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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 V6OPS Working Group P. Matthews 3 Internet-Draft Alcatel-Lucent 4 Intended status: Informational October 22, 2012 5 Expires: April 25, 2013 7 Design Guidelines for IPv6 Networks 8 draft-matthews-v6ops-design-guidelines-01 10 Abstract 12 This document presents advice on the design choices that arise when 13 designing IPv6 networks (both dual-stack and IPv6-only). The 14 intended audience is someone designing an IPv6 network who is 15 knowledgeable about best current practices around IPv4 network 16 design, and wishes to learn the corresponding practices for IPv6. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on April 25, 2013. 35 Copyright Notice 37 Copyright (c) 2012 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Design Choices . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2.1. Mix IPv4 and IPv6 on the Same Link? . . . . . . . . . . . 4 55 2.2. Links with Only Link-Local Addresses? . . . . . . . . . . 4 56 2.3. Link-Local Next-Hop in a Static Route? . . . . . . . . . . 6 57 2.4. Separate or Combined eBGP Sessions? . . . . . . . . . . . 6 58 2.5. eBGP Endpoints: Global or Link-Local Addresses? . . . . . 7 59 3. General Observations . . . . . . . . . . . . . . . . . . . . . 8 60 3.1. Use of Link-Local Addresses . . . . . . . . . . . . . . . 8 61 3.2. Separation of IPv4 and IPv6 . . . . . . . . . . . . . . . 9 62 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 63 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 64 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 65 7. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 66 8. Informative References . . . . . . . . . . . . . . . . . . . . 10 67 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 69 1. Introduction 71 This document presents advice on the design choices that arise when 72 designing IPv6 networks (both dual-stack and IPv6-only). The 73 intended audience is someone designing an IPv6 network who is 74 knowledgeable about best current practices around IPv4 network 75 design, and wishes to learn the corresponding practices for IPv6. 77 The focus of the document is on design choices where there are 78 differences between IPv4 and IPv6, either in the range of possible 79 alternatives (e.g. the extra possibilities introduced by link-local 80 addresses in IPv6) or the recommended alternative. The document 81 presents the alternatives and discusses the pros and cons in detail. 82 Where consensus currently exists around the best practice, this is 83 documented; otherwise the document simply summarizes the current 84 state of the discussion. Thus this document serves to both to 85 document the reasoning behind best current practices for IPv6, and to 86 allow a designer to make an intelligent choice where no such 87 consensus exists. 89 This document does not present advice on strategies for adding IPv6 90 to a network, nor does it discuss transition mechanisms. For advice 91 in these areas, see [RFC6180] for general advice, 92 [I-D.ietf-v6ops-wireline-incremental-ipv6] for wireline service 93 providers, [RFC6342] for mobile network providers, [RFC5963] for 94 exchange point operators, [I-D.ietf-v6ops-icp-guidance] for content 95 providers, and both [RFC4852] and 96 [I-D.ietf-v6ops-enterprise-incremental-ipv6] for enterprises. Nor 97 does the document cover the ins and outs of creating an IPv6 98 addressing plan; for advice in this area, see [RFC5375]. 100 The current version of this document focuses on unicast network 101 design only. It does not cover multicast,, nor supporting 102 infrastructure such as DNS. This may change in future versions. 104 The current version is still work in progress, and it is expected 105 that the presentation and discussion of additional design choices 106 will be added as the document matures. 108 2. Design Choices 110 This section consists of a list of specific design choices a network 111 designer faces when designing an IPv6-only or dual-stack network, 112 along with guidance and advice to the designer when making a choice. 114 2.1. Mix IPv4 and IPv6 on the Same Link? 116 Should IPv4 and IPv6 traffic be logically separated on a link? That 117 is: 119 a. Mix IPv4 and IPv6 traffic on the same layer 2 connection, OR 121 b. Separate IPv4 and IPv6 by using separate physical or logical 122 links (e.g., two physical links or two VLANs on the same link)? 124 Option (a) implies a single layer 3 interface at each end with both 125 IPv4 and IPv6 addresses; while option (b) implies two layer 3 126 interfaces, one for IPv4 addresses and one with IPv6 addresses. 128 The advantages of option (a) include: 130 o Requires only half as many layer 3 interfaces as option (b), thus 131 providing better scaling; 133 o May require fewer physical ports, thus saving money; 135 o Can make the QoS implementation much easier (for example, rate- 136 limiting the combined IPv4 and IPv6 traffic to or from a 137 customer); 139 o Provides better support for the expected future of increasing IPv6 140 traffic and decreasing IPv4 traffic; 142 o And is generally conceptually simpler. 144 For these reasons, there is a pretty strong consensus in the operator 145 community that option (a) is the preferred way to go. 147 However, there can be times when option (b) is the pragmatic choice. 148 Most commonly, option (b) is used to work around limitations in 149 network equipment. One big example is the generally poor level of 150 support today for individual statistics on IPv4 traffic vs IPv6 151 traffic when option (a) is used. Other, device-specific, limitations 152 exist as well. It is expected that these limitations will go away as 153 support for IPv6 matures, making option (b) less and less attractive 154 until the day that IPv4 is finally turned off. 156 Most networks today use option (a) wherever possible. 158 2.2. Links with Only Link-Local Addresses? 160 Should the link: 162 a. Use only link-local addresses ("unnumbered"), OR 164 b. Have global or unique-local addresses assigned in addition to 165 link-locals? 167 There are two advantages of unnumbered links. The first advantage is 168 ease of configuration. In a network with a large number of 169 unnumbered links, the operator can just enable an IGP on each router, 170 without going through the tedious process of assigning and tracking 171 the addresses for each link. The second advantage is security. 172 Since link-local addresses are unroutable, the associated interfaces 173 cannot be attacked from an off-link device. This implies less effort 174 around maintaining security ACLs. 176 Countering this advantage are various disadvantages to unnumbered 177 links in IPv6: 179 o It is not possible to ping an interface that has only a link-local 180 address from a device that is not directly attached to the link. 181 Thus, to troubleshoot, one must typically log into a device that 182 is directly attached to the device in question, and execute the 183 ping from there. 185 o A traceroute passing over the unnumbered link will return the 186 loopback or system address of the router, rather than the address 187 of the interface itself. 189 o On some devices, by default the link-layer address of the 190 interface is derived from the MAC address assigned to interface. 191 When this is done, swapping out the interface hardware (e.g. 192 interface card) will cause the link-layer address to change. In 193 some cases (peering config, ACLs, etc) this may require additional 194 changes. However, many devices allow the link-layer address of an 195 interface to be explicitly configured, which avoids this issue. 197 o The practice of naming router interfaces using DNS names is 198 difficult-to-impossible when using LLAs only. 200 o It is not possible to identify the interface or link (in a 201 database, email, etc) by just giving its address. 203 For more discussion on the pros and cons, see 204 [I-D.ietf-opsec-lla-only]. 206 Today, most operators use numbered links (option b). 208 2.3. Link-Local Next-Hop in a Static Route? 210 What form of next-hop address should one use in a static route? 212 a. Use the far-end's link-local address as the next-hop address, OR 214 b. Use the far-end's GUA/ULA address as the next-hop address? 216 Recall that the IPv6 specs for OSPF [RFC5340] and ISIS [RFC5308] 217 dictate that they always use link-locals for next-hop addresses. For 218 static routes, [RFC4861] section 8 says: 220 A router MUST be able to determine the link-local address for each 221 of its neighboring routers in order to ensure that the target 222 address in a Redirect message identifies the neighbor router by 223 its link-local address. For static routing, this requirement 224 implies that the next-hop router's address should be specified 225 using the link-local address of the router. 227 This implies that using a GUA or ULA as the next hop will prevent a 228 router from sending Redirect messages for packets that "hit" this 229 static route. All this argues for using a link-local as the next-hop 230 address in a static route. 232 However, there are two cases where using a link-local address as the 233 next-hop clearly does not work. One is when the static route is an 234 indirect (or multi-hop) static route. The second is when the static 235 route is redistributed into another routing protocol. In these 236 cases, the above text from RFC 4861 notwithstanding, either a GUA or 237 ULA must be used. 239 Furthermore, many network operators are concerned about the 240 dependency of the default link-local address on an underlying MAC 241 address, as described in the previous section. 243 Today most operators use GUAs as next-hop addresses. 245 2.4. Separate or Combined eBGP Sessions? 247 For a dual-stack peering connection where eBGP is used as the routing 248 protocol, then one can either: 250 a. Use one BGP session to carry both IPv4 and IPv6 routes, OR 252 b. Use two BGP sessions, a session over IPv4 carrying IPv4 routes 253 and a session over IPv6 carrying IPv6 routes. 255 The main advantage of (a) is a reduction in the number of BGP 256 sessions compared with (b). 258 However, there are three main concerns with option (a). First, on 259 most existing implementations, adding or removing an address family 260 to an established BGP session will cause the router to tear down and 261 re-establish the session. Thus adding the IPv6 family to an existing 262 session carrying just IPv4 routes will disrupt the session, and the 263 eventual removal of IPv4 from the dual IPv4/IPv6 session will also 264 disrupt the session. This disruption problem will persist until 265 something similar to [I-D.ietf-idr-dynamic-cap] is widely deployed. 266 Second, there is the question of which protocol to use to carry the 267 dual IPv4/IPv6 session: over IPv4 or over IPv6? Carrying it over 268 IPv4 makes sense initially from a stability and troubleshooting 269 perspective, but will eventually seem out-of-date. Third, carrying 270 (for example) IPv6 routes over IPv4 means that route information is 271 transported over a different transport plane than the data packets 272 themselves. If the IPv6 data plane was to fail, then IPv6 routes 273 would still be exchanged, but any IPv6 traffic resulting from these 274 routes would be dropped. 276 Given these disadvantages, option (b) is the better choice in most 277 situations, and this is the choice selected in most networks today. 279 2.5. eBGP Endpoints: Global or Link-Local Addresses? 281 When running eBGP over IPv6, there are two options for the addresses 282 to use at each end of the eBGP session (or more properly, the 283 underlying TCP session): 285 a. Use link-local addresses for the eBGP session, OR 287 b. Use global addresses for the eBGP session. 289 Note that the choice here is the addresses to use for the eBGP 290 sessions, and not whether the link itself has global (or unique- 291 local) addresses. In particular, it is quite possible for the eBGP 292 session to use link-local addresses even when the link has global 293 addresses. 295 The big attraction for option (a) is security: an eBGP session using 296 link-local addresses is impossible to attack from a device that is 297 off-link. This provides very strong protection against TCP RST and 298 similar attacks. Though there are other ways to get an equivalent 299 level of security (e.g. GTSM [RFC5082], MD5 [RFC5925], or ACLs), 300 these other ways require additional configuration which can be 301 forgotten or potentially mis-configured. 303 However, there are a number of small disadvantages to using link- 304 local addresses: 306 o Using link-local addresses only works for single-hop eBGP 307 sessions; it does not work for multi-hop sessions. 309 o One must use "next-hop self" at both endpoints, otherwise 310 redistributing routes learned via eBGP into iBGP will not work. 311 (Some products enable "next-hop self" in this situation 312 automatically). 314 o Operators and their tools are used to referring to eBGP sessions 315 by address only, something that is not possible with link-local 316 addresses. 318 o If one is configuring parallel eBGP sessions for IPv4 and IPv6 319 routes, then using link-local addresses for the IPv6 session 320 introduces an extra difference between the two sessions which 321 could otherwise be avoided. 323 o On some products, an eBGP session using a link-local address is 324 more complex to configure than a session that use a global 325 address. 327 o Finally, a strict interpretation of RFC 2545 can be seen as 328 forbidding running eBGP between link-local addresses, as RFC 2545 329 requires the BGP next-hop field to contain at least a global 330 address. 332 For these reasons, most operators today choose to have their eBGP 333 sessions use global addresses. 335 3. General Observations 337 There are two themes that run though many of the design choices in 338 this document. This section presents some general discussion on 339 these two themes. 341 3.1. Use of Link-Local Addresses 343 The proper use of link-local addresses is a common theme in the IPv6 344 network design choices. Link-layer addresses are, of course, always 345 present in an IPv6 network, but current network design practice 346 mostly ignores them, despite efforts such as 347 [I-D.ietf-opsec-lla-only]. 349 There are three main reasons for this current practice: 351 o Network operators are concerned about the volitility of link-local 352 addresses based on MAC addresses, despite the fact that this 353 concern can be overcome by manually-configuring link-local 354 addresses; 356 o It is impossible to ping a link-local address from a device that 357 is not on the same subnet. This is a troubleshooting 358 disadvantage, though it can also be viewed as a security 359 advantage. 361 o Most operators are currently running networks that carry both IPv4 362 and IPv6 traffic, and wish to harmonize their IPv4 and IPv6 design 363 and operational practices where possible. 365 3.2. Separation of IPv4 and IPv6 367 Currently, most operators are running or planning to run networks 368 that carry both IPv4 and IPv6 traffic. Hence the question: To what 369 degree should IPv4 and IPv6 be kept separate? As can be seen above, 370 this breaks into two sub-questions: To what degree should IPv4 and 371 IPv6 traffic be kept separate, and to what degree should IPv4 and 372 IPv6 routing information be kept separate? 374 The general consensus around the first question is that IPv4 and IPv6 375 traffic should generally be mixed together. This recommendation is 376 driven by the operational simplicity of mixing the traffic, plus the 377 general observation that the service being offered to the end user is 378 Internet connectivity and most users do not know or care about the 379 differences between IPv4 and IPv6. Thus it is very desirable to mix 380 IPv4 and IPv6 on the same link to the end user. On other links, 381 separation is possible but more operationally complex, though it does 382 occasionally allow the operator to work around limitations on network 383 devices. The situation here is roughly comparable to IP and MPLS 384 traffic: many networks mix the two traffic types on the same links 385 without issues. 387 By contrast, there is more of an argument for carrying IPv6 routing 388 information over IPv6 transport, while leaving IPv4 routing 389 information on IPv4 transport. By doing this, one gets fate-sharing 390 between the control and data plane for each IP protocol version: if 391 the data plane fails for some reason, then often the control plane 392 will too. 394 4. IANA Considerations 396 This document makes no requests of IANA. 398 5. Security Considerations 400 (TBD) 402 6. Acknowledgements 404 Many, many people in the V6OPS working group provided comments and 405 suggestions that made their way into this document. A partial list 406 includes: Rajiv Asati, Fred Baker, Michael Behringer, Marc Blanchet, 407 Ron Bonica, Randy Bush, Cameron Byrne, Brian Carpenter, Tim Chown, 408 Lorenzo Colitti, Gert Doering, Bill Fenner, Kedar K Gaonkar, Chris 409 Grundemann, Steinar Haug, Ray Hunter, Joel Jaeggli, KK, Victor 410 Kuarsingh, Alexandru Petrescu, Mark Smith, Jean-Francois Tremblay, 411 Tina Tsou, Dan York, and Xuxiaohu. There are probably others which 412 are not listed here, likely because they made a helpful comment at 413 the mic during a WG session and I didn't catch the name. 415 I would also like to thank Pradeep Jain and Alastair Johnson for 416 helpful comments on a very preliminary version of this document. 418 7. History 420 Version -01 422 Many, many changes from version -00, too many to document 423 individually. Most of these changes are due to the many helpful 424 comments and suggestions received by email or at the mic during 425 the lengthy discussion at IETF 84 in Vancouver. 427 Version -00 429 Initial, very preliminary, version. 431 8. Informative References 433 [I-D.ietf-idr-dynamic-cap] 434 Ramachandra, S. and E. Chen, "Dynamic Capability for 435 BGP-4", draft-ietf-idr-dynamic-cap-14 (work in progress), 436 December 2011. 438 [I-D.ietf-opsec-lla-only] 439 Behringer, M. and E. Vyncke, "Using Only Link-Local 440 Addressing Inside an IPv6 Network", 441 draft-ietf-opsec-lla-only-01 (work in progress), 442 September 2012. 444 [I-D.ietf-v6ops-enterprise-incremental-ipv6] 445 Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V., 446 Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment 447 Guidelines", 448 draft-ietf-v6ops-enterprise-incremental-ipv6-01 (work in 449 progress), September 2012. 451 [I-D.ietf-v6ops-icp-guidance] 452 Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet 453 Content and Application Service Providers", 454 draft-ietf-v6ops-icp-guidance-04 (work in progress), 455 September 2012. 457 [I-D.ietf-v6ops-wireline-incremental-ipv6] 458 Kuarsingh, V. and L. Howard, "Wireline Incremental IPv6", 459 draft-ietf-v6ops-wireline-incremental-ipv6-06 (work in 460 progress), September 2012. 462 [RFC4852] Bound, J., Pouffary, Y., Klynsma, S., Chown, T., and D. 463 Green, "IPv6 Enterprise Network Analysis - IP Layer 3 464 Focus", RFC 4852, April 2007. 466 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 467 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 468 September 2007. 470 [RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. 471 Pignataro, "The Generalized TTL Security Mechanism 472 (GTSM)", RFC 5082, October 2007. 474 [RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, 475 October 2008. 477 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 478 for IPv6", RFC 5340, July 2008. 480 [RFC5375] Van de Velde, G., Popoviciu, C., Chown, T., Bonness, O., 481 and C. Hahn, "IPv6 Unicast Address Assignment 482 Considerations", RFC 5375, December 2008. 484 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP 485 Authentication Option", RFC 5925, June 2010. 487 [RFC5963] Gagliano, R., "IPv6 Deployment in Internet Exchange Points 488 (IXPs)", RFC 5963, August 2010. 490 [RFC6180] Arkko, J. and F. Baker, "Guidelines for Using IPv6 491 Transition Mechanisms during IPv6 Deployment", RFC 6180, 492 May 2011. 494 [RFC6342] Koodli, R., "Mobile Networks Considerations for IPv6 495 Deployment", RFC 6342, August 2011. 497 Author's Address 499 Philip Matthews 500 Alcatel-Lucent 501 600 March Road 502 Ottawa, Ontario K2K 2E6 503 Canada 505 Phone: +1 613-784-3139 506 Email: philip_matthews@magma.ca