Maximum segment size

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.[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 header size and TCP header sizes.[2] Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576[3] - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280[4] - 40 - 20).

Small MSS values will reduce or eliminate IP fragmentation, but will result in higher overhead.[5]

Each direction of data flow can use a different MSS.

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

Default value

The default TCP Maximum Segment Size is 536.[6] 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.[7]

Inter-Layer Communication

In order to notify MSS to the other end, an inter-layer communication is done as follows:[8]

While sending TCP segments to the other end, an inter-layer communication is done as follows:[9]

MSS and PMTU

MSS is sometimes mistaken for PMTU. MSS is a concept used by TCP in the Transport layer and it specifies the largest amount of data that a computer or communications device can receive in a single TCP segment. While PMTU is used by the IP layer and it specifies the largest packet size that can be sent over this path without suffering fragmentation.

Further reading

See also

References

  1. RFC 879, page 2, Section 3, "The MSS counts only data octets in the segment, it does not count the TCP header or the IP header."
  2. RFC 2460, page 28, Section 8.3
  3. RFC 879, page 1, Section 1
  4. RFC 2460, page 24, Section 5
  5. The TCP/IP Guide, TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size
  6. RFC 879, page 1, Section 1, "The default TCP Maximum Segment Size is 536."
  7. RFC 793, page 19, Section 3.1, "This field must only be sent in the initial connection request (i.e., in segments with the SYN control bit set)."
  8. RFC 879, page 1, Section 11
  9. RFC 879, page 1, Section 11
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