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.
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
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)
 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.
 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
 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.
 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.
 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.
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.
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
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