Caching learned MSS/MTU values

Hannes Frederic Sowa hannes at
Fri Oct 18 09:31:23 CEST 2013

On Thu, Oct 17, 2013 at 09:05:24AM -0700, Jason Fesler wrote:
> I'm once again considering trying to improve on the PMTUD
> failure detection. Due to limitations on the client side I can't use raw
> sockets to generate test packets. The client is JavaScript and runs in a
> browser; all I can do is try fetching urls from multiple locations, each
> with a different MTU.
> I know that the various operating systems tend to cache any PMTUD issues
> that they can detect; future connections to that destination will use
> smaller packets accordingly. What I can not see to find is an adequate
> description of what granularity this gets cached with. /128? /64?   Also, I
> the absence of Packet Too Big messages, what does each OS do?

Linux, too, does cache on /128 basis. In the absence of PTB the connection
will get stuck. ;)



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