Request for Comments: 6598
Category: Best Current Practice
Time Warner Cable
IANA-Reserved IPv4 Prefix for Shared Address Space
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.
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 © 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
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:
- legitimately assigned globally unique address space
o usurped globally unique address space (i.e., squat space)
- [RFC1918] space
- 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:
- 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.
- 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.
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:
- Console gaming -- some games fail when two subscribers using the same outside public IPv4 address try to connect to each other.
- 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.
- 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 126.96.36.199 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,
- 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.
- 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.
- Service Providers MUST NOT include Shared Address Space in external-facing DNS 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.
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.
8.1. 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.
Yamagata, I., Miyakawa, S., Nakagawa, A., Yamaguchi, J., and H. Ashida, "ISP Shared Address", Work in Progress, January 2012.
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:
Jason Weil Time Warner Cable 13820 Sunrise Valley Drive Herndon, VA 20171 USA EMail: firstname.lastname@example.org Victor Kuarsingh Rogers Communications 8200 Dixie Road Brampton, ON L6T 0C1 Canada EMail: email@example.com Chris Donley CableLabs 858 Coal Creek Circle Louisville, CO 80027 USA EMail: firstname.lastname@example.org Christopher Liljenstolpe Telstra Corp. 7/242 Exhibition Street Melbourne, VIC 316 Australia Phone: +61 3 8647 6389 EMail: email@example.com
Phone: +1.360.513.2293 EMail: firstname.lastname@example.org