| TOC |
|
By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.
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.
This Internet-Draft will expire on March 30, 2009.
As we approach IPv4 address depletion, the IETF must provide for IPv4 and IPv6 coexistence: a way for ISPs and enterprises to reduce public IPv4 address consumption and a way for hosts to migrate to IPv6 connectivity -- while providing reasonable access for those IPv6 hosts to access the IPv4 Internet.
This draft compares eight proposals for IPv6 and IPv4 coexistence.
1.
Terminology
2.
Introduction
3.
Overview of Proposals
3.1.
IPv4 hosts in Customer Premise
3.1.1.
Address Plus Port (A+P)
3.1.2.
APB-Revised (APBR)
3.1.3.
Dual-Stack Lite (DS-Lite)
3.1.4.
NAT444
3.2.
IPv6 hosts in Customer Premise
3.2.1.
IVI
3.2.2.
NAT6
3.2.3.
NAT64
3.2.4.
NAT-PT
3.2.5.
sNAT-PT
4.
Changes Required in Network Elements
4.1.
IPv4 and IPv6 Hosts Accessing the IPv4 Internet
4.2.
IPv4 Hosts Accessing the IPv4 Internet
4.3.
IPv4 Internet Accessing IPv6 hosts
5.
Port Forwarding
5.1.
Static Incoming Ports
5.2.
Dynamic Incoming Ports
6.
Transport Protocol Support
7.
Analysis with V6OPS's NAT64 Problem Statement
8.
Comparison of Proposals with NAT-PT Problems
8.1.
Issues Unrelated to an DNS-ALG
8.1.1.
Issues with Protocols Embedding IP Addresses
8.1.2.
NAPT-PT Redirection Issues
8.1.3.
NAT-PT Binding State Decay
8.1.4.
Loss of Information through Incompatible Semantics
8.1.5.
NAT-PT and Fragmentation
8.1.6.
NAT-PT Interaction with SCTP and Multihoming
8.1.7.
NAT-PT as a Proxy Correspondent Node for MIPv6
8.1.8.
NAT-PT and Multicast
8.2.
Issues Exacerbated by the Use of DNS-ALG
8.2.1.
Network Topology Constraints Implied by NAT-PT
8.2.2.
Scalability and Single Point of Failure Concerns
8.2.3.
Issues with Lack of Address Persistence
8.2.4.
DoS Attacks on Memory and Address/Port Pool
8.3.
Issues Directly Related to Use of DNS-ALG
8.3.1.
Address Selection Issues when Communicating with Dual-Stack End-Hosts
8.3.2.
Non-Global Validity of Translated RR Records
8.3.3.
Inappropriate Translation of Responses to A Queries
8.3.4.
DNS-ALG and Multi-Addressed Nodes
8.3.5.
Limitations on Deployment of DNS Security Capabilities
8.4.
Impact on IPv6 Application Development
9.
Security Considerations
9.1.
Address Sharing
10.
Acknowledgements
11.
IANA Considerations
12.
References
12.1.
Normative References
12.2.
Informative References
Appendix A.
Changes
A.1.
Changes from 00 to 01
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
| TOC |
The following terms are used throughout this document.
- Address Family Translation (AFT):
- The function of translating from one IP address family (IPv4 or IPv6) to another (IPv6 or IPv4).
- Carrier Grade NAT (CGN):
- A NAT device used by many subscribers (homes or end sites), where 'many' would be on the order of dozens to hundreds of thousands of subscribers. This might NAT between any combination of IPv4 and IPv6. Typically, the end user does not have the ability to adjust the behavior of the CGN (i.e., no ability to create static port mapping).
- CPE router:
- Customer Premise Equipment router. A device that performs routing functions, located at the customer's premise. This device does not perform NAT functions. (Some referenced specifications use the term 'Home GateWay' (HGW) to mean the same thing. We use CPE router because the subscriber might not be a business and not a 'home'.)
- DNS rewriting:
- The generalized function of synthesizing a DNS AAAA response from a DNS A response. The term "DNS rewriting" instead of "DNS-ALG" because in NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] DNS-ALG meant a DNS rewriting function with an interface to the NAT function.
- NAT:
- Network Address Translation. This translates IP addresses, one-for-one, between two networks, without changing transport protocol ports. The two networks might be IPv4 (NAT44), IPv6 and IPv4 (NAT64), or IPv4 and IPv6 (NAT46). This document follows (the unfortunate) common usage that "NAT" can also mean "NAPT" (Network Address and Port Translator).
- Softwire
- A tunnel for carrying IPv4 and IPv6 traffic over IPv6 and IPv4 networks [RFC4925] (Li, X., Dawkins, S., Ward, D., and A. Durand, “Softwire Problem Statement,” July 2007.).
| TOC |
As the Internet approaches IPv4 address depletion, it will be necessary for Internet Service Providers to continue to simultaneously provide their users with access to the IPv4 Internet, reduce the number of IPv4 public addresses consumed by each subscriber, and provide a way for subscribers to migrate to IPv6.
The proposals have several high-level attributes in common:
Provide access to the IPv4 Internet: There are two approaches to provide access to the v4 Internet. One approach is to have a dual-stack host with some modifications from the classic design [RFC4213] (Nordmark, E. and R. Gilligan, “Basic Transition Mechanisms for IPv6 Hosts and Routers,” October 2005.). The other approach is to have an IPv6-only host and operate an address family translation (AFT) device between the IPv6-only host and the IPv4 Internet.
Reduce consumption of global IPv4 addresses: Network address and port translator (NAPT) technology, and NAPT itself, allows more than one host to simultaneously use a single IPv4 address. NAPT technology is used in all proposals to conserve IPv4 public address space.
IPv6 migration: it is important that a migration path for users and content providers to move to IPv6 is enabled and encouraged. This is necessary because operating a NAT device in order to reduce per-subscriber IPv4 address consumption is not a viable long-term solution: we will still exhaust the IPv4 public address space, and operating NATs is expensive and reduces the reliability of the Internet.
Port limitations: All proposals use a NAPT to provide access to the IPv4 Internet, which reduces the number of ports each subscriber can use. This has negative impacts on some applications (e.g., Apple iTunes, Google Maps). This problem is resolved by the content provider and the subscriber both using IPv6.
This draft is discussed on the [v4v6interm‑interest] (IETF, “v4v6interm-interest mailing list,” .) mailing list. Individual proposals are discussed on the mailing list indicated in this document.
| TOC |
This document classifies the proposals into two categories. The first category provides IPv4 and IPv6 access to the subscriber, and the second category provides only IPv6 access to the subscriber. In both categories, IPv4 addresses are conserved by using a NAT device. This NAT device is placed in the carrier's network ("Carrier Grade NAT") or (in the case of APB-Revised) in the CPE router. In all proposals (except NAT444) a host can obtain native IPv6 connectivity with native IPv6 hosts without regard to the co-existence proposal.
The descriptions below provide a very brief overview of each proposal, in alphabetical order.
| TOC |
For Internet access, the following proposals allow for IPv4 hosts in the customer premise.
| TOC |
Address Plus Port (Maennel, O., Bush, R., Cittadini, L., and S. Bellovin, “A Better Approach than Carrier-Grade-NAT,” .) [A+P] uses a NAT in (or close to) the customer premise for access to the IPv4 Internet. The Service Provider conveys both an IPv4 address (as done today) and a range of TCP/UDP ports to the NAT. Outgoing IPv4 traffic is NATted to that range of TCP/UDP ports, and the Service Provider routes packets to the appropriate customer using both the destination IPv4 address (as done today) and destination TCP/UDP port.
One of A+P's architectures is depicted in Figure 1 (Address Plus Port, v4-capable ISP (A+P-v4)), where the ISP's network supports both IPv4 and IPv6. In this architecture, the NAT's job is straight forward: it NATs to a limited port range and sends the packet upstream to the ISP. Because multiple customers share a single IPv4 address, the aggregation router needs to route return packets to the appropriate customer's NAT using destination IPv4 and destination port. This architecture is denoted as "A+P-v4".
+---+ +-------------+
IPv6 host-----+ | +----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
Dual-stack host--+ |
|NAT| +------+ +-------------+
IPv4 host----+ +--------+router+-------------+IPv4 Internet|
+---+ +------+ +-------------+
|<private IPv4>NAT<-----------------------------public v4----->| Figure 1: Address Plus Port, v4-capable ISP (A+P-v4) |
Another of A+P's architectures is depicted in Figure 2 (Address Plus Port, v6-only ISP (A+P-v6)), where the ISP's network runs only IPv6. In this architecture, the NAT encapsulates the IPv4 packet into an IPv6 packet, where the IPv6 packet has a source address that corresponds to that NAT, and a destination address that corresponds to a well-known prefix which routes to a IPv6 tunnel concentrator. When the packet arrives at the IPv6 tunnel concentrator the IPv6 source address is used to construct the IPv4 source address and TCP/UDP port, and the IPv6 destination address is used to construct the IPv4 destination address; the IPv6 destination TCP/UDP port is taken from the IPv6 packet's destination TCP/UDP header. This architecture is denoted as "A+P-v6".
+---+ +-------------+
IPv6 host-----+ | +----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
Dual-stack host--+ |
|NAT| +--------+ +-------------+
IPv4 host----+ +===IPv6 tunnel===+ tunnel +--+IPv4 Internet|
+---+ |concent.| +-------------+
+--------+
|<private IPv4>NAT<----------------------------public v4----->| Figure 2: Address Plus Port, v6-only ISP (A+P-v6) |
| TOC |
APB-Revised (APBR) (no document yet available) shares each IPv4 address amongst several subscribers through a tunnel aggregation device. APBP was introduced in [I‑D.despres‑v6ops‑apbp] (Despres, R., “A Scalable IPv4-IPv6 Transition Architecture Need for an address-port-borrowing-protocol (APBP),” July 2008.) and APB-Revised further evolves the concept so that mappings, between IPv6 addresses and IPv4-address/port-ranges are static. The static mapping avoids the need for the service provider equipment to NAT.
APBR can be implemented with subscriber site tunnel endpoints either in a router (CPE router or other router) or in the APBR host. In both implementations, each subscriber site is assigned a subset of public IPv4 address range available to the CGN, typically limited to a single address and a restricted port range. Figure 3 (APBPR-CPE, tunnel between CPE and CGN) shows the APBR architecture in the case where the tunnel is established between the CPE router (upgraded to support APBR) and the APB-R-capable softwire tunnel concentrator. Any IPv4 traffic from hosts behind the CPE router is NAT'd (using classic NAT44) and forwarded through the tunnel to the tunnel endpoint. The customer premise NATs using the external port range it is 'borrowing' from the APBR endpoint. This is abbreviated APBR-CPE in this document.
This proposal is discussed in [Softwires] (IETF, “Softwires working group mailing list,” .).
APBR allows for two implementations for IPv4 access. Figure 3 (APBPR-CPE, tunnel between CPE and CGN) shows APBR using a CGN (abbreviated APBR-CGN in this document).
+---+ +-------------+
IPv6 host-----+ | +----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
Dual-stack host--+ | +--------+
|NAT| | APB-R | +-------------+
IPv4 host----+ +===IPv6 tunnel===+softwire+--+IPv4 Internet|
+---+ | tunnel | +-------------+
|concent.|
+--------+
|<private IPv4>NAT<----------------------------public v4------>| Figure 3: APBPR-CPE, tunnel between CPE and CGN |
In the figure above, the IPv6 tunnel is an IPv4-over-IPv6 tunnel.
Figure 4 (APBR-host - tunnel between CPE and APBR tunnel endpoint ) shows another APBR architecture where the tunnel is established directly between the host (upgraded to support APBR) and the APBR tunnel endpoint. Any IPv4 traffic from the APBR host is routed through the tunnel to the APB-R-capable softwire tunnel concentrator. Tunnelling is sufficient; no NAT device is needed between the host and the public IPv4 network. This is abbreviated APBR-host in this document. In Figure 4 (APBR-host - tunnel between CPE and APBR tunnel endpoint ), the customer premise NAT does not NAT traffic to/from the APB-R host; however, it does NAT traffic to/from the IPv4-only host.
+---+ +-------------+
IPv6 host-----+ | +----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
APB-R host--+ | +--------+
|CPE| | APB-R | +-------------+
IPv4 host----+ +===IPv6 tunnel===+softwire+--+IPv4 Internet|
+---+ | tunnel | +-------------+
|concent.|
+--------+
|<private IPv4>NAT<----------------------------public v4------>| Figure 4: APBR-host - tunnel between CPE and APBR tunnel endpoint |
Figure 5 (APBR-HC - tunnels between CPE and APBR-tunnel endpoint and between host and CPE ) shows the APBR architecture with two tunnels. One tunnel is established between the CPE router and the APBR endpoint, and a second tunnel between the subscriber host and the CPE router. In this architecture, the CPE router is upgraded to establish a tunnel to the APB-R-capable softwire tunnel concentrator (external side) and to accept a tunnel from the host (internal side); the APBR host IP stack is upgraded to establish a tunnel to the CPE router. Any traffic from the APBR host is routed by the host's APBR stack and forwarded through the tunnel to the CPE router. Tunnelling is sufficient; no NAT device is needed between the host and the core IPv4 network. This is abbreviated APBR-HC (Host and CPE router) in this document.
+---+ +-------------+
IPv6 host-----+ | +----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
DS host==v4/v4==+ | +--------+
|NAT| | APB-R | +-------------+
IPv4 host----+ +===IPv6 tunnel===+softwire+--+IPv4 Internet|
+---+ | tunnel | +-------------+
|concent.|
+--------+
|<------- public v4 (partially in 2 consecutive tunnels ------>
|<-private v4-->|<--service provider IPv6--->|<----public v4-->| Figure 5: APBR-HC - tunnels between CPE and APBR-tunnel endpoint and between host and CPE |
| TOC |
Dual-Stack Lite (DS-Lite) provides a global IPv4 address that is shared amongst several subscribers through a CGN. Each subscriber network is connected to the CGN through a tunnel, using IPv6 as the tunnel transport. All IPv4 traffic is sent inside of that tunnel. The tunnel endpoint implements Dual-Stack (Nordmark, E. and R. Gilligan, “Basic Transition Mechanisms for IPv6 Hosts and Routers,” October 2005.) [RFC4213]. DS-lite is currently described in two Internet Drafts, [I‑D.durand‑dual‑stack‑lite] (Durand, A., “Dual-stack lite broadband deployments post IPv4 exhaustion,” July 2008.) and [I‑D.droms‑softwires‑snat] (Droms, R. and B. Haberman, “Softwires Network Address Translation (SNAT),” July 2008.), and is discussed in [Softwires] (IETF, “Softwires working group mailing list,” .).
DS-Lite can be implemented with the tunnel endpoints either in a router (CPE router or aggregation router) or in a host. In both cases, a single subscriber IPv4 address or IPv4 prefix may overlap, or even be identical for all subscribers. Addresses from overlapping address spaces are disambiguated by the tunnels between the subscriber networks and the CGN.
Figure 6 (Dual-Stack Lite, tunnel terminated on router (DS-Lite router)) shows the DS-Lite architecture in the case where the tunnel is terminated in a router, which could be the CPE router or an aggregation router. In the diagram, the router terminating the tunnel is a CPE router, but another router could be used as well (e.g., service provider's aggregation router). In this architecture, the router is upgraded to establish a tunnel to the CGN, and does not perform any NAT processing on subscriber traffic. The router provides DHCP service (addresses and other configuration information) to the subscriber hosts. This is abbreviated "DS-Lite router" in this document.
+------+ +-------------+
IPv6 host-----+ | +-----------------+IPv6 Internet|
| +--IPv6------+ +-------------+
Dual-stack host-+ |
|router| +---+ +-------------+
IPv4 host-----+ +===IPv6 tunnel=========+CGN+--+IPv4 Internet|
+------+ +---+ +-------------+
|<--private v4 (partially in tunnel)-->NAT<---public v4---->
|<--service provider IPv6-->|<----public v4----->| Figure 6: Dual-Stack Lite, tunnel terminated on router (DS-Lite router) |
The choice of encapsulation for the IPv6 tunnel is outside the scope of this document.
Figure 7 (Dual-Stack Lite, tunnel terminated on host (DS-Lite host)) shows the DS-Lite architecture when the tunnel is terminated in the subscriber host. In this architecture, the DS-Lite host IP stack is upgraded to establish a tunnel to the CGN, through an unmodified CPE router and across either IPv6 transport. IPv4 traffic from the DS-Lite host is routed through the tunnel to the CGN. This is abbreviated "DS-Lite host" in this document.
+------+ +-------------+
IPv6 host----+ +-------------------------------+IPv6 Internet|
| | +-------------+
|router|
DS-Lite | | +---+ +-------------+
host==================IPv4-over-IPv6 tunnel==+CGN+--+IPv4 Internet|
+------+ +---+ +-------------+
|<--private v4 (in tunnel)------------->NAT<---public v4---->
|<-subscr. IPv6-->|<--service provider IPv6-->|<----public v4---->
| Figure 7: Dual-Stack Lite, tunnel terminated on host (DS-Lite host) |
The choice of encapsulation for the IPv6 tunnel is outside the scope of this document.
| TOC |
NAT444 (no written proposal) would NAT twice: first using a NAT device in the customer premise (as typically deployed today) and another NAT device in the ISP's network (a CGN). This proposal is discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
This proposal does not provide native IPv6 access to the subscriber, but doesn't preclude it if the host or its CPE router wanted to use a tunneling solution (e.g., Teredo (Huitema, C., “Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs),” February 2006.) [RFC4380])
+---+ +---+ +-------------+
IPv4 host----+NAT+------IPv4---------+CGN+--+IPv4 Internet|
+---+ +---+ +-------------+
|<private v4->NAT<-----private v4---->NAT<----public v4--->| Figure 8: NAT444 |
| TOC |
For access to the IPv4 Internet, the following proposals require IPv6 hosts in the customer premise, and do not support IPv4 hosts. These proposals provide access to the IPv4 Internet without requiring dual-stack on client equipment.
| TOC |
IVI ([I‑D.xli‑behave‑ivi] (Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, “The CERNET IVI Translation Design and Deployment for the IPv4/IPv6 Coexistence and Transition,” January 2010.), [I‑D.baker‑behave‑ivi] (Li, X., Bao, C., Baker, F., and K. Yin, “IVI Update to SIIT and NAT-PT,” September 2008.)) uses an address and service architecture designed to facilitate transition from an IPv4 Internet to an IPv6 Internet. This service contains three parts: A DNS Application Layer Gateway, a stateful Network Address Translator that enables IPv6 clients to initiate connections to IPv4 servers and peers, and a stateless Network Address Translator that enables IPv4 and IPv6 systems to interoperate freely.
For an IPv6 host needing access to IPv4 hosts, IVI is similar to both SIIT (Nordmark, E., “Stateless IP/ICMP Translation Algorithm (SIIT),” February 2000.) [RFC2765] and NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] but with a different address format. Rather than using the DNS-ALG described in [RFC2766] (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.), the DNS rewriting function (A to AAAA) is fixed and points to a specific IVI gateway, which removes the relationship between the NAT function and DNS function. The DNS server may be in the IVI gateway or in a separate system related to it.
IVI also allows IPv4 hosts to access a IPv6 host, using a stateless NAT. This is accomplished by providing the IPv6 host an IVI address, which is simply an IPv6 address from a pool of IPv6 addresses. This pool of IPv6 addresses has a fixed IPv4-to-IPv6 mapping algorithm applied to translate between the two address families and the translation is implemented by an IVI gateway. The IPv6 address would be advertised in DNS with an A record, pointing to the IVI gateway. This allows IPv6-only hosts to have a presence on the IPv4 Internet. In this scheme, subsets of the IPv4 addresses are embedded in prefix-specific IPv6 addresses and these IPv6 addresses can therefore communicate with the global IPv6 networks directly and can communicate with the global IPv4 networks via stateless (or almost stateless) gateways. DNS rewriting is not used, or necessary, for this fixed mapping of IPv4 addresses to IPv6 address.
This proposal is discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
+-------------+
+----------------------------+IPv6 Internet|
| +-------------+
| +-----+
+------+ | +----+ IVI |------+
IPv6 host---+ | | / +-----+ \ +-------------+
| CPE +--IPv6-< >--+IPv4 Internet|
|router| \ +-----------+ / +-------------+
IPv6 host---+ | +--+DNS-rewrite|--+
+------+ +-----------+
| Figure 9: IVI |
| TOC |
NAT6 (Jennings, C., “NAT for IPv6-Only Hosts,” November 2008.) [I‑D.jennings‑behave‑nat6] encourages IPv6 host itself to provide necessary DNS rewriting functions (appending a configured IPv6 prefix to an IPv4 address to create an IPv6 address) and have the NAT function (from IPv6 to IPv4) performed in the network. By having the host provide the DNS rewriting function, a DNS rewriting function in the network is avoided. This proposal is discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
+-------------+
+--------------+IPv6 Internet|
| +-------------+
+------+ |
IPv6 host---+ | | +----+ +-------------+
| CPE +---IPv6--+NAT6+-----+IPv4 Internet|
IPv6 host---+router| +----+ +-------------+
+------+
| Figure 10: NAT6 |
| TOC |
For an IPv6 host needing access to IPv4 hosts, NAT64 (Bagnulo, M., Matthews, P., and I. Beijnum, “NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” March 2009.) [I‑D.bagnulo‑behave‑nat64] uses the logic of SIIT (Nordmark, E., “Stateless IP/ICMP Translation Algorithm (SIIT),” February 2000.) [RFC2765] and NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] but with a different address format. This removes the relationship between the NAT function and DNS function. Rather than using the DNS-ALG described in [RFC2766] (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.), the DNS service simply advertises DNS A and AAAA records specifying the IPv6 address of the NAT64 device (which is a CGN device). The DNS server may be in the CGN or in a separate system related to it. This proposal is discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
+-------------+
+-------------------------+IPv6 Internet|
| +-------------+
| +-----+
+------+ | +----+NAT64+----+
IPv6 host-+ | | / +-----+ \ +-------------+
| CPE +--IPv6-< >-+IPv4 Internet|
IPv6 host-+router| \ +-------------+ / +-------------+
+------+ ++DNS rewriting|+
+-------------+
| Figure 11: NAT64 |
Note: the following network architecture is not described in NAT64 (Bagnulo, M., Matthews, P., and I. Beijnum, “NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” March 2009.) [I‑D.bagnulo‑behave‑nat64], but is included here for completeness.
It is also possible to utilize NAT64 to access private IPv4 address (Figure 12 (NAT64 to Private IPv4 Addresses)). This is useful if there are a lot of IPv4 servers and it is too difficult or expensive to put each of them on a global IPv4 address, and it is not possible to upgrade them to IPv6.
IPv4 host
+-----+ /
IPv6------------+NAT64+-------<-IPv4 host
Internet +-----+ \
IPv4 host
NAT<--private IPv4---->| Figure 12: NAT64 to Private IPv4 Addresses |
Note that in this scenario, DNS rewriting is not necessary as all of the IPv4 addresses could be given AAAA records.
| TOC |
This section is provided for reference only, because NAT-PT has been deprecated by [RFC4966] (Aoun, C. and E. Davies, “Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status,” July 2007.).
NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] provides a combined DNS-ALG and NAT function, which share state. This is typically implemented in a single device.
+-------------+
+---------------------+IPv6 Internet|
| +-------------+
| +-----------+
| | +- - -+ |
+------+ | +--+ NAT +--+
IPv6 host---+ | | /| +- + -+ |\ +-------------+
| CPE +--IPv6-< | | | >-+IPv4 Internet|
IPv6 host---+router| \| +- -+- -+ |/ +-------------+
+------+ +-+DNS-ALG|-+
| +- - - -+ |
+-----------+
| Figure 13: NAT-PT |
NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] and [RFC4966] (Aoun, C. and E. Davies, “Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status,” July 2007.) can be discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
| TOC |
For an IPv6 host needing access to IPv4 hosts, sNAT-PT (Miyata, H. and M. Endo, “sNAT-PT: Simplified Network Address Translation - Protocol Translation,” September 2008.) [I‑D.miyata‑v6ops‑snatpt] provides DNS rewriting and NAT functionality. The DNS rewriting component is described in [I‑D.endo‑v6ops‑dnsproxy] (Endo, M. and H. Miyata, “Translator Friendly DNS Proxy,” October 2008.).
sNAT-PT also provides access from the IPv4 Internet to IPv6 hosts with a 1:1 mapping.
This proposal is discussed in [Behave] (IETF, “BEHAVE working group mailing list,” .).
+-------------+
+-----------------------------+IPv6 Internet|
| +-------------+
| +-------+
+------+ | +-----+sNAT-PT|----+
IPv6 host-+ | | / +-------+ \ +-------------+
| CPE +-IPv6-< >--+IPv4 Internet|
IPv6 host-+router| \ +-------------+ / +-------------+
+------+ +--+DNS rewriting|-+
+-------------+
| Figure 14: sNAT-PT |
| TOC |
This section describes changes to network elements for various scenarios. In all cases, the content provider's DNS and content provider's network does not need to change (except due to the problem of port limitations as described in Section 2 (Introduction)).
| TOC |
For the case of an IPv4 host, IPv6 host, or dual-stack host that need to connect to IPv4 hosts on the Internet, the following table summarizes the changes required to subscriber's hosts (when CPE routers are present and when CPE routers are not present) and to some network elements:
| Proposal | Subscriber Hosts w/CPE router | Subscriber Hosts w/o CPE router | CPE router | ISP Access Network |
|---|---|---|---|---|
| A+P-v4 | no change | no change (A+P NAT would be performed by SP) | A+P support | route using destination port |
| A+P-v6 | no change | no change (A+P NAT would be performed by SP) | A+P support | tunnel concentrator |
| APBR-CPE | no change | (not applicable) | APBR CPE | APBR endpoint (stateless) |
| APBR-host | (not applicable) | APBR CPE | APBR CPE | APBR endpoint (stateless) |
| APBR-HC | APBR support | (not applicable) | APBR CPE internal & external | APBR endpoint (stateless) |
| NAT444 | no change | no change | no change | NAT v4v4 |
| DS-Lite router | no change | (not supported; use DS-Lite host) | DS-Lite CPE | NAT v4v4 w/tunnel |
| DS-Lite host | (not supported; use DS-Lite router) | DS-Lite v6 | no change | NAT v4v4 w/tunnel |
| IVI | move to v6 | move to v6 | move to v6 | IVI + DNS rewriting |
| NAT6 | move to v6 | move to v6 | move to v6 | NAT6 |
| NAT64 | move to v6 | move to v6 | move to v6 | NAT64 + DNS rewriting |
| NAT-PT | move to v6 | move to v6 | move to v6 | NAT-PT + DNS-ALG |
| sNAT-PT | move to v6 | move to v6 | move to v6 | sNAT-PT + DNS rewriting |
| Table 1: Changes Required to Network Elements |
For IPv6 hosts that access the IPv4 Internet, the following table describes the high-level technologies used by each proposal.
| Proposal | ISP's Internal Network | DNS Impact | Carrier Grade NAT |
|---|---|---|---|
| A+P-v4 | IPv4 destination port routing | no change | (no CGN, if subscriber's NAT support A+P NAT) |
| A+P-v6 | IPv4/IPv6 tunnel | no change | (no CGN, if subscriber's NAT support A+P NAT) |
| APBR-CGN and APBP-borrow | IPv4/IPv6 tunnel | no change | (no CGN) |
| DS-Lite router | IPv4/IPv6 tunnel | no change | IPv4/IPv4 |
| DS-Lite host | IPv4/IPv6 tunnel | no change | IPv4/IPv4 |
| NAT444 | (v6 not supported) | (v6 not supported) | (v6 not supported) |
| IVI | v4 NATted, native v6 address | DNS rewriting | IPv6/IPv4 |
| NAT64 | v4 NATted, native v6 address | DNS rewriting | IPv6/IPv4 |
| NAT-PT | v4 NATted, native v6 address | DNS-ALG | IPv6/IPv4 |
| sNAT-PT | v4 NATted, native v6 address | DNS rewriting | IPv6/IPv4 |
| Table 2: IPv6 to IPv4 - technologies involved |
| TOC |
The following table compares the five mechanisms that support end hosts running IPv4 to access the IPv4 Internet: APB-Revised, Dual-Stack Lite, NAT444.
| Proposal | CPE router | ISP's Internal Network | Service Provider Equipment |
|---|---|---|---|
| A+P-v4 | IPv6 support + A+P NAT44 | IPv4 and IPv6 | destination port routing |
| A+P-v6 | IPv6 support + IPv4/IPv6 tunnel + A+P NAT44 | IPv6 | IPv6 tunnel termination |
| APBR-CPE | IPv6 support + IPv4/IPv6 tunnel + NAT44 | IPv6 | IPv6 tunnel termination |
| DS-Lite router | IPv6 support + IPv4/IPv6 tunnel | IPv6 | IPv6 tunnel termination, NAT44 (CGN) |
| DS-Lite host | IPv6 support (if using DS-Lite IPv6 tunneling) | IPv6 (if using DS-Lite IPv6 tunneling) | IPv6 tunnel termination, NAT44 |
| NAT444 | no change | multi-realm IPv4 | NAT44 (CGN) |
| Table 3: IPv4 Hosts Accessing the IPv4 Internet |
The proposals IVI, NAT6, NAT64, NAT-PT, and sNAT-PT are not shown in table Table 3 (IPv4 Hosts Accessing the IPv4 Internet) because those proposals provide no support for IPv4-only hosts to access the IPv4 Internet.
| TOC |
IVI, NAT-PT, and sNAT-PT all provide mechanisms for IPv4 hosts on the Internet to access IPv6-only servers. Such mappings consume IPv4 address space.
IVI: IVI allows 1:1 mapping from an IPv4 client to an IPv6 server. IVI also allows 1:n mappings, by utilizing the TCP/UDP port number of the incoming IPv4 packet in the algorithm to determine the destination IPv6 host; this conserves IPv4 address space consumption for those hosts that need a few TCP/UDP ports available from the IPv4 Internet.
NAT-PT: NAT-PT allows 1:n mapping from an IPv4 client to an IPv6 server, which is accomplished by dynamically mapping an IPv4 address to an IPv6 address after a DNS "A" record query.
sNAT-PT: NAT-PT allows 1:1 mapping from an IPv4 client to an IPv6 server.
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Some applications require accepting incoming UDP or TCP traffic. When the remote host is on IPv4, the incoming traffic will be directed towards an IPv4 address. The applications are separated into two broad categories: those requiring static incoming ports and those requiring dynamic incoming ports.
Due to IPv4 NATs and IPv4 firewalls, some applications use [UPnP‑IGD] (UPnP Forum, “Universal Plug and Play Internet Gateway Device,” 2000.) (e.g., XBox) or ICE (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) [I‑D.ietf‑mmusic‑ice] (e.g., SIP, Yahoo!/Google/Microsoft chat networks), other applications have all but completely abandoned incoming connections (e.g., most FTP transfers use passive mode). But some applications rely on ALGs, UPnP IGD, or manual port configuration. Further discussion in the IETF community is necessary to decide how to proceed on this issue.
Note: Placing application awareness (i.e., ALG) in the CGN will cause bug fixes and new features to be delayed by development, testing, and deployment. To prevent such delays, application awareness should be placed elsewhere (e.g., in the CPE router or in the end host).
Note: Extending NAT-PMP (Cheshire, S., “NAT Port Mapping Protocol (NAT-PMP),” April 2008.) [I‑D.cheshire‑nat‑pmp] to support IPv6 could provide provide static port forwarding and dynamic port forwarding for IPv4 and IPv6 hosts needing access from IPv4 hosts.
| TOC |
Static incoming ports are used by applications for multiple sessions.
Note: Some applications (e.g., BitTorrent) can use UPnP IGD to control IPv4 NATs and open a static incoming port. However, technical limitations of UPnP IGD appear to prevent UPnP IGD from being directly implemented in a CGN. The most significant technical limitation is that UPnP IGD expects the control point (the host) to be able to specify the public port; with hundreds of subscribers utilizing the same public IP address, this is untenable. Other UPnP IGD technical limitations may be surmountable (e.g., UPnP IGD's ability to create and destroy mappings for other IP addresses).
Examples of applications that require static incoming ports include:
The solutions proposed for static ports are:
A+P: The subscriber's customer premise NAT can forward ports within the allocated port range. This port could be advertised by the subscriber using DNS SRV resource records or other means.
APBR-host and APBR-HC: assign a port in the available port range; advertise it with the IPv4 address using a DNS SRV resource record.
Dual-Stack Lite: none
NAT444: none
IVI: assign IPv6 IVI address to IPv6 hosts that require incoming IPv4 connections
NAT6: none
NAT64: none
sNAT-PT: assign IPv4 address to IPv6 hosts that require incoming IPv4 sessions.
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Dynamic incoming ports are, generally, used by applications for a single session. Examples of applications that require dynamic incoming ports include:
The solutions proposed for dynamic ports are:
A+P: An ALG can be incorporated into the subscriber's A+P-aware NAT, as done today with subscriber's NAT44 devices.
APBR-host and APBR-HC: assign a port in the available port range.
Dual-Stack Lite: none
NAT444: none (although it is reasonable to expect that ALGs, as they exist in today's IPv4 NATs, might be utilized)
IVI: assign IPv6 IVI address to IPv6 hosts that require incoming IPv4 connections
NAT6: none
NAT64: applications could be modified to support STUN (for TCP and UDP) to learn their public IPv4 address and TCP/UDP port.
sNAT-PT: assign IPv4 address to IPv6 hosts that require incoming IPv4 connections.
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[[Placeholder: discuss how DCCP and SCTP (and other transport protocols) are supported by each proposal. Although existing IPv4 NATs do not support DCCP or SCTP, it is reasonable to expect that new NATs could support those transport protocols if we want those protocols to work between address families.
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This section analyzes how each proposal maps to the requirements in [I‑D.ietf‑v6ops‑nat64‑pb‑statement‑req] (Bagnulo, M., Baker, F., and I. Beijnum, “IPv4/IPv6 Coexistence and Transition: Requirements for solutions,” May 2008.).
[[Placeholder until [I‑D.ietf‑v6ops‑nat64‑pb‑statement‑req] (Bagnulo, M., Baker, F., and I. Beijnum, “IPv4/IPv6 Coexistence and Transition: Requirements for solutions,” May 2008.) becomes more stable.]]
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The following sections analyze how proposals fare against the problems caused by NAT-PT (Tsirtsis, G. and P. Srisuresh, “Network Address Translation - Protocol Translation (NAT-PT),” February 2000.) [RFC2766] as documented in [RFC4966] (Aoun, C. and E. Davies, “Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status,” July 2007.):
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NAT6 requires applications to handle NAT6 traversal themselves.
The other proposals are silent on this issue, but in general using an application layer gateway (ALG), in some device in the network, appears to be the only solution to this problem. See also Section 5 (Port Forwarding).
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All proposals are silent on this issue.
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NAT6 and NAT64 discuss binding lifetimes.
The other proposals are silent on this issue.
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All proposals are silent on this issue.
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[[NAT64, NAT6, DS-Lite, and IVI all mention fragmentation. Need to analyze how they differ.]]
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IVI supports multi-homing if there is a 1:1 mapping between IPv4 and IPv6 addresses. However, 1:1 mapping is not sustainable as we approach IPv4 exhaustion.
APBR (both APBR-host and APBR-HC) support SCTP.
The other proposals are silent on this issue. All proposals seem to be considering only TCP, UDP, and ICMP.
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All proposals are silent on this issue.
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IVI can support Source-Specific Multicast (Holbrook, H. and B. Cain, “Source-Specific Multicast for IP,” August 2006.) [RFC4607] (see Section 7 of [I‑D.xli‑behave‑ivi] (Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, “The CERNET IVI Translation Design and Deployment for the IPv4/IPv6 Coexistence and Transition,” January 2010.)).
Dual-Stack Lite does not support multicast.
NAT6 does not specify how it can work with multicast.
The other proposals are silent on this issue.
Note: it may be possible for IGMP messages to be propagated and proxied (Fenner, B., He, H., Haberman, B., and H. Sandick, “Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying"),” August 2006.) [RFC4605] across their respective NAT device (Wing, D. and T. Eckert, “IP Multicast Requirements for a Network Address Translator (NAT) and a Network Address Port Translator (NAPT),” February 2008.) [RFC5135]. More study on this is needed.
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The separation of NAT and DNS-rewriting reduces the impact of this issue. IVI, NAT64, and sNAT-PT separate the NAT and DNS-rewrite functions, and avoid this constraint.
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The separation of NAT and DNS-rewriting reduces the impact of this issue. IVI, NAT64, and sNAT-PT all separate the NAT and DNS-rewrite functions.
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TBD.
| TOC |
A CGN would only allow a certain subscriber to open a certain number of ports, thereby preventing a single subscriber from DoSing other subscribers ([I‑D.nishitani‑cgn] (Yamagata, I., Nishitani, T., Miyakawa, S., Nakagawa, A., and H. Ashida, “Common requirements for IP address sharing schemes,” March 2010.), "a CGN SHOULD limit the number of the CGN external ports").
| TOC |
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Unlike NAT-PT, all proposals that involve DNS-rewriting (IVI, NAT64, sNAT-PT) do not return synthetic AAAA records if a real AAAA record exists. This prevents the problem. A DNS timeout (of the AAAA query) will prevent the optimum DNS response from being returned (that is, the real AAAA record rather than the synthesized AAAA record pointing to the CGN device), but it is still possible to connect even when such a timeout occurs. In practice, such DNS timeouts are not a common occurance.
In [I‑D.bagnulo‑behave‑nat64] (Bagnulo, M., Matthews, P., and I. Beijnum, “NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” March 2009.), EDNS0 (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) [RFC2671] is proposed as the mechanism for a host to determine if the AAAA record is synthetic (that is, generated by the DNS rewriting function) or if the AAAA record is genuine (that is, was not synthesized).
| TOC |
TBD.
| TOC |
sNAT-PT avoids this problem with its stateful DNS proxy.
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The additional NAT binding state is not created if the DNS rewriting and NAT functions are separate. Thus, this problem is avoided by [I‑D.xli‑behave‑ivi] (Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, “The CERNET IVI Translation Design and Deployment for the IPv4/IPv6 Coexistence and Transition,” January 2010.), [I‑D.bagnulo‑behave‑nat64] (Bagnulo, M., Matthews, P., and I. Beijnum, “NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” March 2009.), and [I‑D.miyata‑v6ops‑snatpt] (Miyata, H. and M. Endo, “sNAT-PT: Simplified Network Address Translation - Protocol Translation,” September 2008.).
| TOC |
DNSSEC is incompatible with synthesized DNS responses (DNS rewriting).
NAT64 recommends that DNSSEC-capable IPv6-only hosts use the EDNS SAS option to ignore synthetic DNS responses. This would allow the IPv6 host to ignore synthetic DNS responses and allows DNSSEC to work for non- synthesized AAAA responses. This means, however, that DNSSEC only works for native IPv6 AAAA responses, and DNSSEC cannot be used for IPv4 A responses.
No other proposal discusses how it would work with DNSSEC.
Note: A proposal that does DNS rewriting only in a validating resolver (after validation), or construct records in the authoritative server, will work fine with DNSsec.
| TOC |
TBD.
| TOC |
| TOC |
When resources are shared it is important they are shared fairly. On today's Internet, the shared resource is bandwidth -- both the service provider's core bandwidth (sharing between subscribers) and subscriber access bandwidth (sharing between a subscriber's own hosts). Subscribers are given an IP address(es) for their exclusive use. With all of the NAT44 and NAT64 mechanisms proposed, an IPv4 address is shared amongst several subscribers.
This address sharing raises some security considerations, including DoS potential (a subscriber might accidentally or purposefully use all available ports, denying ports to other subscribers [I‑D.nishitani‑cgn] (Yamagata, I., Nishitani, T., Miyakawa, S., Nakagawa, A., and H. Ashida, “Common requirements for IP address sharing schemes,” March 2010.) and spoofing (a subscriber might send a packet with the correct IP address, but the port belongs to a different subscriber [A+P] (Maennel, O., Bush, R., Cittadini, L., and S. Bellovin, “A Better Approach than Carrier-Grade-NAT,” .).
For lack of a better identifier, many applications and systems use an IPv4 address as an end-host identifier and take action based on that identity. In the past, IP addresses sometimes provided additional privileges (e.g., the ability to login without a password using Berkeley "r services"). This persists today with some systems (e.g., Sender Policy Framework (SPF)). Conversely, undesired behavior of a certain IP address can cause servers to refuse to provide service. For example, excessive connection attempts or excessive downloading can cause an HTTP server to delay (or refuse) providing service to that IP address. As another example, IP address blacklisting (e.g., DNSBL) might cause e-mail from that IP address to be considered more likely to be spam. Even with consumer NAT44, these systems work reasonably well because excessive connection attempts or spam originating from any host belonging to a subscriber is punished, without harming other subscribers of that ISP. (Of course, some such systems apply their rate limiting to entire subnets in order to purposefully punish other subscribers of that ISP.) However, when an ISP deploys a NAT44 that aggregates many subscribers behind the same public IPv4 address, all of those subscribers will be appear as one identity to the rest of the Internet. This will cause problems with existing systems that equate an IPv4 address with an identity, and take action based on such identities.
| TOC |
Thanks to the authors of the contributions compared in this document, Cullen Jennings (NAT6); Marcelo Bagnulo, Philip Matthews, Iljitsch van Beijnum (NAT64); Xing Li, Maoke Chen, Congxiao Bao, Hong Zhang, Jianping Wu, Fred Baker (IVI); Alain Durand, Ralph Droms, Brian Haberman (DS-Lite); Tomohiro Nishitani, Shin Miyakawa (CGN); Remi Despres (APB-Revised); Hiroshi Miyata, Masahito Endo (sNAT-PT); Olaf Maennel, Randy Bush, Luca Cittadini, Steven M. Bellovin (A+P).
Thanks to Fred Baker, Randy Bush, Thomas Narten, Dave Thaler and Eric Vyncke for their review and suggested improvements to the document.
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This document has no IANA actions.
| TOC |
| TOC |
| [A+P] | Maennel, O., Bush, R., Cittadini, L., and S. Bellovin, “A Better Approach than Carrier-Grade-NAT.” |
| [I-D.bagnulo-behave-nat64] | Bagnulo, M., Matthews, P., and I. Beijnum, “NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers,” draft-bagnulo-behave-nat64-03 (work in progress), March 2009 (TXT). |
| [I-D.baker-behave-ivi] | Li, X., Bao, C., Baker, F., and K. Yin, “IVI Update to SIIT and NAT-PT,” draft-baker-behave-ivi-01 (work in progress), September 2008 (TXT). |
| [I-D.durand-dual-stack-lite] | Durand, A., “Dual-stack lite broadband deployments post IPv4 exhaustion,” draft-durand-dual-stack-lite-00 (work in progress), July 2008 (TXT). |
| [I-D.endo-v6ops-dnsproxy] | Endo, M. and H. Miyata, “Translator Friendly DNS Proxy,” draft-endo-v6ops-dnsproxy-01 (work in progress), October 2008 (TXT). |
| [I-D.ietf-v6ops-nat64-pb-statement-req] | Bagnulo, M., Baker, F., and I. Beijnum, “IPv4/IPv6 Coexistence and Transition: Requirements for solutions,” draft-ietf-v6ops-nat64-pb-statement-req-00 (work in progress), May 2008 (TXT). |
| [I-D.jennings-behave-nat6] | Jennings, C., “NAT for IPv6-Only Hosts,” draft-jennings-behave-nat6-01 (work in progress), November 2008 (TXT). |
| [I-D.miyata-v6ops-snatpt] | Miyata, H. and M. Endo, “sNAT-PT: Simplified Network Address Translation - Protocol Translation,” draft-miyata-v6ops-snatpt-02 (work in progress), September 2008 (TXT). |
| [I-D.nishitani-cgn] | Yamagata, I., Nishitani, T., Miyakawa, S., Nakagawa, A., and H. Ashida, “Common requirements for IP address sharing schemes,” draft-nishitani-cgn-04 (work in progress), March 2010 (TXT). |
| [I-D.xli-behave-ivi] | Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, “The CERNET IVI Translation Design and Deployment for the IPv4/IPv6 Coexistence and Transition,” draft-xli-behave-ivi-07 (work in progress), January 2010 (TXT). |
| [RFC4966] | Aoun, C. and E. Davies, “Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status,” RFC 4966, July 2007 (TXT). |
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| Dan Wing | |
| Cisco Systems, Inc. | |
| 170 West Tasman Drive | |
| San Jose, CA 95134 | |
| USA | |
| Email: | dwing@cisco.com |
| David Ward | |
| Cisco Systems, Inc. | |
| Email: | wardd@cisco.com |
| Alain Durand | |
| Comcast | |
| 1500 Market st | |
| Philadelphia, PA 19102 | |
| USA | |
| Email: | alain_durand@cable.comcast.com |
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