The maximum segment size (MSS) 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 count the TCP header or the IP header (unlike, for example, the MTU for IP datagrams).[1]: §3.7.1  The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment.

To avoid fragmentation in the IP layer, a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes. Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576 - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280 - 40 - 20).[1]: §3.7.1 

Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead.[2]

Each direction of data flow can use a different MSS.

For most computer users, the MSS option is established by the operating system.

TCP options size (Variable 0–320 bits, in units of 32 bits) must be deducted from MSS size if TCP options are enabled. For example, TCP Time Stamps are enabled by default on Linux platforms.

Default value

The default TCP Maximum Segment Size is for IPv4 is 536. For IPv6 it is 1220.[1]: §3.7.1  Where a host wishes to set the maximum segment size to a value other than the default, the maximum segment size is specified as a TCP option, initially in the TCP SYN packet during the TCP handshake. The value cannot be changed after the connection is established.

Inter-Layer Communication

In order to notify MSS to the other end, an inter-layer communication is done as follows:[3]: §11 

While sending TCP segments to the other end, an inter-layer communication is done as follows:[3]: §11 

MSS and MTU

MSS is sometimes conflated with MTU/PMTU, which is a characteristic of the underlying link layer, while MSS applies specifically to TCP and hence the transport layer. The two are similar in that they limit the maximum size of the payload carried by their respective protocol data unit (frame for MTU, TCP segment for MSS), and related since MSS cannot exceed the MTU for its underlying link (taking into account the overhead of any headers added by the layers below TCP). However, the difference, in addition to applying to different layers, is that MSS can have a different value in either direction and also that frames exceeding the MTU may cause packets (which encapsulate segments) to be fragmented by the network layer, while segments exceeding the MSS are simply discarded.

Further reading

See also

References

  1. ^ a b c W. Eddy, ed. (August 2022). Transmission Control Protocol (TCP). Internet Engineering Task Force. doi:10.17487/RFC9293. ISSN 2070-1721. STD 7. RFC 9293. Internet Standard 7. Obsoletes RFC 793, 879, 2873, 6093, 6429, 6528 and 6691. Updates RFC 1011, 1122 and 5961.
  2. ^ The TCP/IP Guide, TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size
  3. ^ a b J. Postel (November 1983). The TCP Maximum Segment Size and Related Topics. Network Working Group. doi:10.17487/RFC0879. RFC 879. Obsolete. Obsoleted by RFC 7805 and 9293. Updated by RFC 6691.
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