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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                                 IPTEL WG
Internet Draft                                      J.Rosenberg,H.Salama
draft-rs-trip-gw-01.txt                               dynamicsoft, Cisco
July 14, 2000
Expires: January 2001


          Usage of TRIP in Gateways for Exporting Phone Routes

STATUS OF THIS MEMO

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as work in progress.

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   This document describes a new application of the Telephony Routing
   over IP (TRIP) protocol. TRIP was engineered as a tool for inter-
   domain exchange of telephone routing information. However, it can
   also be used as a means for gateways and soft switches to export
   their routing information to a Location Server (LS), which may be
   co-resident with a proxy or gatekeeper. This LS can then manage those
   gateway resources. We discuss the motivations for this application,
   the reasons why other mechanims, such as the SIP REGISTER method, are
   not appropriate, and then show how a minimal subset of TRIP is needed
   for this application.


1 Introduction

   In typical VoIP deployments, a service provider runs a network



J.Rosenberg,H.Salama                                          [Page 1]


Internet Draft             TRIP for Gateways               July 14, 2000


   composed of numerous gateways, softswitches, and proxy servers.
   Generally, gateways (both composed and decomposed) are distributed
   geograpically throughout the network. When a gateway terminates a
   call to a number, cost to the service provider is incurred. This cost
   is partly dependent on the cost of making a call over the PSTN from
   the gateway to the destination number. By placing gateways in
   numerous locations over the globe, the service provider can be sure
   it can terminate calls to the PSTN with minimal cost.

   When calls arrive at the network (either from customers of the
   provider, or from peer providers desiring termination service), they
   are first sent to a proxy which acts as a routing engine. Based on
   the knowledge of available gateways, their capacity (in terms of
   circuit and DSP resources) and other attributes, and the telephone
   numbers each gateway can terminate calls to, the proxy forwards the
   call to the appropriate gateway. In essence, the LS/proxy is
   responsible for managing the real-time resources made available by a
   set of gateways.

   This configuration is depicted graphically in Figure 1.


   There are several problems that must be solved in order to enable
   this scenario:

        o Often, the routing tables in the routing proxies are manually
          entered. This makes configuration more complex, particularly
          for large networks with hundreds or even thousands of
          gateways. In the ideal scenario, each gateway is configured
          with the numbers it can terminate calls to, and with the
          address of the routing proxies. The gateways then push their
          routing information to the proxy. No standard mechanism has
          been specified to do this.

        o It is important for the routing proxy to be aware of failures
          of gateways. This way, an alternate gateway can be selected
          for new incoming calls. This requires some kind of keepalive
          between the gateways and the routing proxy. No standard
          mechanism yet exists for this keepalive.

        o The routing proxy will need to route call setup requests to
          the gateways based on dynamic attributes of those gateways. In
          particular, the available capacity, measured in terms of both
          circuit resources and coding resources, is used to properly
          route calls. The proxy can, for example, perform load
          balancing by forwarding call setups to the most lightly loaded
          gateway among a set. No standard mechanism has been specified
          to do this.



J.Rosenberg,H.Salama                                          [Page 2]


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                                                +---------+
                                                |         |
                                                |   GW    |
                                             >  +---------+
                                           //
                                         //
                                       //       +---------+
                                     //         |         |
                                   //           |   GW    |
                                 //             +---------+
                               //
              +----------+   //                                TO PSTN
              |          |  /                   +---------+
              | Routing  |          ------->    |         |  ----->
     -------->| Proxy    |   -------            |   GW    |
              |          |  --                  +---------+
              |          |    --
              +----------+      --
                                  ---           +---------+
                                     --         |         |
                                       --       |   GW    |
                                         --     +---------+
                                           -->

                                                +---------+
                                                |         |
                                                |   GW    |
                                                +---------+


   Figure 1: Gateway and LS Configuration


   This document discusses how TRIP [1] can be used to solve these two
   problems. The first section reviews other mechanisms for
   accomplishing this - namely the SIP [2] REGISTER method, and
   discusses why TRIP is a much better solution. We assume the reader is
   familiar with TRIP. An overview of its architecture can be found in
   [3].

2 Why not SIP REGISTER?

   The SIP REGISTER method has frequently been proposed as a solution
   for these two problems. This is due, in part, to the similarity of
   the REGISTER method to the H.323 [4] RAS messages. In H.323, RAS
   messages are used to allow gateways to register telephone number



J.Rosenberg,H.Salama                                          [Page 3]


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   prefixes with a gatekeeper. Many assume that SIP REGISTER should
   therefore play a similar role.

   Certainly, the REGISTER mechanism is close to this functionality.
   REGISTER allows clients to send address bindings (including for
   telephone numbers) to a proxy. Registrations are also periodically
   refreshed, allowing a proxy to determine if the address binding
   becomes stale, perhaps due to a crash or device failure. The refresh
   timer (present in the Expires header) can even be negotiated by the
   proxy.

   However, the SIP REGISTER mechanism is different from the desired
   mechanisms for gateways in many respects:

        o The REGISTER mechanism is used to bind a single incoming URI
          to one or more outgoing URIs. In the case of a telephony
          gateway, the binding is between a set of telephone prefixes to
          the address of a gateway. This is a significantly different
          binding, and cannot be represented with the syntax or
          semantics of a SIP REGISTER request.

        o The keepalive mechanism in REGISTER refreshes the *binding*,
          not the status of the UA performing the registration. The
          bindings must be sent each time to keep them alive. In the
          case of a gateway, the keepalive is for the state of the
          gateway, not for the routes the gateway terminates. The
          semantics of REGISTER would need to be completely changed in
          order to support this different semantic.

        o There are properties associated with gateway routes that are
          not associated with URIs. For example, a route may have
          information like capacity (how many simultaneous calls can be
          terminated), which does not make much sense for a property of
          a URI.

        o Because gateways can handle so many calls, it is important for
          liveness information to be conveyed very frequently, on the
          order of seconds. SIP registrations are not meant to be sent
          that frequently; they can be fairly large messages
          (particularly if they need to contain the routes when
          refreshed), resulting in uneeded overheads.

   For these reasons, we do not believe the SIP REGISTER request is the
   right tool for gateway route propagation and gateway keepalives.

3 Why TRIP?

   TRIP was engineered as a tool for interdomain route exchange. It is



J.Rosenberg,H.Salama                                          [Page 4]


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   not a simple protocol, and at first glance, does not seem appropriate
   for application in a gateway.

   However, TRIP provides exactly the features needed to solve the
   problem at hand. TRIP allows one entity (in this case, a gateway) to
   convey to another (in this case, a proxy) a set of telephone routes
   which terminate through it. These routes are represented by telephone
   number prefixes. In TRIP, the routing tables are exchanged once. Only
   when they change are updates sent. This is exactly the capability
   needed for a gateway to send its routing information to a proxy.

   TRIP also supports a keepalive between peers. This keepalive is a
   short message, sent fairly frequently. It does not contain routing
   information. The period of the keepalive is negotiated once at
   startup time. If one of the entities crashes, the other flushes all
   routes received from it. This, too, is exactly the mechanism needed
   for keepalives in a gateway.

   TRIP can contain attributes describing a route. New attributes can
   easily be added. The available capacity of a route is needed by the
   proxies to properly load balance and route to a set of gateways. This
   capacity can be expressed as an attribute.

   Another advantage of using TRIP here is that it makes the
   redistribution of local gateway reachability information into inter-
   domain TRIP straightforward, because it's the same protocol.

   While it is true that TRIP is complex, almost all of this complexity
   is related to the processing of routes *received* from other peers.
   An element, such as a gateway, which only *sends* routes to a peer
   (the proxy), does not need to implement any of those functions. In
   particular, any processing related to aggregation, TRIB updates,
   route propagation and advertisement, handling of transitivity and
   unknown routes, or the decision process need not be implemented. The
   resulting set of functions are very small. They are composed of only:

        o The initial OPEN phase, exchange of keepalive timers, and the
          process of bringing up the state machine.

        o Sending of an UPDATE containing the routes and parameters of
          the gateways.

        o Sending of a periodic keepalive.

   Its worth noting that these functions are not substantially more
   complex than sending a periodic REGISTER.

   The rest of this document is organized as follows. Section 4



J.Rosenberg,H.Salama                                          [Page 5]


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   discusses a new attribute, circuit capacity, and section 5 discusses
   another new attribute, DSP capacity. These new attributes contain
   dynamic capacity of gateways that can be propagated to the LS
   managing those gateways. Section 6 describes the processing rules
   needed for a gateway, and section 7 discusses some of the processing
   needed in an LS.

4 CircuitCapacity Attribute



   Mandatory: False.
   Required Flags: optional, non-transitive
   Potential Flags: None.
   TRIP Type Code: TBD.



   The circuit capacity attribute is used only between a gateway and its
   peer LS responsible for managing that gateway. It is for this reason
   that this attribute is non-transitive. If it is received in a route,
   it SHOULD NOT be propagated unless the LS is sure that it is
   relatively static.

   The circuit capacity identifies the number of PSTN circuits that are
   currently available on a route to terminate calls. The number of
   additional calls sent to that gateway for that route can not exceed
   the circuit capacity. If it does, the signaling protocol will likely
   generate errors, and calls will be rejected.

   Circuit capacity is measured in integral number of calls. It changes
   very dynamically.

4.1 CircuitCapacity Syntax

   The CircuitCapacity attribute is a 4-octet unsigned numbeic value. It
   represents the number of circuits remaining for terminating calls to
   this route. This attribute represents a potentially achievable lower
   bound on the number of calls which can additionally forwarded on this
   route.

4.2 Route Origination and CircuitCapacity

   Routes MAY be originated containing the CircuitCapacity attribute.
   Since this attribute is highly dynamic, changing with every call,
   updates MAY be sent as it changes. However, it is RECOMMENDED that a
   gateway originating routes with this attribute use thresholds, and
   that routes are re-originated only when the attribute moves above or



J.Rosenberg,H.Salama                                          [Page 6]


Internet Draft             TRIP for Gateways               July 14, 2000


   below a treshold. It is also RECOMMENDED that the thresholds in each
   direction (going above a threshold and going below a threshold) be
   different, thus achieving a form of hysterisis. Both of these
   measures help reduce the messaging load from route origination.

4.3 Route Selection and CircuitCapacity

   The CircuitCapacity attribute MAY be used for route selection. Since
   one of its primary applications is load balancing, an LS will wish to
   choose a potentially different route (amonst a set of routes for the
   same prefix) on a call by call basis. This can be modeled as re-
   running the decision process on the arrival of each call. The
   aggregation and dissemination rules for routes with this attribute
   ensure that re-running this selection process never results in
   propagation of a new route to other peers.

4.4 Aggregation and CircuitCapacity

   An LS MUST aggregate routes to the same prefix which contain a
   CircuitCapacity attribute. This guarantees that if the decision
   process is rerun, the route that is disseminated to peers is
   unchanged.

4.5 Route Dissemination and CircuitCapacity

   Routes SHOULD NOT be disseminated with the CircuitCapacity attribute.
   The attribute is meant to reflect capacity at the originating gateway
   only. Its highly dynamic nature makes it inappropriate to disseminate
   in most cases.

5 DSPCapacity attribute



   Mandatory: False.
   Required Flags: optional, non-transitive
   Potential Flags: None.
   TRIP Type Code: TBD.



   The DSPCapacity attribute is used only between a gateway and its peer
   LS responsible for managing that gateway. It is for this reason that
   this attribute is non-transitive. If it is received in a route, it
   SHOULD NOT be propagated unless the LS is sure it is relatively
   static in value.

   The DSPcapacity identifies the amount of DSP resources, in MIPS, that



J.Rosenberg,H.Salama                                          [Page 7]


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   are currently available on a route to terminate calls. The metric
   should be considered as only an approximate. The MIPS are computed
   based on a specific processor (TBD); other processors will need to
   perform a conversion to obtain this normalized parameter. It is
   assumed that the LS is aware of the DSP resource requirements for
   each call, based on the set of codecs indicated in the messages
   routed by the LS/proxy.


        There is lots of handwaving here. Can we usefully define
        this metric? How to determine how much DSP resources are
        really required to terminate a call? Also, the codec used
        may not be known at the time the message is to be routed.
        This can happen with both SIP (when the SDP in the INVITE
        is empty), and H.323. How to handle this?

   DSP capacity is measured in integral number of MIPS. It changes very
   dynamically.

5.1 DSPCapacity Syntax

   The DSPCapacity attribute is a 4-octet unsigned numbeic value. It
   represents the number of MIPS remaining for terminating calls to this
   route.

5.2 Route Origination and DSPCapacity

   Routes MAY be originated containing the DSPCapacity attribute. Since
   this attribute is highly dynamic, changing with every call, updates
   MAY be sent as it changes. However, it is RECOMMENDED that a gateway
   originating routes with this attribute use thresholds, and that
   routes are re-originated only when the attribute moves above or below
   a treshold. It is also RECOMMENDED that the tresholds in each
   direction (going above a threshold and going below a threshold) be
   different, thus achieving a form of hysterisis. Both of these
   measures help reduce the messaging load from route origination.

5.3 Route Selection and DSPCapacity

   The DSPCapacity attribute MAY be used for route selection. Since one
   of its primary applications is load balancing, an LS will wish to
   choose a potentially different route (amonst a set of routes for the
   same prefix) on a call by call basis. This can be modeled as re-
   running the decision process on the arrival of each call. The
   aggregation and dissemination rules for routes with this attribute
   ensure that re-running this selection process never results in
   propagation of a new route to other peers.




J.Rosenberg,H.Salama                                          [Page 8]


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5.4 Aggregation and DSPCapacity

   An LS MUST aggregate routes to the same prefix which contain a
   DSPCapacity attribute. This guarantees that if the decision process
   is rerun, the route that is disseminated to peers is unchanged.

5.5 Route Dissemination and DSPCapacity

   Routes SHOULD NOT be disseminated with the DSPCapacity attribute. The
   attribute is meant to reflect capacity at the originating gateway
   only. Its highly dynamic nature makes it inappropriate to
   disseminate.

6 Gateway Operation

   The protocol a gateway uses to advertise its E.164 reachability to
   the its domain's Location Server(s) (LS)is TRIP. The gateway operates
   in TRIP Send Only mode since it is only interested in advertising its
   reachability, but is not interested in learning about the
   reachability of other gateways and other domains. Also, the gateway
   will not create its own call routing database. Therefore, the gateway
   does not need a complete implementation of TRIP. A lightweight
   version of the protocol is sufficient. In this section we describe
   the operation of TRIP on a gateway. We refer to the protocol
   operating in this context as TRIP for Gateways, or TRIP-GW.

   TRIP-GW is a stripped down version of TRIP, but still completely
   interoperable with normal TRIP speakers. It is an implementation
   profile, not an extension or incompatible reduction.

   The reader is assumed to be familiar with TRIP. In our discussion we
   will skip most of the details common to both versions.

6.1 Session Establishment

   When opening a peering session with a TRIP LS, an TRIP-GW gateway
   follows exactly the same procedures as any other TRIP speaker. The
   TRIP-GW gateway sends an OPEN message which includes a Send Receive
   Capability in the Optional Parameters. The Send Receive Capability is
   set by the gateway to Send Only. When the TRIP LS receives the
   gateway's OPEN message, it set its local policy such that no UPDATE
   messages are sent to the TRIP-GW gateway. The remainder of the peer
   session establishment is identical to TRIP.

6.2 UPDATE Messages

   Once the peer session has been established, the gateway sends UPDATE
   messages to the TRIP LS with the gateway's entire E.164 reachability



J.Rosenberg,H.Salama                                          [Page 9]


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   and its ITAD topology.

   If the gateway's E.164 reachability or its ITAD topology changes at
   any point in time, the gateway MUST generate UPDATE message(s) with
   the change. The frequency of successive UPDATE messages MUST follow
   the same rules specified for TRIP [1]. The TRIP-GW gateway MUST at
   least support all mandatory TRIP attributes.

   If the gateway receives an UPDATE message from the TRIP LS, it MUST
   silently discard it as specified in [1]. No further action should be
   taken.

6.3 KEEPALIVE Messages

   KEEPALIVE messages are periodically exchanged over the peering
   session between the TRIP-GW gateway and the TRIP LS as specified in
   Section 5.4 of [1].

6.4 Error Handling and NOTIFICATION Messages

   The same procedures used with TRIP, are used with TRIP-GW for error
   handling and generating NOTIFICATION messages. The only difference is
   that an TRIP-GW gateway will never generate a NOTIFICATION message in
   response to an UPDATE message, irrespective of the contents of the
   UPDATE message. Any UPDATE message is silently discarded.

6.5 TRIP-GW Finite State Machine

   When the TRIP-GW finite state machine is in the Established state and
   an UPDATE message is received, the UPDATE message is silently
   discarded and the TRIP-GW gateway remains in the Established state.
   Other than that the TRIP finite state machine and the TRIP-GW finite
   state machine are identical.

6.6 Call Routing Databases

   A TRIP-GW gateway may maintain simultaneous sessions with more than
   one TRIP LSs. A TRIP-GW gateway maintains one call routing database
   per peer TRIP LS. These databases are equivalent to TRIP's Adj-
   TRIBs-Out, and hence we will call them Adj-TRIB-GWs-Out. An Adj-
   TRIB-GW-Out contains the gateway's E.164 reachability information
   advertised to its peer TRIP LS. How an Adj-TRIB-GW-Out database gets
   populated is outside the scope of this draft (possibly by manual
   configuration).

   The TRIP-GW gateway does not have databases equivalent to TRIP's
   Adj-TRIBs-In and Loc-TRIB, because the TRIP-GW gateway does not learn
   routes from its peer TRIP LSs, and hence it does not run call route



J.Rosenberg,H.Salama                                         [Page 10]


Internet Draft             TRIP for Gateways               July 14, 2000


   selection.

6.7 Route Selection and Aggregation

   TRIP's route selection and aggregation operations SHOULD NOT be
   implemented by TRIP-GW gateways.

7 LS Behavior

   TRIP completely specifies the behavior of the LS as a TRIP speaker.
   However, the primary question is: is an LS connected to a gateway an
   internal or external peer of the gateway?

   The most obvious choice, internal peer, is not the best choice. If an
   LS has ten peer GWs, all of them advertising reachability to 1408*,
   it will flood all ten routes to all other LSs in the same ITAD. This
   won't scale, because each LS in the ITAD will have to create a
   separate Adj-TRIB-In for each GW in that ITAD. In addition, it
   doesn't allow a SIP Proxy server or an H.323 GK to load balance among
   the GWs of its zone/subdomain.

   A similar problem exists when an LS is an external peer to the
   gateways, and has direct peering relationships with one or more
   internal peers. However, an ingress LS to an ITAD does not perform
   aggregation. Only the egress aggregates routes.

   Therefore, it is RECOMMENDED that the appropriate architecture is
   that the LS actually runs two instances of TRIP; one with an external
   peering relationship to the gateways, and the other with an internal
   peering relationship with one or more LS's within the ITAD. The
   interface between these instances is a local matter; routes are
   exported from one and imported to the other. This architecture is
   shown in Figure 2.


8 Conclusion

   We have argued that the problem of managing a set of gateways from a
   location server is critical. This process of management includes
   propagation of routes, liveness determination, and propagation of
   available capacity for the purposes of load balancing. TRIP is
   ideally suited for these problems. As such, we propose here to define
   TRIP-GW, a subset of TRIP functionality (yet still 100% compatible
   with it) for use on gateways to perform this function.

9 Changes since -00

        o Added text to stress the value of this proposal for managing a



J.Rosenberg,H.Salama                                         [Page 11]


Internet Draft             TRIP for Gateways               July 14, 2000






                                                            +-----+
                                                            |     |
  ....................................                    --| GW  |
  .                    +-------------.------------+    ---  +-----+
  .                    | +--------+  .+--------+  |  --
  .                    | |TRIP    |  .|TRIP    |  +--       +-----+
  .                    |/|Instance|  .|Instance|--|         |     |
  .                    / |        |  .|        |--+-------- | GW  |
  .                   /| |        |  .|        |--|         +-----+
  .                  / | +--------+  .+--------+  +---
  .                 /  |           LS.            |   ---   +-----+
  .                /   +-------------.------------+      -- |     |
  .               /                  .                      | GW  |
  .  +----------+                    .                      +-----+
  .  |          |                    .
  .  |          |                    .
  .  |    LS    |                    .
  .  |          |                    .
  .  |          |                    .
  .  +----------+                    .                      +-----+
  .              \                   .                      |     |
  .               \                  .                   -- | GW  |
  .                \   +-------------.------------+    --   +-----+
  .                 \  | +--------+  .+--------+  | ---
  .                  \ | |TRIP    |  .|TRIP    |  +-        +-----+
  .                   \| |Instance|  .|Instance|--|         |     |
  .                    \ |        |  .|        |--+---------| GW  |
  .                    | |        |  .|        |--|         +-----+
  .                    | +--------+  .+--------+  +---
  .                    |           LS.            |   ---   +-----+
  .                    +-------------.------------+      -- |     |
  .       ITAD                       .                      | GW  |
  ....................................                      +-----+







   Figure 2: LS Architecture for TRIP-GW






J.Rosenberg,H.Salama                                         [Page 12]


Internet Draft             TRIP for Gateways               July 14, 2000


          gateway cluster

        o Added attributes for circuit capacity and DSP capacity

        o Added section on LS operation, discussing aggregation issue

10 Authors Addresses



   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Hussein F. Salama
   Cisco Systems
   Mail Stop SJ-6/3
   170 W. Tasman Drive
   San Jose, CA 95134
   email: hsalama@cisco.com




11 Bibliography

   [1] J. Rosenberg, H. Salama, and M. Squire, "Telephony routing over
   IP (TRIP)," Internet Draft, Internet Engineering Task Force, Jan.
   2000.  Work in progress.

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

   [3] J. Rosenberg and H. Schulzrinne, "A framework for telephony
   routing over ip," Request for Comments 2871, Internet Engineering
   Task Force, June 2000.

   [4] International Telecommunication Union, "Packet based multimedia
   communication systems," Recommendation H.323, Telecommunication
   Standardization Sector of ITU, Geneva, Switzerland, Feb. 1998.







J.Rosenberg,H.Salama                                         [Page 13]