Is MSS value fixed in SYN packet?

Question

I wonder how MSS is set in SYN packet? Is it a fixed value in one operating system or the value could be changed in the same operating system? I know that the value is different in different operating systems. Also is the MSS value in SYN related to hardware configuration? Thanks.

Answer 1

RFC 879 describes how MSS is used and specified.

In short, MSS is specified during TCP handshake via SYN packet. However, this value can later be changed by OS itself or by setting a protocol option.

You can set option TCP_MAXSEG via setsockopt.

Answer 2

Whilst the value of MSS in SYN and SYNACK packets are set by the initiator and responder side, respectively, a widely used practice known as MSS clamping can result in the MSS being altered by a network element on the path - this is often used to reduce the MSS of all connections going over some sort of tunnelled link. For example PPPoE is commonly used on residential broadband and requires an MTU of 1492 and corresponding IPv4 MSS of 1452 so whilst the SYN may leave your machine with an MSS of 1460 (assuming you're using Ethernet with an MTU of 1500) but once it passes the MSS clamping ISP router the MSS in SYN packet will subsequently be changed to 1452, and likewise for the incoming/responder's SYNACK packet so the connection proceeds with reduced MSS of 1452. This practice seems to be used instead of Path MTU Discovery which relies upon the use of ICMP Fragmentation Needed responses from the network as these can be lost on poorly configured networks and by certain load balancing techniques.