IPTEL Working Group                             J. Rosenberg, dynamicsoft
Internet Draft                                   H. Salama, Cisco Systems
draft-ietf-iptel-trip-06.txt                          M. Squire, WindWire
May 2001
Expires November 2001


           Telephony Routing over IP (TRIP)

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as 'work in
   progress.' The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

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

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




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Table of Contents
   Status of this Memo                                               1
   Abstract                                                          1
   Table of Contents                                                 2
   1. Terminology and Definitions                                    6
   2. Introduction                                                   6
   3. Summary of Operation                                           8
   3.1  Peering Session Establishment and Maintenance                8
   3.2  Database Exchanges                                           8
   3.3  Internal Versus External Synchronization                     9
   3.4  Advertising TRIP Routes                                      9
   3.5  Telephony Routing Information Bases                         10
   4. Message Formats                                               12
   4.1  Message Header Format                                       12
   4.2  OPEN Message Format                                         13
   4.2.1 Open Message Optional Parameters                           15
   4.2.1.1 Capability Information                                   15
   4.2.1.1.1 Route Types Supported                                  16
   4.2.1.1.2 Send Receive Capability                                16
   4.3  UPDATE Message Format                                       18
   4.3.1 Routing Attributes                                         18
   4.3.2 Attribute Flags                                            19
   4.3.2.1 Attribute Flags and Route Selection                      20
   4.3.2.2 Attribute Flags and Route Dissemination                  20
   4.3.2.3 Attribute Flags and Route Aggregation                    21
   4.3.2.4 Attribute Flags and Encapsulation                        22
   4.3.3 Mandatory Attributes                                       22
   4.3.4 TRIP UPDATE Attributes                                     23
   4.3.4.1 WithdrawnRoutes                                          23
   4.3.4.2 ReachableRoutes                                          23
   4.3.4.3 NextHopServer                                            23
   4.3.4.4 AdvertisementPath                                        23
   4.3.4.5 RoutedPath                                               23
   4.3.4.6 AtomicAggregate                                          24
   4.3.4.7 LocalPreference                                          24
   4.3.4.8 MultiExitDisc                                            24
   4.3.4.9 Communities                                              24
   4.3.4.10 ITAD Topology                                           24
   4.3.4.11 ConvertedRoute                                          24
   4.4  KEEPALIVE Message Format                                    25
   4.5  NOTIFICATION Message Format                                 25
   5. TRIP Attributes                                               27
   5.1  WithdrawnRoutes                                             27


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   5.1.1 Syntax of WithdrawnRoutes                                  27
   5.1.1.1 Generic TRIP Route Format                                27
   5.1.1.2 Decimal Routing Numbers                                  28
   5.1.1.3 PentaDecimal Routing Numbers                             29
   5.1.1.4 E.164 Numbers                                            30
   5.2  ReachableRoutes                                             30
   5.2.1 Syntax of ReachableRoutes                                  31
   5.2.2 Route Origination and ReachableRoutes                      31
   5.2.3 Route Selection and ReachableRoutes                        31
   5.2.4 Aggregation and ReachableRoutes                            31
   5.2.5 Route Dissemination and ReachableRoutes                    31
   5.2.6 Aggregation Specifics for Decimal Routing Numbers, E.164
         Numbers, and PentaDecimal Routing Numbers                  31
   5.3  NextHopServer                                               32
   5.3.1 NextHopServer Syntax                                       32
   5.3.2 Route Origination and NextHopServer                        33
   5.3.3 Route Selection and NextHopServer                          33
   5.3.4 Aggregation and NextHopServer                              33
   5.3.5 Route Dissemination and NextHopServer                      33
   5.4  AdvertisementPath                                           34
   5.4.1 AdvertisementPath Syntax                                   34
   5.4.2 Route Origination and AdvertisementPath                    34
   5.4.3 Route Selection and AdvertisementPath                      35
   5.4.4 Aggregation and AdvertisementPath                          35
   5.4.4.1 Aggregating Routes with Identical Paths                  35
   5.4.4.2 Aggregating Routes with Different Paths                  35
   5.4.4.3 Example Path Aggregation Algorithm                       36
   5.4.5 Route Dissemination and AdvertisementPath                  37
   5.5  RoutedPath                                                  37
   5.5.1 RoutedPath Syntax                                          37
   5.5.2 Route Origination and RoutedPath                           38
   5.5.3 Route Selection and RoutedPath                             38
   5.5.4 Aggregation and RoutedPath                                 38
   5.5.5 Route Dissemination and RoutedPath                         38
   5.6  AtomicAggregate                                             39
   5.6.1 AtomicAggregate Syntax                                     39
   5.6.2 Route Origination and AtomicAggregate                      39
   5.6.3 Route Selection and AtomicAggregate                        39
   5.6.4 Aggregation and AtomicAggregate                            39
   5.6.5 Route Dissemination and AtomicAggregate                    39
   5.7  LocalPreference                                             40
   5.7.1 LocalPreference Syntax                                     40
   5.7.2 Route Origination and LocalPreference                      40
   5.7.3 Route Selection and LocalPreference                        40


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   5.7.4 Aggregation and LocalPreference                            40
   5.7.5 Route Dissemination and LocalPreference                    40
   5.8  MultiExitDisc                                               40
   5.8.1 MultiExitDisc Syntax                                       41
   5.8.2 Route Origination and MultiExitDisc                        41
   5.8.3 Route Selection and MultiExitDisc                          41
   5.8.4 Aggregation and MultiExitDisc                              41
   5.8.5 Route Dissemination and MultiExitDisc                      41
   5.9  Communities                                                 41
   5.9.1 Syntax of Communities                                      42
   5.9.2 Route Origination and Communities                          43
   5.9.3 Route Selection and Communities                            43
   5.9.4 Aggregation and Communities                                44
   5.9.5 Route Dissemination and Communities                        44
   5.10  ITAD Topology                                              44
   5.10.1  ITAD Topology Syntax                                     44
   5.10.2  Route Origination and ITAD Topology                      45
   5.10.3  Route Selection and ITAD Topology                        45
   5.10.4  Aggregation and ITAD Topology                            45
   5.10.5  Route Dissemination and ITAD Topology                    45
   5.11  ConvertedRoute                                             45
   5.11.1  ConvertedRoute Syntax                                    46
   5.11.2  Route Origination and ConvertedRoute                     46
   5.11.3  Route Selection and ConvertedRoute                       46
   5.11.4  Aggregation and ConvertedRoute                           46
   5.11.5  Route Dissemination and ConvertedRoute                   46
   5.12  Considerations for Defining New TRIP Attributes            46
   6. TRIP Error Detection and Handling                             46
   6.1  Message Header Error Detection and Handling                 47
   6.2  OPEN Message Error Detection and Handling                   47
   6.3  UPDATE Message Error Detection and Handling                 49
   6.4  NOTIFICATION Message Error Detection and Handling           50
   6.5  Hold Timer Expired Error Handling                           50
   6.6  Finite State Machine Error Handling                         50
   6.7  Cease                                                       51
   6.8  Connection Collision Detection                              51
   7. TRIP Version Negotiation                                      52
   8. TRIP Capability Negotiation                                   52
   9. TRIP Finite State Machine                                     52
   10.  UPDATE Message Handling                                     58
   10.1  Flooding Process                                           59
   10.1.1  Database Information                                     59
   10.1.2  Determining Newness                                      59
   10.1.3  Flooding                                                 59


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   10.1.4  Sequence Number Considerations                           60
   10.1.5  Purging a Route Within the ITAD                          60
   10.1.6  Receiving Self-Originated Routes                         61
   10.1.7  Removing Withdrawn Routes                                61
   10.2  Decision Process                                           61
   10.2.1  Phase 1: Calculation of Degree of Preference             62
   10.2.2  Phase 2: Route Selection                                 63
   10.2.2.1 Breaking Ties (Phase 2)                                 64
   10.2.3  Phase 3: Route Dissemination                             64
   10.2.4  Overlapping Routes                                       65
   10.3  Update-Send Process                                        65
   10.3.1  Internal Updates                                         66
   10.3.1.1 Breaking Ties (Routes Received from External Peers)     67
   10.3.2  External Updates                                         67
   10.3.3  Controlling Routing Traffic Overhead                     67
   10.3.3.1 Frequency of Route Advertisement                        68
   10.3.3.2 Frequency of Route Origination                          68
   10.3.3.3 Jitter                                                  68
   10.3.4  Efficient Organization of Routing Information            69
   10.3.4.1 Information Reduction                                   69
   10.3.4.2 Aggregating Routing Information                         70
   10.4  Route Selection Criteria                                   70
   10.5  Originating TRIP routes                                    71
   11.  TRIP Transport                                              71
   12.  ITAD Topology                                               71
   13.  IANA Considerations                                         71
   13.1  TRIP Capabilities                                          71
   13.2  TRIP Attributes                                            72
   13.3  Destination Address Families                               72
   13.4  TRIP Application Protocols                                 72
   13.5  ITAD Numbers                                               73
   14.  Security Considerations                                     73
   Appendix 1. TRIP FSM State Transitions and Actions               74
   Appendix 2. Implementation Recommendations                       77
   A.2.1.  Multiple Networks Per Message                            77
   A.2.2.  Processing Messages on a Stream Protocol                 78
   A.2.4.  TRIP Timers                                              78
   A.2.5.  AP_SET Sorting                                           78
   Acknowledgments                                                  79
   References                                                       79
   Authors' Addresses                                               80
   Intellectual Property Notice                                     80
   Full Copyright Statement                                         81



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1.   Terminology and Definitions
   The key words 'MUST,' 'REQUIRED,' 'SHOULD,' 'RECOMMENDED,' and 'MAY'
   in this document are to be interpreted as described in RFC2119 [1].

   A framework for a 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.

   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.

   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


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   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 drafts.

   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].





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   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 ITAD are independent of
   the boundaries of the layer 3 routing autonomous systems.  Neither
   internal nor external TRIP peers need 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.

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 case of an
   external peer, all routes in the LS's Adj-TRIB-Out for that external
   peer. In 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.


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   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).

   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 the 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


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   are three methods by which a given LS can indicate that a route has
   been withdrawn from service:

   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 the 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:

   - External routes: An external route is a route received from an
     external peer LS
   - Internal routes: An internal route is a route received from an
     internal LS in the same ITAD.
   - 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:

   - 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



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     that there is an Adj-TRIBs-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 three parts of the
   routing information base.

                           Loc-TRIB
                              /\
                               |
                       Decision Process
                        /\    /\      |
                        |      |      |
               Adj-TRIBs-In    |     \/
              (Internal LSs)   |   Adj-TRIBs-Out
                               |
                               |
                               |
                            Ext-TRIB
                           /\      /\
                           |        |
                  Adj-TRIB-In      Local Routes
              (External Peers)

                       Figure 1: TRIB Relationships

   Although the conceptual model distinguishes between Adj-TRIBs-In,
   Loc-TRIB, and Adj-TRIBs-Out, this neither implies nor requires that
   an implementation must maintain three separate copies of the routing
   information. The choice of implementation (for example, 3 copies of


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   the information vs. 1 copy with pointers) is not constrained by the
   protocol.

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




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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 14 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.

   My ITAD:



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   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.

   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 is a 2-octet field that unambiguously identifies
   individual parameters.

   Parameter Length is a 2-octet field that contains the length of the
   Parameter Value field in octets.




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   Parameter Value is a variable length field that is interpreted
   according to the value of the Parameter Type field.

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.




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   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.

   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 <POTS, SIP>, indicating a set of POTS
   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,


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   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.

   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 termination 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, both of
   them in either Send Only mode or 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.'



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   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.

   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 11 octets (the TRIP header
   plus at least the WithdrawnRoutes and ReachableRoutes attributes).

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.


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   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.

   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


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   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
   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


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   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).

   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


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   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.

    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 within the servers.  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 UDATE
   message is entirely optional and independent of any other
   attributes.




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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.

4.3.4.2     ReachableRoutes
   This attribute lists 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 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


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   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 over which 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 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 LSs 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 a not well-known attribute 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



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   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.

   If the negotiated Hold Time interval is zero, then periodic
   KEEPALIVE messages MUST NOT be sent.

   KEEPALIVE message consists of only 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


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     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.

   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).



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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 attribute MUST be included in every UPDATE
   message.  It 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.

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:


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   The address family field gives the type of address for the route.
   Two 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.

   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.


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   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
   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,


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   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 'F', 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'

   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 egress 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 MUST be included in every UPDATE
   message.  It specifies a set of routes that are to be added to



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   service by the receiving LS(s).  The set of routes MAY be empty,
   this is 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.

   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 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.




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   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.

   Given a route with application protocol A and destinations D, the
   NextHopServer indicates the next-hop that messages of protocol A
   destined for D should be sent.  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


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   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.

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.





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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 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:

      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:

   - The originating LS shall include its own ITAD number in the
     AdvertisementPath attribute of all advertisements sent to LSs

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     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.

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:

   - All pairs of the type AP_SEQUENCE in the aggregated path MUST
     appear in all of the paths of routes to be aggregated.
   - 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).

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   - 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:

   - 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.

5.4.4.3     Example Path Aggregation Algorithm
   An example algorithm to aggregate two paths works as follows:

   - 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


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     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.

   - 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:

         - 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.

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5.5.2 Route Origination and RoutedPath
   When an LS originates a route it MUST include the RoutedPath
   attribute.

   - 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.

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.

   - When a 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).

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       - 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.

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.



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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.

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.




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   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.

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.



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   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
   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 component information 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 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                 |


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   +---------------+---------------+--------------+----------------+
   |                         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.

   - 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 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.





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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.

   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 to which LSs are active within their domain 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

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    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 a LS changes, it MUST
   originate 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.

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


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   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.

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 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:
   - the use of this attribute,
   - the attribute's flags,
   - the attribute's syntax,
   - how the attribute works with route origination,
   - how the attribute works with route aggregation, and
   - 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.


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   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
   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 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 on 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 Error Code 'OPEN Message
   Error.'  The Error Subcode elaborates on the specific nature of the


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   error. The error checks in this section MUST be performed by each LS
   on 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 less than the version 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.

   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 and can take any value. An LS considers the
   TRIP Identifier invalid if it has an already 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 differrent 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.




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   If the Optional Parameters of the OPEN message include Capability
   Information which do 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 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 on receipt of every UPDATE message.  These
   error checks MUST occur before flooding procedures are invoked with
   internal peers.

   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 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.




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   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, but 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.'

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 '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 Error Code 'Finite State Machine Error' and the TRIP connection
   MUST be closed.




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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 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.

   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 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 accepts the
   TRIP connection initiated by the remote LS:

   1.   Otherwise, the local LS closes newly created TRIP connection
        (the one associated with the newly received OPEN message), and



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        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 of the newly created connection. Note
        that a connection collision cannot be detected with connections
        that are 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.

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 a 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

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   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.

   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.


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   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.

   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.




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   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.

   If the Hold Timer expires, the local LS sends NOTIFICATION message
   with 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 NOTIFICATION message with Error Code
   'Cease' and changes its state to Idle.

   The Start event is ignored in the OpenSent state.



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   In response to any other event the local LS sends NOTIFICATION
   message with 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 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 Error Code
   'Cease' and changes its state to Idle.

   Start event is ignored in the OpenConfirm state.

   In response to any other event the local LS sends NOTIFICATION
   message with Error Code 'Finite State Machine Error' and changes its
   state to Idle.




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   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 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 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 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, releases all resources associated with
   that connection.  Additionally, if the peer is an external peer, the
   LS deletes all routes derived from that connection.



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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 run 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 run 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.



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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).

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.



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   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 on 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


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   greater than that used in the withdraw, or the LS may wait until
   MaxPurgeTime seconds have expired since the route was withdrawn.

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
   individual application of the degree of preference function to each


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   feasible route, followed by the choice of the one with the highest
   degree of preference.

   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:

   - selection of routes to be advertised to internal peers
   - selection of routes to be advertised to external peers
   - route aggregation and route information reduction

   The Decision Process takes place in three distinct phases, each
   triggered by a different event:

   - Phase 1 is responsible for calculating the degree of preference
     for each route received from an external peer, and for advertising
     to all the internal peers the routes from external peers that have
     the highest degree of preference for each distinct destination.
   - 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 completes
   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.



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   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.

   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:


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   - the highest degree of preference, or
   - 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
   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.

   - 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 completes 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.


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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 more
   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.

   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:

   - Install both the less and the more specific routes
   - Install the more specific route only
   - Install the non-overlapping part of the less specific route only
     (that implies disaggregation of the less-specific route)
   - Aggregate the two routes and install the aggregated route
   - Install the less specific route only
   - Install neither route

   If an LS chooses e), 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 a), then it MUST NOT advertise the more general 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.


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   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.

   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:


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   - 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
   destination, all of which were advertised by external peers. The
   local LS shall select one of these routes according to the following
   rules:

   - 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 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.

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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
   to some common set of destinations received from external peers will
   be at least MinRouteAdvertisementInterval, and will also ensure 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

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   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.

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 is 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:

   a) 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.

   b) 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


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   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.

   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:

   - 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.



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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.

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
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 IANA, and are allocated subject to the Specification


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   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 value 0.  Attribute Type Codes 1 through
   11 have already been allocated by this document.

   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 3-32767, 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 addresss family codes 32768-65535 for vendor-specific
   applications.

13.4 TRIP Application Protocols
   This document reserves 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
   application protocol codes 32768-65535 for vendor-specific
   applications.




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13.5 ITAD Numbers
   This document reserves 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:
   - Information about the organization that will administer the ITAD.
   - Contact information (postal and email address).

   IANA may delegate the responsibility for allocating subsets of the
   ITAD number space (from 256 to (2**32-1)) to other organizations
   (similar to the delegation of BGP AS number assignment).

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, in addition,
   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.




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   A Security Association (SA) [12] is a simplex 'connection' that
   affords security services to the traffic carried by it.  Security
   services are afforded to an 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.

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


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                 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

   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



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   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            KEEPALIVE          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
   -----------------------------------------------------------------

   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
         |      |         |        |          |             |


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   13    |  1   |    1    |   1    |     1    |      1      |    1
         |      |         |        |          |             |
         --------------------------------------------------------------

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
   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 a 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.


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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 for received messages 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.

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.


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

References
   [1]  S. Bradner, 'Keywords for use in RFCs to Indicate Requirement
        Levels,' IETF RFC 2119, March 1997.

   [2]  J. Rosenberg and H. Schulzrinne, 'A Framework for a Gateway
        Location Protocol,' IETF RFC 2871, June 2000.

   [3]  Y. Rekhter and T. Li, 'Border Gateway Protocol 4 (BGP-4),' IETF
        RFC 1771, March 1995.

   [4]  J. Moy, 'Open Shortest Path First Version 2,' IETF RFC 2328,
        April, 1998.

   [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]  J. Luciani, et al, 'Server Cache Synchronization Protocol
        (SCSP),' IETF RFC 2334, April, 1998.

   [7]  International Telecommunication Union, 'Visual Telephone
        Systems and Equipment for Local Area Networks which Provide a
        Non-Guaranteed Quality of Service,' Recommendation H.323,
        Telecommunication Standardization Sector of ITU, Geneva,
        Switzerland, May 1996.

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

   [9]  R. Braden, 'Requirements for Internet Hosts -- Application and
        Support,' IETF RFC 1123, October 1989.

   [10] R. Hinden and S. Deering, 'IP Version 6 Addressing
        Architecture,' IETF RFC 2373, July 1998.

   [11] T. Narten and H. Alvestrand, 'Guidelines for Writing an IANA
        Considerations Section in RFCs,' IETF RFC 2434, October 1998.



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   [12] S. Kent and R. Atkinson, 'Security Architecture for the
        Internet Protocol,' IETF RFC 2401, November 1998.

   [13] S. Kent and R. Atkinson, 'IP Authentication Header,' IETF RFC
        2402, November 1998.

   [14] S. Kent and R. Atkinson, 'IP Encapsulating Security Payload
        (ESP),' IETF RFC 2406, November 1998.

   [15] D. Harkins and D. Carrel, 'The Internet Key Exchange (IKE),'
        IETF RFC 2409, November 1998.

Authors' Addresses
   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   973-952-5000
   email: jdrosen@dynamicsoft.com

   Hussein F. Salama
   Cisco Systems
   Mail Stop SJ-6/3
   170 W. Tasman Drive
   San Jose, CA 95134
   408-527-7147
   email: hsalama@cisco.com

   Matt Squire
   WindWire
   4825 Creekstone Drive
   Durham, NC 27703
   919-247-0820
   email: msquire@windwire.com

Intellectual Property Notice
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   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


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   has made any effort to identify any such rights.  Information on the
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