tcp/ip协议的MSS和MTU浅析

978计划

978计划工作组 2009-12-4
1 MSS、MTU与数据及协议头的关系图
以太网MSS的最大可达1460，MTU最大可达1500，MTU不能小于576字节。
2 MSS浅析
MSS的英文缩写为 Maximum Segment Size，在《TCP/IP详解第一卷》（英文版）中有如下的描述：
The maximum segment size (MSS) is the largest "chunk" of data that TCP will send to the other end. When a connection is established, each end can announce its MSS. The values we've seen have all been 1024. The resulting IP datagram is normally 40 bytes larger: 20 bytes for the TCP header and 20 bytes for the IP header.
Some texts refer to this as a "negotiated" option. It is not negotiated in any way. When a connection is established, each end has the option of announcing the MSS it expects to receive. (An MSS option can only appear in a SYN segment.) If one end does not receive an MSS option from the other end, a default of 536 bytes is assumed. (This default allows for a 20-byte IP header and a 20-byte TCP header to fit into a 576-byte IP datagram.)
3 MTU浅析
    MTU是最大传输单元，英文缩写为 Maximum Transmission Unit。


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