rfc791.txt

RFC 相关的技术文档

RFC:  791
                                    
                                    
                                    
                                    
                                    
                                    
                                    
                           INTERNET PROTOCOL
                                    
                                    
                         DARPA INTERNET PROGRAM
                                    
                         PROTOCOL SPECIFICATION
                                    
                                    
                                    
                             September 1981













                              prepared for

               Defense Advanced Research Projects Agency
                Information Processing Techniques Office
                         1400 Wilson Boulevard
                       Arlington, Virginia  22209







                                   by

                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291



September 1981                                                          
                                                       Internet Protocol



                           TABLE OF CONTENTS

    PREFACE ........................................................ iii

1.  INTRODUCTION ..................................................... 1

  1.1  Motivation .................................................... 1
  1.2  Scope ......................................................... 1
  1.3  Interfaces .................................................... 1
  1.4  Operation ..................................................... 2

2.  OVERVIEW ......................................................... 5

  2.1  Relation to Other Protocols ................................... 9
  2.2  Model of Operation ............................................ 5
  2.3  Function Description .......................................... 7
  2.4  Gateways ...................................................... 9

3.  SPECIFICATION ................................................... 11

  3.1  Internet Header Format ....................................... 11
  3.2  Discussion ................................................... 23
  3.3  Interfaces ................................................... 31

APPENDIX A:  Examples & Scenarios ................................... 34
APPENDIX B:  Data Transmission Order ................................ 39

GLOSSARY ............................................................ 41

REFERENCES .......................................................... 45





















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                                PREFACE



This document specifies the DoD Standard Internet Protocol.  This
document is based on six earlier editions of the ARPA Internet Protocol
Specification, and the present text draws heavily from them.  There have
been many contributors to this work both in terms of concepts and in
terms of text.  This edition revises aspects of addressing, error
handling, option codes, and the security, precedence, compartments, and
handling restriction features of the internet protocol.

                                                           Jon Postel

                                                           Editor




































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                                                          September 1981


RFC:  791
Replaces:  RFC 760
IENs 128, 123, 111,
80, 54, 44, 41, 28, 26

                           INTERNET PROTOCOL

                         DARPA INTERNET PROGRAM
                         PROTOCOL SPECIFICATION



                            1.  INTRODUCTION

1.1.  Motivation

  The Internet Protocol is designed for use in interconnected systems of
  packet-switched computer communication networks.  Such a system has
  been called a "catenet" [1].  The internet protocol provides for
  transmitting blocks of data called datagrams from sources to
  destinations, where sources and destinations are hosts identified by
  fixed length addresses.  The internet protocol also provides for
  fragmentation and reassembly of long datagrams, if necessary, for
  transmission through "small packet" networks.

1.2.  Scope

  The internet protocol is specifically limited in scope to provide the
  functions necessary to deliver a package of bits (an internet
  datagram) from a source to a destination over an interconnected system
  of networks.  There are no mechanisms to augment end-to-end data
  reliability, flow control, sequencing, or other services commonly
  found in host-to-host protocols.  The internet protocol can capitalize
  on the services of its supporting networks to provide various types
  and qualities of service.

1.3.  Interfaces

  This protocol is called on by host-to-host protocols in an internet
  environment.  This protocol calls on local network protocols to carry
  the internet datagram to the next gateway or destination host.

  For example, a TCP module would call on the internet module to take a
  TCP segment (including the TCP header and user data) as the data
  portion of an internet datagram.  The TCP module would provide the
  addresses and other parameters in the internet header to the internet
  module as arguments of the call.  The internet module would then
  create an internet datagram and call on the local network interface to
  transmit the internet datagram.

  In the ARPANET case, for example, the internet module would call on a


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                                                          September 1981
Internet Protocol
Introduction



  local net module which would add the 1822 leader [2] to the internet
  datagram creating an ARPANET message to transmit to the IMP.  The
  ARPANET address would be derived from the internet address by the
  local network interface and would be the address of some host in the
  ARPANET, that host might be a gateway to other networks.

1.4.  Operation

  The internet protocol implements two basic functions:  addressing and
  fragmentation.

  The internet modules use the addresses carried in the internet header
  to transmit internet datagrams toward their destinations.  The
  selection of a path for transmission is called routing.

  The internet modules use fields in the internet header to fragment and
  reassemble internet datagrams when necessary for transmission through
  "small packet" networks.

  The model of operation is that an internet module resides in each host
  engaged in internet communication and in each gateway that
  interconnects networks.  These modules share common rules for
  interpreting address fields and for fragmenting and assembling
  internet datagrams.  In addition, these modules (especially in
  gateways) have procedures for making routing decisions and other
  functions.

  The internet protocol treats each internet datagram as an independent
  entity unrelated to any other internet datagram.  There are no
  connections or logical circuits (virtual or otherwise).

  The internet protocol uses four key mechanisms in providing its
  service:  Type of Service, Time to Live, Options, and Header Checksum.

  The Type of Service is used to indicate the quality of the service
  desired.  The type of service is an abstract or generalized set of
  parameters which characterize the service choices provided in the
  networks that make up the internet.  This type of service indication
  is to be used by gateways to select the actual transmission parameters
  for a particular network, the network to be used for the next hop, or
  the next gateway when routing an internet datagram.

  The Time to Live is an indication of an upper bound on the lifetime of
  an internet datagram.  It is set by the sender of the datagram and
  reduced at the points along the route where it is processed.  If the
  time to live reaches zero before the internet datagram reaches its
  destination, the internet datagram is destroyed.  The time to live can
  be thought of as a self destruct time limit.


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                                                       Internet Protocol
                                                            Introduction



  The Options provide for control functions needed or useful in some
  situations but unnecessary for the most common communications.  The
  options include provisions for timestamps, security, and special
  routing.

  The Header Checksum provides a verification that the information used
  in processing internet datagram has been transmitted correctly.  The
  data may contain errors.  If the header checksum fails, the internet
  datagram is discarded at once by the entity which detects the error.

  The internet protocol does not provide a reliable communication
  facility.  There are no acknowledgments either end-to-end or
  hop-by-hop.  There is no error control for data, only a header
  checksum.  There are no retransmissions.  There is no flow control.

  Errors detected may be reported via the Internet Control Message
  Protocol (ICMP) [3] which is implemented in the internet protocol
  module.