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Re: Common operational misconceptions
From: Masataka Ohta <mohta () necom830 hpcl titech ac jp>
Date: Tue, 21 Feb 2012 09:16:53 +0900

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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