Addition to the Very Distant Host specifications
A clarification to the early network host-IMP interface specification.
Walden, D.C., "Addition to the Very Distant Host specifications", RFC 591, DOI 10.17487/RFC0591, November 1973, <https://www.rfc-editor.org/info/rfc591>.
@misc{rfc591,
series = {Request for Comments},
number = 591,
howpublished = {RFC 591},
publisher = {RFC Editor},
doi = {10.17487/RFC0591},
url = {https://www.rfc-editor.org/info/rfc591},
author = {D.C. Walden},
title = {{Addition to the Very Distant Host specifications}},
pagetotal = 1,
year = 1973,
month = nov,
} RFC text
Network Working Group D. Walden
Request for Comments: #591 BBN-Net
NIC: #20327 29 November 1973
Addition to the Very Distant Host Specification
It has been brought to my attention that the following sentence
should be added to the Very Distant Host specification in Appendix F of
BBN report 1822, Specifications for the Interconnection of a Host and an
IMP, to the paragraph beginning on page F-5 and ending on page F-7:
"Null packets do not use a channel (nor a channel number) when sent and
are not acknowledged when received."
Please file this RFC with your copy of BBN report 1822 until 1822
is next updated.
DCW/nlg
RECEIVED AT NIC DECEMBER 3, 1973
AI ELI5
On this page
§1The Missing Clarification
RFC 591 is a brief note that adds a clarifying sentence to the Very Distant Host specification found in Appendix F of BBN Report 1822, which describes how a host computer connects to an IMP (Interface Message Processor) in the early ARPANET.
The specification was incomplete regarding the handling of null packets—messages with no data payload—and this ambiguity needed to be cleared up for implementers building compatible systems.
A computer interface rulebook forgot to say how to send empty messages. This note says: send them without a numbered slot and do not wait for confirmation.
§2Null Packets in the Protocol
In the host-IMP interface, null packets are messages that contain no actual data. The clarification states that null packets do not use a channel (nor a channel number) when sent and are not acknowledged when received. This means the sender does not need to wait for an acknowledgment after sending a null packet.
This rule distinguishes null packets from regular data packets, which follow normal channel allocation and acknowledgment rules.
Empty messages are simple: they skip the fancy routing system and do not expect a reply. That makes them quick and easy to handle.
§3Context and Status
This RFC was published in November 1973 as an addendum to the BBN Report 1822 specification, which was a key document for building ARPANET hosts and IMPs. The author, D. Walden from BBN, noted that readers of the specification should file this RFC with their copy of the report until the report itself was updated to include the clarification.
This was a quick fix to an important rulebook before the rulebook could be officially reprinted with the missing sentence included.