Mixed MTU hosts on a network

[Roman Mamedov]

On Fri, 16 Mar 2018 15:53:43 +0500
Roman Mamedov <rm at romanrm.net> wrote:

> But guess what, turns out that didn't work either. Tried both OUTPUT and
> POSTROUTING chains on the "mangle" table, and set-mss all the way down to
> 1220, no matter what, the iperf3 output looked the same as before.

Actually the iptables bit is easy to explain. Even if initial MSS is forced
to a low value on the sender, it's get negotiated back up to the maximum value
according to MTU on the receiver (changed both IPs since then):

21:13:38.641531 IP6 fd39:30::f5a8:e923:f8cd:24b5.40052 > fd39:30::e84f:942d:7f93:ddc1.5001: Flags [S], seq 2397878391, win 27200, options [mss 1220,sackOK,TS val 566161815 ecr 0,nop,wscale 9], length 0
21:13:38.641574 IP6 fd39:30::e84f:942d:7f93:ddc1.5001 > fd39:30::f5a8:e923:f8cd:24b5.40052: Flags [S.], seq 1221117548, ack 2397878392, win 26800, options [mss 1352,sackOK,TS val 2726162536 ecr 566161815,nop,wscale 9], length 0
21:13:38.716047 IP6 fd39:30::f5a8:e923:f8cd:24b5.40052 > fd39:30::e84f:942d:7f93:ddc1.5001: Flags [.], ack 1, win 54, options [nop,nop,TS val 566161889 ecr 2726162536], length 0
21:13:38.716444 IP6 fd39:30::f5a8:e923:f8cd:24b5.40052 > fd39:30::e84f:942d:7f93:ddc1.5001: Flags [P.], seq 1341:1605, ack 1, win 54, options [nop,nop,TS val 566161889 ecr 2726162536], length 264
21:13:38.716458 IP6 fd39:30::e84f:942d:7f93:ddc1.5001 > fd39:30::f5a8:e923:f8cd:24b5.40052: Flags [.], ack 1, win 55, options [nop,nop,TS val 2726162611 ecr 566161889,nop,nop,sack 1 {1341:1605}], length 0

So the other side really needs to have a proper MTU set. And the highest working
wg0 MTU on PPPoE turned out to be 1408, not 1412 as I assumed. As for why 1412
also works but only if set on the sender side, I've no explanation for that yet.

-- 
With respect,
Roman