Date: Thu, 14 Aug 1997 17:45:01 PDT From: Bill Fenner <fenner@parc.xerox.com> To: "Jordan K. Hubbard" <jkh@time.cdrom.com> Cc: wollman@freebsd.org, current@freebsd.org Subject: Re: Well, I guess it's about time I mentioned this little problem... Message-ID: <97Aug14.174508pdt.177512@crevenia.parc.xerox.com> In-Reply-To: Your message of "Thu, 14 Aug 97 11:32:30 PDT." <1433.871583550@time.cdrom.com>
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My bet is broken path mtu discovery in the opposite direction. >11:30:53.276132 time.cdrom.com.1216 > rocky.sunlabs.com.http: S 2243214515:2243214515(0) win 16384 <ms > s 1460,nop,wscale 0,nop,nop,timestamp[|tcp]> (DF) >11:30:53.501585 rocky.sunlabs.com.http > time.cdrom.com.1216: S 294104724:294104724(0) ack 2243214516 > win 8760 <mss 1460> (DF) >11:30:53.501696 time.cdrom.com.1216 > rocky.sunlabs.com.http: . ack 1 win 16384 (DF) >11:30:53.503656 time.cdrom.com.1216 > rocky.sunlabs.com.http: . 1:513(512) ack 1 win 16384 (DF) >11:30:53.797419 rocky.sunlabs.com.http > time.cdrom.com.1216: . ack 513 win 8760 (DF) >11:30:53.797508 time.cdrom.com.1216 > rocky.sunlabs.com.http: P 513:667(154) ack 1 win 16384 (DF) For some reason, we send only 512 bytes in the first packet, but that's probably not interesting in this case since we send a small first packet the second time. What's interesting is that in the one that works, the return ACK for byte #667 is piggybacked on the other end's MSS-sized data packet -- so, if in this one, the ACK for byte #667 is also piggybacked on the MSS-sized data packet, it's a 1500-byte packet, which is apparently getting dropped without proper notification on the way back to us. >11:30:56.480098 time.cdrom.com.1216 > rocky.sunlabs.com.http: P 513:667(154) ack 1 win 16384 (DF) >11:30:56.698495 rocky.sunlabs.com.http > time.cdrom.com.1216: . ack 667 win 8760 (DF) We retransmit because we haven't gotten an ACK, and rocky replies with a dataless ACK. It doesn't include any data since it's still doing slow-start towards us. rocky keeps retransmitting that first MSS(MTU)-sized packet, it keeps getting dropped and MTU discovery isn't working so we never hear from it again. Bill
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