[c-nsp] MTU and PMTUD

[Marcin Kurek]

> I may have misunderstood you. Ddi you have 8936 configured on both
> ends? I thought you had only 8936 on the CSR.
>
> How I understood it:
>
> *Dec  8 11:17:15.453: TCP0: Connection to 12.0.0.7:179, advertising MSS
8936
> *Dec  8 11:17:15.456: TCP: tcb 7FFB9A6D64C0 connection to
> 12.0.0.7:179, peer MSS 1240, MSS is 1240
>
> Local 12.0.0.13 CSR is advertising 8936 to remote 12.0.0.7
> Remote 12.0.0.7 is advertising 1240
> We settle to 1240

> I guess you are saying the remote 12.0.0.7 is as well configured to
> 8936?

Exactly. This is the configuration I have on 12.0.0.7:

neighbor 12.0.0.13
  remote-as 12
  tcp mss 8936
  update-source Loopback0
  address-family vpnv4 unicast
  !
  address-family ipv4 rt-filter
  !
  address-family l2vpn evpn

So my understanding *was* that this command is enforcing MSS=8936, no
matter if it's SYN or SYN ACK.

> Then I agree, I wouldn't expect that, and can't explain it.
> Almost sounds like a bug where the configuration command is only read
> when IOS-XR establishes the connection, but not when it receives it?

That's my thought as well. But this is strange as I have 7.5.2 and 6.7.3 in
the lab and the behavior is the same.

I guess it's time to ask TAC friends :)

Thanks!

Kind regards,
Marcin

czw., 8 gru 2022 o 10:50 Saku Ytti <saku at ytti.fi> napisał(a):

> On Thu, 8 Dec 2022 at 11:40, Marcin Kurek <md.kurek at gmail.com> wrote:
> >
> > >  https://www.rfc-editor.org/rfc/rfc8654.html
> >
> > Thanks!
> >
> > > But it was [8936, 1240].min - so it was 'negotiated' here to the
> > > smallest? If you change the 8936 end to 1239, then that will be used,
> > > regardless who starts it.
> >
> > Yes, but why would XR advertise 1240 if I'm using 'tcp mss 8936' for
> that neighbor?
> > Initially I thought that the whole point of this command is to set MSS
> to a fixed value of our choice.
>
> I may have misunderstood you. Ddi you have 8936 configured on both
> ends? I thought you had only 8936 on the CSR.
>
> How I understood it:
>
> *Dec  8 11:17:15.453: TCP0: Connection to 12.0.0.7:179, advertising MSS
> 8936
> *Dec  8 11:17:15.456: TCP: tcb 7FFB9A6D64C0 connection to
> 12.0.0.7:179, peer MSS 1240, MSS is 1240
>
> Local 12.0.0.13 CSR is advertising 8936 to remote 12.0.0.7
> Remote 12.0.0.7 is advertising 1240
> We settle to 1240
>
> I guess you are saying the remote 12.0.0.7 is as well configured to
> 8936? Then I agree, I wouldn't expect that, and can't explain it.
> Almost sounds like a bug where the configuration command is only read
> when IOS-XR establishes the connection, but not when it receives it?
>
> --
>   ++ytti
>