IANA-Reserved IPv4 Prefix for Shared Address Space
RFC 6598

Internet Engineering Task Force (IETF)                           J. Weil
Request for Comments: 6598                             Time Warner Cable
BCP: 153                                                    V. Kuarsingh
Updates: 5735                                      Rogers Communications
Category: Best Current Practice                                C. Donley
ISSN: 2070-1721                                                CableLabs
                                                         C. Liljenstolpe
                                                           Telstra Corp.
                                                              M. Azinger
                                                 Frontier Communications
                                                              April 2012

           IANA-Reserved IPv4 Prefix for Shared Address Space

Abstract

   This document requests the allocation of an IPv4 /10 address block to
   be used as Shared Address Space to accommodate the needs of Carrier-
   Grade NAT (CGN) devices.  It is anticipated that Service Providers
   will use this Shared Address Space to number the interfaces that
   connect CGN devices to Customer Premises Equipment (CPE).

   Shared Address Space is distinct from RFC 1918 private address space
   because it is intended for use on Service Provider networks.
   However, it may be used in a manner similar to RFC 1918 private
   address space on routing equipment that is able to do address
   translation across router interfaces when the addresses are identical
   on two different interfaces.  Details are provided in the text of
   this document.

   This document details the allocation of an additional special-use
   IPv4 address block and updates RFC 5735.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in 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/rfc6598.

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IESG Note

   A number of operators have expressed a need for the special-purpose
   IPv4 address allocation described by this document.  During
   deliberations, the IETF community demonstrated very rough consensus
   in favor of the allocation.

   While operational expedients, including the special-purpose address
   allocation described in this document, may help solve a short-term
   operational problem, the IESG and the IETF remain committed to the
   deployment of IPv6.

Copyright Notice

   Copyright (c) 2012 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Language ...........................................3
   3. Alternatives to Shared Address Space ............................3
   4. Use of Shared CGN Space .........................................4
   5. Risk ............................................................5
      5.1. Analysis ...................................................5
      5.2. Empirical Data .............................................6
   6. Security Considerations .........................................7
   7. IANA Considerations .............................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................9
   Appendix A. Acknowledgments .......................................10

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1.  Introduction

   IPv4 address space is nearly exhausted.  However, ISPs must continue
   to support IPv4 growth until IPv6 is fully deployed.  To that end,
   many ISPs will deploy a Carrier-Grade NAT (CGN) device, such as that
   described in [RFC6264].  Because CGNs are used on networks where
   public address space is expected, and currently available private
   address space causes operational issues when used in this context,
   ISPs require a new IPv4 /10 address block.  This address block will
   be called the "Shared Address Space" and will be used to number the
   interfaces that connect CGN devices to Customer Premises Equipment
   (CPE).

   Shared Address Space is similar to [RFC1918] private address space in
   that it is not globally routable address space and can be used by
   multiple pieces of equipment.  However, Shared Address Space has
   limitations in its use that the current [RFC1918] private address
   space does not have.  In particular, Shared Address Space can only be
   used in Service Provider networks or on routing equipment that is
   able to do address translation across router interfaces when the
   addresses are identical on two different interfaces.

   This document requests the allocation of an IPv4 /10 address block to
   be used as Shared Address Space.  In conversations with many ISPs, a
   /10 is the smallest block that will allow them to deploy CGNs on a
   regional basis without requiring nested CGNs.  For instance, as
   described in [ISP-SHARED-ADDR], a /10 is sufficient to service Points
   of Presence in the Tokyo area.

   This document details the allocation of an additional special-use
   IPv4 address block and updates [RFC5735].

2.  Requirements Language

   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 [RFC2119].

3.  Alternatives to Shared Address Space

   The interfaces that connect CGN devices to CPE might conceivably be
   numbered from any of the following address spaces:

   o  legitimately assigned globally unique address space

   o  usurped globally unique address space (i.e., squat space)

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   o  [RFC1918] space

   o  Shared Address Space

   A Service Provider can number the interfaces in question from
   legitimately assigned globally unique address space.  While this
   solution poses the fewest problems, it is impractical because
   globally unique IPv4 address space is in short supply.  While the
   Regional Internet Registries (RIRs) have enough address space to
   allocate a single /10 to be shared by all Service Providers, they do
   not have enough address space to make a unique assignment to each
   Service Provider.

   Service Providers MUST NOT number the interfaces in question from
   usurped globally unique address space (i.e., squat space).  If a
   Service Provider leaks advertisements for squat space into the global
   Internet, the legitimate holders of that address space may be
   adversely impacted, as would those wishing to communicate with them.
   Even if the Service Provider did not leak advertisements for squat
   space, the Service Provider and its subscribers might lose
   connectivity to the legitimate holders of that address space.

   A Service Provider can number the interfaces in question from
   [RFC1918] space if at least one of the following conditions is true:

   o  The Service Provider knows that the CPE/NAT works correctly when
      the same [RFC1918] address block is used on both its inside and
      outside interfaces.

   o  The Service Provider knows that the [RFC1918] address block that
      it uses to number interfaces between the CGN and CPE is not used
      on the subscriber side of the CPE.

   Unless at least one of the conditions above is true, the Service
   Provider cannot safely use [RFC1918] address space and must resort to
   Shared Address Space.  This is typically the case in an unmanaged
   service, where subscribers provide their own CPE and number their own
   internal network.

4.  Use of Shared CGN Space

   Shared Address Space is IPv4 address space designated for Service
   Provider use with the purpose of facilitating CGN deployment.  Also,
   Shared Address Space can be used as additional non-globally routable
   space on routing equipment that is able to do address translation
   across router interfaces when the addresses are identical on two
   different interfaces.

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   Devices MUST be capable of performing address translation when
   identical Shared Address Space ranges are used on two different
   interfaces.

   Packets with Shared Address Space source or destination addresses
   MUST NOT be forwarded across Service Provider boundaries.  Service
   Providers MUST filter such packets on ingress links.  One exception
   to this paragraph's proscription is in the case of business
   relationships, such as hosted CGN services.

   When running a single DNS infrastructure, Service Providers MUST NOT
   include Shared Address Space in zone files.  When running a split DNS
   infrastructure, Service Providers MUST NOT include Shared Address
   Space in external-facing zone files.

   Reverse DNS queries for Shared Address Space addresses MUST NOT be
   forwarded to the global DNS infrastructure.  DNS Providers SHOULD
   filter requests for Shared Address Space reverse DNS queries on
   recursive nameservers.  This is done to avoid having to set up
   something similar to AS112.net for [RFC1918] private address space
   that a host has incorrectly sent for a DNS that reverse-maps queries
   on the public Internet [RFC6304].

   Because CGN service requires non-overlapping address space on each
   side of the home NAT and CGN, entities using Shared Address Space for
   purposes other than for CGN service, as described in this document,
   are likely to experience problems implementing or connecting to CGN
   service at such time as they exhaust their supply of public IPv4
   addresses.

5.  Risk

5.1.  Analysis

   Some existing applications discover the outside address of their
   local CPE, determine whether the address is reserved for special use,
   and behave differently based on that determination.  If a new IPv4
   address block is reserved for special use and that block is used to
   number CPE outside interfaces, some of the above-mentioned
   applications may fail.

   For example, assume that an application requires its peer (or some
   other device) to initiate an incoming connection directly with its
   CPE's outside address.  That application discovers the outside
   address of its CPE and determines whether that address is reserved
   for special use.  If the address is reserved for special use, the
   application rightly concludes that the address is not reachable from

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   the global Internet and behaves in one manner.  If the address is not
   reserved for special use, the application assumes that the address is
   reachable from the global Internet and behaves in another manner.

   While the assumption that a non-special-use address is reachable from
   the global Internet is generally safe, it is not always true (e.g.,
   when the CPE outside interface is numbered from globally unique
   address space but that address is not advertised to the global
   Internet as when it is behind a CGN).  Such an assumption could cause
   certain applications to behave incorrectly in those cases.

5.2.  Empirical Data

   The primary motivation for the allocation of Shared Address Space is
   as address space for CGNs; the use and impact of CGNs has been
   previously described in [RFC6269] and [NAT444-IMPACTS].  Some of the
   services adversely impacted by CGNs are as follows:

   1.  Console gaming -- some games fail when two subscribers using the
       same outside public IPv4 address try to connect to each other.

   2.  Video streaming -- performance is impacted when using one of
       several popular video-streaming technologies to deliver multiple
       video streams to users behind particular CPE routers.

   3.  Peer-to-peer -- some peer-to-peer applications cannot seed
       content due to the inability to open incoming ports through the
       CGN.  Likewise, some SIP client implementations cannot receive
       incoming calls unless they first initiate outgoing traffic or
       open an incoming port through the CGN using the Port Control
       Protocol (PCP) [PCP-BASE] or a similar mechanism.

   4.  Geo-location -- geo-location systems identify the location of the
       CGN server, not the end host.

   5.  Simultaneous logins -- some websites (particularly banking and
       social-networking websites) restrict the number of simultaneous
       logins per outside public IPv4 address.

   6.  6to4 -- 6to4 requires globally reachable addresses and will not
       work in networks that employ addresses with limited topological
       span, such as those employing CGNs.

   Based on testing documented in [NAT444-IMPACTS], the CGN impacts on
   items 1-5 above are comparable regardless of whether globally unique,
   Shared Address Space, or [RFC1918] addresses are used.  There is,
   however, a difference between the three alternatives in the treatment
   of 6to4.

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   As described in [RFC6343], CPE routers do not attempt to initialize
   6to4 tunnels when they are configured with [RFC1918] or [RFC5735] WAN
   addresses.  When configured with globally unique or Shared Address
   Space addresses, such devices may attempt to initiate 6to4, which
   would fail.  Service Providers can mitigate this issue using 6to4
   Provider Managed Tunnels [6to4-PMT] or blocking the route to
   192.88.99.1 and generating an IPv4 'destination unreachable' message
   [RFC6343].  When the address range is well-defined, as with Shared
   Address Space, CPE router vendors can include Shared Address Space in
   their list of special-use addresses (e.g., [RFC5735]) and treat
   Shared Address Space similarly to [RFC1918] space.  When the CGN-CPE
   address range is not well-defined, as in the case of globally unique
   space, it will be more difficult for CPE router vendors to mitigate
   this issue.

   Thus, when comparing the use of [RFC1918] and Shared Address Space,
   Shared Address Space poses an additional impact on 6to4 connectivity,
   which can be mitigated by Service Provider or CPE router vendor
   action.  On the other hand, the use of [RFC1918] address space poses
   more of a challenge vis-a-vis Shared Address Space when the
   subscriber and Service Provider use overlapping [RFC1918] space,
   which will be outside the Service Provider's control in the case of
   unmanaged service.  Service Providers have indicated that it is more
   challenging to mitigate the possibility of overlapping [RFC1918]
   address space on both sides of the CPE router than it is to mitigate
   the 6to4 impacts of Shared Address Space.

6.  Security Considerations

   Similar to other [RFC5735] special-use IPv4 addresses, Shared Address
   Space does not directly raise security issues.  However, the Internet
   does not inherently protect against abuse of these addresses.
   Attacks have been mounted that depend on the unexpected use of
   similar special-use addresses.  Network operators are encouraged to
   review this document and determine what security policies should be
   associated with this address block within their specific operating
   environments.  They should consider including Shared Address Space in
   Ingress Filter lists [RFC3704], unless their Internet service
   incorporates a CGN.

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   To mitigate potential misuse of Shared Address Space, except where
   required for hosted CGN service or a similar business relationship,

   o  routing information about Shared Address Space networks MUST NOT
      be propagated across Service Provider boundaries.  Service
      Providers MUST filter incoming advertisements regarding Shared
      Address Space.

   o  packets with Shared Address Space source or destination addresses
      MUST NOT be forwarded across Service Provider boundaries.  Service
      Providers MUST filter such packets on ingress links.

   o  Service Providers MUST NOT include Shared Address Space in
      external-facing DNS zone files.

   o  reverse DNS queries for Shared Address Space addresses MUST NOT be
      forwarded to the global DNS infrastructure.

   o  DNS Providers SHOULD filter requests for Shared Address Space
      reverse DNS queries on recursive nameservers.

7.  IANA Considerations

   IANA has recorded the allocation of an IPv4 /10 for use as Shared
   Address Space.

   The Shared Address Space address range is 100.64.0.0/10.

8.  References

8.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

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8.2.  Informative References

   [6to4-PMT] Kuarsingh, V., Ed., Lee, Y., and O. Vautrin, "6to4
              Provider Managed Tunnels", Work in Progress,
              February 2012.

   [ISP-SHARED-ADDR]
              Yamagata, I., Miyakawa, S., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "ISP Shared Address", Work in Progress,
              January 2012.

   [NAT444-IMPACTS]
              Donley, C., Howard, L., Kuarsingh, V., Berg, J., and J.
              Doshi, "Assessing the Impact of Carrier-Grade NAT on
              Network Applications", Work in Progress, November 2011.

   [PCP-BASE] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", Work
              in Progress, March 2012.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

   [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
              Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
              June 2011.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              June 2011.

   [RFC6304]  Abley, J. and W. Maton, "AS112 Nameserver Operations",
              RFC 6304, July 2011.

   [RFC6343]  Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
              RFC 6343, August 2011.

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Appendix A.  Acknowledgments

   Thanks to the following people (in alphabetical order) for their
   guidance and feedback:

      Stan Barber
      John Brzozowski
      Isaiah Connell
      Greg Davies
      Owen DeLong
      Kirk Erichsen
      Wes George
      Chris Grundemann
      Tony Hain
      Philip Matthews
      John Pomeroy
      Barbara Stark
      Jean-Francois Tremblay
      Leo Vegoda
      Steven Wright
      Ikuhei Yamagata

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Authors' Addresses

   Jason Weil
   Time Warner Cable
   13820 Sunrise Valley Drive
   Herndon, VA  20171
   USA

   EMail: jason.weil@twcable.com

   Victor Kuarsingh
   Rogers Communications
   8200 Dixie Road
   Brampton, ON  L6T 0C1
   Canada

   EMail: victor.kuarsingh@gmail.com

   Chris Donley
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027
   USA

   EMail: c.donley@cablelabs.com

   Christopher Liljenstolpe
   Telstra Corp.
   7/242 Exhibition Street
   Melbourne, VIC  316
   Australia

   Phone: +61 3 8647 6389
   EMail: cdl@asgaard.org

   Marla Azinger
   Frontier Communications
   Vancouver, WA
   USA

   Phone: +1.360.513.2293
   EMail: marla.azinger@frontiercorp.com

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