Common operational misconceptions

Masataka Ohta mohta at necom830.hpcl.titech.ac.jp
Mon Feb 20 18:16:53 CST 2012


George Bonser wrote:

> I am saying that MTU probing works just fine, even with a
> link in between that has a shorter MTU and doesn't pass
> ICMP.

And I have been saying your statement is unfounded.

> I actually have one of those.

I can't see any.

> It actively probes with packets of varying sizes and learns
> what the path MTU is.

First, it sets eff_pmtu to 1400B. OK?

> It does not rely on ICMP.  Again, it actively probes with
> varying sizes of packets until it believes it has
> "converged".

>     The initial value for search_high SHOULD be the largest possible
>     packet that might be supported by the flow.  This may be limited by
>     the local interface MTU, by an explicit protocol mechanism such as
>     the TCP MSS option,

OK, so, even with local MTU of 7500B and without ICMP PTB, if
local MTU of your peer is 1500B, TCP MSS makes search_high
1500B.

As eff_pmtu of 1400B is close enough to search_high, you are
done.

Eff_pmtu of 1400B will be used forever with no probe packets
sent.

> The timer for Linux is 5 minute by default but you can change it.

Timer timeouts do not affect TCP MSS.

						Masataka Ohta

PS

Your lengthy quotation means you don't see the point.



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