Unusual TCP/IP Packet Size

[Doug Hardie]

On 13 February 2013, at 02:29, Eugene Grosbein <egrosbein at rdtc.ru> wrote:

> 13.02.2013 17:25, Doug Hardie пишет:
>> 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..... at .@.*.....
>> 	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.

Thanks.  Now all the packets are 1500 or under.  They all are received with a SSL header.