SYN with a window size of zero


Walt (Haas@UTAH-20.ARPA)
Thu 16 Jan 86 15:15:37-MST


One of our TCP implementations (TOPS-20) opens a TCP connection
with a segment that contains the usual SYN and a window size of
zero. According to one interpretation of the spec there should be
no legal response to such a segment since the normal response is
SYN+ACK and the SYN in the response would take up sequence number
space, which does not exist according to the sender of the first
segment. According to another intepretation, the window size
refers to data octets only, and the sequence number space taken by
SYNs and FINs shouldn't count.

Various implementations handle this in various ways - some apparantly
assume that it's silly to send SYN with a window size of zero, and just
go ahead and reply with SYN+ACK. One implementation appears to go
into a tight loop in this situation.

Does anybody have any references that would resolve the apparent
ambiguity? It isn't clear to me how to apply the robustness principle
to this case - one could imagine that an implementation could have
an unusual situation where it needed to open a connection but didn't
at the moment have any place to put a reply.

Thanks in advance for any helpful comments -- Walt
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