rfc1547.txt

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Network Working Group                                         D. Perkins
Request for Comments: 1547                    Carnegie Mellon University
Category: Informational                                    December 1993


     Requirements for an Internet Standard Point-to-Point Protocol

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document discusses the evaluation criteria for an Internet
   Standard Data Link Layer protocol to be used with point-to-point
   links.  Although many industry standard protocols and ad hoc
   protocols already exist for the data link layer, none are both
   complete and sufficiently versatile to be accepted as an Internet
   Standard.  In preparation to designing such a protocol, the features
   necessary to qualify a point-to-point protocol as an Internet
   Standard are discussed in detail.  An analysis of the strengths and
   weaknesses of several existing protocols on the basis of these
   requirements demonstrates the failure of each to address key issues.

      Historical Note: This was the design requirements document dated
      June 1989, which was followed for RFC-1134 through the present.
      It is now published for completeness and future guidance.






















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Table of Contents

   1.    Introduction ................................................3
   1.1   Definitions of Terms ........................................4
   2.    Required Features ...........................................6
   2.1   Simplicity ..................................................7
   2.2   Transparency ................................................7
   2.3   Packet Framing ..............................................7
   2.4   Bandwidth Efficiency ........................................8
   2.5   Protocol Processing Efficiency ..............................8
   2.6   Protocol Multiplexing .......................................8
   2.7   Multiple Physical and Data Link Layer Protocols..............8
   2.8   Error Detection .............................................9
   2.9   Standardized Maximum Packet Length (MTU) ....................9
   2.10  Switched and Non-Switched Media .............................9
   2.11  Symmetry ....................................................9
   2.12  Connection Liveness .........................................10
   2.13  Loopback Detection ..........................................10
   2.14  Misconfiguration Detection ..................................11
   2.15  Network Layer Address Negotiation ...........................11
   2.16  Data Compression Negotiation ................................11
   2.17  Extensibility and Option Negotiation ........................12
   3.    Features Not Required .......................................12
   3.1   Error Correction ............................................12
   3.2   Flow Control ................................................13
   3.3   Sequencing ..................................................13
   3.4   Backward Compatibility ......................................13
   3.5   Multi-Point Links ...........................................13
   3.6   Half-Duplex or Simplex Links ................................14
   3.7   7-bit Asynchronous RS-232 Links .............................14
   4.    Prior Work On PPP Protocols .................................14
   4.1   Internet Protocols ..........................................14
   4.1.1 RFC 891 - DCN Local-Network Protocols, Appendix A............14
   4.1.2 RFC 914 - Thinwire Protocols ................................14
   4.1.3 RFC 916 - Reliable Asynchronous Transfer Protocol............15
   4.1.4 RFC 935 - Reliable Link Layer Protocols .....................15
   4.1.5 RFC 1009 - Requirements for Internet Gateways ...............15
   4.1.6 RFC 1055 - Serial Line IP ...................................16
   4.2   International Protocols .....................................16
   4.2.1 ISO 3309 - HDLC Frame Structure .............................16
   4.2.2 ISO 6256 - HDLC Balanced Class of Procedures.................16
   4.2.3 CCITT X.25 and X.25 LAPB ....................................17
   4.2.4 CCITT I.441 LAPD ............................................17
   4.3   Other Protocols .............................................17
   4.3.1 Cisco Systems point-to-point protocols ......................17
   4.3.2 MIT PC/IP framing protocol ..................................18
   4.3.3 Proteon p4200 point-to-point protocol .......................18
   4.3.4 Ungermann Bass point-to-point protocol ......................18



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   4.3.5 Wellfleet point-to-point protocol ...........................19
   4.3.6 XNS Synchronous Point-to-Point Protocol .....................19
   REFERENCES ........................................................20
   SECURITY CONSIDERATION.............................................21
   CHAIR'S ADDRESS ...................................................21
   AUTHOR'S ADDRESS ..................................................21
   EDITOR'S ADDRESS ..................................................21

1. Introduction

   The Internet has seen explosive growth in the number of hosts
   supporting IP [1].  The vast majority of these hosts are connected to
   Local Area Networks (LANs) of various types, Ethernet being the most
   common.  Most of the other hosts are connected through Wide Area
   Networks (WANs), such as X.25 style Public Data Networks (PDNs).

   In the past, relatively few of these hosts were connected with simple
   point-to-point links.  Yet, point-to-point serial links are among the
   oldest methods of data communications, and almost every host supports
   point-to-point connections.  For example, asynchronous RS-232
   interfaces are essentially ubiquitous.

   One reason for the small number of point-to-point IP links was the
   lack of a single established encapsulation protocol.  There were
   plenty of non-standard (and at least one de facto standard)
   encapsulation protocols available, but there was not one which was
   agreed upon as an Internet Standard.

   A number of protocols have been proposed to the Internet community,
   but no consensus was reached as to which protocol should be adopted
   as a standard.  The reason may be that these proposals often
   addressed specific problems rather than providing general purpose
   service.

   For example, one of the most successful protocols to-date was Rick
   Adam's SLIP protocol for BSD UNIX [9].  SLIP provides only the most
   rudimentary support for sending IP datagrams over asynchronous serial
   lines, and ignores issues such as the use of protocols other than IP
   and the use of synchronous links.

   This document proposes a set of requirements for an Internet Standard
   point-to-point protocol (ISPPP).  Its purpose is not to propose any
   one design for the standard; any solutions outlined in the text are
   intended only as examples, and do not preclude other implementations.

   The document is divided into four major sections.  The first section
   defines a number of technical terms used in this document.  The
   second section lists the proposed requirements and details some



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   issues that are ignored by other protocols.  The third section
   attempts to clarify a number of non-requirements.  The fourth section
   analyzes existing protocols in light of the proposed requirements and
   discusses the failure of each to address key issues.

1.1 Definitions of Terms

   This section defines many of the terms which will be used in further
   sections of this document.  The terms "layer" and "level" are used
   extensively and refer to protocol layers as defined by the
   International Organization For Standardization's Reference Model
   (ISORM) standard.  In particular, the terms Physical Layer, Data Link
   Layer and Network Layer refer to layers one, two and three
   respectively of the ISORM.  A "higher layer" refers to one with a
   numerically larger layer number.

    datagram

      The unit of transmission in the network layer (such as IP).  A
      datagram may be encapsulated in one or more packets (q.v.) passed
      to the data link layer.

    data link layer

      Layer two in the ISO reference model.  Defines how bits
      transmitted and received by the physical layer are recognized as
      bytes and frames.  May also define procedures for error detection
      and correction, sequencing and flow control.

    fragment

      The result of fragmentation.  Fragmentation at the network layer
      breaks large datagrams into multiple parts less than or equal to
      the size of the packets passed to the data link layer.
      Fragmentation at the data link layer breaks large packets into
      multiple frames.

    frame

      The unit of transmission at the data link layer.  A frame may
      include a header and/or a trailer along with some number of units
      of data.

    framing protocol

      A protocol at the data link level for marking the beginning and
      end of a frame transmitted across a link.




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    internet

      An interconnected system of networks tied together by a common
      "internet protocol" providing a common and consistent network
      address structure.

    Internet

      Specifically refers to the IP Internet.

    Internet Standard Point-to-Point Protocol (ISPPP)

      A point-to-point protocol which is declared an official Internet
      Standard.  This protocol does not yet exist, but its proposed
      characteristics are presented in this paper.

    Maximum Transmission Unit (MTU)

      The maximum allowable length for a packet (q.v.) transmitted over
      a point-to-point link without incurring network layer
      fragmentation.

    network layer

      Layer three in the ISO reference model.  Responsible for routing
      packets (q.v) between physical networks.

    octet

      A unit of transmission consisting of 8 bits.  On most machines an
      octet is the same as a byte or a character, but this need not be
      true.

    packet

      The unit of transmission passed across the interface between the
      network layer and the data link layer.  A packet is usually mapped
      to a frame (q.v); the exception is when data link layer
      fragmentation is being performed.

    physical layer

      The first layer in the ISO reference model.  Describes electrical,
      mechanical and timing characteristics of a link.

    point-to-point protocol (ppp)

      A data link layer protocol for the transmission of packets (q.v.)



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      over a point-to-point link.  In the following discussion, the
      acronym "ppp" refers to any generic point-to-point protocol.

    serial line IP (slip)

      Often incorrectly used as a synonym for "point-to-point protocol",
      "slip" specifically refers to any protocol for the transmission of
      IP datagrams over a serial point-to-point line.