[GTALUG] Calling all networking and SVN gurus

[James Knott]

On 2019-06-08 04:26 PM, Jamon Camisso via talk wrote:
> First thing I'd look at is MTU between Jenkins and the remote server. If
> there's some route churn you could conceivably end up with different
> MTUs which can lead to inconsistent fragmentation or timeouts. With a
> large SVN repo and lots of propfind requests, the overhead of a bad MTU
> somewhere along the line would be quite noticeable. Try tracepath &
> tracepath6 to see what things look like between the hosts.


IP has been designed to work with different size MTU from the
beginning.  Many years ago, 576 bytes was common on dial up
connections.  These days 1492 is common for ADSL, as well as the
commonly used 1500 on cable modems, etc..  It's even possible to have
9000 bytes on a network and, back when I was at IBM in the late 90s, we
used token ring with 4K bytes MTU, IIRC.  Routers would fragment the
packets to accommodate the changes in MTU along the path and TCP will
negotiate the maximum segment size, based on the smallest MTU at each
end.  These days fragmentation has been largely replaced with path MTU
discovery, where a change to a smaller MTU will cause an ICMP message,
back to the source, advising of the maximum usable MTU.  PMTUD is
mandatory on IPv6.

Bottom line, fragments are unlikely to be an issue as all modern OSs use
PMTUD on TCP and Linux uses it on everything.