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The maximum message segment length (MSS) represents the length of the most big data that the TCP travels to the other end. When a connection is established, both sides of the connection notify their respective MSS. We've met. MSS is 1024. As a result, IP datagrams are usually 40 bytes long: 20-byte TCP header and 20-byte IP header.

In some books, it is seen as a "negotiable" option. It is not negotiable under any conditions. When establishing a connection, each party has an MSS option to advertise that it expects to receive (the MSS option can only appear in the SYN message segment). If one party does not receive the MSS value from the other, MSS locates the default value of 536 bytes (this default allows 20-byte IP headers and 20-byte TCP headers to fit 576-byte IP packets).

In general, if no segmentation occurs, the bigger the MSS, the better (this is not always true). The larger the message segment, the more data each segment is allowed to transmit, which has higher network utilization than the header of IP and TCP. When TCP sends a SYN, either because a local application process wants to initiate a connection, or because the host on the other side receives a connection request, it can set the MTU length on the outgoing interface minus the fixed IP header and TCP header length. For an Ethernet, the MSS value can reach 1460 bytes. Using the encapsulation of IEEE 802.3, its MSS can reach 1452 bytes.

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