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From: CERF@USC-ISI.ARPA
Newsgroups: mod.protocols.tcp-ip
Subject: Re: SYN with a window size of zero
Message-ID: <[USC-ISI.ARPA]16-Jan-86.21:44:33.CERF>
Date: Thu, 16-Jan-86 21:44:00 EST
Article-I.D.: <[USC-ISI.ARPA]16-Jan-86.21:44:33.CERF>
Posted: Thu Jan 16 21:44:00 1986
Date-Received: Sat, 18-Jan-86 01:21:38 EST
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Approved: tcp-ip@sri-nic.arpa

Walt,

since one could not successfully "open" a connection without
getting the matching SYN-ACK, it seems appropriate for the
recipient of such a packet to respond, despite the apparent
violation. As you know, it is allowed to send at least one
octet into a closed (0) window to assure that when it
opens you hear that. The SYN can safely elicit an ACK without
opening the window any further.

Depending on the implementation, some systems don't apply the
window size until AFTER the connection has reached the OPEN state
which it cannot get to without first exchanging SYN-ACKs.

Jon Postel will undoubtedly have a reference to page xx para gg
and sugestion to look in one of Dave Clark's marvelous
tutorial sections...

Vint Cerf