RFC 793, "Transmission Control Protocol", September 1981

Note: This RFC has been obsoleted by RFC 9293

Note: This RFC has been updated by RFC 1122, RFC 3168, RFC 6093, RFC 6528

Source of RFC: Legacy
Area Assignment: tsv

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Errata ID: 3300
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Botong Huang
Date Reported: 2012-07-30
Held for Document Update by: Wes Eddy
Date Held: 2012-09-13

Section 3.9 says:

SEGMENT ARRIVES
SYN-SENT STATE

If the ACK bit is set

 If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
 the RST bit is set, if so drop the segment and return)

 <SEQ=SEG.ACK><CTL=RST>

 and discard the segment. Return.

 If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.

It should say:

SEGMENT ARRIVES
SYN-SENT STATE

If the ACK bit is set

 If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
 the RST bit is set, if so drop the segment and return)

 <SEQ=SEG.ACK><CTL=RST>

 and discard the segment. Return.

 If SND.UNA < SEG.ACK =< SND.NXT then the ACK is acceptable.

Notes:

In SYN-SENT, SND.UNA == ISS, so the first line is contradictory to the last.

Verifier Notes:

This is being Held for Document Update rather than Verified because technically the algorithm is still correct, as the first check against ISS will fail and cause a reset to be generated. The second sentence that has an off-by-one error appears to be merely a paraphrasing.

In practice today, much code seems to be simplifying further and checking that SEG.ACK == SND.NXT, for stacks that are not sending data on the SYN, so I do not believe this text is leading to any significant issue with bugs or interoperability in the wild.

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