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:

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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
       ------                                -------

  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 ->

  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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