Re: Outgoing packets larget than MTU

[Pavel Georgiev]

Yeah, that did the job (at least the part with the packets larger than MTU), I 
still have package disordering but now I can take this to my ISP.

Thanks.

On Wednesday 26 March 2008 18:37:52 Bill Fink wrote:
> 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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