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Internet Engineering Task Force                           Bernhard Petri
INTERNET-DRAFT                                                Siemens AG
<draft-petri-mobileip-pipe-00.txt>
Date:  Jan. 20, 2000

Expires: July 2000






               Private IP Encapsulation within IP (PIPE)


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   RFC 2003 specifies a method by which an IP datagram may be
   encapsulated (carried as payload) within an IP datagram. This is 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.

   This draft allows to extend this encapsulation mechanism also for
   private IP addresses.





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1. Introduction

   [RFC 2003] 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.

   [RFC 2003] only allows to encapsulate public IP addresses within IP.
   However, current IP solutions often use non-unique private IP
   addresses, e.g. taken from the address space reserved for this
   purpose in [RFC 1918].  These private IP addresses typically are
   unique within a private Intranet, e.g. behind a firewall / Network
   Address Translator (NAT), but are not globally unique and not
   globally routable in the Internet.

   This draft therefore outlines an extension of [RFC 2003] which allows
   to encapsulate and decapsulate such private IP addresses in the same
   way as described in [RFC 2003], and  to transfer them across the
   public Internet (also referred to as "tunneling" in RFC 2003).
   Behaviour not explicitly mentioned in this draft applies as specified
   in [RFC 2003].

2. Motivation and Solution Overview

2.1 Motivation: Mobility Applications Using Private IP Addresses


   It is expected that there may be various applications which will be
   able to benefit from the PIPE encapsulation mechanism outlined in
   this draft. An important initial application will be the support of
   mobile nodes having obtained private IP addresses within a foreign
   network and/or using private IP addresses in their home network (see
   also section 2 of [RFC 2003]).

   Figure 1 shows an example of a basic tunneling/encapsulation case
   where a private IP address is translated and encapsulated through a
   public IP tunnel; within the framework of mobility applications, the
   source might e.g.  be a foreign agent registering at the home agent
   as the destination, or the source might e.g. be the home agent
   forwarding data packets to the foreign agent as the destination.




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   The terms "encapsulator" and "decapsulator" are used as defined in
   [RFC 2003]. It should be noted that the encapsulator - as the entry
   point of the tunnel - also performs an address resolution function,
   in this case between private und public IP addressing. The particular
   resolution functions used by the encapsulator are outside the scope
   of this draft.


           private IP         private IP in IP       private IP
              |                     |                   |
      source ---> encapsulator  --------> decapsulator ---> destination

       Figure 1: Example for encapsulation of private IP in IP


   In Figure 1, the "private IP" address realm between source and
   encapsulator may or may not be the same "private IP" address realm as
   the one between decapsulator and destination.

2.2 Identification of Private Addressing Realms

   A major problem of the use of private IP addresses is that they are
   not globally unique, and that a decapsulator receiving an
   encapsulated private IP address would usually not know, to which
   address space a private IP address belongs to. In Figure 1 above, the
   decapsulator will usually not be able to derive the particular
   private addressing realm from its knowledge about the begin of the
   tunnel.

   Similar problems had already emerged within the Internetwork over
   NBMA (ION) area of the IETF, and a solution had been developed using
   a global identification scheme for IP-VPNs which is outlined in [RFC
   2685]. This scheme uses an identifier ("VPN-ID") to identify a
   private network, typically with the objective to provide a related
   VPN service. The scheme is based on a well-known OUI/index mechanism
   as e.g. also used for MAC addresses, or for the Interface ID of IPv6
   addresses. The VPN-ID consists of a 7-octet format which is split
   into a 3-octet OUI of a "VPN authority", followed by a 4-octet index
   allocated by that authority.

   This draft proposes to also use this identification scheme for the
   indication of the particular address realm a private IP address
   belongs to, when being encapsulated and being transferred across the
   internet. Since private IP addresses are assumed to be unique within
   that particular private network, it is easy to attach a VPN-ID to
   them, e.g. by using the VPN-related OUI of the owner of that network.





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2.3. Example: Private IP Interconnection

   This example is intended to illustrate and motivate the generic
   solution specified in sections 3 ff below. In this particular
   example, the configuration shown in Figure 1 is taken, and it is
   assumed that both the source and destination belong to the same
   private address realm "PR1".

   Two different cases for the IP-IP tunnels can be distinguished:

     a) The decapsulator may be configured in a way that all inner IP
     header addresses received via IP-IP tunnels, are assumed to belong
     to one particular IP address space (either a private IP address
     space or the public IP addressing). In this case, the encapsulator
     will only insert IP datagrams from that particular address space
     into the IP-IP tunnel.

     b) No particular IP address space is pre-associated with the IP-IP
     tunnel.  This configuration option applies in cases where the
     encapsulator might send IP datagrams for different address spaces
     (e.g. public and private) via the IP-IP tunnel to a decapsulator.


   In case a), IP-IP encapsulation is applied in a similar way as
   specified in section 3 of [RFC 2003], with the difference that in
   this example, the IP addresses within the inner IP header are
   configured to belong to the private IP address space "PR1" rather
   than to public IP addressing.

   In order to encapsulate a private IP datagram into an IP-IP tunnel in
   case b), an outer IP header is inserted by the encapsulator before
   the datagram's existing private IP header, as follows:



















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                                              Public IP Network

      Private IP Network "PR1"          +---------------------------+
                                        |                           |
                                        |      Outer IP Header      |
                                        |                           |
                                        +--------+------------------+
                                        |  Sel.  |    VPN-OUI       |
                                        +--------+------------------+
                                        |         VPN index         |
    +---------------------------+       +---------------------------+
    |                           |       |                           |
    |  private IP Header (PR1)  |       |  private IP Header (PR1)  |
    |                           |       |                           |
    +---------------------------+ ====> +---------------------------+
    |                           |       |                           |
    |                           |       |                           |
    |       IP Payload          |       |         IP Payload        |
    |                           |       |                           |
    |                           |       |                           |
    +---------------------------+       +---------------------------+

           Figure 2: Example for Private IP-IP Encapsulation


   The detailed formats for the fields shown in Figure 2 are specified
   in section 3 below. The Selector field ("Sel.") serves as
   discriminator / selector indicating which types of addresses are used
   in the inner IP header. The VPN-Identifier (VPN-ID), consisting of
   VPN-OUI and VPN index, in this example shows a value identifying PR1.

3. Private IP-IP Encapsulation (PIPE)

3.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].

   In this document, the following definitions and acronyms are used:

   Default Addressing Realm -- this is the configured addressing realm a
   particular (sent or received) source / destination IP address is
   assumed to belong to, unless another addressing realm is explicitly
   indicated by a VPN-ID with the mechanisms specified below. In case of
   [RFC 2003], the default addressing realm is the public IP addressing.




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   Explicitly Indicated Addressing Realm -- this is the addressing realm
   specified by a VPN-ID related to a particular sent or received source
   / destination IP address.

   VPN-Identifier (VPN-ID) -- this term is specified in [RFC 2685], and
   is used in this document for the identification of a particular
   Explicitly Indicated Addressing Realm.

   Other terms, like e.g. "encapsulator", "decapsulator", "inner IP
   header" and "outer IP header" are used as specified in section 3 of
   [RFC 2003].

3.2 Configuration Options and Backward Compatibility

   This draft outlines a backward compatible extension of [RFC 2003] for
   the use of private IP addresses. Behaviour not explicitly mentioned
   in this draft applies as specified in [RFC 2003].

   As indicated in [RFC 2003], IP-IP tunnels require knowledge about the
   decapsulation capabilities at the endpoint of the tunnel. For this
   document, 3 different configuration options are distinguished which
   determine how source and destination IP addresses, indicated in the
   inner IP header, are interpreted.

     (1)All source and destination addresses, both in the inner and
     outer IP headers, are interpreted as being public IP addresses.
     Support of this configuration option IS REQUIRED for the case of
     communication with legacy [RFC 2003] devices.

     (2)All indicated source and destination IP addresses, indicated in
     the inner or outer header of IP-IP tunnels, are assumed to belong
     to one particular Default Addressing Realm. This configuration
     option MUST e.g. be selected if either the encapsulator or the
     decapsulator is not able to process VPN-IDs.  The Default
     Addressing Realm may be the public Internet addressing (as in RFC
     2003) or any other private IP addressing realm. The Default
     Addressing Realm for the outer IP header is that of the network
     between Encapsulator and Decapsulator.

     (3) Indicated source IP addresses may belong to an Explicitly
     Indicated Addressing Realm or to the Default Addressing Realm;
     indicated destination IP addresses may also belong to an Explicitly
     Indicated Addressing Realm or to the Default Addressing Realm. This
     is the most general case.

   For equipment complying to this specification, it IS REQUIRED to
   support configuration options (1) and (2), and it is RECOMMENDED to
   also support option (3). Support of option (3) IS REQUIRED if more



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   than one addressing realm is to be supported by IP-IP tunnels.

   The main purpose of configuration option (1) is to ensure
   interoperability with legacy [RFC 2003] devices. For configuration
   option (1), the IP in IP encapsulation formats specified in [RFC
   2003] MUST be used.

   The formats for configuration options (2) and (3) are outlined in
   section 3.3 below. For configuration options (2) and (3), Default
   Addressing Realms for the inner source and destination IP addresses
   MUST be preallocated. Note that the Default Addressing Realms for
   source and destination MAY be different.

   For all 3 options, the format of the inner "IP header" and "IP
   payload" fields MUST be coded as specified in section 3 of [RFC
   2003].

3.3 PIPE Encapsulation Formats

   For the formats used in configuration options (2) and (3), 5
   different cases can be distinguished (see subsections 3.3.1 - 3.3.5
   below). For all 5 cases, the "Protocol" field of the outer IP header
   (identifying the next level protocol) MUST be set to the value: ..
   <to be allocated by IANA, e.g. value 129>.

   The Selector Byte ("Sel."), specified below, is used to distinguish
   between these cases:

3.3.1 Default Source and Destination Address

   If for both source and destination address the Default Addressing
   Realm is used, the encapsulation format of [RFC 2003] applies. Note
   that unlike in [RFC 2003], the inner header IP addresses in this case
   are not automatically assumed to be public IP addresses, but are
   interpreted according to the Default Address Realm(s). It is not
   precluded that the Default Address Realms for the source and
   destination addresses are different.

   For configuration option (2), this is the only possible format.


3.3.2 Explicitly Indicated Source and Destination Adress
      (Same Realm)

   If both the source and destination IP address are to be indicated
   explicitly, and belong to the same addressing realm, the following
   encapsulation format SHOULD be used:




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         +---------------------------+
         |                           |
         |      Outer IP Header      |
         |                           |
         +------+--------------------+
         | Sel. |     VPN-OUI        |
         +------+--------------------+
         |         VPN index         |
         +---------------------------+
         |                           |
         |     Inner IP Header       |
         |                           |
         +---------------------------+
         |                           |
         |                           |
         |       IP Payload          |
         |                           |
         |                           |
         +---------------------------+

           Figure 3: Private IP-IP Encapsulation Format (Case 2)


      Outer IP Header:   as in [RFC 2003]
      Sel.:      Selector, value to be allocated by IANA:
                 tentatively:  0xE0
                 (= explicitly indicated source / destination address,
                  same addressing realm)
      VPN-OUI, VPN index:  as in [RFC 2685]
                 (refers to both source and destination IP address)

   In addition to the format specified above, the format specified in
   section 3.3.5 below MAY instead by used in certain cases.



3.3.3 Default Source Adress, Explicit Destination Address

   If only the destination address is to be indicated explicitly, the
   following encapsulation format MUST be used:










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         +---------------------------+
         |                           |
         |      Outer IP Header      |
         |                           |
         +------+--------------------+
         | Sel. |     VPN-OUI        |
         +------+--------------------+
         |         VPN index         |
         +---------------------------+
         |                           |
         |     Inner IP Header       |
         |                           |
         +---------------------------+
         |                           |
         |                           |
         |       IP Payload          |
         |                           |
         |                           |
         +---------------------------+

           Figure 4: Private IP-IP Encapsulation Format (Case 3)


      Outer IP Header:   as in [RFC 2003]
      Sel.:      Selector, value to be allocated by IANA:
                 tentatively:  0xE1
                 (=default source, explicitly indicated destination)
      VPN-OUI, VPN index:  as in [RFC 2685], refers to destination only



3.3.4 Explicit Source Adress, Default Destination Address

   If only the source address is to be indicated explicitly, the
   following encapsulation format MUST be used:















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         +---------------------------+
         |                           |
         |      Outer IP Header      |
         |                           |
         +------+--------------------+
         | Sel. |     VPN-OUI        |
         +------+--------------------+
         |         VPN index         |
         +---------------------------+
         |                           |
         |     Inner IP Header       |
         |                           |
         +---------------------------+
         |                           |
         |                           |
         |       IP Payload          |
         |                           |
         |                           |
         +---------------------------+

           Figure 5: Private IP-IP Encapsulation Format (Case 4)


      Outer IP Header:   as in [RFC 2003]
      Sel.:      Selector, value to be allocated by IANA:
                 tentatively:  0xE2
                 (=explicitly indicated source, default destination)
      VPN-OUI, VPN index:  as in [RFC 2685], refers to source only



3.3.5 Explicitly Indicated Source and Destination Address
      (Different Realms)

   If both the source and the destination address are to be indicated
   explicitly, and belong to different addressing realms, the following
   encapsulation format MUST be used:













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         +---------------------------+
         |                           |
         |      Outer IP Header      |
         |                           |
         +------+--------------------+
         | Sel. |    VPN-OUI (S)     |
         +------+--------------------+
         |      VPN index (S)        |
         +---------------------------+
         | Pad  |    VPN-OUI (D)     |
         +------+--------------------+
         |      VPN index (D)        |
         +---------------------------+
         |                           |
         |     Inner IP Header       |
         |                           |
         +---------------------------+
         |                           |
         |                           |
         |       IP Payload          |
         |                           |
         |                           |
         +---------------------------+

           Figure 6: Private IP-IP Encapsulation Format (Case 5)


      Outer IP Header:   as in [RFC 2003]
      Sel.:      Selector, value to be allocated by IANA:
                 tentatively:  0xE3
                 (=explicitly indicated source and destination,
                  different addressing realm)
      VPN-OUI (S), VPN index (S): as in [RFC 2685], refers to source
      Pad:       Pad field (inserted for 32-bit alignment), this field
                 MUST be coded as 0x00, and is ignored on receipt)
      VPN-OUI (D), VPN index (D): as in [RFC 2685], refers to
   destination


4. Example: A Mobile Node Registers at its Home Agent

   This section provides an example illustrating how the formats
   specified in section 3 above can be applied. In this example, a
   mobile node MN has obtained a temporary private IP address within a
   private IP address realm PR2, where it is currently located. It now
   wants to register this address with its home agent HA which owns a
   private IP address within realm PR3.



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       priv. realm PR2      public IP (or PR4)       private realm PR3
            |                       |                       |
       MN  --->  border gateway A ------> border gateway B --->  HA
                      (BG A)                   (BG B)

       Figure 7: Example for encapsulation of private IP in IP


   Since PR3 is different from PR2, the following Private IP in IP
   Encapsulation (PIPE) formats are used on the way from MN to HA:


   Step 1:  From MN to BG A:
     Outer Header:  Source Address:       MN(PR2)
                    Destination Address:  BG A(PR2)
     Inner Header:  Source Address:       MN(PR2) = default
                    Destination Address:  HA (PR3)
     Selector:      0xE1 (explicitly indicated destination)



   Step 2: From BG A to BG B:
     Outer Header:  Source Address:       BG A(public IP or PR4)
                    Destination Address:  BG B(public IP or PR4)
     Inner Header:  Source Address:       MN (PR2)
                    Destination Address:  HA (PR3)
     Selector:      0xE3 (explicit source and destination,
                          different address realms)




   Step 3: From BG B to HA:
     Outer Header:  Source Address:       BG B(PR3)
                    Destination Address:  HA(PR3)
     Inner Header:  Source Address:       MN(PR2)
                    Destination Address:  HA(PR3) = default
     Selector:      0xE2 (explicitly indicated source)


   It should be noted that in addition to the formats illustrated above,
   a real transfer of a packet from MN to HA also involves a number of
   routing decisions, and address resolution functions which are outside
   the scope of this specification, and may perhaps be specified in
   separate drafts.

   In the example above, the use of the public Internet as a backbone to
   interconnect the address realms PR2 and PR3 seems to be a natural



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   choice, but another private realm (PR4) might also be suitable for
   that purpose. The selection of a transit backbone or a particular
   address resolution may be subject to different criteria, and/or may
   be dependent on particular applications.

5. 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.  More detailed considerations of security implications
   of IP-IP tunnels can be found in section 6 of [RFC 2003].

   Since private addresses are typically administered to prevent access
   to networks inside an enterprise, the transfer of private addresses
   across networks outside this enterprise must be handled with great
   care. It may be required to use authentication and possibly
   encryption to maintain the existing security policy which originally
   dictated the choice of using a private address space within the
   enterprise.

6. IANA Considerations

   It is proposed that IANA establishes and maintains a list of protocol
   values for the selector byte following the outer IP header. The
   following values are an example for this possible list:

     0x00 through 0xDF  as for corresponding IP version number and
                        header field
     0xE0 - 0xE3        as defined in this specification
     0xE4 - 0xFF        reserved

   In addition, the formats for private IP-IP encapsulation specified in
   this document require the allocation of a new value in the "Protocol"
   field (identifying the next header) within the IPv4 header, (e.g.:
   129   PIPE   Private IP-IP Encapsulation    .... ).

7. IPR Considerations

   Siemens may own intellectual property on some of the technologies
   described in this document.

References

   [RFC 1918] Rekhter, Y. et al., "Address Allocation for Private
   Internets", RFC 1918, Febr. 1996

   [RFC 2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
   October 1996



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   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997

   [RFC 2685] Fox, B., Gleeson, B., "Virtual Private Networks
   Identifier", RFC 2685, September 1999.


Author Information


   Bernhard Petri
   Siemens AG
   Hofmannstr. 51
   Munich, Germany, D-81359
   phone: +49 89 722-34578
   email: bernhard.petri@icn.siemens.de



































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