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Versions: (draft-ietf-iptel-trip-gw) 00 01 02 03 04 05 06 07 08 09 RFC 5140

IPTEL Working Group                   Manjunath Bangalore, Cisco Systems
Internet Draft                             Rajneesh Kumar, Cisco Systems
draft-ietf-iptel-tgrep-00.txt            Jonathan Rosenberg, dynamicsoft
October 2002                               Hussein Salama, Cisco Systems
Expiration Date: April 2003                Dhaval N. Shah, Cisco Systems


           A Telephony Gateway REgistration Protocol (TGREP)


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 the Telephony Gateway Registration Protocol
   (TGREP) for registration of telephony prefixes supported by telephony
   gateways and soft switches. The registration mechanism can also be
   used to export resource information. The prefix and resource
   information can then be passed on to a TRIP Location Server, which in
   turn can propogate that routing information within the same, and
   other internet telephony administrative domains (ITAD). TGREP shares
   a lot of similarites with the TRIP Protocol. It has similar
   procedures and Finite State Machine for session establishment. It
   also shares the same format for messaages and a subset of attributes
   with TRIP.







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1. Terminology and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

   Some other useful definitions are:

   Trunk: In a network, a communication path connecting two switching
   systems used in the establishment of an end-to-end connection. In
   selected applications, it may have both its terminations in the same
   switching system.

   TrunkGroup: A set of trunks, traffic engineered as a unit, for the
   establishment of connections within or between switching systems in
   which all of the paths are interchangeable except where subgrouped.

   Carrier: An organization that provides connections for telephony
   services between end-users and/or exchanges.



2. Introduction

   In typical VoIP networks, Internet Telephony Administrative Domains
   (ITADs) will deploy numerous gateways for the purposes of
   geographical diversity, capacity, and redundancy. When a call arrives
   at the domain, it must be routed to one of those gateways.
   Frequently, an ITAD will break their network into geographic POPs,
   with each POP containing some number of gateways, and a proxy server
   element that fronts those gateways. The proxy server is responsible
   for managing the access to the POP, and also for determining which of
   the gateways will receive any given call that arrives at the POP.

   This configuration is depicted graphically in Figure 1.
















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

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


                  Figure 1: Gateway and LS Configuration

   The decision about which gateway to use depends on many factors,
   including their availability, remaining call capacity and call
   success statistics particular POTS destination. For the proxy to do
   this adequately, it needs to have access to this information in
   real-time, as it changes. This means there must be some kind of
   communications between the proxy and the gateways to convey this
   information.

   In this document, we specify a protocol for registration of routes
   (destinations) supported by the gateway to the TRIP Location Server,
   known as Telephony Gateway REgistration Protocol (TGREP). TGREP
   Protocol can be considered an auxiliary protocol to TRIP. Routes
   learnt through TGREP can be injected into and further processed
   and/or propogated by the TRIP Location Server.

   TGREP shares a lot of commonality with the TRIP in various aspects.



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   Particularly, TGREP and TRIP have the same session establishment
   procedures, state machine etc. TGREP also makes use of a similar
   UPDATE message to propogate the routes supported. It uses
   Notification, Keepalive and OPEN message in the same essence as TRIP.
   TGREP defines few new attributes that are needed to specify certain
   characterstics of gateways, like Available Capacity for a destination
   etc. The document aims at specifying all the attributes that can go
   in on the TGREP session. The document also specified some new address
   families which can be useful in advertising the information on the
   GWs. The type codes for the new attributes and address families are
   yet to be obtained from IANA. An attempt will be made to assign the
   codes in such a way that they are in continuation with the codes
   assigned for attributes and families defined in the the TRIP RFC[4].

   As a general rule, because of lot of similarities between TRIP and
   TGREP, frequent reference will be made to the terminologies and
   formats defined in TRIP RFC[4]. It is suggested that the reader be
   familiar with the concepts of TRIP like attributes, flags, route
   types, address families etc.


3. Defining TGREP

   TGREP is a route registration protocol for telephony destinations on
   a gateway. These telephony destinations could be prefixes, trunk
   groups or Carriers. The Speaker of TGREP resides on the GW and
   gathers all the information from the GW to relay to the other side. A
   TGREP Receiver is defined, which receives this information and after
   certain optional operations like consolidation, aggregation etc.
   (defined in Sections 3.10.1 and 3.10.2) hands over the reachability
   information a to TRIP Location Server.

   The TGREP speaker establishes a session to the TGREP Receiver using
   the procedures similar to session establishment in TRIP. The TGREP
   Speaker is however, in "Send only" mode and the receiver is in the
   "Receive only" mode. After the session establishment the TGREP
   speaker sends the reachibility informaton in the UPDATE messages. The
   UPDATE messages have the same format as in TRIP. However, there are
   certain attributes that are not relevant in TGREP. TGREP also defines
   certain new attributes that find a use in a TGREP update.

   In the rest of the document we specify attributes and address
   families used in TGREP. We also specify the operations of
   consolidation and aggregation as they apply to the UPDATEs received
   from the TGREP Gateway by the TGREP Receiver.






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 A TGREP UPDATE supports the following attributes:
   1. WithdrawnRoutes (as defined in TRIP)
   2. ReachableRoutes  (as defined in TRIP)
   3. NexthopServer (as defined in TRIP)
   4. TotalCircuitCapacity
   5. AvailableCircuits
   6. CallSuccess
   7. Prefix
   8. TrunkGroup
   9. Carrier


3.1. TotalCircuitCapacity Attribute

    Mandatory: False.
    Required Flags: Not well-known.
    Potential Flags: None.
    TRIP Type Code: 13 (To be assigned by IANA)

   The TotalCircuitCapacity attribute is used to reflect the
   administratively provisioned capacity as opposed to the availability
   at a given point in time as provided by the AvailableCircuits
   attribute. Because of its relatively static nature, this attribute
   MAY be propogated beyond the LS that receives it.

   The TotalCircuitCapacity identifies the total number of PSTN circuits
   that are available on a route to complete calls. It is used in
   conjunction with the AvailableCircuits attribute in gateway selection
   by the LS. The total number of calls sent to the specified route on
   the gateway cannot exceed this total circuit capacity under steady
   state conditions.

   TotalCircuitCapacity is measured in integral number of calls. This is
   not expected to change frequently. This can change, for instance,
   when certain telephony trunks on the gateway are taken out of service
   for maintenance purposes.


3.1.1. TotalCircuitCapacity Syntax

   The TotalCircuitCapacity attribute is a 4-octet unsigned numeric
   value. It represents the total number of circuits available for
   terminating calls through this advertised route. This attribute
   represents a potentially achievable upper bound on the number of
   calls which can be terminated on this route in total.






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3.1.2. Route Origination and TotalCircuitCapacity

   Routes MAY be originated containing the TotalCircuitCapacity
   attribute.


3.1.3. Route Selection and TotalCircuitCapacity

   The TotalCircuitCapacity 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 (amongst a set of routes
   for the same destination), 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.


3.1.4. Aggregation and TotalCircuitCapacity

   An LS MAY aggregate routes to the same prefix which contain a
   TotalCircuitCapacity attribute. It SHOULD add the values of the
   individual routes to determine the value for the aggregated route in
   the same ITAD.


3.1.5. Route Dissemination and TotalCircuitCapacity

   Since this attribute reflects the static capacity of the gateway's
   circuit resources, it is not expected to change frequently. Hence the
   LS receiving this attribute MAY disseminate it to other peers, both
   internal and external to the ITAD.



3.2. AvailableCircuits Attribute

   Mandatory: False.
   Required Flags: Not well-known.
   Potential Flags: None.
   TRIP Type Code: 14. (To be assigned by IANA)


   The AvailableCircuits attribute is used ONLY between a gateway and
   its peer LS responsible for managing that gateway. If it is received
   in a route, it MUST NOT be propagated unless the LS is sure that it
   is relatively static.




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

   AvailableCircuits is measured in integral number of calls. It is very
   dynamic.


3.2.1. AvailableCircuits Syntax

   The AvailableCircuits attribute is a 4-octet unsigned numeric value.
   It represents the number of circuits remaining for terminating calls
   to this route.


3.2.2. Route Origination and AvailableCircuits

   Routes MAY be originated containing the AvailableCircuits 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 threshold. 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.


3.2.3. Route Selection and AvailableCircuits

   The AvailableCircuits 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 (amongst 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.


3.2.4. Aggregation and AvailableCircuits

   Not applicable






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3.2.5. Route Dissemination and AvailableCircuits

   Routes MUST NOT be disseminated with the AvailableCircuits 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.


3.3. CallSuccess Attribute

   Mandatory: False.
   Required Flags: Not well-known.
   Potential Flags: None.
   TRIP Type Code: 15. (To be assigned by IANA)

   The CallSuccess attribute is an attribute used ONLY between a gateway
   and its peer LS responsible for managing that gateway. If it is
   received in a route, it MUST NOT be propagated.

   The CallSuccess attribute provides information about the number of
   normally terminated calls out of a total number of attempted calls.
   CallSuccess is to be  determined by the gateway based on the
   Disconnect cause code at call termination. For example, a call that
   reaches the Alerting stage but does not get connected because of
   called party being unavailable is conventionally considered a
   successful call. Similar is the case when the called party is busy.
   On the other hand, a call that gets disconnected because of a Circuit
   or Resource being unavailable is conventionally considered a failed
   call. The exact mapping of disconnect causes to CallSuccess is at the
   discretion of the gateway reporting the attribute.

   The CallSuccess attribute is used by the LS to keep track of failures
   in reaching certain telephony destinations through a gateway(s) and
   use that information in the gateway selection process to enhance the
   probability of successful call termination.

   This information can be used by the LS to consider alternative
   gateways to terminate calls to those destinations with a better
   likelihood of success.


3.3.1. CallSuccess Syntax

   The CallSuccess attribute is comprised of two component fields - each
   represented as an unsigned 4-octet numeric value.  The first
   component field represents the total number of calls terminated
   normally for the advertised destination on a given address family.
   The second component field represents the total number of attempted



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   calls for the advertised destination.


3.3.2. Route Origination and CallSuccess

   Routes MAY be originated containing the CallSuccess attribute. This
   attribute is expected to get statistically significant with passage
   of time as more calls are attempted. Therefore, it is RECOMMENDED
   that the gateway make a choice based on local thresholds to determine
   when to report this attribute in an UPDATE.


3.3.3. Route Selection and CallSuccess

   The CallSuccess attribute MAY be used for route selection.  This
   attribute represents a measure of success of terminating calls to the
   advertised destination(s).  This information MAY be used to select
   from amongst a set of alternative routes to increase the probability
   of successful call termination.


3.3.4. Aggregation and CallSuccess

   Not applicable


3.3.5. Route Dissemination and CallSuccess

   Routes MUST NOT be disseminated with the CallSuccess attribute.  Its
   potential to change dynamically does not make it suitable to
   disseminate in most cases.


3.4. Prefix Attributes

   Mandatory: False.
   Required Flags: Not well-known.
   Potential Flags: None.
   TRIP Type Codes: 16 for E164 prefix, 17 for pentadecimal prefix and
   18 for decimal prefix (To be assigned by IANA)

   The Prefix attribute is used to represent the list of prefixes  that
   the respective route can complete calls to.  This attribute is
   intended to be used with the Carrier or Trunkgroup address family
   (discussed in Section 3.7).

   Though it is possible that all prefix ranges may  be reachable
   through a given Carrier, administrative issues could make certain



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   ranges preferable to others.


3.4.1. Prefix Attribute Syntax

   The Prefix attribute could be E.164, Decimal or PentaDecimal (refer
   to TRIP RFC [4]), each of them having it's own type code. The Prefix
   attribute is encoded as a sequence of prefix values in the attribute
   value field. The prefixes are listed sequentially with no padding as
   shown in Figure 2. Each prefix includes a 2-octet length field that
   represents the length of the address field in octets. The order of
   prefixes in the attribute is not significant.

   The presence of Prefix Attribute with the length field of the
   attribute as 0 signifies that the TGREP GW can terminate ALL prefixes
   for the given Reachable route(s).

                        1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 . . . 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-------------------------------+-----------+----------------------------------+-----------
|             Length            | Prefix1...|              Length              | Prefix2...
+-------------------------------+-----------+----------------------------------+-----------

                          Figure 2: Prefix Format


3.4.2. Route Origination and Prefix

   Routes MAY be originated containing the Prefix attribute.


3.4.3. Route Selection and Prefix

   The Prefix attribute MAY be used for route selection.


3.4.4. Aggregation and Prefix

   Routes with differing Prefix attribute MUST NOT be aggregated.
   Routes with the same value in the Prefix attribute MAY be aggregated
   and the same Prefix attribute attached to the aggregated object.










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3.4.5. Route Dissemination and Prefix

   The LS receiving this attribute should disseminate it to other peers,
   both internal and external to the ITAD.


3.5. TrunkGroup Attribute

   Mandatory: False.
   Required Flags: Not well-known.
   Potential Flags: None.
   TRIP Type Code: 20 (To be assigned by IANA)

   The TrunkGroup attribute is used to represent the list of trunkgroups
   on the gateway that the gateway can complete the calls to. It enables
   providers to route calls to destinations through preferred trunks.
   This attribute is relatively static.


3.5.1. TrunkGroup Syntax

   The TrunkGroup attribute is a variable length attribute that is
   composed of a sequence of trunkgroup length-value fields. It
   indicates that the gateway can complete the call through any
   trunkgroup (represented by the trunkgroup label) in the sequence.

   Each trunkgroup is a length-value field (shown in Figure 3 below).
   The length field is a 1-octet unsigned numeric value. The value field
   is a variable length alphanumeric field and is also called the
   trunkgroup label field. The length field represents the size of the
   trunkgroup label in number of octets. The maximum length is 128
   octets.

   The presence of TrunkGroup attribute with the length field of the
   attribute as 0 signifies that the TGREP GW can terminate ALL
   trunkgroup  for the given Reachable route(s).

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 ...  7 8 9 0 1 2 3 4 5 ...
   +---------------+--------------------+---------------+---------------------
   |    Length     | TrunkGroup Label1..|   Length      | TrunkGroup Label2..
   +---------------+--------------------+---------------+---------------------
                        Figure 3: TrunkGroup Syntax








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3.5.2. Route Origination and TrunkGroup

   Routes MAY be originated containing the TrunkGroupattribute.


3.5.3. Route Selection and TrunkGroup

   The TrunkGroup attribute MAY be used for route selection. Certain
   trunkgroups MAY be preferred over others based on provider policy.


3.5.4. Aggregation and TrunkGroup

   Routes with differing TrunkGroup attribute MUST NOT be aggregated.
   Routes with the same value in the TrunkGroup attribute MAY be
   aggregated and the same TrunkGroup attribute attached to the
   aggregated object.


3.5.5. Route Dissemination and TrunkGroup

   This attribute is not expected to change frequently. Hence, the LS
   receiving this attribute MAY disseminate it to other peers, internal
   to ITAD. Routes MUST not be disseminated  external to the ITAD, with
   TrunkGroup attribute.


3.6. Carrier Attribute

   Mandatory: False.
   Required Flags: Not well-known.
   Potential Flags: None.
   TRIP Type Code: 19 (To be assigned by IANA)

   The Carrier attribute is used to represent the list of carriers that
   the gateway can complete calls to. It enables providers to route
   calls to destinations through preferred carriers.  This attribute is
   relatively static.


3.6.1. Carrier Syntax

   The Carrier attribute is a variable length attribute that is composed
   of a sequence of carrier values. It indicates that the route can
   complete the call to any of the carriers represented in the sequence
   of carrier values.

   Each  carrier value has two fields, a 2-octet CountryCode, and a 2-



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   octet CarrierIdCode. The CountryCode field represents the country
   that corresponds to the Carrier identified by CarrierIdCode. It is
   the same as the E.164 country code used to dial internal telephony
   destinations. The CarrierIdCode or CIC represents a unique code
   assigned to the Carrier or Telephony service provider offering the
   service by an administrative body operating in that region.

   The presence of Carrier Attribute with the length field of the
   attribute as 0 signifies that the TGREP GW can terminate ALL Carriers
   for the given Reachable route(s).

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +--------------------------------+-------------------------------+-----
      |        CountryCode             |       CarrierIdCode           | ...
      +--------------------------------+-------------------------------+-----
                         Figure 4: Carrier Syntax


3.6.2. Route Origination and Carrier

   Routes MAY be originated containing the Carrier attribute.


3.6.3. Route Selection and Carrier

   The Carrier attribute MAY be used for route selection. Certain
   carriers MAY be preferred over others based on provider policy.


3.6.4. Aggregation and Carrier

   Routes with differing Carrier attribute MUST NOT be aggregated.
   Routes with the same value in the Carrier attribute MAY be aggregated
   and the same Carrier attribute attached to the aggregated object.


3.6.5. Route Dissemination and Carrier

   This attribute is not expected to change frequently. Hence, the LS
   receiving this attribute MAY disseminate it to other peers, both
   internal and external to the ITAD.









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3.7. TrunkGroup and Carrier Address Families

   When a set of GWs are to managed at the granularity of carriers or
   trunkgroups then it may be more appropriate for a GW can advertise
   routes using the Carrier address family or trunkgroup address family
   respectively. Also, in a TGREP association between the gateway and
   the LS responsible for managing  that gateway, there are some
   attributes that more naturally fit in as advertised properties of
   trunkgroups or carriers rather than that of advertised prefixes, For
   example: the AvailableCircuit information on a particular trunkgroup
   or a particular carrier. To express this relationship, the existing
   TRIP address families are inadequate. We need separate address
   families that can associate certain properties like AvailableCircuits
   information to  trunkgroups or carriers.

   The primary benefits of this model are as follows:

     - It allows a service provider to route calls based strictly on the
       trunkGroups or carriers.
     - it facilitates more accurate reporting of attributes of a dynamic
       nature like AvailableCircuits by providing the ability to manage
       resources at the granularity of a trunkgroup or a carrier.
     - Gateways can get really large with the ability to provision
       hundreds or a few thousand circuits and this can increase the
       potential for traffic that reports dynamic resource information
       between the gateway and the LS. The model introduced can
       potentially alleviate this UPDATE traffic hence increasing
       efficiency and providing a scalable gateway registration model.


3.7.1. Address Family Syntax

   Consider the generic TRIP route format from TRIP[4] shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+--------------+----------------+
      |       Address Family          |      Application Protocol     |
      +---------------+---------------+--------------+----------------+----
      |            Length             |       Address (variable)     ....
      +---------------+---------------+--------------+----------------+----

                    Figure 5: Generic TRIP Route Format

   The Address Family field will be used to represent the type of the
   address associated for this family, which is based on the TrunkGroup
   or Carrier.  The codes for the new address families will be allocated
   by IANA.



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   The Application Protocol field is same as the one for the Decimal,
   PentaDecimal and E.164 address families defined in TRIP[4].  The
   Length field represents the total size of the Address field in bytes.

   The value in the Address field represents either the TrunkGroup or
   the Carrier address families and the syntax is as follows:

   For the TrunkGroup Address Family, the Address field is a variable
   length alphanumeric field (trunkgroup label), where length is
   determined by the length field of the route. The maximum value of the
   length field for this Address Family is 128 bytes.

   For the Carrier Address Family, the length field represents the
   length of the Address field in bytes. The Address field is a fixed
   length field representing Carrier value.  It is encoded as a fixed
   length 4-octet value consisting of two fields : Country code and the
   CarrierId.

   If a gateway supports any of these address families, it should
   include that address family as one of the Route types supported in
   the OPEN message capability negotiation phase.

   The following attributes are currently defined to be used with all
   the address families including the TrunkGroup and Carrier address
   families.

     - AvailableCircuits Attribute
     - TotalCircuitCapacity Attribute
     - CallSuccess Attribute

       It is recommended that the above three attributes be used by the
       gateway with the TrunkGroup or Carrier address families, if
       possible. This will potentially offer a more efficient resource
       reporting framework, and a scalable model for gateway
       provisioning.

       However, when the gateway is not using TrunkGroup or Carrier
       address family, it MAY use the above attributes with the Decimal,
       PentaDecimal and E.164 address families.

       The Prefix, Carrier and TrunkGroup attributes MUST NOT be used
       with their respective address families.









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3.8. Other attribute considerations

3.8.1. Cost/Pricing attribute

   In exploring attributes suitable for the GW-LS communications,
   Pricing is another attribute that was considered. Though at first
   glance, it seems like a useful piece of information to be advertised
   by the gateway to express the price offered by carriers to different
   destinations, in reality, the computation of pricing can get quite
   complex. For example, the price offered by a provider can vary by
   time of day, it can be based on an agreement between two service
   providers interconnecting with each other, it could be based on one
   price for the first 'n' minutes and a different price after that,
   Least Cost routing choices and Grades of service offered by a carrier
   can affect pricing. There could be other factors as well. Expressing
   this complex interplay between different factors that determine
   pricing is non-trivial to represent. It will be a subject of further
   investigation to determine whether advertising pricing associated
   with carriers in its simple form without any dependencies adds value
   to be included as an attribute in the TGREP communications.


3.9. Gateway Operation

   A gateway uses TRIP to advertise its reachability to its domain's
   Location Server(s) (LS, which are closely coupled with proxies).  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.


3.9.1. Session Establishment

    When opening a peering session with a TGREP Receiver, a TGREP
    gateway follows exactly the same procedures as any other TRIP speaker.
    The TGREP 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. The OPEN message also contains the
    address families supported by the gateway. The remainder of the
    peer session establishment is identical to TRIP.





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3.9.2. UPDATE Messages

   Once the peer session has been established, the gateway sends UPDATE
   messages to the TRIP LS with the gateway's entire reachability.  The
   Gateway also sends any attributes associated with the routes.

   If the gateway's reachability 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[4].  The TGREP gateway MUST support all mandatory
   TRIP attributes.

   If the gateway receives an UPDATE message from the TRIP LS, it MUST
   silently discard it as specified for TRIP[4]. No further action
   should be taken.


3.9.3. KEEPALIVE Messages

   KEEPALIVE messages are periodically exchanged over the peering
   session between the TGREP gateway and the TRIP LS as specified in
   Section 4.4 of TRIP RFC[4].


3.9.4. Error Handling and NOTIFICATION Messages

   The same procedures used with TRIP, are used with TGREP for error
   handling and generating NOTIFICATION messages. The only difference is
   that a TGREP 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.


3.9.5. TGREP Finite State Machine

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


3.9.6. Call Routing Databases

   A TGREP gateway may maintain simultaneous sessions with more than one
   TRIP LSs. A TGREP 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



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



3.9.7. Multiple Address Families

   As mentioned above, TGREP supports various address families in order
   to convey the reachibilty of telephony destinations. A TGREP session
   MUST NOT send UPDATEs of more than one of the following catagories
   (a) Prefix Address families (E164, Pentadecimal and decimal) (b)
   Trunkgroup address family (c) Carrier Address family for a given
   established session. TGREP should specify it's choice address family
   through the route-type capability in the OPEN message. And route-type
   specification in the OPEN message violating the above rule should be
   rejected with a NOTIFICATION message.


3.9.8. Route Selection and Aggregation

   TRIP's route selection and aggregation operations MUST NOT be
   implemented by TGREP gateways.


3.10. LS/Proxy Behavior

   As mentioned earlier, TGREP can be considered as a protocol
   complimentary to TRIP in providing reachability information that can
   then be further fed into the Location Server for propogation.  The
   architecture of an LS/Proxy system is as follows: There exists a TRIP
   LS application that functions as a speaker in the I-TRIP/E-TRIP
   network as documented in the TRIP RFC [4]. Then, there is a signaling
   server fronting a set of gateways and receives routing information on
   one or more TGREP sessions. This routing information from the
   gateways is received and processed by a TGREP receiver application.
   Subsequently, this routing information is presented as candidate
   routes (possibly as local routes) to the TRIP LS.  The interface
   between these two applications is entirely a local matter. However,
   before importing these routes into the TRIP LS, certain operations
   may optionally be performed on these routes. The nature of these
   operations and the accompanying motivation are described in the
   subsections below. The order in which the operations are listed



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   represents an implicit logical sequence in which they are applied.
   The architecture for an LS/Proxy entity is shown in Figure 7 below.

















































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    +-------------------------------------------------------+
    |                    +-------------------------------+  |
    |                    |     +-+  +-+                  |  |
    |                    |     |A|  |C|                  |  |       +-----+
    |                    |     |g|  |o|                  |  | TGREP |     |
    |   +-------------+  |     |g|  |n|  +-------------+ |  |    -- | GW  |
    |   |             |  |     |r|  |s|  |             | |  |  --   +-----+
    |   |    TRIP     |  |     |e|  |o|  |             | |  +--
    |   |     LS    <----------|g<--|l<---    TGREP    |-++-|       +-----+
    |   |             |  |     |a|  |i|  |   Session   | |  |       |     |
    |   |  (I-TRIP/   |  |     |t|  |d|  |  Mangement  |-++-+-------| GW  |
    |   |   E-TRIP)   |  |     |i|  |a|  |             | |  |       +-----+
    |   |             |  |     |o|  |t|  |             |-+ -+-
    |   +-----------/-+  |     |n|  |i|  +-------------+ |  | ---   +-----+
    |              /     |     | |  |o|                  |  |    -- |     |
    |             /      |     | |  |n|                  |  |       | GW  |
    |            /       |     +-+  +-+                  |  |       +-----+
    |           /        |              TGREP Receiver   |  |
    |          /         +-------------------------------+  |
    |         /                                             |
    |        /                                              |
    +-------/-----------------------------------------------+
           /                            LS/Proxy
          /
         /
        /
       /
      /
    +/----------------+
    |                 |
    |                 |
    |                 |
    |       LS        |
    |                 |
    |                 |
    |                 |
    |                 |
    |                 |
    +\----------------+
      \
       \
        \
         \
          \
           \
            \
    +--------\---------------------------------------------+
              \


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    |            \       +-------------------------------+  |
    |             \      |     +-+  +-+                  |  |
    |              \     |     |A|  |C|                  |  |       +-----+
    |               \    |     |g|  |o|                  |  | TGREP |     |
    |   +------------\+  |     |g|  |n|  +-------------+ |  |    -- | GW  |
    |   |             |  |     |r|  |s|  |             | |  |  --   +-----+
    |   |    TRIP     |  |     |e|  |o|  |             | |  +--
    |   |     LS    <----------|g<--|l<---    TGREP    |-++-|       +-----+
    |   |             |  |     |a|  |i|  |   Session   | |  |       |     |
    |   |  (I-TRIP/   |  |     |t|  |d|  |  Mangement  |-++-+-------| GW  |
    |   |   E-TRIP)   |  |     |i|  |a|  |             | |  |       +-----+
    |   |             |  |     |o|  |t|  |             |-+ -+-
    |   +-------------+  |     |n|  |i|  +-------------+ |  | ---   +-----+
    |                    |     | |  |o|                  |  |    -- |     |
    |                    |     | |  |n|                  |  |       | GW  |
    |                    |     +-+  +-+                  |  |       +-----+
    |                    |              TGREP Receiver   |  |
    |                    +-------------------------------+  |
    |                                                       |
    |                                                       |
    +-------------------------------------------------------+
                                        LS/Proxy


                   Figure 7: LS Architecture for TRIP-GW


3.10.1. Route consolidation

   The TGREP receiver may receive routing information from one or more
   gateways. It is possible that multiple routes are available for the
   same destination. These different alternative routes may be received
   from the same gateway, or from multiple gateways. It is RECOMMENDED
   that the set of gateway routes for each destination be consolidated,
   before presenting a candidate route, to the TRIP LS.  The motivation
   for this operation should be to define a route that can maximally
   represent the collective routing capabilities of the set of gateways,
   managed by this TGREP receiver.  Let us take an example scenario in
   order to bring out the motivation for this operation.  Let us say,
   the TGREP receiver maintains peering sessions with gateways A, and B.

   o Gateway A advertises a route for destination "SIP 408" on the E.164
   address family with the Carrier attribute value C1.

   o Gateway B advertises a route for destination "SIP 408" on the E.164
   address family with Carrier attribute value C2.



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   The TGREP receiver that receives these routes can consolidate these
   constituent routes into a single route for destination "SIP 408" with
   its Carrier attribute being a union of the the Carrier attribute
   values of the individual routes, namely, "C1 C2". This operation is
   refered to as Consolidation. In the above example, it is possible
   that a route to the destination "SIP 408" through one or more
   carriers may have been lost if the individual routes were not
   consolidated.

   Another example is to consolidate the Prefix attribute from multiple
   Carrier or Trunkgroup updates received from different gateways for
   the same destination. Let us say, there are  Carrier AF updates from
   two gateways for Carrier destination X, and the prefix attribute
   values are {408, 650} from one update and {919, 973} from the other.
   The prefix values from these two updates can be consolidated into a
   single Carrier AF route advertisement with prefix value {408, 650,
   919, 973}.

   In general, there is a potential for loss of gateway routing
   information, when TGREP routes from a set of gateways are not
   consolidated, when a candidate route is presented to the TRIP LS.
   The specifics of applying the consolidation operation to different
   attributes and routes from different address families, is left to the
   individual TGREP receiver implementations.


3.10.2. Aggregation

   The set of gateway routes, that are in a consolidated form or
   otherwise, may be aggregated before importing it to the LS instance
   that is responsible for I-TRIP/E-TRIP processing. This operation
   follows the standard aggregation procedures described in the TRIP RFC
   [4], while adhering to the aggregation rules for each route
   attribute.


4. IANA Considerations

     - The Attribute Type Codes for the new attributes:
       AvailableCircuits, TotalCircuitCapacity, CallSuccess, Prefix,
       TrunkGroup and Carrier described in Sections 4.1, 4.2, 4.3, 4.4,
       4.5 and 4.6 above, respectively, are to be assigned by IANA.
     - The Address Family Codes for the new address families: TrunkGroup
       and Carrier described in Section 4.7, are to be assigned by IANA.







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5. Changes since draft-ietf-iptel-trip-gw-00.txt

     - Changed title of the document to TGREP (Telephony Gateway
       REgistration Protocol)
     - Changed name of protocol described in this document to TGREP
     - Changed Abstract and Introduction sections to position TGREP as
       an auxiliary protocol to TRIP (as opposed to a "subset" of TRIP)
     - Modified the section on LS/Proxy Behavior including the diagram
     - Added an additional example to the Route Consolidation section
     - Changed the format of Carrier (both as an attribute and as an AF)
       to accomodate representation of Country codes in association with
       CICs.
     - Updated text to allow Carrier attribute in TrunkGroup address
       family and TrunkGroup attribute in Carrier address family.


6. Changes since -03

     - Removed Carrier-Trunkgroup attribute and address family and
       references to it.
     - Added Terminology and Definitions section.
     - Updated CallSuccess attribute.
     - Added Prefix attribute.
     - Added Carrier attribute.
     - Added TrunkGroup attribute.
     - Added TrunkGroup Address Family.
     - Added Carrier Address Family.
     - Added some more references.



7. Changes since -02

     - Removed the requirements section.
     - Discussed the motivation for introducing Carrier information into
       TRIP.
     - Defined a new attribute for the E.164 address family.
     - Defined a new address family for CarrierCode-TrunkGroup
       combination .
     - Defined new attributes to advertise dynamic gateway
       characteristics like resource availability, and call success
       rate.
     - Added as section to validate the TGREP solution against the
       requirements in [7].







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8. Changes since -01

     - Added requirements.
     - Added more formal analysis of REGISTER and added analysis of SLP.
     - Removed circuit capacity attribute.


9. Changes since -00

     - Added text to stress the value of this proposal for managing a
       gateway cluster.
     - Added attributes for circuit capacity and DSP capacity.
     - Added section on LS operation, discussing aggregation issue.


Authors' Addresses


   Manjunath Bangalore
   Cisco Systems
   Mail Stop SJC-21/2/2
   771 Alder Drive
   Milpitas, CA 95035
   email: manjax@cisco.com


   Rajneesh Kumar
   Cisco Systems
   Mail Stop SJC-21/2/2
   771 Alder Drive
   Milpitas, CA 95035
   email: rajneesh@cisco.com


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


   Hussein F. Salama
   Cisco Systems
   Mail Stop SJC-24/3/2
   170 W. Tasman Drive
   San Jose, CA 95134
   email: hsalama@cisco.com



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   Dhaval N. Shah
   Cisco Systems
   Mail Stop SJC-21/2/2
   771 Alder Drive
   Milpitas, CA 95035
   email: dhaval@cisco.com



Acknowledgments

   We wish to thank David Oran, Anirudh Sahoo, Kevin McDermott, Jon Peterson,
   Li Li and Bob Penfield for their insightful comments and suggestions.


References

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

   [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] E. Guttman, C. Perkins, J. Veizades, and M. Day, "Service
   location protocol, version 2," Request for Comments 2608, Internet
   Engineering Task Force, June 1999.

   [4] J. Rosenberg, H. Salama, and M. Squire, "Telephony routing over
   IP (TRIP)," Request for Comments 3219, Internet Engineering Task
   Force, January 2002.

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

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

   [7] J. Rosenberg, "Requirements for Gateway Registration," Internet
   Draft, Internet Engineering Task Force, Nov. 2001.  Work in progress.

   [8] ITU-T List of ITU Carrier Codes (published periodically in the
   ITU-T Operational Bulletin).



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   [9] J. Peterson, "An Architecture for Gateway Registration Based on
   Trunk Groups," Internet Draft, Internet Engineering Task Force, Feb.
   2002. Work in progress.


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