IP Encapsulation within IP
RFC 2003

Network Working Group                                         C. Perkins
Request for Comment: 2003                                            IBM
Category: Standards Track                                   October 1996

                       IP Encapsulation within IP

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document specifies a method by which an IP datagram may be
   encapsulated (carried as payload) within an IP datagram.
   Encapsulation is suggested as a means to alter the normal IP routing
   for datagrams, by delivering them to an intermediate destination that
   would otherwise not be selected by the (network part of the) IP
   Destination Address field in the original IP header.  Encapsulation
   may serve a variety of purposes, such as delivery of a datagram to a
   mobile node using Mobile IP.

1. Introduction

   This document specifies a method by which an IP datagram may be
   encapsulated (carried as payload) within an IP datagram.
   Encapsulation is suggested as a means to alter the normal IP routing
   for datagrams, by delivering them to an intermediate destination that
   would otherwise not be selected based on the (network part of the) IP
   Destination Address field in the original IP header.  Once the
   encapsulated datagram arrives at this intermediate destination node,
   it is decapsulated, yielding the original IP datagram, which is then
   delivered to the destination indicated by the original Destination
   Address field.  This use of encapsulation and decapsulation of a
   datagram is frequently referred to as "tunneling" the datagram, and
   the encapsulator and decapsulator are then considered to be the
   "endpoints" of the tunnel.

   In the most general tunneling case we have

      source ---> encapsulator --------> decapsulator ---> destination

   with the source, encapsulator, decapsulator, and destination being
   separate nodes.  The encapsulator node is considered the "entry

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   point" of the tunnel, and the decapsulator node is considered the
   "exit point" of the tunnel.  There in general may be multiple
   source-destination pairs using the same tunnel between the
   encapsulator and decapsulator.

2. Motivation

   The Mobile IP working group has specified the use of encapsulation as
   a way to deliver datagrams from a mobile node's "home network" to an
   agent that can deliver datagrams locally by conventional means to the
   mobile node at its current location away from home [8].  The use of
   encapsulation may also be desirable whenever the source (or an
   intermediate router) of an IP datagram must influence the route by
   which a datagram is to be delivered to its ultimate destination.
   Other possible applications of encapsulation include multicasting,
   preferential billing, choice of routes with selected security
   attributes, and general policy routing.

   It is generally true that encapsulation and the IP loose source
   routing option [10] can be used in similar ways to affect the routing
   of a datagram, but there are several technical reasons to prefer
   encapsulation:

    -  There are unsolved security problems associated with the use of
       the IP source routing options.

    -  Current Internet routers exhibit performance problems when
       forwarding datagrams that contain IP options, including the IP
       source routing options.

    -  Many current Internet nodes process IP source routing options
       incorrectly.

    -  Firewalls may exclude IP source-routed datagrams.

    -  Insertion of an IP source route option may complicate the
       processing of authentication information by the source and/or
       destination of a datagram, depending on how the authentication is
       specified to be performed.

    -  It is considered impolite for intermediate routers to make
       modifications to datagrams which they did not originate.

   These technical advantages must be weighed against the disadvantages
   posed by the use of encapsulation:

    -  Encapsulated datagrams typically are larger than source routed
       datagrams.

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    -  Encapsulation cannot be used unless it is known in advance that
       the node at the tunnel exit point can decapsulate the datagram.

   Since the majority of Internet nodes today do not perform well when
   IP loose source route options are used, the second technical
   disadvantage of encapsulation is not as serious as it might seem at
   first.

3. IP in IP Encapsulation

   To encapsulate an IP datagram using IP in IP encapsulation, an outer
   IP header [10] is inserted before the datagram's existing IP header,
   as follows:

                                         +---------------------------+
                                         |                           |
                                         |      Outer IP Header      |
                                         |                           |
     +---------------------------+       +---------------------------+
     |                           |       |                           |
     |         IP Header         |       |         IP Header         |
     |                           |       |                           |
     +---------------------------+ ====> +---------------------------+
     |                           |       |                           |
     |                           |       |                           |
     |         IP Payload        |       |         IP Payload        |
     |                           |       |                           |
     |                           |       |                           |
     +---------------------------+       +---------------------------+

   The outer IP header Source Address and Destination Address identify
   the "endpoints" of the tunnel.  The inner IP header Source Address
   and Destination Addresses identify the original sender and recipient
   of the datagram, respectively.  The inner IP header is not changed by
   the encapsulator, except to decrement the TTL as noted below, and
   remains unchanged during its delivery to the tunnel exit point.  No
   change to IP options in the inner header occurs during delivery of
   the encapsulated datagram through the tunnel.  If need be, other
   protocol headers such as the IP Authentication header [1] may be
   inserted between the outer IP header and the inner IP header.  Note
   that the security options of the inner IP header MAY affect the
   choice of security options for the encapsulating (outer) IP header.

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3.1. IP Header Fields and Handling

   The fields in the outer IP header are set by the encapsulator as
   follows:

      Version

         4

      IHL

         The Internet Header Length (IHL) is the length of the outer IP
         header measured in 32-bit words [10].

      TOS

         The Type of Service (TOS) is copied from the inner IP header.

      Total Length

         The Total Length measures the length of the entire encapsulated
         IP datagram, including the outer IP header, the inner IP
         header, and its payload.

      Identification, Flags, Fragment Offset

         These three fields are set as specified in [10].  However, if
         the "Don't Fragment" bit is set in the inner IP header, it MUST
         be set in the outer IP header; if the "Don't Fragment" bit is
         not set in the inner IP header, it MAY be set in the outer IP
         header, as described in Section 5.1.

      Time to Live

         The Time To Live (TTL) field in the outer IP header is set to a
         value appropriate for delivery of the encapsulated datagram to
         the tunnel exit point.

      Protocol

         4

      Header Checksum

         The Internet Header checksum [10] of the outer IP header.

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      Source Address

         The IP address of the encapsulator, that is, the tunnel entry
         point.

      Destination Address

         The IP address of the decapsulator, that is, the tunnel exit
         point.

      Options

         Any options present in the inner IP header are in general NOT
         copied to the outer IP header.  However, new options specific
         to the tunnel path MAY be added.  In particular, any supported
         types of security options of the inner IP header MAY affect the
         choice of security options for the outer header.  It is not
         expected that there be a one-to-one mapping of such options to
         the options or security headers selected for the tunnel.

   When encapsulating a datagram, the TTL in the inner IP header is
   decremented by one if the tunneling is being done as part of
   forwarding the datagram; otherwise, the inner header TTL is not
   changed during encapsulation.  If the resulting TTL in the inner IP
   header is 0, the datagram is discarded and an ICMP Time Exceeded
   message SHOULD be returned to the sender.  An encapsulator MUST NOT
   encapsulate a datagram with TTL = 0.

   The TTL in the inner IP header is not changed when decapsulating.
   If, after decapsulation, the inner datagram has TTL = 0, the
   decapsulator MUST discard the datagram.  If, after decapsulation, the
   decapsulator forwards the datagram to one of its network interfaces,
   it will decrement the TTL as a result of doing normal IP forwarding.
   See also Section 4.4.

   The encapsulator may use any existing IP mechanisms appropriate for
   delivery of the encapsulated payload to the tunnel exit point.  In
   particular, use of IP options is allowed, and use of fragmentation is
   allowed unless the "Don't Fragment" bit is set in the inner IP
   header.  This restriction on fragmentation is required so that nodes
   employing Path MTU Discovery [7] can obtain the information they
   seek.

3.2. Routing Failures

   Routing loops within a tunnel are particularly dangerous when they
   cause datagrams to arrive again at the encapsulator.  Suppose a
   datagram arrives at a router for forwarding, and the router

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   determines that the datagram has to be encapsulated before further
   delivery.  Then:

    -  If the IP Source Address of the datagram matches the router's own
       IP address on any of its network interfaces, the router MUST NOT
       tunnel the datagram; instead, the datagram SHOULD be discarded.

    -  If the IP Source Address of the datagram matches the IP address
       of the tunnel destination (the tunnel exit point is typically
       chosen by the router based on the Destination Address in the
       datagram's IP header), the router MUST NOT tunnel the datagram;
       instead, the datagram SHOULD be discarded.

   See also Section 4.4.

4. ICMP Messages from within the Tunnel

   After an encapsulated datagram has been sent, the encapsulator may
   receive an ICMP [9] message from any intermediate router within the
   tunnel other than the tunnel exit point.  The action taken by the
   encapsulator depends on the type of ICMP message received.  When the
   received message contains enough information, the encapsulator MAY
   use the incoming message to create a similar ICMP message, to be sent
   to the originator of the original unencapsulated IP datagram (the
   original sender).  This process will be referred to as "relaying" the
   ICMP message from the tunnel.

   ICMP messages indicating an error in processing a datagram include a
   copy of (a portion of) the datagram causing the error.  Relaying an
   ICMP message requires that the encapsulator strip off the outer IP
   header from this returned copy of the original datagram.  For cases
   in which the received ICMP message does not contain enough data to
   relay the message, see Section 5.

4.1. Destination Unreachable (Type 3)

   ICMP Destination Unreachable messages are handled by the encapsulator
   depending upon their Code field.  The model suggested here allows the
   tunnel to "extend" a network to include non-local (e.g., mobile)
   nodes.  Thus, if the original destination in the unencapsulated
   datagram is on the same network as the encapsulator, certain
   Destination Unreachable Code values may be modified to conform to the
   suggested model.

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      Network Unreachable (Code 0)

         An ICMP Destination Unreachable message SHOULD be returned
         to the original sender.  If the original destination in
         the unencapsulated datagram is on the same network as the
         encapsulator, the newly generated Destination Unreachable
         message sent by the encapsulator MAY have Code 1 (Host
         Unreachable), since presumably the datagram arrived at the
         correct network and the encapsulator is trying to create the
         appearance that the original destination is local to that
         network even if it is not.  Otherwise, if the encapsulator
         returns a Destination Unreachable message, the Code field MUST
         be set to 0 (Network Unreachable).

      Host Unreachable (Code 1)

         The encapsulator SHOULD relay Host Unreachable messages to the
         sender of the original unencapsulated datagram, if possible.

      Protocol Unreachable (Code 2)

         When the encapsulator receives an ICMP Protocol Unreachable
         message, it SHOULD send a Destination Unreachable message with
         Code 0 or 1 (see the discussion for Code 0) to the sender of
         the original unencapsulated datagram.  Since the original
         sender did not use protocol 4 in sending the datagram, it would
         be meaningless to return Code 2 to that sender.

      Port Unreachable (Code 3)

         This Code should never be received by the encapsulator, since
         the outer IP header does not refer to any port number.  It MUST
         NOT be relayed to the sender of the original unencapsulated
         datagram.

      Datagram Too Big (Code 4)

         The encapsulator MUST relay ICMP Datagram Too Big messages to
         the sender of the original unencapsulated datagram.

      Source Route Failed (Code 5)

         This Code SHOULD be handled by the encapsulator itself.
         It MUST NOT be relayed to the sender of the original
         unencapsulated datagram.

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4.2. Source Quench (Type 4)

   The encapsulator SHOULD NOT relay ICMP Source Quench messages to the
   sender of the original unencapsulated datagram, but instead SHOULD
   activate whatever congestion control mechanisms it implements to help
   alleviate the congestion detected within the tunnel.

4.3. Redirect (Type 5)

   The encapsulator MAY handle the ICMP Redirect messages itself.  It
   MUST NOT not relay the Redirect to the sender of the original
   unencapsulated datagram.

4.4. Time Exceeded (Type 11)

   ICMP Time Exceeded messages report (presumed) routing loops within
   the tunnel itself.  Reception of Time Exceeded messages by the
   encapsulator MUST be reported to the sender of the original
   unencapsulated datagram as Host Unreachable (Type 3, Code 1).  Host
   Unreachable is preferable to Network Unreachable; since the datagram
   was handled by the encapsulator, and the encapsulator is often
   considered to be on the same network as the destination address in
   the original unencapsulated datagram, then the datagram is considered
   to have reached the correct network, but not the correct destination
   node within that network.

4.5. Parameter Problem (Type 12)

   If the Parameter Problem message points to a field copied from the
   original unencapsulated datagram, the encapsulator MAY relay the ICMP
   message to the sender of the original unencapsulated datagram;
   otherwise, if the problem occurs with an IP option inserted by the
   encapsulator, then the encapsulator MUST NOT relay the ICMP message
   to the original sender.  Note that an encapsulator following
   prevalent current practice will never insert any IP options into the
   encapsulated datagram, except possibly for security reasons.

4.6. Other ICMP Messages

   Other ICMP messages are not related to the encapsulation operations
   described within this protocol specification, and should be acted on
   by the encapsulator as specified in [9].

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5. Tunnel Management

   Unfortunately, ICMP only requires IP routers to return 8 octets (64
   bits) of the datagram beyond the IP header.  This is not enough to
   include a copy of the encapsulated (inner) IP header, so it is not
   always possible for the encapsulator to relay the ICMP message from
   the interior of a tunnel back to the original sender.  However, by
   carefully maintaining "soft state" about tunnels into which it sends,
   the encapsulator can return accurate ICMP messages to the original
   sender in most cases.  The encapsulator SHOULD maintain at least the
   following soft state information about each tunnel:

    - MTU of the tunnel (Section 5.1)
    - TTL (path length) of the tunnel
    - Reachability of the end of the tunnel

   The encapsulator uses the ICMP messages it receives from the interior
   of a tunnel to update the soft state information for that tunnel.
   ICMP errors that could be received from one of the routers along the
   tunnel interior include:

    - Datagram Too Big
    - Time Exceeded
    - Destination Unreachable
    - Source Quench

   When subsequent datagrams arrive that would transit the tunnel, the
   encapsulator checks the soft state for the tunnel.  If the datagram
   would violate the state of the tunnel (for example, the TTL of the
   new datagram is less than the tunnel "soft state" TTL) the
   encapsulator sends an ICMP error message back to the sender of the
   original datagram, but also encapsulates the datagram and forwards it
   into the tunnel.

   Using this technique, the ICMP error messages sent by the
   encapsulator will not always match up one-to-one with errors
   encountered within the tunnel, but they will accurately reflect the
   state of the network.

   Tunnel soft state was originally developed for the IP Address
   Encapsulation (IPAE) specification [4].

5.1. Tunnel MTU Discovery

   When the Don't Fragment bit is set by the originator and copied into
   the outer IP header, the proper MTU of the tunnel will be learned
   from ICMP Datagram Too Big (Type 3, Code 4) messages reported to the
   encapsulator.  To support sending nodes which use Path MTU Discovery,

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   all encapsulator implementations MUST support Path MTU Discovery [5,
   7] soft state within their tunnels.  In this particular application,
   there are several advantages:

    -  As a benefit of Path MTU Discovery within the tunnel, any
       fragmentation which occurs because of the size of the
       encapsulation header is performed only once after encapsulation.
       This prevents multiple fragmentation of a single datagram, which
       improves processing efficiency of the decapsulator and the
       routers within the tunnel.

    -  If the source of the unencapsulated datagram is doing Path MTU
       Discovery, then it is desirable for the encapsulator to know
       the MTU of the tunnel.  Any ICMP Datagram Too Big messages from
       within the tunnel are returned to the encapsulator, and as noted
       in Section 5, it is not always possible for the encapsulator to
       relay ICMP messages to the source of the original unencapsulated
       datagram.  By maintaining "soft state" about the MTU of the
       tunnel, the encapsulator can return correct ICMP Datagram Too Big
       messages to the original sender of the unencapsulated datagram to
       support its own Path MTU Discovery.  In this case, the MTU that
       is conveyed to the original sender by the encapsulator SHOULD
       be the MTU of the tunnel minus the size of the encapsulating
       IP header.  This will avoid fragmentation of the original IP
       datagram by the encapsulator.

    -  If the source of the original unencapsulated datagram is
       not doing Path MTU Discovery, it is still desirable for the
       encapsulator to know the MTU of the tunnel.  In particular, it is
       much better to fragment the original datagram when encapsulating,
       than to allow the encapsulated datagram to be fragmented.
       Fragmenting the original datagram can be done by the encapsulator
       without special buffer requirements and without the need to
       keep reassembly state in the decapsulator.  By contrast, if
       the encapsulated datagram is fragmented, then the decapsulator
       must reassemble the fragmented (encapsulated) datagram before
       decapsulating it, requiring reassembly state and buffer space
       within the decapsulator.

   Thus, the encapsulator SHOULD normally do Path MTU Discovery,
   requiring it to send all datagrams into the tunnel with the "Don't
   Fragment" bit set in the outer IP header.  However there are problems
   with this approach.  When the original sender sets the "Don't
   Fragment" bit, the sender can react quickly to any returned ICMP
   Datagram Too Big error message by retransmitting the original
   datagram.  On the other hand, suppose that the encapsulator receives
   an ICMP Datagram Too Big message from within the tunnel.  In that
   case, if the original sender of the unencapsulated datagram had not

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   set the "Don't Fragment" bit, there is nothing sensible that the
   encapsulator can do to let the original sender know of the error.
   The encapsulator MAY keep a copy of the sent datagram whenever it
   tries increasing the tunnel MTU, in order to allow it to fragment and
   resend the datagram if it gets a Datagram Too Big response.
   Alternatively the encapsulator MAY be configured for certain types of
   datagrams not to set the "Don't Fragment" bit when the original
   sender of the unencapsulated datagram has not set the "Don't
   Fragment" bit.

5.2. Congestion

   An encapsulator might receive indications of congestion from the
   tunnel, for example, by receiving ICMP Source Quench messages from
   nodes within the tunnel.  In addition, certain link layers and
   various protocols not related to the Internet suite of protocols
   might provide such indications in the form of a Congestion
   Experienced [6] flag.  The encapsulator SHOULD reflect conditions of
   congestion in its "soft state" for the tunnel, and when subsequently
   forwarding datagrams into the tunnel, the encapsulator SHOULD use
   appropriate means for controlling congestion [3]; However, the
   encapsulator SHOULD NOT send ICMP Source Quench messages to the
   original sender of the unencapsulated datagram.

6. Security Considerations

   IP encapsulation potentially reduces the security of the Internet,
   and care needs to be taken in the implementation and deployment of IP
   encapsulation.  For example, IP encapsulation makes it difficult for
   border routers to filter datagrams based on header fields.  In
   particular, the original values of the Source Address, Destination
   Address, and Protocol fields in the IP header, and the port numbers
   used in any transport header within the datagram, are not located in
   their normal positions within the datagram after encapsulation.
   Since any IP datagram can be encapsulated and passed through a
   tunnel, such filtering border routers need to carefully examine all
   datagrams.

6.1. Router Considerations

   Routers need to be aware of IP encapsulation protocols in order to
   correctly filter incoming datagrams.  It is desirable that such
   filtering be integrated with IP authentication [1].  Where IP
   authentication is used, encapsulated packets might be allowed to
   enter an organization when the encapsulating (outer) packet or the
   encapsulated (inner) packet is sent by an authenticated, trusted
   source.  Encapuslated packets containing no such authentication
   represent a potentially large security risk.

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   IP datagrams which are encapsulated and encrypted [2] might also pose
   a problem for filtering routers.  In this case, the router can filter
   the datagram only if it shares the security association used for the
   encryption.  To allow this sort of encryption in environments in
   which all packets need to be filtered (or at least accounted for), a
   mechanism must be in place for the receiving node to securely
   communicate the security association to the border router.  This
   might, more rarely, also apply to the security association used for
   outgoing datagrams.

6.2. Host Considerations

   Host implementations that are capable of receiving encapsulated IP
   datagrams SHOULD admit only those datagrams fitting into one or more
   of the following categories:

    -  The protocol is harmless:  source address-based authentication is
       not needed.

    -  The encapsulating (outer) datagram comes from an authentically
       identified, trusted source.  The authenticity of the source could
       be established by relying on physical security in addition to
       border router configuration, but is more likely to come from use
       of the IP Authentication header [1].

    -  The encapuslated (inner) datagram includes an IP Authentication
       header.

    -  The encapsulated (inner) datagram is addressed to a network
       interface belonging to the decapsulator, or to a node with which
       the decapsulator has entered into a special relationship for
       delivering such encapsulated datagrams.

   Some or all of this checking could be done in border routers rather
   than the receiving node, but it is better if border router checks are
   used as backup, rather than being the only check.

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7. Acknowledgements

   Parts of Sections 3 and 5 of this document were taken from portions
   (authored by Bill Simpson) of earlier versions of the Mobile IP
   Internet Draft [8].  The original text for section 6 (Security
   Considerations) was contributed by Bob Smart.  Good ideas have also
   been included from RFC 1853 [11], also authored by Bill Simpson.
   Thanks also to Anders Klemets for finding mistakes and suggesting
   improvements to the draft.  Finally, thanks to David Johnson for
   going over the draft with a fine-toothed comb, finding mistakes,
   improving consistency, and making many other improvements to the
   draft.

References

   [1] Atkinson, R., "IP Authentication Header", RFC 1826, August 1995.

   [2] Atkinson, R., "IP Encapsulating Security Payload", RFC 1827,
       August 1995.

   [3] Baker, F., Editor, "Requirements for IP Version 4 Routers", RFC
       1812, June 1995.

   [4] Gilligan, R., Nordmark, E., and B. Hinden, "IPAE: The SIPP
       Interoperability and Transition Mechanism", Work in Progress.

   [5] Knowles, S., "IESG Advice from Experience with Path MTU
       Discovery", RFC 1435, March 1993.

   [6] Mankin, A., and K. Ramakrishnan, "Gateway Congestion Control
       Survey", RFC 1254, August 1991.

   [7] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
       November 1990.

   [8] Perkins, C., Editor, "IP Mobility Support", RFC 2002,
       October 1996.

   [9] Postel, J., Editor, "Internet Control Message Protocol", STD 5,
       RFC 792, September 1981.

   [10] Postel, J., Editor, "Internet Protocol", STD 5, RFC 791,
        September 1981.

   [11] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.

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Author's Address

   Questions about this memo can be directed to:

   Charles Perkins
   Room H3-D34
   T. J. Watson Research Center
   IBM Corporation
   30 Saw Mill River Rd.
   Hawthorne, NY  10532

   Work:   +1-914-784-7350
   Fax:    +1-914-784-6205
   EMail: perk@watson.ibm.com

   The working group can be contacted via the current chair:

   Jim Solomon
   Motorola, Inc.
   1301 E. Algonquin Rd.
   Schaumburg, IL  60196

   Work:   +1-847-576-2753
   EMail: solomon@comm.mot.com

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