the TTL Diff and QS Nonce for the
   request packet at the ingress.  This would enable the egress router
   to modify the QS TTL and QS Nonce so that it appeared that all the
   routers along the path had approved the request.  There does not
   appear to be any protection against a colluding ingress and egress
   router.  Even if an intermediate router had deleted the Quick-Start
   Option from the packet, the ingress router could have sent the
   Quick-Start Option to the egress router out-of-band, with the egress
   router inserting the Quick-Start Option, with a modified QS TTL
   field, back in the packet.

   However, unlike ECN, there is somewhat less of an incentive for
   cooperating ingress and egress routers to collude to falsely modify
   the Quick-Start Request so that it appears to have been approved by
   all the routers along the path.  With ECN, a colluding ingress router
   could falsely mark a packet as ECN-capable, with the colluding egress
   router returning the ECN field in the IP header to its original non-
   ECN-capable codepoint, and congested routers along the path could
   have been fooled into not dropping that packet.  This collusion would
   give an unfair competitive advantage to the traffic protected by the
   colluding ingress and egress routers.

   In contrast, with Quick-Start, the collusion of the ingress and
   egress routers to make it falsely appear that a Quick-Start Request
   was approved sometimes would give an advantage to the traffic covered
   by that collusion, and sometimes would give a disadvantage, depending
   on the details of the scenario.  If some router along the path really
   does not have enough available bandwidth to approve the Quick-Start
   Request, then Quick-Start packets sent as a result of the falsely
   approved request could be dropped in the network, to the possible
   disadvantage of the connection.  Thus, while the ingress and egress
   routers could collude to prevent intermediate routers from denying a
   Quick-Start Request, it would not always be to the connection's
   advantage for this to happen.  One defense against such a collusion
   would be for some router between the ingress and egress nodes that
   denied the request to monitor connection performance, penalizing
   connections that seem to be using Quick-Start after a Quick-Start
   Request was denied, or that are reporting an Approved Rate higher
   than that actually approved by that router.

   If the congested router is ECN-capable, and the colluding ingress and
   egress routers are lying about ECN-capability as well as about
   Quick-Start, then the result could be that the Quick-Start Request
   falsely appears to the sender to have been approved, and the Quick-



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   Start packets falsely appear to the congested router to be ECN-
   capable.  In this case, the colluding routers might succeed in giving
   a competitive advantage to the traffic protected by their collusion
   (if no intermediate router is monitoring to catch such misbehavior).

9.5.  Misbehaving Middleboxes and the IP TTL

   One possible difficulty is that of traffic normalizers [HKP01], or
   other middleboxes along that path, that rewrite IP TTLs in order to
   foil other kinds of attacks in the network.  If such a traffic
   normalizer rewrote the IP TTL, but did not adjust the Quick-Start TTL
   by the same amount, then the sender's mechanism for determining if
   the request was approved by all routers along the path would no
   longer be reliable.  Rewriting the IP TTL could result in false
   positives (with the sender incorrectly believing that the Quick-
   Start Request was approved) as well as false negatives (with the
   sender incorrectly believing that the Quick-Start Request was
   denied).

9.6.  Attacks on Quick-Start

   As discussed in [SAF06], Quick-Start is vulnerable to two kinds of
   attacks: (1) attacks to increase the routers' processing and state
   load and (2) attacks with bogus Quick-Start Requests to temporarily
   tie up available Quick-Start bandwidth, preventing routers from
   approving Quick-Start Requests from other connections.  Routers can
   protect against the first kind of attack by applying a simple limit
   on the rate at which Quick-Start Requests will be considered by the
   router.

   The second kind of attack, to tie up the available Quick-Start
   bandwidth, is more difficult to defend against.  As discussed in
   [SAF06], Quick-Start Requests that are not going to be used, either
   because they are from malicious attackers or because they are denied
   by routers downstream, can result in short-term `wasting' of
   potential Quick-Start bandwidth, resulting in routers denying
   subsequent Quick-Start Requests that, if approved, would in fact have
   been used.

   We note that the likelihood of malicious attacks would be minimized
   significantly when Quick-Start was deployed in a controlled
   environment such as an intranet, where there was some form of
   centralized control over the users in the system.  We also note that
   this form of attack could potentially make Quick-Start unusable, but
   it would not do any further damage; in the worst case, the network
   would function as a network without Quick-Start.





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   [SAF06] considers the potential of Extreme Quick-Start algorithms at
   routers, which keep per-flow state for Quick-Start connections, in
   protecting the availability of Quick-Start bandwidth in the face of
   frequent, overly large Quick-Start Requests.

9.7.  Simulations with Quick-Start

   Quick-Start was added to the NS simulator [SH02] by Srikanth
   Sundarrajan, and additional functionality was added by Pasi
   Sarolahti.  The validation test is at `test-all-quickstart' in the
   `tcl/test' directory in NS.  The initial simulation studies from
   [SH02] show a significant performance improvement using Quick-Start
   for moderate-sized flows (between 4 KB and 128 KB) in underutilized
   environments.  These studies are of file transfers, with the
   improvement measured as the relative increase in the overall
   throughput for the file transfer.  The study shows that potential
   improvement from Quick-Start is proportional to the delay-bandwidth
   product of the path.

   The Quick-Start simulations in [SAF06] explore the following: the
   potential benefit of Quick-Start for the connection, the relative
   benefits of different router-based algorithms for approving Quick-
   Start Requests, and the effectiveness of Quick-Start as a function of
   the senders' algorithms for choosing the size of the rate request.

10.  Implementation and Deployment Issues

   This section discusses some of the implementation issues with Quick-
   Start.  This section also discusses some of the key deployment
   issues, such as the chicken-and-egg deployment problems of mechanisms
   that have to be deployed in both routers and end nodes in order to
   work, and the problems posed by the wide deployment of middleboxes
   today that block the use of known or unknown IP Options.

10.1.  Implementation Issues for Sending Quick-Start Requests

   Section 4.7 discusses some of the issues with deciding the initial
   sending rate to request.  Quick-Start raises additional issues about
   the communication between the transport protocol and the application,
   and about the use of past history with Quick-Start in the end node.

   One possibility is that a protocol implementation could provide an
   API for applications to indicate when they want to request Quick-
   Start, and what rate they would like to request.  In the conventional
   socket API, this could be a socket option that is set before a
   connection is established.  Some applications, such as those that use
   TCP for bulk transfers, do not have interest in the transmission
   rate, but they might know the amount of data that can be sent



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   immediately.  Based on this, the sender implementation could decide
   whether Quick-Start would be useful, and what rate should be
   requested.

   We note that when Quick-Start is used, the TCP sender is required to
   save the QS Nonce and the TTL Diff when the Quick-Start Request is
   sent, and to implement an additional timer for the paced transmission
   of Quick-Start packets.

10.2.  Implementation Issues for Processing Quick-Start Requests

   A router or other network host must be able to determine the
   approximate bandwidth of its outbound network interfaces in order to
   process incoming Quick-Start rate requests, including those that
   originate from the host itself.  One possibility would be for hosts
   to rely on configuration information to determine link bandwidths;
   this has the drawback of not being robust to errors in configuration.
   Another possibility would be for network device drivers to infer the
   bandwidth for the interface and to communicate this to the IP layer.

   Particular issues will arise for wireless links with variable
   bandwidth, where decisions will have to be made about how frequently
   the host gets updates of the changing bandwidth.  It seems
   appropriate that Quick-Start Requests would be handled particularly
   conservatively for links with variable bandwidth; to avoid cases
   where Quick-Start Requests are approved, the link bandwidth is
   reduced, and the data packets that are sent end up being dropped.

   Difficult issues also arise for paths with multi-access links (e.g.,
   Ethernet).  Routers or end-nodes with multi-access links should be
   particularly conservative in granting Quick-Start Requests.  In
   particular, for some multi-access links, there may be no procedure
   for an attached node to use to determine whether all parts of the
   multi-access link have been underutilized in the recent past.

10.3.  Possible Deployment Scenarios

   Because of possible problems discussed above concerning using Quick-
   Start over some network paths and the security issues discussed in
   Section 11, the most realistic initial deployment of Quick-Start
   would most likely take place in intranets and other controlled
   environments.  Quick-Start is most useful on high bandwidth-delay
   paths that are significantly underutilized.  The primary initial
   users of Quick-Start would likely be in organizations that provide
   network services to their users and also have control over a large
   portion of the network path.





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   Quick-Start is not currently intended for ubiquitous deployment in
   the global Internet.  In particular, Quick-Start should not be
   enabled by default in end-nodes or in routers; instead, when Quick-
   Start is used, it should be explicitly enabled by users or system
   administrators.

   Below are a few examples of networking environments where Quick-
   Start would potentially be useful.  These are the environments that
   might consider an initial deployment of Quick-Start in the routers
   and end-nodes, where the incentives for routers to deploy Quick-
   Start might be the most clear.

   * Centrally administrated organizational intranets: These intranets
     often have large network capacity, with networks that are
     underutilized for much of the time [PABL+05].  Such intranets might
     also include high-bandwidth and high-delay paths to remote sites.
     In such an environment, Quick-Start would be of benefit to users,
     and there would be a clear incentive for the deployment of Quick-
     Start in routers.  For example, Quick-Start could be quite useful
     in high-bandwidth networks used for scientific computing.

   * Wireless networks: Quick-Start could also be useful in high-delay
     environments of Cellular Wide-Area Wireless Networks, such as the
     GPRS [BW97] and their enhancements and next generations.  For
     example, GPRS EDGE (Enhanced Data for GSM Evolution) is expected to
     provide wireless bandwidth of up to 384 Kbps (roughly 32 1500-byte
     packets per second) while the GPRS round-trip times range typically
     from a few hundred milliseconds to over a second, excluding any
     possible queueing delays in the network [GPAR02].  In addition,
     these networks sometimes have variable additional delays due to
     resource allocation that could be avoided by keeping the connection
     path constantly utilized, starting from initial slow-start.  Thus,
     Quick-Start could be of significant benefit to users in these
     environments.

   * Paths over satellite links: Geostationary Orbit (GEO) satellite
     links have one-way propagation delays on the order of 250 ms while
     the bandwidth can be measured in megabits per second [RFC2488].
     Because of the considerable bandwidth-delay product on the link,
     TCP's slow-start is a major performance limitation in the beginning
     of the connection.  A large initial congestion window would be
     useful to users of such satellite links.

   * Single-hop paths: Quick-Start should work well over point-to-point
     single-hop paths, e.g., from a host to an adjacent server.  Quick-
     Start would work over a single-hop IP path consisting of a multi-
     access link only if the host was able to determine if the path to
     the next IP hop has been significantly underutilized over the



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     recent past.  If the multi-access link includes a layer-2 switch,
     then the attached host cannot necessarily determine the status of
     the other links in the layer-2 network.

10.4.  A Comparison with the Deployment Problems of ECN

   Given the glacially slow rate of deployment of ECN in the Internet to
   date [MAF05], it is disconcerting to note that some of the deployment
   problems of Quick-Start are even greater than those of ECN.  First,
   unlike ECN, which can be of benefit even if it is only deployed on
   one of the routers along the end-to-end path, a connection's use of
   Quick-Start requires Quick-Start deployment on all of the routers
   along the end-to-end path.  Second, unlike ECN, which uses an
   allocated field in the IP header, Quick-Start requires the extra
   complications of an IP Option, which can be difficult to pass through
   the current Internet [MAF05].

   However, in spite of these issues, there is some hope for the
   deployment of Quick-Start, at least in protected corners of the
   Internet, because the potential benefits of Quick-Start to the user
   are considerably more dramatic than those of ECN.  Rather than simply
   replacing the occasional dropped packet by an ECN-marked packet,
   Quick-Start is capable of dramatically increasing the throughput of
   connections in underutilized environments [SAF06].

11.  Security Considerations

   Sections 9.4 and 9.6 discuss the security considerations related to
   Quick-Start.  Section 9.4 discusses the potential abuse of Quick-
   Start by senders or receivers lying about whether the request was
   approved or about the approved rate, and of routers in collusion to
   misuse Quick-Start.  Section 9.5 discusses potential problems with
   traffic normalizers that rewrite IP TTLs in packet headers.  All
   these problems could result in the sender using a Rate Request that
   was inappropriately large, or thinking that a request was approved
   when it was in fact denied by at least one router along the path.
   This inappropriate use of Quick-Start could result in congestion and
   an unacceptable level of packet drops along the path.  Such
   congestion could also be part of a Denial of Service attack.

   Section 9.6 discusses a potential attack on the routers' processing
   and state load from an attack of Quick-Start Requests.  Section 9.6
   also discusses a potential attack on the available Quick-Start
   bandwidth by sending bogus Quick-Start Requests for bandwidth that
   will not, in fact, be used.  While this impacts the global usability
   of Quick-Start, it does not endanger the network as a whole since TCP
   uses standard congestion control if Quick-Start is not available.




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   Section 4.7.2 discusses the potential problem of packets with Quick-
   Start Requests dropped by middleboxes along the path.

   As discussed in Section 5, for IPv4 IPsec Authentication Header
   Integrity Check Value (AH ICV) calculation, the Quick-Start Option is
   a mutable IPv4 option and hence completely zeroed for AH ICV
   calculation purposes.  This is also the treatment required by RFC
   4302 for unrecognized IPv4 options.  The IPv6 Quick-Start Option's
   IANA-allocated option type indicates that it is a mutable option;
   hence, according to RFC 4302, its option data is required to be
   zeroed for AH ICV computation purposes.  See RFC 4302 for further
   explanation.

   Section 6.2 discusses possible problems of Quick-Start used by
   connections carried over simple tunnels that are not compatible with
   Quick-Start.  In this case, it is possible that a Quick-Start Request
   is erroneously considered approved by the sender without the routers
   in the tunnel having individually approved the request, causing a
   false positive.

   We note two high-order points here.  First, the Quick-Start Nonce
   goes a long way towards preventing large-scale cheating.  Second,
   even if a host occasionally uses Quick-Start when it is not approved
   by the entire network path, the network will not collapse.  Quick-
   Start does not remove TCP's basic congestion control mechanisms;
   these will kick in when the network is heavily loaded, relegating any
   Quick-Start mistake to a transient.
























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12.  IANA Considerations

   Quick-Start requires an IP Option and a TCP Option.

12.1.  IP Option

   Quick-Start requires both an IPv4 Option Number (Section 3.1) and an
   IPv6 Option Number (Section 3.2).

   IPv4 Option Number:

   Copy Class Number Value Name
   ---- ----- ------ ----- ----
      0    00     25    25   QS    - Quick-Start


   IPv6 Option Number [RFC2460]:

   HEX         act  chg  rest
   ---         ---  ---  -----
     6          00   1   00110     Quick-Start

   For the IPv6 Option Number, the first two bits indicate that the IPv6
   node may skip over this option and continue processing the header if
   it doesn't recognize the option type, and the third bit indicates
   that the Option Data may change en-route.

   In both cases, this document should be listed as the reference
   document.

12.2.  TCP Option

   Quick-Start requires a TCP Option Number (Section 4.2).

   TCP Option Number:

   Kind Length Meaning
   ---- ------ ------------------------------
     27 8      Quick-Start Response

   This document should be listed as the reference document.










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13.  Conclusions

   We are presenting the Quick-Start mechanism as a simple,
   understandable, and incrementally deployable mechanism that would be
   sufficient to allow some connections to start up with large initial
   rates, or large initial congestion windows, in over-provisioned,
   high-bandwidth environments.  We expect there will be an increasing
   number of over-provisioned, high-bandwidth environments where the
   Quick-Start mechanism, or another mechanism of similar power, could
   be of significant benefit to a wide range of traffic.  We are
   presenting the Quick-Start mechanism as a request for the community
   to provide feedback and experimentation on issues relating to Quick-
   Start.

14.  Acknowledgements

   The authors wish to thank Mark Handley for discussions of these
   issues.  The authors also thank the End-to-End Research Group, the
   Transport Services Working Group, and members of IPAM's program on
   Large-Scale Communication Networks for both positive and negative
   feedback on this proposal.  We thank Srikanth Sundarrajan for the
   initial implementation of Quick-Start in the NS simulator, and for
   the initial simulation study.  Many thanks to David Black and Joe
   Touch for extensive feedback on Quick-Start and IP tunnels.  We also
   thank Mohammed Ashraf, John Border, Bob Briscoe, Martin Duke, Tom
   Dunigan, Mitchell Erblich, Gorry Fairhurst, John Heidemann, Paul
   Hyder, Dina Katabi, and Vern Paxson for feedback.  Thanks also to
   Gorry Fairhurst for the suggestion of adding the QS Nonce to the
   Report of Approved Rate.

   The version of the QS Nonce in this document is based on a proposal
   from Guohan Lu [L05].  Earlier versions of this document contained an
   eight-bit QS Nonce, and subsequent versions discussed the possibility
   of a four-bit QS Nonce.

   This document builds upon the concepts described in [RFC3390],
   [AHO98], [RFC2415], and [RFC3168].  Some of the text on Quick-Start
   in tunnels was borrowed directly from RFC 3168.

   This document is the development of a proposal originally by Amit
   Jain for Initial Window Discovery.










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Appendix A.  Related Work

   The Quick-Start proposal, taken together with HighSpeed TCP [RFC3649]
   or other transport protocols for high-bandwidth transfers, could go a
   significant way towards extending the range of performance for best-
   effort traffic in the Internet.  However, there are many things that
   the Quick-Start proposal would not accomplish.  Quick-Start is not a
   congestion control mechanism, and would not help in making more
   precise use of the available bandwidth -- that is, of achieving the
   goal of high throughput with low delay and low packet-loss rates.
   Quick-Start would not give routers more control over the decrease
   rates of active connections.

   In addition, any evaluation of Quick-Start must include a discussion
   of the relative benefits of approaches that use no explicit
   information from routers, and of approaches that use more fine-
   grained feedback from routers as part of a larger congestion control
   mechanism.  We discuss several classes of proposals in the sections
   below.

A.1.  Fast Start-Ups without Explicit Information from Routers

   One possibility would be for senders to use information from the
   packet streams to learn about the available bandwidth, without
   explicit information from routers.  These techniques would not allow
   a start-up as fast as that available from Quick-Start in an
   underutilized environment; one already has to have sent some packets
   in order to use the packet stream to learn about available bandwidth.
   However, these techniques could allow a start-up considerably faster
   than the current Slow-Start.  While it seems clear that approaches
   *without* explicit feedback from the routers will be strictly less
   powerful than is possible *with* explicit feedback, it is also
   possible that approaches that are more aggressive than Slow-Start are
   possible without the complexity involved in obtaining explicit
   feedback from routers.

   Periodic packet streams:
   [JD02] explores the use of periodic packet streams to estimate the
   available bandwidth along a path.  The idea is that the one-way
   delays of a periodic packet stream show an increasing trend when the
   stream's rate is higher than the available bandwidth (due to an
   increasing queue).  While [JD02] states that the proposed mechanism
   does not cause significant increases in network utilization, losses,
   or delays when done by one flow at a time, the approach could be
   problematic if conducted concurrently by a number of flows.  [JD02]
   also gives an overview of some of the earlier work on inferring the
   available bandwidth from packet trains.




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   Swift-Start:
   The Swift Start proposal from [PRAKS02] combines packet-pair and
   packet-pacing techniques.  An initial congestion window of four
   segments is used to estimate the available bandwidth along the path.
   This estimate is then used to dramatically increase the congestion
   window during the second RTT of data transmission.

   SPAND:
   In the TCP/SPAND proposal from [ZQK00] for speeding up short data
   transfers, network performance information would be shared among many
   co-located hosts to estimate each connection's fair share of the
   network resources.  Based on such estimation and the transfer size,
   the TCP sender would determine the optimal initial congestion window
   size.  The design for TCP/SPAND uses a performance gateway that
   monitors all traffic entering and leaving an organization's network.

   Sharing information among TCP connections:
   The Congestion Manager [RFC3124] and TCP control block sharing
   [RFC2140] both propose sharing congestion information among multiple
   TCP connections with the same endpoints.  With the Congestion
   Manager, a new TCP connection could start with a high initial cwnd,
   if it was sharing the path and the cwnd with a pre-existing TCP
   connection to the same destination that had already obtained a high
   congestion window.  RFC 2140 discusses ensemble sharing, where an
   established connection's congestion window could be `divided up' to
   be shared with a new connection to the same host.  However, neither
   of these approaches addresses the case of a connection to a new
   destination, with no existing or recent connection (and therefore
   congestion control state) to that destination.

   While continued research on the limits of the ability of TCP and
   other transport protocols to learn of available bandwidth without
   explicit feedback from the router seems useful, we note that there
   are several fundamental advantages of explicit feedback from routers.

   (1) Explicit feedback is faster than implicit feedback:
       One advantage of explicit feedback from the routers is that it
       allows the transport sender to reliably learn of available
       bandwidth in one round-trip time.

   (2) Explicit feedback is more reliable than implicit feedback:
       Techniques that attempt to assess the available bandwidth at
       connection start-up using implicit techniques are more error-
       prone than techniques that involve every element in the network
       path.  While explicit information from the network can be wrong,
       it has a much better chance of being appropriate than an end-host
       trying to *estimate* an appropriate sending rate using "block
       box" probing techniques of the entire path.



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A.2.  Optimistic Sending without Explicit Information from Routers

   Another possibility that has been suggested [S02] is for the sender
   to start with a large initial window without explicit permission from
   the routers and without bandwidth estimation techniques and for the
   first packet of the initial window to contain information, such as
   the size or sending rate of the initial window.  The proposal would
   be that congested routers would use this information in the first
   data packet to drop or delay many or all of the packets from that
   initial window.  In this way, a flow's optimistically large initial
   window would not force the router to drop packets from competing
   flows in the network.  Such an approach would seem to require some
   mechanism for the sender to ensure that the routers along the path
   understood the mechanism for marking the first packet of a large
   initial window.

   Obviously, there would be a number of questions to consider about an
   approach of optimistic sending.

   (1) Incremental deployment:
       One question would be the potential complications of incremental
       deployment, where some of the routers along the path might not
       understand the packet information describing the initial window.

   (2) Congestion collapse:
       There could also be concerns about congestion collapse if many
       flows used large initial windows, many packets were dropped from
       optimistic initial windows, and many congested links ended up
       carrying packets that are only going to be dropped downstream.

   (3) Distributed Denial of Service attacks:
       A third question would be the potential role of optimistic
       senders in amplifying the damage done by a Distributed Denial of
       Service (DDoS) attack (assuming attackers use compliant
       congestion control in the hopes of "flying under the radar").

   (4) Performance hits if a packet is dropped:
       A fourth issue would be to quantify the performance hit to the
       connection when a packet is dropped from one of the initial
       windows.

A.3.  Fast Start-Ups with Other Information from Routers

   There have been several proposals somewhat similar to Quick-Start,
   where the transport protocol collects explicit information from the
   routers along the path.





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   An IP Option about the free buffer size:
   In related work, [P00] investigates the use of a slightly different
   IP option for TCP connections to discover the available bandwidth
   along the path.  In that proposal, the IP option would query the
   routers along the path about the smallest available free buffer size.
   Also, the IP option would have been sent after the initial SYN
   exchange, when the TCP sender already had an estimate of the round-
   trip time.

   The Performance Transparency Protocol:
   The Performance Transparency Protocol (PTP) includes a proposal for a
   single PTP packet that would collect information from routers along
   the path from the sender to the receiver [W00].  For example, a
   single PTP packet could be used to determine the bottleneck bandwidth
   along a path.

   ETEN:
   Additional proposals for end nodes to collect explicit information
   from routers include one variant of Explicit Transport Error
   Notification (ETEN), which includes a cumulative mechanism to notify
   endpoints of aggregate congestion statistics along the path [KAPS02].
   (A second variant in [KSEPA04] does not depend on cumulative
   congestion statistics from the network.)

A.4.  Fast Start-Ups with more Fine-Grained Feedback from Routers

   Proposals for more fine-grained, congestion-related feedback from
   routers include XCP [KHR02], MaxNet [MaxNet], and AntiECN marking
   [K03].  Appendix B.6 discusses in more detail the relationship
   between Quick-Start and proposals for more fine-grained per-packet
   feedback from routers.

   XCP:
   Proposals, such as XCP for new congestion control mechanisms based on
   more feedback from routers, are more powerful than Quick-Start, but
   also are more complex to understand and more difficult to deploy.
   XCP routers maintain no per-flow state, but provide more fine-
   grained feedback to end-nodes than the one-bit congestion feedback of
   ECN.  The per-packet feedback from XCP can be positive or negative,
   and specifies the increase or decrease in the sender's congestion
   window when this packet is acknowledged.  XCP is a full-fledge
   congestion control scheme, whereas Quick-Start represents a quick
   check to determine if the network path is significantly underutilized
   such that a connection can start faster and then fall back to TCP's
   standard congestion control algorithms.






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   AntiECN:
   The AntiECN proposal is for a single bit in the packet header that
   routers could set to indicate that they are underutilized.  For each
   TCP ACK arriving at the sender indicating that a packet has been
   received with the Anti-ECN bit set, the sender would be able to
   increase its congestion window by one packet, as it would during
   slow-start.

A.5.  Fast Start-Ups with Lower-Than-Best-Effort Service

   There have been proposals for routers to provide a Lower Effort
   differentiated service that would be lower than best effort
   [RFC3662].  Such a service could carry traffic for which delivery is
   strictly optional, or could carry traffic that is important but that
   has low priority in terms of time.  Because it does not interfere
   with best-effort traffic, Lower Effort services could be used by
   transport protocols that start up faster than slow-start.  For
   example, [SGF05] is a proposal for the transport sender to use low-
   priority traffic for much of the initial traffic, with routers
   configured to use strict priority queueing.

   A separate but related issue is that of below-best-effort TCP,
   variants of TCP that would not rely on Lower Effort services in the
   network, but would approximate below-best-effort traffic by detecting
   and responding to congestion sooner than standard TCP.  TCP Nice
   [V02] and TCP Low Priority (TCP-LP) [KK03] are two such proposals for
   below-best-effort TCP, with the purpose of allowing TCP connections
   to use the bandwidth unused by TCP and other traffic in a non-
   intrusive fashion.  Both TCP Nice and TCP Low Priority use the
   default slow-start mechanisms of TCP.

   We note that Quick-Start is quite different from either a Lower-
   Effort service or a below-best-effort variant of TCP.  Unlike these
   proposals, Quick-Start is intended to be useful for best-effort
   traffic that wishes to receive at least as much bandwidth as
   competing best-effort connections.















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Appendix B.  Design Decisions

B.1.  Alternate Mechanisms for the Quick-Start Request: ICMP and RSVP

   This document has proposed using an IP Option for the Quick-Start
   Request from the sender to the receiver, and using transport
   mechanisms for the Quick-Start Response from the receiver back to the
   sender.  In this section, we discuss alternate mechanisms, and
   consider whether ICMP ([RFC792], [RFC4443]) or RSVP [RFC2205]
   protocols could be used for delivering the Quick-Start Request.

B.1.1.  ICMP

   Being a control protocol used between Internet nodes, one could argue
   that ICMP is the ideal method for requesting permission for faster
   start-up from routers.  The ICMP header is above the IP header.
   Quick-Start could be accomplished with ICMP as follows: If the ICMP
   protocol is used to implement Quick-Start, the equivalent of the
   Quick-Start IP option would be carried in the ICMP header of the ICMP
   Quick-Start Request.  The ICMP Quick-Start Request would have to pass
   by the routers on the path to the receiver, possibly using the IP
   Router Alert option [RFC2113].  A router that approves the Quick-
   Start Request would take the same actions as in the case with the
   Quick-Start IP Option, and forward the packet to the next router
   along the path.  A router that does not approve the Quick-Start
   Request, even with a decreased value for the Requested Rate, would
   delete the ICMP Quick-Start Request, and send an ICMP Reply to the
   sender that the request was not approved.  If the ICMP Reply was
   dropped in the network, and did not reach the receiver, the sender
   would still know that the request was not approved from the absence
   of feedback from the receiver.  If the ICMP Quick-Start Request was
   dropped in the network due to congestion, the sender would assume
   that the request was not approved.  The ICMP message would need the
   source and destination port numbers for demultiplexing at the end
   nodes.  If the ICMP Quick-Start Request reached the receiver, the
   receiver would use transport-level or application-level mechanisms to
   send a response to the sender, exactly as with the IP Option.

   One benefit of using ICMP would be that the delivery of the TCP SYN
   packet or other initial packet would not be delayed by IP option
   processing at routers.  A greater advantage is that if middleboxes
   were blocking packets with Quick-Start Requests, using the Quick-
   Start Request in a separate ICMP packet would mean that the middlebox
   behavior would not affect the connection as a whole.  (To get this
   robustness to middleboxes with TCP using an IP Quick-Start Option,
   one would have to have a TCP-level Quick-Start Request packet that
   could be sent concurrently with, but separately from, the TCP SYN
   packet.)



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   However, there are a number of disadvantages to using ICMP.  Some
   firewalls and middleboxes may not forward the ICMP Quick-Start
   Request packets.  (If an ICMP Reply packet from a router to the
   sender is dropped in the network, the sender would still know that
   the request was not approved, as stated earlier, so this would not be
   as serious of a problem.)  In addition, it would be difficult, if not
   impossible, for a router in the middle of an IP tunnel to deliver an
   ICMP Reply packet to the actual source, for example, when the inner
   IP header is encrypted, as in IPsec ESP tunnel mode [RFC4301].
   Again, however, the ICMP Reply packet would not be essential to the
   correct operation of ICMP Quick-Start.

   Unauthenticated out-of-band ICMP messages could enable some types of
   attacks by third-party malicious hosts that are not possible when the
   control information is carried in-band with the IP packets that can
   only be altered by the routers on the connection path.  Finally, as a
   minor concern, using ICMP would cause a small amount of additional
   traffic in the network, which is not the case when using IP options.

B.1.2.  RSVP

   With some modifications, RSVP [RFC2205] could be used as a bearer
   protocol for carrying the Quick-Start Requests.  Because routers are
   expected to process RSVP packets more extensively than the normal
   transport protocol IP packets, delivering a Quick-Start rate request
   using an RSVP packet would seem an appealing choice.  However, Quick-
   Start with RSVP would require a few differences from the conventional
   usage of RSVP.  Quick-Start would not require periodical refreshing
   of soft state, because Quick-Start does not require per-connection
   state in routers.  Quick-Start Requests would be transmitted
   downstream from the sender to receiver in the RSVP Path messages,
   which is different from the conventional RSVP model where the
   reservations originate from the receiver.  Furthermore, the Quick-
   Start Response would be sent using the transport-level or
   application-level mechanisms, instead of using the RSVP Resv message.

   If RSVP was used for carrying a Quick-Start Request, a new "Quick-
   Start Request" class object would be included in the RSVP Path
   message that is sent from the sender to receiver.  The object would
   contain the rate request field in addition to the common length and
   type fields.  The Send_TTL field in the RSVP common header could be
   used as the equivalent of the QS TTL field.  The Quick-Start capable
   routers along the path would inspect the Quick-Start Request object
   in the RSVP Path message, decrement Send_TTL, and adjust the rate
   request field if needed.  If an RSVP router did not understand the
   Quick-Start Request object, it would reject the entire RSVP message
   and send an RSVP PathErr message back to the sender.  When an RSVP
   message with the Quick-Start Request object reaches the receiver, the



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   receiver sends a Quick-Start Reply message in the corresponding
   transport protocol header in the same way as described in the context
   of IP options earlier.  If the RSVP message with the Quick-Start
   Request object was dropped along the path, the transport sender would
   simply proceed with the normal congestion control procedures.

   Much of the discussion about benefits and drawbacks of using ICMP for
   making the Quick-Start Request also applies to the RSVP case.  If the
   Quick-Start Request was transmitted in a separate packet instead of
   as an IP option, the transport protocol packet delivery would not be
   delayed due to IP option processing at the routers, and the initial
   transport packets would reach their destination more reliably.  The
   possible disadvantages of using ICMP and RSVP are also expected to be
   similar: middleboxes in the network may not be able to forward the
   Quick-Start Request messages, and the IP tunnels might cause problems
   for processing the Quick-Start Requests.

B.2.  Alternate Encoding Functions

   In this section, we look at alternate encoding functions for the Rate
   Request field in the Quick-Start Request.  The main requirements for
   this function is that it should have a sufficiently wide range for
   the requested rate.  There is no need for overly fine-grained
   precision in the requested rate.  Similarly, while it would be
   attractive for the encoding function to be easily computable, it is
   also possible for end-nodes and routers to simply store the table
   giving the mapping between the value N in the Rate Request field, and
   the actual rate request f(N).  In this section, we consider possible
   encoding methods for Rate Request fields of different sizes,
   including four-bit, eight-bit, and larger Rate Request fields.

   Linear functions:
   One possible proposal would be for the Rate Request field to be
   formatted in bits per second, scaled so that one unit equals M Kbps,
   for some fixed value of M.  Thus, for the value N in the Rate Request
   field, the requested rate would be M*N Kbps.

   Powers of two:
   If a granularity of factors of two is sufficient for the Rate
   Request, then the encoding function with the most range would be for
   the requested rate to be K*2^N; for N, the value in the Rate Request
   field; and for K, some constant.  For N=0, the rate request would be
   set to zero, regardless of the encoding function.  For example, for
   K=40,000 and an eight-bit Rate Request field, the request range would
   be from 80 Kbps to 40*2^255 Kbps.  This clearly would be an
   unnecessarily large request range.





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   For a four-bit Rate Request field, the upper limit on the rate
   request is 1.3 Gbps.  It seems to us that an upper limit of 1.3 Gbps
   would be fine for the Quick-Start rate request, and that connections
   wishing to start up with a higher initial sending rate should be
   encouraged to use other mechanisms, such as the explicit reservation
   of bandwidth.  If an upper limit of 1.3 Gbps was not acceptable, then
   five or six bits could be used for the Rate Request field.

   The lower limit of 80 Kbps could be useful for flows with round-trip
   times of a second or more.  For a flow with a round-trip time of one
   second, as is typical in some wireless networks, the TCP initial
   window of 4380 bytes allowed by [RFC3390] (given appropriate packet
   sizes) would translate to an initial sending rate of 35 Kbps.  Thus,
   for TCP flows, a rate request of 80 Kbps could be useful for some
   flows with large round-trip times.

   The lower limit of 80 Kbps could also be useful for some non-TCP
   flows that send small packets, with at most one small packet every 10
   ms.  A rate request of 80 Kbps would translate to a rate of a hundred
   100-byte packets per second (including packet headers).  While some
   small-packet flows with large round-trip times might find a smaller
   rate request of 40 Kbps to be useful, our assumption is that a lower
   limit of 80 Kbps on the rate request will be generally sufficient.
   Again, if the lower limit of 80 kbps was not acceptable, then extra
   bits could be used for the Rate Request field.

   If the granularity of factors of two was too coarse, then the
   encoding function could use a base less than two.  An alternate form
   for the encoding function would be to use a hybrid of linear and
   exponential functions.

   A mantissa and exponent representation:
   Section 4.4 of [B05] suggests a mantissa and exponent representation
   for the Quick-Start encoding function.  With e and f as the binary
   numbers in the exponent and mantissa fields, and with 0 <= f < 1,
   this would represent the rate (1+f)*2^e.  [B05] suggests a mantissa
   field for f of 8, 16, or 24 bits, with an exponent field for e of 8
   bits.  This representation would allow larger rate requests, with an
   encoding that is less coarse than the powers-of-two encoding used in
   this document.

   Constraints of the transport protocol:
   We note that the Rate Request is also constrained by the abilities of
   the transport protocol.  For example, for TCP with Window Scaling,
   the maximum window is at most 2**30 bytes.  For a TCP connection with
   a long, 1 second round-trip time, this would give a maximum sending
   rate of 1.07 Gbps.




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B.3.  The Quick-Start Request: Packets or Bytes?

   One of the design questions is whether the Rate Request field should
   be in bytes per second or in packets per second.  We discuss this
   separately from the perspective of the transport, and from the
   perspective of the router.

   For TCP, the results from the Quick-Start Request are translated into
   a congestion window in bytes, using the measured round-trip time and
   the MSS.  This window applies only to the bytes of data payload, and
   does not include the bytes in the TCP or IP packet headers.  Other
   transport protocols would conceivably use the Quick-Start Request
   directly in packets per second, or could translate the Quick-Start
   Request to a congestion window in packets.

   The assumption of this document is that the router only approves the
   Quick-Start Request when the output link is significantly
   underutilized.  For this, the router could measure the available
   bandwidth in bytes per second, or could convert between packets and
   bytes by some mechanism.

   If the Quick-Start Request was in bytes per second, and applied only
   to the data payload, then the router would have to convert from bytes
   per second of data payload, to bytes per second of packets on the
   wire.  If the Rate Request field was in bytes per second, and the
   sender ended up using very small packets, this could translate to a
   significantly larger number in terms of bytes per second on the wire.
   Therefore, for a Quick-Start Request in bytes per second, it makes
   most sense for this to include the transport and IP headers as well
   as the data payload.  Of course, this will be, at best, a rough
   approximation on the part of the sender; the transport-level sender
   might not know the size of the transport and IP headers in bytes, and
   might know nothing at all about the separate headers added in IP
   tunnels downstream.  This rough estimate seems sufficient, however,
   given the overall lack of fine precision in Quick-Start
   functionality.

   It has been suggested that the router could possibly use information
   from the MSS option in the TCP packet header of the SYN packet to
   convert the Quick-Start Request from packets per second to bytes per
   second, or vice versa.  This would be problematic for several
   reasons.  First, if IPsec is used, the TCP header will be encrypted.
   Second, the MSS option is defined as the maximum MSS that the TCP
   sender expects to receive, not the maximum MSS that the TCP sender
   plans to send [RFC793].  However, it is probably often the case that
   this MSS also applies as an upper bound on the MSS used by the TCP
   sender in sending.




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   We note that the sender does not necessarily know the Path MTU when
   the Quick-Start Request is sent, or when the initial window of data
   is sent.  Thus, with IPv4, packets from the initial window could end
   up being fragmented in the network if the "Don't Fragment" (DF) bit
   is not set [RFC1191].  A Rate Request in bytes per second is
   reasonably robust to fragmentation.  Clearly, a Rate Request in
   packets per second is less robust in the presence of fragmentation.
   Interactions between larger initial windows and Path MTU Discovery
   are discussed in more detail in RFC 3390 [RFC3390].

   For a Quick-Start Request in bytes per second, the transport senders
   would have the additional complication of estimating the bandwidth
   usage added by the packet headers.

   We have chosen a Rate Request field in bytes per second rather than
   in packets per second because it seems somewhat more robust,
   particularly to routers.

B.4.  Quick-Start Semantics: Total Rate or Additional Rate?

   For a Quick-Start Request sent in the middle of a connection, there
   are two possible semantics for the Rate Request field, as follows:

   (1) Total Rate: The requested Rate Request is the requested total
       rate for the connection, including the current rate; or

   (2) Additional Rate: The requested Rate Request is the requested
       increase in the total rate for that connection, over and above
       the current sending rate.

   When the Quick-Start Request is sent after an idle period, the
   current sending rate is zero, and there is no difference between (1)
   and (2) above.  However, a Quick-Start Request can also be sent in
   the middle of a connection that has not been idle, e.g., after a
   mobility event, or after an application-limited period when the
   sender is suddenly ready to send at a much higher rate.  In this
   case, there can be a significant difference between (1) and (2)
   above.  In this section, we consider briefly the tradeoffs between
   these two options, and explain why we have chosen the `Total Rate'
   semantics.

   The Total Rate semantics makes it easier for routers to "allocate"
   the same rate to all connections.  This lends itself to fairness, and
   improves convergence times between old and new connections.  With the
   Additional Rate semantics, the router would not necessarily know the
   current sending rates of the flows requesting additional rates, and
   therefore would not have sufficient information to use fairness as a
   metric in granting rate requests.  With the Total Rate semantics, the



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   fairness is automatic; the router is not granting rate requests for
   *additional* bandwidth without knowing the current sending rates of
   the different flows.

   The Additional Rate semantics also lends itself to gaming by the
   connection, with senders sending frequent Quick-Start Requests in the
   hope of gaining a higher rate.  If the router is granting the same
   maximum rate for all rate requests, then there is little benefit to a
   connection of sending a rate request over and over again.  However,
   if the router is granting an *additional* rate with each rate
   request, over and above the current sending rate, then it is in a
   connection's interest to send as many rate requests as possible, even
   if very few of them are, in fact, granted.

   Appendix E discusses a Report of Current Sending Rate as one possible
   function in the Quick-Start Option.  However, we have not
   standardized this possible use at this time.

B.5.  Alternate Responses to the Loss of a Quick-Start Packet

   Section 4.6 discusses TCP's response to the loss of a Quick-Start
   packet in the initial window.  This section discusses several
   alternate responses.

   One possible alternative to reverting to the default Slow-Start after
   the loss of a Quick-Start packet from the initial window would have
   been to halve the congestion window and continue in congestion
   avoidance.  However, we note that this would not have been a
   desirable response for either the connection or for the network as a
   whole.  The packet loss in the initial window indicates that Quick-
   Start failed in finding an appropriate congestion window, meaning
   that the congestion window after halving may easily also be wrong.

   A more moderate alternate would be to continue in congestion
   avoidance from a window of (W-D)/2, where W is the Quick-Start
   congestion window, and D is the number of packets dropped or marked
   from that window.  However, such an approach would implicitly assume
   that the number of Quick-Start packets delivered is a good indication
   of the appropriate available bandwidth for that flow, even though
   other packets from that window were dropped in the network.  And,
   further, that using half the number of segments that were
   successfully transmitted is conservative enough to account for the
   possibly inaccurate congestion window indication.  We believe that
   such an assumption would require more analysis at this point,
   particularly in a network with a range of packet dropping mechanisms
   at the router, and we cannot recommend it at this time.





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   Another drawback of approaches that don't revert back to slow-start
   when a Quick-Start packet in the initial window is dropped is that
   such approaches could give the TCP receiver a greater incentive to
   lie about the Quick-Start Request.  If the sender reverts to slow-
   start when a Quick-Start packet in the initial window is dropped,
   this diminishes the benefit a receiver would get from a Quick-Start
   request that resulted in a dropped or ECN-marked packet.

B.6.  Why Not Include More Functionality?

   This proposal for Quick-Start is a rather coarse-grained mechanism
   that would allow a connection to use a higher sending rate along
   underutilized paths, but that does not attempt to provide a next-
   generation transport protocol or congestion control mechanism, and
   does not attempt the goal of providing very high throughput with very
   low delay.  Appendix A.4 discusses a number of proposals (such as
   XCP, MaxNet, and AntiECN) that provide more fine-grained per-packet
   feedback from routers than the current congestion control mechanisms
   and that attempt these more ambitious goals.

   Compared to proposals such as XCP and AntiECN, Quick-Start offers
   much less control.  Quick-Start does not attempt to provide a new
   congestion control mechanism, but simply to get permission from
   routers for a higher sending rate at start-up, or after an idle
   period.  Quick-Start can be thought of as an "anti-congestion-
   control" mechanism that is only of any use when all the routers along
   the path are significantly underutilized.  Thus, Quick-Start is of no
   use towards a target of high link utilization, or fairness in a
   high-utilization scenario, or controlling queueing delay during high
   utilization, or anything of the like.

   At the same time, Quick-Start would allow larger initial windows than
   would proposals such as AntiECN, requires less input to routers than
   XCP (e.g., XCP's cwnd and RTT fields), and would require less
   frequent feedback from routers than any new congestion control
   mechanism.  Thus, Quick-Start is significantly less powerful than
   proposals for new congestion control mechanisms, such as XCP and
   AntiECN, but as powerful or more powerful in terms of the specific
   issue of allowing larger initial windows.  Also, (we think) it is
   more amenable to incremental deployment in the current Internet.

   We do not discuss proposals such as XCP in detail, but simply note
   that there are a number of open questions.  One question concerns
   whether there is a pressing need for more sophisticated congestion
   control mechanisms, such as XCP, in the Internet.  Quick-Start is
   inherently a rather crude tool that does not deliver assurances about
   maintaining high link utilization and low queueing delay; Quick-Start
   is designed for use in environments that are significantly



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   underutilized, and addresses the single question of whether a higher
   sending rate is allowed.  New congestion control mechanisms with more
   fine-grained feedback from routers could allow faster start-ups even
   in environments with rather high link utilization.  Is this a
   pressing requirement?  Are the other benefits of more fine-grained
   congestion control feedback from routers a pressing requirement?

   We would argue that even if more fine-grained per-packet feedback
   from routers was implemented, it is reasonable to have a separate
   mechanism, such as Quick-Start, for indicating an allowed initial
   sending rate, or an allowed total sending rate after an idle or
   underutilized period.

   One difference between Quick-Start and current proposals for fine-
   grained per-packet feedback, such as XCP, is that XCP is designed to
   give robust performance even in the case where different packets
   within a connection routinely follow different paths.  XCP achieves
   relatively robust performance in the presence of multipath routing by
   using per-packet feedback, where the feedback carried in a single
   packet is about the relative increase or decrease in the rate or
   window to take effect when that particular packet is acknowledged,
   not about the allowed sending rate for the connection as a whole.

   In contrast, Quick-Start sends a single Quick-Start Request, and the
   answer to that request gives the allowed sending rate for an entire
   window of data.  As a result, Quick-Start could be problematic in an
   environment where some fraction of the packets in a window of data
   take path A, and the rest of the packets take path B; for example,
   the Quick-Start Request could have traveled on path A, while half the
   Quick-Start packets sent in the succeeding round-trip time are routed
   on path B.  We note that [ZDPS01] shows Internet paths to be stable
   on the order of RTTs.

   There are also differences between Quick-Start and some of the
   proposals for per-packet feedback in terms of the number of bits of
   feedback required from the routers to the end-nodes.  Quick-Start
   uses four bits of feedback in the rate request field to indicate the
   allowed sending rate.  XCP allocates a byte for per-packet feedback,
   though there has been discussion of variants of XCP with less per-
   packet feedback.  This would be more like other proposals, such as
   anti-ECN, that use a single bit of feedback from routers to indicate
   that the sender can increase as fast as slow-starting, in response to
   this particular packet acknowledgement.  In general, there is
   probably considerable power in fine-grained proposals with only two
   bits of feedback, indicating that the sender should decrease,
   maintain, or increase the sending rate or window when this packet is
   acknowledged.  However, the power of Quick-Start would be
   considerably limited if it was restricted to only two bits of



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   feedback; it seems likely that determining the initial sending rate
   fundamentally requires more bits of feedback from routers than does
   the steady-state, per-packet feedback to increase or decrease the
   sending rate.

   On a more practical level, one difference between Quick-Start and
   proposals for per-packet feedback is that there are fewer open issues
   with Quick-Start than there would be with a new congestion control
   mechanism.  Because Quick-Start is a mechanism for requesting an
   initial sending rate in an underutilized environment, its fairness
   issues are less severe than those of a general congestion control
   mechanism.  With Quick-Start, there is no need for the end nodes to
   tell the routers the round-trip time and congestion window, as is
   done in XCP; all that is needed is for the end nodes to report the
   requested sending rate.

   Table 3 provides a summary of the differences between Quick-Start and
   proposals for per-packet congestion control feedback.

                                               Proposals for
                         Quick-Start           Per-Packet Feedback
   +------------------+----------------------+----------------------+
    Semantics:        | Allowed sending rate | Change in rate/window,
                      |  per connection.     |  per-packet.
   +------------------+----------------------+----------------------+
    Relationship to   | In addition.         | Replacement.
    congestion ctrl:  |                      |
   +------------------+----------------------+----------------------+
    Frequency:        | Start-up, or after   | Every packet.
                      |  an idle period.     |
   +------------------+----------------------+----------------------+
    Limitations:      | Only useful on       | General congestion
                      |  underutilized paths.|  control mechanism.
   +------------------+----------------------+----------------------+
    Input to routers: | Rate request.        |RTT, cwnd, request (XCP)
                      |                      | None (Anti-ECN).
   +------------------+----------------------+----------------------+
    Bits of feedback: | Four bits for        | A few bits would
                      |   rate request.      |  suffice?
   +------------------+----------------------+----------------------+

        Table 3: Differences between Quick-Start and Proposals for
                     Fine-Grained Per-Packet Feedback.

   A separate question concerns whether mechanisms, such as Quick-Start,
   in combination with HighSpeed TCP and other changes in progress,
   would make a significant contribution towards meeting some of these
   needs for new congestion control mechanisms.  This could be viewed as



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   a positive step towards meeting some of the more pressing current
   needs with a simple and reasonably deployable mechanism, or
   alternately, as a negative step of unnecessarily delaying more
   fundamental changes.  Without answering this question, we would note
   that our own approach tends to favor the incremental deployment of
   relatively simple mechanisms, as long as the simple mechanisms are
   not short-term hacks, but mechanisms that lead the overall
   architecture in the fundamentally correct direction.

B.7.  Alternate Implementations for a Quick-Start Nonce

B.7.1.  An Alternate Proposal for the Quick-Start Nonce

   An alternate proposal for the Quick-Start Nonce from [B05] would be
   for an n-bit field for the QS Nonce, with the sender generating a
   random nonce when it generates a Quick-Start Request.  Each router
   that reduces the Rate Request by r would hash the QS nonce r times,
   using a one-way hash function such as MD5 [RFC1321] or the secure
   hash 1 [SHA1].  The receiver returns the QS nonce to the sender.
   Because the sender knows the original value for the nonce, and the
   original rate request, the sender knows the total number of steps s
   that the rate has been reduced.  The sender then hashes the original
   nonce s times to check whether the result is the same as the nonce
   returned by the receiver.

   This alternate proposal for the nonce would be considerably more
   powerful than the QS nonce described in Section 3.4, but it would
   also require more CPU cycles from the routers when they reduce a
   Quick-Start Request, and from the sender in verifying the nonce
   returned by the receiver.  As reported in [B05], routers could
   protect themselves from processor exhaustion attacks by limiting the
   rate at which they will approve reductions of Quick-Start Requests.

   Both the Function field and the Reserved field in the Quick-Start
   Option would allow the extension of Quick-Start to use Quick-Start
   requests with the alternate proposal for the Quick-Start Nonce, if it
   was ever desired.

B.7.2.  The Earlier Request-Approved Quick-Start Nonce

   An earlier version of this document included a Request-Approved
   Quick-Start Nonce (QS Nonce) that was initialized by the sender to a
   non-zero, `random' eight-bit number, along with a QS TTL that was
   initialized to the same value as the TTL in the IP header.  The
   Request-Approved Quick-Start Nonce would have been returned by the
   transport receiver to the transport sender in the Quick-Start
   Response.  A router could deny the Quick-Start Request by failing to
   decrement the QS TTL field, by zeroing the QS Nonce field, or by



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   deleting the Quick-Start Request from the packet header.  The QS
   Nonce was included to provide some protection against broken
   downstream routers, or against misbehaving TCP receivers that might
   be inclined to lie about whether the Rate Request was approved.  This
   protection is now provided by the QS Nonce, by the use of a random
   initial value for the QS TTL field, and by Quick-Start-capable
   routers hopefully either deleting the Quick-Start Option or zeroing
   the QS TTL and QS Nonce fields when they deny a request.

   With the old Request-Approved Quick-Start Nonce, along with the QS
   TTL field set to the same value as the TTL field in the IP header,
   the Quick-Start Request mechanism would have been self-terminating;
   the Quick-Start Request would terminate at the first participating
   router after a non-participating router had been encountered on the
   path.  This minimizes unnecessary overhead incurred by routers
   because of option processing for the Quick-Start Request.  In the
   current specification, this "self-terminating" property is provided
   by Quick-Start-capable routers hopefully either deleting the Quick-
   Start Option or zeroing the Rate Request field when they deny a
   request.  Because the current specification uses a random initial
   value for the QS TTL, Quick-Start-capable routers can't tell if the
   Quick-Start Request is invalid because of non-Quick-Start-capable
   routers upstream.  This is the cost of using a design that makes it
   difficult for the receiver to cheat about the value of the QS TTL.

Appendix C.  Quick-Start with DCCP

   DCCP is a new transport protocol for congestion-controlled,
   unreliable datagrams, intended for applications such as streaming
   media, Internet telephony, and online games.  In DCCP, the
   application has a choice of congestion control mechanisms, with the
   currently-specified Congestion Control Identifiers (CCIDs) being CCID
   2 for TCP-like congestion control, and CCID 3 for TCP Friendly Rate
   Control (TFRC), an equation-based form of congestion control.  We
   refer the reader to [RFC4340] for a more detailed description of DCCP
   and congestion control mechanisms.

   Because CCID 3 uses a rate-based congestion control mechanism, it
   raises some new issues about the use of Quick-Start with transport
   protocols.  In this document, we don't attempt to specify the use of
   Quick-Start with DCCP.  However, we do discuss some of the issues
   that might arise.

   In considering the use of Quick-Start with CCID 3 for requesting a
   higher initial sending rate, the following questions arise:






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   (1) How does the sender respond if a Quick-Start packet is dropped?

       As in TCP, if an initial Quick-Start packet is dropped, the CCID
       3 sender should revert to the congestion control mechanisms it
       would have used if the Quick-Start Request had not been approved.

   (2) When does the sender decide there has been no feedback from the
       receiver?

       Unlike TCP, CCID 3 does not use acknowledgements for every
       packet, or for every other packet.  In contrast, the CCID 3
       receiver sends feedback to the sender roughly once per round-trip
       time.  In CCID 3, the allowed sending rate is halved if no
       feedback is received from the receiver in at least four round-
       trip times (when the sender is sending at least one packet every
       two round-trip times).  When a Quick-Start Request is used, it
       would seem necessary to use a smaller time interval, e.g., to
       reduce the sending rate if no feedback arrives from the receiver
       in at least two round-trip times.

   The question also arises of how the sending rate should be reduced
   after a period of no feedback from the receiver.  As with TCP, the
   default CCID 3 response of halving the sending rate is not
   necessarily a sufficient response to the absence of feedback; an
   alternative is to reduce the sending rate to the sending rate that
   would have been used if no Quick-Start Request had been approved.
   That is, if a CCID 3 sender uses a Quick-Start Request, special rules
   might be required to handle the sender's response to a period of no
   feedback from the receiver regarding the Quick-Start packets.

   Similarly, in considering the use of Quick-Start with CCID 3 for
   requesting a higher sending rate after an idle period, the following
   questions arise:

   (1) What rate does the sender request?

       As in TCP, there is a straightforward answer to the rate request
       that the CCID 3 sender should use in requesting a higher sending
       rate after an idle period.  The sender knows the current loss
       event rate, either from its own calculations or from feedback
       from the receiver, and can determine the sending rate allowed by
       that loss event rate.  This is the upper bound on the sending
       rate that should be requested by the CCID 3 sender.  A Quick-
       Start Request is useful with CCID 3 when the sender is coming out
       of an idle or underutilized period, because in standard
       operation, CCID 3 does not allow the sender to send more than
       twice as fast as the receiver has reported received in the most
       recent feedback message.



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   (2) What is the response to loss?

       The response to the loss of Quick-Start packets should be to
       return to the sending rate that would have been used if Quick-
       Start had not been requested.

   (3) When does the sender decide there has been no feedback from the
       receiver?

       As in the case of the initial sending rate, it would seem prudent
       to reduce the sending rate if no feedback is received from the
       receiver in at least two round-trip times.  It seems likely that,
       in this case, the sending rate should be reduced to the sending
       rate that would have been used if no Quick-Start Request had been
       approved.

Appendix D.  Possible Router Algorithm

   This specification does not tightly define the algorithm a router
   uses when deciding whether to approve a Quick-Start Rate Request or
   whether and how to reduce a Rate Request.  A range of algorithms is
   likely useful in this space and we consider the algorithm a
   particular router uses to be a local policy decision.  In addition,
   we believe that additional experimentation with router algorithms is
   necessary to have a solid understanding of the dynamics various
   algorithms impose.  However, we provide one particular algorithm in
   this appendix as an example and as a framework for thinking about
   additional mechanisms.

   [SAF06] provides several algorithms routers can use to consider
   incoming Rate Requests.  The decision process involves two
   algorithms.  First, the router needs to track the link utilization
   over the recent past.  Second, this utilization needs to be updated
   by the potential new bandwidth from recent Quick-Start approvals, and
   then compared with the router's notion of when it is underutilized
   enough to approve Quick-Start Requests (of some size).

   First, we define the "peak utilization" estimation technique (from
   [SAF06]).  This mechanism records the utilization of the link every S
   seconds and stores the most recent N of these measurements.  The
   utilization is then taken as the highest utilization of the N
   samples.  This method, therefore, keeps N*S seconds of history.  This
   algorithm reacts rapidly to increases in the link utilization.  In
   [SAF06], S is set to 0.15 seconds, and experiments use values for N
   ranging from 3 to 20.

   Second, we define the "target" algorithm for processing incoming
   Quick-Start Rate Requests (also from [SAF06]).  The algorithm relies



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   on knowing the bandwidth of the outgoing link (which, in many cases,
   can be easily configured), the utilization of the outgoing link (from
   an estimation technique such as given above), and an estimate of the
   potential bandwidth from recent Quick-Start approvals.

   Tracking the potential bandwidth from recent Quick-Start approvals is
   another case where local policy dictates how it should be done.  The
   simplest method, outlined in Section 8.2, is for the router to keep
   track of the aggregate Quick-Start rate requests approved in the most
   recent two or more time intervals, including the current time
   interval, and to use the sum of the aggregate rate requests over
   these time intervals as the estimate of the approved Rate Requests.
   The experiments in [SAF06] keep track of the aggregate approved Rate
   Requests over the most recent two time intervals, for intervals of
   150 msec.

   The target algorithm also depends on a threshold (qs_thresh) that is
   the fraction of the outgoing link bandwidth that represents the
   router's notion of "significantly underutilized".  If the
   utilization, augmented by the potential bandwidth from recent Quick-
   Start approvals, is above this threshold, then no Quick-Start Rate
   Requests will be approved.  If the utilization, again augmented by
   the potential bandwidth from recent Quick-Start approvals, is less
   than the threshold, then Rate Requests can be approved.  The Rate
   Requests will be reduced such that the bandwidth allocated would not
   drive the utilization to more than the given threshold.  The
   algorithm is:

     util_bw = bandwidth * utilization;
     util_bw = util_bw + recent_qs_approvals;
     if (util_bw < (qs_thresh * bandwidth))
     {
         approved = (qs_thresh * bandwidth) - util_bw;
         if (rate_request < approved)
             approved = rate_request;
         approved = round_down (approved);
         recent_qs_approvals += approved;
     }

   Also note that, given that Rate Requests are fairly coarse, the
   approved rate should be rounded down when it does not fall exactly on
   one of the rates allowed by the encoding scheme.

   Routers that wish to keep close track of the allocated Quick-Start
   bandwidth could use Approved Rate reports to learn when rate requests
   had been decremented downstream in the network, and also to learn
   when a sender begins to use the approved Quick-Start Request.




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RFC 4782               Quick-Start for TCP and IP           January 2007


Appendix E.  Possible Additional Uses for the Quick-Start Option

   The Quick-Start Option contains a four-bit Function field in the
   third byte, enabling additional uses to be defined for the Quick-
   Start Option.  In this section, we discuss some of the possible
   additional uses that have been discussed for Quick-Start.  The
   Function field makes it easy to add new functions for the Quick-
   Start Option.

   Report of Current Sending Rate: A Quick-Start Request with the
   `Report of Current Sending Rate' codepoint set in the Function field
   would be using the Rate Request field to report the current estimated
   sending rate for that connection.  This could accompany a second
   Quick-Start Request in the same packet containing a standard rate
   request, for a connection that is using Quick-Start to increase its
   current sending rate.

   Request to Increase Sending Rate: A codepoint for `Request to
   Increase Sending Rate' in the Function field would indicate that the
   connection is not idle or just starting up, but is attempting to use
   Quick-Start to increase its current sending rate.  This codepoint
   would not change the semantics of the Rate Request field.

   RTT Estimate: If a codepoint for `RTT Estimate' was used, a field for
   the RTT Estimate would contain one or more bits giving the sender's
   rough estimate of the round-trip time, if known.  E.g., the sender
   could estimate whether the RTT was greater or less than 200 ms.
   Alternately, if the sender had an estimate of the RTT when it sends
   the Rate Request, the two-bit Reserved field at the end of the
   Quick-Start Option could be used for a coarse-grained encoding of the
   RTT.

   Informational Request: An Informational Request codepoint in the
   Function field would indicate that a request is purely informational,
   for the sender to find out if a rate request would be approved, and
   what size rate request would be approved when the Informational
   Request is sent.  For example, an Informational Request could be
   followed one round-trip time later by a standard Quick-Start Request.
   A router approving an Informational Request would not consider this
   as an approval for Quick-Start bandwidth to be used, and would not be
   under any obligation to approve a similar standard Quick-Start
   Request one round-trip time later.  An Informational Request with a
   rate request of zero could be used by the sender to find out if all
   of the routers along the path supported Quick-Start.

   Use Format X for the Rate Request Field: A Quick-Start codepoint for
   `Use Format X for the Rate Request Field' would indicate that an
   alternate format was being used for the Rate Request field.



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RFC 4782               Quick-Start for TCP and IP           January 2007


   Do Not Decrement: A Do Not Decrement codepoint could be used for a
   Quick-Start Request where the sender would rather have the request
   denied than to have the rate request decremented in the network.
   This could be used if the sender was only interested in using Quick-
   Start if the original rate request was approved.

   Temporary Bandwidth Use: A Temporary codepoint has been proposed to
   indicate that a connection would only use the requested bandwidth for
   a single time interval.

Normative References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
             November 1990.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
             Control", RFC 2581, April 1999.

   [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
             Explicit Congestion Notification (ECN) to IP", RFC 3168,
             September 2001.

   [RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
             Initial Window", RFC 3390, October 2002.

   [RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
             Congestion Windows", RFC 3742, March 2004.

Informative References

   [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
             792, September 1981.

   [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.

   [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
             RFC 1812, June 1995.




Floyd, et al.                 Experimental                     [Page 75]



RFC 4782               Quick-Start for TCP and IP           January 2007


   [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
             October 1996.

   [RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, February
             1997.

   [RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
             April 1997.

   [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
             Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
             Functional Specification", RFC 2205, September 1997.

   [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.

   [RFC2415] Poduri, K. and K. Nichols, "Simulation Studies of Increased
             Initial TCP Window Size", RFC 2415, September 1998.

   [RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP Over
             Satellite Channels using Standard Mechanisms", BCP 28, RFC
             2488, January 1999.

   [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
             and B. Palter, "Layer Two Tunneling Protocol 'L2TP'", RFC
             2661, August 1999.

   [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
             "Generic Routing Encapsulation (GRE)", RFC 2784, March
             2000.

   [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
             Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
             L., and V. Paxson, "Stream Control Transmission Protocol",
             RFC 2960, October 2000.

   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.

   [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
             RFC 3124, June 2001.

   [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
             Issues", RFC 3234, February 2002.

   [RFC3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
             August 2002.




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RFC 4782               Quick-Start for TCP and IP           January 2007


   [RFC3360] Floyd, S., "Inappropriate TCP Resets Considered Harmful",
             BCP 60, RFC 3360, August 2002.

   [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
             RFC 3649, December 2003.

   [RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
             Per-Domain Behavior (PDB) for Differentiated Services", RFC
             3662, December 2003.

   [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
             "IPv6 Flow Label Specification", RFC 3697, March 2004.

   [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
             in IPv6", RFC 3775, June 2004.

   [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig,
             R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood,
             "Advice for Internet Subnetwork Designers", BCP 89, RFC
             3819, July 2004.

   [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
             Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
             3948, January 2005.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, December 2005.

   [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
             2005.

   [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
             4306, December 2005.

   [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
             Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
             Control Protocol (DCCP) Congestion Control ID 2: TCP-like
             Congestion Control", RFC 4341, March 2006.

   [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
             Message Protocol (ICMPv6) for the Internet Protocol Version
             6 (IPv6) Specification", RFC 4443, March 2006.

   [AHO98]   M. Allman, C. Hayes and S. Ostermann. An evaluation of TCP
             with Larger Initial Windows. ACM Computer Communication
             Review, July 1998.



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RFC 4782               Quick-Start for TCP and IP           January 2007


   [B05]     Briscoe, B., "Review: Quick-Start for TCP and IP",
             <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html>,
             November 2005.

   [BW97]    G. Brasche and B. Walke. Concepts, Services and Protocols
             of the new GSM Phase 2+ General Packet Radio Service. IEEE
             Communications Magazine, pages 94--104, August 1997.

   [GPAR02]  A. Gurtov, M. Passoja, O. Aalto, and M. Raitola. Multi-
             Layer Protocol Tracing in a GPRS Network. In Proceedings of
             the IEEE Vehicular Technology Conference (Fall VTC2002),
             Vancouver, Canada, September 2002.

   [H05]     P. Hoffman, email, October 2005.  Citation for
             acknowledgement purposes only.

   [HKP01]   M. Handley, C. Kreibich and V. Paxson, Network Intrusion
             Detection: Evasion, Traffic Normalization, and End-to-End
             Protocol Semantics, Proc. USENIX Security Symposium 2001.

   [Jac88]   V. Jacobson, Congestion Avoidance and Control, ACM SIGCOMM.

   [JD02]    Manish Jain, Constantinos Dovrolis, End-to-End Available
             Bandwidth: Measurement Methodology, Dynamics, and Relation
             with TCP Throughput, SIGCOMM 2002.

   [K03]     S. Kunniyur, "AntiECN Marking: A Marking Scheme for High
             Bandwidth Delay Connections", Proceedings, IEEE ICC '03,
             May 2003.  <http://www.seas.upenn.edu/~kunniyur/>.

   [KAPS02]  Rajesh Krishnan, Mark Allman, Craig Partridge, James P.G.
             Sterbenz. Explicit Transport Error Notification (ETEN) for
             Error-Prone Wireless and Satellite Networks. Technical
             Report No. 8333, BBN Technologies, March 2002.
             <http://www.icir.org/mallman/papers/>.

   [KHR02]   Dina Katabi, Mark Handley, and Charles Rohrs, Internet
             Congestion Control for Future High Bandwidth-Delay Product
             Environments. ACM Sigcomm 2002, August 2002.
             <http://ana.lcs.mit.edu/dina/XCP/>.

   [KK03]    A. Kuzmanovic and E. W. Knightly.  TCP-LP: A Distributed
             Algorithm for Low Priority Data Transfer.  INFOCOM 2003,
             April 2003.







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RFC 4782               Quick-Start for TCP and IP           January 2007


   [KSEPA04] Rajesh Krishnan, James Sterbenz, Wesley Eddy, Craig
             Partridge, Mark Allman. Explicit Transport Error
             Notification (ETEN) for Error-Prone Wireless and Satellite
             Networks. Computer Networks, 46(3), October 2004.

   [L05]     Guohan Lu, Nonce in TCP Quick Start, September 2005.
             <http://www.net-glyph.org/~lgh/nonce-usage.pdf>.

   [MH06]    Mathis, M. and J. Heffner, "Packetization Layer Path MTU
             Discovery", Work in Progress, December 2006.

   [MAF04]   Alberto Medina, Mark Allman, and Sally Floyd, Measuring
             Interactions Between Transport Protocols and Middleboxes,
             Internet Measurement Conference 2004, August 2004.
             <http://www.icir.org/tbit/".

   [MAF05]   Alberto Medina, Mark Allman, and Sally Floyd.  Measuring
             the Evolution of Transport Protocols in the Internet.
             Computer Communications Review, April 2005.

   [MaxNet]  MaxNet Home Page,
             <http://netlab.caltech.edu/~bartek/maxnet.htm>.

   [P00]     Joon-Sang Park, Bandwidth Discovery of a TCP Connection,
             report to John Heidemann, 2000, private communication.
             Citation for acknowledgement purposes only.

   [PABL+05] Ruoming Pang, Mark Allman, Mike Bennett, Jason Lee, Vern
             Paxson, Brian Tierney.  A First Look at Modern Enterprise
             Traffic.  ACM SIGCOMM/USENIX Internet Measurement
             Conference, October 2005.

   [PRAKS02] Craig Partridge, Dennis Rockwell, Mark Allman, Rajesh
             Krishnan, James P.G. Sterbenz. A Swifter Start for TCP.
             Technical Report No. 8339, BBN Technologies, March 2002.
             <http://www.icir.org/mallman/papers/>.

   [RW03]    Mattia Rossi and Michael Welzl, On the Impact of IP Option
             Processing, Preprint-Reihe des Fachbereichs Mathematik -
             Informatik, No. 15, Institute of Computer Science,
             University of Innsbruck, Austria, October 2003.

   [RW04]    Mattia Rossi and Michael Welzl, On the Impact of IP Option
             Processing -   Part 2, Preprint-Reihe des Fachbereichs
             Mathematik - Informatik, No. 26, Institute of Computer
             Science, University of Innsbruck, Austria, July 2004.





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RFC 4782               Quick-Start for TCP and IP           January 2007


   [S02]     Ion Stoica, private communication, 2002.  Citation for
             acknowledgement purposes only.

   [SAF06]   Pasi Sarolahti, Mark Allman, and Sally Floyd.  Determining
             an Appropriate Sending Rate Over an Underutilized Network
             Path.  February 2006.
             <http://www.icir.org/floyd/quickstart.html>.

   [SGF05]   Singh, M., Guha, S., and P. Francis, "Utilizing spare
             network bandwidth to improve TCP performance", ACM SIGCOMM
             2005 Work in Progress session, August 2005,
             <https://www.guha.cc/saikat/pub/sigcomm05-lowtcp.pdf>.

   [SHA1]    "Secure Hash Standard", FIPS, U.S. Department of Commerce,
             Washington, D.C., publication 180-1, April 1995.

   [SH02]    Srikanth Sundarrajan and John Heidemann.  Study of TCP
             Quick Start with NS-2.  Class Project, December 2002.  Not
             publicly available; citation for acknowledgement purposes
             only.

   [V02]     A. Venkataramani, R. Kokku, and M. Dahlin.  TCP Nice: A
             Mechanism for Background Transfers.  OSDI 2002.

   [VH97]    V. Visweswaraiah and J. Heidemann, Improving Restart of
             Idle TCP Connections, Technical Report 97-661, University
             of Southern California, November 1997.

   [W00]     Michael Welzl: PTP: Better Feedback for Adaptive
             Distributed Multimedia Applications on the Internet, IPCCC
             2000 (19th IEEE International Performance, Computing, And
             Communications Conference), Phoenix, Arizona, USA, 20-22
             February 2000.
             <http://www.welzl.at/research/publications/>.

   [ZDPS01]  Y. Zhang, N. Duffield, V. Paxson, and S. Shenker,  On the
             Constancy of Internet Path Properties, Proc. ACM SIGCOMM
             Internet Measurement Workshop, November 2001.

   [ZQK00]   Y. Zhang, L. Qiu, and S. Keshav, Speeding Up Short Data
             Transfers: Theory, Architectural Support, and Simulation
             Results, NOSSDAV 2000, June 2000.









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RFC 4782               Quick-Start for TCP and IP           January 2007


Authors' Addresses

   Sally Floyd
   Phone: +1 (510) 666-2989
   ICIR (ICSI Center for Internet Research)
   EMail: floyd@icir.org
   URL: http://www.icir.org/floyd/

   Mark Allman
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704-1198
   Phone: (440) 235-1792
   EMail: mallman@icir.org
   URL: http://www.icir.org/mallman/

   Amit Jain
   F5 Networks
   EMail: a.jain@f5.com

   Pasi Sarolahti
   Nokia Research Center
   P.O. Box 407
   FI-00045 NOKIA GROUP
   Finland
   Phone: +358 50 4876607
   EMail: pasi.sarolahti@iki.fi
























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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,
   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
   IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
   PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






Floyd, et al.                 Experimental                     [Page 82]



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