Date: Thu, 28 Oct 1999 11:07:22 -0400 (EDT) From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> To: "Craig Critchley" <craigc@nwlink.com> Cc: <net@FreeBSD.ORG> Subject: Re: FTP Net Performance Message-ID: <199910281507.LAA91582@khavrinen.lcs.mit.edu> In-Reply-To: <218001bf211b$791005b0$0201010a@fuzzer.com> References: <Pine.BSF.4.10.9910280237560.30200-100000@jade.chc-chimes.com> <218001bf211b$791005b0$0201010a@fuzzer.com>
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<<On Thu, 28 Oct 1999 01:07:26 -0700, "Craig Critchley" <craigc@nwlink.com> said: > 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. In any event, duplicate acknowledgements are a sign of two possible problems: 1) Packets are getting re-ordered (not likely in your configuration). 2) Packets are getting dropped (which, as you already stated, was happening about 25% of the time). TCP will always perform poorly when large numbers of packets are being dropped, because it must wait for a retransmission timeout (minimum value 0.5 seconds in -current and 1.5 +/- 0.5 seconds in -stable) and then slow-start. -GAWollman -- Garrett A. Wollman | O Siem / We are all family / O Siem / We're all the same wollman@lcs.mit.edu | O Siem / The fires of freedom Opinions not those of| Dance in the burning flame MIT, LCS, CRS, or NSA| - Susan Aglukark and Chad Irschick To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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