Date: Thu, 28 Oct 1999 08:31:46 -0700 From: bmah@CA.Sandia.GOV (Bruce A. Mah) To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> Cc: "Craig Critchley" <craigc@nwlink.com>, net@FreeBSD.ORG Subject: Re: FTP Net Performance Message-ID: <199910281531.IAA97032@nimitz.ca.sandia.gov> In-Reply-To: Your message of "Thu, 28 Oct 1999 11:07:22 EDT." <199910281507.LAA91582@khavrinen.lcs.mit.edu>
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--==_Exmh_-804607468P Content-Type: text/plain; charset=us-ascii If memory serves me right, Garrett Wollman wrote: > <<On Thu, 28 Oct 1999 01:07:26 -0700, "Craig Critchley" <craigc@nwlink.com> s > aid: > > > Ah, I've misunderstood the sequence number being ack'd - it's the next byte > > expected by the reciever, eh? Damn. I've probably been tearing apart the > > wrong machine. > > No, it's the sequence number of the last octet (or control bit) > successfully received *and desegmented*. The SYN and FIN bits occupy > one unit in sequence space. TCP's acknowledgements are cumulative, so > ACK(1234) means that all data up to and including sequence number 1234 > have been received successfully. If sequence 1222 is lost, then > regardless of how many later packets are successfully received, TCP > will send ACK(1221) until a segment containing 1222 is received. Possibly contributing to the confusion is the way that tcpdump prints out TCP sequence numbers. If you have a line that looks like... 08:22:19.252316 foo.ssh > bar.935: P 1:37(36) ack 20 win 8760 (DF) [tos 0x10] ...the segment displayed really contained octets with sequence numbers 1 through 36 inclusive, despite the fact that the line reads "1:37" (36 objects numbered starting from 1 will end at 36, not 37). Note that the next segment from this example conversation was... 08:22:20.472454 foo.ssh > bar.935: P 37:73(36) ack 40 win 8760 (DF) [tos 0x10] ...showing that byte 37 really went in the second segment, not the first. If a person didn't know this fact, it'd be easy to interpret the acks wrongly. Bruce. --==_Exmh_-804607468P Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 5.0i for non-commercial use MessageID: PxW59vcMMOAxFckYIAHWJKhNVEf9nMri iQA/AwUBOBhsYtjKMXFboFLDEQLB+wCfT89VgdmYJKcoubXQp9TK1MWX2AoAoOeY UrTtKo16RwOBIwqgY2DcjztA =AwR2 -----END PGP SIGNATURE----- --==_Exmh_-804607468P-- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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