Path: utzoo!utgpu!watserv1!watmath!att!ucbvax!animal.clearpoint.com!solensky
From: solensky@animal.clearpoint.com (Frank T. Solensky)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  do FINs count as 1 like SYNs ?
Message-ID: <9103022237.AA00192@animal>
Date: 2 Mar 91 22:37:14 GMT
References: <6515@acdhq.north.de>
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The Internet
Lines: 17

>Hi!
>Sending a SYN advances the sequence number by one since it must be ACKed.
>A FIN is ACKed, too, so does it advanced the sequence number?
>I browsed through RFC761 but found nothing which tells me. :-(

Kai --
	RFC 791 (which updated 763) describes the closing sequence pretty
well and illustrates this case in Figure 13 (page 39).  The packet that ACKs
the FIN will have an ACK number one greater than the FIN's sequence number.
So, yes, the sequence number is incremented after the FIN is transmitted
(so that ACK will == snd.NXT).
	You may want to also get a copy of RFC-1122 (Requirements for Internet
hosts): this document clarifies a number of points in both the TCP and IP
protocol specifications that have been interpreted in different (and frequently
incompatible) ways.

				-- Frank Solensky / Clearpoint Research Corp.