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From: Doug Hardie <bc979@lafn.org>
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Subject: Unusual TCP/IP Packet Size
Date: Wed, 13 Feb 2013 02:25:46 -0800
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Monitoring a tcpdump between two systems, a FreeBSD 9.1 system has the =
following interface:

msk0: flags=3D8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu =
1500
	=
options=3Dc011b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,TSO4,VLAN_HWTSO,LINK=
STATE>
	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=20
	nd6 options=3D29<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...=85=85.


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?