Re: Outgoing packets larget than MTU

[Bill Fink]

On Wed, 26 Mar 2008, Pavel Georgiev wrote:

> I have several linux boxes hooked to a gigabit switch and accidentally noticed 
> that tcpdump reports packages with size larger than 1500:
> 
> tcpdump greater 1600 -n
> 
> This produces many lines of output with packets with size > 1500 (2960, 4420, 
> etc.)
> 
> The MTU of the interface is 1500 (reported by ifconfig), the MTU on the other 
> boxes hooked to the same switch is also 1500.
> 
> This would be OK if it was limited to the intranet only (although strange as 
> the MTU is 1500 on all boxes), but this also happens for packets going out to 
> the Internet. I traced a tcp connection to an outside host on both sides and 
> notices that both hosts send an mss of 1460 during connection initialization, 
> but the first packet (and all other) that the box in question sends is with 
> length 2960, which on the other side is received and acknowledged as two 
> packets with length 1500. This suggest that someone on the way (probably the 
> first router) does fragmentation, which is OK. The problem is that eventually 
> packets on the remote host start to arrive with changed order, which tcp 
> handles as expected, but this somehow prevents the tcp window from growing 
> and as a result the transfer rate never gets high enough to utilize the line. 
> 
> What I`m trying to find out is how come packets leaving my box have larger 
> size than the MTU of interface (I`m guessing that if I solve that, no 
> additional fragmentation will occur, packets will arrive in the right order 
> and tcp windows will grow as expected).
> 
> Few notes:
> - kernel on the boxes in question:   2.6.14.6 and  2.6.15
> - the line to the boxes is wide enough to handle 100Mbit/s, if I try the same 
> test that I describe but in the other direction, I get the full 100Mbit/s 
> speed as tcp window grows high enough to allow this even with latency > 
> 100ms.
> - boxes in question use e1000 driver.

Perhaps try disabling TSO on the interface with ethtool.

					-Bill
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