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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 octets, that a computer or communications device can receive in a single TCP segment. It does not count the TCP header or the IP header.〔(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."〕 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. The default TCP Maximum Segment Size is 536.〔(RFC 879 ), page 1, Section 1, "The default TCP Maximum Segment Size is 536."〕 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.〔(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)."〕 Each direction of data flow can use a different MSS. 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.〔(RFC 2460 ), page 28, Section 8.3〕 Therefore IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576〔(RFC 879 ), page 1, Section 1〕 - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280〔(RFC 2460 ), page 24, Section 5〕 - 40 - 20). Small MSS values will reduce or eliminate IP fragmentation, but will result in higher overhead.〔(The TCP/IP Guide ), TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size〕 For most computer users, the MSS option is established by the operating system. ==Further reading== * * 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Maximum segment size」の詳細全文を読む スポンサード リンク
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