[clue] a networking riddle

[Mike Bean]

Well, I don't mean OU in the windows sense, I mean logical groupings of
servers.  Clusters.   I'm trying to get my head around this, because
intuitively, it just doesn't make any sense.

If you're coming home from the grocery store, and you have more groceries
then you can carry, you make multiple trips to the car.

but I don't think it's quite that simple.  Think networks aren't as smart
as I think they are,  intuitively, something configured to send as 1500,
should be able to receive 1500 only, not necessary 9000, but something
that's configured as 9000 should be able to send and receive as 9000 or
1500.   (scratches head.)

In actuality, B (9000) can send to A (1500)
and A (1500) can send to B (9000)

but if we set A to 9000, it can no longer send to B (9000)
(scratches head)

Boy, if you know someone who's in school looking for a secure job, tell
them to become a network person.    REALLY GOOD network people are worth
their weight in gold.


On Mon, Oct 14, 2013 at 3:26 PM, Quentin Hartman <qhartman at gmail.com> wrote:

> 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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