IP in IP Tunneling
RFC 1853

Document Type RFC - Informational (October 1995; No errata)
Last updated 2013-03-02
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Network Working Group                                         W. Simpson
Request for Comments: 1853                                    Daydreamer
Category: Informational                                     October 1995

                           IP in IP Tunneling

Status of this Memo

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

IESG Note:

   Note that this memo is an individual effort of the author.  This
   document reflects a current informal practice in the internet.  There
   is an effort underway within the IETF Mobile-IP Working Group to
   provide an appropriate proposed standard to address this issue.

Abstract

   This document discusses implementation techniques for using IP
   Protocol/Payload number 4 Encapsulation for tunneling with IP
   Security and other protocols.

Table of Contents

     1.     Introduction ..........................................    2

     2.     Encapsulation .........................................    3

     3.     Tunnel Management .....................................    5
        3.1       Tunnel MTU Discovery ............................    5
        3.2       Congestion ......................................    6
        3.3       Routing Failures ................................    6
        3.4       Other ICMP Messages .............................    6

     SECURITY CONSIDERATIONS ......................................    7
     REFERENCES ...................................................    7
     ACKNOWLEDGEMENTS .............................................    8
     AUTHOR'S ADDRESS .............................................    8

Simpson                      Informational                      [Page 1]
RFC 1853                     IP Tunnelling                  October 1995

1.  Introduction

   The IP in IP encapsulation Protocol/Payload number 4 [RFC-1700] has
   long been used to bridge portions of the Internet which have disjoint
   capabilities or policies.  This document describes implementation
   techniques used for many years by the Amateur Packet Radio network
   for joining a large mobile network, and also by early implementations
   of IP Security protocols.

   Use of IP in IP encapsulation differs from later tunneling techniques
   (for example, protocol numbers 98 [RFC-1241], 94 [IDM91a], 53
   [swIPe], and 47 [RFC-1701]) in that it does not insert its own
   special glue header between IP headers.  Instead, the original
   unadorned IP Header is retained, and simply wrapped in another
   standard IP header.

   This information applies principally to encapsulation of IP version
   4.  Other IP versions will be described in separate documents.

Simpson                      Informational                      [Page 2]
RFC 1853                     IP Tunnelling                  October 1995

2.  Encapsulation

   The encapsulation technique is fairly simple.  An outer IP header is
   added before the original IP header.  Between them are any other
   headers for the path, such as security headers specific to the tunnel
   configuration.

   The outer IP header Source and Destination identify the "endpoints"
   of the tunnel.  The inner IP header Source and Destination identify
   the original sender and recipient of the datagram.

   Each header chains to the next using IP Protocol values [RFC-1700].

                                          +---------------------------+
                                          |      Outer IP Header      |
                                          +---------------------------+
                                          |      Tunnel Headers       |
      +---------------------------+       +---------------------------+
      |         IP Header         |       |      Inner IP Header      |
      +---------------------------+ ====> +---------------------------+
      |                           |       |                           |
      |         IP Payload        |       |         IP Payload        |
      |                           |       |                           |
      +---------------------------+       +---------------------------+

   The format of IP headers is described in [RFC-791].

   Type Of Service  copied from the inner IP header.  Optionally,
                    another TOS may be used between cooperating peers.

                    This is in keeping with the transparency principle
                    that if the user was expecting a given level of
                    service, then the tunnel should provide the same
                    service.  However, some tunnels may be constructed
                    specifically to provide a different level of service
                    as a matter of policy.

   Identification   A new number is generated for each outer IP header.

                    The encapsulated datagram may have already been
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