[clue] a networking riddle

Quentin Hartman qhartman at gmail.com
Mon Oct 14 15:26:04 MDT 2013


By OU, do you mean broadcast domain? ie - two LANs connected by a router?
If so, it sounds like you have the (usually default) MTU of 1500 set on
side A, but the 9000 set on B, and your router isn't correctly doing MTU
management when crossing from one to the other.

Going the other way works because the fragments are already small enough to
be handled correctly at the other end. The complexity of making this work
reliably is the primary reason so few people bother to use jumbo frames,
even though they technically are better most of the time anymore.

The wikipedia article on MTU is actually pretty good:
http://en.wikipedia.org/wiki/Maximum_transmission_unit

QH


On Mon, Oct 14, 2013 at 3:15 PM, Mike Bean <beandaemon at gmail.com> wrote:

>
> So I've been scratching my head for a long time trying to understand this,
> and it makes no sense to me.   We have to discrete OU's.   SCP
> transmissions of any real size tend to fail from A to B, but not from B to
> A.
>
> My colleagues have been arguing that the fix is to set the MTU at side B
> to 1500.  And Lo, and behold, when we do, it works, increase the MTU to
> 9000, and it fails again.
>
>
> http://unix.stackexchange.com/questions/14187/why-does-scp-hang-on-copying-files-larger-than-1405-bytes
>
> What I have difficulty understanding, is if a jumbo frame carries, say,
> 9000 bytes, and hypothetically, if it's a 5,000 bytes file + your MTU =
> 1500, which means you split it up into 3 transmissions.
>
> Same size transmission; capped packet size,  more packets.   So I would
> naturally conclude that at an MTU of 9000 it could get done in 1 what an
> MTU of 1500 would do in 3?
>
> (scratches head)
>
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