INTERNET-DRAFT                                               Lixia Zhang
<draft-ietf-rsvp-diagnostic-msgs-04.txt>                  Andreas Terzis
Expiration: February 1999                                           UCLA
                                                     Subramaniam Vincent

                                                             August 1998
                         RSVP Diagnostic Messages


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   Distribution of this memo is unlimited.

   This Internet Draft expires in February, 1999.


   This document specifies the RSVP diagnosis facility.  As the
   deployment of RSVP is spreading out, it becomes clear that a method
   for collecting information about the RSVP state along the path is
   needed.  This specification describes the functionality, diagnostic
   message formats, and processing rules.

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1.  Introduction

   In the original design of the RSVP protocol, error messages are the
   only means for the end hosts to receive feedback information
   regarding a specific request that has failed, a failure in setting up
   either a PATH state or reservation state.  In the absence of
   failures, one receives no feedback regarding the details of a
   reservation that has been put in place, such as whether, or where, or
   how, one's own reservation request is merged with that of others.  In
   case of a failure, the 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.  Such missing
   information, however, can be highly desirable for debugging purpose,
   or for network resource management in general.

   This document specifies RSVP diagnostic messages that allows one to
   collect information of RSVP state along the path from a receiver to a
   specific sender.  Diagnostic messages are independent from any other
   RSVP control messages and produce no side-effects.  That is, they do
   not change any RSVP state at either routers or hosts.  Similarly,
   they do not represent an error report but a collection of RSVP state
   information as requested.

   We have the following design goals in mind:

     - To be able to collect RSVP state information at every hop along
        the path where the PATH state has been set up, either for an
        existing reservation or before a reservation request is made;
        here the "hop" means RSVP-capable routers.

        More specifically, we want to be able to collect information
        about flowspec, refresh timer values, and reservation merging at
        each hop along the path.

     - To be able to collect the routing hop count for each non-RSVP

     - To avoid diagnostic packet implosion or explosion.

   The following are specifically identified as non-goals:

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

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2.  Overview

   We define two types of RSVP diagnostic packets, diagnostic request
   (DREQ) and reply (DREP).  This diagnostic tool can be invoked by a
   client from any host that may or may not be a participant of the RSVP
   session to be diagnosed.  Thus generally speaking three nodes are
   involved in performing the diagnostic function:  the requester, the
   starting and the ending nodes of the diagnosis, as shown in Figure 1.
   It is possible that the client invoking the diagnosis function may
   reside directly on the LAST-HOP, in which case that the first two
   nodes are the the same.  The starting node of the diagnosis is named
   "LAST-HOP", meaning the last-hop of the path segment to be diagnosed,
   which can be either the receiving end or an intermediate router along
   a reserved path.  The ending node is the sender host in general,
   although one can also limit the length of the path segment to be
   diagnosed by specifying a hop-count limit for the diagnosis messages.
   To avoid packet implosion or explosion, all diagnostic packets are
   forwarded via unicast only.

   A client invokes RSVP diagnostic functions by generating a DREQ
   packet and sending to the LAST-HOP node which should be on the RSVP
   path to be diagnosed.  This DREQ packet specifies the RSVP session
   and a sender host to that session.  The DREQ packet starts collecting
   information at the LAST-HOP node and proceeds toward the sender (see
   Figure 1).

    Receiver        LAST-HOP                                  Sender
           __           __         __     __     __     __       __
          |  |---------|  |------>|  |-->|  |-->|  |-->|  |---->|  |
          |__|         |__| DREQ  |__|   |__|   |__|   |__|     |__|
                        |            RSVP routers
                      |   | Requester

                           Figure 1

   Each RSVP-capable router receiving the DREQ packet adds to the packet
   a response data object containing the router's RSVP state for the
   specified RSVP session, and then forwards the request via unicast to
   the router that it believes to be the previous hop for the given
   sender.  Each subsequent RSVP router attaches its own response data

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   object to the end of the DREQ packet, then forwards via unicast to
   the previous hop.  When the DREQ packet reaches the sender, the
   sender changes the packet type to Diagnostic Reply (DREP) and sends
   the completed response to the original requester.  Partial response
   may also be returned before the DREQ packet reaches the sender if any
   error condition along the path, such as "no path state", prevents
   further forwarding of the DREQ packet, or if the specified hop-count
   for the diagnosis has been reached.

   DREP packets can be unicast back to the requester either directly, or
   in a hop-by-hop manner by reversing the exact path that the DREQ
   packet has taken.  The former is faster and more efficient, but the
   latter may be the only choice if the packets have to cross firewalls,
   due to the way that firewalls operate.

   To facilitate the latter case, a DREQ packet may optionally carry a
   ROUTE object, which is a list of router addresses that the DREQ
   packet has passed through on the way to the sender; this ROUTE object
   is built incrementally as the DREQ packet passes through the
   intermediate routers.  The DREP packet can then be returned to the
   requester by reversing the path.

   When the path consists of many hops, it is possible that the total
   length of a DREP packet will exceed the path MTU size before reaching
   the sender, thus the packet has to be fragmented.  Relying on IP
   fragmentation and reassembly, however, can be problematic, especially
   when DREP packets 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 which is carried in every DREQ packet. If an
   intermediate router finds that the default MTU size is bigger than
   that of the outgoing  link, it returns the DREQ packet with the
   corresponding error bit set.  If an intermediate router detects that
   a DREQ packet size reaches the MTU size, it sends a partial DREP,
   consisting of the collected responses back, to the requester and then
   continues to forward the trimmed DREQ packet to the next hop towards
   the sender.

   Through out this document we use the word "DREQ packet", rather the
   word "message" to call a diagnostic request since it always consists
   of a single packet.  On the other hand, one DREQ packet can generate
   multiple DREP packets, each containing a fragment of the total reply.
   Furthermore, when discussing diagnostic packet handling, the
   terminology used refers to the direction of data packet flow, thus
   "outgoing interface" of a router is the interface a DREQ packet comes
   from. THE DREQ then gets forwarded to an "incoming interface",
   because DREQ packets travel in the reverse direction of the data

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   Notice that one can forward DREQ packets 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.

3.  Diagnostic Packet Format

   A diagnostic packet consists of the following parts:

           |        RSVP common header         |
           |  Diagnostic packet header object  |
           |         session object            |
           |    (optional) SELECT object       |
           |    (optional) ROUTE object        |
           |    zero or more Response Object   |

3.1.  RSVP Message Common Header

   In the RSVP message common header,

                   0             1              2             3
     | Vers | Flags|    Type     |       RSVP Checksum       |
     |   Send_TTL  |   reserved  |          RSVP Length      |

   The Flags field is unused for now and must be set to zero.

   Type = 8: DREQ

   Type = 9: DREP

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   The RSVP Checksum is the 16-bit one's complement of the one's
   complement sum of the whole diagnosis message (including this
   header).  For computing the checksum, the checksum field is set to
   zero.  When receiving packets, the checksum MUST be verified before
   processing a packet.

   Send_TTL: the TTL value that a router puts in the IP packet header
   when forwarding the DREQ packet to the previous hop.

   RSVP length: the total length of this diagnostic packet in bytes,
   including the common header.  If this is a DREP packet and the MF
   flag in the diagnostic packet header (see below) is set, this length
   field indicate the length of this single DREP fragment, rather than
   the total length of the the complete DREP reply (which may not be
   known in advance).

3.2.  RSVP Diagnostic Packet Header Object

   Both DREQ and DREP headers are a concatenation of Diagnostic Packet
   Header Object and an RSVP Session object, as defined below:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |           length              |    class      |     c-type    |
    | Max-RSVP-hops | RSVP-hop-count|         Reserved          |H|MF|
    |                          Message ID                           |
    |           path MTU            |     Fragment offset           |
    |                                                               |
    |                         Sender Filter-Spec                    |
    |                                                               |
    |                         LAST-HOP Address                      |
    |                                                               |
    |                  Response Address Filter-Spec                 |
    |                                                               |
    |                                                               |
    |                         Next-Hop RSVP_HOP Object              |
    |                                                               |
    |                                                               |
    +               Followed by RSVP Session Object                 |
    |                                                               |

   Length is the length of this diagnostic header object.

   Class = 30.

   C-type field is used to distinguish between IPv4 (C-type = 1) and
   IPv6 (Ctype = 2).  In the IPv6 case addresses will be 16 bytes each.

   Max-RSVP-hops specifies the maximum number of RSVP hops that the
   requester wants to collect information from.  In case an error
   condition in the middle of the path prevents the DREQ packet from
   reaching the specified sender, one may use this field to perform an
   expanding-length search to reach the point just before the problem.

   The fragment offset field indicates where in the total reply this

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   fragment belongs. The fragment offset is measured in octets. The
   first fragment has offset zero.

   RSVP-hop-count field records the number of RSVP hops that have been
   traversed so far.

   The 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 to the LAST-HOP address in a hop-
   by-hop way.  The node specified by the LAST-HOP address then forwards
   DREP packets to the response address.

   The MF flag means "more fragments".  It must be set to zero (0) on
   all DREQ packets, and set to one (1) on all DREP packets that carry
   partial results and are returned by intermediate routers due to the
   MTU limit.  When the sender converts a DREQ packet to DREP, the MF
   flag remains zero.  An intermediate router may also converts a DREQ
   packet to DREP when the DREQ packet has traversed the specified
   number of Max-RSVP-hops, in which case the MF flag remains zero.

   Message ID identifies an individual DREQ packet and the corresponding
   reply (or all the fragments of the reply). A possible way of using
   the message ID is the 16 bits to specify the ID of the process doing
   the query and the low 16 bits to be the sequence number of the query.
   This way processes on the same machine can distinguish between each
   other's replies and between different copies of the same query.

   The path MTU is a 16-bit field that specifies a default MTU size, in
   number of bytes, that all diagnostic packets must fit within.

   Sender Filter-Spec is the IP address plus the port of the sender
   being traced.  The DREQ packet proceeds hop-by-hop towards this

   LAST-HOP address is the IP address of the last hop at the receiving
   end for the path being traced.  The DREQ packet starts collecting
   information at this node and proceeds toward the sender.

   Response Address Filter-Spec contains the IP address and the port to
   which the DREP packet(s) should be sent. This Response Address
   Filter-Spec specifies the process originating the request.

   The Next-Hop RSVP_HOP object carries the IP address of the interface
   to which the DREQ must be forwarded to. This object is updated on a
   hop by hop basis, and is used for the same reasons that a RESV
   message contains an RSVP_HOP object. That is, to distinguish logical
   interfaces and avoid problems caused by routing asymmetries.

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   The session object identifies the RSVP session for which the state
   information is being collected.

   Optionally, the diagnostic packet may contain a SELECT object which
   carries a list of [Class, C-type] pairs, each pair specifies one type
   of RSVP object the diagnosis invoking client wants to examine.  When
   a SELECT object is included in the DREQ packet, each RSVP router
   along the way should attach to the response object each type of the
   objects specified in the SELECT list.  In the absence of a SELECT
   object, the router will attach a set of default objects.

   The SELECT object has the following format:

    |          length               |    class      |     c-type    |
    |    class      |     c-type    |    class      |     c-type    |
    |            ...................                                |

   The Length field represents the total length of the object in number
   of bytes.

   Class = 33

   C-type field is not used at the moment and must be set to zero.

   The object payload part carries a list of  [Class, C-type] pairs.  In
   case where the requested number of objects is an odd number, the last
   two bytes must be set to zero.

   Optionally, the diagnostic packet may also contain a ROUTE object, as
   defined below. The ROUTE object is to be used to return DREP packets

    |          length               |  class        |     c-type    |
    |             reserved                          |    R-pointer  |
    |                                                               |
    +                     List of RSVP routers                      |
    |                                                               |

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   The Length field represents the total length of the object in number
   of bytes, from which the number of addresses in the RSVP router list
   can be easily computed.

   Class = 31.

   C-type field is used to distinguish between IPv4 (C-type = 1) and
   IPv6 (Ctype = 2) ROUTE object.

   R-pointer is used in DREP packets only (see Section 4.2 for details),
   but is incremented as each hop adds its incoming interface address in
   the ROUTE object.

   In a DREQ packet, the List of RSVP routers lists all the RSVP hops
   between the LAST-HOP address, as specified in the Diagnostic packet
   header object, and the last RSVP router the DREQ packet has visited.
   In a DREP packet, List of RSVP routers lists all the RSVP hops
   between the LAST-HOP and the router that returns this DREP packet.

3.3.  Response Data Object

   When receiving a DREQ packet, each RSVP router attaches a "response
   data" object to it before forwarding on.  The response data object is
   defined as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |          length               |     class     |   C-type      |
    |                       DREQ Arrival Time                       |
    |                  Incoming Interface Address                   |
    |                  Outgoing Interface Address                   |
    |                 Previous-RSVP-Hop Router Address              |
    |                      reservation style                        |
    |   D-TTL       |M|R-err|  K    |      timer value              |
    |                                                               |
    |                         (TUNNEL object)                       |
    |                                                               |
    |                                                               |
    |                         Tspec object                          |
    |                                                               |
    |                                                               |
    |                        filter spec object                     |
    |                                                               |
    |                                                               |
    |                          flowspec object                      |
    |                                                               |

   Class = 32.

   Ctype 1 and 2 specify whether this is an IPv4 or IPv6 response data,

   DREQ Arrival Time is a 32-bit NTP timestamp specifying the arrival
   time of the DREQ packet at this router.  The 32-bit form of an NTP
   timestamp 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

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   on which packets from the sender, as defined in the Diagnostic Packet
   Header, are expected to arrive, or 0 if unknown.

   Outgoing Interface Address specifies the IP address of the interface
   from which the DREQ packet comes, and to which packets from the given
   sender and for the specified session address flow, or 0 if unknown.

   Previous-RSVP-Hop Router Address specifies the router from which this
   router receives RSVP PATH messages for this source, or 0 if unknown.

   Notice that the response object format as shown above assumes IPv4
   addresses of 4-byte each; in case of IPv6 (indicated by C-type = 2),
   these three addresses will be 16 bytes each.

   Reservation style is the 4-byte value of RSVP Style Object as defined
   in the RSVP specification.

   D-TTL contains the routing hop count this DREQ packet traveled from
   the down-stream RSVP router to the current router.

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

   R-error is a 3-bit field that indicates error conditions at a router.
   Currently defined values are
           0x00: no error
           0x01: no PATH state
           0x02: MTU too big
           0x04: ROUTE object too big

   K is the refresh timer parameter defined in RSVP, and timer value is
   the local refresh timer value in seconds.

   The next part, TUNNEL object, is an optional one which should be
   inserted when a DREQ packet arrives at an RSVP router 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 when so
   desired, 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 which may have totally different parameter

   The tunnel object is defined in the RSVP Tunnel Specification
   [RSVPTUN], with the following format:

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    |          length               |  class        |     c-type    |
    |                                                               |
    |           Session object (for the end-to-end session)         |
    |                                                               |
    |                                                               |
    |           Sender Filter-Spec (for the tunnel sender)          |
    |                                                               |

                                           SESSION_ASSOC Object

   Class=192.  Ctype 1 specifies IPv4 sessions, Ctype 2 specifies IPv6
   sessions, and Ctypes 3 and 4 specify sessions with IPSEC Generalized
   Port Id for IPv4 and IPv6 respectively.

   The remaining parts, Tspec, filter spec, and flowspec objects follow
   the definitions given in RSVP specification. The latter two may be
   absent (see Section 4.1 on DREQ forwarding). In the case of a SE
   reservation the filter spec is actually the set of all filter specs
   that share the reservation. The flowspec describes the actual
   reservation in place.

   Also note that the length of these object is varying so the lengths
   used on the diagram above are not representative.

4.  Diagnostic Packet Forwarding Rules

4.1.  DREQ Packet Forwarding

   DREQ packets are forwarded via hop-by-hop unicast from the LAST-HOP
   address to the Sender address as specified in the diagnostic packet
   header.  Each hop performs the following processing before forwarding
   the packet to the next hop towards the sender:

     1. Compute the routing 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 RSVP common header.  The result is then
        saved in the D-TTL field of the response data object.

     2. If no PATH state exists for the specified session, set R-error =

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        0x01 in the Response Data 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 PATH messages for the current router.

     4. Attach the response data object to the end of the DREQ packet.
        If the DREQ packet contains a SELECT object, attach one copy of
        each of the objects specified in the SELECT.  Otherwise attach
        Tspec, filter spec, and flowspec objects to the response object.
        Tspec, filter spec, and flowspec objects describe the
        reservation in place at the Outgoing Interface for the specified

        If no reservation state exists for the specified RSVP session,
        the response object will contain no filter-spec or flowspec

        If neither PATH nor reservation state exists for the specified
        RSVP session, then the response object contains none of the
        Tspec, filter or flow spec object.

     5. Increment the RSVP-hop-count field in the diagnostic packet
        header by one.  If the resulting value is equal to that of Max-
        RSVP-hops, or if the current hop is the sender as identified by
        the "Source Address" in the RSVP diagnostic header, 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
        packet back to either the LAST-HOP address (if H = 1), or to the
        response address directly via unicast (if H = 0).

     7. If the resulting DREQ packet 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 packet
        length field in the RSVP  common header accordingly. Finally
        forward the DREQ packet to the next hop towards the source,
        after recomputing the checksum.


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

     2. Make a copy of the DREQ packet 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 response data object) > path MTU, then set the "route too
        big" error bit, recompute the checksum, send the response packet
        and go to 4, else recompute the checksum and send the response

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

     4. Return.

4.2.  DREP Forwarding

   When the H flag is off, DREP packets 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 router receives a DREP packet, it simply decreases R-pointer
   by one (address length), and forward the packet to the address
   pointed by R-pointer in the route list.

   When the LAST-HOP router receives a DREP packet, it sends the packet
   to the Response address.

4.3.  MTU Selection and Adjustment

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

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

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   Whenever a DREQ packet size approaches the specified MTU value, an
   intermediate RSVP router makes a copy of the packet, converts it to a
   DREP packet to send back, and then trims off the partial results from
   DREQ packet 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 router must check the MTU value when
   processing a DREQ packet.  If the specified MTU value is larger than
   the MTU of the incoming interface (that the DREQ packet will be
   forwarded to), the router

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

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

   When this happens, the router follows steps (1) and (2) as outlined
   above, and trims off the extra part of the DREP packet to fit in the
   smaller MTU of the link.  The trimming must be done at response
   object boundaries.  Such trimming of packets 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 packet from being forwarded
   further, the packet is simply dropped.

   If an error condition, such as lack of PATH state, prevents a DREQ
   packet from being forwarded further, the router must change the
   current packet 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 packet forwarding.  Let us
   look at two different cases.

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   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 packets 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 packets.  The querier
   may start by setting the H flag off, which can give a faster response
   delivery and reduced overhead at intermediate routers.  However if no
   response is received, the querier may resend the DREQ packet 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 router 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

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

5.3.  Discovering Non-RSVP Clouds

   The D-TTL field in each response data block shows the number of
   routing hops between adjacent RSVP routers.  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 routers by running either unicast or multicast
   trace route.

5.4.  Discovering Reservation Merges

   The flowspec value in a response data block 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 response
   data blocks differs, that is, a downstream router Rd has a smaller
   value than its immediate upstream router Ru, it indicates a merge of
   reservation with RSVP request(s) from other down stream interface(s)

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   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 packet 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 packets 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

   Given that this diagnosis function is developed and added to RSVP
   after a number of RSVP implementations have been in place, it is
   possible, or even likely, that when performing RSVP diagnosis, one
   may encounter one or more RSVP-capable routers that do not understand
   diagnostic packets, thus drop them.  When this happens, the invoking
   client will get no response from its requests.

   One way to by-pass such "legacy" RSVP routers is running an iteration
   of RSVP diagnosis by using 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 routers
   along the path, and then gradually increases the value of Max-RSVP-
   hops field in the DREQ packet, starting from a low value until one no
   longer receives a response.  One can then try RSVP diagnosis again by
   starting with the first router (which is further upstream towards the
   sender) after the unresponding one.

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6.  Comments on Diagnostic Client Implementation.

   Following the design principle that routers in the network should not
   hold more than necessary state,  RSVP nodes are responsible only for
   forwarding Diagnostic packets and filling Response Data Objects.
   Additional diagnostic functionalities should be carried out by the
   Diagnostic Clients.  Furthermore, if the diagnostic function is
   invoked from a third-party host, we should not 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 SELECT list, create a DREQ packet and send to the LAST-HOP
        RSVP node using raw IP packet with protocol number 46 (RSVP).
        The port of the UDP socket that the Diagnostic Client is
        listening to for replies, should be included in the Response
        Address Filter-Spec.

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

        The LAST-HOP RSVP node upon receiving DREP packets sends them to
        the the Diagnostic Client as UDP packets, using the port
        supplied to in the Response Address Filter-Spec.

     3. Upon receiving a DREP packet 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 increasing hop
        count as suggested in Section 5.6 in order to cross RSVP-capable

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        but diagnosis-incapable routers.

     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 routers. 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.

9.  References

   [RSVPTUN] L. Zhang, A. Terzis, "RSVP Operation Over IP Tunnels ",
   Internet Draft draft-ietf-rsvp-tunnel-02.txt, November, 1997.

10.  Authors' Addresses

      Lixia Zhang
      4531G Boelter Hall
      Los Angeles, CA  90095

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      Phone:    310-825-2695

      Andreas Terzis
      4677 Boelter Hall
      Los Angeles, CA 90095

      Phone:    310-267-2190

      Subramaniam Vincent

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