What is MSS clamping OpenWrt?

I found this video really helpful in explaining why wan firewall zones use MSS Clamping by default in OpenWrt. In short, it prevents round-trips to a remote server when possible by marking TCP packets with the MTU size of the local client when they go out of the router.

What is MSS clamping?

TCP MSS clamping is a feature that sets the maximum segment size used by a TCP session. The way that it achieves this is during the TCP 3 way handshake, a server can set the MSS in the outgoing TCP SYN packets signalling the maximum segment size of the data packets that it can receive.

What is MSS in router?

MSS, or maximum segment size, is the largest data payload that a device will accept from a network connection.

What is the difference between MTU and MSS?

TCP uses MTU (Maximum Transmission Unit) for identifying the maximum size of any data packet and MSS (maximum segment size) for specifying the maximum size of packets that can be sent over a network.

How is MSS calculated?

This is the MTU minus the IP header length (MDDS = MTU – IPHdrLen). When opening a connection, TCP can send an MSS option with the value equal to: MDDS – TCPHdrLen. In other words, the MSS value to send is: MSS = MTU – TCPHdrLen – IPHdrLen.

Can MSS be bigger than MTU?

MTU is maximum IP packet size of a given link. MSS is Maximum TCP segment size. MTU is used for fragmentation i.e packet larger than MTU is fragmented. But in case of MSS, packet larger than MSS is discarded.

Is MSS negotiated?

Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host .”

Does MSS include TCP header?

TCP MSS, the maximum segment size, is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not include the TCP header or the IP header.