Common operational misconceptions
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
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
> 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
As eff_pmtu of 1400B is close enough to search_high, you are
Eff_pmtu of 1400B will be used forever with no probe packets
> The timer for Linux is 5 minute by default but you can change it.
Timer timeouts do not affect TCP MSS.
Your lengthy quotation means you don't see the point.
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