TCP/IP详解 卷1 2.3

2.3 Trailer Encapsulation

尾部封装

RFC 893 [Leffler and Karels 1984] describes another form of encapsulation used on Ethernets, called trailer encapsulation. It was an experiment with early BSD systems on DEC VAXes that improved performance by rearranging the order of the fields in the IP datagram. The variable-length fields at the beginning of the data portion of the Ethernet frame (the IP header and the TCP header) were moved to the end (right before the CRC). This allows the data portion of the frame to be mapped to a hardware page, saving a memory-to-memory copy when the data is copied in the kernel. TCP data that is a multiple of 512 bytes in size can be moved by just manipulating the kernel's page tables. Two hosts negotiated the use of trailer encapsulation using an extension of ARP. Different Ethernet frame type values are defined for these frames.

RFC 893 描述了以太网的另一种封装形式，叫做尾部封装。它是早期 BSD 系统在 DEC VAXes 上的实验，通过重排列 IP 数据报的区域顺序以提高性能。以太帧中数据部分前面（ IP 头部和 TCP 头部）的变长区域被移动到了尾部（正好在 CRC 前面）。这允许帧中数据部分被映射成一个硬件页，当数据被拷贝到内核中时，存储一个内存到内存的备份。 TCP 数据是 512 字节大小的倍数，能通过操作内核页表来移动。两台主机通过扩展 ARP 协商使用尾部封装。此种帧定义了不同的以太帧类型值。

Nowadays trailer encapsulation is deprecated, so we won't show any examples of it. Interested readers are referred to RFC 893 and Section 11.8 of [Leffler et al. 1989] for additional details.

如今，尾部封装已经不支持，我们不出示其任何实例。感兴趣的读者可参阅 RFC 893 和 11.8 节。