Maximum Segment Size

The Maximum Segment Size (MSS ) is defined in a computer network the maximum number of bytes that can be sent as payload in a TCP segment.

Background

While connecting, first tells the client to the server with its MSS value. The server then responds with its MSS value. Both MSS requirements are not synchronized. As in IPv4, the IP and TCP header together at least 40 bytes in size, the MSS must be there at least 40 bytes smaller than the Maximum Transmission Unit (MTU ) to avoid fragmentation of the IP packets. With IPv6, IP and TCP headers are together at least 60 bytes in size, so here the MSS must be smaller than the MTU of 60 bytes.

MSS Clamping

On routers that connect networks with different MTUs, MSS Clamping is often used. This is a process in which the router is reduced in the TCP packet to establish a connection, the MSS. They must be reduced either to a specified value or determined by the MTU of the network with the smaller MTU. By this method, only the packets are dispatched to the TCP connection in both directions, which must not be fragmented or discarded because they were too big.

However, this method is a hack. It violates the layer model of the Internet protocol suite, because a router changed on network layer data of the transport layer. Moreover, this method only works for TCP, other protocols such as UDP do not use MSS.

With DSL routers MSS Clamping is used as the MTU of the Internet connection is reduced by the PPPoE.