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rfc:rfc3219

Network Working Group J. Rosenberg Request for Comments: 3219 dynamicsoft Category: Standards Track H. Salama

                                                         Cisco Systems
                                                             M. Squire
                                                     Hatteras Networks
                                                          January 2002
                  Telephony Routing over IP (TRIP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This document presents the Telephony Routing over IP (TRIP).  TRIP is
 a policy driven inter-administrative domain protocol for advertising
 the reachability of telephony destinations between location servers,
 and for advertising attributes of the routes to those destinations.
 TRIP's operation is independent of any signaling protocol, hence TRIP
 can serve as the telephony routing protocol for any signaling
 protocol.
 The Border Gateway Protocol (BGP-4) is used to distribute routing
 information between administrative domains.  TRIP is used to
 distribute telephony routing information between telephony
 administrative domains.  The similarity between the two protocols is
 obvious, and hence TRIP is modeled after BGP-4.

Table of Contents

 1    Terminology and Definitions  ..............................   3
 2    Introduction  .............................................   4
 3    Summary of Operation  .....................................   5
 3.1  Peering Session Establishment and Maintenance  ............   5
 3.2  Database Exchanges  .......................................   6
 3.3  Internal Versus External Synchronization  .................   6
 3.4  Advertising TRIP Routes  ..................................   6

Rosenberg, et. al. Standards Track [Page 1] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 3.5  Telephony Routing Information Bases  ......................   7
 3.6  Routes in TRIP  ...........................................   9
 3.7  Aggregation  ..............................................   9
 4    Message Formats  ..........................................  10
 4.1  Message Header Format  ....................................  10
 4.2  OPEN Message Format  ......................................  11
 4.3  UPDATE Message Format  ....................................  15
 4.4  KEEPALIVE Message Format   ................................  22
 4.5  NOTIFICATION Message Format   .............................  23
 5    TRIP Attributes   .........................................  24
 5.1  WithdrawnRoutes  ..........................................  24
 5.2  ReachableRoutes  ..........................................  28
 5.3  NextHopServer   ...........................................  29
 5.4  AdvertisementPath   .......................................  31
 5.5  RoutedPath  ...............................................  35
 5.6  AtomicAggregate   .........................................  36
 5.7  LocalPreference   .........................................  37
 5.8  MultiExitDisc  ............................................  38
 5.9  Communities  ..............................................  39
 5.10 ITAD Topology    ..........................................  41
 5.11 ConvertedRoute  ...........................................  43
 5.12 Considerations for Defining New TRIP Attributes   .........  44
 6    TRIP Error Detection and Handling   .......................  44
 6.1  Message Header Error Detection and Handling   .............  45
 6.2  OPEN Message Error Detection and Handling   ...............  45
 6.3  UPDATE Message Error Detection and Handling   .............  46
 6.4  NOTIFICATION Message Error Detection and Handling   .......  48
 6.5  Hold Timer Expired Error Handling   .......................  48
 6.6  Finite State Machine Error Handling   .....................  48
 6.7  Cease   ...................................................  48
 6.8  Connection Collision Detection   ..........................  48
 7    TRIP Version Negotiation   ................................  49
 8    TRIP Capability Negotiation   .............................  50
 9    TRIP Finite State Machine   ...............................  50
 10   UPDATE Message Handling   .................................  55
 10.1 Flooding Process   ........................................  56
 10.2 Decision Process   ........................................  58
 10.3  Update-Send Process   ..................................... 62
 10.4  Route Selection Criteria   ................................ 67
 10.5  Originating TRIP Routes   ................................. 67
 11    TRIP Transport   .......................................... 68
 12    ITAD Topology   ........................................... 68
 13    IANA Considerations  ...................................... 68
 13.1  TRIP Capabilities   ....................................... 68
 13.2  TRIP Attributes    ........................................ 69
 13.3  Destination Address Families   ............................ 69
 13.4  TRIP Application Protocols   .............................. 69
 13.5  ITAD Numbers   ............................................ 70

Rosenberg, et. al. Standards Track [Page 2] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 14    Security Considerations   ................................. 70
 A1    Appendix 1: TRIP FSM State Transitions and Actions   ...... 71
 A2    Appendix 2: Implementation Recommendations   .............. 73
 Acknowledgments  ................................................ 75
 References  ..................................................... 75
 Intellectual Property Notice  ................................... 77
 Authors' Addresses  ............................................. 78
 Full Copyright Statement  ....................................... 79

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].
 A framework for Telephony Routing over IP (TRIP) is described in [2].
 We assume the reader is familiar with the framework and terminology
 of [2].  We define and use the following terms in addition to those
 defined in [2].
 Telephony Routing Information Base (TRIB): The database of reachable
 telephony destinations built and maintained at an LS as a result of
 its participation in TRIP.
 IP Telephony Administrative Domain (ITAD): The set of resources
 (gateways, location servers, etc.) under the control of a single
 administrative authority.  End users are customers of an ITAD.
 Less/More Specific Route: A route X is said to be less specific than
 a route Y if every destination in Y is also a destination in X, and X
 and Y are not equal.  In this case, Y is also said to be more
 specific than X.
 Aggregation: Aggregation is the process by which multiple routes are
 combined into a single less specific route that covers the same set
 of destinations.  Aggregation is used to reduce the size of the TRIB
 being synchronized with peer LSs by reducing the number of exported
 TRIP routes.
 Peers: Two LSs that share a logical association (a transport
 connection).  If the LSs are in the same ITAD, they are internal
 peers.  Otherwise, they are external peers.  The logical association
 between two peer LSs is called a peering session.

Rosenberg, et. al. Standards Track [Page 3] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Telephony Routing Information Protocol (TRIP): The protocol defined
 in this specification.  The function of TRIP is to advertise the
 reachability of telephony destinations, attributes associated with
 the destinations, as well as the attributes of the path towards those
 destinations.
 TRIP destination: TRIP can be used to manage routing tables for
 multiple protocols (SIP, H323, etc.).  In TRIP, a destination is the
 combination of (a) a set of addresses (given by an address family and
 address prefix), and (b) an application protocol (SIP, H323, etc).

2. Introduction

 The gateway location and routing problem has been introduced in [2].
 It is considered one of the more difficult problems in IP telephony.
 The selection of an egress gateway for a telephony call, traversing
 an IP network towards an ultimate destination in the PSTN, is driven
 in large part by the policies of the various parties along the path,
 and by the relationships established between these parties.  As such,
 a global directory of egress gateways in which users look up
 destination phone numbers is not a feasible solution.  Rather,
 information about the availability of egress gateways is exchanged
 between providers, and subject to policy, made available locally and
 then propagated to other providers in other ITADs, thus creating
 routes towards these egress gateways.  This would allow each provider
 to create its own database of reachable phone numbers and the
 associated routes - such a database could be very different for each
 provider depending on policy.
 TRIP is an inter-domain (i.e., inter-ITAD) gateway location and
 routing protocol.  The primary function of a TRIP speaker, called a
 location server (LS), is to exchange information with other LSs.
 This information includes the reachability of telephony destinations,
 the routes towards these destinations, and information about gateways
 towards those telephony destinations residing in the PSTN.  The TRIP
 requirements are set forth in [2].
 LSs exchange sufficient routing information to construct a graph of
 ITAD connectivity so that routing loops may be prevented.  In
 addition, TRIP can be used to exchange attributes necessary to
 enforce policies and to select routes based on path or gateway
 characteristics.  This specification defines TRIP's transport and
 synchronization mechanisms, its finite state machine, and the TRIP
 data.  This specification defines the basic attributes of TRIP.  The
 TRIP attribute set is extendible, so additional attributes may be
 defined in future documents.

Rosenberg, et. al. Standards Track [Page 4] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 TRIP is modeled after the Border Gateway Protocol 4 (BGP-4) [3] and
 enhanced with some link state features, as in the Open Shortest Path
 First (OSPF) protocol [4], IS-IS [5], and the Server Cache
 Synchronization Protocol (SCSP) [6].  TRIP uses BGP's inter-domain
 transport mechanism, BGP's peer communication, BGP's finite state
 machine, and similar formats and attributes as BGP.  Unlike BGP
 however, TRIP permits generic intra-domain LS topologies, which
 simplifies configuration and increases scalability in contrast to
 BGP's full mesh requirement of internal BGP speakers.  TRIP uses an
 intra-domain flooding mechanism similar to that used in OSPF [4],
 IS-IS [5], and SCSP [6].
 TRIP permits aggregation of routes as they are advertised through the
 network.  TRIP does not define a specific route selection algorithm.
 TRIP runs over a reliable transport protocol.  This eliminates the
 need to implement explicit fragmentation, retransmission,
 acknowledgment, and sequencing.  The error notification mechanism
 used in TRIP assumes that the transport protocol supports a graceful
 close, i.e., that all outstanding data will be delivered before the
 connection is closed.
 TRIP's operation is independent of any particular telephony signaling
 protocol.  Therefore, TRIP can be used as the routing protocol for
 any of these protocols, e.g., H.323 [7] and SIP [8].
 The LS peering topology is independent of the physical topology of
 the network.  In addition, the boundaries of an ITAD are independent
 of the boundaries of the layer 3 routing autonomous systems.  Neither
 internal nor external TRIP peers need to be physically adjacent.

3. Summary of Operation

 This section summarizes the operation of TRIP.  Details are provided
 in later sections.

3.1. Peering Session Establishment and Maintenance

 Two peer LSs form a transport protocol connection between one
 another.  They exchange messages to open and confirm the connection
 parameters, and to negotiate the capabilities of each LS as well as
 the type of information to be advertised over this connection.
 KeepAlive messages are sent periodically to ensure adjacent peers are
 operational.  Notification messages are sent in response to errors or
 special conditions.  If a connection encounters an error condition, a
 Notification message is sent and the connection is closed.

Rosenberg, et. al. Standards Track [Page 5] RFC 3219 Telephony Routing over IP (TRIP) January 2002

3.2. Database Exchanges

 Once the peer connection has been established, the initial data flow
 is a dump of all routes relevant to the new peer (In the case of an
 external peer, all routes in the LS's Adj-TRIB-Out for that external
 peer.  In the case of an internal peer, all routes in the Ext-TRIB
 and all Adj-TRIBs-In).  Note that the different TRIBs are defined in
 Section 3.5.
 Incremental updates are sent as the TRIP routing tables (TRIBs)
 change.  TRIP does not require periodic refresh of the routes.
 Therefore, an LS must retain the current version of all routing
 entries.
 If a particular ITAD has multiple LSs and is providing transit
 service for other ITADs, then care must be taken to ensure a
 consistent view of routing within the ITAD.  When synchronized the
 TRIP routing tables, i.e., the Loc-TRIBs, of all internal peers are
 identical.

3.3. Internal Versus External Synchronization

 As with BGP, TRIP distinguishes between internal and external peers.
 Within an ITAD, internal TRIP uses link-state mechanisms to flood
 database updates over an arbitrary topology.  Externally, TRIP uses
 point-to-point peering relationships to exchange database
 information.
 To achieve internal synchronization, internal peer connections are
 configured between LSs of the same ITAD such that the resulting
 intra-domain LS topology is connected and sufficiently redundant.
 This is different from BGP's approach that requires all internal
 peers to be connected in a full mesh topology, which may result in
 scaling problems.  When an update is received from an internal peer,
 the routes in the update are checked to determine if they are newer
 than the version already in the database.  Newer routes are then
 flooded to all other peers in the same domain.

3.4. Advertising TRIP Routes

 In TRIP, a route is defined as the combination of (a) a set of
 destination addresses (given by an address family indicator and an
 address prefix), and (b) an application protocol (e.g. SIP, H323,
 etc.).  Generally, there are additional attributes associated with
 each route (for example, the next-hop server).

Rosenberg, et. al. Standards Track [Page 6] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 TRIP routes are advertised between a pair of LSs in UPDATE messages.
 The destination addresses are included in the ReachableRoutes
 attribute of the UPDATE, while other attributes describe things like
 the path or egress gateway.
 If an LS chooses to advertise a TRIP route, it may add to or modify
 the attributes of the route before advertising it to a peer.  TRIP
 provides mechanisms by which an LS can inform its peer that a
 previously advertised route is no longer available for use.  There
 are three methods by which a given LS can indicate that a route has
 been withdrawn from service:
  1. Include the route in the WithdrawnRoutes Attribute in an UPDATE

message, thus marking the associated destinations as being no

       longer available for use.
    -  Advertise a replacement route with the same set of destinations
       in the ReachableRoutes Attribute.
    -  For external peers where flooding is not in use, the LS-to-LS
       peer connection can be closed, which implicitly removes from
       service all routes which the pair of LSs had advertised to each
       other over that peer session.  Note that terminating an
       internal peering session does not necessarily remove the routes
       advertised by the peer LS as the same routes may have been
       received from multiple internal peers because of flooding.  If
       an LS determines that another internal LS is no longer active
       (from the ITAD Topology attributes of the UPDATE messages from
       other internal peers), then it MUST remove all routes
       originated into the LS by that LS and rerun its decision
       process.

3.5. Telephony Routing Information Bases

 A TRIP LS processes three types of routes:
  1. External routes: An external route is a route received from an

external peer LS

  1. Internal routes: An internal route is a route received from an

internal LS in the same ITAD.

  1. Local routes: A local route is a route locally injected into

TRIP, e.g. by configuration or by route redistribution from

       another routing protocol.
 The Telephony Routing Information Base (TRIB) within an LS consists
 of four distinct parts:

Rosenberg, et. al. Standards Track [Page 7] RFC 3219 Telephony Routing over IP (TRIP) January 2002

  1. Adj-TRIBs-In: The Adj-TRIBs-In store routing information that

has been learned from inbound UPDATE messages. Their contents

       represent TRIP routes that are available as an input to the
       Decision Process.  These are the "unprocessed" routes received.
       The routes from each external peer LS and each internal LS are
       maintained in this database independently, so that updates from
       one peer do not affect the routes received from another LS.
       Note that there is an Adj-TRIB-In for every LS within the
       domain, even those with which the LS is not directly peered.
    -  Ext-TRIB: There is only one Ext-TRIB database per LS.  The LS
       runs the route selection algorithm on all external routes
       (stored in the Adj-TRIBs-In of the external peers) and local
       routes (may be stored in an Adj-TRIB-In representing the local
       LS) and selects the best route for a given destination and
       stores it in the Ext-TRIB.  The use of Ext-TRIB will be
       explained further in Section 10.3.1
    -  Loc-TRIB: The Loc-TRIB contains the local TRIP routing
       information that the LS has selected by applying its local
       policies to the routing information contained in its Adj-
       TRIBs-In of internal LSs and the Ext-TRIB.
    -  Adj-TRIBs-Out:  The Adj-TRIBs-Out store the information that
       the local LS has selected for advertisement to its external
       peers.  The routing information stored in the Adj-TRIBs-Out
       will be carried in the local LS's UPDATE messages and
       advertised to its peers.
 Figure 1 illustrates the relationship between the four parts of the
 routing information base.
                          Loc-TRIB
                              ^
                              |
                      Decision Process
                       ^      ^      |
                       |      |      |
              Adj-TRIBs-In    |      V
             (Internal LSs)   |   Adj-TRIBs-Out
                              |
                              |
                              |
                           Ext-TRIB
                          ^        ^
                          |        |
                 Adj-TRIB-In      Local Routes
             (External Peers)
                   Figure 1: TRIB Relationships

Rosenberg, et. al. Standards Track [Page 8] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Although the conceptual model distinguishes between Adj-TRIBs-In,
 Ext-TRIB, Loc-TRIB, and Adj-TRIBs-Out, this neither implies nor
 requires that an implementation must maintain four separate copies of
 the routing information.  The choice of implementation (for example,
 4 copies of the information vs. 1 copy with pointers) is not
 constrained by the protocol.

3.6. Routes in TRIP

 A route in TRIP specifies a range of numbers by being a prefix of
 those numbers (the exact definition & syntax of route are in 5.1.1).
 Arbitrary ranges of numbers are not atomically representable by a
 route in TRIP.  A prefix range is the only type of range supported
 atomically.  An arbitrary range can be accomplished by using multiple
 prefixes in a ReachableRoutes attribute (see Section 5.1 & 5.2).  For
 example, 222-xxxx thru 999-xxxx could be represented by including the
 prefixes 222, 223, 224,...,23,24,...,3,4,...,9 in a ReachableRoutes
 attribute.

3.7. Aggregation

 Aggregation is a scaling enhancement used by an LS to reduce the
 number of routing entries that it has to synchronize with its peers.
 Aggregation may be performed by an LS when there is a set of routes
 {R1, R2, ...} in its TRIB such that there exists a less specific
 route R where every valid destination in R is also a valid
 destination in {R1, R2, ...} and vice-versa.  Section 5 includes a
 description of how to combine each attribute (by type) on the {R1,
 R2, ...} routes into an attribute for R.
 Note that there is no mechanism within TRIP to communicate that a
 particular address prefix is not used or valid within a particular
 address family, and thus that these addresses could be skipped during
 aggregation.  LSs may use methods outside of TRIP to learn of invalid
 prefixes that may be ignored during aggregation.
 An LS is not required to perform aggregation, however it is
 recommended whenever maintaining a smaller TRIB is important.  An LS
 decides based on its local policy whether or not to aggregate a set
 of routes into a single aggregate route.
 Whenever an LS aggregates multiple routes where the NextHopServer is
 not identical in all aggregated routes, the NextHopServer attribute
 of the aggregate route must be set to a signalling server in the
 aggregating LS's domain.

Rosenberg, et. al. Standards Track [Page 9] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 When an LS resets the NextHopServer of any route, and this may be
 performed because of aggregation or other reasons, it has the effect
 of adding another signalling server along the signalling path to
 these destinations.  The end result is that the signalling path
 between two destinations may consist of multiple signalling servers
 across multiple domains.

4. Message Formats

 This section describes message formats used by TRIP.  Messages are
 sent over a reliable transport protocol connection.  A message MUST
 be processed only after it is entirely received.  The maximum message
 size is 4096 octets.  All implementations MUST support this maximum
 message size.  The smallest message that MAY be sent consists of a
 TRIP header without a data portion, or 3 octets.

4.1. Message Header Format

 Each message has a fixed-size header.  There may or may not be a data
 portion following the header, depending on the message type.  The
 layout of the header fields is shown in Figure 2.
       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +--------------+----------------+---------------+
       |          Length               |      Type     |
       +--------------+----------------+---------------+
                    Figure 2: TRIP Header
 Length:  This 2-octet unsigned integer indicates the total length of
 the message, including the header, in octets.  Thus, it allows one to
 locate, in the transport-level stream, the beginning of the next
 message.  The value of the Length field must always be at least 3 and
 no greater than 4096, and may be further constrained depending on the
 message type.  No padding of extra data after the message is allowed,
 so the Length field must have the smallest value possible given the
 rest of the message.
 Type:  This 1-octet unsigned integer indicates the type code of the
 message.  The following type codes are defined:
    1 - OPEN
    2 - UPDATE
    3 - NOTIFICATION
    4 - KEEPALIVE

Rosenberg, et. al. Standards Track [Page 10] RFC 3219 Telephony Routing over IP (TRIP) January 2002

4.2. OPEN Message Format

 After a transport protocol connection is established, the first
 message sent by each side is an OPEN message.  If the OPEN message is
 acceptable, a KEEPALIVE message confirming the OPEN is sent back.
 Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
 messages may be exchanged.
 The minimum length of the OPEN message is 17 octets (including
 message header).  OPEN messages not meeting this minimum requirement
 are handled as defined in Section 6.2.
 In addition to the fixed-size TRIP header, the OPEN message contains
 the following fields:
     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
    +---------------+---------------+--------------+----------------+
    |    Version    |    Reserved   |          Hold Time            |
    +---------------+---------------+--------------+----------------+
    |                            My ITAD                            |
    +---------------+---------------+--------------+----------------+
    |                        TRIP Identifier                        |
    +---------------+---------------+--------------+----------------+
    |    Optional Parameters Len    |Optional Parameters (variable)...
    +---------------+---------------+--------------+----------------+
                      Figure 3: TRIP OPEN Header
 Version:
 This 1-octet unsigned integer indicates the protocol version of the
 message.  The current TRIP version number is 1.
 Hold Time:
 This 2-octet unsigned integer indicates the number of seconds that
 the sender proposes for the value of the Hold Timer.  Upon receipt of
 an OPEN message, an LS MUST calculate the value of the Hold Timer by
 using the smaller of its configured Hold Time and the Hold Time
 received in the OPEN message.  The Hold Time MUST be either zero or
 at least three seconds.  An implementation MAY reject connections on
 the basis of the Hold Time.  The calculated value indicates the
 maximum number of seconds that may elapse between the receipt of
 successive KEEPALIVE and/or UPDATE messages by the sender.
 This 4-octet unsigned integer indicates the ITAD number of the
 sender.  The ITAD number must be unique for this domain within this
 confederation of cooperating LSs.

Rosenberg, et. al. Standards Track [Page 11] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 ITAD numbers are assigned by IANA as specified in Section 13.  This
 document reserves ITAD number 0.  ITAD numbers from 1 to 255 are
 designated for private use.
 TRIP Identifier:
 This 4-octet unsigned integer indicates the TRIP Identifier of the
 sender.  The TRIP Identifier MUST uniquely identify this LS within
 its ITAD.  A given LS MAY set the value of its TRIP Identifier to an
 IPv4 address assigned to that LS.  The value of the TRIP Identifier
 is determined on startup and MUST be the same for all peer
 connections.  When comparing two TRIP identifiers, the TRIP
 Identifier is interpreted as a numerical 4-octet unsigned integer.
 Optional Parameters Length:
 This 2-octet unsigned integer indicates the total length of the
 Optional Parameters field in octets.  If the value of this field is
 zero, no Optional Parameters are present.
 Optional Parameters:
 This field may contain a list of optional parameters, where each
 parameter is encoded as a <Parameter Type, Parameter Length,
 Parameter Value> triplet.
     0                   1                   2
     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
    +---------------+---------------+--------------+----------------+
    |       Parameter Type          |       Parameter Length        |
    +---------------+---------------+--------------+----------------+
    |                  Parameter Value (variable)...
    +---------------+---------------+--------------+----------------+
                  Figure 4: Optional Parameter Encoding
 Parameter Type:
 This is a 2-octet field that unambiguously identifies individual
 parameters.
 Parameter Length:
 This is a 2-octet field that contains the length of the Parameter
 Value field in octets.
 Parameter Value:
 This is a variable length field that is interpreted according to the
 value of the Parameter Type field.

Rosenberg, et. al. Standards Track [Page 12] RFC 3219 Telephony Routing over IP (TRIP) January 2002

4.2.1. Open Message Optional Parameters

 This document defines the following Optional Parameters for the OPEN
 message.

4.2.1.1. Capability Information

 Capability Information uses Optional Parameter type 1.  This is an
 optional parameter used by an LS to convey to its peer the list of
 capabilities supported by the LS.  This permits an LS to learn of the
 capabilities of its peer LSs.  Capability negotiation is defined in
 Section 8.
 The parameter contains one or more triples <Capability Code,
 Capability Length, Capability Value>, where each triple is encoded as
 shown below:
  0                   1                   2
  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
 +---------------+---------------+--------------+----------------+
 |       Capability Code         |       Capability Length       |
 +---------------+---------------+--------------+----------------+
 |       Capability Value (variable)...
 +---------------+---------------+--------------+----------------+
         Figure 5:  Capability Optional Parameter
 Capability Code:
 Capability Code is a 2-octet field that unambiguously identifies
 individual capabilities.
 Capability Length:
 Capability Length is a 2-octet field that contains the length of the
 Capability Value field in octets.
 Capability Value:
 Capability Value is a variable length field that is interpreted
 according to the value of the Capability Code field.
 Any particular capability, as identified by its Capability Code, may
 appear more than once within the Optional Parameter.
 This document reserves Capability Codes 32768-65535 for vendor-
 specific applications (these are the codes with the first bit of the
 code value equal to 1).  This document reserves value 0.  Capability
 Codes (other than those reserved for vendor specific use) are
 controlled by IANA.  See Section 13 for IANA considerations.

Rosenberg, et. al. Standards Track [Page 13] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 The following Capability Codes are defined by this specification:
    Code           Capability
    1              Route Types Supported
    2              Send Receive Capability

4.2.1.1.1. Route Types Supported

 The Route Types Supported Capability Code lists the route types
 supported in this peering session by the transmitting LS.  An LS MUST
 NOT use route types that are not supported by the peer LS in any
 particular peering session.  If the route types supported by a peer
 are not satisfactory, an LS SHOULD terminate the peering session.
 The format for a Route Type is:
  0                   1                   2
  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      |
 +---------------+---------------+--------------+----------------+
          Figure 6: Route Types Supported Capability
 The Address Family and Application Protocol are as defined in Section
 5.1.1.  Address Family gives the address family being routed (within
 the ReachableRoutes attribute).  The application protocol lists the
 application for which the routes apply.  As an example, a route type
 for TRIP could be <E.164, SIP>, indicating a set of E.164
 destinations for the SIP protocol.
 The Route Types Supported Capability MAY contain multiple route types
 in the capability.  The number of route types within the capability
 is the maximum number that can fit given the capability length.  The
 Capability Code is 1 and the length is variable.

4.2.1.1.2. Send Receive Capability

 This capability specifies the mode in which the LS will operate with
 this particular peer.  The possible modes are: Send Only mode,
 Receive Only mode, or Send Receive mode.  The default mode is Send
 Receive mode.
 In Send Only mode, an LS transmits UPDATE messages to its peer, but
 the peer MUST NOT transmit UPDATE messages to that LS.  If an LS in
 Send Only mode receives an UPDATE message from its peer, it MUST
 discard that message, but no further action should be taken.

Rosenberg, et. al. Standards Track [Page 14] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 The UPDATE messages sent by an LS in Send Only mode to its intra-
 domain peer MUST include the ITAD Topology attribute whenever the
 topology changes.  A useful application of an LS in Send Only mode
 with an external peer is to enable gateway registration services.
 If a service provider terminates calls to a set of gateways it owns,
 but never initiates calls, it can set its LSs to operate in Send Only
 mode, since they only ever need to generate UPDATE messages, not
 receive them.  If an LS in Send Receive mode has a peering session
 with a peer in Send Only mode, that LS MUST set its route
 dissemination policy such that it does not send any UPDATE messages
 to its peer.
 In Receive Only mode, the LS acts as a passive TRIP listener.  It
 receives and processes UPDATE messages from its peer, but it MUST NOT
 transmit any UPDATE messages to its peer.  This is useful for
 management stations that wish to collect topology information for
 display purposes.
 The behavior of an LS in Send Receive mode is the default TRIP
 operation specified throughout this document.
 The Send Receive capability is a 4-octet unsigned numeric value.  It
 can only take one of the following three values:
    1 - Send Receive mode
    2 - Send only mode
    3 - Receive Only mode
 A peering session MUST NOT be established between two LSs if both of
 them are  in Send Only mode or if both of them are in Receive Only
 mode.  If a peer LS detects such a capability mismatch when
 processing an OPEN message, it MUST respond with a NOTIFICATION
 message and close the peer session.  The error code in the
 NOTIFICATION message must be set to "Capability Mismatch."
 An LS MUST be configured in the same Send Receive mode for all peers.

4.3. UPDATE Message Format

 UPDATE messages are used to transfer routing information between LSs.
 The information in the UPDATE packet can be used to construct a graph
 describing the relationships between the various ITADs.  By applying
 rules to be discussed, routing information loops and some other
 anomalies can be prevented.

Rosenberg, et. al. Standards Track [Page 15] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 An UPDATE message is used to both advertise and withdraw routes from
 service.  An UPDATE message may simultaneously advertise and withdraw
 TRIP routes.
 In addition to the TRIP header, the TRIP UPDATE contains a list of
 routing attributes as shown in Figure 7.  There is no padding between
 routing attributes.
       +------------------------------------------------+--...
       | First Route Attribute | Second Route Attribute |  ...
       +------------------------------------------------+--...
                  Figure 7: TRIP UPDATE Format
 The minimum length of an UPDATE message is 3 octets (there are no
 mandatory attributes in TRIP).

4.3.1. Routing Attributes

 A variable length sequence of routing attributes is present in every
 UPDATE message.  Each attribute is a triple <attribute type,
 attribute length, attribute value> of variable length.
     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
    +---------------+---------------+--------------+----------------+
    |  Attr. Flags  |Attr. Type Code|         Attr. Length          |
    +---------------+---------------+--------------+----------------+
    |                   Attribute Value (variable)                  |
    +---------------+---------------+--------------+----------------+
                  Figure 8: Routing Attribute Format
 Attribute Type is a two-octet field that consists of the Attribute
 Flags octet followed by the Attribute Type Code octet.
 The Attribute Type Code defines the type of attribute.  The basic
 TRIP-defined Attribute Type Codes are discussed later in this
 section.  Attributes MUST appear in the UPDATE message in numerical
 order of the Attribute Type Code.  An attribute MUST NOT be included
 more than once in the same UPDATE message.  Attribute Flags are used
 to control attribute processing when the attribute type is unknown.
 Attribute Flags are further defined in Section 4.3.2.

Rosenberg, et. al. Standards Track [Page 16] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 This document reserves Attribute Type Codes 224-255 for vendor-
 specific applications (these are the codes with the first three bits
 of the code equal to 1).  This document reserves value 0.  Attribute
 Type Codes (other than those reserved for vendor specific use) are
 controlled by IANA.  See Section 13 for IANA considerations.
 The third and the fourth octets of the route attribute contain the
 length of the attribute value field in octets.
 The remaining octets of the attribute represent the Attribute Value
 and are interpreted according to the Attribute Flags and the
 Attribute Type Code.  The basic supported attribute types, their
 values, and their uses are defined in this specification.  These are
 the attributes necessary for proper loop free operation of TRIP, both
 inter-domain and intra-domain.  Additional attributes may be defined
 in future documents.

4.3.2. Attribute Flags

 It is clear that the set of attributes for TRIP will evolve over
 time.  Hence it is essential that mechanisms be provided to handle
 attributes with unrecognized types.  The handling of unrecognized
 attributes is controlled via the flags field of the attribute.
 Recognized attributes should be processed according to their specific
 definition.
 The following are the attribute flags defined by this specification:
          Bit       Flag
          0         Well-Known Flag
          1         Transitive Flag
          2         Dependent Flag
          3         Partial Flag
          4         Link-state Encapsulated Flag
 The high-order bit (bit 0) of the Attribute Flags octet is the Well-
 Known Bit.  It defines whether the attribute is not well-known (if
 set to 1) or well-known (if set to 0).  Implementations are not
 required to support not well-known attributes, but MUST support
 well-known attributes.
 The second high-order bit (bit 1) of the Attribute Flags octet is the
 Transitive bit.  It defines whether a not well-known attribute is
 transitive (if set to 1) or non-transitive (if set to 0).  For well-
 known attributes, the Transitive bit MUST be zero on transmit and
 MUST be ignored on receipt.
 The third high-order bit (bit 2) of the Attribute Flags octet is the
 Dependent bit.  It defines whether a transitive attribute is

Rosenberg, et. al. Standards Track [Page 17] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 dependent (if set to 1) or independent (if set to 0).  For well-known
 attributes and for non-transitive attributes, the Dependent bit is
 irrelevant, and MUST be set to zero on transmit and MUST be ignored
 on receipt.
 The fourth high-order bit (bit 3) of the Attribute Flags octet is the
 Partial bit.  It defines whether the information contained in the not
 well-known transitive attribute is partial (if set to 1) or complete
 (if set to 0).  For well-known attributes and for non-transitive
 attributes the Partial bit MUST be set to 0 on transmit and MUST be
 ignored on receipt.
 The fifth high-order bit (bit 4) of the Attribute Flags octet is the
 Link-state Encapsulation bit.  This bit is only applicable to certain
 attributes (ReachableRoutes and WithdrawnRoutes) and determines the
 encapsulation of the routes within those attributes.  If this bit is
 set, link-state encapsulation is used within the attribute.
 Otherwise, standard encapsulation is used within the attribute.  The
 Link-state Encapsulation technique is described in Section 4.3.2.4.
 This flag is only valid on the ReachableRoutes and WithdrawnRoutes
 attributes.  It MUST be cleared on transmit and MUST be ignored on
 receipt for all other attributes.
 The other bits of the Attribute Flags octet are unused.  They MUST be
 zeroed on transmit and ignored on receipt.

4.3.2.1. Attribute Flags and Route Selection

 Any recognized attribute can be used as input to the route selection
 process, although the utility of some attributes in route selection
 is minimal.

4.3.2.2. Attribute Flags and Route Dissemination

 TRIP provides for two variations of transitivity due to the fact that
 intermediate LSs need not modify the NextHopServer when propagating
 routes.  Attributes may be non-transitive, dependent transitive, or
 independent transitive.  An attribute cannot be both dependent
 transitive and independent transitive.
 Unrecognized independent transitive attributes may be propagated by
 any intermediate LS.  Unrecognized dependent transitive attributes
 MAY only be propagated if the LS is NOT changing the next-hop server.
 The transitivity variations permit some unrecognized attributes to be
 carried end-to-end (independent transitive), some to be carried
 between adjacent next-hop servers (dependent transitive), and other
 to be restricted to peer LSs (non-transitive).

Rosenberg, et. al. Standards Track [Page 18] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 An LS that passes an unrecognized transitive attribute to a peer MUST
 set the Partial flag on that attribute.  Any LS along a path MAY
 insert a transitive attribute into a route.  If any LS except the
 originating LS inserts a new independent transitive attribute into a
 route, then it MUST set the Partial flag on that attribute.  If any
 LS except an LS that modifies the NextHopServer inserts a new
 dependent transitive attribute into a route, then it MUST set the
 Partial flag on that attribute.  The Partial flag indicates that not
 every LS along the relevant path has processed and understood the
 attribute.  For independent transitive attributes, the "relevant
 path" is the path given in the AdvertisementPath attribute.  For
 dependent transitive attributes, the relevant path consists only of
 those domains thru which this object has passed since the
 NextHopServer was last modified.  The Partial flag in an independent
 transitive attribute MUST NOT be unset by any other LS along the
 path.  The Partial flag in a dependent transitive attribute MUST be
 reset whenever the NextHopServer is changed, but MUST NOT be unset by
 any LS that is not changing the NextHopServer.
 The rules governing the addition of new non-transitive attributes are
 defined independently for each non-transitive attribute.  Any
 attribute MAY be updated by an LS in the path.

4.3.2.3. Attribute Flags and Route Aggregation

 Each attribute defines how it is to be handled during route
 aggregation.
 The rules governing the handling of unknown attributes are guided by
 the Attribute Flags.  Unrecognized transitive attributes are dropped
 during aggregation.  There should be no unrecognized non-transitive
 attributes during aggregation because non-transitive attributes must
 be processed by the local LS in order to be propagated.

4.3.2.4. Attribute Flags and Encapsulation

 Normally attributes have the simple format as described in Section
 4.3.1.  If the Link-state Encapsulation Flag is set, then the two
 additional fields are added to the attribute header as shown in
 Figure 9.

Rosenberg, et. al. Standards Track [Page 19] RFC 3219 Telephony Routing over IP (TRIP) January 2002

  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
 +---------------+---------------+--------------+----------------+
 |  Attr. Flags  |Attr. Type Code|          Attr. Length         |
 +---------------+---------------+--------------+----------------+
 |                  Originator TRIP Identifier                   |
 +---------------+---------------+--------------+----------------+
 |                        Sequence Number                        |
 +---------------+---------------+--------------+----------------+
 |                   Attribute Value (variable)                  |
 +---------------+---------------+--------------+----------------+
               Figure 9: Link State Encapsulation
 The Originator TRIP ID and Sequence Number are used to control the
 flooding of routing updates within a collection of servers.  These
 fields are used to detect duplicate and old routes so that they are
 not further propagated to other LSs.  The use of these fields is
 defined in Section 10.1.

4.3.3. Mandatory Attributes

 There are no Mandatory attributes in TRIP.  However, there are
 Conditional Mandatory attributes.  A conditional mandatory attribute
 is an attribute, which MUST be included in an UPDATE message if
 another attribute is included in that message.  For example, if an
 UPDATE message includes a ReachableRoutes attribute, it MUST include
 an AdvertisementPath attribute as well.
 The three base attributes in TRIP are WithdrawnRoutes,
 ReachableRoutes, and ITAD Topology.  Their presence in an UPDATE
 message is entirely optional and independent of any other attributes.

4.3.4. TRIP UPDATE Attributes

 This section summarizes the attributes that may be carried in an
 UPDATE message.  Attributes MUST appear in the UPDATE message in
 increasing order of the Attribute Type Code.  Additional details are
 provided in Section 5.

4.3.4.1. WithdrawnRoutes

 This attribute lists a set of routes that are being withdrawn from
 service.  The transmitting LS has determined that these routes should
 no longer be advertised, and is propagating this information to its
 peers.

Rosenberg, et. al. Standards Track [Page 20] RFC 3219 Telephony Routing over IP (TRIP) January 2002

4.3.4.2. ReachableRoutes

 This attribute lists a set of routes that are being added to service.
 These routes will have the potential to be inserted into the Adj-
 TRIBs-In of the receiving LS and the route selection process will be
 applied to them.

4.3.4.3. NextHopServer

 This attribute gives the identity of the entity to which messages
 should be sent along this routed path.  It specifies the identity of
 the next hop server as either a host domain name or an IP address.
 It MAY optionally specify the UDP/TCP port number for the next hop
 signaling server.  If not specified, then the default port SHOULD be
 used.  The NextHopServer is specific to the set of destinations and
 application protocol defined in the ReachableRoutes attribute.  Note
 that this is NOT necessarily the address to which media (voice,
 video, etc.)  should be transmitted, it is only for the application
 protocol as given in the ReachableRoutes attribute.

4.3.4.4. AdvertisementPath

 The AdvertisementPath is analogous to the AS_PATH in BGP4 [3].  The
 attribute records the sequence of domains through which this
 advertisement has passed.  The attribute is used to detect when the
 routing advertisement is looping.  This attribute does NOT reflect
 the path through which messages following this route would traverse.
 Since the next-hop need not be modified by each LS, the actual path
 to the destination might not have to traverse every domain in the
 AdvertisementPath.

4.3.4.5. RoutedPath

 The RoutedPath attribute is analogous to the AdvertisementPath
 attribute, except that it records the actual path (given by the list
 of domains) *to* the destinations.  Unlike AdvertisementPath, which
 is modified each time the route is propagated, RoutedPath is only
 modified when the NextHopServer attribute changes.  Thus, it records
 the subset of the AdvertisementPath which signaling messages
 following this particular route would traverse.

4.3.4.6. AtomicAggregate

 The AtomicAggregate attribute indicates that a route may actually
 include domains not listed in the RoutedPath.  If an LS, when
 presented with a set of overlapping routes from a peer LS, selects a
 less specific route without selecting the more specific route, then
 the LS MUST include the AtomicAggregate attribute with the route.  An

Rosenberg, et. al. Standards Track [Page 21] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 LS receiving a route with an AtomicAggregate attribute MUST NOT make
 the set of destinations more specific when advertising it to other
 LSs.

4.3.4.7. LocalPreference

 The LocalPreference attribute is an intra-domain attribute used to
 inform other LSs of the local LS's preference for a given route.  The
 preference of a route is calculated at the ingress to a domain and
 passed as an attribute with that route throughout the domain.  Other
 LSs within the same ITAD use this attribute in their route selection
 process.  This attribute has no significance between domains.

4.3.4.8. MultiExitDisc

 There may be more than one LS peering relationship between
 neighboring domains.  The MultiExitDisc attribute is used by an LS to
 express a preference for one link between the domains over another
 link between the domains.  The use of the MultiExitDisc attribute is
 controlled by local policy.

4.3.4.9. Communities

 The Communities attribute is not a well-known attribute.  It is used
 to facilitate and simplify the control of routing information by
 grouping destinations into communities.

4.3.4.10. ITAD Topology

 The ITAD topology attribute is an intra-domain attribute that is used
 by LSs to indicate their intra-domain topology to other LSs in the
 domain.

4.3.4.11. ConvertedRoute

 The ConvertedRoute attribute indicates that an intermediate LS has
 altered the route by changing the route's Application Protocol.

4.4. KEEPALIVE Message Format

 TRIP does not use any transport-based keep-alive mechanism to
 determine if peers are reachable.  Instead, KEEPALIVE messages are
 exchanged between peers often enough as not to cause the Hold Timer
 to expire.  A reasonable maximum time between KEEPALIVE messages
 would be one third of the Hold Time interval.  KEEPALIVE messages
 MUST NOT be sent more than once every 3 seconds.  An implementation
 SHOULD adjust the rate at which it sends KEEPALIVE messages as a
 function of the negotiated Hold Time interval.

Rosenberg, et. al. Standards Track [Page 22] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
 messages MUST NOT be sent.
 The KEEPALIVE message consists of only a message header and has a
 length of 3 octets.

4.5. NOTIFICATION Message Format

 A NOTIFICATION message is sent when an error condition is detected.
 The TRIP transport connection is closed immediately after sending a
 NOTIFICATION message.
 In addition to the fixed-size TRIP header, the NOTIFICATION message
 contains the following fields:
  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
 +---------------+---------------+--------------+----------------+
 |  Error Code   | Error Subcode |       Data... (variable)
 +---------------+---------------+--------------+----------------+
              Figure 10: TRIP NOTIFICATION Format
 Error Code:
 This 1-octet unsigned integer indicates the type of NOTIFICATION.
 The following Error Codes have been defined:
 Error Code       Symbolic Name               Reference
   1         Message Header Error             Section 6.1
   2         OPEN Message Error               Section 6.2
   3         UPDATE Message Error             Section 6.3
   4         Hold Timer Expired               Section 6.5
   5         Finite State Machine Error       Section 6.6
   6         Cease                            Section 6.7
 Error Subcode:
 This 1-octet unsigned integer provides more specific information
 about the nature of the reported error.  Each Error Code may have one
 or more Error Subcodes associated with it.  If no appropriate Error
 Subcode is defined, then a zero (Unspecific) value is used for the
 Error Subcode field.
 Message Header Error Subcodes:
    1  - Bad Message Length.
    2  - Bad Message Type.

Rosenberg, et. al. Standards Track [Page 23] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 OPEN Message Error Subcodes:
    1  - Unsupported Version Number.
    2  - Bad Peer ITAD.
    3  - Bad TRIP Identifier.
    4  - Unsupported Optional Parameter.
    5  - Unacceptable Hold Time.
    6  - Unsupported Capability.
    7  - Capability Mismatch.
 UPDATE Message Error Subcodes:
    1 - Malformed Attribute List.
    2 - Unrecognized Well-known Attribute.
    3 - Missing Well-known Mandatory Attribute.
    4 - Attribute Flags Error.
    5 - Attribute Length Error.
    6 - Invalid Attribute.
 Data:
 This variable-length field is used to diagnose the reason for the
 NOTIFICATION.  The contents of the Data field depend upon the Error
 Code and Error Subcode.
 Note that the length of the data can be determined from the message
 length field by the formula:
          Data Length = Message Length - 5
 The minimum length of the NOTIFICATION message is 5 octets (including
 message header).

5. TRIP Attributes

 This section provides details on the syntax and semantics of each
 TRIP UPDATE attribute.

5.1. WithdrawnRoutes

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: Link-State Encapsulation (when flooding).
 TRIP Type Code: 1
 The WithdrawnRoutes specifies a set of routes that are to be removed
 from service by the receiving LS(s).  The set of routes MAY be empty,
 indicated by a length field of zero.

Rosenberg, et. al. Standards Track [Page 24] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.1.1. Syntax of WithdrawnRoutes

 The WithdrawnRoutes Attribute encodes a sequence of routes in its
 value field.  The format for individual routes is given in Section
 5.1.1.1.  The WithdrawnRoutes Attribute lists the individual routes
 sequentially with no padding as shown in Figure 11.  Each route
 includes a length field so that the individual routes within the
 attribute can be delineated.
          +---------------------+---------------------+...
          |  WithdrawnRoute1... |  WithdrawnRoute2... |...
          +---------------------+---------------------+...
               Figure 11: WithdrawnRoutes Format

5.1.1.1. Generic TRIP Route Format

 The generic format for a TRIP route is given in Figure 12.
  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 12: Generic TRIP Route Format
 Address Family:
 The address family field gives the type of address for the route.
 Three address families are defined in this Section:
          Code              Address Family
          1                 Decimal Routing Numbers
          2                 PentaDecimal Routing Numbers
          3                 E.164 Numbers
 This document reserves address family code 0.  This document reserves
 address family codes 32768-65535 for vendor-specific applications
 (these are the codes with the first bit of the code value equal to
 1).  Additional address families may be defined in the future.
 Assignment of address family codes is controlled by IANA.  See
 Section 13 for IANA considerations.

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 Application Protocol:
 The application protocol gives the protocol for which this routing
 table is maintained.  The currently defined application protocols
 are:
          Code              Protocol
          1                 SIP
          2                 H.323-H.225.0-Q.931
          3                 H.323-H.225.0-RAS
          4                 H.323-H.225.0-Annex-G
 This document reserves application protocol code 0.  This document
 reserves application protocol codes 32768-65535 for vendor-specific
 applications (these are the codes with the first bit of the code
 value equal to 1).  Additional application protocols may be defined
 in the future.  Assignment of application protocol codes is
 controlled by IANA.  See Section 13 for IANA considerations.
 Length:
 The length of the address field, in bytes.
 Address:
 This is an address (prefix) of the family type given by Address
 Family.  The octet length of the address is variable and is
 determined by the length field of the route.

5.1.1.2. Decimal Routing Numbers

 The Decimal Routing Numbers address family is a super set of all
 E.164 numbers, national numbers, local numbers, and private numbers.
 It can also be used to represent the decimal routing numbers used in
 conjunction with Number Portability in some countries/regions.  A set
 of telephone numbers is specified by a Decimal Routing Number prefix.
 Decimal Routing Number prefixes are represented by a string of
 digits, each digit encoded by its ASCII character representation.
 This routing object covers all phone numbers starting with this
 prefix.  The syntax for the Decimal Routing Number prefix is:
    Decimal-routing-number  = *decimal-digit
    decimal-digit           = DECIMAL-DIGIT
    DECIMAL-DIGIT           = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
 This DECIMAL Routing Number prefix is not bound in length.  This
 format is similar to the format for a global telephone number as
 defined in SIP [8] without visual separators and without the "+"
 prefix for international numbers.  This format facilitates efficient
 comparison when using TRIP to route SIP or H323, both of which use
 character based representations of phone numbers.  The prefix length

Rosenberg, et. al. Standards Track [Page 26] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 is determined from the length field of the route.  The type of
 Decimal Routing Number (private, local, national, or international)
 can be deduced from the first few digits of the prefix.

5.1.1.3. PentaDecimal Routing Numbers

 This address family is used to represent PentaDecimal Routing Numbers
 used in conjunction with Number Portability in some
 countries/regions.  PentaDecimal Routing Number prefixes are
 represented by a string of digits, each digit encoded by its ASCII
 character representation.  This routing object covers all routing
 numbers starting with this prefix.  The syntax for the PentaDecimal
 Routing Number prefix is:
    PentaDecimal-routing-number   = *pentadecimal-digit
    pentadecimal-routing-digit    = PENTADECIMAL-DIGIT
    PENTADECIMAL-DIGIT            = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|
                                    "8"|"9"|"A"|"B"|"C"|"D"|"E"
 Note the difference in alphabets between Decimal Routing Numbers and
 PentaDecimal Routing Numbers.  A PentaDecimal Routing Number prefix
 is not bound in length.
 Note that the address family, which suits the routing numbers of a
 specific country/region depends on the alphabets used for routing
 numbers in that country/region.  For example, North American routing
 numbers SHOULD use the Decimal Routing Numbers address family,
 because their alphabet is limited to the digits "0" through "9".
 Another example, in most European countries routing numbers use the
 alphabet "0" through "9" and "A" through "E", and hence these
 countries SHOULD use the PentaDecimal Routing Numbers address family.

5.1.1.4. E.164 Numbers

 The E.164 Numbers address family is dedicated to fully qualified
 E.164 numbers.  A set of telephone numbers is specified by a E.164
 prefix.  E.164 prefixes are represented by a string of digits, each
 digit encoded by its ASCII character representation.  This routing
 object covers all phone numbers starting with this prefix.  The
 syntax for the E.164 prefix is:
    E164-number          = *e164-digit
    E164-digit           = E164-DIGIT
    E164-DIGIT           = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"

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 This format facilitates efficient comparison when using TRIP to route
 SIP or H323, both of which use character based representations of
 phone numbers.  The prefix length is determined from the length field
 of the route.
 The E.164 Numbers address family and the Decimal Routing Numbers
 address family have the same alphabet.  The E.164 Numbers address
 family SHOULD be used whenever possible.  The Decimal Routing Numbers
 address family can be used in case of private numbering plans or
 applications that do not desire to advertise fully expanded, fully
 qualified telephone numbers.  If Decimal Routing Numbers are used to
 advertise non-fully qualified prefixes, the prefixes may have to be
 manipulated (e.g. expanded) at the boundary between ITADs.  This adds
 significant complexity to the ITAD-Border LS, because, it has to map
 the prefixes from the format used in its own ITAD to the format used
 in the peer ITAD.

5.2. ReachableRoutes

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: Link-State Encapsulation (when flooding).
 TRIP Type Code: 2
 The ReachableRoutes attribute specifies a set of routes that are to
 be added to service by the receiving LS(s).  The set of routes MAY be
 empty, as indicated by setting the length field to zero.

5.2.1. Syntax of ReachableRoutes

 The ReachableRoutes Attribute has the same syntax as the
 WithdrawnRoutes Attribute.  See Section 5.1.1.

5.2.2. Route Origination and ReachableRoutes

 Routes are injected into TRIP by a method outside the scope of this
 specification.  Possible methods include a front-end protocol, an
 intra-domain routing protocol, or static configuration.

5.2.3. Route Selection and ReachableRoutes

 The routes in ReachableRoutes are necessary for route selection.

5.2.4. Aggregation and ReachableRoutes

 To aggregate multiple routes, the set of ReachableRoutes to be
 aggregated MUST combine to form a less specific set.

Rosenberg, et. al. Standards Track [Page 28] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 There is no mechanism within TRIP to communicate that a particular
 address prefix is not used and thus that these addresses could be
 skipped during aggregation.  LSs MAY use methods outside of TRIP to
 learn of invalid prefixes that may be ignored during aggregation.
 If an LS advertises an aggregated route, it MUST include the
 AtomicAggregate attribute.

5.2.5. Route Dissemination and ReachableRoutes

 The ReachableRoutes attribute is recomputed at each LS except where
 flooding is being used (e.g., within a domain).  It is therefore
 possible for an LS to change the Application Protocol field of a
 route before advertising that route to an external peer.
 If an LS changes the Application Protocol of a route it advertises,
 it MUST include the ConvertedRoute attribute in the UPDATE message.

5.2.6. Aggregation Specifics for Decimal Routing Numbers, E.164 Numbers,

     and PentaDecimal Routing Numbers
 An LS that has routes to all valid numbers in a specific prefix
 SHOULD advertise that prefix as the ReachableRoutes, even if there
 are more specific prefixes that do not actually exist on the PSTN.
 Generally, it takes 10 Decimal Routing/E.164 prefixes, or 15
 PentaDecimal Routing prefixes, of length n to aggregate into a prefix
 of length n-1.  However, if an LS is aware that a prefix is an
 invalid Decimal Routing/E.164 prefix, or PentaDecimal Routing prefix,
 then the LS MAY aggregate by skipping this prefix.  For example, if
 the Decimal Routing prefix 19191 is known not to exist, then an LS
 can aggregate to 1919 without 19191.  A prefix representing an
 invalid set of PSTN destinations is sometimes referred to as a
 "black-hole."  The method by which an LS is aware of black-holes is
 not within the scope of TRIP, but if an LS has such knowledge, it can
 use the knowledge when aggregating.

5.3. NextHopServer

 Conditional Mandatory: True (if ReachableRoutes and/or
 WithdrawnRoutes attribute is present).
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 3.

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 Given a route with application protocol A and destinations D, the
 NextHopServer indicates to the next-hop that messages of protocol A
 destined for D should be sent to it.  This may or may not represent
 the ultimate destination of those messages.

5.3.1. NextHopServer Syntax

 For generality, the address of the next-hop server may be of various
 types (domain name, IPv4, IPv6, etc).  The NextHopServer attribute
 includes the ITAD number of next-hop server, a length field, and a
 next-hop name or address.
 The syntax for the NextHopServer is given in Figure 13.
  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
 +---------------+---------------+--------------+----------------+
 |                         Next Hop ITAD                         |
 +---------------+---------------+--------------+----------------+
 |             Length            |         Server (variable)    ...
 +---------------+---------------+--------------+----------------+
                Figure 13: NextHopServer Syntax
 The Next-Hop ITAD indicates the domain of the next-hop.  Length field
 gives the number of octets in the Server field, and the Server field
 contains the name or address of the next-hop server.  The server
 field is represented as a string of ASCII characters.  It is defined
 as follows:
 Server  = host [":" port ]
 host    = <   A legal Internet host domain name
            or an IPv4 address using the textual representation
               defined in Section 2.1 of RFC 1123 [9]
            or an IPv6 address using the textual representation
               defined in Section 2.2 of RFC 2373 [10].  The IPv6
               address MUST be enclosed in "[" and "]"
               characters.>
 port    = *DIGIT
 If the port is empty or not given, the default port is assumed (e.g.,
 port 5060 if the application protocol is SIP).

5.3.2. Route Origination and NextHopServer

 When an LS originates a routing object into TRIP, it MUST include a
 NextHopServer within its domain.  The NextHopServer could be an
 address of the egress gateway or of a signaling proxy.

Rosenberg, et. al. Standards Track [Page 30] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.3.3. Route Selection and NextHopServer

 LS policy may prefer certain next-hops or next-hop domains over
 others.

5.3.4. Aggregation and NextHopServer

 When aggregating multiple routing objects into a single routing
 object, an LS MUST insert a new signaling server from within its
 domain as the new NextHopServer unless all of the routes being
 aggregated have the same next-hop.

5.3.5. Route Dissemination and NextHopServer

 When propagating routing objects to peers, an LS may choose to insert
 a signaling proxy within its domain as the new next-hop, or it may
 leave the next-hop unchanged.  Inserting a new next-hop will cause
 the signaling messages to be sent to that address, and will provide
 finer control over the signaling path.  Leaving the next-hop
 unchanged will yield a more efficient signaling path (fewer hops).
 It is a local policy decision of the LS to decide whether to
 propagate or change the NextHopServer.

5.4. AdvertisementPath

 Conditional Mandatory: True (if ReachableRoutes and/or
 WithdrawnRoutes attribute is present).
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 4.
 This attribute identifies the ITADs through which routing information
 carried in an advertisement has passed.  The AdvertisementPath
 attribute is analogous to the AS_PATH attribute in BGP.  The
 attributes differ in that BGP's AS_PATH also reflects the path to the
 destination.  In TRIP, not every domain need modify the next-hop, so
 the AdvertisementPath may include many more hops than the actual path
 to the destination.  The RoutedPath attribute (Section 5.5) reflects
 the actual signaling path to the destination.

5.4.1. AdvertisementPath Syntax

 AdvertisementPath is a variable length attribute that is composed of
 a sequence of ITAD path segments.  Each ITAD path segment is
 represented by a type-length-value triple.
 The path segment type is a 1-octet long field with the following
 values defined:

Rosenberg, et. al. Standards Track [Page 31] RFC 3219 Telephony Routing over IP (TRIP) January 2002

    Value      Segment Type
    1          AP_SET: unordered set of ITADs a route in the
               advertisement message has traversed
    2          AP_SEQUENCE: ordered set of ITADs a route in
               the advertisement message has traversed
 The path segment length is a 1-octet long field containing the number
 of ITADs in the path segment value field.
 The path segment value field contains one or more ITAD numbers, each
 encoded as a 4-octets long field.  ITAD numbers uniquely identify an
 Internet Telephony Administrative Domain, and must be obtained from
 IANA.  See Section 13 for procedures to obtain an ITAD number from
 IANA.

5.4.2. Route Origination and AdvertisementPath

 When an LS originates a route then:
  1. The originating LS shall include its own ITAD number in the

AdvertisementPath attribute of all advertisements sent to LSs

       located in neighboring ITADs.  In this case, the ITAD number of
       the originating LS's ITAD will be the only entry in the
       AdvertisementPath attribute.
    -  The originating LS shall include an empty AdvertisementPath
       attribute in all advertisements sent to LSs located in its own
       ITAD.  An empty AdvertisementPath attribute is one whose length
       field contains the value zero.

5.4.3. Route Selection and AdvertisementPath

 The AdvertisementPath may be used for route selection.  Possible
 criteria to be used are the number of hops on the path and the
 presence or absence of particular ITADs on the path.
 As discussed in Section 10, the AdvertisementPath is used to prevent
 routing information from looping.  If an LS receives a route with its
 own ITAD already in the AdvertisementPath, the route MUST be
 discarded.

5.4.4. Aggregation and AdvertisementPath

 The rules for aggregating AdvertisementPath attributes are given in
 the following sections, where the term "path" used in Section 5.4.4.1
 and 5.4.4.2 is understood to mean AdvertisementPath.

Rosenberg, et. al. Standards Track [Page 32] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.4.4.1. Aggregating Routes with Identical Paths

 If all routes to be aggregated have identical path attributes, then
 the aggregated route has the same path attribute as the individual
 routes.

5.4.4.2. Aggregating Routes with Different Paths

 For the purpose of aggregating path attributes we model each ITAD
 within the path as a pair <type, value>, where "type" identifies a
 type of the path segment (AP_SEQUENCE or AP_SET), and "value" is the
 ITAD number.  Two ITADs are said to be the same if their
 corresponding <type, value> are the same.
 If the routes to be aggregated have different path attributes, then
 the aggregated path attribute shall satisfy all of the following
 conditions:
  1. All pairs of the type AP_SEQUENCE in the aggregated path MUST

appear in all of the paths of routes to be aggregated.

  1. All pairs of the type AP_SET in the aggregated path MUST appear

in at least one of the paths of the initial set (they may

       appear as either AP_SET or AP_SEQUENCE types).
    -  For any pair X of the type AP_SEQUENCE that precedes pair Y in
       the aggregated path, X precedes Y in each path of the initial
       set that contains Y, regardless of the type of Y.
    -  No pair with the same value shall appear more than once in the
       aggregated path, regardless of the pair's type.
 An implementation may choose any algorithm that conforms to these
 rules.  At a minimum, a conformant implementation MUST be able to
 perform the following algorithm that meets all of the above
 conditions:
  1. Determine the longest leading sequence of tuples (as defined

above) common to all the paths of the routes to be aggregated.

       Make this sequence the leading sequence of the aggregated path.
    -  Set the type of the rest of the tuples from the paths of the
       routes to be aggregated to AP_SET, and append them to the
       aggregated path.
    -  If the aggregated path has more than one tuple with the same
       value (regardless of tuple's type), eliminate all but one such
       tuple by deleting tuples of the type AP_SET from the aggregated
       path.
 An implementation that chooses to provide a path aggregation
 algorithm that retains significant amounts of path information may
 wish to use the procedure of Section 5.4.4.3.

Rosenberg, et. al. Standards Track [Page 33] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.4.4.3. Example Path Aggregation Algorithm

 An example algorithm to aggregate two paths works as follows:
  1. Identify the ITADs (as defined in Section 5.4.1) within each

path attribute that are in the same relative order within both

       path attributes.  Two ITADs, X and Y, are said to be in the
       same order if either X precedes Y in both paths, or if Y
       precedes X in both paths.
    -  The aggregated path consists of ITADs identified in (a) in
       exactly the same order as they appear in the paths to be
       aggregated.  If two consecutive ITADs identified in (a) do not
       immediately follow each other in both of the paths to be
       aggregated, then the intervening ITADs (ITADs that are between
       the two consecutive ITADs that are the same) in both attributes
       are combined into an AP_SET path segment that consists of the
       intervening ITADs from both paths; this segment is then placed
       in between the two consecutive ITADs identified in (a) of the
       aggregated attribute.  If two consecutive ITADs identified in
       (a) immediately follow each other in one attribute, but do not
       follow in another, then the intervening ITADs of the latter are
       combined into an AP_SET path segment; this segment is then
       placed in between the two consecutive ITADs identified in (a)
       of the aggregated path.
 If as a result of the above procedure a given ITAD number appears
 more than once within the aggregated path, all but the last instance
 (rightmost occurrence) of that ITAD number should be removed from the
 aggregated path.

5.4.5. Route Dissemination and AdvertisementPath

 When an LS propagates a route which it has learned from another LS,
 it shall modify the route's AdvertisementPath attribute based on the
 location of the LS to which the route will be sent.
  1. When a LS advertises a route to another LS located in its own

ITAD, the advertising LS MUST NOT modify the AdvertisementPath

       attribute associated with the route.
    -  When a LS advertises a route to an LS located in a neighboring
       ITAD, then the advertising LS MUST update the AdvertisementPath
       attribute as follows:

Rosenberg, et. al. Standards Track [Page 34] RFC 3219 Telephony Routing over IP (TRIP) January 2002

  • If the first path segment of the AdvertisementPath is of

type AP_SEQUENCE, the local system shall prepend its own

          ITAD number as the last element of the sequence (put it in
          the leftmost position).
  • If the first path segment of the AdvertisementPath is of

type AP_SET, the local system shall prepend a new path

          segment of type AP_SEQUENCE to the AdvertisementPath,
          including its own ITAD number in that segment.

5.5. RoutedPath

 Conditional Mandatory: True
 (if ReachableRoutes attribute is present).
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 5.
 This attribute identifies the ITADs through which messages sent using
 this route would pass.  The ITADs in this path are a subset of those
 in the AdvertisementPath.

5.5.1. RoutedPath Syntax

 The syntax of the RoutedPath attribute is the same as that of the
 AdvertisementPath attribute.  See Section 5.4.1.

5.5.2. Route Origination and RoutedPath

 When an LS originates a route it MUST include the RoutedPath
 attribute.
  1. The originating LS shall include its own ITAD number in the

RoutedPath attribute of all advertisements sent to LSs located

       in neighboring ITADs.  In this case, the ITAD number of the
       originating LS's ITAD will be the only entry in the RoutedPath
       attribute.
    -  The originating LS shall include an empty RoutedPath attribute
       in all advertisements sent to LSs located in its own ITAD.  An
       empty RoutedPath attribute is one whose length field contains
       the value zero.

5.5.3. Route Selection and RoutedPath

 The RoutedPath MAY be used for route selection, and in most cases is
 preferred over the AdvertisementPath for this role.  Some possible
 criteria to be used are the number of hops on the path and the
 presence or absence of particular ITADs on the path.

Rosenberg, et. al. Standards Track [Page 35] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.5.4. Aggregation and RoutedPath

 The rules for aggregating RoutedPath attributes are given in Section
 5.4.4.1 and 5.4.4.2, where the term "path" used in Section 5.4.4.1
 and 5.4.4.2 is understood to mean RoutedPath.

5.5.5. Route Dissemination and RoutedPath

 When an LS propagates a route that it learned from another LS, it
 modifies the route's RoutedPath attribute based on the location of
 the LS to which the route is sent.
  1. When an LS advertises a route to another LS located in its own

ITAD, the advertising LS MUST NOT modify the RoutedPath

       attribute associated with the route.
    -  If the LS has not changed the NextHopServer attribute, then the
       LS MUST NOT change the RoutedPath attribute.
    -  Otherwise, the LS changed the NextHopServer and is advertising
       the route to an LS in another ITAD.  The advertising LS MUST
       update the RoutedPath attribute as follows:
  • If the first path segment of the RoutedPath is of type

AP_SEQUENCE, the local system shall prepend its own ITAD

          number as the last element of the sequence (put it in the
          leftmost position).
  • If the first path segment of the RoutedPath is of type

AP_SET, the local system shall prepend a new path segment of

          type AP_SEQUENCE to the RoutedPath, including its own ITAD
          number in that segment.

5.6. AtomicAggregate

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 6.
 The AtomicAggregate attribute indicates that a route may traverse
 domains not listed in the RoutedPath.  If an LS, when presented with
 a set of overlapping routes from a peer LS, selects the less specific
 route without selecting the more specific route, then the LS includes
 the AtomicAggregate attribute with the routing object.

5.6.1. AtomicAggregate Syntax

 This attribute has length zero (0); the value field is empty.

Rosenberg, et. al. Standards Track [Page 36] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.6.2. Route Origination and AtomicAggregate

 Routes are never originated with the AtomicAggregate attribute.

5.6.3. Route Selection and AtomicAggregate

 The AtomicAggregate attribute may be used in route selection - it
 indicates that the RoutedPath may be incomplete.

5.6.4. Aggregation and AtomicAggregate

 If any of the routes to aggregate has the AtomicAggregate attribute,
 then so MUST the resultant aggregate.

5.6.5. Route Dissemination and AtomicAggregate

 If an LS, when presented with a set of overlapping routes from a peer
 LS, selects the less specific route (see Section 0) without selecting
 the more specific route, then the LS MUST include the AtomicAggregate
 attribute with the routing object (if it is not already present).
 An LS receiving a routing object with an AtomicAggregate attribute
 MUST NOT make the set of destinations more specific when advertising
 it to other LSs, and MUST NOT remove the attribute when propagating
 this object to a peer LS.

5.7. LocalPreference

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 7.
 The LocalPreference attribute is only used intra-domain, it indicates
 the local LS's preference for the routing object to other LSs within
 the same domain.  This attribute MUST NOT be included when
 communicating to an LS in another domain, and MUST be included over
 intra-domain links.

5.7.1. LocalPreference Syntax

 The LocalPreference attribute is a 4-octet unsigned numeric value.  A
 higher value indicates a higher preference.

Rosenberg, et. al. Standards Track [Page 37] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.7.2. Route Origination and LocalPreference

 Routes MUST NOT be originated with the LocalPreference attribute to
 inter-domain peers.  Routes to intra-domain peers MUST be originated
 with the LocalPreference attribute.

5.7.3. Route Selection and LocalPreference

 The LocalPreference attribute allows one LS in a domain to calculate
 a preference for a route, and to communicate this preference to other
 LSs within the domain.

5.7.4. Aggregation and LocalPreference

 The LocalPreference attribute is not affected by aggregation.

5.7.5. Route Dissemination and LocalPreference

 An LS MUST include the LocalPreference attribute when communicating
 with peer LSs within its own domain.  An LS MUST NOT include the
 LocalPreference attribute when communicating with LSs in other
 domains.  LocalPreference attributes received from inter-domain peers
 MUST be ignored.

5.8. MultiExitDisc

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 8.
 When two ITADs are connected by more than one set of peers, the
 MultiExitDisc attribute may be used to specify preferences for routes
 received over one of those links versus routes received over other
 links.  The MultiExitDisc parameter is used only for route selection.

5.8.1. MultiExitDisc Syntax

 The MultiExitDisc attribute carries a 4-octet unsigned numeric value.
 A higher value represents a more preferred routing object.

5.8.2. Route Origination and MultiExitDisc

 Routes originated to intra-domain peers MUST NOT be originated with
 the MultiExitDisc attribute.  When originating a route to an inter-
 domain peer, the MultiExitDisc attribute may be included.

Rosenberg, et. al. Standards Track [Page 38] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.8.3. Route Selection and MultiExitDisc

 The MultiExitDisc attribute is used to express a preference when
 there are multiple links between two domains.  If all other factors
 are equal, then a route with a higher MultiExitDisc attribute is
 preferred over a route with a lower MultiExitDisc attribute.

5.8.4. Aggregation and MultiExitDisc

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

5.8.5. Route Dissemination and MultiExitDisc

 If received from a peer LS in another domain, an LS MAY propagate the
 MultiExitDisc to other LSs within its domain.  The MultiExitDisc
 attribute MUST NOT be propagated to LSs in other domains.
 An LS may add the MultiExitDisc attribute when propagating routing
 objects to an LS in another domain.  The inclusion of the
 MultiExitDisc attribute is a matter of policy, as is the value of the
 attribute.

5.9. Communities

 Conditional Mandatory: False.
 Required Flags: Not Well-Known, Independent Transitive.
 Potential Flags: None.
 TRIP Type Code: 9.
 A community is a group of destinations that share some common
 property.
 The Communities attribute is used to group destinations so that the
 routing decision can be based on the identity of the group.  Using
 the Communities attribute should significantly simplify the
 distribution of routing information by providing an administratively
 defined aggregation unit.
 Each ITAD administrator may define the communities to which a
 particular route belongs.  By default, all routes belong to the
 general Internet Telephony community.
 As an example, the Communities attribute could be used to define an
 alliance between a group of Internet Telephony service providers for
 a specific subset of routing information.  In this case, members of

Rosenberg, et. al. Standards Track [Page 39] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 that alliance would accept only routes for destinations in this group
 that are advertised by other members of the alliance.  Other
 destinations would be more freely accepted.  To achieve this, a
 member would tag each route with a designated Community attribute
 value before disseminating it.  This relieves the members of such an
 alliance, from the responsibility of keeping track of the identities
 of all other members of that alliance.
 Another example use of the Communities attribute is with aggregation.
 It is often useful to advertise both the aggregate route and the
 component more-specific routes that were used to form the aggregate.
 These information components are only useful to the neighboring TRIP
 peer, and perhaps the ITAD of the neighboring TRIP peer, so it is
 desirable to filter out the component routes.  This can be achieved
 by specifying a Community attribute value that the neighboring peers
 will match and filter on.  That way it can be assured that the more
 specific routes will not propagate beyond their desired scope.

5.9.1. Syntax of Communities

 The Communities attribute is of variable length.  It consists of a
 set of 8-octet values, each of which specifies a community.  The
 first 4 octets of the Community value are the Community ITAD Number
 and the next 4 octets are the Community ID.
 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
 +---------------+---------------+--------------+----------------+
 |                       Community ITAD Number 1                 |
 +---------------+---------------+--------------+----------------+
 |                         Community ID 1                        |
 +---------------+---------------+--------------+----------------+
 |                       . . . . . . . . .
 +---------------+---------------+--------------+----------------+
                  Figure 14: Communities Syntax
 For administrative assignment, the following assumptions may be made:
    The Community attribute values starting with a Community ITAD
    Number of 0x00000000 are hereby reserved.
 The following communities have global significance and their
 operation MUST be implemented in any Community attribute-aware TRIP
 LS.

Rosenberg, et. al. Standards Track [Page 40] RFC 3219 Telephony Routing over IP (TRIP) January 2002

  1. NO_EXPORT (Community ITAD Number = 0x00000000 and Community ID

= 0xFFFFFF01). Any received route with a community attribute

       containing this value MUST NOT be advertised outside of the
       receiving TRIP ITAD.
 Other community values MUST be encoded using an ITAD number in the
 four most significant octets.  The semantics of the final four octets
 (the Community ID octets) may be defined by the ITAD (e.g., ITAD 690
 may define research, educational, and commercial community IDs that
 may be used for policy routing as defined by the operators of that
 ITAD).

5.9.2. Route Origination and Communities

 The Communities attribute is not well-known.  If a route has a
 Communities attribute associated with it, the LS MUST include that
 attribute in the advertisement it originates.

5.9.3. Route Selection and Communities

 The Communities attribute may be used for route selection.  A route
 that is a member of a certain community may be preferred over another
 route that is not a member of that community.  Likewise, routes
 without a certain community value may be excluded from consideration.

5.9.4. Aggregation and Communities

 If a set of routes is to be aggregated and the resultant aggregate
 does not carry an Atomic_Aggregate attribute, then the resulting
 aggregate should have a Communities attribute that contains the union
 of the Community attributes of the aggregated routes.

5.9.5. Route Dissemination and Communities

 An LS may manipulate the Communities attribute before disseminating a
 route to a peer.  Community attribute manipulation may include adding
 communities, removing communities, adding a Communities attribute (if
 none exists), deleting the Communities attribute, etc.

5.10. ITAD Topology

 Conditional Mandatory: False.
 Required Flags: Well-known, Link-State encapsulated.
 Potential Flags: None.
 TRIP Type Code: 10.

Rosenberg, et. al. Standards Track [Page 41] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Within an ITAD, each LS must know the status of other LSs so that LS
 failure can be detected.  To do this, each LS advertises its internal
 topology to other LSs within the domain.  When an LS detects that
 another LS is no longer active, the information sourced by that LS
 can be deleted (the Adj-TRIB-In for that peer may be cleared).  The
 ITAD Topology attribute is used to communicate this information to
 other LSs within the domain.
 An LS MUST send a topology update each time it detects a change in
 its internal peer set.  The topology update may be sent in an UPDATE
 message by itself or it may be piggybacked on an UPDATE message which
 includes ReachableRoutes and/or WithdrawnRoutes information.
 When an LS receives a topology update from an internal LS, it MUST
 recalculate which LSs are active within the ITAD via a connectivity
 algorithm on the topology.

5.10.1. ITAD Topology Syntax

 The ITAD Topology attribute indicates the LSs with which the LS is
 currently peering.  The attribute consists of a list of the TRIP
 Identifiers with which the LS is currently peering, the format is
 given in  Figure 15.  This attribute MUST use the link-state
 encapsulation as defined in Section 4.3.2.4.
  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
 +---------------+---------------+--------------+----------------+
 |                        TRIP Identifier 1                      |
 +---------------+---------------+--------------+----------------+
 |                        TRIP Identifier 2 ...                  |
 +---------------+---------------+--------------+----------------+
                 Figure 15: ITAD Topology Syntax

5.10.2. Route Origination and ITAD Topology

 The ITAD Topology attribute is independent of any routes in the
 UPDATE.  Whenever the set of internal peers of an LS changes, it MUST
 create an UPDATE with the ITAD Topology Attribute included listing
 the current set of internal peers.  The LS MUST include this
 attribute in the first UPDATE it sends to a peer after the peering
 session is established.

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5.10.3. Route Selection and ITAD Topology

 This attribute is independent of any routing information in the
 UPDATE.  When an LS receives an UPDATE with an ITAD Topology
 attribute, it MUST compute the set of LSs currently active in the
 domain by performing a connectivity test on the ITAD topology as
 given by the set of originated ITAD Topology attributes.  The LS MUST
 locally purge the Adj-TRIB-In for any LS that is no longer active in
 the domain.  The LS MUST NOT propagate this purging information to
 other LSs as they will make a similar decision.

5.10.4. Aggregation and ITAD Topology

 This information is not aggregated.

5.10.5. Route Dissemination and ITAD Topology

 An LS MUST ignore the attribute if received from a peer in another
 domain.  An LS MUST NOT send this attribute to an inter-domain peer.

5.11. ConvertedRoute

 Conditional Mandatory: False.
 Required Flags: Well-known.
 Potential Flags: None.
 TRIP Type Code: 12.
 The ConvertedRoute attribute indicates that an intermediate LS has
 altered the route by changing the route's Application Protocol.  For
 example, if an LS receives a route with Application Protocol X and
 changes the Application Protocol to Y before advertising the route to
 an external peer, the LS MUST include the ConvertedRoute attribute.
 The attribute is an indication that the advertised application
 protocol will not be used end-to-end, i.e., the information
 advertised about this route is not complete.

5.11.1. ConvertedRoute Syntax

 This attribute has length zero (0); the value field is empty.

5.11.2. Route Origination and ConvertedRoute

 Routes are never originated with the ConvertedRoute attribute.

Rosenberg, et. al. Standards Track [Page 43] RFC 3219 Telephony Routing over IP (TRIP) January 2002

5.11.3. Route Selection and ConvertedRoute

 The ConvertedRoute attribute may be used in route selection - it
 indicates that advertised routing information is not complete.

5.11.4. Aggregation and ConvertedRoute

 If any of the routes to aggregate has the ConvertedRoute attribute,
 then so MUST the resultant aggregate.

5.11.5. Route Dissemination and ConvertedRoute

 If an LS changes the Application Protocol of a route before
 advertising the route to an external peer, the LS MUST include the
 ConvertedRoute attribute.

5.12. Considerations for Defining New TRIP Attributes

 Any proposal for defining new TRIP attributes should specify the
 following:
  1. the use of this attribute,
  2. the attribute's flags,
  3. the attribute's syntax,
  4. how the attribute works with route origination,
  5. how the attribute works with route aggregation, and
  6. how the attribute works with route dissemination and the

attribute's scope (e.g., intra-domain only like

       LocalPreference)
 IANA will manage the assignment of TRIP attribute type codes to new
 attributes.

6. TRIP Error Detection and Handling

 This section describes errors to be detected and the actions to be
 taken while processing TRIP messages.
 When any of the conditions described here are detected, a
 NOTIFICATION message with the indicated Error Code, Error Subcode,
 and Data fields MUST be sent, and the TRIP connection MUST be closed.
 If no Error Subcode is specified, then a zero Subcode MUST be used.
 The phrase "the TRIP connection is closed" means that the transport
 protocol connection has been closed and that all resources for that
 TRIP connection have been de-allocated.  If the connection was
 inter-domain, then routing table entries associated with the remote
 peer MUST be marked as invalid.  Routing table entries MUST NOT be

Rosenberg, et. al. Standards Track [Page 44] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 marked as invalid if an internal peering session is terminated.  The
 fact that the routes have been marked as invalid is passed to other
 TRIP peers before the routes are deleted from the system.
 Unless specified explicitly, the Data field of the NOTIFICATION
 message that is sent to indicate an error MUST be empty.

6.1. Message Header Error Detection and Handling

 All errors detected while processing the Message Header are indicated
 by sending the NOTIFICATION message with the Error Code Message
 Header Error.  The Error Subcode elaborates on the specific nature of
 the error.  The error checks in this section MUST be performed by
 each LS upon receipt of every message.
 If the Length field of the message header is less than 3 or greater
 than 4096, or if the Length field of an OPEN message is less than the
 minimum length of the OPEN message, or if the Length field of an
 UPDATE message is less than the minimum length of the UPDATE message,
 or if the Length field of a KEEPALIVE message is not equal to 3, or
 if the Length field of a NOTIFICATION message is less than the
 minimum length of the NOTIFICATION message, then the Error Subcode
 MUST be set to Bad Message Length.  The Data field contains the
 erroneous Length field.
 If the Type field of the message header is not recognized, then the
 Error Subcode MUST be set to "Bad Message Type."  The Data field
 contains the erroneous Type field.

6.2. OPEN Message Error Detection and Handling

 All errors detected while processing the OPEN message are indicated
 by sending the NOTIFICATION message with the Error Code "OPEN Message
 Error."  The Error Subcode elaborates on the specific nature of the
 error.  The error checks in this section MUST be performed by each LS
 upon receipt of every OPEN message.
 If the version number contained in the Version field of the received
 OPEN message is not supported, then the Error Subcode MUST be set to
 "Unsupported Version Number."  The Data field is a 1-octet unsigned
 integer, which indicates the largest locally supported version
 number, which is less than the version of the remote TRIP peer bid
 (as indicated in the received OPEN message).
 If the ITAD field of the OPEN message is unacceptable, then the Error
 Subcode MUST be set to "Bad Peer ITAD."  The determination of
 acceptable ITAD numbers is outside the scope of this protocol.

Rosenberg, et. al. Standards Track [Page 45] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If the Hold Time field of the OPEN message is unacceptable, then the
 Error Subcode MUST be set to "Unacceptable Hold Time."  An
 implementation MUST reject Hold Time values of one or two seconds.
 An implementation MAY reject any proposed Hold Time.  An
 implementation that accepts a Hold Time MUST use the negotiated value
 for the Hold Time.
 If the TRIP Identifier field of the OPEN message is not valid, then
 the Error Subcode MUST be set to "Bad TRIP Identifier."  A TRIP
 identifier is 4-octets in length and can take any value.  An LS
 considers the TRIP Identifier invalid if it already has an open
 connection with another peer LS that has the same ITAD and TRIP
 Identifier.
 Any two LSs within the same ITAD MUST NOT have equal TRIP Identifier
 values.  This restriction does not apply to LSs in different ITADs
 since the purpose is to uniquely identify an LS using its TRIP
 Identifier and its ITAD number.
 If one of the Optional Parameters in the OPEN message is not
 recognized, then the Error Subcode MUST be set to "Unsupported
 Optional Parameters."
 If the Optional Parameters of the OPEN message include Capability
 Information with an unsupported capability (unsupported in either
 capability type or value), then the Error Subcode MUST be set to
 "Unsupported Capability," and the entirety of the unsupported
 capabilities MUST be listed in the Data field of the NOTIFICATION
 message.
 If the Optional Parameters of the OPEN message include Capability
 Information which does not match the receiving LS's capabilities,
 then the Error Subcode MUST be set to "Capability Mismatch," and the
 entirety of the mismatched capabilities MUST be listed in the Data
 field of the NOTIFICATION message.

6.3. UPDATE Message Error Detection and Handling

 All errors detected while processing the UPDATE message are indicated
 by sending the NOTIFICATION message with the Error Code "UPDATE
 Message Error."  The Error Subcode elaborates on the specific nature
 of the error.  The error checks in this section MUST be performed by
 each LS upon receipt of every UPDATE message.  These error checks
 MUST occur before flooding procedures are invoked with internal
 peers.

Rosenberg, et. al. Standards Track [Page 46] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If any recognized attribute has Attribute Flags that conflict with
 the Attribute Type Code, then the Error Subcode MUST be set to
 "Attribute Flags Error."  The Data field contains the erroneous
 attribute (type, length and value).
 If any recognized attribute has an Attribute Length that conflicts
 with the expected length (based on the attribute type code), then the
 Error Subcode MUST be set to "Attribute Length Error."  The Data
 field contains the erroneous attribute (type, length and value).
 If any of the mandatory (i.e., conditional mandatory attribute and
 the conditions for including it in the UPDATE message are fulfilled)
 well-known attributes are not present, then the Error Subcode MUST be
 set to "Missing Well-known Mandatory Attribute."  The Data field
 contains the Attribute Type Code of the missing well-known
 conditional mandatory attributes.
 If any of the well-known attributes are not recognized, then the
 Error Subcode MUST be set to "Unrecognized Well-known Attribute."
 The Data field contains the unrecognized attribute (type, length and
 value).
 If any attribute has a syntactically incorrect value, or an undefined
 value, then the Error Subcode is set to "Invalid Attribute."  The
 Data field contains the incorrect attribute (type, length and value).
 Such a NOTIFICATION message is sent, for example, when a
 NextHopServer attribute is received with an invalid address.
 The information carried by the AdvertisementPath attribute is checked
 for ITAD loops.  ITAD loop detection is done by scanning the full
 AdvertisementPath, and checking that the ITAD number of the local
 ITAD does not appear in the AdvertisementPath.  If the local ITAD
 number appears in the AdvertisementPath, then the route MAY be stored
 in the Adj-TRIB-In.  However unless the LS is configured to accept
 routes with its own ITAD in the advertisement path, the route MUST
 not be passed to the TRIP Decision Process.  The operation of an LS
 that is configured to accept routes with its own ITAD number in the
 advertisement path are outside the scope of this document.
 If the UPDATE message was received from an internal peer and either
 the WithdrawnRoutes, ReachableRoutes, or ITAD Topology attribute does
 not have the Link-State Encapsulation flag set, then the Error
 Subcode is set to "Invalid Attribute" and the data field contains the
 attribute.  Likewise, the attribute is invalid if received from an
 external peer and the Link-State Flag is set.
 If any attribute appears more than once in the UPDATE message, then
 the Error Subcode is set to "Malformed Attribute List."

Rosenberg, et. al. Standards Track [Page 47] RFC 3219 Telephony Routing over IP (TRIP) January 2002

6.4. NOTIFICATION Message Error Detection and Handling

 If a peer sends a NOTIFICATION message, and there is an error in that
 message, there is unfortunately no means of reporting this error via
 a subsequent NOTIFICATION message.  Any such error, such as an
 unrecognized Error Code or Error Subcode, should be noticed, logged
 locally, and brought to the attention of the administration of the
 peer.  The means to do this, however, are outside the scope of this
 document.

6.5. Hold Timer Expired Error Handling

 If a system does not receive successive messages within the period
 specified by the negotiated Hold Time, then a NOTIFICATION message
 with a "Hold Timer Expired" Error Code MUST be sent and the TRIP
 connection MUST be closed.

6.6. Finite State Machine Error Handling

 An error detected by the TRIP Finite State Machine (e.g., receipt of
 an unexpected event) MUST result in sending a NOTIFICATION message
 with the Error Code "Finite State Machine Error" and the TRIP
 connection MUST be closed.

6.7. Cease

 In the absence of any fatal errors (that are indicated in this
 section), a TRIP peer MAY choose at any given time to close its TRIP
 connection by sending the NOTIFICATION message with the Error Code
 "Cease."  However, the Cease NOTIFICATION message MUST NOT be used
 when a fatal error indicated by this section exists.

6.8. Connection Collision Detection

 If a pair of LSs try simultaneously to establish a transport
 connection to each other, then two parallel connections between this
 pair of speakers might well be formed.  We refer to this situation as
 connection collision.  Clearly, one of these connections must be
 closed.
 Based on the value of the TRIP Identifier, a convention is
 established for detecting which TRIP connection is to be preserved
 when a collision occurs.  The convention is to compare the TRIP
 Identifiers of the peers involved in the collision and to retain only
 the connection initiated by the LS with the higher-valued TRIP
 Identifier.

Rosenberg, et. al. Standards Track [Page 48] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Upon receipt of an OPEN message, the local LS MUST examine all of its
 connections that are in the OpenConfirm state.  An LS MAY also
 examine connections in an OpenSent state if it knows the TRIP
 Identifier of the peer by means outside of the protocol.  If among
 these connections there is a connection to a remote LS, whose TRIP
 Identifier equals the one in the OPEN message, then the local LS MUST
 perform the following collision resolution procedure:
 The TRIP Identifier and ITAD of the local LS is compared to the TRIP
 Identifier and ITAD of the remote LS (as specified in the OPEN
 message).  TRIP Identifiers are treated as 4-octet unsigned integers
 for comparison.
 If the value of the local TRIP Identifier is less than the remote
 one, or if the two TRIP Identifiers are equal and the value of the
 ITAD of the local LS is less than value of the ITAD of the remote LS,
 then the local LS MUST close the TRIP connection that already exists
 (the one that is already in the OpenConfirm state), and accept the
 TRIP connection initiated by the remote LS:
    1. Otherwise, the local LS closes the newly created TRIP
       connection and continues to use the existing one (the one that
       is already in the OpenConfirm state).
    2. If a connection collision occurs with an existing TRIP
       connection that is in the Established state, then the LS MUST
       unconditionally close off the newly created connection.  Note
       that a connection collision cannot be detected with connections
       in Idle, Connect, or Active states.
    3. To close the TRIP connection (that results from the collision
       resolution procedure), an LS MUST send a NOTIFICATION message
       with the Error Code "Cease" and the TRIP connection MUST be
       closed.

7. TRIP Version Negotiation

 Peer LSs may negotiate the version of the protocol by making multiple
 attempts to open a TRIP connection, starting with the highest version
 number each supports.  If an open attempt fails with an Error Code
 "OPEN Message Error" and an Error Subcode "Unsupported Version
 Number," then the LS has available the version number it tried, the
 version number its peer tried, the version number passed by its peer
 in the NOTIFICATION message, and the version numbers that it
 supports.  If the two peers support one or more common versions, then
 this will allow them to rapidly determine the highest common version.
 In order to support TRIP version negotiation, future versions of TRIP
 must retain the format of the OPEN and NOTIFICATION messages.

Rosenberg, et. al. Standards Track [Page 49] RFC 3219 Telephony Routing over IP (TRIP) January 2002

8. TRIP Capability Negotiation

 An LS MAY include the Capabilities Option in its OPEN message to a
 peer to indicate the capabilities supported by the LS.  An LS
 receiving an OPEN message MUST NOT use any capabilities that were not
 included in the OPEN message of the peer when communicating with that
 peer.

9. TRIP Finite State Machine

 This section specifies TRIP operation in terms of a Finite State
 Machine (FSM).  Following is a brief summary and overview of TRIP
 operations by state as determined by this FSM.  A condensed version
 of the TRIP FSM is found in Appendix 1.  There is one TRIP FSM per
 peer and these FSMs operate independently.
 Idle state:
 Initially TRIP is in the Idle state for each peer.  In this state,
 TRIP refuses all incoming connections.  No resources are allocated to
 the peer.  In response to the Start event (initiated by either the
 system or the operator), the local system initializes all TRIP
 resources, starts the ConnectRetry timer, initiates a transport
 connection to the peer, starts listening for a connection that may be
 initiated by the remote TRIP peer, and changes its state to Connect.
 The exact value of the ConnectRetry timer is a local matter, but
 should be sufficiently large to allow TCP initialization.
 If an LS detects an error, it closes the transport connection and
 changes its state to Idle.  Transitioning from the Idle state
 requires generation of the Start event.  If such an event is
 generated automatically, then persistent TRIP errors may result in
 persistent flapping of the LS.  To avoid such a condition, Start
 events MUST NOT be generated immediately for a peer that was
 previously transitioned to Idle due to an error.  For a peer that was
 previously transitioned to Idle due to an error, the time between
 consecutive Start events, if such events are generated automatically,
 MUST exponentially increase.  The value of the initial timer SHOULD
 be 60 seconds, and the time SHOULD be at least doubled for each
 consecutive retry up to some maximum value.
 Any other event received in the Idle state is ignored.
 Connect State:
 In this state, an LS is waiting for a transport protocol connection
 to be completed to the peer, and is listening for inbound transport
 connections from the peer.

Rosenberg, et. al. Standards Track [Page 50] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If the transport protocol connection succeeds, the local LS clears
 the ConnectRetry timer, completes initialization, sends an OPEN
 message to its peer, sets its Hold Timer to a large value, and
 changes its state to OpenSent.  A Hold Timer value of 4 minutes is
 suggested.
 If the transport protocol connect fails (e.g., retransmission
 timeout), the local system restarts the ConnectRetry timer, continues
 to listen for a connection that may be initiated by the remote LS,
 and changes its state to Active state.
 In response to the ConnectRetry timer expired event, the local LS
 cancels any outstanding transport connection to the peer, restarts
 the ConnectRetry timer, initiates a transport connection to the
 remote LS, continues to listen for a connection that may be initiated
 by the remote LS, and stays in the Connect state.
 If the local LS detects that a remote peer is trying to establish a
 connection to it and the IP address of the peer is not an expected
 one, then the local LS rejects the attempted connection and continues
 to listen for a connection from its expected peers without changing
 state.
 If an inbound transport protocol connection succeeds, the local LS
 clears the ConnectRetry timer, completes initialization, sends an
 OPEN message to its peer, sets its Hold Timer to a large value, and
 changes its state to OpenSent.  A Hold Timer value of 4 minutes is
 suggested.
 The Start event is ignored in the Connect state.
 In response to any other event (initiated by either the system or the
 operator), the local system releases all TRIP resources associated
 with this connection and changes its state to Idle.
 Active state:
 In this state, an LS is listening for an inbound connection from the
 peer, but is not in the process of initiating a connection to the
 peer.
 If an inbound transport protocol connection succeeds, the local LS
 clears the ConnectRetry timer, completes initialization, sends an
 OPEN message to its peer, sets its Hold Timer to a large value, and
 changes its state to OpenSent.  A Hold Timer value of 4 minutes is
 suggested.

Rosenberg, et. al. Standards Track [Page 51] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 In response to the ConnectRetry timer expired event, the local system
 restarts the ConnectRetry timer, initiates a transport connection to
 the TRIP peer, continues to listen for a connection that may be
 initiated by the remote TRIP peer, and changes its state to Connect.
 If the local LS detects that a remote peer is trying to establish a
 connection to it and the IP address of the peer is not an expected
 one, then the local LS rejects the attempted connection and continues
 to listen for a connection from its expected peers without changing
 state.
 Start event is ignored in the Active state.
 In response to any other event (initiated by either the system or the
 operator), the local system releases all TRIP resources associated
 with this connection and changes its state to Idle.
 OpenSent state:
 In this state, an LS has sent an OPEN message to its peer and is
 waiting for an OPEN message from its peer.  When an OPEN message is
 received, all fields are checked for correctness.  If the TRIP
 message header checking or OPEN message checking detects an error
 (see Section 6.2) or a connection collision (see Section 6.8), the
 local system sends a NOTIFICATION message and changes its state to
 Idle.
 If there are no errors in the OPEN message, TRIP sends a KEEPALIVE
 message and sets a KeepAlive timer.  The Hold Timer, which was
 originally set to a large value (see above), is replaced with the
 negotiated Hold Time value (see Section 4.2).  If the negotiated Hold
 Time value is zero, then the Hold Time timer and KeepAlive timers are
 not started.  If the value of the ITAD field is the same as the local
 ITAD number, then the connection is an "internal" connection;
 otherwise, it is "external" (this will affect UPDATE processing).
 Finally, the state is changed to OpenConfirm.
 If the local LS detects that a remote peer is trying to establish a
 connection to it and the IP address of the peer is not an expected
 one, then the local LS rejects the attempted connection and continues
 to listen for a connection from its expected peers without changing
 state.
 If a disconnect notification is received from the underlying
 transport protocol, the local LS closes the transport connection,
 restarts the ConnectRetry timer, continues to listen for a connection
 that may be initiated by the remote TRIP peer, and goes into the
 Active state.

Rosenberg, et. al. Standards Track [Page 52] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If the Hold Timer expires, the local LS sends a NOTIFICATION message
 with the Error Code "Hold Timer Expired" and changes its state to
 Idle.
 In response to the Stop event (initiated by either system or
 operator) the local LS sends a NOTIFICATION message with the Error
 Code "Cease" and changes its state to Idle.
 The Start event is ignored in the OpenSent state.
 In response to any other event the local LS sends a NOTIFICATION
 message with the Error Code "Finite State Machine Error" and changes
 its state to Idle.
 Whenever TRIP changes its state from OpenSent to Idle, it closes the
 transport connection and releases all resources associated with that
 connection.
 OpenConfirm state:
 In this state, an LS has sent an OPEN to its peer, received an OPEN
 from its peer, and sent a KEEPALIVE in response to the OPEN.  The LS
 is now waiting for a KEEPALIVE or NOTIFICATION message in response to
 its OPEN.
 If the local LS receives a KEEPALIVE message, it changes its state to
 Established.
 If the Hold Timer expires before a KEEPALIVE message is received, the
 local LS sends NOTIFICATION message with the Error Code "Hold Timer
 Expired" and changes its state to Idle.
 If the local LS receives a NOTIFICATION message, it changes its state
 to Idle.
 If the KeepAlive timer expires, the local LS sends a KEEPALIVE
 message and restarts its KeepAlive timer.
 If a disconnect notification is received from the underlying
 transport protocol, the local LS closes the transport connection,
 restarts the ConnectRetry timer, continues to listen for a connection
 that may be initiated by the remote TRIP peer, and goes into the
 Active state.
 In response to the Stop event (initiated by either the system or the
 operator) the local LS sends NOTIFICATION message with the Error Code
 "Cease" and changes its state to Idle.
 The Start event is ignored in the OpenConfirm state.

Rosenberg, et. al. Standards Track [Page 53] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 In response to any other event the local LS sends a NOTIFICATION
 message with the Error Code "Finite State Machine Error" and changes
 its state to Idle.
 Whenever TRIP changes its state from OpenConfirm to Idle, it closes
 the transport connection and releases all resources associated with
 that connection.
 Established state:
 In the Established state, an LS can exchange UPDATE, NOTIFICATION,
 and KEEPALIVE messages with its peer.
 If the negotiated Hold Timer is zero, then no procedures are
 necessary for keeping a peering session alive.  If the negotiated
 Hold Time value is non-zero, the procedures of this paragraph apply.
 If the Hold Timer expires, the local LS sends a NOTIFICATION message
 with the Error Code "Hold Timer Expired" and changes its state to
 Idle.  If the KeepAlive Timer expires, then the local LS sends a
 KeepAlive message and restarts the KeepAlive Timer.  If the local LS
 receives an UPDATE or KEEPALIVE message, then it restarts its Hold
 Timer.  Each time the LS sends an UPDATE or KEEPALIVE message, it
 restarts its KeepAlive Timer.
 If the local LS receives a NOTIFICATION message, it changes its state
 to Idle.
 If the local LS receives an UPDATE message and the UPDATE message
 error handling procedure (see Section6.3) detects an error, the local
 LS sends a NOTIFICATION message and changes its state to Idle.
 If a disconnect notification is received from the underlying
 transport protocol, the local LS changes its state to Idle.
 In response to the Stop event (initiated by either the system or the
 operator), the local LS sends a NOTIFICATION message with the Error
 Code "Cease" and changes its state to Idle.
 The Start event is ignored in the Established state.
 In response to any other event, the local LS sends a NOTIFICATION
 message with Error Code "Finite State Machine Error" and changes its
 state to Idle.
 Whenever TRIP changes its state from Established to Idle, it closes
 the transport connection and releases all resources associated with
 that connection.  Additionally, if the peer is an external peer, the
 LS deletes all routes derived from that connection.

Rosenberg, et. al. Standards Track [Page 54] RFC 3219 Telephony Routing over IP (TRIP) January 2002

10. UPDATE Message Handling

 An UPDATE message may be received only in the Established state.
 When an UPDATE message is received, each field is checked for
 validity as specified in Section 6.3.  The rest of this section
 presumes that the UPDATE message has passed the error-checking
 procedures of Section 6.3.
 If the UPDATE message was received from an internal peer, the
 flooding procedures of Section 10.1 MUST be applied.  The flooding
 process synchronizes the Loc-TRIBs of all LSs within the domain.
 Certain routes within the UPDATE may be marked as old or duplicates
 by the flooding process and are ignored during the rest of the UPDATE
 processing.
 If the UPDATE message contains withdrawn routes, then the
 corresponding previously advertised routes shall be removed from the
 Adj-TRIB-In.  This LS MUST rerun its Decision Process since the
 previously advertised route is no longer available for use.
 If the UPDATE message contains a route, then the route MUST be placed
 in the appropriate Adj-TRIB-In, and the following additional actions
 MUST be taken:
    1. If its destinations are identical to those of a route currently
       stored in the Adj-TRIB-In, then the new route MUST replace the
       older route in the Adj-TRIB-In, thus implicitly withdrawing the
       older route from service.  The LS MUST rerun its Decision
       Process since the older route is no longer available for use.
    2. If the new route is more specific than an earlier route
       contained in the Adj-TRIB-In and has identical attributes, then
       no further actions are necessary.
    3. If the new route is more specific than an earlier route
       contained in the Adj-TRIB-In but does not have identical
       attributes, then the LS MUST run its Decision Process since the
       more specific route has implicitly made a portion of the less
       specific route unavailable for use.
    4. If the new route has destinations that are not present in any
       of the routes currently stored in the Adj-TRIB-In, then the LS
       MUST run its Decision Process.
    5. If the new route is less specific than an earlier route
       contained in the Adj-TRIB-In, the LS MUST run its Decision
       Process on the set of destinations that are described only by
       the less specific route.

Rosenberg, et. al. Standards Track [Page 55] RFC 3219 Telephony Routing over IP (TRIP) January 2002

10.1. Flooding Process

 When an LS receives an UPDATE message from an internal peer, the LS
 floods the new information from that message to all of its other
 internal peers.  Flooding is used to efficiently synchronize all of
 the LSs within a domain without putting any constraints on the
 domain's internal topology.  The flooding mechanism is based on the
 techniques used in OSPF [4] and SCSP [6].  One may argue that TRIP's
 flooding process is in reality a controlled broadcast mechanism.

10.1.1. Database Information

 The LS MUST maintain the sequence number and originating TRIP
 identifier for each link-state encapsulated attribute in an internal
 Adj-TRIB-In.  These values are included with the route in the
 ReachableRoutes, WithdrawnRoutes, and ITAD Topology attributes.  The
 originating TRIP identifier gives the internal LS that originated
 this route into the ITAD, the sequence number gives the version of
 this route at the originating LS.

10.1.2. Determining Newness

 For each route in the ReachableRoutes or WithdrawnRoutes field, the
 LS decides if the route is new or old.  This is determined by
 comparing the Sequence Number of the route in the UPDATE with the
 Sequence Number of the route saved in the Adj-TRIB-In.  The route is
 new if either the route does not exist in the Adj-TRIB-In for the
 originating LS, or if the route does exist in the Adj-TRIB-In but the
 Sequence Number in the UPDATE is greater than the Sequence Number
 saved in the Adj-TRIBs-In.  Note that the newness test is
 independently applied to each link-state encapsulated attribute in
 the UPDATE (WithdrawnRoutes or ReachableRoutes or ITAD Topology).

10.1.3. Flooding

 Each route in the ReachableRoutes or WithdrawnRoutes field that is
 determined to be old is ignored in further processing.  If the route
 is determined to be new then the following actions occur.
 If the route is being withdrawn, then the LS MUST flood the withdrawn
 route to all other internal peers, and MUST mark the route as
 withdrawn.  An LS MUST maintain routes marked as withdrawn in its
 databases for MaxPurgeTime seconds.
 If the route is being updated, then the LS MUST update the route in
 the Adj-TRIB-In and MUST flood it to all other internal peers.

Rosenberg, et. al. Standards Track [Page 56] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If these procedures result in changes to the Adj-TRIB-In, then the
 route is also made available for local route processing as described
 early in Section 10.
 To implement flooding, the following is recommended.  All routes
 received in a single UPDATE message that are determined to be new
 should be forwarded to all other internal peers in a single UPDATE
 message.  Other variations of flooding are possible, but the local LS
 MUST ensure that each new route (and any associated attributes)
 received from an internal peer get forwarded to every other internal
 peer.

10.1.4. Sequence Number Considerations

 The Sequence Number is used to determine when one version of a Route
 is newer than another version of a route.  A larger Sequence Number
 indicates a newer version.  The Sequence Number is assigned by the LS
 originating the route into the local ITAD.  The Sequence Number is an
 unsigned 4-octet integer in the range of 1 thru 2^31-1 MinSequenceNum
 thru MaxSequenceNum).  The value 0 is reserved.  When an LS first
 originates a route (including when the LS restarts/reboots) into its
 ITAD, it MUST originate it with a Sequence Number of MinSequenceNum.
 Each time the route is updated within the ITAD by the originator, the
 Sequence Number MUST be increased.
 If it is ever the case that the sequence number is MaxSequenceNum-1
 and it needs to be increased, then the TRIP module of the LS MUST be
 disabled for a period of TripDisableTime so that all routes
 originated by this LS with high sequence numbers can be removed.

10.1.5. Purging a Route Within the ITAD

 To withdraw a route that it originated within the ITAD, an LS
 includes the route in the WithdrawnRoutes field of an UPDATE message.
 The Sequence Number MUST be greater than the last valid version of
 the route.  The LS MAY choose to use a sequence number of
 MaxSequenceNum when withdrawing routes within its ITAD, but this is
 not required.
 After withdrawing a route, an LS MUST mark the route as "withdrawn"
 in its database, and maintain the withdrawn route in its database for
 MaxPurgeTime seconds.  If the LS needs to re-originate a route that
 had been purged but is still in its database, it can either re-
 originate the route immediately using a Sequence Number that is
 greater than that used in the withdraw, or the LS may wait until
 MaxPurgeTime seconds have expired since the route was withdrawn.

Rosenberg, et. al. Standards Track [Page 57] RFC 3219 Telephony Routing over IP (TRIP) January 2002

10.1.6. Receiving Self-Originated Routes

 It is common for an LS to receive UPDATES for routes that it
 originated within the ITAD via the flooding procedure.  If the LS
 receives an UPDATE for a route that it originated that is newer (has
 a higher sequence number) than the LSs current version, then special
 actions must be taken.  This should be a relatively rare occurrence
 and indicates that a route still exists within the ITAD since the LSs
 last restart/reboot.
 If an LS receives a self-originated route update that is newer than
 the current version of the route at the LS, then the following
 actions MUST be taken.  If the LS still wishes to advertise the
 information in the route, then the LS MUST increase the Sequence
 Number of the route to a value greater than that received in the
 UPDATE and re-originate the route.  If the LS does not wish to
 continue to advertise the route, then it MUST purge the route as
 described in Section 10.1.5.

10.1.7. Removing Withdrawn Routes

 An LS SHOULD ensure that routes marked as withdrawn are removed from
 the database in a timely fashion after the MaxPurgeTime has expired.
 This could be done, for example, by periodically sweeping the
 database, and deleting those entries that were withdrawn more than
 MaxPurgeTime seconds ago.

10.2. Decision Process

 The Decision Process selects routes for subsequent advertisement by
 applying the policies in the local Policy Information Base (PIB) to
 the routes stored in its Adj-TRIBs-In.  The output of the Decision
 process is the set of routes that will be advertised to all peers;
 the selected routes will be stored in the local LS's Adj-TRIBs-Out.
 The selection process is formalized by defining a function that takes
 the attributes of a given route as an argument and returns a non-
 negative integer denoting the degree of preference for the route.
 The function that calculates the degree of preference for a given
 route shall not use as its inputs any of the following:  the
 existence of other routes, the non-existence of other routes, or the
 attributes of other routes.  Route selection then consists of an
 individual application of the degree of preference function to each
 feasible route, followed by the choice of the one with the highest
 degree of preference.

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 All internal LSs in an ITAD MUST run the Decision Process and apply
 the same decision criteria, otherwise it will not be possible to
 synchronize their Loc-TRIBs.
 The Decision Process operates on routes contained in each Adj-TRIBs-
 In, and is responsible for:
  1. selection of routes to be advertised to internal peers
  2. selection of routes to be advertised to external peers
  3. route aggregation and route information reduction
 The Decision Process takes place in three distinct phases, each
 triggered by a different event:
  1. Phase 1 is responsible for calculating the degree of preference

for each route received from an external peer.

  1. Phase 2 is invoked on completion of phase 1. It is responsible

for choosing the best route out of all those available for each

       distinct destination, and for installing each chosen route into
       the Loc-TRIB.
    -  Phase 3 is invoked after the Loc-TRIB has been modified.  It is
       responsible for disseminating routes in the Loc-TRIB to each
       external peer, according to the policies contained in the PIB.
       Route aggregation and information reduction can optionally be
       performed within this phase.

10.2.1. Phase 1: Calculation of Degree of Preference

 The Phase 1 decision function shall be invoked whenever the local LS
 receives from a peer an UPDATE message that advertises a new route, a
 replacement route, or a withdrawn route.
 The Phase 1 decision function is a separate process that is completed
 when it has no further work to do.
 The Phase 1 decision function shall lock an Adj-TRIB-In prior to
 operating on any route contained within it, and shall unlock it after
 operating on all new or replacement routes contained within it.
 The local LS MUST determine a degree of preference for each newly
 received or replacement route.  If the route is learned from an
 internal peer, the value of the LocalPreference attribute MUST be
 taken as the degree of preference.  If the route is learned from an
 external peer, then the degree of preference MUST be computed based
 on pre-configured policy information and used as the LocalPreference
 value in any intra-domain TRIP advertisement.  The exact nature of
 this policy information and the computation involved is a local
 matter.

Rosenberg, et. al. Standards Track [Page 59] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 The output of the degree of preference determination process is the
 local preference of a route.  The local LS computes the local
 preference of routes learned from external peers or originated
 internally at that LS.  The local preference of a route learned from
 an internal peer is included in the LocalPreference attribute
 associated with that route.

10.2.2. Phase 2: Route Selection

 The Phase 2 decision function shall be invoked on completion of Phase
 1.  The Phase 2 function is a separate process that completes when it
 has no further work to do.  Phase 2 consists of two sub-phases: 2a
 and 2b.  The same route selection function is applied in both sub-
 phases, but the inputs to each phase are different.  The Phase 2a
 process MUST consider as inputs all external routes, that are present
 in the Adj-TRIBs-In of external peers, and all local routes.  The
 output of Phase 2a is inserted into the Ext-TRIB.  The Phase 2b
 process shall be invoked upon completion of Phase 2a and it MUST
 consider as inputs all routes in the Ext-TRIB and all routes that are
 present in the Adj-TRIBs-In of internal LSs.  The output of Phase 2b
 is stored in the Loc-TRIB.
 The Phase 2 decision function MUST be blocked from running while the
 Phase 3 decision function is in process.  The Phase 2 function MUST
 lock all Adj-TRIBs-In and the Ext-TRIB prior to commencing its
 function, and MUST unlock them on completion.
 If the LS determines that the NextHopServer listed in a route is
 unreachable, then the route MAY be excluded from the Phase 2 decision
 function.  The means by which such a determination is made is not
 mandated here.
 For each set of destinations for which one or more routes exist, the
 local LS's route selection function MUST identify the route that has:
  1. the highest degree of preference, or
  2. is selected as a result of the tie breaking rules specified in

10.2.2.1.

 Withdrawn routes MUST be removed from the Loc-TRIB, Ext-TRIB, and the
 Adj-TRIBs-In.

10.2.2.1. Breaking Ties (Phase 2)

 Several routes to the same destination that have the same degree of
 preference may be input to the Phase 2 route selection function.  The
 local LS can select only one of these routes for inclusion in the

Rosenberg, et. al. Standards Track [Page 60] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 associated Ext-TRIB (Phase 2a) or Loc-TRIB (Phase 2b).  The local LS
 considers all routes with the same degrees of preference.  The
 following algorithm shall be used to break ties.
  1. If the local LS is configured to use the MultiExitDisc

attribute to break ties, and candidate routes received from the

       same neighboring ITAD differ in the value of the MultiExitDisc
       attribute, then select the route that has the larger value of
       MultiExitDisc.
    -  If at least one of the routes was originated by an internal LS,
       select the route route that was advertised by the internal LS
       that has the lowest TRIP ID.
    -  Otherwise, select the route that was advertised by the neighbor
       domain that has the lowest ITAD number.

10.2.3. Phase 3: Route Dissemination

 The Phase 3 decision function MUST be invoked upon completion of
 Phase 2 if Phase 2 results in changes to the Loc-TRIB or when a new
 LS-to-LS peer session is established.
 The Phase 3 function is a separate process that is completed when it
 has no further work to do.  The Phase 3 routing decision function
 MUST be blocked from running while the Phase 2 decision function is
 in process.
 All routes in the Loc-TRIB shall be processed into a corresponding
 entry in the associated Adj-TRIBs-Out.  Route aggregation and
 information reduction techniques (see 10.3.4) MAY optionally be
 applied.
 When the updating of the Adj-TRIBs-Out is complete, the local LS MUST
 run the external update process of 10.3.2.

10.2.4. Overlapping Routes

 When overlapping routes are present in the same Adj-TRIB-In, the more
 specific route shall take precedence, in order, from most specific to
 least specific.
 The set of destinations described by the overlap represents a portion
 of the less specific route that is feasible, but is not currently in
 use.  If a more specific route is later withdrawn, the set of
 destinations described by the more specific route will still be
 reachable using the less specific route.

Rosenberg, et. al. Standards Track [Page 61] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If an LS receives overlapping routes, the Decision Process MUST take
 into account the semantics of the overlapping routes.  In particular,
 if an LS accepts the less specific route while rejecting the more
 specific route from the same peer, then the destinations represented
 by the overlap may not forward along the domains listed in the
 AdvertisementPath attribute of that route.  Therefore, an LS has the
 following choices:
    1. Install both the less and the more specific routes
    2. Install the more specific route only
    3. Install the non-overlapping part of the less specific route
       only (that implies disaggregation of the less-specific route)
    4. Aggregate the two routes and install the aggregated route
    5. Install the less specific route only
    6. Install neither route
 If an LS chooses 5), then it SHOULD add AtomicAggregate attribute to
 the route.  A route that carries AtomicAggregate attribute MUST NOT
 be de-aggregated.  That is, the route cannot be made more specific.
 Forwarding along such a route does not guarantee that route traverses
 only domains listed in the RoutedPath of the route.  If an LS chooses
 1), then it MUST NOT advertise the less specific route without the
 more specific route.

10.3. Update-Send Process

 The Update-Send process is responsible for advertising UPDATE
 messages to all peers.  For example, it distributes the routes chosen
 by the Decision Process to other LSs that may be located in either
 the same ITAD or a neighboring ITAD.  Rules for information exchange
 between peer LSs located in different ITADs are given in 10.3.2;
 rules for information exchange between peer LSs located in the same
 ITAD are given in 10.3.1.
 Before forwarding routes to peers, an LS MUST determine which
 attributes should be forwarded along with that route.  If a not
 well-known non-transitive attribute is unrecognized, it is quietly
 ignored.  If a not well-known dependent-transitive attribute is
 unrecognized, and the NextHopServer attribute has been changed by the
 LS, the unrecognized attribute is quietly ignored.  If a not well-
 known dependent-transitive attribute is unrecognized, and the
 NextHopServer attribute has not been modified by the LS, the Partial
 bit in the attribute flags octet is set to 1, and the attribute is
 retained for propagation to other TRIP speakers.  Similarly, if an
 not well-known independent-transitive attribute is unrecognized, the
 Partial bit in the attribute flags octet is set to 1, and the
 attribute is retained for propagation to other TRIP speakers.

Rosenberg, et. al. Standards Track [Page 62] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 If a not well-known attribute is recognized, and has a valid value,
 then, depending on the type of the not well-known attribute, it is
 updated, if necessary, for possible propagation to other TRIP
 speakers.

10.3.1. Internal Updates

 The Internal update process is concerned with the distribution of
 routing information to internal peers.
 When an LS receives an UPDATE message from another TRIP LS located in
 its own ITAD, it is flooded as described in Section 10.1.
 When an LS receives a new route from an LS in a neighboring ITAD, or
 if a local route is injected into TRIP, the LS determines the
 preference of that route.  If the new route has the highest degree of
 preference for all external routes and local routes to a given
 destination (or if the route was selected via a tie-breaking
 procedure as specified in 10.3.1.1), the LS MUST insert that new
 route into the Ext-TRIB database and the LS MUST advertise that route
 to all other LSs in its ITAD by means of an UPDATE message.  The LS
 MUST advertise itself as the Originator of that route within the
 ITAD.
 When an LS receives an UPDATE message with a non-empty
 WithdrawnRoutes attribute from an external peer, or if a local route
 is withdrawn from TRIP, the LS MUST remove from its Adj-TRIB-In all
 routes whose destinations were carried in this field.  If the
 withdrawn route was previously selected into the Ext-TRIB, the LS
 MUST take the following additional steps:
  1. If a new route is selected for advertisement for those

destinations, then the LS MUST insert the replacement route

       into Ext-TRIB to replace the withdrawn route and advertise it
       to all internal LSs.
    -  If a replacement route is not available for advertisement, then
       the LS MUST include the destinations of the route in the
       WithdrawnRoutes attribute of an UPDATE message, and MUST send
       this message to each internal peer.  The LS MUST also remove
       the withdrawn route from the Ext-TRIB.

10.3.1.1. Breaking Ties (Routes Received from External Peers)

 If an LS has connections to several external peers, there will be
 multiple Adj-TRIBs-In associated with these peers.  These databases
 might contain several equally preferable routes to the same

Rosenberg, et. al. Standards Track [Page 63] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 destination, all of which were advertised by external peers.  The
 local LS shall select one of these routes according to the following
 rules:
  1. If the LS is configured to use the MultiExitDisc attribute to

break ties, and the candidate routes differ in the value of the

       MultiExitDisc attribute, then select the route that has the
       lowest value of MultiExitDisc, else
    -  Select the route that was advertised by the external LS that
       has the lowest TRIP Identifier.

10.3.2. External Updates

 The external update process is concerned with the distribution of
 routing information to external peers.  As part of the Phase 3 route
 selection process, the LS has updated its Adj-TRIBs-Out.  All newly
 installed routes and all newly unfeasible routes for which there is
 no replacement route MUST be advertised to external peers by means of
 UPDATE messages.
 Any routes in the Loc-TRIB marked as withdrawn MUST be removed.
 Changes to the reachable destinations within its own ITAD SHALL also
 be advertised in an UPDATE message.

10.3.3. Controlling Routing Traffic Overhead

 The TRIP protocol constrains the amount of routing traffic (that is,
 UPDATE messages) in order to limit both the link bandwidth needed to
 advertise UPDATE messages and the processing power needed by the
 Decision Process to digest the information contained in the UPDATE
 messages.

10.3.3.1. Frequency of Route Advertisement

 The parameter MinRouteAdvertisementInterval determines the minimum
 amount of time that must elapse between advertisements of routes to a
 particular destination from a single LS.  This rate limiting
 procedure applies on a per-destination basis, although the value of
 MinRouteAdvertisementInterval is set on a per LS peer basis.
 Two UPDATE messages sent from a single LS that advertise feasible
 routes to some common set of destinations received from external
 peers MUST be separated by at least MinRouteAdvertisementInterval.
 Clearly, this can only be achieved precisely by keeping a separate
 timer for each common set of destinations.  This would be unwarranted
 overhead.  Any technique which ensures that the interval between two
 UPDATE messages sent from a single LS that advertise feasible routes

Rosenberg, et. al. Standards Track [Page 64] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 to some common set of destinations received from external peers will
 be at least MinRouteAdvertisementInterval, and will also ensure that
 a constant upper bound on the interval is acceptable.
 Two UPDATE messages, sent from a single LS to an external peer, that
 advertise feasible routes to some common set of destinations received
 from internal peers MUST be separated by at least
 MinRouteAdvertisementInterval.
 Since fast convergence is needed within an ITAD, this rate limiting
 procedure does not apply to routes received from internal peers and
 being broadcast to other internal peers.  To avoid long-lived black
 holes, the procedure does not apply to the explicit withdrawal of
 routes (that is, routes whose destinations explicitly withdrawn by
 UPDATE messages).
 This procedure does not limit the rate of route selection, but only
 the rate of route advertisement.  If new routes are selected multiple
 times while awaiting the expiration of MinRouteAdvertisementInterval,
 the last route selected shall be advertised at the end of
 MinRouteAdvertisementInterval.

10.3.3.2. Frequency of Route Origination

 The parameter MinITADOriginationInterval determines the minimum
 amount of time that must elapse between successive advertisements of
 UPDATE messages that report changes within the advertising LS's own
 ITAD.

10.3.3.3. Jitter

 To minimize the likelihood that the distribution of TRIP messages by
 a given LS will contain peaks, jitter should be applied to the timers
 associated with MinITADOriginationInterval, KeepAlive, and
 MinRouteAdvertisementInterval.  A given LS shall apply the same
 jitter to each of these quantities regardless of the destinations to
 which the updates are being sent; that is, jitter will not be applied
 on a "per peer" basis.
 The amount of jitter to be introduced shall be determined by
 multiplying the base value of the appropriate timer by a random
 factor that is uniformly distributed in the range from 0.75 to 1.0.

Rosenberg, et. al. Standards Track [Page 65] RFC 3219 Telephony Routing over IP (TRIP) January 2002

10.3.4. Efficient Organization of Routing Information

 Having selected the routing information that it will advertise, a
 TRIP speaker may use methods to organize this information in an
 efficient manner.  These methods are discussed in the following
 sections.

10.3.4.1. Information Reduction

 Information reduction may imply a reduction in granularity of policy
 control - after information has collapsed, the same policies will
 apply to all destinations and paths in the equivalence class.
 The Decision Process may optionally reduce the amount of information
 that it will place in the Adj-TRIBs-Out by any of the following
 methods:
  1. ReachableRoutes: A set of destinations can be usually

represented in compact form. For example, a set of E.164 phone

       numbers can be represented in more compact form using E.164
       prefixes.
    -  AdvertisementPath: AdvertisementPath information can be
       represented as ordered AP_SEQUENCEs or unordered AP_SETs.
       AP_SETs are used in the route aggregation algorithm described
       in Section 5.4.4.  They reduce the size of the AP_PATH
       information by listing each ITAD number only once, regardless
       of how many times it may have appeared in multiple
       advertisement paths that were aggregated.
 An AP_SET implies that the destinations advertised in the UPDATE
 message can be reached through paths that traverse at least some of
 the constituent ITADs.  AP_SETs provide sufficient information to
 avoid route looping; however their use may prune potentially feasible
 paths, since such paths are no longer listed individually as in the
 form of AP_SEQUENCEs.  In practice this is not likely to be a
 problem, since once a call arrives at the edge of a group of ITADs,
 the LS at that point is likely to have more detailed path information
 and can distinguish individual paths to destinations.

10.3.4.2. Aggregating Routing Information

 Aggregation is the process of combining the characteristics of
 several different routes in such a way that a single route can be
 advertised.  Aggregation can occur as part of the decision process to
 reduce the amount of routing information that is placed in the Adj-
 TRIBs-Out.

Rosenberg, et. al. Standards Track [Page 66] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Aggregation reduces the amount of information an LS must store and
 exchange with other LSs.  Routes can be aggregated by applying the
 following procedure separately to attributes of like type.
 Routes that have the following attributes shall not be aggregated
 unless the corresponding attributes of each route are identical:
 MultiExitDisc, NextHopServer.
 Attributes that have different type codes cannot be aggregated.
 Attributes of the same type code may be aggregated.  The rules for
 aggregating each attribute MUST be provided together with attribute
 definition.  For example, aggregation rules for TRIP's basic
 attributes, e.g., ReachableRoutes and AdvertisementPath, are given in
 Section 5.

10.4. Route Selection Criteria

 Generally speaking, additional rules for comparing routes among
 several alternatives are outside the scope of this document.  There
 are two exceptions:
  1. If the local ITAD appears in the AdvertisementPath of the new

route being considered, then that new route cannot be viewed as

       better than any other route.  If such a route were ever used, a
       routing loop could result (see Section 6.3).
    -  In order to achieve successful distributed operation, only
       routes with a likelihood of stability can be chosen.  Thus, an
       ITAD must avoid using unstable routes, and it must not make
       rapid spontaneous changes to its choice of route.  Quantifying
       the terms "unstable" and "rapid" in the previous sentence will
       require experience, but the principle is clear.

10.5. Originating TRIP Routes

 An LS may originate local routes by injecting routing information
 acquired by some other means (e.g. via an intra-domain routing
 protocol or through manual configuration or some dynamic registration
 mechanism/protocol) into TRIP.  An LS that originates TRIP routes
 shall assign the degree of preference to these routes by passing them
 through the Decision Process (see Section 10.2).  To TRIP, local
 routes are identical to external routes and are subjected to the same
 two phase route selection mechanism.  A local route which is selected
 into the Ext-TRIB MUST be advertised to all internal LSs.  The
 decision whether to distribute non-TRIP acquired routes within an
 ITAD via TRIP or not depends on the environment within the ITAD (e.g.
 type of intra-domain routing protocol) and should be controlled via
 configuration.

Rosenberg, et. al. Standards Track [Page 67] RFC 3219 Telephony Routing over IP (TRIP) January 2002

11. TRIP Transport

 This specification defines the use of TCP as the transport layer for
 TRIP.  TRIP uses TCP port 6069.  Running TRIP over other transport
 protocols is for further study.

12. ITAD Topology

 There are no restrictions on the intra-domain topology of TRIP LSs.
 For example, LSs in an ITAD can be configured in a full mesh, star,
 or any other connected topology.  Similarly, there are no
 restrictions on the topology of TRIP ITADs.  For example, the ITADs
 can be organized in a flat topology (mesh or ring) or in multi-level
 hierarchy or any other topology.
 The border between two TRIP ITADs may be located either on the link
 between two TRIP LSs or it may coincide on a TRIP LS.  In the latter
 case, the same TRIP LS will be member in more than one ITAD, and it
 appears to be an internal peer to LSs in each ITAD it is member of.

13. IANA Considerations

 This document creates a new IANA registry for TRIP parameters.  The
 following TRIP parameters are included in the registry:
  1. TRIP Capabilities
  2. TRIP Attributes
  3. TRIP Address Families
  4. TRIP Application Protocols
  5. TRIP ITAD Numbers
 Protocol parameters are frequently initialized/reset to 0.  This
 document reserves the value 0 of each of the above TRIP parameters in
 order to clearly distinguish between an unset parameter and any other
 registered values for that parameter.
 The sub-registries for each of the above parameters are discussed in
 the sections below.

13.1. TRIP Capabilities

 Requests to add TRIP capabilities other than those defined in Section
 4.2.1.1 must be submitted to iana@iana.org.  Following the assigned
 number policies outlined in [11], Capability Codes in the range
 32768-65535 are reserved for Private Use (these are the codes with
 the first bit of the code value equal to 1).  This document reserves
 value 0.  Capability Codes 1 and 2 have been assigned in Section
 4.2.1.1.  Capability Codes in the range 2-32767 are controlled by

Rosenberg, et. al. Standards Track [Page 68] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 IANA, and are allocated subject to the Specification Required (IETF
 RFC or equivalent) condition.  The specification MUST include a
 description of the capability, the possible values it may take, and
 what constitutes a capability mismatch.

13.2. TRIP Attributes

 This document reserves Attribute Type Codes 224-255 for Private Use
 (these are the codes with the first three bits of the code equal to
 1).  This document reserves the value 0.  Attribute Type Codes 1
 through 11 have already been allocated by this document.  Attribute
 Type Codes 1 through 11 are defined in Sections 5.1 through 5.11.
 Attribute Type Codes in the range 12-223 are controlled by IANA, and
 require a Specification document (RFC or equivalent).  The
 specification MUST provide all information required in Section 5.12
 of this document.
 Attribute Type Code registration requests must be sent to
 iana@iana.org.  In addition to the specification requirement, the
 request MUST include an indication of who has change control over the
 attribute and contact information (postal and email address).

13.3. Destination Address Families

 This document reserves address family 0. Requests to add TRIP address
 families other than those defined in Section 5.1.1.1 ( address
 families 1, 2, and 3), i.e., in the range 4-32767, must be submitted
 to iana@iana.org.  The request MUST include a brief description of
 the address family, its alphabet, and special processing rules and
 guidelines, such as guidelines for aggregation, if any.  The requests
 are subject to Expert Review.  This document reserves the address
 family codes 32768-65535 for vendor-specific applications.

13.4. TRIP Application Protocols

 This document creates a new IANA registry for TRIP application
 protocols.  This document reserves the application protocol code 0.
 Requests to add TRIP application protocols other than those defined
 in Section 5.1.1.1 (application protocols 1 through 4), i.e., in the
 range 5-32767, must be submitted to iana@iana.org.  The request MUST
 include a brief background on the application protocol, and a
 description of how TRIP can be used to advertise routes for that
 protocol.  The requests are subject to Expert Review.  This document
 reserves the application protocol codes 32768-65535 for vendor-
 specific applications.

Rosenberg, et. al. Standards Track [Page 69] RFC 3219 Telephony Routing over IP (TRIP) January 2002

13.5. ITAD Numbers

 This document reserves the ITAD number 0.  ITAD numbers in the range
 1-255 are designated for Private Use.  ITAD numbers in the range from
 256 to (2**32)-1 are allocated by IANA on a First-Come-First-Serve
 basis.  Requests for ITAD numbers must be submitted to iana@iana.org.
 The requests MUST include the following:
  1. Information about the organization that will administer the

ITAD.

  1. Contact information (postal and email address).

14. Security Considerations

 This section covers security between peer TRIP LSs when TRIP runs
 over TCP in an IP environment.
 A security mechanism is clearly needed to prevent unauthorized
 entities from using the protocol defined in this document for setting
 up unauthorized peer sessions with other TRIP LSs or interfering with
 authorized peer sessions.  The security mechanism for the protocol,
 when transported over TCP in an IP network, is IPsec [12].  IPsec
 uses two protocols to provide traffic security: Authentication Header
 (AH) [13] and Encapsulating Security Payload (ESP) [14].
 The AH header affords data origin authentication, connectionless
 integrity and optional anti-replay protection of messages passed
 between the peer LSs.  The ESP header provides origin authentication,
 connectionless integrity, anti-replay protection, and confidentiality
 of messages.
 Implementations of the protocol defined in this document employing
 the ESP header SHALL comply with section 5 of [14], which defines a
 minimum set of algorithms for integrity checking and encryption.
 Similarly, implementations employing the AH header SHALL comply with
 section 5 of [13], which defines a minimum set of algorithms for
 integrity checking using manual keys.
 Implementations SHOULD use IKE [15] to permit more robust keying
 options.  Implementations employing IKE SHOULD support authentication
 with RSA signatures and RSA public key encryption.
 A Security Association (SA) [12] is a simplex "connection" that
 affords security services to the traffic carried by it.  Security
 services are afforded to a SA by the use of AH, or ESP, but not both.
 Two types of SAs are defined: transport mode and tunnel mode [12].  A
 transport mode SA is a security association between two hosts, and is
 appropriate for protecting the TRIP session between two peer LSs.

Rosenberg, et. al. Standards Track [Page 70] RFC 3219 Telephony Routing over IP (TRIP) January 2002

A1. Appendix 1: TRIP FSM State Transitions and Actions

 This Appendix discusses the transitions between states in the TRIP
 FSM in response to TRIP events.  The following is the list of these
 states and events when the negotiated Hold Time value is non-zero.
 TRIP States:
    1 - Idle
    2 - Connect
    3 - Active
    4 - OpenSent
    5 - OpenConfirm
    6 - Established
 TRIP Events:
    1 - TRIP Start
    2 - TRIP Stop
    3 - TRIP Transport connection open
    4 - TRIP Transport connection closed
    5 - TRIP Transport connection open failed
    6 - TRIP Transport fatal error
    7 - ConnectRetry timer expired
    8 - Hold Timer expired
    9 - KeepAlive timer expired
    10 - Receive OPEN message
    11 - Receive KEEPALIVE message
    12 - Receive UPDATE messages
    13 - Receive NOTIFICATION message
 The following table describes the state transitions of the TRIP FSM
 and the actions triggered by these transitions.
 Event                Actions              Message Sent    Next State
 --------------------------------------------------------------------
 Idle (1)
  1            Initialize resources            none             2
               Start ConnectRetry timer
               Initiate a transport connection
  others               none                    none             1
 Connect(2)
  1                    none                    none             2
  3            Complete initialization         OPEN             4
               Clear ConnectRetry timer
  5            Restart ConnectRetry timer      none             3
  7            Restart ConnectRetry timer      none             2
               Initiate a transport connection
  others       Release resources               none             1

Rosenberg, et. al. Standards Track [Page 71] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 Active (3)
  1                    none                    none             3
  3            Complete initialization         OPEN             4
               Clear ConnectRetry timer
  5            Close connection                                 3
               Restart ConnectRetry timer
  7            Restart ConnectRetry timer      none             2
               Initiate a transport connection
  others       Release resources               none             1
 OpenSent(4)
  1                    none                    none             4
  4            Close transport connection      none             3
               Restart ConnectRetry timer
  6            Release resources               none             1
 10            Process OPEN is OK            KEEPALIVE          5
               Process OPEN failed           NOTIFICATION       1
 others        Close transport connection    NOTIFICATION       1
               Release resources
 OpenConfirm (5)
  1                   none                     none             5
  4            Release resources               none             1
  6            Release resources               none             1
  9            Restart KeepAlive timer       KEEPALIVE          5
 11            Complete initialization         none             6
               Restart Hold Timer
 13            Close transport connection                       1
               Release resources
 others        Close transport connection    NOTIFICATION       1
               Release resources
 Established (6)
  1                   none                     none             6
  4            Release resources               none             1
  6            Release resources               none             1
  9            Restart KeepAlive timer       KEEPALIVE          6
 11            Restart Hold Timer              none             6
 12            Process UPDATE is OK          UPDATE             6
               Process UPDATE failed         NOTIFICATION       1
 13            Close transport connection                       1
               Release resources
 others        Close transport connection    NOTIFICATION       1
               Release resources
 -----------------------------------------------------------------

Rosenberg, et. al. Standards Track [Page 72] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 The following is a condensed version of the above state transition
 table.
 Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
       | (1)  |   (2)   |  (3)   |    (4)   |     (5)     |   (6)
       |----------------------------------------------------------
  1    |  2   |    2    |   3    |     4    |      5      |    6
       |      |         |        |          |             |
  2    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  3    |  1   |    4    |   4    |     1    |      1      |    1
       |      |         |        |          |             |
  4    |  1   |    1    |   1    |     3    |      1      |    1
       |      |         |        |          |             |
  5    |  1   |    3    |   3    |     1    |      1      |    1
       |      |         |        |          |             |
  6    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  7    |  1   |    2    |   2    |     1    |      1      |    1
       |      |         |        |          |             |
  8    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  9    |  1   |    1    |   1    |     1    |      5      |    6
       |      |         |        |          |             |
 10    |  1   |    1    |   1    |  1 or 5  |      1      |    1
       |      |         |        |          |             |
 11    |  1   |    1    |   1    |     1    |      6      |    6
       |      |         |        |          |             |
 12    |  1   |    1    |   1    |     1    |      1      | 1 or 6
       |      |         |        |          |             |
 13    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
       --------------------------------------------------------------

A2. Appendix 2: Implementation Recommendations

 This section presents some implementation recommendations.

A.2.1: Multiple Networks Per Message

 The TRIP protocol allows for multiple address prefixes with the same
 advertisement path and next-hop server to be specified in one
 message.  Making use of this capability is highly recommended.  With
 one address prefix per message there is a substantial increase in
 overhead in the receiver.  Not only does the system overhead increase
 due to the reception of multiple messages, but the overhead of
 scanning the routing table for updates to TRIP peers is incurred
 multiple times as well.  One method of building messages containing

Rosenberg, et. al. Standards Track [Page 73] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 many address prefixes per advertisement path and next hop from a
 routing table that is not organized per advertisement path is to
 build many messages as the routing table is scanned.  As each address
 prefix is processed, a message for the associated advertisement path
 and next hop is allocated, if it does not exist, and the new address
 prefix is added to it.  If such a message exists, the new address
 prefix is just appended to it.  If the message lacks the space to
 hold the new address prefix, it is transmitted, a new message is
 allocated, and the new address prefix is inserted into the new
 message.  When the entire routing table has been scanned, all
 allocated messages are sent and their resources released.  Maximum
 compression is achieved when all the destinations covered by the
 address prefixes share the same next hop server and common
 attributes, making it possible to send many address prefixes in one
 4096-byte message.
 When peering with a TRIP implementation that does not compress
 multiple address prefixes into one message, it may be necessary to
 take steps to reduce the overhead from the flood of data received
 when a peer is acquired or a significant network topology change
 occurs.  One method of doing this is to limit the rate of updates.
 This will eliminate the redundant scanning of the routing table to
 provide flash updates for TRIP peers.  A disadvantage of this
 approach is that it increases the propagation latency of routing
 information.  By choosing a minimum flash update interval that is not
 much greater than the time it takes to process the multiple messages,
 this latency should be minimized.  A better method would be to read
 all received messages before sending updates.

A.2.2: Processing Messages on a Stream Protocol

 TRIP uses TCP as a transport mechanism.  Due to the stream nature of
 TCP, all the data of a received message does not necessarily arrive
 at the same time.  This can make it difficult to process the data as
 messages, especially on systems where it is not possible to determine
 how much data has been received but not yet processed.
 One method that can be used in this situation is to first try to read
 just the message header.  For the KEEPALIVE message type, this is a
 complete message; for other message types, the header should first be
 verified, in particular the total length.  If all checks are
 successful, the specified length, minus the size of the message
 header is the amount of data left to read.  An implementation that
 would "hang" the routing information process while trying to read
 from a peer could set up a message buffer (4096 bytes) per peer and
 fill it with data as available until a complete message has been
 received.

Rosenberg, et. al. Standards Track [Page 74] RFC 3219 Telephony Routing over IP (TRIP) January 2002

A.2.3: Reducing Route Flapping

 To avoid excessive route flapping an LS which needs to withdraw a
 destination and send an update about a more specific or less specific
 route SHOULD combine them into the same UPDATE message.

A.2.4: TRIP Timers

 TRIP employs seven timers: ConnectRetry, Hold Time, KeepAlive,
 MaxPurgeTime, TripDisableTime, MinITADOriginationInterval, and
 MinRouteAdvertisementInterval.  The suggested value for the
 ConnectRetry timer is 120 seconds.  The suggested value for the Hold
 Time is 90 seconds.  The suggested value for the KeepAlive timer is
 30 seconds.  The suggested value for the MaxPurgeTime timer is 10
 seconds.  The suggested value for the TripDisableTime timer is 180
 seconds.  The suggested value for the MinITADOriginationInterval is
 30 seconds.  The suggested value for the
 MinRouteAdvertisementInterval is 30 seconds.
 An implementation of TRIP MUST allow these timers to be configurable.

A.2.5: AP_SET Sorting

 Another useful optimization that can be done to simplify this
 situation is to sort the ITAD numbers found in an AP_SET.  This
 optimization is entirely optional.

Acknowledgments

 We wish to thank Dave Oran for his insightful comments and
 suggestions.

References

 [1]   Bradner, S., "Keywords for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [2]   Rosenberg, J. and H. Schulzrinne, "A Framework for a Gateway
       Location Protocol", RFC 2871, June 2000.
 [3]   Rekhter, Y. and T. Li, "Border Gateway Protocol 4 (BGP-4)," RFC
       1771, March 1995.
 [4]   Moy, J., "Open Shortest Path First Version 2", STD 54, RFC
       2328, April 1998.

Rosenberg, et. al. Standards Track [Page 75] RFC 3219 Telephony Routing over IP (TRIP) January 2002

 [5]   "Intermediate System to Intermediate System Intra-Domain
       Routing Exchange Protocol for use in Conjunction with the
       Protocol for Providing the Connectionless-mode Network Service
       (ISO 8473)," ISO DP 10589, February 1990.
 [6]   Luciani, J., Armitage, G., Halpern, J. and N. Doraswamy,
       "Server Cache Synchronization Protocol (SCSP)", RFC 2334, April
       1998.
 [7]   International Telecommunication Union, "Packet-Based Multimedia
       Communication Systems," Recommendation H.323, Version 3
       Telecommunication Standardization Sector of ITU, Geneva,
       Switzerland, November 2000.
 [8]   Handley, H., Schulzrinne, H., Schooler, E. and J. Rosenberg,
       "SIP:  Session Initiation Protocol", RFC 2543, March 1999.
 [9]   Braden, R., "Requirements for Internet Hosts -- Application and
       Support", STD 3, RFC 1123, October 1989.
 [10]  Hinden, R. and S. Deering, "IP Version 6 Addressing
       Architecture", RFC 2373, July 1998.
 [11]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October
       1998.
 [12]  Kent, S. and R. Atkinson, "Security Architecture for the
       Internet Protocol", RFC 2401, November 1998.
 [13]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
       November 1998.
 [14]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
       (ESP)", RFC 2406, November 1998.
 [15]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
       RFC 2409, November 1998.

Rosenberg, et. al. Standards Track [Page 76] RFC 3219 Telephony Routing over IP (TRIP) January 2002

Intellectual Property Notice

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 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
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 proprietary rights by implementers or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
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 The IETF has been notified of intellectual property rights claimed in
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 document.  For more information consult the online list of claimed
 rights.

Rosenberg, et. al. Standards Track [Page 77] RFC 3219 Telephony Routing over IP (TRIP) January 2002

Authors' Addresses

 Jonathan Rosenberg
 dynamicsoft
 72 Eagle Rock Avenue
 First Floor
 East Hanover, NJ 07936
 Phone: 973-952-5000
 EMail: jdrosen@dynamicsoft.com
 Hussein F. Salama
 Cisco Systems
 170 W. Tasman Drive
 San Jose, CA 95134
 Phone: 408-527-7147
 EMail: hsalama@cisco.com
 Matt Squire
 Hatteras Networks
 639 Davis Drive
 Suite 200
 Durham, NC 27713
 EMail: mattsquire@acm.org

Rosenberg, et. al. Standards Track [Page 78] RFC 3219 Telephony Routing over IP (TRIP) January 2002

Full Copyright Statement

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Acknowledgement

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Rosenberg, et. al. Standards Track [Page 79]

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