Subject: [tsvwg] MSS clamping interfering with path MTU discovery (draft-ietf-tsvwg-transport-encrypt-01)

From: Frode Kileng <[email-masked]>
Date: Mon, 5 Nov 2018 08:55:34 +0100

Hi.

I wonder if the authors could provide examples of MSS clamping 
interfering with MTU discovery as was mentioned at the meeting today and 
was added to the -01 version of the draft. If it's just a theoretical 
possibility I would suggest to remove this.

AFAIK MSS clamping is a practice only used by network operators who do 
know their fixed MTU within their network segment and clamps according 
to this value.

In the presentation today, and also at the previous tsvwg meeting, there 
was an expression of surprise that the value used in MSS clamping in 
mobile network is lower than in many other networks. This is just a 
result of tunneling within the mobile core where you need to take into 
consideration the (variable) size GTP header. Some mobile networks may 
use additional tunneling and thereby clamping to an even lower value.

5 of 13 mobile networks I tested did MSS clamping with a value ranging 
from 1360-1420.

Best regards

Frode Kileng