RFC 6619

Scalable Operation of Address Translators with Per-Interface Bindings 🎀

Okay bestie, this is RFC 6619 💖 It's a Standards Track doc from the IETF (Internet Engineering Task Force), so it's an official spec, not just vibes. It was written by J. Arkko (Ericsson), L. Eggert (NetApp), and M. Townsley (Cisco). It's dated June 2012, and its ISSN is 2070-1721. The title block doesn't list anything it obsoletes or updates, so no drama there 🙈

Abstract ✨

This document explains how to employ address translation in networks that serve a large number of individual customers without requiring a correspondingly large amount of private IPv4 address space.

Status of This Memo 📜

This is an Internet Standards Track document.

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 Internet Standards 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/rfc6619.

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.

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1. Introduction ✨

Okay so this doc explains how to use address translation without eating up a huge chunk of private address space 💅 This matters a lot in networks that serve a ton of individual customers. Networks serving more than 2^24 (16 million) users cannot give each user a unique private IPv4 address, because the largest reserved private address block is 10/8 [RFC1918]. Lots of networks are already hitting these limits today, like in the consumer Internet service market. Even some single devices can run into them, like cellular network gateways or mobile IP home agents 😭

If there were plenty of IPv4 address space, this wouldn't even be a problem. Giving every user a public IPv4 address would still be totally viable, and you could skip all the complications that come with the more limited private address space. But the IPv4 address pool is running low, so that approach is getting harder and harder to keep up with 💀

People have suggested converting more of the unassigned IPv4 space into private use, so bigger networks could run on private IPv4 addresses. At the time of this writing, the IANA "free pool" had only 12 unallocated unicast IPv4 /8 prefixes. Reserving a few of those for private use would give such deployments some breathing room, but it wouldn't be a long-term solution. It would cause significant operational and management overhead, and it would further reduce the number of available IPv4 addresses 🙅‍♀️

Segmenting a network into areas of overlapping private address space is another possible technique, but it seriously complicates the design and operation of a network 🥴

Finally, the transition to IPv6 will eventually get rid of these addressing limits 🦋 But during the migration period, when IPv4 and IPv6 have to coexist, address or protocol translation will be needed in order to reach IPv4 destinations.

The rest of this document is organized as follows. Section 2 gives an outline of the solution, Section 3 introduces some terms, Section 4 specifies the required behavior for managing NAT bindings, and Section 5 discusses the use of this technique with IPv6.

2. Solution Outline 💅

The need for address or protocol translation during the migration period to IPv6 creates the chance to deploy these mechanisms in a way that supports a large user base without needing a correspondingly large IPv4 address block. It's giving efficiency 🌸

A Network Address Translator (NAT) is typically configured to connect a network domain that uses private IPv4 addresses to the public Internet. The NAT device, which is configured with a public IPv4 address, creates and maintains a mapping for each communication session from a device inside the domain it serves to devices in the public Internet. It does this by translating the packet flow of each session so that the externally visible traffic uses only public addresses.

In many NAT deployments, the network domain connected by the NAT to the public Internet is a broadcast network sharing the same media, where each individual device has to have a private IPv4 address that is unique within this network. In these deployments, it's natural to also implement the NAT functionality so that it uses the private IPv4 address when looking up which mapping should be used to translate a given communication session 👀

Here's the thing though: this isn't an inherent requirement 💖 When other methods of identifying the correct mapping are available, and the NAT is not connecting a shared-media broadcast network to the Internet, there's no need to assign each device in the domain a unique IPv4 address ✨

This is the case, for example, when the NAT connects devices to the Internet over individual point-to-point links. In this case, it becomes possible to use the same private addresses many times, which means you can support any number of devices behind a NAT using very few IPv4 addresses 🤯

There are tunneling-based techniques that can get the same benefits by establishing new tunnels over any IP network [RFC6333]. But where point-to-point links already exist, adding another layer of tunneling is unnecessary (and can even be potentially harmful because of effects on the Maximum Transfer Unit (MTU) settings) 🙃 The approach in this document can be implemented and deployed within a single device and has no effect on hosts behind it. Also, since no additional layers of tunneling are introduced, there's no effect on the MTU. You also don't need to implement tunnel endpoint discovery, security mechanisms, or other parts of a tunneling solution. Honestly, there are no changes to the devices behind the NAT 🫶

Note that existing tunnels are a common special case of point-to-point links. Cellular network gateways, for instance, terminate a large number of tunnels that are already needed for mobility management reasons. This approach is especially attractive in environments like that, since no additional tunneling mechanisms, negotiation, or host changes are required. Packets also keep taking the same path they'd normally take, because there's no extra tunneling.

Other commonly used network technologies that might be of interest include Point-to-Point Protocol (PPP) [RFC1661] links, PPP over Ethernet (PPPoE) [RFC2516] encapsulation, Asynchronous Transfer Mode (ATM) Permanent Virtual Circuits (PVCs), and per-subscriber virtual LAN (VLAN) allocation in consumer broadband networks 🍓

The approach described here also results in overlapping private address space, similar to segmenting the network into different areas. However, this overlap is applied only at the network edges, so it doesn't negatively impact routing or reachability of servers 💜

3. Terms 📖

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 [RFC2119].

"NAT" in this document includes both "Basic NAT" and "Network Address Port Translation (NAPT)" as defined by [RFC2663]. The term "NAT Session" is adapted from [RFC5382] and is defined as follows.

NAT Session - A NAT session is an association between a transport layer session as seen in the internal realm and a session as seen in the external realm, by virtue of NAT translation. The NAT session will provide the translation glue between the two session representations.

This document uses the term "mapping" as defined in [RFC4787] to refer to state at the NAT necessary for network address and port translation of sessions. 🧚

4. Per-Interface Bindings 🦋

To support a mode of operation that uses a fixed number of IPv4 addresses to serve an arbitrary number of devices, a NAT MUST manage its mappings on a per-interface basis. It does this by associating a particular NAT session not only with the five tuples used for the transport connection on both sides of the NAT, but also with the internal interface on which the user device is connected to the NAT. This approach allows each internal interface to use the same private IPv4 address range. Note that the interface doesn't have to be physical 🙌 It may also correspond to a tunnel, VLAN, or other identifiable communications channel.

For deployments where exactly one user device is connected with a separate tunnel interface and all tunnels use the same IPv4 address for the user devices, it's redundant to store this address in the mapping in addition to the internal interface identifier. When the internal interface identifier is shorter than a 32-bit IPv4 address, this may decrease the storage requirements of a mapping entry by a small amount, which may help NAT scalability. For other deployments, it's likely necessary to store both the user device IPv4 address and the internal interface identifier, which slightly increases the size of the mapping entry.

This mode of operation is only suitable in deployments where user devices connect to the NAT over point-to-point links. If supported, this mode of operation SHOULD be configurable, and it should be disabled by default in general-purpose NAT devices. 🚫

All address translators make it hard to address devices behind them. The same is true of the particular NAT variant described in this document. An additional constraint comes from using the same address space for different devices behind the NAT, which prevents the use of unique private addresses for communication between devices behind the same NAT. Part 2 is coming with Section 5 and everything after it, so stay tuned bestie 💖

5. IPv6 Considerations ✨

okay so private address space conservation is still a BIG deal during the IPv6 migration 🌸 bestie, we're going to be talking to the IPv4 internet for a looong time, so we can't just ghost it 💀 this doc lays out two recommended deployment models for IPv6, and the vibe is totally different for each one 🦋

in the first model, the mechanisms in this document are super useful 💖 this one uses dual stack [RFC4213]. the IPv6 side runs on global addresses with direct end-to-end communication, which is so clean 😭 but the IPv4 side is still stuck with private addressing and NATs, no way around it 🙈 so for the IPv4 side, per-interface NAT mappings are RECOMMENDED 🫶 per-interface mappings help the NAT scale, and dual-stack helps take pressure off the NAT device by moving key traffic over to IPv6, so the NAT doesn't have to process it at all 👑

the second model uses address and protocol translation, like the one defined in [RFC6146] ✨ here there is NO IPv4 in the internal network at all 💅 this model only works when every relevant device and application is IPv6 capable, so make sure your whole squad is ready first 🧚 in this setup, per-interface mappings could be used the way we described above, but they're generally unnecessary 🌈 the IPv6 address space is huge, so there are plenty of mappings already, periodt 💜

6. Security Considerations 🔥

okay this section is serious, bestie, so read it carefully 💀

the practices in this document do NOT affect the security properties of address translation 🎀 the binding method in here is not observable to a device on the outside of the NAT, so a regular NAT and a NAT specified here cannot be told apart 🙈 point-to-point links mean devices behind the NAT cannot talk to each other directly without going through the NAT (or a router) 😭 and since they all share the same address space, a NAT operation MUST occur between two devices for them to communicate, ngl that's a lot of translating 🍓

the security implications of address translation in general have been discussed in lots of earlier documents, including [RFC2663], [RFC2993], [RFC4787], and [RFC5382] 📚 go check those out if you want the full tea ☕ iykyk

7. References 🦋

7.1. Normative References 💜

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

7.2. Informative References ✨

[L2NAT]    Miles, D., Ed., and M. Townsley, "Layer2-Aware NAT", Work
           in Progress, March 2009.

[RFC1661]  Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
           STD 51, RFC 1661, July 1994.

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

[RFC2516]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
           and R. Wheeler, "A Method for Transmitting PPP Over
           Ethernet (PPPoE)", RFC 2516, February 1999.

[RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
           Translator (NAT) Terminology and Considerations",
           RFC 2663, August 1999.

[RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
           November 2000.

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

[RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
           Translation (NAT) Behavioral Requirements for Unicast
           UDP", BCP 127, RFC 4787, January 2007.

[RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
           Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
           RFC 5382, October 2008.

[RFC6127]  Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6
           Co-Existence Scenarios", RFC 6127, May 2011.

[RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
           NAT64: Network Address and Protocol Translation from IPv6
           Clients to IPv4 Servers", RFC 6146, April 2011.

[RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
           Stack Lite Broadband Deployments Following IPv4
           Exhaustion", RFC 6333, August 2011.

[TRILOGY]  "Trilogy Project", <http://www.trilogy-project.org/>.

Appendix A. Contributors 🎀

the ideas in this document were first presented in [RFC6333] 🧚 this doc is also indebted to [RFC6127] and [L2NAT] 💖 but all of those focused on extra components, like tunneling protocols or allocating special IP address ranges 🌸 we wanted a spec that just focuses on the core functionality of per-interface NAT mappings ✨ still, David Miles and Alain Durand deserve all the credit for coming up with the ideas in this memo 👑

Appendix B. Acknowledgments 🫶

the authors would also like to thank Randy Bush, Fredrik Garneij, Dan Wing, Christian Vogt, Marcelo Braun, Joel Halpern, Wassim Haddad, Alan Kavanaugh, and others for interesting discussions in this problem space 💜

Lars Eggert is partly funded by the Trilogy Project [TRILOGY], a research project supported by the European Commission under its Seventh Framework Program 🌈

Authors' Addresses 📮

Jari Arkko
Ericsson
Jorvas  02420
Finland

EMail: jari.arkko@piuha.net


Lars Eggert
NetApp
Sonnenallee 1
85551 Kirchheim
Germany

Phone: +49 151 12055791
EMail: lars@netapp.com
URI:   http://eggert.org/


Mark Townsley
Cisco
Paris  75006
France

EMail: townsley@cisco.com