TCP question

Joe Weening (JJW@SU-AI.ARPA)
20 Jan 86 1504 PST

Suppose the following happens:

        Host A Host B
        ---------- ----------
        Sends SYN
        Enters SYN-SENT state
                                Receives SYN
                                Enters SYN-RECEIVED state
                                Sends SYN ACK, but packet is lost
        Retransmits SYN
                                Receives SYN

The second SYN received by host B is out of the window, so my reading of
RFC 793 is that host B should send an ACK and drop the segment (bottom of
page 69). But this way, host A never receives a SYN from host B.

Have I missed something? It seems that Host B should reply with another
SYN ACK when it receives a SYN in the SYN-RECEIVED state (maybe after
checking to make sure the sequence number on the SYN is one less than the
current RCV.NXT), but I can't find this anywhere. Our implementation
actually acts as described above, and I'd like to fix it.

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