TCP SYN-ACK MSS wrong?

[rdhw]

RB450G, RouterOS 5.11

For good reason, I have assigned an MTU of 1432 to an interface in the RouterBoard, instead of the default 1500 (the interface is on an internal bridge). I can verify that this setting has been acted upon, because that interface now sends TCP packets with a maximum size of 1392 (=1432-40). All well and good.

However, when I send that interface a TCP SYN request, it responds with a SYN-ACK offering an MSS of 1460.

This seems wrong to me. It ought to be sending a SYN-ACK offering an MSS of 1392.

Is this a bug in RouterOS? Or is there a setting that I have missed?

[jandafields]

@rdhw

RB450G, RouterOS 5.11

For good reason, I have assigned an MTU of 1432 to an interface in the RouterBoard, instead of the default 1500 (the interface is on an internal bridge). I can verify that this setting has been acted upon, because that interface now sends TCP packets with a maximum size of 1392 (=1432-40). All well and good.

However, when I send that interface a TCP SYN request, it responds with a SYN-ACK offering an MSS of 1460.

This seems wrong to me. It ought to be sending a SYN-ACK offering an MSS of 1392.

Is this a bug in RouterOS? Or is there a setting that I have missed?

I think that changing the MTU does not automatically change the MSS. So, the MSS of 1460 is accurate for an MTU of 1500 (default).

You can change the MSS manually to whatever you want. See this post for some similiar examples:
http://forum.mikrotik.com/t/pppoe-and-mtu-mru-mss-settings/49115/1