Transmission of IPv4 Packets over Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Interfaces
RFC 5579

Independent Submission                                   F. Templin, Ed.
Request for Comments: 5579                  Boeing Research & Technology
Category: Informational                                    February 2010
ISSN: 2070-1721

                   Transmission of IPv4 Packets over
  Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Interfaces

Abstract

   The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
   specifies a Non-Broadcast, Multiple Access (NBMA) interface type for
   the transmission of IPv6 packets over IPv4 networks using automatic
   IPv6-in-IPv4 encapsulation.  The original specifications make no
   provisions for the encapsulation and transmission of IPv4 packets,
   however.  This document specifies a method for transmitting IPv4
   packets over ISATAP interfaces.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5579.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

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RFC 5579                IPv4 Packets over ISATAP           February 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. ISATAP Interface Model ..........................................3
   4. ISATAP Interface MTU ............................................4
   5. IPv6 Operation ..................................................4
   6. IPv4 Operation ..................................................4
      6.1. ISATAP IPv4 Address Configuration ..........................4
      6.2. IPv4 Route Configuration ...................................5
      6.3. ISATAP Interface Determination .............................5
      6.4. Next-Hop Resolution ........................................5
      6.5. Encapsulation and Transmission .............................6
      6.6. IPv4 Multicast Mapping .....................................6
      6.7. Recursive Encapsulation Avoidance ..........................7
   7. Security Considerations .........................................7
   8. Acknowledgements ................................................7
   9. References ......................................................7
      9.1. Normative References .......................................7
      9.2. Informative References .....................................8
   Appendix A. Encapsulation Avoidance ................................9

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1.  Introduction

   The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
   [RFC5214] specifies a Non-Broadcast, Multiple Access (NBMA) interface
   type for the transmission of IPv6 packets over IPv4 networks using
   automatic IPv6-in-IPv4 encapsulation.  ISATAP interfaces therefore
   typically configure IPv6 addresses and prefixes, but they do not
   configure IPv4 addresses and prefixes.  In typical implementations
   and deployments, an ISATAP interface therefore appears as an ordinary
   interface configured for IPv6 operation but with a null IPv4
   configuration.  This places an unnecessary limitation on the ISATAP
   domain of applicability.

   ISATAP interfaces perform automatic IPv6-in-IPv4 encapsulation over a
   virtual IPv6 overlay that spans a region within a connected IPv4
   routing topology (i.e., a "site") comprising ordinary IPv4 routers.
   ISATAP interfaces configure IPv6 link-local addresses that
   encapsulate an IPv4 address assigned to an underlying IPv4 interface
   within the IPv6 link-local prefix "fe80::/10", as specified in
   Sections 6 and 7 of [RFC5214].  ISATAP interfaces may additionally
   configure IPv6 addresses from a non-link-local IPv6 prefix in exactly
   the same fashion.  As a result, [RFC5214] extends the basic
   transition mechanisms specified in [RFC4213].

   This document specifies mechanisms and operational practices that
   enable automatic IPv4-in-IPv4 encapsulation over ISATAP interfaces in
   the same manner as for IPv6-in-IPv4 encapsulation.  As a result, this
   document also extends the IPv4-in-IPv4 tunneling mechanisms specified
   in [RFC2003].  These mechanisms are useful in a wide variety of
   enterprise network scenarios, e.g., as discussed in the RANGER
   [RANGER] and VET [VET] proposals.

   The following sections specify IPv4 operation over ISATAP interfaces.
   A working knowledge of the IPv4 and IPv6 protocols ([RFC0791] and
   [RFC2460]), IPv4-in-IPv4 encapsulation [RFC2003], and IPv6-in-IPv4
   encapsulation ([RFC4213] and [RFC5214]) is assumed.

2.  Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

3.  ISATAP Interface Model

   ISATAP interfaces use automatic IPv6-in-IPv4 encapsulation to span a
   region within a connected IPv4 routing topology (i.e., a "site") in a
   single IPv6 hop.  That is to say that the site comprises border nodes

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   with ISATAP interfaces that forward IPv6-in-IPv4 packets across the
   site in a single IPv6 hop, and ordinary IPv4 routers between the
   border nodes that decrement the Time to Live (TTL) in the outer IPv4
   header.  Border nodes that configure ISATAP interfaces within the
   same site therefore see each other as single-hop neighbors.

   ISATAP interfaces are configured over one or more of the node's
   underlying IPv4 interfaces connected to the site.  These underlying
   IPv4 interfaces configure site- or greater-scoped IPv4 addresses, and
   the underlying IPv4 interfaces of two "neighboring" ISATAP interfaces
   may be separated by many IPv4 hops within the site.

   This specification simply extends the ISATAP interface model to also
   support IPv4-in-IPv4 encapsulation.  When IPv4-in-IPv4 encapsulation
   is used, the ISATAP interface spans exactly the same underlying site
   as for IPv6-in-IPv4 encapsulation.

4.  ISATAP Interface MTU

   ISATAP interface MTU considerations are exactly as specified in
   Section 3.2 of [RFC4213] and apply equally for both IPv6 and IPv4
   operation.

5.  IPv6 Operation

   IPv6 operations over ISATAP interfaces are exactly as specified in
   [RFC5214].

6.  IPv4 Operation

   The following sections specify IPv4 operation over ISATAP interfaces:

6.1.  ISATAP IPv4 Address Configuration

   As for IPv6 operation, IPv4 operation requires that all ISATAP
   interfaces connected to the same site configure a unique Layer 3 IPv4
   address ("L3ADDR") taken from an IPv4 prefix for the site.  L3ADDR is
   used for next-hop determination, but it may also be used as the
   source address for packets that originate from the ISATAP interface
   itself.

   When a unique "name" for the ISATAP site is required (e.g., to
   distinguish it from other ISATAP sites), L3ADDR is taken from a
   public IPv4 prefix.  Otherwise, it may be taken from a link-local-
   scoped IPv4 prefix (e.g., 169.254/16 [RFC3927]).

   Methods for ensuring L3ADDR uniqueness include dynamic allocation
   using DHCP [RFC2131], manual configuration, etc.

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6.2.  IPv4 Route Configuration

   As for any IPv4 interface, IPv4 Forwarding Information Base (FIB)
   entries (i.e., IPv4 routes) are configured over ISATAP interfaces via
   either administrative or dynamic mechanisms.

   Next-hop addresses in FIB entries configured over an ISATAP interface
   correspond to the L3ADDR assigned to the ISATAP interface of a
   neighbor.

6.3.  ISATAP Interface Determination

   When the node's IPv4 layer has a packet to send, it performs an IPv4
   FIB lookup to determine the outgoing ISATAP interface and the next-
   hop L3ADDR.  The node then checks the packet length against the MTU
   configured on the ISATAP interface.

   If the packet is no larger than the MTU, the node admits it into the
   ISATAP interface without fragmentation.  If the packet is larger than
   the MTU, the node examines the "Don't Fragment (DF)" flag in the IPv4
   header.  If DF=1, it drops the packet and returns an ICMPv4
   "fragmentation needed" message to the original source [RFC1191];
   otherwise, it fragments the packet using IPv4 fragmentation and
   admits each fragment into the ISATAP interface.

6.4.  Next-Hop Determination and Address Mapping

   As for ISATAP for IPv6, ISATAP for IPv4 requires a means for
   determining the L3ADDR of neighbors on the ISATAP interface that can
   provide a next-hop toward the final destination.  Next-hop
   determination for default routes is through the Potential Router List
   (PRL) discovery procedures specified in Section 8.3.2 of [RFC5214].
   Next-hop determination methods for more-specific routes include
   forwarding initial packets via a default router until a redirect is
   received, name service lookup (e.g., as described in [VET]), etc.

   After a next-hop L3ADDR is discovered, the node admits IPv4
   packets/fragments into the ISATAP interface and maps the next-hop
   L3ADDR into a next-hop Layer 2 address ("L2ADDR"), which in reality
   is the IPv4 address of an underlying interface of the ISATAP neighbor
   that may be many IPv4 hops away.

   Address mapping for IPv4 differs from the IPv6 version in that no
   algorithmic mapping between L3ADDR and L2ADDR is possible.  ISATAP
   for IPv4 therefore requires an L3ADDR->L2ADDR address mapping method
   that is coordinated on a per-site basis such that all nodes in the
   site follow the same convention.  Examples include name service
   lookup (e.g., as described in [VET]), static mapping table lookup,

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   etc.

   The node next performs an IPv4 FIB lookup on the next-hop L2ADDR to
   determine the correct underlying IPv4 interface.  If the FIB lookup
   fails, the node drops the packet and returns an ICMPv4 "Destination
   Unreachable" message to the original source [RFC0792]; otherwise, it
   encapsulates the packet and submits it to the IPv4 layer as described
   below.

6.5.  Encapsulation and Transmission

   After performing the IPv4 FIB lookup on the next-hop L2ADDR, the node
   encapsulates the packet as specified in [RFC2003] with the IPv4
   address of the underlying interface as the outer IPv4 source address
   and the next-hop L2ADDR as the outer IPv4 destination address.  The
   node sets the DF flag in the outer IPv4 header according to Section
   3.2 of [RFC4213].  The node also sets the IP protocol field in the
   outer IPv4 header to 4 (i.e., ip-protocol-4).

   The node then submits the encapsulated packet to the IPv4 layer.  The
   IPv4 layer fragments the packet if necessary, then forwards each
   fragment to the underlying IPv4 interface.  The underlying IPv4
   interface then performs address resolution on the outer IPv4
   destination address (i.e., the next-hop L2ADDR) and submits the
   packet for transmission on the underlying link layer.

6.6.  IPv4 Multicast Mapping

   In many aspects, ISATAP is simply a unicast-only derivative of
   "6over4" [RFC2529].  For various reasons, however, ISATAP has seen
   practical wide-scale deployment while the 6over4 approach has been
   silently carried forward through ongoing research efforts.  This
   specification extends the ISATAP interface model to support IPv4
   multicast mapping in a manner that exactly parallels IPv6 multicast
   mapping in 6over4 (see [RFC2529], Section 6).  Indeed, the approach
   might more aptly be named "4over4" were it not for the fact that the
   name "ISATAP" has already become ingrained in the widely published
   terminology.

   IPv4 multicast mapping is available only on ISATAP interfaces
   configured over sites that support IPv4 multicast.  For such sites,
   an IPv4 packet sent on an ISATAP interface with a multicast
   destination address DST MUST be encapsulated for transmission on an
   underlying IPv4 interface to the IPv4 multicast address of
   Organization-Local Scope using the mapping below.  The mapped address
   SHOULD be taken from the block 239.193.0.0/16, a different sub-block
   of the Organization-Local Scope address block, or -- if none of those
   are available -- from the expansion blocks defined in [RFC2365].

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   Note that when they are formed using the expansion blocks, they use
   only a /16-sized block.

   +-------+-------+-------+-------+
   |  239  |  OLS  | DST2  | DST3  |
   +-------+-------+-------+-------+

        DST2, DST3          Last two bytes of IPv4 multicast address.

        OLS                 From the configured Organization-Local
                            Scope address block.  SHOULD be 193;
                            see [RFC2365] for details.

                   Figure 1: ISATAPv4 Multicast Mapping

   No new IANA registration procedures are required for the above.

6.7.  Recursive Encapsulation Avoidance

   The node must take care in managing its IPv4 FIB table entries in
   order to avoid looping through recursive encapsulations.

7.  Security Considerations

   The security considerations specified in [RFC2003] apply equally to
   this document.  The security considerations specified in ISATAP
   [RFC5214] and 6over4 [RFC2529] also apply, with the exception that
   ip-protocol-4 encapsulation is used instead of ip-protocol-41.

   Updated tunnel security considerations are found in [SECURITY].

8.  Acknowledgements

   This work extends the ISATAP interface model, which has evolved
   through the insights of many contributers over the course of many
   decades.  Special thanks to Brian Carpenter and Jari Arkko for their
   helpful review input.

9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

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   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927, May
              2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

9.2.  Informative References

   [SECURITY] Hoagland, J., Krishnan, S., and D. Thaler, "Security
              Concerns With IP Tunneling", Work in Progress, October
              2008.

   [VET]      Templin, F., "Virtual Enterprise Traversal (VET)", RFC
              5558, February 2010.

   [RANGER]   Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720, February
              2010.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC2365]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
              RFC 2365, July 1998.

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Appendix A.  Encapsulation Avoidance

   In some instances, an ISATAP interface may be configured over a site
   in which the L3ADDRs and L2ADDRs of all ISATAP neighbors are also
   known to be routable within the underlying site.  In that case, the
   ISATAP interface MAY avoid encapsulation and submit the
   unencapsulated packets directly to the IPv4 layer.  Note however that
   this approach does not provide for the use of indirection afforded
   through encapsulation.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707 MC 7L-49
   Seattle, WA  98124
   USA

   EMail: fltemplin@acm.org

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