tcp/ip协议的MSS和MTU浅析

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。在《TCP/IP详解第一卷》（英文版）中有如下的描述：

    As we can see from , there is a limit on the size of the frame for both Ethernet encapsulation and 802.3 encapsulation. This limits the number of bytes of data to 1500 and 1492, respectively. This characteristic of the link layer is called the MTU, its maximum transmission unit. Most types of networks have an upper limit.

If IP has a datagram to send, and the datagram is larger than the link layer's MTU, IP performs fragmentation, breaking the datagram up into smaller pieces (fragments), so that each fragment is smaller than the MTU. We discuss IP fragmentation in .