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Trailer encapsulations (RFC0893)

IP.com Disclosure Number: IPCOM000003942D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2019-Feb-13
Document File: 6 page(s) / 9K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

S. Leffler: AUTHOR [+1]

Related Documents

10.17487/RFC0893: DOI

Abstract

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.

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This is the abbreviated version, containing approximately 26% of the total text.

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 [Page 1]

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 protocol header information is fixed in size, alignment to a page boundary is virtually impossible. Protocol header information may vary in size due to the use of multiple protocols (each with a different header), or it may vary in size by agreement (for example, when optional information i...

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