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

Errata ID: 3301
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
Otherwise (Other States)
fifth check the ACK field
p71

     if the ACK bit is on

        SYN-RECEIVED STATE

          If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state
          and continue processing.


            If the segment acknowledgment is not acceptable, form a
            reset segment,

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

            and send it.

It should say:

SEGMENT ARRIVES
Otherwise (Other States)
fifth check the ACK field
p71

     if the ACK bit is on

        SYN-RECEIVED STATE

          If SND.UNA < SEG.ACK =< SND.NXT then enter ESTABLISHED state
          and continue processing.


            If the segment acknowledgment is not acceptable, form a
            reset segment,

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

            and send it.

Notes:

In SYN-RECEIVE, SND.UNA == ISS. When the first ACK arrive with SEG.ACK == SND.UNA, it should be not accepted and thus transfer to ESTABLISHED state. This doesn't seem to be in rfc1122. Not sure if it is reported somewhere.


Verifier Note:
The side sending generating a packet that would erroneously check out would either be in error itself for generating the packet (not following the standard), or the packet would be extremely old (from a prior connection), and the responses should generate RSTs. Existing code seems to already be implementing this correctly, so I do not think there is an interoperability issue.

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