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TCP Maximum Segment Size (MSS)

During session connection establishment, two peers, or hosts, engage in negotiations to determine the IP
segment size of packets that they will exchange during their communication. The segment size is based on
the MSS option (maximum segment size) value set in the TCP SYN (synchronize) packets that the peers
exchange during session negotiation. The MSS field value to be used is largely determined by the maximum
transmission unit (MTU) of the interfaces that the peers are directly connected to.

About TCP and MSS

The TCP protocol is designed to limit the size of segments of data to a maximum of number of bytes. The
purpose for this is to constrain the need to fragment segments of data for transmission at the IP level. The
TCP MSS specifies the maximum number of bytes that a TCP packet’s data field, or segment, can contain.
It refers to the maximum amount of TCP data in a single IP datagram that the local system can accept and
reassemble.
A TCP packet includes data for headers as well as data contained in the segment. If the MSS value is set too
low, the result is inefficient use of bandwidth; more packets are required to transmit the data. An MSS value
that is set too high could result in an IP datagram that is too large to send and that must be fragmented.
Typically a host bases its MSS value on its outgoing interface’s maximum transmission unit (MTU) size. The
MTU is the maximum frame size along the path between peers. A packet is fragmented when it exceeds the
MTU size. Because of variation of the MTU size of the interfaces of hosts in the path taken by TCP packets
between two peers, some packets that are within the negotiated MSS size of the two peers might be
fragmented but instead are dropped and an ICMP error message is sent to the source host of the packet.
To diminish the likelihood of fragmentation and to protect against packet loss, you can decrease the TCP MSS.