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Versions: 00 01 02 03 04 05 06 07 08 09                                 
Network Working Group                                   P. Ashwood-Smith
Internet Draft                                              A. Paraschiv
Expiration Date: April 2001                              Nortel Networks

                                                            October 2000


                   MPLS LDP Query Message Description


                    draft-ietf-mpls-lsp-query-00.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in an Internet-Drafts Shadow
   Directory, see http://www.ietf.org/shadow.html.

Abstract

   This document describes the encoding and procedures for three new LDP
   messages, the Query Message and Query-Reply Message and Partial
   Query-Reply Message (the last one is almost identical to the Query-
   Reply message; therefore all references to the Query-Reply messages
   imply the Partial Query-Reply messages as well, unless otherwise
   specified).  An LER sends a Query message when it needs to find out
   information about an LSP. The Query message is sent for an
   established LSP.  The Query message can be used for LDP LSPs as well
   as for CR-LSPs.  The queried data is encoded into the Query-Reply
   messages. The Query Message carries only the list of hops.










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Contents

    1    Introduction  .............................................   3
    2    Overview  .................................................   3
    2.1  LDP Overview  .............................................   3
    2.2  CR-LDP Overview  ..........................................   4
    3    LDP Message Structure Overview  ...........................   4
    4    Behavior of LSRs with constraints in handling the query messages  5
    4.1  LSR does not support the query messages  ..................   6
    4.2  LSR cannot share any information  .........................   6
    4.3  LSR cannot share some of the queried information   ........   6
    4.4  LSR can share the queried information   ...................   7
    5    Query Message  ............................................   7
    5.1  Query Message encoding     ................................   8
    5.2  Query Message Procedures  .................................   9
    6    Reply Messages   ..........................................  10
    6.1  Query-Reply Message encoding     ..........................  10
    6.2  Query-Reply Message Procedures  ...........................  12
    6.3  Partial Query-Reply Message encoding     ..................  13
    6.4  Partial Query-Reply Message Procedures  ...................  13
    7    Query TLVs  ...............................................  14
    7.1  Query Label TLV  ..........................................  15
    7.2  Query Merge Flags TLV  ....................................  15
    7.3  Label TLV  ................................................  16
    8    Acknowledgments  ..........................................  17
    9    References  ...............................................  17
   10    Author's Addresses  .......................................  18






















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   Changes from previous version:

   o  First revision.



1. Introduction

   The original Multiprotocol Label Switching (MPLS) architecture
   [MPLS-ARCH] was been defined to support the forwarding of data based
   on a label. The MPLS architecture does not assume a single label
   distribution protocol. A number of different label distribution
   protocols are being standardized. This draft describes the query
   mechanism for an LSP or CR-LSP. It specifies procedures and encodings
   for the new messages added for the query mechanism.  Extensions to
   the RSVP-TE to provide the same functionality are subject for future
   study and will be covered in future draft versions.

   The new LDP messages are: Query Message, Query-Reply Message and
   Partial Query-Reply Message.  The following new TLVs are added to
   accommodate the encodings for the new query messages:
      - Query TLV
      - Query Label TLV
      - Query Merge Flags TLV
      - Label TLV.

   LDP uses the TCP transport for session, advertisement and
   notification messages; i.e., for everything but the UDP-based
   discovery mechanism. The messages which are added to support the
   query mechanism are sent over TCP as well.



2. Overview

2.1. LDP Overview

   Label Distribution Protocol (LDP) defined in [LDP Specification]
   contains a set of procedures and messages by which Label Switched
   Routers (LSR) establish Label Switch Paths (LSP) through a network by
   mapping network layer routing information directly to data-link layer
   switched paths. LDP associates a Forwarding Equivalence Class (FEC)
   with each LSP it creates. The FEC associated with an LSP specifies
   which packets are mapped to that LSP.







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2.2. CR-LDP Overview

   As described in [Constraint-Base LSP Setup using LDP], Constraint
   Base Routing (CR-LDP) offers the opportunity to extend the
   information used to setup paths beyond what is available for the
   routing protocol.



3. LDP Message Structure Overview

   As described in LDP Specification draft, all LDP  messages  have  the
   following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|   Message Type              |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                     Mandatory Parameters                      |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                     Optional Parameters                       |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   U bit
      Unknown message bit.  Upon receipt of an unknown message, if U is
      clear (=0), a notification is returned to the message originator;
      if U is set (=1), the unknown message is silently ignored.  The
      sections following that define messages specify a value for the U-
      bit.

   Message Type
      Identifies the type of message

   Message Length
      Specifies the cumulative length in octets of the Message ID,
      Mandatory Parameters, and Optional Parameters.



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   Message ID
      32-bit value used to identify this message.  Used by the sending
      LSR to facilitate identifying notification messages that may apply
      to this message.  An LSR sending a notification message in
      response to this message should include this Message Id in the
      Status TLV carried by the notification message; see Section
      "Notification Message".

   Mandatory Parameters
      Variable length set of required message parameters.  Some messages
      have no required parameters.

   For messages that have required parameters, the required parameters
   MUST appear in the order specified by the individual message
   specifications in the sections that follow.

   Optional Parameters
      Variable length set of optional message parameters.  Many messages
      have no optional parameters.

   For messages that have optional parameters, the optional parameters
   may appear in any order.


4. Behavior of LSRs with constraints in handling the query messages

   Upon receiving a Query message, an LSR has to behave according to its
   configuration constraints in handling the query messages and
   returning the queried information. The following cases were
   identified:
           - the LSR does not support the code to handle the messages
             for the query mechanism
           - the LSR supports the code to handle the messages for the
             query mechanism, but it is configured not to return any data
           - the LSR supports the code to handle the messages for the
             query mechanism, but it is configured not to return part of
             the queried data
           - the LSR supports the code to handle the messages for the
             query mechanism, and it is configured to return all the data
             which is queried.

   The draft provides flexibility to handle each of the above cases.









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4.1. LSR does not support the query messages

   In this case, the LSR has to behave as if it received an unknown
   message type. It has therefore to reply with a Notification message.
   The notification message is sent upstream and the status is "Unknown
   Message Type".


4.2. LSR cannot share any information

   In this case, the LSR is able to decode and process the query
   messages. However, it is configured to hide all the data. It should
   propagate the message after it encodes a zero-length TLV for its hop
   in the hop list in the Query message. When Query-Reply message is
   received from downstream, the LSR is requested to propagate the reply
   message upstream after it encodes the zero-length tlvs for the
   queried data. When the ingress receives back the reply, it can
   identify which TLVs are empty; it can therefore ignore the zero-
   length TLVs and process the rest of the data.

   Note: zero-length TLV encoding can be used for all types of queried
   information except the merge information.  The LSR is requested to
   signal the fact that the merging information is private by encoding a
   special value in the corresponding merge bits (for more information
   on the merge flags values please refer to Query Merge Flags TLV
   section).


4.3. LSR cannot share some of the queried information

   In this case, the LSR is able to decode and process the query
   messages. It has to propagate the query messages. It has to encode
   values for the data types that it is willing to return and zero-
   length TLVs for values for the data that is hidden.

   Note: zero-length TLV encoding can be used for all types of queried
   information except the merge information.  The LSR is requested to
   signal the fact that the merging information is private by encoding a
   special value in the corresponding merge bits (for more information
   on the merge flags values please refer to Query Merge Flags TLV
   section).










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4.4. LSR can share the queried information

   This is the normal case for an LSR. In this case, the LSR's behavior
   has to follow the query and replies procedures described in the
   following sections of the draft.

   In order to have consistency among data encoded in the query and
   reply messages, each LSR which can propagate the messages has to
   encode its information in the query and in the reply messages.

   The decision that an LSR can share the queried information has to be
   controlled through configuration flags. This way each node along the
   path can protect its data if they consider it private.

   Note: It would be more efficient to control/restrict the private data
   per MPLS cloud (inter MPLS domain) and not per LSR node (inter and
   intra MPLS domain).  When there are different MPLS clouds which have
   nodes belonging to different vendors, the control of the private
   information could be restricted to the boundary nodes. Within an MPLS
   domain, there should be no restrictions on the queried information.
   It would be useful to have some knowledge on which are the nodes on
   the boundaries and have only those hiding the queried data. Because
   there is no mechanism to identify which are the boundary nodes, this
   is subject for future study.



5. Query Message

   This sections describes the Query message and its encodings and
   procedures.  This message is meant to be used to gather information
   about an LSP.  It can be sent at any time for an established LSP.
   The draft currently describes the procedures for the cases when the
   Query Message is initiated by the ingress LER.  Future versions of
   the draft may add the procedures for the query message when issued
   from a core LSR or from egress.

   The Query Message can be used to gather information about:
           - LSRs which form the LSP
           - labels along the LSP
           - information on what LSRs are merging points along the path
           - unused bandwidth (as described in "Improving Topology Data Base Accuracy with LSP Feedback")
           - congestion status
           - round trip delay
           - anything that is needed in the future and can be computed and
             encoded in a TLV.





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   The queried information is going to be encoded in the Query-Reply
   message which is sent back upstream, as a response to the Query
   message.


5.1. Query Message encoding

      The encoding for the Query message is:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|          Query (0x0409)     |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label TLV                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Query TLV                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Hop Count TLV                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message ID
      32-bit value used to identify this message.

   Label TLV
      The label associated to the LSP which is queried. This TLV is a
      list of Generalized Label TLVs [OPTICAL reference].  The
      Generalized Label TLV provides a more generic encoding for
      different types of labels.  Most of the time the list has one
      element; this is the case when the LSP is not tunneled.  For
      tunneled LSPs, the Label TLV has more that one element; it has to
      behave like a label stack (it contains the previous label and the
      tunnel's label). See Section Label TLV for more information on
      Label TLV encoding.

   Query TLV
      What to query. See Section Query TLV for encoding.

   Hop Count TLV
      Specifies the number of LSR hops that can still be traversed
      before the message is dropped due to loop detection. It is
      initialized to the max value of 255 (or the configured value, if
      any). Every LSR that receives the Query Message has to subtract 1



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      from the Hop Count value.  The Query message should be dropped if
      the hop count value becomes zero; a Notification signaling Loop
      Detection should be sent in reply to the sender of the message.
      See [LDP Specification] for Hop Count TLV encoding.

   Optional Parameters
      This variable length field contains 0 or more parameters, each
      encoded as a TLV.

               Optional Parameter     Length       Value

               ER      TLV            var          See CR-LDP

   The ER TLV is a list of hops. It is used when the Query flag Q3 is
   set. Every LSR should add its IP address. The address to be added
   should be the outgoing interface address. Addresses  are organized as
   a last-in-fist-out stack (the first address in TLV is considered the
   top). By carrying this TLV in the Query Message and preserving this
   order for the hops, we allow the possibility to interwork the Query
   Message with the RSVP Path message.



5.2. Query Message Procedures

   The LER ingress initiates the Query message. It populates the Query
   TLV Parameters according to what kind of information it wants to
   gather. The query for an LSP is done by its label. The only data that
   the Query Message carries is the list of hops. This way, each node
   along the path will have a complete route from source to destination.
   The hop list information is useful for network management.

   Upon receiving a Query Message, an LSR decodes the label to identify
   which LSP is queried. If it cannot find the LSP which is using the
   label, it sends back a Notification message. Otherwise, it checks
   which is the out label which is bound to the queried in-label and
   which is the downstream LSR peer. It replaces the in-label from Label
   TLV with the out-label used by the LSP. It then passes the Query
   message to the downstream peer. When the Query message gets to a
   tunnel, it has to be able to handle both the previous label and the
   tunnel's label. The Label TLV behaves like a label stack. The
   previous label is pushed and the tunnel label is used. At the end of
   the tunnel, we need to pop the stack and start substituting the lower
   level labels.

   Upon receiving the Query message, the egress node has to reply with a
   Query Reply Message. The Query-Reply Message contains the Query TLV
   which was received in the Query Message. The Query TLV tells the LSRs



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   along the path which information is being queried and allows
   intermediate LSRs to piggy back their own queried information on the
   Query reply message.



6. Reply Messages

   These messages are propagated upstream.  There are situations in
   which the Query message does not reach the end point of the queried
   LSP (e.g. when an LSR along the path is congested, or when it does
   not support the Query messages). In these scenarios it would be
   useful if the ingress LSR gathered at least some information about
   the LSRs which are along the path, up to the one that failed. The
   Partial Query-Reply message provides this mechanism. It is
   recommended to use the Partial Query-Reply messages when a Query
   message fails.  There are two types of reply messages:

           - Query-Reply message (final reply)
           - Partial Query-Reply message.

   Both of the reply messages are described in the following sections.



6.1. Query-Reply Message encoding

   This message is generated by the end point of the LSP. It is
   propagated upstream, by each LSR along the path.

   The encoding for the Query-Reply message is:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|    Query-Reply (0x0410)     |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Query TLV                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     MessageId TLV                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Hop Count TLV                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Message ID
      32-bit value used to identify this message.

   Query TLV
      What is to be queried. See Section Query TLV for encoding.

   Message Id TLV
      The value of this parameter is the message id of the corresponding
      Query message.

   Hop Count TLV
      Specifies the number of LSR hops that can still be traversed
      before the message is dropped due to loop detection. It is
      initialized to the max value of 255 (or the configured value, if
      any). Every LSR that receives the Query Message has to subtract 1
      from the Hop Count value.  The Query message should be dropped if
      the hop count value becomes zero. See [LDP Specification] for Hop
      Count TLV encoding.

   Optional Parameters
      This variable length field contains 0 or more parameters, each
      encoded as a TLV. The optional parameters are:

               Optional Parameter     Length       Value

               ER      TLV            var          See CR-LDP
               Query Label TLV        var          See Query Label TLV section
               IPV4 specified link feedback TLV    See Improving Topology ...
               Query Merge Flags TLV   var         See Query Merge Flags TLV
                                                   section

   For simplicity we reuse here TLV types defined for CR-LDP and LDP.
   They are:

           - IPV4 specified link feedback TLV
           - ER TLV
           - Generalized Label TLV (used in the Query Label TLV encoding)
           - Hop Count TLV.

   The IPV4 specified link feedback TLV is used when the Q1 flag from
   the Query TLV is set. It is used by the egress LER to encode the
   bandwidth.  For more information on query flags, Q1, ... Q6, refer to
   Query TLV section.

   The ER TLV is a list of hops. It is used when the Query flag Q3 is
   set. Every LSR should add its IP address. The address to be added
   should be the outgoing interface address. Addresses  are organized as
   a last-in-fist-out stack (the first address in TLV is considered the



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   top).

   The Query Label TLV is a list of labels. It is used when the Query
   flag Q2 is set. It is populated with the labels used for the path
   which is queried. For tunneled LSPs, the Query Label TLV represents a
   list of labels associated to the lowest level tunnel.

   If Q3 and Q2 flags are set, the labels should be encoded in the same
   order as the hops.

   Query Merge Flags TLV is a bit mask. It has variable length and every
   two bits in the mask will correspond to an LSR along the path. Its
   length is rounded up to the next byte. If Q6 is set, every LSP along
   the path will have to set its corresponding bits in the mask. The
   bits have to be set in the same order as the labels and hops.
   Usually, Q6 is set when Q2 set and/or Q3 set.

   For more information for the TLV encodings of the TLVs which are
   reused, please see CR-LDP draft, LDP draft and IMPROVING TOPOLOGY
   DATA BASE ACCURACY WITH LSP FEEDBACK VIA CR_LDP draft.



6.2. Query-Reply Message Procedures

   A Query-Reply message is initiated by an egress node which receives a
   Query message, if the egress is able to identify the queried LSP.  If
   not, the egress replies with a Notification message.

   Upon receiving the Query message, the egress node has to reply with a
   Query Reply message. The egress node has to encode into the Query-
   Reply message a MessageId Tlv. The mapping between a Query and a
   Query-Reply Message is done based on the message id. Besides the
   MessageId Tlv, the egress has to encode the information that was
   queried (bandwidth, path, etc).

   After the encoding is done, the query reply message is sent back, on
   the reversed path, toward the ingress. Every LSR across the LSP has
   to encode its information according to what query flags are set.












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6.3. Partial Query-Reply Message encoding

   The Partial Query-Reply message is initiated by tandem LSRs along the
   queried path. The message is generated only if the following rules
   apply:
           - if the Query message asked for partial
             replies (the Query message signals this request
             through Q8 bit)
           - if the tandem LSR is configured to provide partial replies.

   The encoding for the Partial Query-Reply message is identical to  the
   Query-Reply,  except  the  message  type.   For  more  details on the
   encoding please refer to the Query-Reply encoding.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|    Query-Reply (0x0411)     |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Query TLV                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     MessageId TLV                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Hop Count TLV                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



6.4. Partial Query-Reply Message Procedures

   The procedures are similar to the Query-Reply's procedures.  Upon
   receiving a Query message, a tandem LSR will check the flag from the
   Query message (Q8) which signals if the partial replies are requested
   by the ingress node. If the flag is set, the LSR has to check next if
   it is configured to fulfill this request. If the LSR supports partial
   replies, it has to create a Partial Query-Reply and encode the
   queried data and send it upstream like any Query-Reply messages. It
   has then to process the Query message according to the Query message
   procedures. When an LSR receives a Partial Query-Reply from upstream,
   it should encode its information according to what is queried and
   propagate the message. It is recommended to use the Partial Query
   mechanism when the Query message fails (when the ingress LER does not
   receive a Query-Reply message in response to a Query message).




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7. Query TLVs

   The Query TLV is used to specify the information being queried.  The
   Query TLV travels in the Query message to the egress node, where it
   is copied into a reverse flowing Query-Reply messages and used by the
   egress and intermediate LSRs to know what information is being
   queried.

   The format for the Query TLV is:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0|  Query TLV  (0x0840)      |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Query  Flags  |            Reserved                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Query Flags can be set according to what the Query is used for.

      +--+--+--+--+--+--+--+--+
      |Q8|Re|Q6|Q5|Q4|Q3|Q2|Q1|
      +--+--+--+--+--+--+--+--+

   They can be:
           - Q1 : query the bandwidth; if set, the LSR that
                  receives the Query message has to encode the bandwidth
                  that is available on the link (unused bandwidth);
           - Q2 : query the labels which are associated to each hop in the
                  path;
           - Q3 : query the LSRs which form the LSP which is queried;
                  if set, the LSR that received the Query-Reply message
                  has to encode the current hop in the ER-TLV
           - Q4 : reserved for congestion status; < format - TBD >
           - Q5 : query the round trip delay; if set, the LSR should fill in
                  the link delay information if available; < format - TBD >
           - Q6 : query which LSPs along the path are merging points; if set,
                  the LSR that receives the Query message has to encode
                  if it is a merging point; the encoding is done in the
                  Query Merge flags TLV.

           - Q8 : if set, the ingress requests partial query-replies;









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7.1. Query Label TLV

   The Query Label TLV is used to encode the labels used along the path
   which is queried.

   The format for the Query Label TLV is:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Query  Label TLV  (0x0841)|      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Generalized Label TLV 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Generalized Label TLV n                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Generalized Label TLV is used to encode labels along the path. Please
    refer to [OPTICAL reference] for more information on the Generalized
   Label TLV encoding. If the Q2 flag is set, every LSR has to encode
   the out-label corresponding to the queried LSP.  In the Query Label
   TLV, labels are organized as a last-in-fist-out stack (the first
   label in TLV is considered the top). They should be encoded in the
   same order as the hops and the merge flags.

   Note: The Query Label TLV could use the Label TLV which is defined in
   LDP draft. The same note applies to the Label TLV that encodes the
   label which is queried.


7.2. Query Merge Flags TLV

   The Query Merge Flags TLV is used to encode the information about
   which LSRs along the path the queried LSP is being merged into.















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   The format for the Query Label TLV is:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Merge Flags TLV  (0x0842) |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Number of merge flags                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Merge flags   |               |               |               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Query Merge Flags TLV has 4 bytes field to store the number of
   merge flags. This number is equal to the number of LSRs which are
   traversed by the Query-Reply Message.  Each 2 bits in the Merge flags
   field represents the merge info for an LSR. The bit is set to 1 if
   the LSR does not do merge for the queried LSP and is set to 2
   otherwise. If the LSR want to hide the merging information, it has to
   set the merging flags to 0.  The length is going to be rounded up to
   the next byte.  Every LSR which is asked to encode the merging info
   into this TLV has to update the Number of merge flags and to set its
   corresponding bits accordingly.



7.3. Label TLV

   The Label TLV is used to encode the label stack of the labels which
   are queried (along the queried LSP).

   The format for the Query Label TLV is:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0|        Label TLV  (0x0843)|      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Generalized Label TLV 1                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Generalized Label TLV n                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The Label TLV is a stack of Generalized Labels. The top of the stack
   is in the right octets of the TLV encoding.  The Label TLV has to be
   updated every time the Query message is processed by an LSR. The
   encoding of the labels should follow the last-in-first-out stack
   model.

   Generalized Label TLV is used to encode labels. Please refer to
   [OPTICAL reference] for more information on the Generalized Label TLV
   encoding.

   Note: The Label TLV could use the Label TLV which is defined in LDP
   draft.



8. Acknowledgments

   The authors would like to acknowledge the careful review and comments
   of Jean-Pierre Coupal, Steve Hamilton, Don Fedyk and Gregory Wright.



9. References

[CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
draft-ietf-mpls-cr-ldp-03.txt, October 1999

[LDP] Andersson et al., "LDP Specification", draft-ietf-mpls-ldp-07.txt,
May 2000.

[RSVP-RR]  Berger L., Gan D., Swallow G., Pan P., Tommasi F., Molendini
S., "RSVP Refresh Overhead Reduction Extensions", draft-ietf-rsvp-
refresh-reduct-04.txt.

[CR-LDP] Peter Ashwood-Smith et al., "Improving Topology Data Base
Accuracy with LSP Feedback", draft-ietf-mpls-te-feed-01.txt

Ashwood-Smith P., Berger L., "Generalized MPLS-Signaling Functional
Description" draft-ashwood-generalized-mpls-signaling-00.txt












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10. Author's Addresses

   Peter Ashwood-Smith               Antonela Paraschiv
   Nortel Networks Corp.             Nortel Networks Corp.
   P.O. Box 3511 Station C,          600 Technology Park Drive
   Ottawa, ON K1Y 4H7                Billerica, MA 01821
   Canada                            USA
   Phone: +1 613-763-4534            phone: +1 978-288-6136
   petera@nortelnetworks.com         antonela@nortelnetworks.com


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