Internet DRAFT - draft-ietf-iptel-gwloc-framework


Internet Engineering Task Force                                 iptel wg
Internet Draft                                 J.Rosenberg,H.Schulzrinne
draft-ietf-iptel-gwloc-framework-05.txt            Bell Labs/Columbia U.
October 21, 1999
Expires: April 22, 2000

               A Framework for Telephony Routing over IP


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   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.

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

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   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, 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 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

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             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 ITAD.

        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.

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

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

        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.

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

J.Rosenberg,H.Schulzrinne                                     [Page 3]

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   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 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 large.

        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 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

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        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:

        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.

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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":

        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
   network, in order to determine the IP address of the PC corresponding
   to the user with the given phone number.

   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.

   As a result for both of these cases, the mapping function is a name

J.Rosenberg,H.Schulzrinne                                     [Page 6]

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   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.

   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.

        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

J.Rosenberg,H.Schulzrinne                                     [Page 7]

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          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

6 Example Applications of TRIP

   TRIP is a general purpose tool for exchanging of 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
   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

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

   Figure 1: TRIP in the Clearinghouse Application

          ------        ------
         |      |------|      |
         | M1   |      |  M2  |
         |      |\    /|      |
          ------  \  /  ------
            |      \/     |
            |      /\     |<-----GLP
          ------  /  \  ------
         |      |/    \|      |
         | 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

J.Rosenberg,H.Schulzrinne                                     [Page 9]

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   relationship, as shown in Figure 3.

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

   Figure 3: TRIP for Wholesalers

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

7 Architecture

   Figure 4 gives the overall architecture of TRIP.

   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

J.Rosenberg,H.Schulzrinne                                    [Page 10]

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

   Figure 4: TRIP Architecture

   (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

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   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.

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 set 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.,

J.Rosenberg,H.Schulzrinne                                    [Page 12]

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   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 are 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.

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

J.Rosenberg,H.Schulzrinne                                    [Page 13]

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   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 Gateway Information Base (GIB). 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.

                       |Intra-domain protocol

GLP-->  Gateways     POLICY     Gateways -->GLP
          IN                     Out
                Information Base

   Figure 5: Flow of Information in TRIP

   The GIB 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.

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   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.

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 is 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

J.Rosenberg,H.Schulzrinne                                    [Page 15]

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   local configuration. 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.

   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 set up administratively. There
   is no autodiscovery procedure. Two LS's are configured to communicate
   with each other when their administrators have an agreement in place
   to exchange gateway information. The syntax and semantics of the
   messages exchanged over this association 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 itself.

   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 which is the layer 7
   "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.
   How the decision is made is at the discretion of the administrator,
   and is embodied in the LS policy. The decision can be made based on
   information learned through TRIP, or through any out of band means.

   A routing object may have additional information which characterizes

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

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   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 cost metrics. These might
   represent the amount a particular gateway might charge for a call.
   Unfortunately, the set of cost structures is potentially unbounded,
   ranging from the simple (flat rate), to the arbitrarily complex (time
   and caller dependent, past usage dependent, destination dependent,
   etc.). For this reason, TRIP does not attempt to define specific cost

10 The Front End

   As a result of TRIP, the LS builds up a database (the GIB) 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 GIB. 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.

        Administrators: Administrators may need to access the GIB for

J.Rosenberg,H.Schulzrinne                                    [Page 18]

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             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 about
   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 [9]). 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

   Some of the possible protocols for the front end are:

        Service Location Protocol (SLP): SLP has been designed to fit

J.Rosenberg,H.Schulzrinne                                    [Page 19]

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             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.

        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 has some
   set of phone numbers they are willing to terminate IP calls towards.
   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.

   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

J.Rosenberg,H.Schulzrinne                                    [Page 20]

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

   Since the relationships between LS's are based on pre-arranged
   administrative relationships, shared secrets seem an appropriate
   mechanism for the authentication and integrity.

   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

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   aggregation to the receiving LS's.

   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.

13 Acknowledgments

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

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] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments (Proposed
   Standard) 2543, Internet Engineering Task Force, Mar. 1999.

   [3] M. Arango, A. Dugan, I. Elliott, C. Huitema, and S. Pickett,
   "Media gateway control protocol (MGCP) version 1.0," Request for
   Comments (Informational) 2705, Internet Engineering Task Force, Oct.

   [4] R. Droms, "Dynamic host configuration protocol," Request for
   Comments (Draft Standard) 2131, Internet Engineering Task Force, Mar.

   [5] W. Simpson and Editor, "The point-to-point protocol (PPP),"
   Request for Comments (Standard) 1661, Internet Engineering Task
   Force, July 1994.

   [6] Y. Rekhter and T. Li, "A border gateway protocol 4 (BGP-4),"
   Request for Comments (Draft Standard) 1771, Internet Engineering Task
   Force, Mar. 1995.

   [7] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan, "Service
   location protocol," Request for Comments (Proposed Standard) 2165,
   Internet Engineering Task Force, June 1997.

   [8] W. Yeong, T. Howes, and S. Kille, "Lightweight directory access
   protocol," Request for Comments (Draft Standard) 1777, Internet
   Engineering Task Force, Mar. 1995.

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   [9] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
   callee capabilities," Internet Draft, Internet Engineering Task
   Force, Mar. 1999.  Work in progress.

15 Authors Addresses

   Jonathan Rosenberg
   Lucent Technologies, Bell Laboratories
   101 Crawfords Corner Rd.
   Holmdel, NJ 07733
   Rm. 4C-526

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

                           Table of Contents

   1          Introduction ........................................    1
   2          Terminology .........................................    2
   3          Motivation and Problem Definition ...................    3
   4          Related Problems ....................................    6
   5          Relationship with BGP ...............................    7
   6          Example Applications of TRIP ........................    8
   6.1        Clearinghouses ......................................    8
   6.2        Confederations ......................................    8
   6.3        Gateway Wholesalers .................................    8
   7          Architecture ........................................   10
   8          Elements ............................................   11
   8.1        IT Administrative Domain ............................   11
   8.2        Gateway .............................................   12
   8.3        End Users ...........................................   13
   8.4        Location Server .....................................   14
   9          Element Interactions ................................   15
   9.1        Gateways and Location Servers .......................   15
   9.2        Location Server to Location Server ..................   16

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   9.2.1      Nature of Exchanged Information .....................   16
   9.2.2      Quality of Service ..................................   17
   9.2.3      Cost Information ....................................   18
   10         The Front End .......................................   18
   10.1       Front End Customers .................................   18
   10.2       Front End Protocols .................................   19
   11         Number Translations .................................   20
   12         Security Considerations .............................   21
   13         Acknowledgments .....................................   22
   14         Bibliography ........................................   22
   15         Authors Addresses ...................................   23

J.Rosenberg,H.Schulzrinne                                    [Page 24]