Common operational misconceptions

George Bonser gbonser at
Tue Feb 21 02:11:03 UTC 2012

> -----Original Message-----
> From: Masataka Ohta

> First, it sets eff_pmtu to 1400B. OK? 

Where did you get 1400 from?  Are you talking specifically with the linux implementation?  

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

I suppose that depends on a specific implementation of "close enough" is.  I haven't looked at the specific code linux uses to implement this and "close enough" is fairly subjective and can be interpreted in different ways by different people.  But even 1400 on, say, a 1420 MSS ICMP black hole is one heck of a lot better than running no PMTUD at all and running at something under 600 bytes.

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

I am wondering where you got this magic 1400 value from.  It should basically zero in on a number pretty close to the real path MSS in a few iterations.  Maybe that one specific implementation stops at the first successful value, but that isn't the way the recommendation is written.

Did I say it was "perfect"?  No, but the notion that PMTUD is "broken" or "doesn't work" isn't exactly true.  With the old mechanism, such a connection would simply hang or force people to turn off PMTUD completely.  The new mechanism actually seems to perform rather well in the face of an ICMP black hole.

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