RFC 893 (rfc893) - Page 1 of 6
Trailer encapsulations
Alternative Format: Original Text Document
Network Working Group Samuel J. Leffler
Request for Comments: 893 Michael J. Karels
University of California at Berkeley
April 1984
Trailer Encapsulations
Status of this Memo
This RFC discusses the motivation for use of "trailer encapsulations"
on local-area networks and describes the implementation of such an
encapsulation on various media. This document is for information
only. This is NOT an official protocol for the ARPA Internet
community.
Introduction
A trailer encapsulation is a link level packet format employed by
4.2BSD UNIX (among others). A trailer encapsulation, or "trailer",
may be generated by a system under certain conditions in an effort to
minimize the number and size of memory-to-memory copy operations
performed by a receiving host when processing a data packet.
Trailers are strictly a link level packet format and are not visible
(when properly implemented) in any higher level protocol processing.
This note cites the motivation behind the trailer encapsulation and
describes the trailer encapsulation packet formats currently in use
on 3 Mb/s Experimental Ethernet, 10 Mb/s Ethernet, and 10 Mb/s V2LNI
ring networks [1].
The use of a trailer encapsulation was suggested by Greg Chesson, and
the encapsulation described here was designed by Bill Joy.
Motivation
Trailers are motivated by the overhead which may be incurred during
protocol processing when one or more memory to memory copies must be
performed. Copying can be required at many levels of processing,
from moving data between the network medium and the host's memory, to
passing data between the operating system and user address spaces.
An optimal network implementation would expect to incur zero copy
operations between delivery of a data packet into host memory and
presentation of the appropriate data to the receiving process. While
many packets may not be processed without some copying operations,
when the host computer provides suitable memory management support it
may often be possible to avoid copying simply by manipulating the
appropriate virtual memory hardware.
In a page mapped virtual memory environment, two prerequisites are
usually required to achieve the goal of zero copy operations during
packet processing. Data destined for a receiving agent must be
Leffler & Karels