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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Message Formatting and Data Transfer

TCP Checksum Calculation and the TCP "Pseudo Header" 1 2 3 TCP Sliding Window Data Transfer and Acknowledgement Mechanics

Specifying a Non-Default MSS Value

Naturally, there are likely to be cases where the default MSS is non-ideal, so TCP provides a means for a device to specify that the MSS it wants to use is either smaller or larger than the default value of 536. A device can inform the other of the MSS it wants to use during the connection establishment process. A device that chooses to do so includes in its SYN message the TCP option called, appropriately, Maximum Segment Size. The other device receives this option and records the MSS for the connection. Each device can specify the MSS it wants for the segments it receives independently.

Devices may wish to use a larger MSS if they know for a fact that the MTUs of the networks the segments will pass over are larger than the IP minimum of 576. This is most commonly the case when large amounts of data are sent on a local network; the process of path MTU discovery is used to determine the appropriate MSS. A smaller MSS might be advisable if it were known that a particular optional feature was in place that would consistently increase the size of the IP header. Employing IPSec for security would be a good example.

TCP Checksum Calculation and the TCP "Pseudo Header" 1 2 3 TCP Sliding Window Data Transfer and Acknowledgement Mechanics