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Request For Comments - RFC2871

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Network Working Group                                       J. Rosenberg
Request for Comments: 2871                                   dynamicsoft
Category: Informational                                   H. Schulzrinne
                                                     Columbia University
                                                               June 2000

               A Framework for Telephony Routing over IP

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   This document serves as a framework for Telephony Routing over IP
   (TRIP), which supports the discovery and exchange of IP telephony
   gateway routing tables between providers. The document defines the
   problem of telephony routing exchange, and motivates the need for the
   protocol. It presents an architectural framework for TRIP, defines
   terminology, specifies the various protocol elements and their
   functions, overviews the services provided by the protocol, and
   discusses how it fits into the broader context of Internet telephony.

Table of Contents

   1      Introduction ........................................    2
   2      Terminology .........................................    2
   3      Motivation and Problem Definition ...................    4
   4      Related Problems ....................................    6
   5      Relationship with BGP ...............................    7
   6      Example Applications of TRIP ........................    8
   6.1    Clearinghouses ......................................    8
   6.2    Confederations ......................................    9
   6.3    Gateway Wholesalers .................................    9
   7      Architecture ........................................   11
   8      Elements ............................................   12
   8.1    IT Administrative Domain ............................   12
   8.2    Gateway .............................................   13
   8.3    End Users ...........................................   14
   8.4    Location Server .....................................   14
   9      Element Interactions ................................   16

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   9.1    Gateways and Location Servers .......................   16
   9.2    Location Server to Location Server ..................   16
   9.2.1  Nature of Exchanged Information .....................   17
   9.2.2  Quality of Service ..................................   18
   9.2.3  Cost Information ....................................   19
   10     The Front End .......................................   19
   10.1   Front End Customers .................................   19
   10.2   Front End Protocols .................................   20
   11     Number Translations .................................   21
   12     Security Considerations .............................   22
   13     Acknowledgments .....................................   23
   14     Bibliography ........................................   23
   15     Authors' Addresses ..................................   24
   16     Full Copyright Statement ............................   25

1 Introduction

   This document serves as a framework for Telephony Routing over IP
   (TRIP), which supports the discovery and exchange of IP telephony
   gateway routing tables between providers. The document defines the
   problem of telephony routing exchange, and motivates the need for the
   protocol. It presents an architectural framework for TRIP, defines
   terminology, specifies the various protocol elements and their
   functions, overviews the services provided by the protocol, and
   discusses how it fits into the broader context of Internet telephony.

2 Terminology

   We define the following terms. Note that there are other definitions
   for these terms, outside of the context of gateway location. Our
   definitions aren't general, but refer to the specific meaning here:

     Gateway: A device with some sort of circuit switched network
        connectivity and IP connectivity, capable of initiating and
        terminating IP telephony signaling protocols, and capable of
        initiating and terminating telephone network signaling

     End User: The end user is usually (but not necessarily) a human
        being, and is the party who is the ultimate initiator or
        recipient of calls.

     Calling Device: The calling device is a physical entity which has
        IP connectivity. It is under the direction of an end user who
        wishes to place a call. The end user may or may not be directly
        controlling the calling device. If the calling device is a PC,

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        the end user is directly controlling it. If, however, the
        calling device is a telephony gateway, the end user may be
        accessing it through a telephone.

     Gatekeeper: The H.323 gatekeeper element, defined in [1].

     SIP Server: The Session Initiation Protocol proxy or redirect
        server defined in [2].

     Call Agent: The MGCP call agent, defined in [3].

     GSTN: The Global Switched Telephone Network, which is the worldwide
        circuit switched network.

     Signaling Server: A signaling server is an entity which is capable
        of receiving and sending signaling messages for some IP
        telephony signaling protocol, such as H.323 or SIP.  Generally
        speaking, a signaling server is a gatekeeper, SIP server, or
        call agent.

     Location Server (LS): A logical entity with IP connectivity which
        has knowledge of gateways that can be used to terminate calls
        towards the GSTN. The LS is the main entity that participates in
        Telephony Routing over IP. The LS is generally a point of
        contact for end users for completing calls to the telephony
        network. An LS may also be responsible for propagation of
        gateway information to other LS's. An LS may be coresident with
        an H.323 gatekeeper or SIP server, but this is not required.

     Internet Telephony Administrative Domain (ITAD): The set of
        resources (gateways and Location Servers) under the control of a
        single administrative authority. End users are customers of an

     Provider: The administrator of an ITAD.

     Location Server Policy: The set of rules which dictate how a
        location server processes information it sends and receives via
        TRIP. This includes rules for aggregating, propagating,
        generating, and accepting information.

     End User Policy: Preferences that an end user has about how a call
        towards the GSTN should be routed.

     Peers: Two LS's are peers when they have a persistent association
        between them over which gateway information is exchanged.

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     Internal peers: Peers that both reside within the same ITAD.

     External peers: Peers that reside within different ITADs.

     Originating Location Server: A Location Server which first
        generates a route to a gateway in its ITAD.

     Telephony Routing Information Base (TRIB): The database of gateways
        an LS builds up as a result of participation in TRIP.

3 Motivation and Problem Definition

   As IP telephony gateways grow in terms of numbers and usage, managing
   their operation will become increasingly complex. One of the
   difficult tasks is that of gateway location, also known as gateway
   selection, path selection, gateway discovery, and gateway routing.
   The problem occurs when a calling device (such as a telephony gateway
   or a PC with IP telephony software) on an IP network needs to
   complete a call to a phone number that represents a terminal on a
   circuit switched telephone network. Since the intended target of the
   call resides in a circuit switched network, and the caller is
   initiating the call from an IP host, a telephony gateway must be
   used. The gateway functions as a conversion point for media and
   signaling, converting between the protocols used on the IP network,
   and those used in the circuit switched network.

   The gateway is, in essence, a relaying point for an application layer
   signaling protocol. There may be many gateways which could possibly
   complete the call from the calling device on the IP network to the
   called party on the circuit switched network. Choosing such a gateway
   is a non-trivial process. It is complicated because of the following

     Number of Candidate Gateways: It is anticipated that as IP
        telephony becomes widely deployed, the number of telephony
        gateways connecting the Internet to the GSTN will become large.
        Attachment to the GSTN means that the gateway will have
        connectivity to the nearly one billion terminals reachable on
        this network. This means that every gateway could theoretically
        complete a call to any terminal on the GSTN.  As such, the
        number of candidate gateways for completing a call may be very

     Business Relationships: In reality, the owner of a gateway is
        unlikely to make the gateway available to any user who wishes to
        connect to it. The gateway provides a useful service, and incurs
        cost when completing calls towards the circuit switched network.
        As a result, providers of gateways will, in many cases, wish to

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        charge for use of these gateways. This may restrict usage of the
        gateway to those users who have, in some fashion, an established
        relationship with the gateway provider.

     Provider Policy: In all likelihood, an end user who wishes to make
        use of a gateway service will not compensate the gateway
        provider directly. The end user may have a relationship with an
        IP telephony service provider which acts as an intermediary to
        providers of gateways. The IP telephony service provider may
        have gateways of its own as well. In this case, the IP telephony
        service provider may have policies regarding the usage of
        various gateways from other providers by its customers. These
        policies must figure into the selection process.

     End User Policy: In some cases, the end user may have specific
        requirements regarding the gateway selection. The end user may
        need a specific feature, or have a preference for a certain
        provider. These need to be taken into account as well.

     Capacity: All gateways are not created equal. Some are large,
        capable of supporting hundreds or even thousands of simultaneous
        calls. Others, such as residential gateways, may only support
        one or two calls. The process for selecting gateways should
        allow gateway capacity to play a role. It is particularly
        desirable to support some form of load balancing across gateways
        based on their capacities.

     Protocol and Feature Compatibilities: The calling party may be
        using a specific signaling or media protocol that is not
        supported by all gateways.

   From these issues, it becomes evident that the selection of a gateway
   is driven in large part by the policies of various parties, and by
   the relationships established between these parties. As such, there
   cannot be a global "directory of gateways" in which users look up
   phone numbers. Rather, information on availability of gateways must
   be exchanged by providers, and subject to policy, made available
   locally and then propagated to other providers. This would allow each
   provider to build up its own local database of available gateways -
   such a database being very different for each provider depending on

   From this, we can conclude that a protocol is needed between
   administrative domains for exchange of gateway routing information.
   The protocol that provides these functions is Telephony Routing over
   IP (TRIP). TRIP provides a specific set of functions:

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      o Establishment and maintenance of peering relationships between

      o Exchange and synchronization of telephony gateway routing
        information between providers;

      o Prevention of stable routing loops for IP telephony signaling

      o Propagation of learned gateway routing information to other
        providers in a timely and scalable fashion;

      o Definition of the syntax and semantics of the data which
        describe telephony gateway routes.

   TRIP can be generally summarized as an inter-domain IP telephony
   gateway routing protocol.

4 Related Problems

   At a high level, the problem TRIP solves appears to be a mapping
   problem: given an input telephone number, determine, based on some
   criteria, the address of a telephony gateway. For this reason, the
   gateway location problem is often called a "phone number to IP
   address translation problem". This is an over-simplification,
   however. There are at least three separate problems, all of which can
   be classified as a "phone number to IP address translation problem",
   and only one of which is addressed by TRIP:

      o Given a phone number that corresponds to a terminal on a
        circuit switched network, determine the IP address of a
        gateway capable of completing a call to that phone number.

      o Given a phone number that corresponds to a specific host on
        the Internet (this host may have a phone number in order to
        facilitate calls to it from the circuit switched network),
        determine the IP address of this host.

      o Given a phone number that corresponds to a user of a terminal
        on a circuit switched network, determine the IP address of an
        IP terminal which is owned by the same user.

   The last of these three mapping functions is useful for services
   where the PC serves as an interface for the phone. One such service
   is the delivery of an instant message to a PC when the user's phone
   rings. To deliver this service, a switch in the GSTN is routing a
   call towards a phone number. It wishes to send an Instant Message to
   the PC for this user. This switch must somehow have access to the IP

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   network, in order to determine the IP address of the PC corresponding
   to the user with the given phone number. The mapping function is a
   name to address translation problem, where the name happens to be
   represented by a string of digits. Such a translation function is
   best supported by directory protocols. This problem is not addressed
   by TRIP.

   The second of these mappings is needed to facilitate calls from
   traditional phones to IP terminals. When a user on the GSTN wishes to
   call a user with a terminal on the IP network, they need to dial a
   number identifying that terminal. This number could be an IP address.
   However, IP addresses are often ephemeral, assigned on demand by DHCP
   [4] or by dialup network access servers using PPP [5]. The number
   could be a hostname, obtained through some translation of groups of
   numbers to letters. However, this is cumbersome. It has been proposed
   instead to assign phone numbers to IP telephony terminals. A caller
   on the GSTN would then dial this number as they would any other. This
   number serves as an alternate name for the IP terminal, in much the
   same way its hostname serves as a name. A switch in the GSTN must
   then access the IP network, and obtain the mapping from this number
   to an IP address for the PC. Like the previous case, this problem is
   a name to address translation problem, and is best handled by a
   directory protocol. It is not addressed by TRIP.

   The first mapping function, however, is fundamentally an address to
   route translation problem. It is this problem which is considered by
   TRIP. As discussed in Section 3, this mapping depends on local
   factors such as policies and provider relationships. As a result, the
   database of available gateways is substantially different for each
   provider, and needs to be built up through specific inter-provider
   relationships. It is for this reason that a directory protocol is not
   appropriate for TRIP, whereas it is appropriate for the others.

5 Relationship with BGP

   TRIP can be classified as a close cousin of inter-domain IP routing
   protocols, such as BGP [6]. However, there are important differences
   between BGP and TRIP:

      o TRIP runs at the application layer, not the network layer,
        where BGP resides.

      o TRIP runs between servers which may be separated by many
        intermediate networks and IP service providers. BGP runs
        between routers that are usually adjacent.

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      o The information exchanged between TRIP peers describes routes
        to application layer devices, not IP routers, as is done with

      o TRIP assumes the existence of an underlying IP transport
        network. This means that servers which exchange TRIP routing
        information need not act as forwarders of signaling messages
        that are routed based on this information. This is not true in
        BGP, where the peers must also act as forwarding points (or
        name an adjacent forwarding hop) for IP packets.

      o The purpose of TRIP is not to establish global connectivity
        across all ITADs. It is perfectly reasonable for there to be
        many small islands of TRIP connectivity. Each island
        represents a closed set of administrative relationships.
        Furthermore, each island can still have complete connectivity
        to the entire GSTN. This is in sharp contrast to BGP, where
        the goal is complete connectivity across the Internet. If a
        set of AS's are isolated from some other set because of a BGP
        disconnect, no IP network connectivity exists between them.

      o Gateway routes are far more complex than IP routes (since they
        reside at the application, not the network layer), with many
        more parameters which may describe them.

      o BGP exchanges prefixes which represent a portion of the IP
        name space. TRIP exchanges phone number ranges, representing a
        portion of the GSTN numbering space. The organization and
        hierarchies in these two namespaces are different.

   These differences means that TRIP borrows many of the concepts from
   BGP, but that it is still a different protocol with its own specific
   set of functions.

6 Example Applications of TRIP

   TRIP is a general purpose tool for exchanging IP telephony routes
   between providers. TRIP does not, in any way, dictate the structure
   or nature of the relationships between those providers. As a result,
   TRIP has applications for a number of common cases for IP telephony.

6.1 Clearinghouses

   A clearinghouse is a provider that serves as an exchange point
   between a number of other providers, called the members of the
   clearinghouse. Each member signs on with the clearinghouse. As part
   of the agreement, the member makes their gateways available to the
   other members of the clearinghouse. In exchange, the members have

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   access to the gateways owned by the other members of the
   clearinghouse. When a gateway belonging to one member makes a call,
   the clearinghouse plays a key role in determining which member
   terminates the call.

   TRIP can be applied here as the tool for exchanging routes between
   the members and the clearinghouse. This is shown in Figure 1.

   There are 6 member companies, M1 through M6. Each uses TRIP to send
   and receive gateway routes with the clearinghouse provider.

6.2 Confederations

   We refer to a confederation as a group of providers which all agree
   to share gateways with each other in a full mesh, without using a
   central clearinghouse. Such a configuration is shown in Figure 2.
   TRIP would run between each pair of providers.

6.3 Gateway Wholesalers

          ------                                  ------
         |      |                                |      |
         | M1   |    TRIP                 TRIP   |  M2  |
         |      |\    |                    |    /|      |
          ------  \   |                    |   /  ------
                   \ \ /   -------------- \ / /
          ------    \----|              |----/    ------
         |      |        |              |        |      |
         | M3   |--------| Clearinghouse|--------|  M4  |
         |      |        |              |        |      |
          ------    /----|              |----\    ------
                   /      --------------      \
          ------  /                            \  ------
         |      |/                              \|      |
         | M5   |                                |  M6  |
         |      |                                |      |
          ------                                  ------

          Figure 1: TRIP in the Clearinghouse Application

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                       ------        ------
                      |      |------|      |
                      | M1   |      |  M2  |
                      |      |\    /|      |
                       ------  \  /  ------
                         |      \/     |
                         |      /\     |<-----TRIP
                       ------  /  \  ------
                      |      |/    \|      |
                      | M3   |      |  M4  |
                      |      |------|      |
                       ------        ------

                 Figure 2: TRIP for Confederations

   In this application, there are a number of large providers of
   telephony gateways. Each of these resells its gateway services to
   medium sized providers. These, in turn, resell to local providers who
   sell directly to consumers. This is effectively a pyramidal
   relationship, as shown in Figure 3.

                            |      |
                            |  M1  |
                            |      |
                           /       \ <------- TRIP
                      ------        ------
                     |      |      |      |
                     |  M2  |      |  M3  |
                     |      |      |      |
                      ------        ------
                     /      \      /      \
               ------        ------        ------
              |      |      |      |      |      |
              | M4   |      | M5   |      | M6   |
              |      |      |      |      |      |
               ------        ------        ------

                Figure 3: TRIP for Wholesalers

   Note that in this example, provider M5 resells gateways from both M2
   and M3.

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7 Architecture

   Figure 4 gives the overall architecture of TRIP.

           ITAD1                                ITAD2
      -----------------                ------------------
     |                  |             |                  |
     |  ----            |             |           ----   |
     | | GW |           |             |          | EU |  |
     |  ----  \  ----   |             |  ----  /  ----   |
     |          | LS | ---------------- | LS |           |
     |  ----     ----   |             /  ----  \  ----   |
     | | GW | /         |            /|          | EU |  |
     |  ----            |           / |           ----   |
     |                  |          /  |                  |
      ------------------          /    ------------------
                     --------- /----------
                    |         |           |
                    |        ----         |
                    |       | LS |        |
                    |     /  ---- \       |
                    |  ----   ||   ----   |
                    | | GW |  ||  | EU |  |
                    |  ----   ||   ----   |
                    |  ----   ||   ----   |
                    | | GW | /  \ | EU |  |
                    |  ----        ----   |
                    |                     |

                  Figure 4: TRIP Architecture

   There are a number of Internet Telephony administrative domains
   (ITAD's), each of which has at least one Location Server (LS). The
   LS's, through an out-of-band means, called the intra-domain protocol,
   learn about the gateways in their domain. The intra-domain protocol
   is represented by the lines between the GW and LS elements in ITAD1
   in the Figure. The LS's have associations with other LS's, over which
   they exchange gateway information. These associations are established
   administratively, and are set up when the IT administrative domains
   have some kind of agreements in place regarding exchange of gateway
   information. In the figure, the LS in ITAD1 is connected to the LS in
   ITAD2, which is in turn connected to the LS in ITAD3. Through
   Telephony Routing over IP (TRIP), the LS in ITAD2 learns about the
   two gateways in ITAD1. This information is accessed by end users

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   (EUs) in ITAD2 through the front-end. The front-end is a non-TRIP
   protocol or mechanism by which the LS databases are accessed. In
   ITAD3, there are both EUs and gateways. The LS in ITAD3 learns about
   the gateways in ITAD1 through a potentially aggregated advertisement
   from the LS in ITAD2.

8 Elements

   The architecture in Figure 4 consists of a number of elements. These
   include the IT administrative domain, end user, gateway, and location

8.1 IT Administrative Domain

   An IT administrative domain consists of zero or more gateways, at
   least one Location Server, and zero or more end users. The gateways
   and LS's are those which are under the administrative control of a
   single authority. This means that there is one authority responsible
   for dictating the policies and configuration of the gateways and

   An IT administrative domain need not be the same as an autonomous
   system. While an AS represents a set of physically connected
   networks, an IT administrative domain may consist of elements on
   disparate networks, and even within disparate autonomous systems.

   The end users within an IT administrative domain are effectively the
   customers of that IT administrative domain. They are interested in
   completing calls towards the telephone network, and thus need access
   to gateways. An end user may be a customer of one IT administrative
   domain for one call, and then a customer of a different one for the
   next call.

   An IT administrative domain need not have any gateways. In this case,
   its LS learns about gateways in other domains, and makes these
   available to the end users within its domain. In this case, the IT
   administrative domain is effectively a virtual IP telephony gateway
   provider. This is because it provides gateway service, but may not
   actually own or administer any gateways.

   An IT administrative domain need not have any end users. In this
   case, it provides "wholesale" gateway service, making its gateways
   available to customers in other IT administrative domains.

   An IT administrative domain need not have gateways nor end users. In
   this case, the ITAD only has LS's. The ITAD acts as a reseller,
   learning about other gateways, and then aggregating and propagating
   this information to other ITAD's which do have customers.

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8.2 Gateway

   A gateway is a logical device which has both IP connectivity and
   connectivity to some other network, usually a public or private
   telephone network. The function of the gateway is to translate the
   media and signaling protocols from one network technology to the
   other, achieving a transparent connection for the users of the

   A gateway has a number of attributes which characterize the service
   it provides. Most fundamental among these are the range of phone
   numbers to which it is willing to provide service. This range may be
   broken into subranges, and associated with each, some cost metric or
   cost token. This token indicates some notion of cost or preference
   for completing calls for this part of the telephone number range.

   A gateway has attributes which characterize the volume of service
   which it can provide. These include the number of ports it has (i.e.,
   the number of simultaneous phone calls it can support), and the
   access link speed. These two together represent some notion of the
   capacity of the gateway. The metric is useful for allowing Location
   Servers to decide to route calls to gateways in proportion to the
   value of the metric, thus achieving a simple form of load balancing.

   A gateway also has attributes which characterize the type of service
   it provides. This includes, but is not limited to, signaling
   protocols supported, telephony features provided, speech codecs
   understood, and encryption algorithms which are implemented. These
   attributes may be important in selecting a gateway. In the absence of
   baseline required features across all gateways (an admirable, but
   difficult goal), such a set of attributes is required in order to
   select a gateway with which communications can be established. End
   users which have specific requirements for the call (such as a user
   requesting a business class call, in which case certain call features
   may need to be supported) may wish to make use of such information as

   Some of these attributes are transported in TRIP to describe
   gateways, and others are not. This depends on whether the metric can
   be reasonably aggregated, and whether it is something which must be
   conveyed in TRIP before the call is set up (as opposed to negotiated
   or exchanged by the signaling protocols themselves). The philosophy
   of TRIP is to keep it simple, and to favor scalability above
   abundance of information. TRIP's attribute set is readily extensible.
   Flags provide information that allow unknown attributes to be
   reasonably processed by an LS.

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8.3 End Users

   An end user is an entity (usually a human being) which wishes to
   complete a call through a gateway from an IP network to a terminal on
   a telephone network. An end user may be a user logged on at a PC with
   some Internet telephony software. The end user may also be connected
   to the IP network through an ingress telephone gateway, which the
   user accessed from telephone handset. This is the case for what is
   referred to as "phone to phone" service with the IP network used for
   interexchange transport.

   End users may, or may not be aware that there is a telephony routing
   service running when they complete a call towards the telephone
   network. In cases where they are aware, end users may have
   preferences for how a call is completed. These preferences might
   include call features which must be supported, quality metrics, owner
   or administrator, and cost preferences.

   TRIP does not dictate how these preferences are combined with those
   of the provider to yield the final gateway selection. Nor does TRIP
   support the transport of these preferences to the LS. This transport
   can be accomplished using the front end, or by some non-protocol

8.4 Location Server

   The Location Server (LS) is the main functional entity of TRIP.  It
   is a logical device which has access to a database of gateways,
   called the Telephony Routing Information Base (TRIB). This database
   of gateways is constructed by combining the set of locally available
   gateways and the set of remote gateways (learned through TRIP) based
   on policy. The LS also exports a set of gateways to its peer LS's in
   other ITAD's. The set of exported gateways is constructed from the set
   of local gateways and the set of remote gateways (learned through
   TRIP) based on policy. As such, policy plays a central role in the LS
   operation. This flow of information is shown in Figure 5.

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                          |Intra-domain protocol
                         \ /

   TRIP-->  Gateways    POLICY     Gateways -->TRIP
                IN                     Out
                            \ /
                      Telephony Routing
                      Information Base

            Figure 5: Flow of Information in TRIP

   The TRIB built up in the LS allows it to make decisions about IP
   telephony call routing. When a signaling message arrives at a
   signaling server, destined for a telephone network address, the LS's
   database can provide information which is useful for determining a
   gateway or an additional signaling server to forward the signaling
   message to. For this reason, an LS may be coresident with a signaling
   server. When they are not coresident, some means of communication
   between the LS and the signaling server is needed. This communication
   is not specifically addressed by TRIP, although it is possible that
   TRIP might meet the needs of such a protocol.

   An ITAD must have at least one LS in order to participate in TRIP.
   An ITAD may have more than one LS, for purposes of load balancing,
   ease of management, or any other reason. In that case, communications
   between these LS's may need to take place in order to synchronize
   databases and share information learned from external peers. This is
   often referred to as the interior component of an inter-domain
   protocol. TRIP includes such a function.

   Figure 5 shows an LS learning about gateways within the ITAD by means
   of an intra-domain protocol. There need not be an intra-domain
   protocol. An LS may operate without knowledge of any locally run
   gateways. Or, it may know of locally run gateways, but through static
   configuration. An LS may also be co-resident with a gateway, in which
   case it would know about the gateway that it is co-resident with.

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9 Element Interactions

9.1 Gateways and Location Servers

   Gateways must somehow propagate information about their
   characteristics to an LS within the same ITAD. This LS may, in turn,
   further propagate this information outside of the ITAD by means of
   TRIP. This LS is called an originating LS for that gateway. When an
   LS nis not coresident with the gateway, the means by which the
   information gets propagated is not within the scope of TRIP.  The
   protocol used to accomplish this is generally called an intra-domain

   One way in which the information can be propagated is with the
   Service Location Protocol (SLP) [7]. The gateway can contain a
   Service Agent (SA), and the LS can act as a Directory Agent (DA). SLP
   defines procedures by which service information is automatically
   propagated to DA's from SA's. In this fashion, an LS can learn about
   gateways in the ITAD.

   An alternate mechanism for the intra-domain protocol is via the
   registration procedures of SIP or H.323. The registration procedures
   provide a means by which users inform a gatekeeper or SIP server
   about their address. Such a registration procedure could be extended
   to allow a gateway to effectively register as well.

   LDAP [8] might also be used for the intra-domain protocol.  A gateway
   can use LDAP to add an entry for itself into the database. If the LS
   also plays the role of the LDAP server, it will be able to learn
   about all those gateways in its ITAD.

   The intra-domain protocol which is used may be different from IT
   administrative domain to IT administrative domain, and is a matter of
   local configuration. There may also be more than one intra-domain
   protocol in a particular ITAD. An LS can also function without an
   intra-domain protocol. It may learn about gateways through static
   configuration, or may not know of any local gateways.

9.2 Location Server to Location Server

   The interaction between LS's is what is defined by TRIP.  LS's within
   the same ITAD use TRIP to synchronize information amongst themselves.
   LS's within different ITADs use TRIP to exchange gateway information
   according to policy. In the former case the LS's are referred to as
   internal peers, and in the latter case, external peers.

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   LS's communicate with each other through persistent associations. An
   LS may be connected to one or more other LS's. LS's need not be
   physically adjacent or part of the same autonomous system. The
   association between a pair of LS's is normally set up
   administratively. Two LS's are configured to communicate with each
   other when their administrators have an agreement in place to
   exchange gateway information. While TRIP does not provide an
   autodiscovery procedure for peer LS's to discover each other, one
   could possibly be used. Such a procedure might be useful for finding
   a backup peer LS when a crash occurs. Alternatively, in an
   environment where the business relationships between peers become
   more standardized, peers might be allowed to discover each other
   through protocols like the Service Location Protocol (SLP) [9].
   Determination about whether autodiscovery should or should not be
   used is at the discretion of the administrator.

   The syntax and semantics of the messages exchanged over the
   association between LS's are dictated by TRIP.  The protocol does not
   dictate the nature of the agreements which must be in place. TRIP
   merely provides a transport means to exchange whatever gateway
   routing information is deemed appropriate by the administrators of
   the system. Details are provided in the TRIP protocol specification

   The rules which govern which gateway information is generated,
   propagated, and accepted by a gateway is called a location server
   policy. TRIP does not dictate or mandate any specific policy.

9.2.1 Nature of Exchanged Information

   The information exchanged by the LS's is a set of routing objects.
   Each routing object minimally consists of a range of telephone
   numbers which are reachable, and an IP address or host name which is
   the application-layer "next hop" towards a gateway which can reach
   that range. Routing objects are learned from the intra-domain
   protocol, static configuration, or from LS's in remote ITAD's. An LS
   may aggregate these routing objects together (merging ranges of
   telephone numbers, and replacing the IP address with its own IP
   address, or with the IP address of a signaling server with which the
   LS is communicating) and then propagate them to another LS. The
   decision about which objects to aggregate and propagate is known as a
   route selection operation. The administrator has great latitude in
   selecting which objects to aggregate and propagate, so long as they
   are within the bounds of correct protocol operation (i.e., no loops
   are formed). The selection can be made based on information learned
   through TRIP, or through any out of band means.

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   A routing object may have additional information which characterizes
   the service at the gateway. These attributes include things like
   protocols, features supported, and capacity. Greater numbers of
   attributes can provide useful information, however, they come at a
   cost. Aggregation becomes difficult with more and more information,
   impacting the scalability of the protocol.

   Aggregation plays a central role in TRIP. In order to facilitate
   scalability, routing objects can be combined into larger aggregates
   before being propagated. The mechanisms by which this is done are
   specified in TRIP. Aggregation of application layer routes to
   gateways is a non-trivial problem. There is a fundamental tradeoff
   between aggregatability and verbosity. The more information that is
   present in a TRIP routing object, the more difficult it is to

   Consider a simple example of two gateways, A and B, capable of
   reaching some set of telephone numbers, X and Y, respectively. C is
   an LS for the ITAD in which A and B are resident. C learns of A and B
   through some other means. As it turns out, X and Y can be combined
   into a single address range, Z. C has several options. It can
   propagate just the advertisement for A, just the advertisement for B,
   propagate both, or combine them and propagate the aggregate
   advertisement. In this case C chooses the latter approach, and sends
   a single routing object to one of its peers, D, containing address
   range Z and its own address, since it is also a signaling server. D
   is also a signaling server.

   Some calling device, E, wishes to place a phone call to telephone
   number T, which happens to be in the address range X. E is configured
   to use D as its default H.323 gatekeeper. So, E sends a call setup
   message to D, containing destination address T. D determines that the
   address T is within the range Z. As D had received a routing object
   from C containing address range Z, it forwards the call setup message
   to C. C, in turn, sees that T is within range X, and so it forwards
   the call setup to A, which terminates the call signaling and
   initiates a call towards the telephone network.

9.2.2 Quality of Service

   One of the factors which is useful to consider when selecting a
   gateway is "QoS" - will a call through this gateway suffer
   sufficiently low loss, delay, and jitter? The quality of a call
   depends on two components - the QoS on the path between the caller
   and gateway, and the capacity of the gateway itself (measured in
   terms of number of circuits available, link capacity, DSP resources,
   etc.). Determination of the latter requires intricate knowledge of

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   underlying network topologies, and of where the caller is located.
   This is something handled by QoS routing protocols, and is outside
   the scope of TRIP.

   However, gateway capacity is not dependent on the caller location or
   path characteristics. For this reason, a capacity metric of some form
   is supported by TRIP. This metric represents the static capacity of
   the gateway, not the dynamic available capacity which varies
   continuously during the gateways operation. LS's can use this metric
   as a means of load balancing of calls among gateways. It can also be
   used as an input to any other policy decision.

9.2.3 Cost Information

   Another useful attribute to propagate is a pricing metric. This might
   represent the amount a particular gateway might charge for a call.
   The metric can be an index into a table that defines a pricing
   structure according to a pre-existing business arrangement, or it can
   contain a representation of the price itself. TRIP itself does not
   define a pricing metric, but one can and should be defined as an
   extension. Using an extension for pricing means more than one such
   metric can be defined.

10 The Front End

   As a result of TRIP, the LS builds up a database (the TRIB) of
   gateway routes. This information is made available to various
   entities within the ITAD. The way in which this information is made
   available is called the front end. It is the visible means by which
   TRIP services are exposed outside of the protocol.

10.1 Front End Customers

   There are several entities which might use the front end to access
   the TRIB. These include, but are not limited to:

     Signaling Servers: Signaling servers receive signaling messages
        (such as H.323 or SIP messages) whose purpose is the initiation
        of IP telephony calls. The destination address of these calls
        may be a phone number corresponding to a terminal on the GSTN.
        In order to route these calls to an appropriate gateway, the
        signaling server will need access to the database built up in
        the LS.

     End Users: End users can directly query the LS to get routing
        information. This allows them to provide detailed information on
        their requirements. They can then go and contact the next hop
        signaling server or gateway towards that phone number.

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     Administrators: Administrators may need to access the TRIB for
        maintenance and management functions.

   When a signaling server contacts the LS to route a phone number, it
   is usually doing so because a calling device (on behalf of an end
   user) has attempted to set up a call. As a result, signaling servers
   effectively act as proxies for end users when accessing the LS
   database. The communication between the calling devices and their
   proxies (the signaling servers) is through the signaling protocol.

   The advantage of this proxy approach is that the actual LS
   interaction is hidden from the calling device. Therefore, whether the
   call is to a phone number or IP address is irrelevant. The routing in
   the case of phone numbers takes place transparently. Proxy mode is
   also advantageous for thin clients (such as standalone IP telephones)
   which do not have the interfaces or processing power for a direct
   query of the LS.

   The disadvantage of the proxy approach is the same as its advantage -
   the LS interaction is hidden from the calling device (and thus the
   end user). In some cases, the end user may have requirements as to
   how they would like the call to be routed. These include preferences
   about cost, quality, administrator, or call services and protocols.
   These requirements are called the end user policy. In the proxy
   approach, the user effectively accesses the service through the
   signaling protocol. The signaling protocol is not likely to be able
   to support expression of complex call routing preferences from end
   users (note however, that SIP does support some forms of caller
   preferences for call routing [10]). Therefore, direct access from the
   end user to the LS can provide much richer call routing services.

   When the end user policy is presented to the LS (either directly or
   through the signaling protocol), it is at the discretion of the LS
   how to make use of it. The location server may have its own policies
   regarding how end user preferences are handled.

10.2 Front End Protocols

   There are numerous protocols that can be used in the front end to
   access the LS database. TRIP does not specify or restrict the
   possibilities for the front end. It is not clear that it is necessary
   or even desirable for there to be a single standard for the front
   end. The various protocols have their strengths and weaknesses. One
   may be the right solution in some cases, and another in different

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   Some of the possible protocols for the front end are:

     Service Location Protocol (SLP): SLP has been designed to fit
        exactly this kind of function. SLP is ideal for locating servers
        described by a set of attributes. In this case, the server is a
        gateway (or next hop towards the gateway), and the attributes
        are the end user policy. The end user is an SLP UA, and the LS
        is an SLP DA. The Service Query is used to ask for a gateway
        with a particular set of attributes.

     Open Settlements Protocol (OSP): OSP [11] is a client server
        protocol. It allows the client to query a server with a phone
        number, and get back the address of a next hop, along with
        authorization tokens to use for the call. In this case, the
        server can be an LS. The routing table it uses to respond to OSP
        queries is the one built up using TRIP.

     Lightweight Directory Access Protocol (LDAP): LDAP is used for
        accessing distributed databases. Since the LS server contains a
        database, LDAP could be used to query it.

     Web Page: The LS could have a web front end. Users could enter
        queries into a form, and the matching gateways returned in the
        response. This access mechanism is more appropriate for human
        access, however. A signaling server would not likely access the
        front end through a web page.

     TRIP: The protocols discussed above are all of the query-response
        type. There is no reason why the LS access must be of this form.
        It is perfectly acceptable for the access to be through complete
        database synchronization, so that the entity accessing the LS
        database effectively has a full copy of it. If this approach
        were desired, TRIP itself is an appropriate mechanism. This
        approach has obvious drawbacks, but nothing precludes it from
        being done.

11 Number Translations

   The model for TRIP is that of many gateways, each of which is willing
   to terminate calls towards some set of phone numbers. Often, this set
   will be based on the set of telephone numbers which are in close
   geographic proximity to the gateway. For example, a gateway in New
   York might be willing to terminate calls to the 212 and 718 area
   codes. Of course, it is up to the administrator to decide on what
   phone numbers the gateway is willing to call.

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   However, certain phone numbers don't represent GSTN terminals at all,
   but rather they represent services or virtual addresses. An example
   of such numbers are freephone and LNP numbers. In the telephone
   network, these are actually mapped to routable telephone numbers,
   often based on complex formulae. A classic example is time-of-day-
   based translation.

   While nothing prevents a gateway from advertising reachability to
   these kinds of numbers, this usage is highly discouraged. Since TRIP
   is a routing protocol, the routes it propagates should be to routable
   numbers, not to names which are eventually translated to routable
   numbers. Numerous problems arise when TRIP is used to propagate
   routes to these numbers:

      o Often, these numbers have only local significance. Calls to a
        freephone number made from New York might terminate in a New
        York office of a company, while calls made from California
        will terminate in a California branch. If this freephone
        number is injected into TRIP by a gateway in New York, it
        could be propagated to other LS's with end users in
        California. If this route is used, calls may be not be routed
        as intended.

      o The call signaling paths might be very suboptimal. Consider a
        gateway in New York that advertises a ported number that maps
        to a phone in California. This number is propagated by TRIP,
        eventually being learned by an LS with end users in
        California. When one of them dials this number, the call is
        routed over the IP network towards New York, where it hits the
        gateway, and then is routed over the GSTN back to California.
        This is a waste of resources. Had the ported number been
        translated before the gateway routing function was invoked, a
        California gateway could have been accessed directly.

   As a result, it is more efficient to perform translations of these
   special numbers before the LS routing databases are accessed. How
   this translation is done is outside the scope of TRIP. It can be
   accomplished by the calling device before making the call, or by a
   signaling server before it accesses the LS database.

12 Security Considerations

   Security is an important component in TRIP. The TRIP model assumes a
   level of trust between peer LS's that exchange information. This
   information is used to propagate information which determines where
   calls will be routed. If this information were incorrect, it could
   cause complete misrouting of calls. This enables a significant denial
   of service attack. The information might also be propagated to other

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   ITADs, causing the problem to potentially spread. As a result, mutual
   authentication of peer LS's is critical. Furthermore, message
   integrity is required.

   TRIP messages may contain potentially sensitive information. They
   represent the routing capabilities of an ITAD. Such information might
   be used by corporate competitors to determine the network topology
   and capacity of the ITAD. As a result, encryption of messages is also
   supported in TRIP.

   As routing objects can be passed via one LS to another, there is a
   need for some sort of end to end authentication as well. However,
   aggregation will cause the routing objects to be modified, and
   therefore authentication can only take place from the point of last
   aggregation to the receiving LS's.

13 Acknowledgments

   The authors would like to thank Randy Bush, Mark Foster, Dave Oran,
   Hussein Salama, and Matt Squire for their useful comments on this

14 Bibliography

   [1]  International Telecommunication Union, "Visual telephone systems
        and equipment for local area networks which provide a non-
        guaranteed quality of service," Recommendation H.323,
        Telecommunication Standardization Sector of ITU, Geneva,
        Switzerland, May 1996.

   [2]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP:  Session Initiation Protocol", RFC 2543, March 1999.

   [3]  Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett,
        "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 2705,
        October 1999.

   [4]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [5]  Simpson, W., "The Point-to-Point Protocol (PPP)," STD 51, RFC
        1661, July 1994.

   [6]  Rekhter Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
        1771, March 1995.

   [7]  Veizades, J., Guttman, E., Perkins, C. and S. Kaplan, "Service
        Location Protocol", RFC 2165, June 1997.

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   [8]  Yeong, W., Howes, T. and S. Kille, "Lightweight Directory Access
        Protocol", RFC 1777, March 1995.

   [9]  Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
        Location Protocol, Version 2", RFC 2608, June 1999.

   [10] Schulzrinne H. and J. Rosenberg, "SIP caller preferences and
        callee capabilities", Work in progress.

   [11] European Telecommunications Standards Institute (ETSI),
        Telecommunications and Internet Protocol Harmonization Over
        Networks (TIPHON), "Inter-domain pricing, authorization, and
        usage exchange," Technical Specification 101 321 version 1.4.2,
        ETSI, 1998.

15 Authors' Addresses

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936

   Email: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003

   Email: schulzrinne@cs.columbia.edu

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16.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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

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©2018 Martin Webb