[lwip-devel] [bug #49717] Window size in received SYN and SYN-ACK assumed scaled but is not

[Ambroz Bizjak]

URL:
  <http://savannah.nongnu.org/bugs/?49717>

                 Summary: Window size in received SYN and SYN-ACK assumed
scaled but is not
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: abizjak
            Submitted on: Sun 27 Nov 2016 07:01:49 PM GMT
                Category: None
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

>From looking at the code I believe lwIP incorrectly interprets the window size
values in received SYN and SYN-ACK segments.

RFC 7323 states: 

"The window field in a segment where the SYN bit is set (i.e., a <SYN> or
<SYN,ACK>) MUST NOT be scaled."

In the code this is in tcp_in.c lines 605 (SYN) 754 (SYN-ACK). In both cases
pcb->snd_wnd is set to SND_WND_SCALE(pcb, tcphdr->wnd) after tcp_parseopt is
called which would have set pcb->snd_scale if the window scale option is
present.




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