Date: Thu, 14 Feb 2013 10:37:23 +0900 From: YongHyeon PYUN <pyunyh@gmail.com> To: Jeremy Chadwick <jdc@koitsu.org> Cc: freebsd-stable@freebsd.org, Eugene Grosbein <egrosbein@rdtc.ru>, yongari@freebsd.org Subject: Re: Unusual TCP/IP Packet Size Message-ID: <20130214013723.GB2945@michelle.cdnetworks.com> In-Reply-To: <20130213130059.GA57337@icarus.home.lan> References: <96AE8BD1-79C2-4743-854F-B8386C54E4A1@lafn.org> <511B6B21.5030606@rdtc.ru> <20130213130059.GA57337@icarus.home.lan>
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On Wed, Feb 13, 2013 at 05:00:59AM -0800, Jeremy Chadwick wrote: > On Wed, Feb 13, 2013 at 05:29:53PM +0700, Eugene Grosbein wrote: > > 13.02.2013 17:25, Doug Hardie ??????????: > > > Monitoring a tcpdump between two systems, a FreeBSD 9.1 system has the following interface: > > > > > > msk0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500 > > > options=c011b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,TSO4,VLAN_HWTSO,LINKSTATE> > > > ether 00:11:2f:2a:c7:03 > > > inet 10.0.1.199 netmask 0xffffff00 broadcast 10.0.1.255 > > > inet6 fe80::211:2fff:fe2a:c703%msk0 prefixlen 64 scopeid 0x1 > > > nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL> > > > media: Ethernet autoselect (100baseTX <full-duplex,flowcontrol,rxpause,txpause>) > > > status: active > > > > > > > > > It sent the following packet: (data content abbreviated) > > > > > > 02:14:42.081617 IP 10.0.1.199.443 > 10.0.1.2.61258: Flags [P.], seq 930:4876, ack 846, win 1040, options [nop,nop,TS val 401838072 ecr 920110183], length 3946 > > > 0x0000: 4500 0f9e ea89 4000 4006 2a08 0a00 01c7 E.....@.@.*..... > > > 0x0010: 0a00 0102 01bb ef4a ece1 680b ae37 1bbc .......J..h..7.. > > > 0x0020: 8018 0410 3407 0000 0101 080a 17f3 8ff8 ....4...??????. > > > > > > > > > The indicated packet length is 3946 and the load of data shown is that size. The MTU on both interfaces is 1500. The receiving system received 3 packets. There is a router and switch between them. One of them fragmented that packet. This is part of a SSL/TLS exchange and one side or the other is hanging on this and just dropping the connection. I suspect the packet size is the issue. ssldump complains about the packet too and stops monitoring. Could this possibly be related to the hardware checksums? > > > > You have TSO enabled on the interface, so large outgoing TCP packet is pretty normal. > > It will be split by the NIC. Disable TSO with ifconfig if it interferes with your ssldump. > > This is not the behaviour I see with em(4) on a 82573E with all defaults > used (which includes TSO4). Note that Doug is using msk(4). > > I can provide packet captures on both ends of a LAN segment using both > tcpdump (on the FreeBSD side) and Wireshark (on the Windows side) that > show a difference in behaviour compared to what Doug sees. This is strange. tcpdump sees a (big) TCP segment right before controller actually transmits it. So if TSO is active for the TCP segment, you should see a series of small TCP packets on receiver side(i.e. 3 TCP packets in Doug's case). If you don't see a big TCP segment with tcpdump on TX path, probably TSO was not used for the TCP segment. It's possible for controller to corrupt the TCP segment during segmentation but Doug's tcpdump looks completely normal to me since tcpdump sees the segment before TCP segmentation. > > What I see on the FreeBSD side with tcpdump is repeated "bad-len 0" > messages for payloads which are chunked or segmented as a result of TSO. > I do not see a 1:1 ratio of "bad-len" entries to chunked payloads; I > only see one "bad-len" entry for all chunks (up until the next ACK or > PSH+ACK of course). > I vaguely recall that some users reported similar TSO issues on various drivers. The root cause of the issue was not identified though. Personally I couldn't reproduce the issue at that time. It could be a driver or network stack bug. > The important part: I do not see captured TCP packets reporting a length > greater than MTU (or MSS for that matter (remember: IP header + TCP > header + MSS <= MTU). > > Also note for Doug: remember that if you're doing packet captures > between two devices that have NAT involved, you may see different > behaviour. Example case: > > 03:58:47.907582 IP 67.180.84.87.2983 > 206.125.172.42.80: Flags [.], ack 13419, win 64240, length 0 > 03:58:47.907649 IP 206.125.172.42.80 > 67.180.84.87.2983: Flags [.], seq 17799:19259, ack 292, win 1026, length 1460 > 03:58:47.907679 IP 206.125.172.42.80 > 67.180.84.87.2983: Flags [.], seq 19259:20719, ack 292, win 1026, length 1460 > 03:58:47.912546 IP 67.180.84.87.2983 > 206.125.172.42.80: Flags [.], ack 16339, win 64240, length 0 > > In the above example there's a Linux NAT router (67.180.84.87) with a > client (192.168.1.50) behind it, talking to 206.125.172.42. MTU is 1500 > (I obviously didn't include the initial SYN :-) ). > > -- > | Jeremy Chadwick jdc@koitsu.org | > | UNIX Systems Administrator http://jdc.koitsu.org/ | > | Mountain View, CA, US | > | Making life hard for others since 1977. PGP 4BD6C0CB |
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