[NANOG] Microsoft.com PMTUD black hole?

Nathan Anderson/FSR nathana at fsr.com
Wed May 7 21:47:13 UTC 2008


Iljitsch van Beijnum wrote:

> The problem is in the direction from M$ to you, so you can't fix that 
> from your end. I wonder if they've installed SP3 on their servers...

Ah, you are right.  I re-read the section on black-hole detection in
http://technet.microsoft.com/en-us/library/bb878081.aspx
more closely this time, and found that, yes, it only helps if the host 
trying to send the large packets has the feature enabled:

"When PMTU black hole router detection is enabled, TCP tries to send 
segments with the DF flag set to 0 after several retransmissions of a 
segment are not acknowledged. If a segment with the DF flag set to 0 is 
acknowledged, the MSS is decreased and the DF flag is set to 1 in 
subsequent segments on the connection. Enabling PMTU black hole 
detection increases the maximum number of retransmissions that are 
performed for a given segment, and therefore has an effect on overall 
performance."

I for some reason interpreted the advertisement of the black hole 
detection feature as being a help to clients impacted by the inability 
of the server to perform PMTUD.

-- 
Nathan Anderson
First Step Internet, LLC
nathana at fsr.com




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