[lwip-devel] [patch #9039] tcp: Avoid 3-second delay for a half-open connection

[Joel Cunningham]

Follow-up Comment #5, patch #9039 (project lwip):

I reviewed RFC 793 and I think the only check we aren't doing is if the ACK is
acceptable and no SYN is contained, then the packet is silently ignored.

I honestly can't think of a situation where the remote TCP would only ACK our
SYN, but regardless we aren't exactly following the steps listed:

https://tools.ietf.org/html/rfc793#page-66

If in SYN-SENT:
1. Check for ACK
2. Check for RST
3. Check security precedence
4. Check for SYN
5. If neither SYN or RST, drop

I also noticed during this analysis that it looks like we don't handle
simultaneous opens.  This is when both TCPs perform an active open at the same
time, going from SYN-SENT to SYN-RCVD by receiving only a SYN packet.  This
can be seen on page 32: https://tools.ietf.org/html/rfc793#page-32.  In our
handling of SYN-SENT we only check for both SYN and ACK flags.  If my static
analysis is correct, I can open a new bug

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