RFC 893 (rfc893) - Page 1 of 6

Trailer encapsulations

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