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RFC 8171

 
 
 

Transparent Interconnection of Lots of Links (TRILL): Edge Directory Assistance Mechanisms

Part 2 of 3, p. 17 to 42
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3.  Pull Model Directory Assistance Mechanisms

   In the Pull Model [RFC7067], a TRILL switch (RBridge) pulls directory
   information from an appropriate directory server when needed.

   A TRILL switch that makes use of Pull Directory services must
   implement appropriate connections between its directory utilization
   and its link-state database and link-state updating.  For example,
   Pull Directory servers for a particular Data Label X are found by
   looking in the core TRILL IS-IS link-state database for
   data-reachable [RFC7780] TRILL switches that advertise themselves by
   setting the Pull Directory flag (PUL) to 1 in their Interested VLANs
   sub-TLV or Interested Labels sub-TLV (see Section 7.3) for that Data
   Label.  The set of such switches can change with configuration
   changes by network management, such as the following:

   o  the startup or shutdown of Pull Directory servers

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   o  changes in network topology, such as the connection or
      disconnection of TRILL switches that are Pull Directory servers

   o  network partition or merger

   As described in Section 3.7, a TRILL switch MUST be able to detect
   that a Pull Directory from which it has cached data is no longer
   data reachable so that it can discard such cached data.

   If multiple data-reachable TRILL switches indicate in the link-state
   database that they are Pull Directory servers for a particular Data
   Label, pull requests can be sent to any one or more of them, but it
   is RECOMMENDED that pull requests be preferentially sent to the
   server or servers that are lowest cost from the requesting TRILL
   switch.

   Pull Directory requests are sent by encapsulating them in an RBridge
   Channel [RFC7178] message using the Pull Directory channel protocol
   number (see Section 7.2).  Responses are returned in an RBridge
   Channel message using the same channel protocol number.  See
   Section 3.2 for Query and Response Message formats.  For cache
   consistency or notification purposes, Pull Directory servers, under
   certain conditions, MUST send unsolicited Update Messages to client
   TRILL switches they believe may be holding old data.  Those clients
   can acknowledge such updates, as described in Section 3.3.  All these
   messages have a common header, as described in Section 3.1.  Errors
   are returned as described in Section 3.6.

   The requests to Pull Directory servers are typically derived from
   ingressed ARP [RFC826], ND [RFC4861], RARP [RFC903], or SEND
   [RFC3971] messages, or data frames with unknown unicast destination
   MAC addresses, intercepted by an ingress TRILL switch, as described
   in Section 1.1.

   Pull Directory responses include an amount of time for which the
   response should be considered valid.  This includes negative
   responses that indicate that no data is available.  It is RECOMMENDED
   that both positive responses with data and negative responses be
   cached and used to locally handle ARP, ND, RARP, unknown destination
   MAC frames, or the like [ARPND], until the responses expire.  If
   information previously pulled is about to expire, a TRILL switch MAY
   try to refresh it by issuing a new pull request but, to avoid
   unnecessary requests, SHOULD NOT do so unless it has been recently
   used.  The validity timer of cached Pull Directory responses is NOT
   reset or extended merely because that cache entry is used.

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3.1.  Pull Directory Message: Common Format

   All Pull Directory messages are transmitted as the Channel
   Protocol-specific payload of RBridge Channel messages [RFC7178].
   Pull Directory messages are formatted as described herein, starting
   with the following common 8-byte header:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Ver  | Type  | Flags | Count |      Err      |    SubErr     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type Specific Payload - variable length
      +-+-+- ...

      Ver: Version of the Pull Directory protocol.  An unsigned integer.
         Version 0 (zero) is specified in this document.  See
         Section 3.1.1 for a discussion of version negotiation.

      Type: The Pull Directory message type, as follows:

                  Type   Section    Name
                  ----   -------   ------------
                     0    -         Reserved
                     1    3.2.1     Query
                     2    3.2.2     Response
                     3    3.3.1     Update
                     4    3.3.2     Acknowledge
                  5-14    -         Unassigned
                    15    -         Reserved

      Flags: Four flag bits whose meaning depends on the Pull Directory
         message type.  Flags whose meanings are not specified are
         reserved, MUST be sent as zero, and MUST be ignored on receipt.

      Count: Some Pull Directory message types specified herein have
         zero or more occurrences of a Record as part of the
         type-specific payload.  The Count field is the number of
         occurrences of that Record and is expressed as an unsigned
         integer.  For any Pull Directory messages not structured with
         such occurrences, this field MUST be sent as zero and ignored
         on receipt.

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      Err, SubErr: A two-part error code.  These fields are only used in
         Reply Messages.  In messages that are requests or updates,
         these fields MUST be sent as zero and ignored on receipt.  An
         Err field containing the value zero means no error.  The
         meaning of values in the SubErr field depends on the value of
         the Err field, but in all cases, a zero SubErr field is allowed
         and provides no additional information beyond the value of the
         Err field.

      Sequence Number: An identifying 32-bit quantity set by the TRILL
         switch sending a request or other unsolicited message and
         returned in every corresponding reply or acknowledgment.  It is
         used to match up responses with the message to which they
         respond.

      Type Specific Payload: Format depends on the Pull Directory
         message type.

3.1.1.  Version Negotiation

   The version number (Ver) in the Pull Directory message header is
   incremented for a future version with changes such that TRILL
   directory messages cannot be parsed correctly by an earlier version.
   Ver is not incremented for minor changes such as defining a new field
   value for an existing field.

   Pull Directory messages come in pairs (Request-Response,
   Update-Acknowledgment).  The version number in the Request/Update
   (Ver1) indicates the format of that message and the format of the
   corresponding returned Response/Acknowledgment.  The version number
   in the returned Response/Acknowledgment (Ver2) indicates the highest
   version number that the sender of that Response/Acknowledgment
   understands.

   In the most common case -- a well-configured network -- Ver1 and Ver2
   will be equal.

   If Ver2 is less than Ver1, the returned Response/Acknowledgment will
   be an error message saying that the version is not understood.

   If Ver2 is greater than Ver1 and the responder understands Ver1, it
   responds normally in Ver1 format.  However, if the responder does not
   understand Ver1, it MUST send a "Version not understood" error
   message (Section 3.6.2) correctly formatted for Ver1.  Thus, all
   implementations that support some version X MUST be able to send a
   Version not understood error message correctly formatted for all
   lower versions down to version 0.

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3.2.  Pull Directory Query and Response Messages

   The formats of Pull Directory Query Messages and Pull Directory
   Response Messages are specified in Sections 3.2.1 and 3.2.2.1,
   respectively.

3.2.1.  Pull Directory Query Message Format

   A Pull Directory Query Message is sent as the Channel
   Protocol-specific content of an RBridge Channel message [RFC7178]
   TRILL Data packet or as a native RBridge Channel data frame (see
   Section 3.5).  The Data Label of the packet is the Data Label in
   which the query is being made.  The priority of the RBridge Channel
   message is a mapping of the priority of the ingressed frame that
   caused the query.  The default mapping depends, per Data Label, on
   the strategy (see Section 4) or a configured priority (see
   "DirGenQPriority" in Section 3.9) for generated queries.  (Generated
   queries are those queries that are not the result of a mapping -- for
   example, a query to refresh a cache entry.)  The Channel
   Protocol-specific data is formatted as a header and a sequence of
   zero or more QUERY Records as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Ver  | Type  | Flags | Count |      Err      |    SubErr     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | QUERY 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | QUERY 2
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | QUERY K
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

      Ver, Sequence Number: See Section 3.1.

      Type: 1 for Query.  Queries received by a TRILL switch that is not
         a Pull Directory for the relevant Data Label result in an error
         response (see Section 3.6) unless inhibited by rate limiting.
         (See [RFC7178] for information on the Response Message that is
         generated if the recipient implements the RBridge Channel
         features but does not implement the Pull Directory RBridge
         Channel Protocol.)

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      Flags, Err, and SubErr: MUST be sent as zero and ignored on
         receipt.

      Count: Count is the number of QUERY Records present.  A
         Query Message Count of 0 is explicitly allowed, for the purpose
         of pinging a Pull Directory server to see if it is responding.
         On receipt of such an empty Query Message, a Response Message
         that also has a Count of 0 is returned unless inhibited by rate
         limiting.

      QUERY: Each QUERY Record within a Pull Directory Query Message is
         formatted as follows:

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |        SIZE           |FR|  RESV  |   QTYPE   |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             If QTYPE = 1
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |                      AFN                      |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |  Query Address ...
               +--+--+--+--+--+--+--+--+--+--+--...
             If QTYPE = 2 or 5
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |  Query Frame ...
               +--+--+--+--+--+--+--+--+--+--+--...

         SIZE: Size of the QUERY Record in bytes, expressed as an
            unsigned integer and not including the SIZE field and
            following byte.  A value of SIZE so large that the material
            doesn't fit in the Query Message indicates a malformed
            QUERY Record.  A QUERY Record with such an illegal SIZE
            value, and any subsequent QUERY Records, MUST be ignored,
            and the entire Query Message MAY be ignored.

         FR: The Flood Record flag.  Ignored if QTYPE is 1.  If QTYPE is
            2 or 5 and the directory information sought is not found,
            the frame provided is flooded; otherwise, it is not
            forwarded.  See Section 3.2.2.2.  For QTYPEs other than 2 or
            5, the FR flag has no effect.

         RESV: A block of three reserved bits.  MUST be sent as zero and
            ignored on receipt.

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         QTYPE: There are several types of QUERY Records currently
            defined in two classes, as follows: (1) a QUERY Record that
            provides an explicit address and asks for all addresses for
            the interface specified by the Query Address and (2) a
            QUERY Record that includes a frame.  The fields of each are
            specified below.  Values of QTYPE are as follows:

            QTYPE   Description
            -----   -------------------------------
               0    Reserved
               1    Address query
               2    Frame query
             3-4    Unassigned
               5    Unknown unicast MAC Query Frame
            6-14    Unassigned
              15    Reserved

         AFN: Address Family Number of the Query Address.

         Query Address: The query is asking for any other addresses, and
            the nickname of the TRILL switch from which they are
            reachable, that correspond to the same interface as this
            address, within the Data Label of the query of the address
            provided.  A typical Query Address would be something like
            the following:

            1. A 48-bit MAC address, with the querying TRILL switch
               primarily interested in either

               a. the RBridge by which that MAC address is reachable, so
                  that the querying RBridge can forward an unknown
                  (before the query) destination MAC address native
                  frame as a unicast TRILL Data packet rather than
                  flooding it, or

               b. the IP address corresponding to the MAC address, so
                  that the RBridge can locally respond to a RARP
                  [RFC903] native frame.

            2. An IPv4 or IPv6 address, with the querying RBridge
               interested in the corresponding MAC address so it can
               locally respond to an ARP [RFC826] or ND [RFC4861] native
               frame [ARPND].

            But the Query Address could be some other address type for
            which an AFN has been assigned, such as a 64-bit MAC address
            [RFC7042] or a CLNS (connectionless-mode network service)
            [X.233] address.

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         Query Frame: Where a QUERY Record is the result of an ARP, ND,
            RARP, SEND, or unknown unicast MAC destination address, the
            ingress TRILL switch MAY send the frame to a Pull Directory
            server if the frame is small enough that the resulting Query
            Message fits into a TRILL Data packet within the campus MTU.
            The full frame is included, starting with the destination
            and source MAC addresses, but does not include the Frame
            Check Sequence (FCS).

   If no response to a Pull Directory Query Message is received within a
   configurable timeout (see "DirQueryTimeout" in Section 3.9), then the
   Query Message should be retransmitted with the same Sequence Number
   (up to a configurable number of times (see "DirQueryRetries" in
   Section 3.9)).  If there are multiple QUERY Records in a
   Query Message, responses to various subsets of these QUERY Records
   can be received before the timeout.  In that case, the remaining
   unanswered QUERY Records should be resent in a new Query Message with
   a new Sequence Number.  If a TRILL switch is not capable of handling
   partial responses to queries with multiple QUERY Records, it MUST NOT
   send a Request Message with more than one QUERY Record in it.

   See Section 3.6 for a discussion of how Query Message errors are
   handled.

3.2.2.  Pull Directory Responses

   A Pull Directory Query Message results in a Pull Directory Response
   Message as described in Section 3.2.2.1.

   In addition, if the QUERY Record QTYPE was 2 or 5, the frame included
   in the Query may be modified and forwarded by the Pull Directory
   server as described in Section 3.2.2.2.

3.2.2.1.  Pull Directory Response Message Format

   Pull Directory Response Messages are sent as the
   Channel Protocol-specific content of an RBridge Channel message
   [RFC7178] TRILL Data packet or as a native RBridge Channel data frame
   (see Section 3.5).  Responses are sent with the same Data Label and
   priority as the Query Message to which they correspond, except that
   the Response Message priority is limited to be no more than the
   configured value DirRespMaxPriority (Section 3.9).

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   The RBridge Channel Protocol-specific data format is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Ver  | Type  | Flags | Count |      Err      |    SubErr     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | RESPONSE 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | RESPONSE 2
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      | RESPONSE K
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

      Ver, Sequence Number: As specified in Section 3.1.

      Type: 2 = Response.

      Flags: MUST be sent as zero and ignored on receipt.

      Count: Count is the number of RESPONSE Records present in the
         Response Message.

      Err, SubErr: A two-part error code.  Zero, unless there was an
         error in the Query Message (in which case, see Section 3.6).

      RESPONSE: Each RESPONSE Record within a Pull Directory Response
         Message is formatted as follows:

           0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         |         SIZE          |OV|  RESV  |   Index   |
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         |                   Lifetime                    |
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         |                Response Data ...
         +--+--+--+--+--+--+--+--+--+--+--...

         SIZE: The size of the RESPONSE Record is an unsigned integer
            number of bytes, not including the SIZE field and following
            byte.  A value of SIZE so large that the material doesn't
            fit in the Query Message indicates a malformed

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            RESPONSE Record.  A RESPONSE Record with such an excessive
            SIZE value, and any subsequent RESPONSE Records, MUST be
            ignored, and the entire Response Message MAY be ignored.

         OV: The overflow flag.  Indicates, as described below, that
            there was too much Response Data to include in one Response
            Message.

         RESV: Three reserved bits that MUST be sent as zero and ignored
            on receipt.

         Index: The relative index of the QUERY Record in the Query
            Message to which this RESPONSE Record corresponds.  The
            Index will always be 1 for Query Messages containing a
            single QUERY Record.  If the Index is larger than the Count
            was in the corresponding Query, that RESPONSE Record MUST be
            ignored, and subsequent RESPONSE Records or the entire
            Response Message MAY be ignored.

         Lifetime: The length of time, in units of 100 milliseconds,
            for which the response should be considered valid, except
            that the values zero and 2**16 - 1 are special.  If zero,
            the response can only be used for the particular query from
            which it resulted and MUST NOT be cached.  If 2**16 - 1, the
            response MAY be kept indefinitely but not after the Pull
            Directory server goes down or becomes unreachable.  (The
            maximum definite time that can be expressed is a little over
            1.8 hours.)

         Response Data: There are three types of RESPONSE Records:

         -  If the Err field of the encapsulating Response Message has a
            message-level error code in it, then the RESPONSE Records
            are omitted and Count will be 0.  See Section 3.6 for
            additional information on errors.

         -  If the Err field of the encapsulating Response Message has a
            record-level error code in it, then the RESPONSE Records are
            those having that error, as further described in
            Section 3.6.

         -  If the Err field of the encapsulating Response Message is 0,
            then the Response Data in each RESPONSE Record is formatted
            as the value part of an Interface Addresses APPsub-TLV
            [RFC7961].  The maximum size of such contents is 255 bytes,
            in which case the RESPONSE Record SIZE field is 255.

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   Multiple RESPONSE Records can appear in a Response Message with the
   same Index if an answer to the QUERY Record consists of multiple
   Interface Addresses APPsub-TLV values.  This would be necessary if,
   for example, a MAC address within a Data Label appears to be
   reachable by multiple TRILL switches.  However, all RESPONSE Records
   to any particular QUERY Record MUST occur in the same Response
   Message.  If a Pull Directory holds more mappings for a queried
   address than will fit into one Response Message, it selects which
   mappings to include, by some method outside the scope of this
   document, and sets the overflow flag (OV) in all of the
   RESPONSE Records responding to that Query Address.

   See Section 3.6 for a discussion of how errors are handled.

3.2.2.2.  Pull Directory Forwarding

   Query Messages with QTYPEs 2 and 5 are interpreted and handled as
   described below.  In these cases, if the information implicitly
   sought is not in the directory and the FR flag in the Query Message
   was 1 (one), the provided frame is forwarded by the Pull Directory
   server as a multi-destination TRILL Data packet with the ingress
   nickname of the Pull Directory server (or proxy, if it is hosted on
   an end station) in the TRILL Header.  If the FR flag is 0, the frame
   is not forwarded in this case.

   If there was no error in the handling of the encapsulating
   Query Message, the Pull Directory server forwards the frame inside
   that QUERY Record, after modifying it in some cases, as described
   below:

   ARP: When QTYPE is 2 and the Ethertype in the QUERY Record indicates
      that an ARP [RFC826] frame is included in the Record:
      The ar$op field MUST be ares_op$REQUEST, and for the response
      described in Section 3.2.2.1, this is treated as a query for the
      target protocol address, where the AFN of that address is given by
      ar$pro.  (ARP fields and value names with embedded dollar signs
      ("$") are specified in [RFC826].)  If (1) ar$op is not
      ares_op$REQUEST, (2) the ARP is malformed, or (3) the query fails,
      an error is returned.  Otherwise, the ARP is modified into the
      appropriate ARP response, which is then sent by the Pull Directory
      server as a TRILL Data packet.

   ND/SEND: When QTYPE is 2 and the Ethertype in the QUERY Record
      indicates that an IPv6 ND [RFC4861] or SEND [RFC3971] frame is
      included in the Record:
      Only Neighbor Solicitation ND frames (corresponding to an ARP
      query) are allowed.  An error is returned for other ND frames or
      if the target address is not found.  Otherwise, if the ND is not a

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      SEND, an ND Neighbor Advertisement response is returned by the
      Pull Directory server as a TRILL Data packet.  In the case of
      SEND, an error is returned indicating that a SEND frame was
      received by the Pull Directory, and the Pull Directory then either
      (1) forwards the SEND frame to the holder of the IPv6 address if
      that information is in the directory or (2) multicasts the
      SEND frame.

   RARP: When QTYPE is 2 and the Ethertype in the QUERY Record indicates
      that a RARP [RFC903] frame is included in the Record:
      If the ar$op field is ares_op$REQUEST, the frame is handled as an
      ARP, as described above.  Otherwise, the ar$op field MUST be
      "reverse request", and for the response described in
      Section 3.2.2.1, this is treated as a query for the target
      hardware address, where the AFN of that address is given by
      ar$hrd.  (See [RFC826] for RARP fields.)  If (1) ar$op is not one
      of these values, (2) the RARP is malformed, or (3) the query
      fails, an error is returned.  Otherwise, the RARP is modified into
      the appropriate RARP response, which is then unicast by the Pull
      Directory server as a TRILL Data packet to the source hardware MAC
      address.

   MacDA: When QTYPE is 5, indicating that a frame is provided in the
      QUERY Record whose destination MAC address TRILL switch attachment
      is unknown, the only requirement is that this MAC address has to
      be unicast.  The Ethertype in the QUERY Record is ignored.  If
      this MAC address is a group address, an error is returned.  In the
      case of Pull Directory Response Messages (Section 3.2.2.1), this
      MAC address is treated as a query for the MacDA.  In the creation
      of the response described in Section 3.2.2.1, the query is treated
      as a query for this MAC address.  If the Pull Directory contains
      TRILL switch attachment information for the MAC address in the
      Data Label of the Query Message, it forwards the frame to that
      switch in a unicast TRILL Data packet.

3.3.  Cache Consistency

   Unless it sends all responses with a Lifetime of 0, a Pull Directory
   MUST take action, by sending Update Messages, to minimize the amount
   of time that a TRILL switch will continue to use stale information
   from that Pull Directory.  The formats of Update Messages and the
   Acknowledge Messages used to respond to Update Messages are given in
   Sections 3.3.1 and 3.3.2, respectively.

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   A Pull Directory server MUST maintain one of three sets of records
   concerning possible cached data at clients of that server.  These are
   numbered and listed below in order of increasing specificity:

   Method 1, Least Specific.  An overall record, per Data Label, of when
      the last positive Response Data sent will expire and when the last
      negative response sent will expire; the records are retained until
      such expiration.

      Pro: Minimizes the record-keeping burden on the Pull Directory
         server.

      Con: Increases the volume of and overhead due to (1) spontaneous
         Update Messages and (2) unnecessarily invalidating cached
         information.

   Method 2, Medium Specificity.  For each unit of data (Interface
      Addresses APPsub-TLV (IA APPsub-TLV) Address Set [RFC7961]) held
      by the server, record when the last response sent with that
      positive Response Data will expire.  In addition, record each
      address about which a negative response was sent by the server and
      when the last such negative response will expire.  Each such
      record of a positive or negative response is discarded upon
      expiration.

      Pros/Cons: An intermediate level of detail in server
         record-keeping; also, an intermediate volume of, and overhead
         due to, spontaneous Update Messages with some unnecessary
         invalidation of cached information.

   Method 3, Most Specific.  For each unit of data held by the server
      (IA APPsub-TLV Address Set [RFC7961]) and each address about which
      a negative response was sent, a list of TRILL switches that were
      sent that data as a positive response or sent a negative response
      for the address, and the expected time to expiration for that data
      or address at each such TRILL switch, assuming that the requester
      cached the response.  Each list entry is retained until such
      expiration time.

      Pros: Minimizes spontaneous Update Messages sent to update pull
         client TRILL switch caches, and minimizes unnecessary
         invalidation of cached information.

      Con: Increased record-keeping burden on the Pull Directory server.

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   RESPONSE Records sent with a zero Lifetime are considered to have
   already expired and so do not need to be tracked.  In all cases,
   there may still be brief periods of time when directory information
   has changed, but information that a pull client has cached has not
   yet been updated or expunged.

   A Pull Directory server might have a limit as to (1) how many TRILL
   switches for which it can maintain detailed expiry information using
   method 3 or (2) how many data units or addresses for which it can
   maintain expiry information using method 2 or the like.  If such
   limits are exceeded, it MUST transition to a lower-numbered method
   but, in all cases, MUST support, at a minimum, method 1 and SHOULD
   support methods 2 and 3.  The use of method 1 may be quite
   inefficient, due to large amounts of cached positive and negative
   information being unnecessarily discarded.

   When data at a Pull Directory is changed, deleted, or added and there
   may be unexpired stale information at a requesting TRILL switch, the
   Pull Directory MUST send an Update Message as discussed below.  The
   sending of such an Update Message MAY be delayed by a configurable
   number of milliseconds (see "DirUpdateDelay" in Section 3.9) to await
   other possible changes that could be included in the same
   Update Message.

   1. If method 1, the least detailed method, is being followed, then
      when any Pull Directory information in a Data Label is changed or
      deleted and there are outstanding cached positive data
      response(s), an all-addresses flush positive data Update Message
      is flooded within that Data Label as an RBridge Channel message.
      If data is added and there are outstanding cached negative
      responses, an all-addresses flush negative message is similarly
      flooded.  A Count field value of 0 in an Update Message indicates
      "all-addresses".  On receiving an all-addresses flooded flush
      positive Update from a Pull Directory server it has used,
      indicated by the F (Flood) and P (Positive) bits being 1 and the
      Count being 0, a TRILL switch discards the cached data responses
      it has for that Data Label.  Similarly, on receiving an
      all-addresses flush negative Update, indicated by the F and
      N (Negative) bits being 1 and the Count being 0, it discards all
      cached negative replies for that Data Label.  A combined flush
      positive and negative can be flooded by having all of the F, P,
      and N bits (see Section 3.3.1) set to 1 and the Count field 0,
      resulting in the discard of all positive and negative cached
      information for the Data Label.

   2. If method 2 is being followed, then a TRILL switch floods
      address-specific positive Update Messages when data that might be
      cached by a querying TRILL switch is changed or deleted and floods

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      address-specific negative Update Messages when data that might be
      cached by a querying TRILL switch is added.  Such messages are
      sent as RBridge Channel messages.  The F bit will be 1; however,
      the Count field will be non-zero, and either the P bit or the
      N bit, but not both, will be 1.  There are actually four possible
      message types that can be flooded:

      a. If data that might still be cached is updated:
         An unsolicited Update Message is sent with the P flag set and
         the Err field 0.  On receipt, the addresses in the RESPONSE
         Records are compared to the addresses for which the receiving
         TRILL switch is holding cached positive information from that
         server.  If they match, the cached information is updated.

      b. If data that might still be cached is deleted:
         An unsolicited Update Message is sent with the P flag set and
         the Err field non-zero, giving the error that would now be
         encountered in attempting to pull information for the relevant
         address from the Pull Directory server.  In this non-zero Err
         field case, the RESPONSE Record(s) differs from non-zero Err
         Reply Message RESPONSE Records in that they do include an
         interface address set.  Any cached positive information for the
         addresses given is deleted, and the negative response is cached
         as per the Lifetime given.

      c. If data for an address for which a negative response was sent
         is added, so that negative response that might still be cached
         is now incorrect:
         An unsolicited Update Message is sent with the N flag set to 1
         and the Err field 0.  The addresses in the RESPONSE Records are
         compared to the addresses for which the receiving TRILL switch
         is holding cached negative information from that server; if
         they match, the cached negative information is deleted, and the
         positive information provided is cached as per the Lifetime
         given.

      d. In the rare case where it is desired to change the Lifetime or
         error associated with negative information that might still be
         cached:
         An unsolicited Update Message is sent with the N flag set to 1
         and the Err field non-zero.  As in case b above, the RESPONSE
         Record(s) gives the relevant addresses.  Any cached negative
         information for the addresses is updated.

   3. If method 3 is being followed, unsolicited Update Messages of the
      same sort are sent as with method 2 above, except that they are
      not normally flooded but unicast only to the specific TRILL
      switches the directory server believes may be holding the cached

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      positive or negative information that needs deletion or updating.
      However, a Pull Directory server MAY flood unsolicited updates
      using method 3 -- for example, if it determines that a
      sufficiently large fraction of the TRILL switches in some Data
      Label are requesters that need to be updated so that flooding is
      more efficient than unicast.

   A Pull Directory server tracking cached information with method 3
   MUST NOT clear the indication that it needs to update cached
   information at a querying TRILL switch until it has either (a) sent
   an Update Message and received a corresponding Acknowledge Message or
   (b) sent a configurable number of updates at a configurable interval
   where these parameters default to three updates 100 milliseconds
   apart (see Section 3.9).

   A Pull Directory server tracking cached information with method 1 or
   method 2 SHOULD NOT clear the indication that it needs to update
   cached information until it has sent an Update Message and received a
   corresponding Acknowledge Message from all of its ESADI neighbors or
   it has sent a number of updates at a configurable interval, as
   specified in the paragraph above.

3.3.1.  Update Message Format

   An Update Message is formatted as a Response Message, with the
   differences described in Section 3.3 above and the following:

   o  The Type field in the message header is set to 3.

   o  The Index field in the RESPONSE Record(s) is set to 0 on
      transmission and ignored on receipt (but the Count field in the
      Update Message header MUST still correctly indicate the number of
      RESPONSE Records present).

   o  The priority with which the message is sent, DirUpdatePriority, is
      configurable and defaults to 5 (see Section 3.9).

   Update Messages are initiated by a Pull Directory server.  The
   Sequence Number space used is controlled by the originating Pull
   Directory server.  This Sequence Number space for Update Messages is
   different from the Sequence Number space used in a Query and the
   corresponding Response that are controlled by the querying
   TRILL switch.

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   The 4-bit Flags field of the message header for an Update Message is
   as follows:

            +---+---+---+---+
            | F | P | N | R |
            +---+---+---+---+

      F: The Flood bit.  If F = 0, the Update Message is unicast.  If
         F = 1, it is multicast to All-Egress-RBridges.

      P, N: Flags used to indicate positive or negative Update Messages.
         P = 1 indicates "positive".  N = 1 indicates "negative".  Both
         may be 1 for a flooded all-addresses Update.

      R: Reserved.  MUST be sent as zero and ignored on receipt.

   For tracking methods 2 and 3 in Section 3.3, a particular Update
   Message MUST have either the P flag or the N flag set, but not both.
   If both are set, the Update Message MUST be ignored, as this
   combination is only valid for method 1.

3.3.2.  Acknowledge Message Format

   An Acknowledge Message is sent in response to an Update Message to
   confirm receipt or indicate an error, unless response is inhibited by
   rate limiting.  It is formatted as a Response Message, but the Type
   is set to 4.

   If there are no errors in the processing of an Update Message or if
   there is an overall message-level error or a header error in an
   Update Message, the message is echoed back with the Err and
   SubErr fields set appropriately, the Type changed to Acknowledge, and
   a null Records section with the Count field set to 0.

   If there is a record-level error in an Update Message, one or more
   Acknowledge Messages may be returned with the erroneous record(s)
   indicated as discussed in Section 3.6.

   An Acknowledge Message is sent with the same priority as the Update
   Message it acknowledges but not more than a configured priority
   called "DirAckMaxPriority", which defaults to 5 (see Section 3.9).

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3.4.  Summary of Record Formats in Messages

   As specified in Sections 3.2 and 3.3, the Query, Response, Update,
   and Acknowledge Messages can have zero or more repeating Record
   structures under different circumstances, as summarized below.  The
   "Err" column abbreviations in this table have the meanings listed
   below.  "IA APPsub-TLV value" means the value part of the
   IA APPsub-TLV specified in [RFC7961].

                 MBZ = MUST be zero
                 Z   = zero
                 NZ  = non-zero
                 NZM = non-zero message-level error
                 NZR = non-zero record-level error

       Message    Err  Section  Record Structure    Response Data
     -----------  ---  -------  ----------------  -------------------
     Query        MBZ  3.2.1    QUERY Record       -
     Response     Z    3.2.2.1  RESPONSE Record   IA APPsub-TLV value
     Response     NZM  3.2.2.1  null               -
     Response     NZR  3.2.2.1  RESPONSE Record   Records with error
     Update       MBZ  3.3.1    RESPONSE Record   IA APPsub-TLV value
     Acknowledge  Z    3.3.2    null               -
     Acknowledge  NZM  3.3.2    null               -
     Acknowledge  NZR  3.3.2    RESPONSE Record   Records with error

   See Section 3.6 for further details on errors.

3.5.  End Stations and Pull Directories

   A Pull Directory can be hosted on an end station as specified in
   Section 3.5.1.

   An end station can use a Pull Directory as specified in
   Section 3.5.2.  This capability would be useful in supporting an end
   station that performs directory-assisted encapsulation [DirAsstEncap]
   or that is a "Smart Endnode" [SmartEN].

   The native Pull Directory messages used in these cases are as
   specified in Section 3.5.3.  In these cases, the edge RBridge(s) and
   end station(s) involved need to detect each other and exchange some
   control information.  This is accomplished with the TRILL End System
   to Intermediate System (ES-IS) mechanism specified in Section 5.

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3.5.1.  Pull Directory Hosted on an End Station

   Optionally, a Pull Directory actually hosted on an end station MAY be
   supported.  In that case, one or more TRILL switches must act as
   indirect Pull Directory servers.  That is, they host a Pull Directory
   server, which is seen by other TRILL switches in the campus, and a
   Pull Directory client, which fetches directory information from one
   or more end-station Pull Directory servers, where at least some of
   the information provided by the Pull Directory server may be
   information fetched from an end station to which it is directly
   connected by the co-located Pull Directory client.  ("Direct
   connection" means a connection not involving any intermediate TRILL
   switches.)

   End stations hosting a Pull Directory server MUST support TRILL ES-IS
   (see Section 5) and advertise the Data Labels for which they are
   providing service in one or more Interested VLANs sub-TLVs or
   Interested Labels sub-TLVs by setting the PUL flag (see Section 7.3).

                                                *  *  *  *  *  *  *
      +---------------+                         *                 *
      | End Station 1 |              +---------------+            *
      | Pull Directory+--------------+ RB1, Pull     |            *
      | Server        |              |      Directory|            *
      +---------------+      +-------+ Client|Server |         +----+
                             |       +---------------+         |RB99|
      +---------------+      |                  *              +----+
      | End Station 2 |   +--+---+   +---------------+            *
      | Pull Directory+---+Bridge+---+ RB2, Pull     |            *
      | Server        |   +--+---+   |      Directory|            *
      +---------------+      |       | Client|Server |            *
                             |       +---------------+            *
                             |                  *        TRILL    *
                             .                  *        Campus   *
                             .                  *                 *
                             .                  *  *  *  *  *  *  *

               Figure 2: End-Station Pull Directory Example

   Figure 2 gives an example where RB1 and RB2 advertise themselves to
   the rest of the TRILL campus, such as RB99, as Pull Directory servers
   and obtain at least some of the information they are providing by
   issuing Pull Directory queries to End Stations 1 and/or 2.  This
   example is specific, but many variations are possible.  The box
   labeled "Bridge" in Figure 2 could be replaced by a complex bridged
   LAN or could be a bridgeless LAN through the use of a hub or
   repeater.  Or, end stations might be connected via point-to-point
   links (as shown for End Station 1), including multi-ported

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   end stations connected by point-to-point links to multiple RBridges.
   Although Figure 2 shows two end stations and two RBridges, there
   could be one or more than two RBridges having such indirect Pull
   Directory servers.  Furthermore, there could be one or more than two
   end stations with Pull Directory servers on them.  Each TRILL switch
   acting as an indirect Pull Directory server could then be differently
   configured as to the Data Labels for which it is providing indirect
   service selected from the union of the Data Labels supported by the
   end-station hosted servers and could select from among those
   end-station hosted servers supporting each Data Label the indirect
   server is configured to provide.

   When an indirect Pull Directory server receives Query Messages from
   other TRILL switches, it answers from information it has cached or
   issues Pull Directory requests to end-station Pull Directory servers
   with which it has TRILL ES-IS adjacency to obtain the information.
   Any Response sent by an indirect Pull Directory server MUST NOT have
   a validity time longer than the validity period of the data on which
   it is based.  When an indirect Pull Directory server receives Update
   Messages, it updates its cached information and MUST originate Update
   Messages to any clients that may have mirrors of the cached
   information so updated.

   Since an indirect Pull Directory server discards information it has
   cached from queries to an end-station Pull Directory server if it
   loses adjacency to the server (Section 3.7), if it detects that such
   information may be cached at RBridge clients and has no other source
   for the information, it MUST send Update Messages to those clients
   withdrawing the information.  For this reason, indirect Pull
   Directory servers may wish to query multiple sources, if available,
   and cache multiple copies of returned information from those multiple
   sources.  Then, if one end-station source becomes inaccessible or
   withdraws the information but the indirect Pull Directory server has
   the information from another source, it need not originate Update
   Messages.

3.5.2.  Use of Pull Directory by End Stations

   Some special end stations, such as those discussed in [DirAsstEncap]
   and [SmartEN], may need to access directory information.  How edge
   RBridges provide this optional service is specified below.

   When Pull Directory support is provided by an edge RBridge to end
   stations, the messages used are as specified in Section 3.5.3 below.
   The edge RBridge MUST support TRILL ES-IS (Section 5) and advertises
   the Data Labels for which it offers this service to end stations by

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   setting the Pull Directory flag (PUL) to 1 in its Interested VLANs
   sub-TLV or Interested Labels sub-TLV (see Section 7.3) for that Data
   Label advertised through TRILL ES-IS.

3.5.3.  Native Pull Directory Messages

   The Pull Directory messages used between TRILL switches and end
   stations are native RBridge Channel messages [RFC7178].  These
   RBridge Channel messages use the same Channel Protocol number as the
   inter-RBridge Pull Directory RBridge Channel messages.  The
   Outer.VLAN ID used is the TRILL ES-IS Designated VLAN (see Section 5)
   on the link to the end station.  Since there is no TRILL Header or
   inner Data Label for native RBridge Channel messages, that
   information is added to the Pull Directory message header as
   specified below.

   The protocol-dependent data part of the native RBridge Channel
   message is the same as for inter-RBridge Channel messages, except
   that the 8-byte header described in Section 3.1 is expanded to 12 or
   16 bytes, as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Ver  | Type  | Flags | Count |      Err      |    SubErr     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Data Label ... (4 or 8 bytes)                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+
      | Type Specific Payload - variable length
      +-+-+- ...

   Fields other than the Data Label field are as defined in Section 3.1.
   The Data Label that normally appears right after the Inner.MacSA of
   the RBridge Channel Pull Directory message appears in the Data Label
   field of the Pull Directory message header in the native RBridge
   Channel message version.  This Data Label appears in a native Query
   Message, to be reflected in a Response Message, or it might appear in
   a native Update to be reflected in an Acknowledge Message.  Since the
   appropriate VLAN or FGL [RFC7172] Ethertype is included, the length
   of the Data Label can be determined from the first 2 bytes.

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3.6.  Pull Directory Message Errors

   A non-zero Err field in the Pull Directory Response or Acknowledge
   Message header indicates an error message.

   If there is an error that applies to an entire Query or Update
   Message or its header, as indicated by the range of the value of the
   Err field, then the QUERY Records probably were not even looked at by
   the Pull Directory server and would provide no additional information
   in the Response or Acknowledge Message.  Therefore, the Records
   section of the response to a Query Message or Update Message is
   omitted, and the Count field is set to 0 in the Response or
   Acknowledge Message.

   If errors occur at the QUERY Record level for a Query Message, they
   MUST be reported in a Response Message separate from the results of
   any successful non-erroneous QUERY Records.  If multiple
   QUERY Records in a Query Message have different errors, they MUST be
   reported in separate Response Messages.  If multiple QUERY Records in
   a Query Message have the same error, this error response MAY be
   reported in one or multiple Response Messages.  In an error Response
   Message, the QUERY Record or Records being responded to appear,
   expanded by the Lifetime for which the server thinks the error might
   persist (usually 2**16 - 1, which indicates "indefinitely") and with
   their Index inserted, as the RESPONSE Record or Records.

   If errors occur at the RESPONSE Record level for an Update Message,
   they MUST be reported in an Acknowledge Message separate from the
   acknowledgment of any non-erroneous RESPONSE Records.  If multiple
   RESPONSE Records in an Update have different errors, they MUST be
   reported in separate Acknowledge Messages.  If multiple
   RESPONSE Records in an Update Message have the same error, this error
   response MAY be reported in one or multiple Acknowledge Messages.  In
   an error Acknowledge Message, the RESPONSE Record or Records being
   responded to appear, expanded by the time for which the server thinks
   the error might persist and with their Index inserted, as a
   RESPONSE Record or Records.

   Err values 1 through 126 are available for encoding errors at the
   Request Message or Update Message level.  Err values 128 through 254
   are available for encoding errors at the QUERY Record or
   RESPONSE Record level.  The SubErr field is available for providing
   more detail on errors.  The meaning of a SubErr field value
   depends on the value of the Err field.

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3.6.1.  Error Codes

   The following table lists error code values for the Err field, their
   meanings, and whether they apply at the message level or the
   record level.

    Err       Level     Meaning
   -------   -------    -----------------------------------------------
       0        -       No Error

       1     Message    Unknown or reserved Query Message field value
       2     Message    Request Message/data too short
       3     Message    Unknown or reserved Update Message field value
       4     Message    Update Message/data too short
   5-126     Message    Unassigned
     127        -       Reserved

     128     Record     Unknown or reserved QUERY Record field value
     129     Record     QUERY Record truncated
     130     Record     Address not found
     131     Record     Unknown or reserved RESPONSE Record field value
     132     Record     RESPONSE Record truncated
   133-254   Record     Unassigned
     255        -       Reserved

   Note that some error codes are for overall message-level errors,
   while some are for errors in the repeating records that occur in
   messages.

3.6.2.  Sub-errors under Error Codes 1 and 3

   The following sub-errors are specified under error codes 1 and 3:

      SubErr   Field with Error
      ------   -------------------------------------------
          0     Unspecified
          1     Version not understood (see Section 3.1.1)
          2     Unknown Type field value
          3     Specified Data Label not being served
      4-254     Unassigned
        255     Reserved

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3.6.3.  Sub-errors under Error Codes 128 and 131

   The following sub-errors are specified under error codes 128 and 131:

      SubErr   Field with Error
      ------   ----------------------------------------------------
          0     Unspecified
          1     Unknown AFN field value
          2     Unknown or Reserved QTYPE field value
          3     Invalid or inconsistent SIZE field value
          4     Invalid frame for QTYPE 2 (other than SEND)
          5     SEND frame sent as QTYPE 2
          6     Invalid frame for QTYPE 5 (such as multicast MacDA)
      7-254     Unassigned
        255     Reserved

3.7.  Additional Pull Details

   A Pull Directory client MUST be able to detect, by tracking
   link-state changes, when a Pull Directory server is no longer
   accessible (data reachable [RFC7780] for the inter-RBridge case or
   TRILL ES-IS (Section 5) adjacent for the end-station-to-RBridge case)
   and MUST promptly discard all pull responses it is retaining from
   that server, as it can no longer receive cache consistency Update
   Messages from the server.

   A secondary Pull Directory server is one that obtains its data from a
   primary directory server.  See the discussion in Section 2.6
   regarding the transfer of directory information from the
   primary server to the secondary server.

3.8.  The "No Data" Flag

   In the TRILL base protocol [RFC6325] as extended for FGL [RFC7172],
   the mere presence of any Interested VLANs sub-TLVs or Interested
   Labels sub-TLVs in the LSP of a TRILL switch indicates connection to
   end stations in the VLAN(s) or FGL(s) listed and thus a need to
   receive multi-destination traffic in those Data Labels.  However,
   with Pull Directories, advertising that you are a directory server
   requires using these sub-TLVs to indicate the Data Label(s) you are
   serving.

   If a directory server does not wish to receive multi-destination
   TRILL Data packets for the Data Labels it lists in one of the
   Interested VLANs or Interested Labels (FGLs) sub-TLVs (see
   Section 1.2), it sets the No Data (NOD) bit to 1 (see Section 7.3).
   This means that data on a distribution tree may be pruned so as not
   to reach the "No Data" TRILL switch as long as there are no TRILL

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   switches interested in the Data Label that are beyond the No Data
   TRILL switch on that distribution tree.  The NOD bit is backward
   compatible, as TRILL switches ignorant of it will simply not prune
   when they could; this is safe, although it may cause increased link
   utilization by some TRILL switches sending multi-destination traffic
   where it is not needed.

   Push Directories advertise themselves inside ESADI, which normally
   requires the ability to send and receive multi-destination TRILL Data
   packets but can be implemented with serial unicast.

   An example of a TRILL switch serving as a directory that might not
   want multi-destination traffic in some Data Labels would be a TRILL
   switch that does not offer end-station service for any of the Data
   Labels for which it is serving as a directory and is

   -  a Pull Directory and/or

   -  a Push Directory for one or more Data Labels, where all of the
      ESADI traffic for those Data Labels will be handled by unicast
      ESADI [RFC7357].

   A Push Directory MUST NOT set the NOD bit for a Data Label if it
   needs to communicate via multi-destination ESADI or RBridge Channel
   PDUs in that Data Label, since such PDUs look like TRILL Data packets
   to transit TRILL switches and are likely to be incorrectly pruned if
   the NOD bit was set.

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3.9.  Pull Directory Service Configuration

   The following per-RBridge scalar configuration parameters are
   available for controlling Pull Directory service behavior.  In
   addition, there is a configurable mapping, per Data Label, from the
   priority of a native frame being ingressed to the priority of any
   Pull Directory query it causes.  The default mapping depends on the
   client strategy, as described in Section 4.

             Name         Default            Section   Note Below
      ------------------  ----------------   -------   ----------

      DirQueryTimeout     100 milliseconds   3.2.1          1
      DirQueryRetries       3                3.2.1          1
      DirGenQPriority       5                3.2.1          2

      DirRespMaxPriority    6                3.2.2.1        3

      DirUpdateDelay       50 milliseconds   3.3
      DirUpdatePriority     5                3.3.1
      DirUpdateTimeout    100 milliseconds   3.3.3
      DirUpdateRetries      3                3.3.3

      DirAckMaxPriority     5                3.3.2          4

      Note 1: Pull Directory Query client timeout waiting for response
         and maximum number of retries.

      Note 2: Priority for client-generated requests (such as a query to
         refresh cached information).

      Note 3: Pull Directory Response Messages SHOULD NOT be sent with
         priority 7, as that priority SHOULD be reserved for messages
         critical to network connectivity.

      Note 4: Pull Directory Acknowledge Messages SHOULD NOT be sent
         with priority 7, as that priority SHOULD be reserved for
         messages critical to network connectivity.



(page 42 continued on part 3)

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