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RFC 3219

 
 
 

Telephony Routing over IP (TRIP)

Part 2 of 3, p. 24 to 54
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5. TRIP Attributes

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

5.1. WithdrawnRoutes

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

   The WithdrawnRoutes 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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