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Trailer encapsulations (RFC0893)

IP.com Disclosure Number: IPCOM000003942D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 5 page(s) / 13K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

S. Leffler: AUTHOR [+2]

Abstract

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].

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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

RFC 893 April 1984

aligned on a page boundary and must have a size which is a multiple

of the hardware page size (or filled to a page boundary). The latter

restriction assumes virtual memory protection is maintained at the

page level; different architectures may alter these prerequisites.

Data to be transmitted across a network may easily be segmented in

the appropriate size, but unless the encapsulating proto...