Date: Tue, 2 Jan 2007 00:25:22 -0800 From: steve@Watt.COM (Steve Watt) To: Julian Elischer <julian@elischer.org> Cc: hackers@freebsd.org Subject: Re: Interesting TCP issue Message-ID: <200701020825.l028PMBK034571@wattres.watt.com> In-Reply-To: steve@wattres.Watt.COM (Steve Watt) "Re: Interesting TCP issue" (Jan 2, 0:06)
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On Jan 2, 0:06, Steve Watt wrote: } Subject: Re: Interesting TCP issue } On Jan 1, 23:56, Julian Elischer wrote: } } Subject: Re: Interesting TCP issue } } Steve Watt wrote: } } > One of my users is having trouble receiving mail from Skype. So, } } > after some sniffing, I discovered this: } } > } } > # tcpdump -vv -s 1500 -i dc0 -X net 213.244.128.0/18 } } > tcpdump: lestening on dc0, link-type EN10MB (Ethernet), capture size 1500 bytes } } > 13:18:13.607493 IP (tos 0x20, ttl 58, id 12896, offset 0, flags [DF], proto: TCP (6), length: 74) share.skype.net.50406 > wattres.watt.com.smtp: P, cksum 0x9297 (correct), 4072464914:4072464936(22) ack 1248591103 win 46 <nop,nop,timestamp 2511885672 520058954> } [ sneck ] } } > } } > And no responses from my system. } } > } } > Interesting. I presume it has something to do with the } } > idiotically small window the remote server is advertising. So I } } > set net.inet.tcp.minmss down to 46, and that resulted in a RST } } > being spit back to skype's server when its retransmit happened. } } > [...] } } } } turn off window scaling (I forget the sysctl) and see if that helps } } It's broken in some versions of freeBSD at least. } } Duh, should've mentioned the version: } } FreeBSD wattres.Watt.COM 6.2-PRERELEASE FreeBSD 6.2-PRERELEASE #6: Tue Dec 26 11:46:36 PST 2006 root@wattres.Watt.COM:/usr/obj/usr/src/sys/WATTRES i386 } } I did the cvsup just before the build time above. } } I just turned off net.inet.tcp.rfc1323; we'll see if that helps on the } next polling attempt by skype's server. We have a winner -- setting net.inet.tcp.rfc1323=0 let the mail message come in on the next try. p0f's guess at the remote machine is that it's a newer Linux 2.6 box; that doesn't seem like an interoperability problem that should've slipped through -BETA, given how common those are... The exchange with rfc1323 looks completely normal, with the remote end giving windows of 5840. But it ended the conversation in a very Windows way by sending a couple of RSTs after the FIN exchange. Or has that brokenness now extended itself to Linux as well? -- Steve Watt KD6GGD PP-ASEL-IA ICBM: 121W 56' 57.5" / 37N 20' 15.3" Internet: steve @ Watt.COM Whois: SW32-ARIN Free time? There's no such thing. It just comes in varying prices...
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