RFC142 - Time-Out Mechanism in the Host-Host Protocol

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　　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 RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Gert Doering 4/97]

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