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RFC 5961, "Improving TCP's Robustness to Blind In-Window Attacks", August 2010

Source of RFC: tcpm (tsv)

Errata ID: 5068

Status: Reported
Type: Technical

Reported By: Wesley Eddy
Date Reported: 2017-07-12

Section 3.2 says:

   [RFC0793] currently requires handling of a segment with the RST bit
   when in a synchronized state to be processed as follows:
...
   Instead, implementations SHOULD implement the following steps in
   place of those specified in [RFC0793] (as listed above).

It should say:

   [RFC0793] currently requires handling of a segment with the RST bit
   when in a synchronized state to be processed as follows:
...
   Instead, when in either a synchronized state or SYN-RECEIVED,
   implementations SHOULD implement the following steps in place of
   those specified in [RFC0793] (as listed above, for the synchronized
   states).  Note that transition from SYN-RECEIVED to either LISTEN or
   CLOSED and user signaling is still subject to whether the connection
   was originated by a passive OPEN (as described in RFC 793).

Notes:

The text in section 3.2 begins by stating a change from RFC 793 for RST bit handling "when in a synchronized state" (which means all states except for LISTEN, SYN-SENT, and SYN-RECEIVED). Later in the section, the same change is described more loosely and text states that it's applicable "In all states except SYN-SENT", and separate behavior is provided for SYN-SENT, but the earlier text leaves uncertainty if the former is supposed to apply to SYN-RECEIVED as well, since the earlier text in the section section begins by discussing only "synchronized" states.

Since the check is totally valid for SYN-RECEIVED, and the behavior in steps 1, 2, and 3 are valid for SYN-RECEIVED, it seems appropriate to make sure this is clarified in the earlier text.

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