DARPA Internet gateway
RFC 823

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Request for Comments:  823
Obsoletes IEN-30 and IEN-109

                        THE DARPA INTERNET GATEWAY

                                  RFC 823

                               Robert Hinden
                               Alan Sheltzer

                       Bolt Beranek and Newman Inc.
                              10 Moulton St.
                      Cambridge, Massachusetts 02238

                              September 1982

                               Prepared for

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

This RFC is a status report on the Internet Gateway developed by BBN. It
describes the Internet Gateway as of September 1982.  This memo presents
detailed descriptions of message formats and gateway procedures, however
this is not an implementation specification, and such details are 
subject to change.




     DARPA Internet Gateway                             September 1982
     RFC 823

                             Table of Contents

     1   INTRODUCTION.......................................... 1
     2   BACKGROUND............................................ 2
     3   FORWARDING INTERNET DATAGRAMS......................... 5
     3.1   Input............................................... 5
     3.2   IP Header Checks.................................... 6
     3.3   Routing............................................. 7
     3.4   Redirects........................................... 9
     3.5   Fragmentation....................................... 9
     3.6   Header Rebuild..................................... 10
     3.7   Output............................................. 10
     4   PROTOCOLS SUPPORTED BY THE GATEWAY................... 12
     4.1   Cross-Net Debugging Protocol....................... 12
     4.2   Host Monitoring Protocol........................... 12
     4.3   ICMP............................................... 14
     4.4   Gateway-to-Gateway Protocol........................ 14
     4.4.1   Determining Connectivity to Networks............. 14
     4.4.2   Determining Connectivity to Neighbors............ 16
     4.4.3   Exchanging Routing Information................... 17
     4.4.4   Computing Routes................................. 19
     4.4.5   Non-Routing Gateways............................. 22
     4.4.6   Adding New Neighbors and Networks................ 23
     4.5   Exterior Gateway Protocol.......................... 24
     5   GATEWAY SOFTWARE..................................... 26
     5.1   Software Structure................................. 26
     5.1.1   Device Drivers................................... 27
     5.1.2   Network Software................................. 27
     5.1.3   Shared Gateway Software.......................... 29
     5.2   Gateway Processes.................................. 29
     5.2.1   Network Processes................................ 29
     5.2.2   GGP Process...................................... 30
     5.2.3   HMP Process...................................... 31
     APPENDIX A. GGP Message Formats.......................... 32
     APPENDIX B. Information Maintained by Gateways........... 39
     APPENDIX C. GGP Events and Responses..................... 41
     REFERENCES............................................... 43

                                    -i-




     DARPA Internet Gateway                             September 1982
     RFC 823

     1  INTRODUCTION

          This document explains the design of  the  Internet  gateway

     used  in  the  Defense  Advanced  Research Project Agency (DARPA)

     Internet program.  The gateway design was  originally  documented

     in  IEN-30,  "Gateway  Routing:  An Implementation Specification"

     [2], and was later updated in IEN-109, "How to Build  a  Gateway"

     [3].   This  document  reflects changes made both in the internet

     protocols and in the gateway design since  these  documents  were

     released.  It supersedes both IEN-30 and IEN-109.

          The Internet gateway described in this document is based  on

     the  work  of many people; in particular, special credit is given

     to V. Strazisar, M. Brescia, E. Rosen, and J. Haverty.

          The gateway's primary purpose is to route internet datagrams

     to their destination networks.  These datagrams are generated and

     processed as described in RFC 791,  "Internet  Protocol  -  DARPA

     Internet  Program  Protocol  Specification"  [1].   This document

     describes  how  the  gateway  forwards  datagrams,  the   routing

     algorithm  and  protocol  used  to  route  them, and the software

     structure  of  the  current   gateway.    The   current   gateway

     implementation  is written in macro-11 assembly language and runs

     in the DEC PDP-11 or LSI-11 16-bit processor.