large ethernet frames on performance test

[idata]

Hi,

I was testing throughput using iperf between 2 servers directly connected. I'm quite happy with the results (~800Mbps) but while using TCP I capture one throuput test and I saw the frame size increasing up to 65KB. Because I capture both sides I can confirm this large frames are arribing to the receiving server.

Perhaps my knowledge of ethernet is outdated, but I thought this was not posible, as the MTU on the NIC of both servers is 1500, and jumbo frames are only 9000, right?

Here is some random samples of a much larger tcpdump. The frame size started at 1514 and increases from there:

on the sender:

05:13:59.580852 IP (tos 0x0, ttl 64, id 12328, offset 0, flags [DF], proto TCP (6), length 65194)

10.195.2.2.60165 > 10.195.1.2.commplex-link: Flags [P.], cksum 0x1727 (incorrect -> 0xd7e9), seq 452419:517561, ack 1, win 46, options [nop,nop,TS val 154182207 ecr 2053727526], length 65142 05:13:59.581697 IP (tos 0x0, ttl 64, id 12375, offset 0, flags [DF], proto TCP (6), length 65194) 10.195.2.2.60165 > 10.195.1.2.commplex-link: Flags [.], cksum 0x1727 (incorrect -> 0x3c62), seq 517561:582703, ack 1, win 46, options [nop,nop,TS val 154182208 ecr 2053727527], length 65142

on the receiver:

05:17:13.679315 IP (tos 0x0, ttl 64, id 31139, offset 0, flags [DF], proto TCP (6), length 65194) 10.195.2.2.60165 > 10.195.1.2.commplex-link: Flags [P.], cksum 0x1727 (incorrect -> 0xe989), seq 208190257:208255399, ack 1, win 46, options [nop,nop,TS val 154184264 ecr 2053729583], length 6514205:17:13.691667 IP (tos 0x0, ttl 64, id 32029, offset 0, flags [DF], proto TCP (6), length 65194)

10.195.2.2.60165 > 10.195.1.2.commplex-link: Flags [.], cksum 0x1727 (incorrect -> 0x77ca), seq 209423797:209488939, ack 1, win 46, options [nop,nop,TS val 154184276 ecr 2053729595], length 65142

Both servers have an Intel 82576 gigabit Ethernet NIC (dual port). Both servers are running a Linux distribution with kernel 2.6.18

Cheers.