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Perhaps my knowledge of ethernet is outdated, but I thought this was not posible, as the MTU on the NIC of both servers is 1500, and jumbo frames are only 9000, right?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