Internet Engineering Task Force                               C. Perkins
INTERNET DRAFT                                                       IBM
                                                            05 July
                                                         21 October 1995

                       IP Encapsulation within IP

Status of This Memo

   This document is a submission by the Mobile-IP Working Group of the
   Internet Engineering Task Force (IETF). Comments should be submitted
   to the mailing list.

   Distribution of this memo is unlimited.

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   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 effect "re-addressing"
   datagrams (i.e, (i.e., delivering them to an intermediate destination
   other than that specified in the IP destination field) for any of a
   variety of reasons, but particularly those useful for adherence to
   the mobile-IP specification.

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 effect "re-addressing"
   datagrams -- that is, delivering them to an intermediate destination
   other than that specified in the IP destination field.  The process
   of encapsulation and decapsulation a datagram is frequently referred
   to as "tunneling" the datagram, and the encapsulator and decapsulator
   are then considered to be the the "endpoints" of the tunnel.

   In the most general encapsulation case we have
      source ----> encapsulator --------> decapsulator ----> destination
   with these being separate machines.  There may in general be multiple
   source-destination pairs using the same tunnel.

2. Motivation

   The mobile-IP working group has specified the use of encapsulation as
   a way to deliver packets from a mobile host's "home network" to an
   agent which can deliver packets to the mobile host by conventional
   means [1]. [4].  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 include
   preferential billing, choice of routes with selected security
   attributes, and general policy routing.

   It is generally true that encapsulation and source routing techniques
   can both be used to re-address datagrams whenever that is necessary,
   but there are several technical reasons to prefer encapsulation:

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

    -  Current internet routers exhibit performance problems when
       forwarding packets which use the IP source routing option.

    -  Too many internet hosts process source routing options

    -  Firewalls may exclude source-routed packets.

    -  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 the packets which they did not originate.

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

    -  Encapsulated packets typically are longer than source routed

    -  Encapsulation cannot be used unless it is known in advance that
       the tunnel endpoint for the encapsulated datagram can decapsulate
       the packet.

   Since the majority of internet hosts 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

3. IP in IP Encapsulation

   An outer IP header is inserted before the datagram's IP header:

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

   The format of the IP header is described in RFC 791[4]. 791[9].  The outer
   IP header source and destination addresses identify the "endpoints"
   of the tunnel.  The inner IP header source and destination addresses
   identify the sender and recipient of the datagram.  The inner IP
   header is not changed by the node which encapsulates it inside the
   outer IP header, and it, except
   to decrement the TTL before delivery.  The inner header remains
   unchanged during its delivery to the tunnel endpoint.  No change
   to IP options in the inner header occurs during delivery of the
   encapsulated packet through the tunnel.

3.1. IP Header Fields and Handling




         The Internet Header Length measures the length (in bytes) of
         the outer IP header exclusive of its payload, but including any
         options which the encapsulating agent may insert.


         The type of service is copied from the inner IP header.

      Total Length

         The length measures the length of the outer IP header along
         with its payload, that is to say the inner IP header and the
         original datagram.



      Fragment Offset

         These three fields are set in accordance with the procedures
         specified in [4]. [9].  The "Don't Fragment" bit in the outer IP
         header is copied from the corresponding flag in the inner IP

      Time to Live

         The Time To Live (TTL) field in the outer IP header is set to
         be a
         value appropriate for delivery of the same as encapsulated datagram to
         the original datagram. tunnel endpoint.


         The protocol field in the outer IP header is set to protocol
         number 4 for the encapsulation protocol.

      Header Checksum

         The Header Checksum is computed over the length (in bytes) of
         the outer IP header exclusive of its payload, but including any
         options which the encapsulating endpoint may insert.

      Source Address

         The IP address of the encapsulating agent, that is, the tunnel
         starting point.

      Destination Address

         The IP address of the decapsulating agent, that is, the tunnel
         completion point.

   When decapsulating,


         not copied from the outer TTL minus one is inserted into inner IP header.  However, new options
         particular to the path MAY be added.  In particular, any
         supported flavors of security options of the inner IP TTL. Thus, hops are counted, but header
         MAY affect the actual routers interior choice of security options for the tunnel.  It
         is not expected that there be a one-to-one mapping of such
         options to the options or security headers selected for the

   The inner TTL is decremented by one.  If the resulting TTL is 0,
   the datagram is not tunneled.  An encapsulating agent MUST NOT
   encapsulate a packet with TTL = 0 for delivery to a tunnel are endpoint.
   The TTL is not identified.  This provides loop protection. changed when decapsulating.  If, after decapsulation,
   the inner packet has TTL zero, a tunnel endpoint MUST discard the
   packet.  If the decapsulator forwards the datagram to some network
   interface, it will decrement the TTL as a result of doing normal IP
   forwarding.  See also subsection 4.4.

   The encapsulating agent is free to use any existing IP mechanisms
   appropriate for delivery of the encapsulated payload to the tunnel
   endpoint.  In particular, this means that use of IP options and
   fragmentation are allowed, unless the "Don't Fragment" bit is set in
   the inner IP header.  This is required so that hosts employing Path
   MTU discovery [2] [7] can obtain the information they seek.

3.2. Routing Failures

   Routing loops within a tunnel are particularly dangerous when they
   arrive again at the encapsulator.  If the IP Source matches any of
   its interfaces, an implementation MUST NOT further encapsulate.  If
   the IP Source matches the tunnel destination, an implementation
   SHOULD NOT further encapsulate.  See also subsection 4.4.

4. ICMP messages from within the tunnel

   After an encapsulated datagram has been sent, the encapsulating
   agent may receive an ICMP [8] message from any intermediate router
   within the tunnel, except for the tunnel endpoint.  The action taken
   by the encapsulating agent depends on the type of ICMP message
   received.  In many cases,  When the received message contains enough information, the
   encapsulating agent will may use the incoming message to create a similar
   ICMP message, which then is to be sent to the originator of the inner IP datagram.
   This process will be referred to as "relaying" the ICMP message to
   the source of the original uncapsulated unencapsulated datagram.  Relaying an ICMP
   message requires that the encapsulator must strip off the outer IP
   header which it receives from the sender of the ICMP message.  For
   cases where the received message does not contain enough data, see
   section 5.

4.1. Destination Unreachable (Type 3)

   Destination Unreachable messages are handled depending upon their
   type.  The model suggested here allows the tunnel to "extend" a
   network to include non-local (e.g, (e.g., mobile) hosts.  Thus, if the
   original destination in the unencapsulated datagram is on the same
   network as the encapsulating agent, certain Destination Unreachable
   codes may be modified to conform to the suggested model.

      Code 0

      Network Unreachable (Code 0)

         A Destination Unreachable message may be returned to
         the original sender.  If the original destination in
         the unencapsulated datagram is on the same network as
         the encapsulating agent, the newly generated Destination
         Unreachable message sent by the encapsulating agent can have
         type 1,
         code 1 (Host Unreachable), since presumably the packet arrived
         to the correct network and the encapsulating agent is trying to
         create the appearance that the original destination is local
         even if it's not.  Otherwise, the encapsulating agent must
         return a Destination Unreachable with code 0 message to the
         original sender.

      Code 1

      Host Unreachable (Code 1)

         The encapsulating agent must relay Destination Host Unreachable messages of code 1 (host unreachable) to
         the source of the original unencapsulated datagram.

      Code 2

      Protocol Unreachable (Code 2)

         When the encapsulating agent receives a Destination Protocol Unreachable
         ICMP message with code 2 (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
         necessarily might only rarely use protocol 4, it would be usually be
         meaningless to return code 2 to that sender.

      Code 3

      Port Unreachable (Code 3)

         This code (port unreachable) should never be received by the encapsulating
         agent, since the outer IP header does not refer to any port
         number.  It must not be relayed to the source of the original
         unencapsulated datagram.

      Code 4

      Datagram Too Big (Code 4)

         The encapsulating agent must relay Destination Unreachable Datagram Too Big messages of code 4 (fragmentation needed and DF set) to
         the source of the original unencapsulated datagram.

      Code 5

      Source Route Failed (Code 5)

         This code (source route failed) should be treated by the encapsulating agent
         itself.  It must not be relayed to the source of the original
         unencapsulated datagram.

4.2. Time Exceeded Source Quench (Type 4)

   The encapsulating agent may relay Time Exceeded Source Quench messages to the
   source of the original unencapsulated datagram.

4.3. Redirect (Type 5)

   The encapsulating agent may act on the Redirect message if it is
   possible, but it should not relay the Redirect back to the source of
   the datagram which was encapsulated.

4.4. Time Exceeded (Type 11)

   ICMP Time Exceeded messages report routing loops within the tunnel
   itself.  Reception of Time Exceeded messages by the encapsulator
   MUST be reported to the originator as Host Unreachable (Type 3 Code
   1).  Host Unreachable is preferable to Network Unreachable; since the
   packet 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 packet, the packet is considered to have
   reached the correct network, but not the correct destination host
   within that network.

4.5. Parameter Problem (Type 12)

   If the parameter problem points to a field copied from the original
   unencapsulated datagram, the encapsulating agent may relay the ICMP
   message to the source; otherwise, if the problem occurs with an IP
   option inserted by the encapsulating agent, then the encapsulating
   agent must not relay the ICMP message to the source.  Note that an
   encapsulating agent following prevalent current practice will never
   insert any IP options into the encapsulated datagram.

4.4. Redirect

   The encapsulating agent may act on the Redirect message if it is
   possible, but it should not relay the Redirect back to the source
   of the datagram which was encapsulated.  Acting upon the Redirect
   message would require that the encapsulating agent maintain storage
   for encapsulated packets, and this may be an unpopular design choice datagram, except possibly
   for many purposes.

4.5. Source Quench

   The encapsulating agent may relay Source Quench messages to the
   source of the original unencapsulated datagram. security reasons.

4.6. Other messages

   Other ICMP messages are not related to the encapsulation operations
   described within this protocol specification, and should be acted on
   as specified in [3]. [8].

5. Security Considerations

   Security considerations are Tunnel Management

   Unfortunately, ICMP only requires IP routers to return 8 bytes (64
   bits) of the datagram beyond the IP header.  This is not addressed in this document.

6. Acknowledgements

   The text enough to
   include the encapsulated header, so it is not always possible for parts of section 3 was taken the
   home agent to immediately reflect the ICMP message from the mobile-IP

References interior
   of a tunnel back to the source host.

   However, by carefully maintaining "soft state" about its tunnels,
   the encapsulating router can return accurate ICMP messages in most
   cases.  The router SHOULD maintain at least the following soft state
   information about each tunnel:

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

   The router 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 router checks the soft state for the tunnel.  If the datagram
   would violate the state of the tunnel (such as, the TTL is less than
   the tunnel TTL) the router sends an ICMP error message back to the
   source, but also forwards the datagram into the tunnel.

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

   Tunnel soft state was originally developed for the IP address
   encapsulation (IPAE) specification [3].

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 (Type 3 Code 4) "Datagram Too Big" errors
   reported to the encapsulator.  To support originating hosts
   which use this capability, all implementations MUST support Path
   MTU Discovery([6, 7]) within their tunnels.  In this particular
   application there are several advantages:

    -  As a benefit of Tunnel MTU Discovery, any fragmentation which
       occurs because of the size of the encapsulation header is done
       only once after encapsulation.  This prevents more than one
       fragmentation of a single datagram, which improves processing
       efficiency of the path routers and tunnel decapsulator.

    -  If the source of the unencapsulated packet is doing MTU discovery
       then it is desirable for the encapsulator to know the MTU to the
       decapsulator.  If it doesn't know the MTU then it can transfer
       the DF bit to the outer packet; however, if that triggers ICMP
       Datagram Too Big from within the tunnel (and hence returned
       to the encapsulator) the encapsulator cannot always return a
       correct ICMP response to the source unless it has kept state
       information about recently sent packets.  If the tunnel MTU is
       returned to the source by the encapsulator in a Datagram Too Big
       message, the MTU that is conveyed 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,
       something that is otherwise certain to occur.

    -  If the source is not doing MTU discovery it is still desirable
       for the encapsulator to know the MTU to the decapsulator.  In
       particular it is much better to fragment the inner packet than
       to allow the outer packet to be fragmented.  Fragmenting the
       inner packet can be done without special buffer requirements and
       without the need to keep state in the decapsulator.  By contrast
       if the outer packet is fragmented then the decapsulator needs to
       keep state and buffer space on behalf of the destination.

   The encapsulator SHOULD in normal circumstances do MTU discovery and
   try to send packets with the DF bit set.  However there are problems
   with this approach.  When the source sets the DF bit it can react
   quickly to resend the information if it gets a ICMP Datagram Too
   Big.  When the encapsulator gets a ICMP Datagram Too Big, but the
   source had not set the DF bit, then there is nothing sensible that
   the encapsulator can do to let the source know.  The encapsulator
   MAY keep a copy of the sent packet whenever it tries increasing the
   MTU - this will allow it to resend the packet fragmented if it gets
   a packet too big response.  Alternatively the encapsulator MAY be
   configured for certain classes of input to not set the DF bit when
   the source has not set the DF bit.

5.2. Congestion

   Tunnel soft state will collect indications of congestion, such as
   an ICMP (Type 4) Source Quench or a Congestion Experienced flag in
   datagrams from the decapsulator (tunnel peer).  When forwarding
   another datagram into the tunnel, it is appropriate to send Source
   Quench messages to the originator.

6. Security Considerations

   IP encapsulation potentially reduces the security of the Internet.
   For this reason care needs to be taken in the implementation and

   Assume an organization has good physical control of a secure subset
   of its network.  Assume that the routers connecting that secure
   network do not allow in packets with source addresses belonging to
   interfaces on that secure network.  In that situation it is possible
   to safely deploy protocols within that network which depend on the
   source address of packets for authentication purposes.

   Networks with physical security can still be used to run protocols
   which are convenient, but which have implementation or protocol bugs
   which would make them dangerous to use if external sources have
   access to the protocol.  The external sources can be excluded using
   router packet filtering.

   IP encapsulation protocols may allow packets to bypass the checks in
   the border routers.  There are two cases to consider:

    -  The case where the people controlling the border routers are
       trying to protect inner machines from themselves.

    -  The case where the inner machine is looking after its own

   An uncooperative inner machine cannot be protected by the border
   router except by barring all packets to that machine.  There is
   nothing to stop encapsulated IP coming in to that inner machine
   in otherwise harmless packets such as port 53 UDP packets (i.e.,
   apparently DNS packets).  So there is a strong case for placing the
   security controls at the host rather than the router.  However, in
   situations where the administrative control of the inner machine is
   cooperative but lacks thoroughness or competence, security can be
   enhanced by also putting protection in the border routers.

6.1. Router Considerations

   Routers need to be aware of IP encapsulation protocols so they can
   correctly filter incoming packets.

   Beyond that it is desirable that filtering be integrated with IP
   authentication [1].  In the case of IP encapsulation this can have
   2 forms:  Encapsulation might be allowed (in some cases) as long
   as the encapsulating packets authentically come from an expected
   encapsulator.  Alternatively encapsulation might be allowed if the
   encapsulated packets have authentication.

   The next problem is with packets which are encapsulated and
   encrypted [2].  In this case the router can only filter the packet if
   it knows the security association.  To allow this sort of encryption
   in environments where all packets need to be filtered (or at least
   accounted for) a mechanism must be in place for the receiving host
   to securely communicate the association to the border router.  This
   might, more rarely, also apply to the association used for outgoing

6.2. Host Considerations

   Receiving IP encapsulation software SHOULD classify incoming packets
   and only allow packets fitting one of the following categories:

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

    -  The packet can be trusted because of trust in the authentically
       identified encapsulating host.  That authentic identification
       could come from physical security plus border router
       configuration but is more likely to come from AH authentication.

    -  The inner packet has AH authentication.

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

6.3. Using Security Options

   The security options of the inner IP header MAY affect the choice of
   security options for the encapsulating IP header.

7. Acknowledgements

   Parts of sections 3 and 5 were taken from the mobile-IP draft [5].
   Good ideas have also been included from RFC 1853 [10].  "Security
   Considerations" (section 6) was largely contributed by Bob Smart.
   Thanks also to Anders Klemets for finding mistakes and suggesting
   many improvements to the draft.


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

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

    [3] R. Gilligan, E. Nordmark, and B. Hinden.  IPAE: The SIPP
        Interoperability and Transition Mechanism.  Internet Draft --
        work in progress, March 1994.

    [4] IETF Mobile-IP Working Group.  IPv4 Mobility Support.
       ietf-draft-mobileip-protocol-10.txt --
        ietf-draft-mobileip-protocol-12.txt - work in progress,
        September 1995.

    [5] IETF Mobile-IP Working Group.  IPv4 Mobility Support.
        ietf-draft-mobileip-protocol-10.txt -- outdated draft, May 1995.


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

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


    [8] J. Postel.  Internet Control Message Protocol.  RFC 792,
        September 1981.


    [9] J. Postel.  Internet Protocol.  RFC 791, September 1981.

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

Author's Address

   Questions about this memo can also be directed to:

          Charles Perkins
          Room J1-A25
          T. J. Watson Research Center
          IBM Corporation
          30 Saw Mill River Rd.
          Hawthorne, NY  10532

          Work:   +1-914-784-7350
          Fax:    +1-914-784-7007

   The working group can be contacted via the current chairs:

        Jim Solomon                       Tony Li
        Motorola, Inc.                    cisco systems
        1301 E. Algonquin Rd.             170 W. Tasman Dr.
        Schaumburg, IL  60196             San Jose, CA  95134

        Work:   +1-708-576-2753           Work:   +1-408-526-8186
        E-mail:      E-mail: