[GTALUG] Calling all networking and SVN gurus

[James Knott]

On 2019-06-08 05:08 PM, Jamon Camisso via talk wrote:
> On 6/8/19 4:50 PM, James Knott via talk wrote:
>> Bottom line, fragments are unlikely to be an issue as all modern OSs use
>> PMTUD on TCP and Linux uses it on everything.
> True enough, but it is also easy to check and determine whether it is an
> issue. I get a ticket or two a month with remote employees who are
> connecting from strange places, or have issues with VPNs, and quite a
> few are MTU related.
>
> I'm curious about PMTUD now: my understanding is that ICMP needs to be
> unrestricted between server & client. If something is blocking that
> traffic how does it work? Also how does PTMUD handle asymmetric paths?
>

The ICMP message would be sent to the source, so asymmetric paths would
not be an issue.  There is also provisions for when ICMP is blocked. 
Take a look at IPv4 traffic with Wireshark.  You'll see the do not
fragment flag is set on TCP in Windows and on everything in Linux.  This
means routers are not supposed to fragment.

https://en.wikipedia.org/wiki/Path_MTU_Discovery.