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Eggert 5 Expires: April 10, 2011 Nokia 6 October 7, 2010 8 Scalable Operation of Address Translators with Per-Interface Bindings 9 draft-arkko-dual-stack-extra-lite-01 11 Abstract 13 This document explains how to employ address translation in networks 14 that serve a large number of individual customers without requiring a 15 correspondingly large amount of private IPv4 address space. 17 Status of this Memo 19 This Internet-Draft is submitted to IETF in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 This Internet-Draft will expire on April 10, 2011. 40 Copyright Notice 42 Copyright (c) 2010 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the BSD License. 55 1. Introduction 57 This document explains how to employ address translation without 58 consuming a large amount of private address space. This is important 59 in networks that serve a large number of individual customers. 60 Networks that serve more than 2^24 (16 million) users cannot assign a 61 unique private IPv4 address to each user, because the largest 62 reserved private address block reserved is 10/8 [RFC1918]. Many 63 networks are already hitting these limits today, for instance, in the 64 consumer Internet service market. Even some individual devices may 65 approach these limits, for instance, cellular network gateways or 66 mobile IP home agents. 68 If ample IPv4 address space was available, this would be a non-issue, 69 because the current practice of assigning public IPv4 addresses to 70 each user would remain viable, and the complications associated with 71 using the more limited private address space could be avoided. 72 However, as the IPv4 address pool is becoming depleted, this practice 73 is becoming increasingly difficult to sustain. 75 It has been suggested that more of the unassigned IPv4 space should 76 be converted for private use, in order to allow the provisioning of 77 larger networks with private IPv4 address space. At the time of 78 writing, the IANA "free pool" contained only 24 unallocated unicast 79 IPv4 /8 prefixes. Although reserving a few of those for private use 80 would create some breathing room for such deployments, it would not 81 result in a solution with long-term viability, would result in 82 significant operational and management overheads, and would further 83 reduce the number of available IPv4 addresses. 85 Segmenting a network into areas of overlapping private address space 86 is another possible technique, but it severely complicates the design 87 and operation of a network. 89 Finally, the transition to IPv6 will eventually eliminate these 90 addressing limitations. However, during the migration period when 91 IPv4 and IPv6 have to co-exist, there will be the need to reach IPv4 92 destinations, which involves the use of address or protocol 93 translation. 95 The rest of this document is organized as follows. Section 2 gives 96 an outline of the solution, Section 3 introduces some terms, 97 Section 4 specifies the required behavior for managing NAT bindings 98 and Section 5 discusses the use of this technique with IPv6. 100 2. Solution Outline 102 The need for address or protocol translation during the migration 103 period to IPv6 creates the opportunity to deploy these mechanisms in 104 a way that allows the support of a large user base without the need 105 for a correspondingly large IPv4 address block. 107 A Network Address Translator (NAT) is typically configured to connect 108 a network domain that uses private IPv4 addresses to the public 109 Internet. The NAT device - which is configured with a public IPv4 110 address - creates and maintains a mapping for each communication 111 session from a device inside the domain it serves to devices in the 112 public Internet. It does that by translating the packet flow of each 113 session such that the externally visible traffic uses only public 114 addresses. 116 In most NAT deployments, the network domain connected by the NAT to 117 the public Internet is a broadcast network sharing the same media, 118 where each individual device must have a unique IPv4 address. In 119 such deployments it is natural to also implement the NAT 120 functionality such that it uses this unique IPv4 address when looking 121 up which mapping should be used to translate a given communication 122 session. 124 It is important to note, however, that this is not an inherent 125 requirement. When other methods of identifying the correct mapping 126 are available, and the NAT is not connecting a shared-media broadcast 127 network to the Internet, there is no need to assign each device in 128 the domain a unique IPv4 address. 130 This is the case, for example, when the NAT connects devices to the 131 Internet that connect to it with individual point-to-point links. In 132 this case, it becomes possible to use the same private addresses many 133 times, making it possible to support any number of devices behind a 134 NAT using very few IPv4 addresses. 136 There are tunneling-based techniques to reach the same benefits, by 137 establishing new tunnels over any IP network 138 [I-D.ietf-softwire-dual-stack-lite]. However, where the point-to- 139 point links already exists, creating an additional layer of tunneling 140 is unnecessary. The approach described in this document can be 141 implemented and deployed within a single device and has no effect to 142 hosts behind it. In addition, as no additional layers of tunneling 143 are introduced, there is no effect to the Maximum Transfer Unit (MTU) 144 settings. 146 Note, however, that existing tunnels are a common special case of 147 point-to-point links. For instance, cellular network gateways 148 terminate a large number of tunnels that are already needed for 149 mobility management reasons. Implementing the approach described in 150 this document is particularly attractive in such environments, given 151 that no additional tunneling mechanisms, negotiation, or host changes 152 are required. In addition, since there is no additional tunneling, 153 packets continue to take the same path as they would normally take. 154 Other commonly appearing network technology that may be of interest 155 include Point-to-Point Protocol (PPP) [RFC1661] links, PPP over 156 Ethernet (PPPoE) [RFC2516] encapsulation, Asynchronous Transfer Mode 157 (ATM) Permanent Virtual Circuits (PVCs), and per-subscriber virtual 158 LAN (VLAN) allocation in consumer broadband networks. 160 The approach described here also results overlapping private address 161 space, like the segmentation of the network to different areas. 162 However, this overlap is applied only at the network edges, and does 163 not impact routing or reachability of servers in a negative way. 165 3. Terms 167 In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL", 168 "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as 169 described in [RFC2119]. 171 "NAT" in this document includes both "Basic NAT" and "Network 172 Address/Port Translator (NAPT)" as defined by [RFC2663]. The term 173 "NAT Session" is adapted from [RFC5382] and is defined as follows. 175 NAT Session - A NAT session is an association between a transport 176 layer session as seen in the internal realm and a session as seen in 177 the external realm, by virtue of NAT translation. The NAT session 178 will provide the translation glue between the two session 179 representations. 181 This document uses the term mapping as defined in [RFC4787] to refer 182 to state at the NAT necessary for network address and port 183 translation of sessions. 185 4. Per-Interface Bindings 187 To support a mode of operation that uses a fixed number of IPv4 188 addresses to serve an arbitrary number of devices, a NAT MUST manage 189 its mappings on a per-interface basis, by associating a particular 190 NAT session not only with the five tuples used for the transport 191 connection on both sides of the NAT, but also with the internal 192 interface on which the user device is connected to the NAT. This 193 approach allows each internal interface to use the same private IPv4 194 address range. 196 For deployments where exactly one user device is connected with a 197 separate tunnel interface and all tunnels use the same IPv4 address 198 for the user devices, it is redundant to store this address in the 199 mapping in addition to the internal interface identifier. When the 200 internal interface identifier is shorter than a 32-bit IPv4 address, 201 this may decrease the storage requirements of a mapping entry by a 202 small measure, which may aid NAT scalability. For other deployments, 203 it is likely necessary to store both the user device IPv4 address and 204 the internal interface identifier, which slightly increases the size 205 of the mapping entry. 207 This mode of operation is only suitable in deployments where user 208 devices connect to the NAT over point-to-point links. If supported, 209 this mode of operation SHOULD be configurable, and it should be 210 disabled by default. 212 5. IPv6 Considerations 214 Private address space conservation is important even during the 215 migration to IPv6, because it will be necessary to communicate with 216 the IPv4 Internet for a long time. This document specifies two 217 recommended deployment models for IPv6. In the first deployment 218 model the mechanisms specified in this document are useful. In the 219 second deployment model no additional mechanisms are needed, because 220 IPv6 addresses are already sufficient to distinguish mappings from 221 each other. 223 The first deployment model employs dual stack [RFC4213]. The IPv6 224 side of dual stack operates based on global addresses and direct end- 225 to-end communication. However, on the IPv4 side private addressing 226 and NATs are a necessity. The use of per-interface NAT mappings is 227 RECOMMENDED for the IPv4 side under these circumstances. Per- 228 interface mappings help the NAT scale, while dual stack operation 229 helps reduce the pressure on the NAT device by moving key types of 230 traffic to IPv6, eliminating the need for NAT processing. 232 The second deployment model involves the use of address and protocol 233 translation, such as the one defined in 234 [I-D.ietf-behave-v6v4-xlate-stateful]. In this deployment model 235 there is no IPv4 in the internal network at all. This model is 236 applicable only in situations where all relevant devices and 237 applications are IPv6-capable. In this situation, per-interface 238 mappings could be employed as specified above, but they are generally 239 unnecessary as the IPv6 address space is large enough to provide a 240 sufficient number of mappings. 242 6. Security Considerations 244 This practices outlined in this document do not affect the security 245 properties of address translation. 247 7. IANA Considerations 249 This document has no IANA implications. 251 8. References 253 8.1. Normative References 255 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 256 Requirement Levels", BCP 14, RFC 2119, March 1997. 258 8.2. Informative References 260 [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, 261 RFC 1661, July 1994. 263 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 264 E. Lear, "Address Allocation for Private Internets", 265 BCP 5, RFC 1918, February 1996. 267 [RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., 268 and R. Wheeler, "A Method for Transmitting PPP Over 269 Ethernet (PPPoE)", RFC 2516, February 1999. 271 [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address 272 Translator (NAT) Terminology and Considerations", 273 RFC 2663, August 1999. 275 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 276 for IPv6 Hosts and Routers", RFC 4213, October 2005. 278 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 279 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 280 RFC 4787, January 2007. 282 [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 283 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 284 RFC 5382, October 2008. 286 [I-D.ietf-softwire-dual-stack-lite] 287 Durand, A., Droms, R., Haberman, B., Woodyatt, J., Lee, 288 Y., and R. Bush, "Dual-stack lite broadband deployments 289 post IPv4 exhaustion", 290 draft-ietf-softwire-dual-stack-lite-03 (work in progress), 291 February 2010. 293 [I-D.arkko-townsley-coexistence] 294 Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6 Co- 295 Existence Scenarios", draft-arkko-townsley-coexistence-03 296 (work in progress), July 2009. 298 [I-D.ietf-behave-v6v4-xlate-stateful] 299 Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful 300 NAT64: Network Address and Protocol Translation from IPv6 301 Clients to IPv4 Servers", 302 draft-ietf-behave-v6v4-xlate-stateful-08 (work in 303 progress), January 2010. 305 [I-D.miles-behave-l2nat] 306 Miles, D. and M. Townsley, "Layer2-Aware NAT", 307 draft-miles-behave-l2nat-00 (work in progress), 308 March 2009. 310 Appendix A. Acknowledgments 312 The ideas in this draft were first presented in 313 [I-D.ietf-softwire-dual-stack-lite]. This document also in debt to 314 [I-D.arkko-townsley-coexistence] and [I-D.miles-behave-l2nat]. 315 However, all of these documents focused on additional components, 316 such as tunneling protocols or the allocation of special IP address 317 ranges. We wanted to publish a specification that just focuses on 318 the core functionality of a per-interface NAT mappings. 320 The authors would also like to thank Alain Durand, Randy Bush, 321 Fredrik Garneij, Dan Wing, Christian Vogt, Marcelo Braun, Joel 322 Halpern, Wassim Haddad and others for interesting discussions in this 323 problem space. 325 Authors' Addresses 327 Jari Arkko 328 Ericsson 329 Jorvas 02420 330 Finland 332 Email: jari.arkko@piuha.net 334 Lars Eggert 335 Nokia Research Center 336 P.O. Box 407 337 Nokia Group 00045 338 Finland 340 Phone: +358 50 48 24461 341 Email: lars.eggert@nokia.com 342 URI: http://research.nokia.com/people/lars_eggert/