Understanding TCP MSS on ASR9K IOS XR

[EVERTON DA SILVA MARQUES]

I am seeing this information for an specific iBGP session:

ASR9K#show tcp detail pcb all

(...snip...)

Datagrams (in bytes): MSS 1240, peer MSS 9152, min MSS 1240, max MSS 1240

Does it mean that IOS XR is actually limiting its own outgoing TCP segments to a maximum size of MSS 1240 (the first MSS value in the output line) ?

Thanks,

Everton

[Jean-Francois Dube]

Hi Everton,

Yes! ASR9K is using 1240 bytes MSS for this TCP session.

My understanding is that IOS XR default MSS is 1240 bytes.

You can enable PMTUD (tcp path-mtu-discovery) to let the device find MTU of path to foreign host. With this knowledge the device should use MSS appropriate for MTU.

Hope this helps.

Cheers,

JFD