Network Working Group                                       J. Rosenberg
Request for Comments: 3219                                   dynamicsoft
Category: Standards Track                                      H. Salama
                                                          Cisco Systems
                                                              M. Squire
                                                      Hatteras Networks
                                                           January 2002


                   Telephony Routing over IP (TRIP)

Status of this Memo

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

Copyright Notice

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

Abstract

  This document presents the Telephony Routing over IP (TRIP).  TRIP is
  a policy driven inter-administrative domain protocol for advertising
  the reachability of telephony destinations between location servers,
  and for advertising attributes of the routes to those destinations.
  TRIP's operation is independent of any signaling protocol, hence TRIP
  can serve as the telephony routing protocol for any signaling
  protocol.

  The Border Gateway Protocol (BGP-4) is used to distribute routing
  information between administrative domains.  TRIP is used to
  distribute telephony routing information between telephony
  administrative domains.  The similarity between the two protocols is
  obvious, and hence TRIP is modeled after BGP-4.

Table of Contents

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



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RFC 3219            Telephony Routing over IP (TRIP)        January 2002


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



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RFC 3219            Telephony Routing over IP (TRIP)        January 2002


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

1. Terminology and Definitions

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

  A framework for Telephony Routing over IP (TRIP) is described in [2].
  We assume the reader is familiar with the framework and terminology
  of [2].  We define and use the following terms in addition to those
  defined in [2].

  Telephony Routing Information Base (TRIB): The database of reachable
  telephony destinations built and maintained at an LS as a result of
  its participation in TRIP.

  IP Telephony Administrative Domain (ITAD): The set of resources
  (gateways, location servers, etc.) under the control of a single
  administrative authority.  End users are customers of an ITAD.

  Less/More Specific Route: A route X is said to be less specific than
  a route Y if every destination in Y is also a destination in X, and X
  and Y are not equal.  In this case, Y is also said to be more
  specific than X.

  Aggregation: Aggregation is the process by which multiple routes are
  combined into a single less specific route that covers the same set
  of destinations.  Aggregation is used to reduce the size of the TRIB
  being synchronized with peer LSs by reducing the number of exported
  TRIP routes.

  Peers: Two LSs that share a logical association (a transport
  connection).  If the LSs are in the same ITAD, they are internal
  peers.  Otherwise, they are external peers.  The logical association
  between two peer LSs is called a peering session.








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  Telephony Routing Information Protocol (TRIP): The protocol defined
  in this specification.  The function of TRIP is to advertise the
  reachability of telephony destinations, attributes associated with
  the destinations, as well as the attributes of the path towards those
  destinations.

  TRIP destination: TRIP can be used to manage routing tables for
  multiple protocols (SIP, H323, etc.).  In TRIP, a destination is the
  combination of (a) a set of addresses (given by an address family and
  address prefix), and (b) an application protocol (SIP, H323, etc).

2. Introduction

  The gateway location and routing problem has been introduced in [2].
  It is considered one of the more difficult problems in IP telephony.
  The selection of an egress gateway for a telephony call, traversing
  an IP network towards an ultimate destination in the PSTN, is driven
  in large part by the policies of the various parties along the path,
  and by the relationships established between these parties.  As such,
  a global directory of egress gateways in which users look up
  destination phone numbers is not a feasible solution.  Rather,
  information about the availability of egress gateways is exchanged
  between providers, and subject to policy, made available locally and
  then propagated to other providers in other ITADs, thus creating
  routes towards these egress gateways.  This would allow each provider
  to create its own database of reachable phone numbers and the
  associated routes - such a database could be very different for each
  provider depending on policy.

  TRIP is an inter-domain (i.e., inter-ITAD) gateway location and
  routing protocol.  The primary function of a TRIP speaker, called a
  location server (LS), is to exchange information with other LSs.
  This information includes the reachability of telephony destinations,
  the routes towards these destinations, and information about gateways
  towards those telephony destinations residing in the PSTN.  The TRIP
  requirements are set forth in [2].

  LSs exchange sufficient routing information to construct a graph of
  ITAD connectivity so that routing loops may be prevented.  In
  addition, TRIP can be used to exchange attributes necessary to
  enforce policies and to select routes based on path or gateway
  characteristics.  This specification defines TRIP's transport and
  synchronization mechanisms, its finite state machine, and the TRIP
  data.  This specification defines the basic attributes of TRIP.  The
  TRIP attribute set is extendible, so additional attributes may be
  defined in future documents.





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  TRIP is modeled after the Border Gateway Protocol 4 (BGP-4) [3] and
  enhanced with some link state features, as in the Open Shortest Path
  First (OSPF) protocol [4], IS-IS [5], and the Server Cache
  Synchronization Protocol (SCSP) [6].  TRIP uses BGP's inter-domain
  transport mechanism, BGP's peer communication, BGP's finite state
  machine, and similar formats and attributes as BGP.  Unlike BGP
  however, TRIP permits generic intra-domain LS topologies, which
  simplifies configuration and increases scalability in contrast to
  BGP's full mesh requirement of internal BGP speakers.  TRIP uses an
  intra-domain flooding mechanism similar to that used in OSPF [4],
  IS-IS [5], and SCSP [6].

  TRIP permits aggregation of routes as they are advertised through the
  network.  TRIP does not define a specific route selection algorithm.

  TRIP runs over a reliable transport protocol.  This eliminates the
  need to implement explicit fragmentation, retransmission,
  acknowledgment, and sequencing.  The error notification mechanism
  used in TRIP assumes that the transport protocol supports a graceful
  close, i.e., that all outstanding data will be delivered before the
  connection is closed.

  TRIP's operation is independent of any particular telephony signaling
  protocol.  Therefore, TRIP can be used as the routing protocol for
  any of these protocols, e.g., H.323 [7] and SIP [8].

  The LS peering topology is independent of the physical topology of
  the network.  In addition, the boundaries of an ITAD are independent
  of the boundaries of the layer 3 routing autonomous systems.  Neither
  internal nor external TRIP peers need to be physically adjacent.

3. Summary of Operation

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

3.1. Peering Session Establishment and Maintenance

  Two peer LSs form a transport protocol connection between one
  another.  They exchange messages to open and confirm the connection
  parameters, and to negotiate the capabilities of each LS as well as
  the type of information to be advertised over this connection.

  KeepAlive messages are sent periodically to ensure adjacent peers are
  operational.  Notification messages are sent in response to errors or
  special conditions.  If a connection encounters an error condition, a
  Notification message is sent and the connection is closed.




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3.2. Database Exchanges

  Once the peer connection has been established, the initial data flow
  is a dump of all routes relevant to the new peer (In the case of an
  external peer, all routes in the LS's Adj-TRIB-Out for that external
  peer.  In the case of an internal peer, all routes in the Ext-TRIB
  and all Adj-TRIBs-In).  Note that the different TRIBs are defined in
  Section 3.5.

  Incremental updates are sent as the TRIP routing tables (TRIBs)
  change.  TRIP does not require periodic refresh of the routes.
  Therefore, an LS must retain the current version of all routing
  entries.

  If a particular ITAD has multiple LSs and is providing transit
  service for other ITADs, then care must be taken to ensure a
  consistent view of routing within the ITAD.  When synchronized the
  TRIP routing tables, i.e., the Loc-TRIBs, of all internal peers are
  identical.

3.3. Internal Versus External Synchronization

  As with BGP, TRIP distinguishes between internal and external peers.
  Within an ITAD, internal TRIP uses link-state mechanisms to flood
  database updates over an arbitrary topology.  Externally, TRIP uses
  point-to-point peering relationships to exchange database
  information.

  To achieve internal synchronization, internal peer connections are
  configured between LSs of the same ITAD such that the resulting
  intra-domain LS topology is connected and sufficiently redundant.
  This is different from BGP's approach that requires all internal
  peers to be connected in a full mesh topology, which may result in
  scaling problems.  When an update is received from an internal peer,
  the routes in the update are checked to determine if they are newer
  than the version already in the database.  Newer routes are then
  flooded to all other peers in the same domain.

3.4. Advertising TRIP Routes

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






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  TRIP routes are advertised between a pair of LSs in UPDATE messages.
  The destination addresses are included in the ReachableRoutes
  attribute of the UPDATE, while other attributes describe things like
  the path or egress gateway.

  If an LS chooses to advertise a TRIP route, it may add to or modify
  the attributes of the route before advertising it to a peer.  TRIP
  provides mechanisms by which an LS can inform its peer that a
  previously advertised route is no longer available for use.  There
  are three methods by which a given LS can indicate that a route has
  been withdrawn from service:

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

3.5. Telephony Routing Information Bases

  A TRIP LS processes three types of routes:

     -  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:







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

  Figure 1 illustrates the relationship between the four parts of the
  routing information base.

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

                    Figure 1: TRIB Relationships




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RFC 3219            Telephony Routing over IP (TRIP)        January 2002


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

3.6. Routes in TRIP

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

3.7. Aggregation

  Aggregation is a scaling enhancement used by an LS to reduce the
  number of routing entries that it has to synchronize with its peers.
  Aggregation may be performed by an LS when there is a set of routes
  {R1, R2, ...} in its TRIB such that there exists a less specific
  route R where every valid destination in R is also a valid
  destination in {R1, R2, ...} and vice-versa.  Section 5 includes a
  description of how to combine each attribute (by type) on the {R1,
  R2, ...} routes into an attribute for R.

  Note that there is no mechanism within TRIP to communicate that a
  particular address prefix is not used or valid within a particular
  address family, and thus that these addresses could be skipped during
  aggregation.  LSs may use methods outside of TRIP to learn of invalid
  prefixes that may be ignored during aggregation.

  An LS is not required to perform aggregation, however it is
  recommended whenever maintaining a smaller TRIB is important.  An LS
  decides based on its local policy whether or not to aggregate a set
  of routes into a single aggregate route.

  Whenever an LS aggregates multiple routes where the NextHopServer is
  not identical in all aggregated routes, the NextHopServer attribute
  of the aggregate route must be set to a signalling server in the
  aggregating LS's domain.






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

4. Message Formats

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

4.1. Message Header Format

  Each message has a fixed-size header.  There may or may not be a data
  portion following the header, depending on the message type.  The
  layout of the header fields is shown in Figure 2.

        0                   1                   2
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
        +--------------+----------------+---------------+
        |          Length               |      Type     |
        +--------------+----------------+---------------+

                     Figure 2: TRIP Header

  Length:  This 2-octet unsigned integer indicates the total length of
  the message, including the header, in octets.  Thus, it allows one to
  locate, in the transport-level stream, the beginning of the next
  message.  The value of the Length field must always be at least 3 and
  no greater than 4096, and may be further constrained depending on the
  message type.  No padding of extra data after the message is allowed,
  so the Length field must have the smallest value possible given the
  rest of the message.

  Type:  This 1-octet unsigned integer indicates the type code of the
  message.  The following type codes are defined:

     1 - OPEN
     2 - UPDATE
     3 - NOTIFICATION
     4 - KEEPALIVE





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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 17 octets (including
  message header).  OPEN messages not meeting this minimum requirement
  are handled as defined in Section 6.2.

  In addition to the fixed-size TRIP header, the OPEN message contains
  the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+--------------+----------------+
     |    Version    |    Reserved   |          Hold Time            |
     +---------------+---------------+--------------+----------------+
     |                            My ITAD                            |
     +---------------+---------------+--------------+----------------+
     |                        TRIP Identifier                        |
     +---------------+---------------+--------------+----------------+
     |    Optional Parameters Len    |Optional Parameters (variable)...
     +---------------+---------------+--------------+----------------+

                       Figure 3: TRIP OPEN Header

  Version:
  This 1-octet unsigned integer indicates the protocol version of the
  message.  The current TRIP version number is 1.

  Hold Time:
  This 2-octet unsigned integer indicates the number of seconds that
  the sender proposes for the value of the Hold Timer.  Upon receipt of
  an OPEN message, an LS MUST calculate the value of the Hold Timer by
  using the smaller of its configured Hold Time and the Hold Time
  received in the OPEN message.  The Hold Time MUST be either zero or
  at least three seconds.  An implementation MAY reject connections on
  the basis of the Hold Time.  The calculated value indicates the
  maximum number of seconds that may elapse between the receipt of
  successive KEEPALIVE and/or UPDATE messages by the sender.

  This 4-octet unsigned integer indicates the ITAD number of the
  sender.  The ITAD number must be unique for this domain within this
  confederation of cooperating LSs.




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  ITAD numbers are assigned by IANA as specified in Section 13.  This
  document reserves ITAD number 0.  ITAD numbers from 1 to 255 are
  designated for private use.

  TRIP Identifier:
  This 4-octet unsigned integer indicates the TRIP Identifier of the
  sender.  The TRIP Identifier MUST uniquely identify this LS within
  its ITAD.  A given LS MAY set the value of its TRIP Identifier to an
  IPv4 address assigned to that LS.  The value of the TRIP Identifier
  is determined on startup and MUST be the same for all peer
  connections.  When comparing two TRIP identifiers, the TRIP
  Identifier is interpreted as a numerical 4-octet unsigned integer.

  Optional Parameters Length:
  This 2-octet unsigned integer indicates the total length of the
  Optional Parameters field in octets.  If the value of this field is
  zero, no Optional Parameters are present.

  Optional Parameters:
  This field may contain a list of optional parameters, where each
  parameter is encoded as a <Parameter Type, Parameter Length,
  Parameter Value> triplet.

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+--------------+----------------+
     |       Parameter Type          |       Parameter Length        |
     +---------------+---------------+--------------+----------------+
     |                  Parameter Value (variable)...
     +---------------+---------------+--------------+----------------+

                   Figure 4: Optional Parameter Encoding

  Parameter Type:
  This is a 2-octet field that unambiguously identifies individual
  parameters.

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

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







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4.2.1. Open Message Optional Parameters

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

4.2.1.1. Capability Information

  Capability Information uses Optional Parameter type 1.  This is an
  optional parameter used by an LS to convey to its peer the list of
  capabilities supported by the LS.  This permits an LS to learn of the
  capabilities of its peer LSs.  Capability negotiation is defined in
  Section 8.

  The parameter contains one or more triples <Capability Code,
  Capability Length, Capability Value>, where each triple is encoded as
  shown below:

   0                   1                   2
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |       Capability Code         |       Capability Length       |
  +---------------+---------------+--------------+----------------+
  |       Capability Value (variable)...
  +---------------+---------------+--------------+----------------+

          Figure 5:  Capability Optional Parameter

  Capability Code:
  Capability Code is a 2-octet field that unambiguously identifies
  individual capabilities.

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

  Capability Value:
  Capability Value is a variable length field that is interpreted
  according to the value of the Capability Code field.

  Any particular capability, as identified by its Capability Code, may
  appear more than once within the Optional Parameter.

  This document reserves Capability Codes 32768-65535 for vendor-
  specific applications (these are the codes with the first bit of the
  code value equal to 1).  This document reserves value 0.  Capability
  Codes (other than those reserved for vendor specific use) are
  controlled by IANA.  See Section 13 for IANA considerations.




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  The following Capability Codes are defined by this specification:

     Code           Capability
     1              Route Types Supported
     2              Send Receive Capability

4.2.1.1.1. Route Types Supported

  The Route Types Supported Capability Code lists the route types
  supported in this peering session by the transmitting LS.  An LS MUST
  NOT use route types that are not supported by the peer LS in any
  particular peering session.  If the route types supported by a peer
  are not satisfactory, an LS SHOULD terminate the peering session.
  The format for a Route Type is:

   0                   1                   2
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |        Address Family         |     Application Protocol      |
  +---------------+---------------+--------------+----------------+

           Figure 6: Route Types Supported Capability

  The Address Family and Application Protocol are as defined in Section
  5.1.1.  Address Family gives the address family being routed (within
  the ReachableRoutes attribute).  The application protocol lists the
  application for which the routes apply.  As an example, a route type
  for TRIP could be <E.164, SIP>, indicating a set of E.164
  destinations for the SIP protocol.

  The Route Types Supported Capability MAY contain multiple route types
  in the capability.  The number of route types within the capability
  is the maximum number that can fit given the capability length.  The
  Capability Code is 1 and the length is variable.

4.2.1.1.2. Send Receive Capability

  This capability specifies the mode in which the LS will operate with
  this particular peer.  The possible modes are: Send Only mode,
  Receive Only mode, or Send Receive mode.  The default mode is Send
  Receive mode.

  In Send Only mode, an LS transmits UPDATE messages to its peer, but
  the peer MUST NOT transmit UPDATE messages to that LS.  If an LS in
  Send Only mode receives an UPDATE message from its peer, it MUST
  discard that message, but no further action should be taken.





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  The UPDATE messages sent by an LS in Send Only mode to its intra-
  domain peer MUST include the ITAD Topology attribute whenever the
  topology changes.  A useful application of an LS in Send Only mode
  with an external peer is to enable gateway registration services.

  If a service provider terminates calls to a set of gateways it owns,
  but never initiates calls, it can set its LSs to operate in Send Only
  mode, since they only ever need to generate UPDATE messages, not
  receive them.  If an LS in Send Receive mode has a peering session
  with a peer in Send Only mode, that LS MUST set its route
  dissemination policy such that it does not send any UPDATE messages
  to its peer.

  In Receive Only mode, the LS acts as a passive TRIP listener.  It
  receives and processes UPDATE messages from its peer, but it MUST NOT
  transmit any UPDATE messages to its peer.  This is useful for
  management stations that wish to collect topology information for
  display purposes.

  The behavior of an LS in Send Receive mode is the default TRIP
  operation specified throughout this document.

  The Send Receive capability is a 4-octet unsigned numeric value.  It
  can only take one of the following three values:

     1 - Send Receive mode
     2 - Send only mode
     3 - Receive Only mode

  A peering session MUST NOT be established between two LSs if both of
  them are  in Send Only mode or if both of them are in Receive Only
  mode.  If a peer LS detects such a capability mismatch when
  processing an OPEN message, it MUST respond with a NOTIFICATION
  message and close the peer session.  The error code in the
  NOTIFICATION message must be set to "Capability Mismatch."

  An LS MUST be configured in the same Send Receive mode for all peers.

4.3. UPDATE Message Format

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






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  An UPDATE message is used to both advertise and withdraw routes from
  service.  An UPDATE message may simultaneously advertise and withdraw
  TRIP routes.

  In addition to the TRIP header, the TRIP UPDATE contains a list of
  routing attributes as shown in Figure 7.  There is no padding between
  routing attributes.

        +------------------------------------------------+--...
        | First Route Attribute | Second Route Attribute |  ...
        +------------------------------------------------+--...

                   Figure 7: TRIP UPDATE Format

  The minimum length of an UPDATE message is 3 octets (there are no
  mandatory attributes in TRIP).

4.3.1. Routing Attributes

  A variable length sequence of routing attributes is present in every
  UPDATE message.  Each attribute is a triple <attribute type,
  attribute length, attribute value> of variable length.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+--------------+----------------+
     |  Attr. Flags  |Attr. Type Code|         Attr. Length          |
     +---------------+---------------+--------------+----------------+
     |                   Attribute Value (variable)                  |
     +---------------+---------------+--------------+----------------+

                   Figure 8: Routing Attribute Format

  Attribute Type is a two-octet field that consists of the Attribute
  Flags octet followed by the Attribute Type Code octet.

  The Attribute Type Code defines the type of attribute.  The basic
  TRIP-defined Attribute Type Codes are discussed later in this
  section.  Attributes MUST appear in the UPDATE message in numerical
  order of the Attribute Type Code.  An attribute MUST NOT be included
  more than once in the same UPDATE message.  Attribute Flags are used
  to control attribute processing when the attribute type is unknown.
  Attribute Flags are further defined in Section 4.3.2.








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  This document reserves Attribute Type Codes 224-255 for vendor-
  specific applications (these are the codes with the first three bits
  of the code equal to 1).  This document reserves value 0.  Attribute
  Type Codes (other than those reserved for vendor specific use) are
  controlled by IANA.  See Section 13 for IANA considerations.

  The third and the fourth octets of the route attribute contain the
  length of the attribute value field in octets.

  The remaining octets of the attribute represent the Attribute Value
  and are interpreted according to the Attribute Flags and the
  Attribute Type Code.  The basic supported attribute types, their
  values, and their uses are defined in this specification.  These are
  the attributes necessary for proper loop free operation of TRIP, both
  inter-domain and intra-domain.  Additional attributes may be defined
  in future documents.

4.3.2. Attribute Flags

  It is clear that the set of attributes for TRIP will evolve over
  time.  Hence it is essential that mechanisms be provided to handle
  attributes with unrecognized types.  The handling of unrecognized
  attributes is controlled via the flags field of the attribute.
  Recognized attributes should be processed according to their specific
  definition.

  The following are the attribute flags defined by this specification:
           Bit       Flag
           0         Well-Known Flag
           1         Transitive Flag
           2         Dependent Flag
           3         Partial Flag
           4         Link-state Encapsulated Flag

  The high-order bit (bit 0) of the Attribute Flags octet is the Well-
  Known Bit.  It defines whether the attribute is not well-known (if
  set to 1) or well-known (if set to 0).  Implementations are not
  required to support not well-known attributes, but MUST support
  well-known attributes.

  The second high-order bit (bit 1) of the Attribute Flags octet is the
  Transitive bit.  It defines whether a not well-known attribute is
  transitive (if set to 1) or non-transitive (if set to 0).  For well-
  known attributes, the Transitive bit MUST be zero on transmit and
  MUST be ignored on receipt.

  The third high-order bit (bit 2) of the Attribute Flags octet is the
  Dependent bit.  It defines whether a transitive attribute is



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  dependent (if set to 1) or independent (if set to 0).  For well-known
  attributes and for non-transitive attributes, the Dependent bit is
  irrelevant, and MUST be set to zero on transmit and MUST be ignored
  on receipt.

  The fourth high-order bit (bit 3) of the Attribute Flags octet is the
  Partial bit.  It defines whether the information contained in the not
  well-known transitive attribute is partial (if set to 1) or complete
  (if set to 0).  For well-known attributes and for non-transitive
  attributes the Partial bit MUST be set to 0 on transmit and MUST be
  ignored on receipt.

  The fifth high-order bit (bit 4) of the Attribute Flags octet is the
  Link-state Encapsulation bit.  This bit is only applicable to certain
  attributes (ReachableRoutes and WithdrawnRoutes) and determines the
  encapsulation of the routes within those attributes.  If this bit is
  set, link-state encapsulation is used within the attribute.
  Otherwise, standard encapsulation is used within the attribute.  The
  Link-state Encapsulation technique is described in Section 4.3.2.4.
  This flag is only valid on the ReachableRoutes and WithdrawnRoutes
  attributes.  It MUST be cleared on transmit and MUST be ignored on
  receipt for all other attributes.

  The other bits of the Attribute Flags octet are unused.  They MUST be
  zeroed on transmit and ignored on receipt.

4.3.2.1. Attribute Flags and Route Selection

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

4.3.2.2. Attribute Flags and Route Dissemination

  TRIP provides for two variations of transitivity due to the fact that
  intermediate LSs need not modify the NextHopServer when propagating
  routes.  Attributes may be non-transitive, dependent transitive, or
  independent transitive.  An attribute cannot be both dependent
  transitive and independent transitive.

  Unrecognized independent transitive attributes may be propagated by
  any intermediate LS.  Unrecognized dependent transitive attributes
  MAY only be propagated if the LS is NOT changing the next-hop server.
  The transitivity variations permit some unrecognized attributes to be
  carried end-to-end (independent transitive), some to be carried
  between adjacent next-hop servers (dependent transitive), and other
  to be restricted to peer LSs (non-transitive).




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

  The rules governing the addition of new non-transitive attributes are
  defined independently for each non-transitive attribute.  Any
  attribute MAY be updated by an LS in the path.

4.3.2.3. Attribute Flags and Route Aggregation

  Each attribute defines how it is to be handled during route
  aggregation.

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

4.3.2.4. Attribute Flags and Encapsulation

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











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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |  Attr. Flags  |Attr. Type Code|          Attr. Length         |
  +---------------+---------------+--------------+----------------+
  |                  Originator TRIP Identifier                   |
  +---------------+---------------+--------------+----------------+
  |                        Sequence Number                        |
  +---------------+---------------+--------------+----------------+
  |                   Attribute Value (variable)                  |
  +---------------+---------------+--------------+----------------+

                Figure 9: Link State Encapsulation

  The Originator TRIP ID and Sequence Number are used to control the
  flooding of routing updates within a collection of servers.  These
  fields are used to detect duplicate and old routes so that they are
  not further propagated to other LSs.  The use of these fields is
  defined in Section 10.1.

4.3.3. Mandatory Attributes

  There are no Mandatory attributes in TRIP.  However, there are
  Conditional Mandatory attributes.  A conditional mandatory attribute
  is an attribute, which MUST be included in an UPDATE message if
  another attribute is included in that message.  For example, if an
  UPDATE message includes a ReachableRoutes attribute, it MUST include
  an AdvertisementPath attribute as well.

  The three base attributes in TRIP are WithdrawnRoutes,
  ReachableRoutes, and ITAD Topology.  Their presence in an UPDATE
  message is entirely optional and independent of any other attributes.

4.3.4. TRIP UPDATE Attributes

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

4.3.4.1. WithdrawnRoutes

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





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4.3.4.2. ReachableRoutes

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

4.3.4.3. NextHopServer

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

4.3.4.4. AdvertisementPath

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

4.3.4.5. RoutedPath

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

4.3.4.6. AtomicAggregate

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



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

4.3.4.7. LocalPreference

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

4.3.4.8. MultiExitDisc

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

4.3.4.9. Communities

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

4.3.4.10. ITAD Topology

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

4.3.4.11. ConvertedRoute

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

4.4. KEEPALIVE Message Format

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



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

  The KEEPALIVE message consists of only a message header and has a
  length of 3 octets.

4.5. NOTIFICATION Message Format

  A NOTIFICATION message is sent when an error condition is detected.
  The TRIP transport connection is closed immediately after sending a
  NOTIFICATION message.

  In addition to the fixed-size TRIP header, the NOTIFICATION message
  contains the following fields:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |  Error Code   | Error Subcode |       Data... (variable)
  +---------------+---------------+--------------+----------------+

               Figure 10: TRIP NOTIFICATION Format

  Error Code:
  This 1-octet unsigned integer indicates the type of NOTIFICATION.
  The following Error Codes have been defined:

  Error Code       Symbolic Name               Reference
    1         Message Header Error             Section 6.1
    2         OPEN Message Error               Section 6.2
    3         UPDATE Message Error             Section 6.3
    4         Hold Timer Expired               Section 6.5
    5         Finite State Machine Error       Section 6.6
    6         Cease                            Section 6.7

  Error Subcode:
  This 1-octet unsigned integer provides more specific information
  about the nature of the reported error.  Each Error Code may have one
  or more Error Subcodes associated with it.  If no appropriate Error
  Subcode is defined, then a zero (Unspecific) value is used for the
  Error Subcode field.

  Message Header Error Subcodes:
     1  - Bad Message Length.
     2  - Bad Message Type.






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  OPEN Message Error Subcodes:
     1  - Unsupported Version Number.
     2  - Bad Peer ITAD.
     3  - Bad TRIP Identifier.
     4  - Unsupported Optional Parameter.
     5  - Unacceptable Hold Time.
     6  - Unsupported Capability.
     7  - Capability Mismatch.

  UPDATE Message Error Subcodes:
     1 - Malformed Attribute List.
     2 - Unrecognized Well-known Attribute.
     3 - Missing Well-known Mandatory Attribute.
     4 - Attribute Flags Error.
     5 - Attribute Length Error.
     6 - Invalid Attribute.

  Data:
  This variable-length field is used to diagnose the reason for the
  NOTIFICATION.  The contents of the Data field depend upon the Error
  Code and Error Subcode.

  Note that the length of the data can be determined from the message
  length field by the formula:

           Data Length = Message Length - 5

  The minimum length of the NOTIFICATION message is 5 octets (including
  message header).

5. TRIP Attributes

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

5.1. WithdrawnRoutes

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: Link-State Encapsulation (when flooding).
  TRIP Type Code: 1

  The WithdrawnRoutes specifies a set of routes that are to be removed
  from service by the receiving LS(s).  The set of routes MAY be empty,
  indicated by a length field of zero.






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5.1.1. Syntax of WithdrawnRoutes

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

           +---------------------+---------------------+...
           |  WithdrawnRoute1... |  WithdrawnRoute2... |...
           +---------------------+---------------------+...

                Figure 11: WithdrawnRoutes Format

5.1.1.1. Generic TRIP Route Format

  The generic format for a TRIP route is given in Figure 12.

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

               Figure 12: Generic TRIP Route Format

  Address Family:
  The address family field gives the type of address for the route.
  Three address families are defined in this Section:

           Code              Address Family
           1                 Decimal Routing Numbers
           2                 PentaDecimal Routing Numbers
           3                 E.164 Numbers

  This document reserves address family code 0.  This document reserves
  address family codes 32768-65535 for vendor-specific applications
  (these are the codes with the first bit of the code value equal to
  1).  Additional address families may be defined in the future.
  Assignment of address family codes is controlled by IANA.  See
  Section 13 for IANA considerations.







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  Application Protocol:
  The application protocol gives the protocol for which this routing
  table is maintained.  The currently defined application protocols
  are:

           Code              Protocol
           1                 SIP
           2                 H.323-H.225.0-Q.931
           3                 H.323-H.225.0-RAS
           4                 H.323-H.225.0-Annex-G

  This document reserves application protocol code 0.  This document
  reserves application protocol codes 32768-65535 for vendor-specific
  applications (these are the codes with the first bit of the code
  value equal to 1).  Additional application protocols may be defined
  in the future.  Assignment of application protocol codes is
  controlled by IANA.  See Section 13 for IANA considerations.

  Length:
  The length of the address field, in bytes.

  Address:
  This is an address (prefix) of the family type given by Address
  Family.  The octet length of the address is variable and is
  determined by the length field of the route.

5.1.1.2. Decimal Routing Numbers

  The Decimal Routing Numbers address family is a super set of all
  E.164 numbers, national numbers, local numbers, and private numbers.
  It can also be used to represent the decimal routing numbers used in
  conjunction with Number Portability in some countries/regions.  A set
  of telephone numbers is specified by a Decimal Routing Number prefix.
  Decimal Routing Number prefixes are represented by a string of
  digits, each digit encoded by its ASCII character representation.
  This routing object covers all phone numbers starting with this
  prefix.  The syntax for the Decimal Routing Number prefix is:

     Decimal-routing-number  = *decimal-digit
     decimal-digit           = DECIMAL-DIGIT
     DECIMAL-DIGIT           = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"

  This DECIMAL Routing Number prefix is not bound in length.  This
  format is similar to the format for a global telephone number as
  defined in SIP [8] without visual separators and without the "+"
  prefix for international numbers.  This format facilitates efficient
  comparison when using TRIP to route SIP or H323, both of which use
  character based representations of phone numbers.  The prefix length



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

5.1.1.3. PentaDecimal Routing Numbers

  This address family is used to represent PentaDecimal Routing Numbers
  used in conjunction with Number Portability in some
  countries/regions.  PentaDecimal Routing Number prefixes are
  represented by a string of digits, each digit encoded by its ASCII
  character representation.  This routing object covers all routing
  numbers starting with this prefix.  The syntax for the PentaDecimal
  Routing Number prefix is:

     PentaDecimal-routing-number   = *pentadecimal-digit
     pentadecimal-routing-digit    = PENTADECIMAL-DIGIT
     PENTADECIMAL-DIGIT            = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|
                                     "8"|"9"|"A"|"B"|"C"|"D"|"E"

  Note the difference in alphabets between Decimal Routing Numbers and
  PentaDecimal Routing Numbers.  A PentaDecimal Routing Number prefix
  is not bound in length.

  Note that the address family, which suits the routing numbers of a
  specific country/region depends on the alphabets used for routing
  numbers in that country/region.  For example, North American routing
  numbers SHOULD use the Decimal Routing Numbers address family,
  because their alphabet is limited to the digits "0" through "9".
  Another example, in most European countries routing numbers use the
  alphabet "0" through "9" and "A" through "E", and hence these
  countries SHOULD use the PentaDecimal Routing Numbers address family.

5.1.1.4. E.164 Numbers

  The E.164 Numbers address family is dedicated to fully qualified
  E.164 numbers.  A set of telephone numbers is specified by a E.164
  prefix.  E.164 prefixes are represented by a string of digits, each
  digit encoded by its ASCII character representation.  This routing
  object covers all phone numbers starting with this prefix.  The
  syntax for the E.164 prefix is:

     E164-number          = *e164-digit
     E164-digit           = E164-DIGIT
     E164-DIGIT           = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"







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  This format facilitates efficient comparison when using TRIP to route
  SIP or H323, both of which use character based representations of
  phone numbers.  The prefix length is determined from the length field
  of the route.

  The E.164 Numbers address family and the Decimal Routing Numbers
  address family have the same alphabet.  The E.164 Numbers address
  family SHOULD be used whenever possible.  The Decimal Routing Numbers
  address family can be used in case of private numbering plans or
  applications that do not desire to advertise fully expanded, fully
  qualified telephone numbers.  If Decimal Routing Numbers are used to
  advertise non-fully qualified prefixes, the prefixes may have to be
  manipulated (e.g. expanded) at the boundary between ITADs.  This adds
  significant complexity to the ITAD-Border LS, because, it has to map
  the prefixes from the format used in its own ITAD to the format used
  in the peer ITAD.

5.2. ReachableRoutes

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: Link-State Encapsulation (when flooding).
  TRIP Type Code: 2

  The ReachableRoutes attribute specifies a set of routes that are to
  be added to service by the receiving LS(s).  The set of routes MAY be
  empty, as indicated by setting the length field to zero.

5.2.1. Syntax of ReachableRoutes

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

5.2.2. Route Origination and ReachableRoutes

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

5.2.3. Route Selection and ReachableRoutes

  The routes in ReachableRoutes are necessary for route selection.

5.2.4. Aggregation and ReachableRoutes

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




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  There is no mechanism within TRIP to communicate that a particular
  address prefix is not used and thus that these addresses could be
  skipped during aggregation.  LSs MAY use methods outside of TRIP to
  learn of invalid prefixes that may be ignored during aggregation.

  If an LS advertises an aggregated route, it MUST include the
  AtomicAggregate attribute.

5.2.5. Route Dissemination and ReachableRoutes

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

  If an LS changes the Application Protocol of a route it advertises,
  it MUST include the ConvertedRoute attribute in the UPDATE message.

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

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

5.3. NextHopServer

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








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

5.3.1. NextHopServer Syntax

  For generality, the address of the next-hop server may be of various
  types (domain name, IPv4, IPv6, etc).  The NextHopServer attribute
  includes the ITAD number of next-hop server, a length field, and a
  next-hop name or address.

  The syntax for the NextHopServer is given in Figure 13.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |                         Next Hop ITAD                         |
  +---------------+---------------+--------------+----------------+
  |             Length            |         Server (variable)    ...
  +---------------+---------------+--------------+----------------+

                 Figure 13: NextHopServer Syntax

  The Next-Hop ITAD indicates the domain of the next-hop.  Length field
  gives the number of octets in the Server field, and the Server field
  contains the name or address of the next-hop server.  The server
  field is represented as a string of ASCII characters.  It is defined
  as follows:

  Server  = host [":" port ]
  host    = <   A legal Internet host domain name
             or an IPv4 address using the textual representation
                defined in Section 2.1 of RFC 1123 [9]
             or an IPv6 address using the textual representation
                defined in Section 2.2 of RFC 2373 [10].  The IPv6
                address MUST be enclosed in "[" and "]"
                characters.>
  port    = *DIGIT

  If the port is empty or not given, the default port is assumed (e.g.,
  port 5060 if the application protocol is SIP).

5.3.2. Route Origination and NextHopServer

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



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5.3.3. Route Selection and NextHopServer

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

5.3.4. Aggregation and NextHopServer

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

5.3.5. Route Dissemination and NextHopServer

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

5.4. AdvertisementPath

  Conditional Mandatory: True (if ReachableRoutes and/or
  WithdrawnRoutes attribute is present).
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 4.

  This attribute identifies the ITADs through which routing information
  carried in an advertisement has passed.  The AdvertisementPath
  attribute is analogous to the AS_PATH attribute in BGP.  The
  attributes differ in that BGP's AS_PATH also reflects the path to the
  destination.  In TRIP, not every domain need modify the next-hop, so
  the AdvertisementPath may include many more hops than the actual path
  to the destination.  The RoutedPath attribute (Section 5.5) reflects
  the actual signaling path to the destination.

5.4.1. AdvertisementPath Syntax

  AdvertisementPath is a variable length attribute that is composed of
  a sequence of ITAD path segments.  Each ITAD path segment is
  represented by a type-length-value triple.

  The path segment type is a 1-octet long field with the following
  values defined:



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     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
        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.






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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).
     -  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.



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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 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:









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        *  If the first path segment of the AdvertisementPath is of
           type AP_SEQUENCE, the local system shall prepend its own
           ITAD number as the last element of the sequence (put it in
           the leftmost position).

        *  If the first path segment of the AdvertisementPath is of
           type AP_SET, the local system shall prepend a new path
           segment of type AP_SEQUENCE to the AdvertisementPath,
           including its own ITAD number in that segment.

5.5. RoutedPath

  Conditional Mandatory: True
  (if ReachableRoutes attribute is present).
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 5.

  This attribute identifies the ITADs through which messages sent using
  this route would pass.  The ITADs in this path are a subset of those
  in the AdvertisementPath.

5.5.1. RoutedPath Syntax

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

5.5.2. Route Origination and RoutedPath

  When an LS originates a route it MUST include the RoutedPath
  attribute.

     -  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.



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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 an LS advertises a route to another LS located in its own
        ITAD, the advertising LS MUST NOT modify the RoutedPath
        attribute associated with the route.
     -  If the LS has not changed the NextHopServer attribute, then the
        LS MUST NOT change the RoutedPath attribute.
     -  Otherwise, the LS changed the NextHopServer and is advertising
        the route to an LS in another ITAD.  The advertising LS MUST
        update the RoutedPath attribute as follows:

        *  If the first path segment of the RoutedPath is of type
           AP_SEQUENCE, the local system shall prepend its own ITAD
           number as the last element of the sequence (put it in the
           leftmost position).

        *  If the first path segment of the RoutedPath is of type
           AP_SET, the local system shall prepend a new path segment of
           type AP_SEQUENCE to the RoutedPath, including its own ITAD
           number in that segment.

5.6. AtomicAggregate

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 6.

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

5.6.1. AtomicAggregate Syntax

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




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5.6.2. Route Origination and AtomicAggregate

  Routes are never originated with the AtomicAggregate attribute.

5.6.3. Route Selection and AtomicAggregate

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

5.6.4. Aggregation and AtomicAggregate

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

5.6.5. Route Dissemination and AtomicAggregate

  If an LS, when presented with a set of overlapping routes from a peer
  LS, selects the less specific route (see Section 0) without selecting
  the more specific route, then the LS MUST include the AtomicAggregate
  attribute with the routing object (if it is not already present).

  An LS receiving a routing object with an AtomicAggregate attribute
  MUST NOT make the set of destinations more specific when advertising
  it to other LSs, and MUST NOT remove the attribute when propagating
  this object to a peer LS.

5.7. LocalPreference

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 7.

  The LocalPreference attribute is only used intra-domain, it indicates
  the local LS's preference for the routing object to other LSs within
  the same domain.  This attribute MUST NOT be included when
  communicating to an LS in another domain, and MUST be included over
  intra-domain links.

5.7.1. LocalPreference Syntax

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








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5.7.2. Route Origination and LocalPreference

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

5.7.3. Route Selection and LocalPreference

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

5.7.4. Aggregation and LocalPreference

  The LocalPreference attribute is not affected by aggregation.

5.7.5. Route Dissemination and LocalPreference

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

5.8. MultiExitDisc

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 8.

  When two ITADs are connected by more than one set of peers, the
  MultiExitDisc attribute may be used to specify preferences for routes
  received over one of those links versus routes received over other
  links.  The MultiExitDisc parameter is used only for route selection.

5.8.1. MultiExitDisc Syntax

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

5.8.2. Route Origination and MultiExitDisc

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





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5.8.3. Route Selection and MultiExitDisc

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

5.8.4. Aggregation and MultiExitDisc

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

5.8.5. Route Dissemination and MultiExitDisc

  If received from a peer LS in another domain, an LS MAY propagate the
  MultiExitDisc to other LSs within its domain.  The MultiExitDisc
  attribute MUST NOT be propagated to LSs in other domains.

  An LS may add the MultiExitDisc attribute when propagating routing
  objects to an LS in another domain.  The inclusion of the
  MultiExitDisc attribute is a matter of policy, as is the value of the
  attribute.

5.9. Communities

  Conditional Mandatory: False.
  Required Flags: Not Well-Known, Independent Transitive.
  Potential Flags: None.
  TRIP Type Code: 9.

  A community is a group of destinations that share some common
  property.

  The Communities attribute is used to group destinations so that the
  routing decision can be based on the identity of the group.  Using
  the Communities attribute should significantly simplify the
  distribution of routing information by providing an administratively
  defined aggregation unit.

  Each ITAD administrator may define the communities to which a
  particular route belongs.  By default, all routes belong to the
  general Internet Telephony community.

  As an example, the Communities attribute could be used to define an
  alliance between a group of Internet Telephony service providers for
  a specific subset of routing information.  In this case, members of



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  that alliance would accept only routes for destinations in this group
  that are advertised by other members of the alliance.  Other
  destinations would be more freely accepted.  To achieve this, a
  member would tag each route with a designated Community attribute
  value before disseminating it.  This relieves the members of such an
  alliance, from the responsibility of keeping track of the identities
  of all other members of that alliance.

  Another example use of the Communities attribute is with aggregation.
  It is often useful to advertise both the aggregate route and the
  component more-specific routes that were used to form the aggregate.
  These information components are only useful to the neighboring TRIP
  peer, and perhaps the ITAD of the neighboring TRIP peer, so it is
  desirable to filter out the component routes.  This can be achieved
  by specifying a Community attribute value that the neighboring peers
  will match and filter on.  That way it can be assured that the more
  specific routes will not propagate beyond their desired scope.

5.9.1. Syntax of Communities

  The Communities attribute is of variable length.  It consists of a
  set of 8-octet values, each of which specifies a community.  The
  first 4 octets of the Community value are the Community ITAD Number
  and the next 4 octets are the Community ID.

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |                       Community ITAD Number 1                 |
  +---------------+---------------+--------------+----------------+
  |                         Community ID 1                        |
  +---------------+---------------+--------------+----------------+
  |                       . . . . . . . . .
  +---------------+---------------+--------------+----------------+

                   Figure 14: Communities Syntax

  For administrative assignment, the following assumptions may be made:

     The Community attribute values starting with a Community ITAD
     Number of 0x00000000 are hereby reserved.

  The following communities have global significance and their
  operation MUST be implemented in any Community attribute-aware TRIP
  LS.






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     -  NO_EXPORT (Community ITAD Number = 0x00000000 and Community ID
        = 0xFFFFFF01).  Any received route with a community attribute
        containing this value MUST NOT be advertised outside of the
        receiving TRIP ITAD.

  Other community values MUST be encoded using an ITAD number in the
  four most significant octets.  The semantics of the final four octets
  (the Community ID octets) may be defined by the ITAD (e.g., ITAD 690
  may define research, educational, and commercial community IDs that
  may be used for policy routing as defined by the operators of that
  ITAD).

5.9.2. Route Origination and Communities

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

5.9.3. Route Selection and Communities

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

5.9.4. Aggregation and Communities

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

5.9.5. Route Dissemination and Communities

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

5.10. ITAD Topology

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






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  Within an ITAD, each LS must know the status of other LSs so that LS
  failure can be detected.  To do this, each LS advertises its internal
  topology to other LSs within the domain.  When an LS detects that
  another LS is no longer active, the information sourced by that LS
  can be deleted (the Adj-TRIB-In for that peer may be cleared).  The
  ITAD Topology attribute is used to communicate this information to
  other LSs within the domain.

  An LS MUST send a topology update each time it detects a change in
  its internal peer set.  The topology update may be sent in an UPDATE
  message by itself or it may be piggybacked on an UPDATE message which
  includes ReachableRoutes and/or WithdrawnRoutes information.

  When an LS receives a topology update from an internal LS, it MUST
  recalculate which LSs are active within the ITAD via a connectivity
  algorithm on the topology.

5.10.1. ITAD Topology Syntax

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

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +---------------+---------------+--------------+----------------+
  |                        TRIP Identifier 1                      |
  +---------------+---------------+--------------+----------------+
  |                        TRIP Identifier 2 ...                  |
  +---------------+---------------+--------------+----------------+

                  Figure 15: ITAD Topology Syntax

5.10.2. Route Origination and ITAD Topology

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








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5.10.3. Route Selection and ITAD Topology

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

5.10.4. Aggregation and ITAD Topology

  This information is not aggregated.

5.10.5. Route Dissemination and ITAD Topology

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

5.11. ConvertedRoute

  Conditional Mandatory: False.
  Required Flags: Well-known.
  Potential Flags: None.
  TRIP Type Code: 12.

  The ConvertedRoute attribute indicates that an intermediate LS has
  altered the route by changing the route's Application Protocol.  For
  example, if an LS receives a route with Application Protocol X and
  changes the Application Protocol to Y before advertising the route to
  an external peer, the LS MUST include the ConvertedRoute attribute.
  The attribute is an indication that the advertised application
  protocol will not be used end-to-end, i.e., the information
  advertised about this route is not complete.

5.11.1. ConvertedRoute Syntax

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

5.11.2. Route Origination and ConvertedRoute

  Routes are never originated with the ConvertedRoute attribute.








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5.11.3. Route Selection and ConvertedRoute

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

5.11.4. Aggregation and ConvertedRoute

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

5.11.5. Route Dissemination and ConvertedRoute

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

5.12. Considerations for Defining New TRIP Attributes

  Any proposal for defining new TRIP attributes should specify the
  following:

     -  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.

  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



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  marked as invalid if an internal peering session is terminated.  The
  fact that the routes have been marked as invalid is passed to other
  TRIP peers before the routes are deleted from the system.

  Unless specified explicitly, the Data field of the NOTIFICATION
  message that is sent to indicate an error MUST be empty.

6.1. Message Header Error Detection and Handling

  All errors detected while processing the Message Header are indicated
  by sending the NOTIFICATION message with the Error Code Message
  Header Error.  The Error Subcode elaborates on the specific nature of
  the error.  The error checks in this section MUST be performed by
  each LS upon receipt of every message.

  If the Length field of the message header is less than 3 or greater
  than 4096, or if the Length field of an OPEN message is less than the
  minimum length of the OPEN message, or if the Length field of an
  UPDATE message is less than the minimum length of the UPDATE message,
  or if the Length field of a KEEPALIVE message is not equal to 3, or
  if the Length field of a NOTIFICATION message is less than the
  minimum length of the NOTIFICATION message, then the Error Subcode
  MUST be set to Bad Message Length.  The Data field contains the
  erroneous Length field.

  If the Type field of the message header is not recognized, then the
  Error Subcode MUST be set to "Bad Message Type."  The Data field
  contains the erroneous Type field.

6.2. OPEN Message Error Detection and Handling

  All errors detected while processing the OPEN message are indicated
  by sending the NOTIFICATION message with the Error Code "OPEN Message
  Error."  The Error Subcode elaborates on the specific nature of the
  error.  The error checks in this section MUST be performed by each LS
  upon receipt of every OPEN message.

  If the version number contained in the Version field of the received
  OPEN message is not supported, then the Error Subcode MUST be set to
  "Unsupported Version Number."  The Data field is a 1-octet unsigned
  integer, which indicates the largest locally supported version
  number, which is less than the version of the remote TRIP peer bid
  (as indicated in the received OPEN message).

  If the ITAD field of the OPEN message is unacceptable, then the Error
  Subcode MUST be set to "Bad Peer ITAD."  The determination of
  acceptable ITAD numbers is outside the scope of this protocol.




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  If the Hold Time field of the OPEN message is unacceptable, then the
  Error Subcode MUST be set to "Unacceptable Hold Time."  An
  implementation MUST reject Hold Time values of one or two seconds.
  An implementation MAY reject any proposed Hold Time.  An
  implementation that accepts a Hold Time MUST use the negotiated value
  for the Hold Time.

  If the TRIP Identifier field of the OPEN message is not valid, then
  the Error Subcode MUST be set to "Bad TRIP Identifier."  A TRIP
  identifier is 4-octets in length and can take any value.  An LS
  considers the TRIP Identifier invalid if it already has an open
  connection with another peer LS that has the same ITAD and TRIP
  Identifier.

  Any two LSs within the same ITAD MUST NOT have equal TRIP Identifier
  values.  This restriction does not apply to LSs in different ITADs
  since the purpose is to uniquely identify an LS using its TRIP
  Identifier and its ITAD number.

  If one of the Optional Parameters in the OPEN message is not
  recognized, then the Error Subcode MUST be set to "Unsupported
  Optional Parameters."

  If the Optional Parameters of the OPEN message include Capability
  Information with an unsupported capability (unsupported in either
  capability type or value), then the Error Subcode MUST be set to
  "Unsupported Capability," and the entirety of the unsupported
  capabilities MUST be listed in the Data field of the NOTIFICATION
  message.

  If the Optional Parameters of the OPEN message include Capability
  Information which does not match the receiving LS's capabilities,
  then the Error Subcode MUST be set to "Capability Mismatch," and the
  entirety of the mismatched capabilities MUST be listed in the Data
  field of the NOTIFICATION message.

6.3. UPDATE Message Error Detection and Handling

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






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  If any recognized attribute has Attribute Flags that conflict with
  the Attribute Type Code, then the Error Subcode MUST be set to
  "Attribute Flags Error."  The Data field contains the erroneous
  attribute (type, length and value).

  If any recognized attribute has an Attribute Length that conflicts
  with the expected length (based on the attribute type code), then the
  Error Subcode MUST be set to "Attribute Length Error."  The Data
  field contains the erroneous attribute (type, length and value).

  If any of the mandatory (i.e., conditional mandatory attribute and
  the conditions for including it in the UPDATE message are fulfilled)
  well-known attributes are not present, then the Error Subcode MUST be
  set to "Missing Well-known Mandatory Attribute."  The Data field
  contains the Attribute Type Code of the missing well-known
  conditional mandatory attributes.

  If any of the well-known attributes are not recognized, then the
  Error Subcode MUST be set to "Unrecognized Well-known Attribute."
  The Data field contains the unrecognized attribute (type, length and
  value).

  If any attribute has a syntactically incorrect value, or an undefined
  value, then the Error Subcode is set to "Invalid Attribute."  The
  Data field contains the incorrect attribute (type, length and value).
  Such a NOTIFICATION message is sent, for example, when a
  NextHopServer attribute is received with an invalid address.

  The information carried by the AdvertisementPath attribute is checked
  for ITAD loops.  ITAD loop detection is done by scanning the full
  AdvertisementPath, and checking that the ITAD number of the local
  ITAD does not appear in the AdvertisementPath.  If the local ITAD
  number appears in the AdvertisementPath, then the route MAY be stored
  in the Adj-TRIB-In.  However unless the LS is configured to accept
  routes with its own ITAD in the advertisement path, the route MUST
  not be passed to the TRIP Decision Process.  The operation of an LS
  that is configured to accept routes with its own ITAD number in the
  advertisement path are outside the scope of this document.

  If the UPDATE message was received from an internal peer and either
  the WithdrawnRoutes, ReachableRoutes, or ITAD Topology attribute does
  not have the Link-State Encapsulation flag set, then the Error
  Subcode is set to "Invalid Attribute" and the data field contains the
  attribute.  Likewise, the attribute is invalid if received from an
  external peer and the Link-State Flag is set.

  If any attribute appears more than once in the UPDATE message, then
  the Error Subcode is set to "Malformed Attribute List."



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6.4. NOTIFICATION Message Error Detection and Handling

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

6.5. Hold Timer Expired Error Handling

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

6.6. Finite State Machine Error Handling

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

6.7. Cease

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

6.8. Connection Collision Detection

  If a pair of LSs try simultaneously to establish a transport
  connection to each other, then two parallel connections between this
  pair of speakers might well be formed.  We refer to this situation as
  connection collision.  Clearly, one of these connections must be
  closed.

  Based on the value of the TRIP Identifier, a convention is
  established for detecting which TRIP connection is to be preserved
  when a collision occurs.  The convention is to compare the TRIP
  Identifiers of the peers involved in the collision and to retain only
  the connection initiated by the LS with the higher-valued TRIP
  Identifier.





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  Upon receipt of an OPEN message, the local LS MUST examine all of its
  connections that are in the OpenConfirm state.  An LS MAY also
  examine connections in an OpenSent state if it knows the TRIP
  Identifier of the peer by means outside of the protocol.  If among
  these connections there is a connection to a remote LS, whose TRIP
  Identifier equals the one in the OPEN message, then the local LS MUST
  perform the following collision resolution procedure:

  The TRIP Identifier and ITAD of the local LS is compared to the TRIP
  Identifier and ITAD of the remote LS (as specified in the OPEN
  message).  TRIP Identifiers are treated as 4-octet unsigned integers
  for comparison.

  If the value of the local TRIP Identifier is less than the remote
  one, or if the two TRIP Identifiers are equal and the value of the
  ITAD of the local LS is less than value of the ITAD of the remote LS,
  then the local LS MUST close the TRIP connection that already exists
  (the one that is already in the OpenConfirm state), and accept the
  TRIP connection initiated by the remote LS:

     1. Otherwise, the local LS closes the newly created TRIP
        connection and continues to use the existing one (the one that
        is already in the OpenConfirm state).
     2. If a connection collision occurs with an existing TRIP
        connection that is in the Established state, then the LS MUST
        unconditionally close off the newly created connection.  Note
        that a connection collision cannot be detected with connections
        in Idle, Connect, or Active states.
     3. To close the TRIP connection (that results from the collision
        resolution procedure), an LS MUST send a NOTIFICATION message
        with the Error Code "Cease" and the TRIP connection MUST be
        closed.

7. TRIP Version Negotiation

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





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8. TRIP Capability Negotiation

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

9. TRIP Finite State Machine

  This section specifies TRIP operation in terms of a Finite State
  Machine (FSM).  Following is a brief summary and overview of TRIP
  operations by state as determined by this FSM.  A condensed version
  of the TRIP FSM is found in Appendix 1.  There is one TRIP FSM per
  peer and these FSMs operate independently.

  Idle state:
  Initially TRIP is in the Idle state for each peer.  In this state,
  TRIP refuses all incoming connections.  No resources are allocated to
  the peer.  In response to the Start event (initiated by either the
  system or the operator), the local system initializes all TRIP
  resources, starts the ConnectRetry timer, initiates a transport
  connection to the peer, starts listening for a connection that may be
  initiated by the remote TRIP peer, and changes its state to Connect.
  The exact value of the ConnectRetry timer is a local matter, but
  should be sufficiently large to allow TCP initialization.

  If an LS detects an error, it closes the transport connection and
  changes its state to Idle.  Transitioning from the Idle state
  requires generation of the Start event.  If such an event is
  generated automatically, then persistent TRIP errors may result in
  persistent flapping of the LS.  To avoid such a condition, Start
  events MUST NOT be generated immediately for a peer that was
  previously transitioned to Idle due to an error.  For a peer that was
  previously transitioned to Idle due to an error, the time between
  consecutive Start events, if such events are generated automatically,
  MUST exponentially increase.  The value of the initial timer SHOULD
  be 60 seconds, and the time SHOULD be at least doubled for each
  consecutive retry up to some maximum value.

  Any other event received in the Idle state is ignored.

  Connect State:
  In this state, an LS is waiting for a transport protocol connection
  to be completed to the peer, and is listening for inbound transport
  connections from the peer.





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  If the transport protocol connection succeeds, the local LS clears
  the ConnectRetry timer, completes initialization, sends an OPEN
  message to its peer, sets its Hold Timer to a large value, and
  changes its state to OpenSent.  A Hold Timer value of 4 minutes is
  suggested.

  If the transport protocol connect fails (e.g., retransmission
  timeout), the local system restarts the ConnectRetry timer, continues
  to listen for a connection that may be initiated by the remote LS,
  and changes its state to Active state.

  In response to the ConnectRetry timer expired event, the local LS
  cancels any outstanding transport connection to the peer, restarts
  the ConnectRetry timer, initiates a transport connection to the
  remote LS, continues to listen for a connection that may be initiated
  by the remote LS, and stays in the Connect state.

  If the local LS detects that a remote peer is trying to establish a
  connection to it and the IP address of the peer is not an expected
  one, then the local LS rejects the attempted connection and continues
  to listen for a connection from its expected peers without changing
  state.

  If an inbound transport protocol connection succeeds, the local LS
  clears the ConnectRetry timer, completes initialization, sends an
  OPEN message to its peer, sets its Hold Timer to a large value, and
  changes its state to OpenSent.  A Hold Timer value of 4 minutes is
  suggested.

  The Start event is ignored in the Connect state.

  In response to any other event (initiated by either the system or the
  operator), the local system releases all TRIP resources associated
  with this connection and changes its state to Idle.

  Active state:
  In this state, an LS is listening for an inbound connection from the
  peer, but is not in the process of initiating a connection to the
  peer.

  If an inbound transport protocol connection succeeds, the local LS
  clears the ConnectRetry timer, completes initialization, sends an
  OPEN message to its peer, sets its Hold Timer to a large value, and
  changes its state to OpenSent.  A Hold Timer value of 4 minutes is
  suggested.






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  In response to the ConnectRetry timer expired event, the local system
  restarts the ConnectRetry timer, initiates a transport connection to
  the TRIP peer, continues to listen for a connection that may be
  initiated by the remote TRIP peer, and changes its state to Connect.

  If the local LS detects that a remote peer is trying to establish a
  connection to it and the IP address of the peer is not an expected
  one, then the local LS rejects the attempted connection and continues
  to listen for a connection from its expected peers without changing
  state.

  Start event is ignored in the Active state.

  In response to any other event (initiated by either the system or the
  operator), the local system releases all TRIP resources associated
  with this connection and changes its state to Idle.

  OpenSent state:
  In this state, an LS has sent an OPEN message to its peer and is
  waiting for an OPEN message from its peer.  When an OPEN message is
  received, all fields are checked for correctness.  If the TRIP
  message header checking or OPEN message checking detects an error
  (see Section 6.2) or a connection collision (see Section 6.8), the
  local system sends a NOTIFICATION message and changes its state to
  Idle.

  If there are no errors in the OPEN message, TRIP sends a KEEPALIVE
  message and sets a KeepAlive timer.  The Hold Timer, which was
  originally set to a large value (see above), is replaced with the
  negotiated Hold Time value (see Section 4.2).  If the negotiated Hold
  Time value is zero, then the Hold Time timer and KeepAlive timers are
  not started.  If the value of the ITAD field is the same as the local
  ITAD number, then the connection is an "internal" connection;
  otherwise, it is "external" (this will affect UPDATE processing).
  Finally, the state is changed to OpenConfirm.

  If the local LS detects that a remote peer is trying to establish a
  connection to it and the IP address of the peer is not an expected
  one, then the local LS rejects the attempted connection and continues
  to listen for a connection from its expected peers without changing
  state.

  If a disconnect notification is received from the underlying
  transport protocol, the local LS closes the transport connection,
  restarts the ConnectRetry timer, continues to listen for a connection
  that may be initiated by the remote TRIP peer, and goes into the
  Active state.




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

  In response to the Stop event (initiated by either system or
  operator) the local LS sends a NOTIFICATION message with the Error
  Code "Cease" and changes its state to Idle.

  The Start event is ignored in the OpenSent state.

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

  Whenever TRIP changes its state from OpenSent to Idle, it closes the
  transport connection and releases all resources associated with that
  connection.

  OpenConfirm state:
  In this state, an LS has sent an OPEN to its peer, received an OPEN
  from its peer, and sent a KEEPALIVE in response to the OPEN.  The LS
  is now waiting for a KEEPALIVE or NOTIFICATION message in response to
  its OPEN.

  If the local LS receives a KEEPALIVE message, it changes its state to
  Established.

  If the Hold Timer expires before a KEEPALIVE message is received, the
  local LS sends NOTIFICATION message with the Error Code "Hold Timer
  Expired" and changes its state to Idle.

  If the local LS receives a NOTIFICATION message, it changes its state
  to Idle.

  If the KeepAlive timer expires, the local LS sends a KEEPALIVE
  message and restarts its KeepAlive timer.

  If a disconnect notification is received from the underlying
  transport protocol, the local LS closes the transport connection,
  restarts the ConnectRetry timer, continues to listen for a connection
  that may be initiated by the remote TRIP peer, and goes into the
  Active state.

  In response to the Stop event (initiated by either the system or the
  operator) the local LS sends NOTIFICATION message with the Error Code
  "Cease" and changes its state to Idle.

  The Start event is ignored in the OpenConfirm state.



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

  Whenever TRIP changes its state from OpenConfirm to Idle, it closes
  the transport connection and releases all resources associated with
  that connection.

  Established state:
  In the Established state, an LS can exchange UPDATE, NOTIFICATION,
  and KEEPALIVE messages with its peer.

  If the negotiated Hold Timer is zero, then no procedures are
  necessary for keeping a peering session alive.  If the negotiated
  Hold Time value is non-zero, the procedures of this paragraph apply.
  If the Hold Timer expires, the local LS sends a NOTIFICATION message
  with the Error Code "Hold Timer Expired" and changes its state to
  Idle.  If the KeepAlive Timer expires, then the local LS sends a
  KeepAlive message and restarts the KeepAlive Timer.  If the local LS
  receives an UPDATE or KEEPALIVE message, then it restarts its Hold
  Timer.  Each time the LS sends an UPDATE or KEEPALIVE message, it
  restarts its KeepAlive Timer.

  If the local LS receives a NOTIFICATION message, it changes its state
  to Idle.

  If the local LS receives an UPDATE message and the UPDATE message
  error handling procedure (see Section6.3) detects an error, the local
  LS sends a NOTIFICATION message and changes its state to Idle.

  If a disconnect notification is received from the underlying
  transport protocol, the local LS changes its state to Idle.

  In response to the Stop event (initiated by either the system or the
  operator), the local LS sends a NOTIFICATION message with the Error
  Code "Cease" and changes its state to Idle.

  The Start event is ignored in the Established state.

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

  Whenever TRIP changes its state from Established to Idle, it closes
  the transport connection and releases all resources associated with
  that connection.  Additionally, if the peer is an external peer, the
  LS deletes all routes derived from that connection.




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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 rerun its Decision Process since the
  previously advertised route is no longer available for use.

  If the UPDATE message contains a route, then the route MUST be placed
  in the appropriate Adj-TRIB-In, and the following additional actions
  MUST be taken:

     1. If its destinations are identical to those of a route currently
        stored in the Adj-TRIB-In, then the new route MUST replace the
        older route in the Adj-TRIB-In, thus implicitly withdrawing the
        older route from service.  The LS MUST rerun its Decision
        Process since the older route is no longer available for use.
     2. If the new route is more specific than an earlier route
        contained in the Adj-TRIB-In and has identical attributes, then
        no further actions are necessary.
     3. If the new route is more specific than an earlier route
        contained in the Adj-TRIB-In but does not have identical
        attributes, then the LS MUST run its Decision Process since the
        more specific route has implicitly made a portion of the less
        specific route unavailable for use.
     4. If the new route has destinations that are not present in any
        of the routes currently stored in the Adj-TRIB-In, then the LS
        MUST run its Decision Process.
     5. If the new route is less specific than an earlier route
        contained in the Adj-TRIB-In, the LS MUST run its Decision
        Process on the set of destinations that are described only by
        the less specific route.







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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 or ITAD Topology).

10.1.3. Flooding

  Each route in the ReachableRoutes or WithdrawnRoutes field that is
  determined to be old is ignored in further processing.  If the route
  is determined to be new then the following actions occur.

  If the route is being withdrawn, then the LS MUST flood the withdrawn
  route to all other internal peers, and MUST mark the route as
  withdrawn.  An LS MUST maintain routes marked as withdrawn in its
  databases for MaxPurgeTime seconds.

  If the route is being updated, then the LS MUST update the route in
  the Adj-TRIB-In and MUST flood it to all other internal peers.





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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 of flooding are possible, but the local LS
  MUST ensure that each new route (and any associated attributes)
  received from an internal peer get forwarded to every other internal
  peer.

10.1.4. Sequence Number Considerations

  The Sequence Number is used to determine when one version of a Route
  is newer than another version of a route.  A larger Sequence Number
  indicates a newer version.  The Sequence Number is assigned by the LS
  originating the route into the local ITAD.  The Sequence Number is an
  unsigned 4-octet integer in the range of 1 thru 2^31-1 MinSequenceNum
  thru MaxSequenceNum).  The value 0 is reserved.  When an LS first
  originates a route (including when the LS restarts/reboots) into its
  ITAD, it MUST originate it with a Sequence Number of MinSequenceNum.
  Each time the route is updated within the ITAD by the originator, the
  Sequence Number MUST be increased.

  If it is ever the case that the sequence number is MaxSequenceNum-1
  and it needs to be increased, then the TRIP module of the LS MUST be
  disabled for a period of TripDisableTime so that all routes
  originated by this LS with high sequence numbers can be removed.

10.1.5. Purging a Route Within the ITAD

  To withdraw a route that it originated within the ITAD, an LS
  includes the route in the WithdrawnRoutes field of an UPDATE message.
  The Sequence Number MUST be greater than the last valid version of
  the route.  The LS MAY choose to use a sequence number of
  MaxSequenceNum when withdrawing routes within its ITAD, but this is
  not required.

  After withdrawing a route, an LS MUST mark the route as "withdrawn"
  in its database, and maintain the withdrawn route in its database for
  MaxPurgeTime seconds.  If the LS needs to re-originate a route that
  had been purged but is still in its database, it can either re-
  originate the route immediately using a Sequence Number that is
  greater than that used in the withdraw, or the LS may wait until
  MaxPurgeTime seconds have expired since the route was withdrawn.





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10.1.6. Receiving Self-Originated Routes

  It is common for an LS to receive UPDATES for routes that it
  originated within the ITAD via the flooding procedure.  If the LS
  receives an UPDATE for a route that it originated that is newer (has
  a higher sequence number) than the LSs current version, then special
  actions must be taken.  This should be a relatively rare occurrence
  and indicates that a route still exists within the ITAD since the LSs
  last restart/reboot.

  If an LS receives a self-originated route update that is newer than
  the current version of the route at the LS, then the following
  actions MUST be taken.  If the LS still wishes to advertise the
  information in the route, then the LS MUST increase the Sequence
  Number of the route to a value greater than that received in the
  UPDATE and re-originate the route.  If the LS does not wish to
  continue to advertise the route, then it MUST purge the route as
  described in Section 10.1.5.

10.1.7. Removing Withdrawn Routes

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

10.2. Decision Process

  The Decision Process selects routes for subsequent advertisement by
  applying the policies in the local Policy Information Base (PIB) to
  the routes stored in its Adj-TRIBs-In.  The output of the Decision
  process is the set of routes that will be advertised to all peers;
  the selected routes will be stored in the local LS's Adj-TRIBs-Out.

  The selection process is formalized by defining a function that takes
  the attributes of a given route as an argument and returns a non-
  negative integer denoting the degree of preference for the route.
  The function that calculates the degree of preference for a given
  route shall not use as its inputs any of the following:  the
  existence of other routes, the non-existence of other routes, or the
  attributes of other routes.  Route selection then consists of an
  individual application of the degree of preference function to each
  feasible route, followed by the choice of the one with the highest
  degree of preference.






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  All internal LSs in an ITAD MUST run the Decision Process and apply
  the same decision criteria, otherwise it will not be possible to
  synchronize their Loc-TRIBs.

  The Decision Process operates on routes contained in each Adj-TRIBs-
  In, and is responsible for:

     -  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.
     -  Phase 2 is invoked on completion of phase 1.  It is responsible
        for choosing the best route out of all those available for each
        distinct destination, and for installing each chosen route into
        the Loc-TRIB.
     -  Phase 3 is invoked after the Loc-TRIB has been modified.  It is
        responsible for disseminating routes in the Loc-TRIB to each
        external peer, according to the policies contained in the PIB.
        Route aggregation and information reduction can optionally be
        performed within this phase.

10.2.1. Phase 1: Calculation of Degree of Preference

  The Phase 1 decision function shall be invoked whenever the local LS
  receives from a peer an UPDATE message that advertises a new route, a
  replacement route, or a withdrawn route.

  The Phase 1 decision function is a separate process that is completed
  when it has no further work to do.

  The Phase 1 decision function shall lock an Adj-TRIB-In prior to
  operating on any route contained within it, and shall unlock it after
  operating on all new or replacement routes contained within it.

  The local LS MUST determine a degree of preference for each newly
  received or replacement route.  If the route is learned from an
  internal peer, the value of the LocalPreference attribute MUST be
  taken as the degree of preference.  If the route is learned from an
  external peer, then the degree of preference MUST be computed based
  on pre-configured policy information and used as the LocalPreference
  value in any intra-domain TRIP advertisement.  The exact nature of
  this policy information and the computation involved is a local
  matter.



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  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:

     -  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




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  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 is completed when it
  has no further work to do.  The Phase 3 routing decision function
  MUST be blocked from running while the Phase 2 decision function is
  in process.

  All routes in the Loc-TRIB shall be processed into a corresponding
  entry in the associated Adj-TRIBs-Out.  Route aggregation and
  information reduction techniques (see 10.3.4) MAY optionally be
  applied.

  When the updating of the Adj-TRIBs-Out is complete, the local LS MUST
  run the external update process of 10.3.2.

10.2.4. Overlapping Routes

  When overlapping routes are present in the same Adj-TRIB-In, the more
  specific route shall take precedence, in order, from most specific to
  least specific.

  The set of destinations described by the overlap represents a portion
  of the less specific route that is feasible, but is not currently in
  use.  If a more specific route is later withdrawn, the set of
  destinations described by the more specific route will still be
  reachable using the less specific route.






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  If an LS receives overlapping routes, the Decision Process MUST take
  into account the semantics of the overlapping routes.  In particular,
  if an LS accepts the less specific route while rejecting the more
  specific route from the same peer, then the destinations represented
  by the overlap may not forward along the domains listed in the
  AdvertisementPath attribute of that route.  Therefore, an LS has the
  following choices:

     1. Install both the less and the more specific routes
     2. Install the more specific route only
     3. Install the non-overlapping part of the less specific route
        only (that implies disaggregation of the less-specific route)
     4. Aggregate the two routes and install the aggregated route
     5. Install the less specific route only
     6. Install neither route

  If an LS chooses 5), then it SHOULD add AtomicAggregate attribute to
  the route.  A route that carries AtomicAggregate attribute MUST NOT
  be de-aggregated.  That is, the route cannot be made more specific.
  Forwarding along such a route does not guarantee that route traverses
  only domains listed in the RoutedPath of the route.  If an LS chooses
  1), then it MUST NOT advertise the less specific route without the
  more specific route.

10.3. Update-Send Process

  The Update-Send process is responsible for advertising UPDATE
  messages to all peers.  For example, it distributes the routes chosen
  by the Decision Process to other LSs that may be located in either
  the same ITAD or a neighboring ITAD.  Rules for information exchange
  between peer LSs located in different ITADs are given in 10.3.2;
  rules for information exchange between peer LSs located in the same
  ITAD are given in 10.3.1.

  Before forwarding routes to peers, an LS MUST determine which
  attributes should be forwarded along with that route.  If a not
  well-known non-transitive attribute is unrecognized, it is quietly
  ignored.  If a not well-known dependent-transitive attribute is
  unrecognized, and the NextHopServer attribute has been changed by the
  LS, the unrecognized attribute is quietly ignored.  If a not well-
  known dependent-transitive attribute is unrecognized, and the
  NextHopServer attribute has not been modified by the LS, the Partial
  bit in the attribute flags octet is set to 1, and the attribute is
  retained for propagation to other TRIP speakers.  Similarly, if an
  not well-known independent-transitive attribute is unrecognized, the
  Partial bit in the attribute flags octet is set to 1, and the
  attribute is retained for propagation to other TRIP speakers.




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  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:

     -  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





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  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 the Phase 3 route
  selection process, the LS has updated its Adj-TRIBs-Out.  All newly
  installed routes and all newly unfeasible routes for which there is
  no replacement route MUST be advertised to external peers by means of
  UPDATE messages.

  Any routes in the Loc-TRIB marked as withdrawn MUST be removed.
  Changes to the reachable destinations within its own ITAD SHALL also
  be advertised in an UPDATE message.

10.3.3. Controlling Routing Traffic Overhead

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

10.3.3.1. Frequency of Route Advertisement

  The parameter MinRouteAdvertisementInterval determines the minimum
  amount of time that must elapse between advertisements of routes to a
  particular destination from a single LS.  This rate limiting
  procedure applies on a per-destination basis, although the value of
  MinRouteAdvertisementInterval is set on a per LS peer basis.

  Two UPDATE messages sent from a single LS that advertise feasible
  routes to some common set of destinations received from external
  peers MUST be separated by at least MinRouteAdvertisementInterval.
  Clearly, this can only be achieved precisely by keeping a separate
  timer for each common set of destinations.  This would be unwarranted
  overhead.  Any technique which ensures that the interval between two
  UPDATE messages sent from a single LS that advertise feasible routes




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  to some common set of destinations received from external peers will
  be at least MinRouteAdvertisementInterval, and will also ensure that
  a constant upper bound on the interval is acceptable.

  Two UPDATE messages, sent from a single LS to an external peer, that
  advertise feasible routes to some common set of destinations received
  from internal peers MUST be separated by at least
  MinRouteAdvertisementInterval.

  Since fast convergence is needed within an ITAD, this rate limiting
  procedure does not apply to routes received from internal peers and
  being broadcast to other internal peers.  To avoid long-lived black
  holes, the procedure does not apply to the explicit withdrawal of
  routes (that is, routes whose destinations explicitly withdrawn by
  UPDATE messages).

  This procedure does not limit the rate of route selection, but only
  the rate of route advertisement.  If new routes are selected multiple
  times while awaiting the expiration of MinRouteAdvertisementInterval,
  the last route selected shall be advertised at the end of
  MinRouteAdvertisementInterval.

10.3.3.2. Frequency of Route Origination

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

10.3.3.3. Jitter

  To minimize the likelihood that the distribution of TRIP messages by
  a given LS will contain peaks, jitter should be applied to the timers
  associated with MinITADOriginationInterval, KeepAlive, and
  MinRouteAdvertisementInterval.  A given LS shall apply the same
  jitter to each of these quantities regardless of the destinations to
  which the updates are being sent; that is, jitter will not be applied
  on a "per peer" basis.

  The amount of jitter to be introduced shall be determined by
  multiplying the base value of the appropriate timer by a random
  factor that is uniformly distributed in the range from 0.75 to 1.0.









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10.3.4. Efficient Organization of Routing Information

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

10.3.4.1. Information Reduction

  Information reduction may imply a reduction in granularity of policy
  control - after information has collapsed, the same policies will
  apply to all destinations and paths in the equivalence class.

  The Decision Process may optionally reduce the amount of information
  that it will place in the Adj-TRIBs-Out by any of the following
  methods:

     -  ReachableRoutes: A set of destinations can be usually
        represented in compact form.  For example, a set of E.164 phone
        numbers can be represented in more compact form using E.164
        prefixes.
     -  AdvertisementPath: AdvertisementPath information can be
        represented as ordered AP_SEQUENCEs or unordered AP_SETs.
        AP_SETs are used in the route aggregation algorithm described
        in Section 5.4.4.  They reduce the size of the AP_PATH
        information by listing each ITAD number only once, regardless
        of how many times it may have appeared in multiple
        advertisement paths that were aggregated.

  An AP_SET implies that the destinations advertised in the UPDATE
  message can be reached through paths that traverse at least some of
  the constituent ITADs.  AP_SETs provide sufficient information to
  avoid route looping; however their use may prune potentially feasible
  paths, since such paths are no longer listed individually as in the
  form of AP_SEQUENCEs.  In practice this is not likely to be a
  problem, since once a call arrives at the edge of a group of ITADs,
  the LS at that point is likely to have more detailed path information
  and can distinguish individual paths to destinations.

10.3.4.2. Aggregating Routing Information

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





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  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.

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.




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11. TRIP Transport

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

12. ITAD Topology

  There are no restrictions on the intra-domain topology of TRIP LSs.
  For example, LSs in an ITAD can be configured in a full mesh, star,
  or any other connected topology.  Similarly, there are no
  restrictions on the topology of TRIP ITADs.  For example, the ITADs
  can be organized in a flat topology (mesh or ring) or in multi-level
  hierarchy or any other topology.

  The border between two TRIP ITADs may be located either on the link
  between two TRIP LSs or it may coincide on a TRIP LS.  In the latter
  case, the same TRIP LS will be member in more than one ITAD, and it
  appears to be an internal peer to LSs in each ITAD it is member of.

13. IANA Considerations

  This document creates a new IANA registry for TRIP parameters.  The
  following TRIP parameters are included in the registry:

     - TRIP Capabilities
     - TRIP Attributes
     - TRIP Address Families
     - TRIP Application Protocols
     - TRIP ITAD Numbers

  Protocol parameters are frequently initialized/reset to 0.  This
  document reserves the value 0 of each of the above TRIP parameters in
  order to clearly distinguish between an unset parameter and any other
  registered values for that parameter.

  The sub-registries for each of the above parameters are discussed in
  the sections below.

13.1. TRIP Capabilities

  Requests to add TRIP capabilities other than those defined in Section
  4.2.1.1 must be submitted to [email protected].  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



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

13.2. TRIP Attributes

  This document reserves Attribute Type Codes 224-255 for Private Use
  (these are the codes with the first three bits of the code equal to
  1).  This document reserves the value 0.  Attribute Type Codes 1
  through 11 have already been allocated by this document.  Attribute
  Type Codes 1 through 11 are defined in Sections 5.1 through 5.11.

  Attribute Type Codes in the range 12-223 are controlled by IANA, and
  require a Specification document (RFC or equivalent).  The
  specification MUST provide all information required in Section 5.12
  of this document.

  Attribute Type Code registration requests must be sent to
  [email protected].  In addition to the specification requirement, the
  request MUST include an indication of who has change control over the
  attribute and contact information (postal and email address).

13.3. Destination Address Families

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

13.4. TRIP Application Protocols

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





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13.5. ITAD Numbers

  This document reserves the ITAD number 0.  ITAD numbers in the range
  1-255 are designated for Private Use.  ITAD numbers in the range from
  256 to (2**32)-1 are allocated by IANA on a First-Come-First-Serve
  basis.  Requests for ITAD numbers must be submitted to [email protected].
  The requests MUST include the following:

     -  Information about the organization that will administer the
        ITAD.
     -  Contact information (postal and email address).

14. Security Considerations

  This section covers security between peer TRIP LSs when TRIP runs
  over TCP in an IP environment.

  A security mechanism is clearly needed to prevent unauthorized
  entities from using the protocol defined in this document for setting
  up unauthorized peer sessions with other TRIP LSs or interfering with
  authorized peer sessions.  The security mechanism for the protocol,
  when transported over TCP in an IP network, is IPsec [12].  IPsec
  uses two protocols to provide traffic security: Authentication Header
  (AH) [13] and Encapsulating Security Payload (ESP) [14].

  The AH header affords data origin authentication, connectionless
  integrity and optional anti-replay protection of messages passed
  between the peer LSs.  The ESP header provides origin authentication,
  connectionless integrity, anti-replay protection, and confidentiality
  of messages.

  Implementations of the protocol defined in this document employing
  the ESP header SHALL comply with section 5 of [14], which defines a
  minimum set of algorithms for integrity checking and encryption.
  Similarly, implementations employing the AH header SHALL comply with
  section 5 of [13], which defines a minimum set of algorithms for
  integrity checking using manual keys.

  Implementations SHOULD use IKE [15] to permit more robust keying
  options.  Implementations employing IKE SHOULD support authentication
  with RSA signatures and RSA public key encryption.

  A Security Association (SA) [12] is a simplex "connection" that
  affords security services to the traffic carried by it.  Security
  services are afforded to a SA by the use of AH, or ESP, but not both.
  Two types of SAs are defined: transport mode and tunnel mode [12].  A
  transport mode SA is a security association between two hosts, and is
  appropriate for protecting the TRIP session between two peer LSs.



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A1. Appendix 1: TRIP FSM State Transitions and Actions

  This Appendix discusses the transitions between states in the TRIP
  FSM in response to TRIP events.  The following is the list of these
  states and events when the negotiated Hold Time value is non-zero.

  TRIP States:
     1 - Idle
     2 - Connect
     3 - Active
     4 - OpenSent
     5 - OpenConfirm
     6 - Established

  TRIP Events:
     1 - TRIP Start
     2 - TRIP Stop
     3 - TRIP Transport connection open
     4 - TRIP Transport connection closed
     5 - TRIP Transport connection open failed
     6 - TRIP Transport fatal error
     7 - ConnectRetry timer expired
     8 - Hold Timer expired
     9 - KeepAlive timer expired
     10 - Receive OPEN message
     11 - Receive KEEPALIVE message
     12 - Receive UPDATE messages
     13 - Receive NOTIFICATION message

  The following table describes the state transitions of the TRIP FSM
  and the actions triggered by these transitions.

  Event                Actions              Message Sent    Next State
  --------------------------------------------------------------------
  Idle (1)
   1            Initialize resources            none             2
                Start ConnectRetry timer
                Initiate a transport connection
   others               none                    none             1

  Connect(2)
   1                    none                    none             2
   3            Complete initialization         OPEN             4
                Clear ConnectRetry timer
   5            Restart ConnectRetry timer      none             3
   7            Restart ConnectRetry timer      none             2
                Initiate a transport connection
   others       Release resources               none             1



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  Active (3)
   1                    none                    none             3
   3            Complete initialization         OPEN             4
                Clear ConnectRetry timer
   5            Close connection                                 3
                Restart ConnectRetry timer
   7            Restart ConnectRetry timer      none             2
                Initiate a transport connection
   others       Release resources               none             1

  OpenSent(4)
   1                    none                    none             4
   4            Close transport connection      none             3
                Restart ConnectRetry timer
   6            Release resources               none             1
  10            Process OPEN is OK            KEEPALIVE          5
                Process OPEN failed           NOTIFICATION       1
  others        Close transport connection    NOTIFICATION       1
                Release resources

  OpenConfirm (5)
   1                   none                     none             5
   4            Release resources               none             1
   6            Release resources               none             1
   9            Restart KeepAlive timer       KEEPALIVE          5
  11            Complete initialization         none             6
                Restart Hold Timer
  13            Close transport connection                       1
                Release resources
  others        Close transport connection    NOTIFICATION       1
                Release resources

  Established (6)
   1                   none                     none             6
   4            Release resources               none             1
   6            Release resources               none             1
   9            Restart KeepAlive timer       KEEPALIVE          6
  11            Restart Hold Timer              none             6
  12            Process UPDATE is OK          UPDATE             6
                Process UPDATE failed         NOTIFICATION       1
  13            Close transport connection                       1
                Release resources
  others        Close transport connection    NOTIFICATION       1
                Release resources
  -----------------------------------------------------------------






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  The following is a condensed version of the above state transition
  table.

  Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
        | (1)  |   (2)   |  (3)   |    (4)   |     (5)     |   (6)
        |----------------------------------------------------------
   1    |  2   |    2    |   3    |     4    |      5      |    6
        |      |         |        |          |             |
   2    |  1   |    1    |   1    |     1    |      1      |    1
        |      |         |        |          |             |
   3    |  1   |    4    |   4    |     1    |      1      |    1
        |      |         |        |          |             |
   4    |  1   |    1    |   1    |     3    |      1      |    1
        |      |         |        |          |             |
   5    |  1   |    3    |   3    |     1    |      1      |    1
        |      |         |        |          |             |
   6    |  1   |    1    |   1    |     1    |      1      |    1
        |      |         |        |          |             |
   7    |  1   |    2    |   2    |     1    |      1      |    1
        |      |         |        |          |             |
   8    |  1   |    1    |   1    |     1    |      1      |    1
        |      |         |        |          |             |
   9    |  1   |    1    |   1    |     1    |      5      |    6
        |      |         |        |          |             |
  10    |  1   |    1    |   1    |  1 or 5  |      1      |    1
        |      |         |        |          |             |
  11    |  1   |    1    |   1    |     1    |      6      |    6
        |      |         |        |          |             |
  12    |  1   |    1    |   1    |     1    |      1      | 1 or 6
        |      |         |        |          |             |
  13    |  1   |    1    |   1    |     1    |      1      |    1
        |      |         |        |          |             |
        --------------------------------------------------------------

A2. Appendix 2: Implementation Recommendations

  This section presents some implementation recommendations.

A.2.1: Multiple Networks Per Message

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



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  many address prefixes per advertisement path and next hop from a
  routing table that is not organized per advertisement path is to
  build many messages as the routing table is scanned.  As each address
  prefix is processed, a message for the associated advertisement path
  and next hop is allocated, if it does not exist, and the new address
  prefix is added to it.  If such a message exists, the new address
  prefix is just appended to it.  If the message lacks the space to
  hold the new address prefix, it is transmitted, a new message is
  allocated, and the new address prefix is inserted into the new
  message.  When the entire routing table has been scanned, all
  allocated messages are sent and their resources released.  Maximum
  compression is achieved when all the destinations covered by the
  address prefixes share the same next hop server and common
  attributes, making it possible to send many address prefixes in one
  4096-byte message.

  When peering with a TRIP implementation that does not compress
  multiple address prefixes into one message, it may be necessary to
  take steps to reduce the overhead from the flood of data received
  when a peer is acquired or a significant network topology change
  occurs.  One method of doing this is to limit the rate of updates.
  This will eliminate the redundant scanning of the routing table to
  provide flash updates for TRIP peers.  A disadvantage of this
  approach is that it increases the propagation latency of routing
  information.  By choosing a minimum flash update interval that is not
  much greater than the time it takes to process the multiple messages,
  this latency should be minimized.  A better method would be to read
  all received messages before sending updates.

A.2.2: Processing Messages on a Stream Protocol

  TRIP uses TCP as a transport mechanism.  Due to the stream nature of
  TCP, all the data of a received message does not necessarily arrive
  at the same time.  This can make it difficult to process the data as
  messages, especially on systems where it is not possible to determine
  how much data has been received but not yet processed.

  One method that can be used in this situation is to first try to read
  just the message header.  For the KEEPALIVE message type, this is a
  complete message; for other message types, the header should first be
  verified, in particular the total length.  If all checks are
  successful, the specified length, minus the size of the message
  header is the amount of data left to read.  An implementation that
  would "hang" the routing information process while trying to read
  from a peer could set up a message buffer (4096 bytes) per peer and
  fill it with data as available until a complete message has been
  received.




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A.2.3: Reducing Route Flapping

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

A.2.4: TRIP Timers

  TRIP employs seven timers: ConnectRetry, Hold Time, KeepAlive,
  MaxPurgeTime, TripDisableTime, MinITADOriginationInterval, and
  MinRouteAdvertisementInterval.  The suggested value for the
  ConnectRetry timer is 120 seconds.  The suggested value for the Hold
  Time is 90 seconds.  The suggested value for the KeepAlive timer is
  30 seconds.  The suggested value for the MaxPurgeTime timer is 10
  seconds.  The suggested value for the TripDisableTime timer is 180
  seconds.  The suggested value for the MinITADOriginationInterval is
  30 seconds.  The suggested value for the
  MinRouteAdvertisementInterval is 30 seconds.

  An implementation of TRIP MUST allow these timers to be configurable.

A.2.5: AP_SET Sorting

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

Acknowledgments

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

References

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

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

  [3]   Rekhter, Y. and T. Li, "Border Gateway Protocol 4 (BGP-4)," RFC
        1771, March 1995.

  [4]   Moy, J., "Open Shortest Path First Version 2", STD 54, RFC
        2328, April 1998.






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  [5]   "Intermediate System to Intermediate System Intra-Domain
        Routing Exchange Protocol for use in Conjunction with the
        Protocol for Providing the Connectionless-mode Network Service
        (ISO 8473)," ISO DP 10589, February 1990.

  [6]   Luciani, J., Armitage, G., Halpern, J. and N. Doraswamy,
        "Server Cache Synchronization Protocol (SCSP)", RFC 2334, April
        1998.

  [7]   International Telecommunication Union, "Packet-Based Multimedia
        Communication Systems," Recommendation H.323, Version 3
        Telecommunication Standardization Sector of ITU, Geneva,
        Switzerland, November 2000.

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

  [9]   Braden, R., "Requirements for Internet Hosts -- Application and
        Support", STD 3, RFC 1123, October 1989.

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

  [11]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October
        1998.

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

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

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

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













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Intellectual Property Notice

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Authors' Addresses

  Jonathan Rosenberg
  dynamicsoft
  72 Eagle Rock Avenue
  First Floor
  East Hanover, NJ 07936

  Phone: 973-952-5000
  EMail: [email protected]


  Hussein F. Salama
  Cisco Systems
  170 W. Tasman Drive
  San Jose, CA 95134

  Phone: 408-527-7147
  EMail: [email protected]


  Matt Squire
  Hatteras Networks
  639 Davis Drive
  Suite 200
  Durham, NC 27713

  EMail: [email protected]























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Full Copyright Statement

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Acknowledgement

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