Scheme for an internet encapsulation protocol: Version 1
RFC 1241

Document Type RFC - Experimental (July 1991; No errata)
Authors Robert Woodburn  , David Mills 
Last updated 2013-03-02
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Network Working Group                                       R. Woodburn
Request for Comments: 1241                                         SAIC
                                                               D. Mills
                                                 University of Delaware
                                                              July 1991

            A Scheme for an Internet Encapsulation Protocol:
                               Version 1

1. Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  Discussion and suggestions for improvement are requested.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

2. Glossary

   Clear Datagram -
     The unmodified IP datagram in the User Space before

   Clear Header -
     The header portion of the Clear Datagram before
     Encapsulation.  This header includes the IP header and
     possibly part or all of the next layer protocol header,
     i.e., the TCP header.

   Decapsulation -
     The stripping of the Encapsulation Header and forwarding
     of the Clear Datagram by the Decapsulator.

   Decapsulator -
     The entity responsible for receiving an Encapsulated
     Datagram, decapsulating it, and delivering it to the
     destination User Space.  Delivery may be direct, or via
     Encapsulation.  A Decapsulator may be a host or a gateway.

   Encapsulated Datagram -
     The datagram consisting of a Clear Datagram prepended with
     an Encapsulation Header.

   Encapsulation -
     The process of mapping a Clear Datagram to the
     Encapsulation Space, prepending an Encapsulation Header to
     the Clear Datagram and routing the Encapsulated Datagram

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RFC 1241                 Internet Encapsulation                July 1991

     to a Decapsulator.

   Encapsulation Header -
     The header for the Encapsulation Protocol prepended to the
     Clear Datagram during Encapsulation.  This header consists
     of an IP header followed by an Encapsulation Protocol

   Encapsulation Protocol Header -
     The Encapsulation Protocol specific portion of the
     Encapsulation Header.

   Encapsulation Space -
     The address and routing space within which the
     Encapsulators and Decapsulators reside.  Routing within
     this space is accomplished via Flows.  Encapsulation
     Spaces do not overlap, that is, the address of any
     Encapsulator or Decapsulator is unique for all
     Encapsulation Spaces.

   Encapsulator -
     The entity responsible for mapping a given User Space
     datagram to the Encapsulation Space, encapsulating the
     datagram, and forwarding the Encapsulated Datagram to a
     Decapsulator.  An Encapsulator may be a host or a gateway.

   Flow -
     Also called a "tunnel."  A flow is the end-to-end path in
     the Encapsulation Space over which Encapsulated Datagrams
     travel.  There may be several Encapsulator/Decapsulator
     pairs along a given flow.  Note that a Flow does not
     denote what User Space gateways are traversed along the

   Flow ID -
     A 32-bit identifier which uniquely distinguishes a flow in
     a given Encapsulator or Decapsulator.  Flow IDs are
     specific to a single Encapsulator/Decapsulator Entity and
     are not global quantities.

   Mapping Function -
     This is the function of mapping a Clear Header to a
     particular Flow.  All encapsulators along a given Flow are
     required to map a given Clear Header to the same Flow.

   User Address -
     The address or identifier uniquely identifying an entity
     within a User Space.

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RFC 1241                 Internet Encapsulation                July 1991

   Source Route -
     A complete end-to-end route which is computed at the
     source and enumerates transit gateways.

   User Space -
     The address and routing space within which the users
     reside.  Routing within this space provides reachability
     between all address pairs within the space.  User Spaces
     do not overlap, that is, a given User Address is unique in
     all User Spaces.

3. Background

   For several years researchers in the Internet community have needed a
   means of "tunneling" between networks.  A tunnel is essentially a
   Source Route that circumvents conventional routing mechanisms.
   Tunnels provide the means to bypass routing failures, avoid broken
   gateways and routing domains, or establish deterministic paths for

   There are several means of accomplishing tunneling.  In the past,
   tunneling has been accomplished through source routing options in the
   IP header which allow gateways along a given path to be enumerated.
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