[c-nsp] MTU and PMTUD

[Marcin Kurek]

> We set the 'physical' MTU (In IOS-XR+Junos L2 but no CRC is in this
> humber) as high as it went when the decision was made. Today you can
> do I think 10k in Cisco8k and 16k in Juniper. We do not seet MPLS or
> IP MTUs separately in Core. On the edge you should always set L3 MTU,
> because you want to have the ability to add subinterfaces with large
> MTU, so physical MTU must be large, as change will affect all
> subinterfaces. This way you can later add big MTU subint, without
> affecting other subints.

Thanks. Actually, I should have said "core-facing" interfaces in the edge :)

XR without enabled PMTUD (default setting) means ~1240 bytes available 
for TCP payload.

That seems to be a bit small, did you perform any kind of performance 
testing to see the difference between defaults and let's say 9000 for BGP?

I'm thinking about RRs in particular, higher MTU (9000 vs 1200) should 
result in some performance benefit, at least from CPU point of view. I 
haven't tested this though.


> No, not at this time, our BGP transfer performance is limited by TCP
> window-size, so larger packets would not do anything for us. And LDP
> has a trivial amount of stable data so it doesn't matter.

Agree. Thing is, PMTUD on XR is a global setting, so it does affect all 
TCP based protocols.