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rfc:rfc142

Network Working Group Charley Kline Request for Comments #142 Johnny Wong NIC #6727 UCLA (NMC) Categories: C.1, C.2, C.3, C.5 3 May 71 Updates: none Obsoletes: none

            Time-out Mechanism in the Host-Host Protocol

On sending a message to a foreign site, the following situations can occur:

1. Destination IMP down - Type 7 message is returned

2. Destination IMP up but destination IMP-HOST interface is

 down - Type 7 message is returned.

3. Destination IMP and IMP-HOST interface up, but IMP-HOST inter-

 face is not taking messages - Type 9 message is returned after
 IMP time-out (ask BBN for time).

4. Destination IMP and IMP-HOST interface up and IMP-HOST inter-

 face is taking messages - Type 5 (rfnm) message is returned.

A suggestion for handling type 7 and type 9 messages has been made in NWG/RFC #117. In this document we would like to discuss in detail the problem: what should happen to the HOST-HOST protocol on receiving a rfnm?

When a NCP sends out a STR or RTS control command on a pair of sockets and gets a rfnm back, this pair of sockets will be in a wait-match state. Everything is fine if a matching RTS or STR, or CLS is returned after a reasonable amount of delay. Trouble will arise when nothing is returned after a long time.

This can happen if the NCP is not running at all but its host is taking in messages (e.g. UCLA's host will receive messages even if the NCP is not running), or if the NCP is running very slowly. The same problem exists on sending out a CLS control command and a matching CLS is never returned. The trouble is that resources are tied up, e.g. sockets, links and table space in the NCP; and one would like to release these resources. In our implementation, when a user does a CLOSE, we can't release the sockets until the matching CLS is returned. This protects us from getting confused if a seconds request is made for the same pair of sockets. This problem can be solved by including a time-out mechanism in the Host-Host protocol. This operates as follows:

                                                              [Page 1]

ne 5 a. On sending out a STR or RTS and if you do not get back a match-

  ing RTS or STR, or a CLS in T time units a CLS will be sent.
  After sending the time-out CLS race condition can be avoided
  by ignoring the matching RTS or STR that arrives before the
  matching CLS.

b. On sending out a CLS (any kind, including the time-out CLS),

  and if you do not get back a matching CLS in T time units, the
  matching CLS is assumed to have returned.  However, if a RTS or
  STR is sent on the same pair of sockets anytime after the time
  out and before a CLS is returned, and then we receive the CLS,
  there is no way to determine whether this returning CLS is for
  matching the previous CLS or for refusing the RTS or STR.  (See
  the figure for detail).  So far we could not solve this race
  condition except by assigning sequence number to connection
  throughout the Network which we don't think is a good solution
  at all.  Hence, we would like to bring the attention of the
  Host-Host Protocol Glitch Cleaning Committe to this problem.
  The time limit T should be a Network Standard and its value should
  be decided also.
     Reason                                Our NCP
     ------                                -------
  1. User requests connection              1. RTS ->
  2. User gets tired requests CLS
     (or NCP timeout)                      2. CLS ->
  3. No matching CLS returned in
     T time units                          3. CLS assumed returned
                                              free socket and other
                                              resources
  4. User requests another connection
     over same socket pair                 4. RTS ->
  5. CLS received                          ?? does it belong to
                                           2 or 4?
                                 Figure
       [ This RFC was put into machine readable form for entry ]
          [ into the online RFC archives by Gert Doering 4/97]
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