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: 1571
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Constantin Hagemeier
Date Reported: 2008-10-11
Held for Document Update by: Wesley Eddy

Section 3.1 says:

Maximum Segment Size Option Data:  16 bits

          If this option is present, then it communicates the maximum
          receive segment size at the TCP which sends this segment.
          This field must only be sent in the initial connection request
          (i.e., in segments with the SYN control bit set).  If this
          option is not used, any segment size is allowed.

It should say:

Maximum Segment Size Option Data:  16 bits

          If this option is present, then it communicates the maximum
          receive segment size at the TCP which sends this segment.
          This field may be sent in the initial connection request
          (i.e., in segments with the SYN control bit set) and must not
          be sent in other segments.  If this option is not used, any
          segment size is allowed.

Notes:

'must only' is ambiguous or even senseless.

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