RFC 2745 (rfc2745) - Page 3 of 23

RSVP Diagnostic Messages

RFC 2745                RSVP Diagnostic Messages            January 2000


   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   |   |