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Versions: 00 01 02 03 04 05 06 07 08 rfc2745                            
INTERNET-DRAFT                                               Lixia Zhang
Expires: May 1999                                         Andreas Terzis
<draft-ietf-rsvp-diagnostic-msgs-05.txt>                            UCLA
                                                     Subramaniam Vincent
                                                                   Cisco
                                                              Bob Braden
                                                                     ISI
                                                           November 1998
                         RSVP Diagnostic Messages

                 <draft-ietf-rsvp-diagnostic-msgs-05.txt>


Status of this Memo

   This document is an Internet-Draft.  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
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   To view the entire list of current Internet-Drafts, please check the
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   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.




Abstract

   This document specifies the RSVP diagnostic facility, which allows a
   user to collect information about the RSVP state along the path.
   This specification describes the functionality, diagnostic message
   formats, and processing rules.









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1.  Introduction

   In the basic RSVP protocol [RSVP], error messages are the only means
   for an end host to receive feedback regarding a failure in setting up
   either path state or reservation state.  An error message carries
   back only the information from the failed point, without any
   information about the state at other hops before or after the
   failure.  In the absence of failures, a host receives no feedback
   regarding the details of a reservation that has been put in place,
   such as whether, or where, or how, its own reservation request is
   being merged with that of others.  Such missing information can be
   highly desirable for debugging purpose, or for network resource
   management in general.

   This document specifies the RSVP diagnostic facility, which is
   designed to fill this information gap.  The diagnostic facility can
   be used to collect and report RSVP state information along the path
   from a receiver to a specific sender.  It uses Diagnostic messages
   that are independent of other RSVP control messages and produce no
   side-effects; that is, they do not change any RSVP state at either
   nodes or hosts.  Similarly, they provide not an error report but
   rather a collection of requested RSVP state information.

   The RSVP diagnostic facility was designed with the following goals:
     - To collect RSVP state information from every RSVP-capable hop
        along a path defined by path state, either for an existing
        reservation or before a reservation request is made.  More
        specifically, we want to be able to collect information about
        flowspecs, refresh timer values, and reservation merging at each
        hop along the path.

     - To collect the IP hop count across each non-RSVP cloud.

     - To avoid diagnostic packet implosion or explosion.

   The following is specifically identified as a non-goal:

     - Checking the resource availability along a path.  Such
        functionality may be useful for future reservation requests, but
        it would require modifications to existing admission control
        modules that is beyond the scope of RSVP.


2.  Overview

   The diagnostic facility introduces two new RSVP message types:
   Diagnostic Request (DREQ) and Diagnostic Reply (DREP).  A DREQ
   message can be originated by a client in a "requester" host, which



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   may or may not be a participant of the RSVP session to be diagnosed.
   A client in the requester host invokes the RSVP diagnostic facility
   by generating a DREQ packet and sending it to the starting node,
   which should be on the RSVP path to be diagnosed.  This DREQ packet
   specifies the RSVP session and a sender host for that session.  The
   DREQ packet collects information hop-by-hop as it is forwarded
   towards the sender (see Figure 1), until it reaches the ending node.
   Specifically, each RSVP-capable hop adds to the DREQ message a
   response (DIAG_RESPONSE) object containing local RSVP state for the
   specified RSVP session.  When the DREQ packet reaches the ending
   node, the message type is changed to Diagnostic Reply (DREP) and the
   completed response is sent to the original requester node.  Partial
   responses may also be returned before the DREQ packet reaches the
   ending node if an error condition along the path, such as "no path
   state", prevents further forwarding of the DREQ packet.  To avoid
   packet implosion or explosion, all diagnostic packets are forwarded
   via unicast only.

   Thus, there are generally three nodes (hosts and/or routers) involved
   in performing the diagnostic function: the requester node, the
   starting node, and the ending node, as shown in Figure 1.  It is
   possible that the client invoking the diagnosis function may reside
   directly on the starting node, in which case that the first two nodes
   are the same.  The starting node is named "LAST-HOP", meaning the
   last-hop of the path segment to be diagnosed.  The LAST-HOP node can
   be either a receiver node or an intermediate node along the path.
   The ending node is usually the specified sender host.  However, the
   client can limit the length of the path segment to be diagnosed by
   specifying a hop-count limit in the DREQ message.


                       LAST-HOP                  Ending
          Receiver        node                     node           Sender
              __           __         __            __              __
             |  |---------|  |------>|  |--> ...-->|  |--> ...---->|  |
             |__|         |__| DREQ  |__|   DREQ   |__|   DREQ     |__|
                           ^                         .              |
                           |                         .              |
                           | DREQ                    . DREP         | DREP
                           |                         .              |
                          _|_               DREP     V              V
             Requester   |   | <------------------------------------
             (client)    |___|

                              Figure 1






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   DREP packets can be unicast from the ending node back to the
   requester either directly or hop-by-hop along the reverse of the path
   taken by the DREQ message to the LAST-HOP, and thence to the
   requester.  The direct return is faster and more efficient, but the
   hop-by-hop reverse-path route may be the only choice if the packets
   have to cross firewalls.  Hop-by-hop return is accomplished using an
   optional ROUTE object, which is built incrementally to contain a list
   of node addresses that the DREQ packet has passed through.  The ROUTE
   object is then used in reverse as a source route to forward the DREP
   hop-by-hop back to the LAST-HOP node.

   A DREQ message always consists of a single unfragmented IP datagram.
   On the other hand, one DREQ message can generate multiple DREP
   packets, each containing a fragment of the total DREQ message.  When
   the path consists of many hops, the total length of a DREP message
   will exceed the MTU size before reaching the sender; thus, the
   message has to be fragmented.  Relying on IP fragmentation and
   reassembly, however, can be problematic, especially when DREP
   messages are returned to the requester hop-by-hop, in which case
   fragmentation/reassembly would have to be performed at every hop.  To
   avoid such excessive overhead, we let the requester define a default
   path MTU size that is carried in every DREQ packet.  If an
   intermediate node finds that the default MTU size is bigger than the
   MTU of the outgoing  link, it returns the DREQ packet with the
   corresponding error bit set.  If an intermediate node detects that a
   DREQ packet size has reached the MTU size, it returns to the
   requester (in either manner described above) a DREP fragment
   containing accumulated responses.  It then removes these responses
   from the DREQ and continues to forward it.  The requester node can
   reassemble the resulting DREP fragments into a complete DREP message.

   When discussing diagnostic packet handling, this document uses
   direction terminology that is consistent with the RSVP functional
   specification [RSVP], relative to the direction of data packet flow.
   Thus, a DREQ packet enters a node through an "outgoing interface" and
   is forwarded towards the sender through an "incoming interface",
   because DREQ packets travel in the reverse direction to the data
   flow.

   Notice that DREQ packets can be forwarded only after the RSVP path
   state has been set up.  If no path state exists, one may resort to
   the traceroute or mtrace facility to examine whether the
   unicast/multicast routing is working correctly.








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3.  Diagnostic Packet Format

   Like other RSVP messages, DREQ and DREP messages consist of an RSVP
   Common Header followed by a variable set of typed RSVP data objects.
   The following sequence must be used:


              +-----------------------------------+
              |        RSVP Common Header         |
              +-----------------------------------+
              |         Session object            |
              +-----------------------------------+
              |       DIAGNOSTIC object           |
              +-----------------------------------+
              |    (optional) DIAG_SELECT object  |
              +-----------------------------------+
              |    (optional) ROUTE object        |
              +-----------------------------------+
              | zero or more DIAG_RESPONSE objects|
              +-----------------------------------+



   The session object identifies the RSVP session for which the state
   information is being collected.  We describe each of the other parts.


3.1.  RSVP Message Common Header

   The RSVP message common header is defined in [RSVP].  The following
   specific exceptions and extensions are needed for DREP and DREQ.

   Type field: define:

         Type = 8: DREQ     Diagnostic Request

         Type = 9: DREP     Diagnostic Reply

   RSVP length:

      If this is a DREP message and the MF flag in the DIAGNOSE object
      (see below) is set, this field indicates the length of this single
      DREP fragment rather than the total length of the complete DREP
      reply message (which cannot generally be known in advance).







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3.2.  DIAGNOSTIC Object

   A DIAGNOSTIC object contains the common diagnostic control
   information in both DREQ and DREP messages.

   o IPv4 DIAGNOSTIC object: Class = 30, C-Type = 1


    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Max-RSVP-hops | RSVP-hop-count|         Reserved          |H|MF|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Request ID                           |
    +---------------+---------------+---------------+---------------+
    |           Path MTU            |     Fragment offset           |
    +---------------+---------------+---------------+---------------+
    |                                                               |
    |                     SENDER_TEMPLATE object                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         LAST-HOP Address                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                  Requester FILTER_SPEC object                 |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                       Next-Hop RSVP_HOP Object                |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   Here all IP addresses use the 4 byte IPv4 format, both explicitly in
   the LAST-HOP Address and by using the IPv4 forms of the embedded
   FILTER_SPEC and RSVP_HOP objects.

   o IPv6 DIAGNOSTIC object: Class = 30, C-Type = 2

   The format is the same, except all explicit and embedded IP addresses
   are 16 byte IPv6 addresses.

   The fields are as follows:

   Max-RSVP-hops

      An octet specifying the maximum number of RSVP hops over which
      information will be collected.  If an error condition in the
      middle of the path prevents the DREQ packet from reaching the



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INTERNET-DRAFT                                                  May 1999


      specified ending node, the Max-RSVP-hops field may be used to
      perform an expanding-length search to reach the point just before
      the problem.  If this value is 1, the LAST-HOP node and the ending
      node will be the same.  If it is zero, there is no hop limit.

   RSVP-hop-count

      Records the number of RSVP hops that have been traversed so far.
      If LAST-HOP and ending nodes are the same, this value will be 1 in
      the resulting DREP message.

   Fragment Offset

      Indicates where this DREP fragment belongs in the complete DREP
      message, measured in octets.  The first fragment has offset zero.
      Ignored in a DREQ message.

   H flag

      Indicates how the reply should be returned.  When H = 0, DREP
      packets should be sent to the response address directly.  If H =
      1, DREP packets must be returned hop-by-hop along the reverse path
      to the LAST-HOP node and thence to the requester node.

   MF flag

      Flag means "more fragments".  It must be set to zero (0) in all
      DREQ messages.  It must be set to one (1) in all DREP packets that
      carry partial results and are returned by intermediate nodes due
      to the MTU limit.  When the DREQ message is converted to a DREP
      message in the ending node, the MF flag must remain zero.

   Request ID

      Identifies an individual DREQ message and the corresponding DREP
      message (or all the fragments of the reply message).

      One possible way to defining the Request ID would use 16 bits to
      specify the ID of the process making the query and 16 bits to
      distinguish different queries from this process.

   Path MTU

      Specifies a default MTU size in octets for DREP and DREQ messages.

   SENDER_TEMPLATE object

      This IPv4/IPv6 SENDER_TEMPLATE object contains the IP address and



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INTERNET-DRAFT                                                  May 1999


      the port of a sender for the session being diagnosed.  The DREQ
      packet is forwarded hop-by-hop towards this address.

   LAST-HOP Address

      The IP address of the LAST-HOP node.  The DREQ message starts
      collecting information at this node and proceeds toward the
      sender.

   Requester FILTER_SPEC Object

      This IPv4/IPv6 FILTER_SPEC object contains the IP address and the
      port from which the request originated and to which the DREP
      message(s) should be sent.

   Next-Hop RSVP_HOP Object

      An RSVP_HOP object carrying the IP address and LIH of the
      interface through which the DREQ will be received.  This object is
      updated hop-by hop.  It is used for the same reasons that a RESV
      message contains an RSVP_HOP object:  to distinguish logical
      interfaces and avoid problems caused by routing asymmetries.


3.3.  DIAG_SELECT Object

   o DIAG_SELECT Class = 33, C-Type = 0.

   A Diagnostic message may optionally contain a DIAG_SELECT object to
   specify which specific RSVP objects should be reported in a
   DIAG_RESPONSE object.  In the absence of a DIAG_SELECT object, the
   DIAG_RESPONSE object added by the node will contain a default set of
   object types (see DIAG_RESPONSE object below).

   The DIAG_SELECT object contains a list of [Class, C-type] pairs, in
   the following format:


    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    class      |     C-Type    |    class      |     C-Type    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    class      |     C-Type    |    class      |     C-Type    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   When a DIAG_SELECT object is included in a DREQ message, each RSVP



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   node along the path will add a DIAG_RESPONSE object containing
   response objects (see below) whose classes and C-Types match entries
   in the DIAG_SELECT list (and are from matching path and reservation
   state).  A C-type octet of zero is a 'wildcard', matching any C-Type
   associated with the associated class.

   If the number of [Class, C-Type] pairs is odd, the last two octets of
   the DIAG_SELECT object must be  zero.


3.4.  ROUTE Object

   A diagnostic message may contain a ROUTE object, which is used to
   record the route of the DREQ message and as a source route for
   returning the DREP message(s) hop-by-hop.

   o IPv4 ROUTE object: Class = 31, C-Type = 1.


    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             reserved                          |    R-pointer  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                     RSVP Node List                            |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   RSVP Node List

      A list of RSVP node IPv4 addresses.  The number of addresses in
      this list can be computed from the object size.

   R-pointer

      Used in DREP messages only (see Section 4.2 for details), but it
      is incremented as each hop adds its incoming interface address in
      the ROUTE object.


   o IPv6 ROUTE object: Class = 31, C-Type = 2

   The same, except RSVP Node List contains IPv6 addresses.

   In a DREQ message, RSVP Node List specifies all RSVP hops between the
   LAST-HOP address specified in the DIAGNOSTIC object, and the last
   RSVP node the DREQ message has visited.  In a DREP message, RSVP Node



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INTERNET-DRAFT                                                  May 1999


   List specifies all RSVP hops between the LAST-HOP and the node that
   returns this DREP message.


3.5.  DIAG_RESPONSE Object

   Each RSVP node attaches DIAG_RESPONSE object to each DREQ message it
   receives, before forwarding the message.  The DIAG_RESPONSE object
   contains the state to be reported for this node.  It has a fixed-
   format header and then a variable list of RSVP state objects, or
   "response objects".

   o IPv4 DIAG_RESPONSE object: Class = 32, C-Type = 1.


    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       DREQ Arrival Time                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Incoming Interface Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Outgoing Interface Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Previous-RSVP-Hop Router Address              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   D-TTL       |M|R-err|  K    |      Timer value              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                  (optional) TUNNEL object                     |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    //                       Response objects                      //
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   o IPv6 DIAG_RESPONSE object: Class = 32, C-Type = 2.

   This object has the same format, except that all explicit and
   embedded IP addresses are IPv6 addresses.

   The fields are as follows:

   DREQ Arrival Time

      A 32-bit NTP timestamp specifying the time the DREQ message
      arrived at this node.  The 32-bit form of an NTP timestamp



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INTERNET-DRAFT                                                  May 1999


      consists of the middle 32 bits of the full 64-bit form, that is,
      the low 16 bits of the integer part and the high 16 bits of the
      fractional part.

   Incoming Interface Address

      Specifies the IP address of the interface on which messages from
      the sender are expected to arrive, or 0 if unknown.

   Outgoing Interface Address

      Specifies the IP address of the interface through which the DREQ
      message arrived and to which messages from the given sender and
      for the specified session address flow, or 0 if unknown.

   Previous-RSVP-Hop Router Address

      Specifies the node from which this node receives RSVP PATH
      messages for this source, or 0 if unknown.  This is also the
      interface through which the DREQ will be forwarded.

   D-TTL

      The number of IP hops this DREQ message traveled from the down-
      stream RSVP node to the current node.

   M flag

      A single-bit flag which indicates whether the reservation
      described by the response objects, is merged with reservations
      from other downstream interfaces when being forwarded upstream.

   R-error

      A 3-bit field that indicates error conditions at a node.
      Currently defined values are:


           0x00: no error
           0x01: no PATH state
           0x02: MTU too big
           0x04: ROUTE object too big

   K

      The refresh timer multiple (defined in RSVP spec).

   Timer value



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      The local refresh timer value in seconds.

   The set of response objects to be included at the end of the
   DIAG_RESPONSE object is determined by a DIAG_SELECT object, if one is
   present.  If no DIAG_SELECT object is present, the response objects
   belong to the default list of classes:

           SENDER_TSPEC object         FILTER_SPEC object
           FLOWSPEC object         STYLE object

   Any C-Type present in the local RSVP state will be used.  These
   response objects may be in any order but they must all be at the end
   of the DIAG_RESPONSE object.


3.6.  TUNNEL Object

   The optional TUNNEL object should be inserted when a DREQ message
   arrives at an RSVP node that acts as a tunnel exit point.

   The TUNNEL object provides mapping between the end-to-end RSVP
   session that is being diagnosed and the RSVP session over the tunnel.
   This mapping information allows the diagnosis client to conduct
   diagnosis over the involved tunnel session, by invoking a separate
   Diagnostic query for the corresponding Tunnel Session and Tunnel
   Sender.  Keep in mind, however, that multiple end-to-end sessions may
   all map to one pre-configured tunnel session that may have totally
   different parameter settings.

   The tunnel object is defined in the RSVP Tunnel Specification
   [RSVPTUN].


4.  Diagnostic Packet Forwarding Rules


4.1.  DREQ Packet Forwarding

   DREQ messages are forwarded  hop-by-hop via unicast from the LAST-HOP
   address to the Sender address, as specified in the DIAGNOSTIC object.
   At each hop, the following processing is performed before the DREQ
   message is forwarded to the next hop towards the sender:

     1. Compute the IP hop count from the previous RSVP hop.  This is
        done by subtracting the value of the TTL value in the IP header
        from Send_TTL in the RSVP common header.  Save the result in the
        D-TTL field of the DIAG_RESPONSE object.




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INTERNET-DRAFT                                                  May 1999


     2. If no PATH state exists for the specified session, set R-error =
        0x01 in the DIAG_RESPONSE object.

     3. If the path MTU value is too large, set "MTU too large" error
        bit, and change the MTU value to the MTU value of the incoming
        interface for the sender.

     4. Attach the DIAG_RESPONSE object to the end of the DREQ message,
        and append response objects that describe the reservation in
        place at the Outgoing Interface for the specified session.

        If the DREQ message contains a DIAG_SELECT object, the response
        object classes are those specified in the DIAG_SELECT;
        otherwise, they are SENDER_TSPEC, FILTER_SPEC, STYLE, and
        FLOWSPEC objects.  If no reservation state exists for the
        specified RSVP session, the DIAG_RESPONSE object will contain no
        FILTER_SPEC or FLOWSPEC or STYLE object. If neither PATH nor
        reservation state exists for the specified RSVP session, then no
        response objects will be appended to the DIAG_RESPONSE object.

     5. Increment the RSVP-hop-count field in the diagnostic message
        header by one.

        If the resulting value is equal to Max-RSVP-hops, or if the
        current hop is the sender as identified by the Sender
        FILTER_SPEC object field in the DIAGNOSTIC object, go to
        Send_DREP(), and then return.

     6. If any error bit is set, change the type field in RSVP common
        header from DREQ to DREP, recompute the checksum, and send the
        message back to the requester.  It is sent either hop-by-hop to
        the LAST-HOP address (if H = 1), or directly to the requester
        address (if H = 0).

     7. If the resulting DREQ message size exceeds the MTU limit, minus
        some margin to hold the address list object as described below,
        go to Send_DREP().

     8. If no error bit set, then if the H-bit is set, append the
        "Incoming Interface Address" to the end of the ROUTE object,
        increment R-Pointer by one, update the Next-Hop RSVP_HOP object
        to be the Previous Hop from the Path State and update the
        message length field in the RSVP common header accordingly.
        Finally, forward the DREQ message to the next hop towards the
        source, after recomputing the checksum.


   Send_DREP():



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INTERNET-DRAFT                                                  May 1999


     1. If the H flag is off in the DIAGNOSTIC object, set
        target=response address given in the DREQ header, else set
        target = the last address in ROUTE.

     2. Make a copy of the DREQ message and change the type field in
        RSVP common header from DREQ to DREP.  If this host is not the
        source, set the MF flag on.

        If the ROUTE object is so large such that (size of ROUTE +  size
        of DIAG_RESPONSE object) > path MTU, then set the "route too
        big" error bit, recompute the checksum, send the response
        message and go to 4, else recompute the checksum and send the
        response message.

     3. If this host is not the source, then trim off all the
        DIAG_RESPONSE objects from the original DREQ message, adjust the
        "Fragment offset" value in the RSVP common header accordingly,
        recompute the checksum, and forward the modified DREQ message
        towards the source.

     4. Return.


4.2.  DREP Forwarding

   When the H flag is off, DREP messages are sent directly to the
   original requester.  When H flag is on, however, they are forwarded
   hop-by-hop towards the requester, by reversing the route as listed in
   the Route object.

   When a node receives a DREP message, it simply decreases R-pointer by
   one (address length), and forwards the message to the address pointed
   by R-pointer in the route list.

   When the LAST-HOP node receives a DREP message, it sends the message
   to the Response address.


4.3.  MTU Selection and Adjustment

   Because the DREQ message carries the allowed MTU size of previous
   hops that the DREP messages will later traverse, this unique feature
   allows the easy semantic fragmentation as described above.  Whenever
   the DREQ message grows to approach the size of MTU, it can be trimmed
   before being forwarded again.

   When a requester sends a DREQ message, the path MTU field in the RSVP
   Diagnostic Packet header can be set to a configured default value.



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INTERNET-DRAFT                                                  May 1999


   Whenever a DREQ message size approaches the specified MTU value, an
   intermediate RSVP node makes a copy of the message, converts it to a
   DREP message to send back, and then trims off the partial results
   from DREQ message and forwards it.

   It is possible that the original MTU value is chosen larger than the
   actual MTU value along some portion of the path being traced.
   Therefore each intermediate RSVP node must check the MTU value when
   processing a DREQ message.  If the specified MTU value is larger than
   the MTU of the incoming interface (that the DREQ message will be
   forwarded to), the node

     (1) sets the R-error value,
     (2) changes the MTU value in the header to the smaller value, and
     (3) converts the DREQ message to a DREP and sends it back to the
        requester.

   In the rare case where some intermediate nodes do not check, or
   enforce upon, the MTU value carried in the diagnostic messages, it is
   possible that on the way back to the requester, a DREP message may
   encounter a link of smaller MTU.

   When this happens, the node follows steps (1) and (2) as outlined
   above, and trims off the extra part of the DREP message to fit in the
   smaller MTU of the link.  The trimming must be done at response
   object boundaries.  Such trimming of messages results in information
   loss.  However because the requester learns what is the available MTU
   size, it can either ignore the loss, or otherwise try again with the
   smaller MTU value.


4.4.  Errors

   If an error condition prevents a DREP message from being forwarded
   further, the message is simply dropped.

   If an error condition, such as lack of PATH state, prevents a DREQ
   message from being forwarded further, the node must change the
   current message to DREP type and return it to the response address.


5.  Problem Diagnosis by Using RSVP Diagnostic Facility


5.1.  Across Firewalls

   Firewalls may cause problems in diagnostic message forwarding.  Let
   us look at two different cases.



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INTERNET-DRAFT                                                  May 1999


   First, let us assume that the querier resides on a receiving host of
   the session to be examined.  In this case, firewalls should not
   prevent the forwarding of the diagnostic messages in a hop-by-hop
   manner, assuming that proper holes have been punched on the firewall
   to allow hop-by-hop forwarding of other RSVP messages.  The querier
   may start by setting the H flag off, which can give a faster response
   delivery and reduced overhead at intermediate nodes.  However if no
   response is received, the querier may resend the DREQ message with H
   flag turned on.

   If the requester is a third party host and is separated from the
   LAST-HOP address by a firewall (either the requester is behind a
   firewall, or the LAST-HOP is a node behind a firewall, or both), at
   this time we do not know any other solution but to change the LAST-
   HOP to a node that is on the same side of the firewall as the
   requester.


5.2.  Examination of RSVP Timers

   One can easily collect information about the current timer value at
   each RSVP hop along the way.  This will be very helpful in situations
   when the reservation state goes up and down frequently, to find out
   whether the state changes are due to improper setting of timer
   values, or K values (when across lossy links), or frequent routing
   changes.


5.3.  Discovering Non-RSVP Clouds

   The D-TTL field in each DIAG_RESPONSE object shows the number of
   routing hops between adjacent RSVP nodes.  Therefore any value
   greater than one indicates a non-RSVP clouds in between.  Together
   with the arrival timestamps (assuming NTP works), this value can also
   give some vague, though not necessarily accurate, indication of how
   big that cloud might be.  One might also find out all the
   intermediate non-RSVP nodes by running either unicast or multicast
   trace route.


5.4.  Discovering Reservation Merges

   The flowspec value in a DIAG_RESPONSE object specifies the amount of
   resources being reserved for the data stream defined by the filter
   spec in the same data block.  When this value of adjacent
   DIAG_RESPONSE objects differs, that is, a downstream node Rd has a
   smaller value than its immediate upstream node Ru, it indicates a
   merge of reservation with RSVP request(s) from other down stream



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INTERNET-DRAFT                                                  May 1999


   interface(s) at Rd.  Further, in case of SE style reservation, one
   can examine how the different SE scopes get merged at each hop.

   In particular, if a receiver sends a DREQ message before sending its
   own reservation, it can discover (1) how many RSVP hops there are
   along the path between the specified sender and itself, (2) how many
   of the hops already have some reservation by other receivers, and (3)
   possibly a rough prediction of how its reservation request might get
   merged with other existing ones.


5.5.  Error Diagnosis

   In addition to examining the state of a working reservation, RSVP
   diagnostic messages are more likely to be invoked when things are not
   working correctly.  For example, a receiver has reserved an adequate
   pipe for a specified incoming data stream, yet the observed delay or
   loss ratio is much higher than expected.  In this case the receiver
   can use the diagnostic facility to examine the reservation state at
   each RSVP hop along the way to find out whether the RSVP state is set
   up correctly, whether there is any blackhole along the way that
   caused RSVP message losses, or whether there are non-RSVP clouds, and
   where they are, that may have caused the performance problem.


5.6.  Crossing "Legacy" RSVP Routers

   Since this diagnosis facility was developed and added to RSVP after a
   number of RSVP implementations were in place, it is possible, or even
   likely, that when performing RSVP diagnosis, one may encounter one or
   more RSVP-capable nodes that do not understand diagnostic messages
   and drop them.  When this happens, the invoking client will get no
   response from its requests.

   One way to by-pass such "legacy" RSVP nodes is to perform RSVP
   diagnosis repeatedly, guided by information from traceroute, or
   mtrace in case of multicast.  When an RSVP diagnostic query times out
   (see next section), one may first use traceroute to get the list of
   nodes along the path, and then gradually increase the value of Max-
   RSVP-hops field in the DREQ message, starting from a low value until
   one no longer receives a response.  One can then try RSVP diagnosis
   again by starting with the first node (which is further upstream
   towards the sender) after the unresponding one.

   There are two problem with the method mentioned above in the case of
   unicast sessions. Both problems are related to the fact that
   traceroute information provides the path from the requester to the
   sender. The first problem is that the LAST_HOP may not on the path



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INTERNET-DRAFT                                                  May 1999


   from the requester to the sender. In this case we can get information
   only from the portion of the path from the LAST_HOP to the sender
   which intersects with the path from the requester to the sender. If
   routers that are not on the intersection of the two paths don't have
   PATH state for the session being diagnosed then they will reply with
   R-error=0x01. The requester can overcome this problem by sending a
   DREQ to every router on the path (from itself to the sender) until it
   reaches the first router that belongs to the path from the sender to
   the LAST_HOP.

   The second problem is that traceroute provides the path from the
   requester to the sender which, due to routing asymmetries, may be
   different than the path traffic from the sender to the LAST_HOP uses.
   There are (at least) one case where this asymmetry will cause the
   diagnosis to fail. We present this case below.



                                   Downstream Path                Sender
                                   __         __            __       __
      Receiver             +------|  |<------|  |<-- ...---|  |-----|  |
         __          __   /       |__|       |__|          |__|     |__|
        |  |--....--|X |_/                    ^
        |__|        |__|      Router B       |
                   Black          __         |
                   Hole    +----->|  |---->---+
                                  |__| Upstream Path

                                Router A

                              Figure 2


   Here the first hop upstream of the black hole is different on the
   upstream path and the downstream path. Traceroute will indicate
   router A as the previous hop (instead of router B which is the right
   one). Sending a DREQ to router A will result in A responding with R-
   error 0x01 (No PATH State). If the two paths converge again then the
   requester can use the solution proposed above to get any (partial)
   information from the rest of the path.

   We don't have, for the moment, any complete solutions for the
   problematic scenarios described here.








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INTERNET-DRAFT                                                  May 1999


6.  Comments on Diagnostic Client Implementation.

   Following the design principle that nodes in the network should not
   hold more than necessary state,  RSVP nodes are responsible only for
   forwarding Diagnostic messages and filling DIAG_RESPONSE objects.
   Additional diagnostic functionality should be carried out by the
   diagnostic clients.  Furthermore, if the diagnostic function is
   invoked from a third-party host, we should not require that host be
   running RSVP daemon to perform the function.  Below we sketch out the
   basic functions that a diagnostic client daemon should carry out.

     1. Take input from the user about the session to be diagnosed, the
        last-hop and the sender address, the Max-RSVP-hops, and possibly
        the DIAG_SELECT list, create a DREQ message and send to the
        LAST-HOP RSVP node using raw IP message with protocol number 46
        (RSVP).  The port of the UDP socket on which the Diagnostic
        Client is listening for replies should be included in the
        Requester FILTER_SPEC object.

     2. Set a retransmission timer, waiting for the reply (one or more
        DREP messages).  Listen to the specified UDP port for responses
        from the LAST-HOP RSVP node.

        The LAST-HOP RSVP node, upon receiving DREP messages, sends them
        to the the Diagnostic Client as UDP packets, using the port
        supplied in the Requester FILTER_SPEC object.

     3. Upon receiving a DREP message to an outstanding diagnostic
        request, the client should clear the retransmission timer, check
        to see if the reply contains the complete result of the
        requested diagnosis.  If so, it should pass the result up to the
        invoking entity immediately.

     4. Reassemble DREP fragments.  If the first reply to an outstanding
        diagnostic request contains only a fragment of the expected
        result, the client should set up a reassembly timer in a way
        similar to IP packet reassembly timer.  If the timer goes off
        before all fragments arrive, the client should pass the partial
        result to the invoking entity.

     5. Use retransmission and reassembly timers to gracefully handle
        packet losses and reply fragment scenarios.

        In the absence of response to the first diagnostic request, a
        client should retransmit the request a few times.  If all the
        retransmissions also fail, the client should invoke traceroute
        or mtrace to obtain the list of hops along the path segment to
        be diagnosed, and then perform an iteration of diagnosis with



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INTERNET-DRAFT                                                  May 1999


        increasing hop count as suggested in Section 5.6 in order to
        cross RSVP-capable but diagnosis-incapable nodes.

     6. If all the above efforts fail, the client must notify the
        invoking entity.



7.  Security Considerations

   RSVP Diagnostics, as any other diagnostic tool, can be a security
   threat since it can reveal possibly sensitive RSVP state information
   to unwanted third parties.

   We feel that the threat is minimal, since as explained in the
   Introduction Diagnostics messages produce no side-effects and
   therefore they cannot change RSVP state in the nodes. In this respect
   RSVP Diagnostics is less a security threat than other diagnostic
   tools and protocols such as SNMP.

   Furthermore, processing of Diagnostic messages can be disabled if it
   is felt that is a security threat.


8.  Acknowledgments

   The idea of developing a diagnostic facility for RSVP was first
   suggested by Mark Handley of UCL.  Many thanks to Lee Breslau of
   Xerox PARC and John Krawczyk of Baynetworks for their valuable
   comments on the first draft of this memo.  Lee Breslau, Bob Braden,
   and John Krawczyk contributed further comments after March 1996 IETF.
   Steven Berson provided valuable comments on various drafts of the
   memo. We would also like to acknowledge Intel for providing a
   research grant as a partial support for this work. Subramaniam
   Vincent did most of this work while a graduate research assistant at
   the USC Information Sciences Institute (ISI).


9.  References

   [RSVPTUN] A. Terzis, J. Krawczyk, J. Wroclawski, L. Zhang. "RSVP
   Operation Over IP Tunnels ", Internet Draft draft-ietf-rsvp-tunnel-
   03.txt, August, 1998.








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INTERNET-DRAFT                                                  May 1999


10.  Authors' Addresses

      Lixia Zhang
      UCLA
      4531G Boelter Hall
      Los Angeles, CA  90095

      Phone:    310-825-2695
      EMail:    lixia@cs.ucla.edu

      Andreas Terzis
      UCLA
      4677 Boelter Hall
      Los Angeles, CA 90095

      Phone:    310-267-2190
      Email:    terzis@cs.ucla.edu

      Subramaniam Vincent
      Cisco Systems
      275, E Tasman Drive, MS SJC04/2/1
      San Jose, CA 95134.
      Phone:    408 525 3474
      Email:    svincent@cisco.com

      Bob Braden
      USC Information Sciences Institute
      4676 Admiralty Way
      Marina del Rey, CA 90292

      Phone:    310 822-1511
      EMail:    braden@isi.edu



















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