2.3 Trailer Encapsulation

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 .

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.