RFC209 - Host/IMP interface documentation

Network Working Group B. Cosell

Request for Comments: 209 Bolt Beranek and Newman

NIC: 7187 13 August 1971

HOST/IMP Interface Documentation

On 25 June 1971 we put a new version of the IMP system, Version 2500,

up in the network. Among other changes, the new system corrected the

problem of Hosts receiving spurious "Destination Dead" [Type 7]

messages. The correction involved making the IMP refuse to accept

any messages from its Host for about 30 seconds after the Host

indicated its intention to come alive. This delay has caused trouble

to a number of NCPs because they had imposed an upper limit on the

time an IMP may take to accept a message, and when that time was

exceeded the IMP was declared inoperative and the NCP shut itself

down, leaving the site personnel annoyed and perplexed.

We're sorry to have caused so mUCh trouble, but we thought that the

new version modified no advertised property of the system. To

prevent misunderstandings of this type in the future, the IMP/Host

interface should be well documented, well understood and invariant.

The only way to set up and maintain such an interface is to presume

that BBN Report No. 1822 is complete and correct. We understand that

it is neither, but operating on the basis of this assumption is the

best way to force 1822 to improve. In our view, the Hosts should

proceed under the constraint that anything not specifically

guaranteed by 1822 does not exist and should not be used. If there

are problems using this approach, please don't "code around" the

problem or treat your IMP as a "black box" and extrapolate its

characteristics from a series of eXPeriments. Instead, send your

comments and problems to the NCC at BBN, and we will fix the IMP

system, or 1822, whichever is necessary.

We are, at the moment, working n an update to 1822. It will reflect

the deletion of the "cease" mechanism [as per RFC#107].

We are also considering augmenting the IMP/Host protocol with a

mechanism to allow a Host to detect a change in the "version" of the

IMP system, and to provide the Hosts with status information as to

who is alive in the net.

[ This RFCwas put into machine readable form for entry ]

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