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Maximum segment size
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<p>The maximum segment size (MSS) is a parameter of the <i>Options</i> field of the <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP</a> header that specifies the largest amount of data, specified in <a href="/facts/Bytes/BNHUp9Qq">bytes</a>, that a computer or communications device can receive in a single <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP segment</a>. It does not count the <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP header</a> or the <a href="/facts/IP_header/GnYmfPLq">IP header</a> (unlike, for example, the <a href="/facts/Maximum_transmission_unit/l2FdKoLa">MTU</a> for IP datagrams).: §3.7.1 The <a href="/facts/IP_datagram/TMHVctMz">IP datagram</a> containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several <a href="/facts/IP_fragmentation/efmuF7f2">fragmented</a> pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment. </p><p>To avoid fragmentation in the <a href="/facts/Internet_layer/336eMWJu">IP layer</a>, 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. Though there is no minimum required MSS defined in <a href="/facts/Request_for_Comments/esftTvE7">IETF RFCs</a>, there is a minimum MTU, and so a default MSS is calculated by subtracting the minimum IP and TCP header sizes from the MTU. For example, <a href="/facts/IPv4/bnQ5MEqm">IPv4</a> hosts typically default to an MSS of 536 octets (= 576 − 20 − 20) and <a href="/facts/IPv6/d4dg7kIL">IPv6</a> hosts generally are able to handle an MSS of 1220 octets (= 1280 − 40 − 20).: §3.7.1 </p><p>Small MSS values will reduce or eliminate IP fragmentation but will result in higher <a href="/facts/Protocol_overhead/e2jgPrHr">overhead</a>. </p><p>Each direction of data flow can use a different MSS. </p><p>For most computer users, the MSS option is established by the <a href="/facts/Operating_system/8XTuvMh2">operating system</a>. </p><p>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. </p>