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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 1338 (Obsoleted by RFC 1519) -- Obsolete informational reference (is this intentional?): RFC 1519 (Obsoleted by RFC 4632) -- Obsolete informational reference (is this intentional?): RFC 2178 (Obsoleted by RFC 2328) Summary: 5 errors (**), 0 flaws (~~), 4 warnings (==), 11 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 GROW V. Fuller 3 Internet-Draft Cisco Systems 4 Expires: July 5, 2006 T. Li 5 Li Consulting 6 January 2006 8 Classless Inter-Domain Routing (CIDR): The Internet Address Assignment 9 and Aggregation Plan 10 draft-ietf-grow-rfc1519bis-04 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on July 5, 2006. 37 Copyright Notice 39 Copyright (C) The Internet Society (2006). 41 Abstract 43 This memo discusses the strategy for address assignment of the 44 existing 32-bit IPv4 address space with a view toward conserving the 45 address space and limiting the growth rate of global routing state. 46 This document obsoletes the original CIDR spec [RFC1519], with 47 changes made both to clarify the concepts it introduced and, after 48 more than twelve years, to update the Internet community on the 49 results of deploying the technology described. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. History and Problem Description . . . . . . . . . . . . . . . 3 55 3. Classless addressing as a solution . . . . . . . . . . . . . . 4 56 3.1. Basic concept and prefix notation . . . . . . . . . . . . 5 57 4. Address assignment and routing aggregation . . . . . . . . . . 8 58 4.1. Aggregation efficiency and limitations . . . . . . . . . . 8 59 4.2. Distributed assignment of address space . . . . . . . . . 9 60 5. Routing implementation considerations . . . . . . . . . . . . 10 61 5.1. Rules for route advertisement . . . . . . . . . . . . . . 11 62 5.2. How the rules work . . . . . . . . . . . . . . . . . . . . 12 63 5.3. A note on prefix filter formats . . . . . . . . . . . . . 13 64 5.4. Responsibility for and configuration of aggregation . . . 13 65 5.5. Route propagation and routing protocol considerations . . 14 66 6. Example of new address assignments and routing . . . . . . . . 15 67 6.1. Address delegation . . . . . . . . . . . . . . . . . . . . 15 68 6.2. Routing advertisements . . . . . . . . . . . . . . . . . . 17 69 7. Domain Name Service considerations . . . . . . . . . . . . . . 18 70 8. Transition to a long term solution . . . . . . . . . . . . . . 18 71 9. Analysis of CIDR's effect on global routing state . . . . . . 18 72 10. Conclusions and Recommendations . . . . . . . . . . . . . . . 20 73 11. Status updates to CIDR documents . . . . . . . . . . . . . . . 21 74 12. Security Considerations . . . . . . . . . . . . . . . . . . . 23 75 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 76 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 77 15. Appendix A: Area Director Comments and Responses . . . . . . . 24 78 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 79 16.1. Normative References . . . . . . . . . . . . . . . . . . . 26 80 16.2. Informative References . . . . . . . . . . . . . . . . . . 26 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 82 Intellectual Property and Copyright Statements . . . . . . . . . . 29 84 1. Introduction 86 This memo discusses the strategy for address assignment of the 87 existing 32-bit IPv4 address space with a view toward conserving the 88 address space and limiting the growth rate of global routing state. 89 This document obsoletes the original CIDR spec [RFC1519], with 90 changes made both to clarify the concepts it introduced and, after 91 more than twelve years, to update the Internet community on the 92 results of deploying the technology described. 94 2. History and Problem Description 96 What is now known as the Internet started as a research project in 97 the 1970s to design and develop a set of protocols that could be used 98 with many different network technologies to provide a seamless, end- 99 to-end facility for interconnecting a diverse set of end systems. 100 When determining how the 32-bit address space would be used, certain 101 assumptions were made about the number of organizations to be 102 connected, the number of end systems per organization, and total 103 number of end systems on the network. The end result was the 104 establishment (see [RFC791]) of three classes of networks: class A 105 (most significant address bits '00'), with 128 possible networks each 106 with 16777216 end systems (minus special bit values reserved for 107 network/broadcast addresses); class B (MSB '10'), with 16384 possible 108 networks each with 65536 end systems (less reserved values); and 109 class C (MSB '110'), with 2097152 possible networks each with 254 end 110 systems (256 bit combinations minus the reserved all-zeros and all- 111 ones patterns). The set of addresses with MSB '111' was reserved for 112 future use; parts of this were eventually defined (MSB '1110') for 113 use with IPv4 multicast and parts are still reserved as of the 114 writing of this document. 116 In the late 1980s, the expansion and commercialization of the former 117 research network resulted in the connection of many new organizations 118 to the rapidly-growing Internet and each new organization required an 119 address assignment according to the class A/B/C addressing plan. As 120 demand for new network numbers, particularly in the class B space 121 started to take on what appeared to be an exponential growth rate, 122 some members of the operations and engineering community started to 123 have concerns over the long-term scaling properties of the class 124 A/B/C system and began thinking about how to modify network number 125 assignment policy and routing protocols to better accommodate the 126 growth. In November, 1991, the IETF created the ROAD (Routing and 127 Addressing) group to examine the situation. This group met in 128 January, 1992 and identified three major problems: 130 1. Exhaustion of the class B network address space. One fundamental 131 cause of this problem is the lack of a network class of a size 132 which is appropriate for mid-sized organization; class C, with a 133 maximum of 254 host addresses, is too small, while class B, which 134 allows up to 65534 host addresses, is too large for most 135 organizations but was the best fit available for use with 136 subnetting. 138 2. Growth of routing tables in Internet routers beyond the ability 139 of current software, hardware, and people to effectively manage. 141 3. Eventual exhaustion of the 32-bit IPv4 address space. 143 It was clear that then-current rates of Internet growth would cause 144 the first two problems to become critical some time between 1993 and 145 1995. Work already in progress on topological assignment of 146 addressing for CLNS, which was presented to the community at the 147 Boulder IETF in December of 1990, led to thoughts on how to re- 148 structure the 32-bit IPv4 address space to increase its lifespan. 149 Work in the ROAD group followed and eventually resulted in the 150 publication of [RFC1338] and later [RFC1519]. 152 The design and deployment of CIDR was intended to solve these 153 problems by providing a mechanism to slow the growth of global 154 routing tables and to reduce the rate of consumption of IPv4 address 155 space. It did not and does not attempt to solve the third problem, 156 which is of a more long-term nature, but instead endeavors to ease 157 enough of the short to mid-term difficulties to allow the Internet to 158 continue to function efficiently while progress is made on a longer- 159 term solution. 161 More historical background on this effort and on the ROAD group may 162 be found in [RFC1380] and at [LWRD]. 164 3. Classless addressing as a solution 166 The solution that the community created was to deprecate the Class 167 A/B/C network address assignment system in favor of using 168 "classless", hierarchical blocks of IP addresses (referred to as 169 prefixes). The assignment of prefixes is intended to roughly follow 170 the underlying Internet topology so that aggregation can be used to 171 facilitate scaling of the global routing system. One implication of 172 this strategy is that prefix assignment and aggregation is generally 173 done according to provider-subscriber relationships, since that is 174 how the Internet topology is determined. 176 When originally proposed in [RFC1338] and [RFC1519], this addressing 177 plan was intended to be a relatively short-term response, lasting 178 approximately three to five years during which a more permanent 179 addressing and routing architecture would be designed and 180 implemented. As can be inferred from the dates on the original 181 documents, CIDR has far outlasted its anticipated lifespan and has 182 become the mid-term solution to the problems described above. 184 It should be noted that in the following text, we describe the 185 current policies and procedures that have been put in place to 186 implement the allocation architecture discussed here. This 187 description is not intended to be interpreted as direction to IANA. 189 Coupled with address management strategies implemented by the 190 Regional Internet Registries (see [NRO] for details), the deployment 191 of CIDR-style addressing has also reduced the rate at which IPv4 192 address space has been consumed, thus providing short-to-medium-term 193 relief to problem #3 described above. 195 Note that, as defined, this plan neither requires nor assumes the re- 196 assignment of those parts of the legacy "class C" space that are not 197 amenable to aggregation (sometimes called "the swamp"). Doing so 198 would somewhat reduce routing table sizes (current estimate is that 199 "the swamp" contains approximately 15,000 entries) though at a 200 significant renumbering cost. Similarly, there is no hard 201 requirement that any end site renumber when changing transit service 202 provider but end sites are encouraged do so to eliminate the need for 203 explicit advertisement of their prefixes into the global routing 204 system. 206 3.1. Basic concept and prefix notation 208 In the simplest sense, the change from Class A/B/C network numbers to 209 classless prefixes is to make explicit which bits in a 32-bit IPv4 210 address are interpreted as the network number (or prefix) associated 211 with a site and which are the used to number individual end systems 212 within the site. In CIDR notation, a prefix is shown as a 4-octet 213 quantity, just like a traditional IPv4 address or network number, 214 followed by the "/" (slash) character, followed by a decimal value 215 between 0 and 32 that describes the number of significant bits. 217 For example, the legacy "class B" network 172.16.0.0, with an implied 218 network mask of 255.255.0.0, is defined as the prefix 172.16.0.0/16, 219 the "/16" indicating that the mask to extract the network portion of 220 the prefix is a 32-bit value where the most significant 16 bits are 221 ones and the least significant 16 bits are zeros. Similarly, the 222 legacy "class C" network number 192.168.99.0 is defined as the prefix 223 192.168.99.0/24 - the most significant 24 bits are ones and the least 224 significant 8 bits are zeros. 226 Using classless prefixes with explicit prefix lengths allows much 227 more flexible matching of address space blocks to actual need. Where 228 formerly only three network sizes were available, prefixes may be 229 defined to describe any power-of-two-sized block of between one and 230 2^32 end system addresses. In practice, the unallocated pool of 231 addresses is administered by the Internet Assigned Numbers Authority 232 ([IANA]). The IANA makes allocations from this pool to Regional 233 Internet Registries, as required. These allocations are made in 234 contiguous bit-aligned blocks of 2^24 addresses (a.k.a. /8 prefixes). 235 The RIRs, in turn, allocate or assign smaller address blocks to Local 236 Internet Registries (LIRs) or Internet Service Providers (ISPs). 237 These entities may make direct use of the assignment (as would 238 commonly be the case for an ISP) or may make further sub-allocations 239 of addresses to their customers. These RIR address assignments vary 240 according to the needs of each ISP or LIR. For example, a large ISP 241 might be allocated an address block of 2^17 addresses (a /15 prefix) 242 while a smaller ISP may be allocated an address block of 2^11 243 addresses (a /21 prefix). 245 Note that the terms "allocate" and "assign" have specific meaning in 246 the Internet address registry system; "allocate" refers to the 247 delegation of a block of address space to an organization which is 248 expected to perform further sub-delegations while "assign" is used 249 for sites that directly use (i.e. number individual hosts) the block 250 of addresses received. 252 The following table provides a convenient short-cut to all of the 253 CIDR prefix sizes, showing the number of addresses possible in each 254 prefix and the number of prefixes of that size that may be numbered 255 in the 32-bit IPv4 address space: 257 notation addrs/block # blocks 258 -------- ----------- ---------- 259 n.n.n.n/32 1 4294967296 "host route" 260 n.n.n.x/31 2 2147483648 "p2p link" 261 n.n.n.x/30 4 1073741824 262 n.n.n.x/29 8 536870912 263 n.n.n.x/28 16 268435456 264 n.n.n.x/27 32 134217728 265 n.n.n.x/26 64 67108864 266 n.n.n.x/25 128 33554432 267 n.n.n.0/24 256 16777216 legacy "class C" 268 n.n.x.0/23 512 8388608 269 n.n.x.0/22 1024 4194304 270 n.n.x.0/21 2048 2097152 271 n.n.x.0/20 4096 1048576 272 n.n.x.0/19 8192 524288 273 n.n.x.0/18 16384 262144 274 n.n.x.0/17 32768 131072 275 n.n.0.0/16 65536 65536 legacy "class B" 276 n.x.0.0/15 131072 32768 277 n.x.0.0/14 262144 16384 278 n.x.0.0/13 524288 8192 279 n.x.0.0/12 1048576 4096 280 n.x.0.0/11 2097152 2048 281 n.x.0.0/10 4194304 1024 282 n.x.0.0/9 8388608 512 283 n.0.0.0/8 16777216 256 legacy "class A" 284 x.0.0.0/7 33554432 128 285 x.0.0.0/6 67108864 64 286 x.0.0.0/5 134217728 32 287 x.0.0.0/4 268435456 16 288 x.0.0.0/3 536870912 8 289 x.0.0.0/2 1073741824 4 290 x.0.0.0/1 2147483648 2 291 0.0.0.0/0 4294967296 1 "default route" 293 n is an 8-bit, decimal octet value. Point to point links are 294 discussed in more detail in [RFC3021]. 296 x is a 1 to 7 bit value, base on the prefix length, shifted into the 297 most significant bits of the octet and converted into decimal form; 298 the least significant bits of the octet are zero. 300 In practice, prefixes of length shorter than 8 have not been 301 allocated or assigned to date, although routes to such short prefixes 302 may exist in routing tables if or when aggressive aggregation is 303 performed. As of the writing of this document, no such routes are 304 seen in the global routing system but operator error and other events 305 have caused some of them (i.e. 128.0.0.0/1 and 192.0.0.0/2) to be 306 observed in some networks at some times in the past. 308 4. Address assignment and routing aggregation 310 Classless addressing and routing was initially developed primarily to 311 improve the scaling properties of routing on the global Internet. 312 Because the scaling of routing is very tightly coupled to the way 313 that addresses are used, deployment of CIDR had implications for the 314 way in which addresses were assigned. 316 4.1. Aggregation efficiency and limitations 318 The only commonly-understood method for reducing routing state on a 319 packet-switched network is through aggregation of information. For 320 CIDR to succeed in reducing the size and growth rate of the global 321 routing system, the IPv4 address assignment process needed to be 322 changed to make possible the aggregation of routing information along 323 topological lines. Since, in general, the topology of the network is 324 determined by the service providers who have built it, topologically- 325 significant address assignments are necessarily service-provider 326 oriented. 328 Aggregation is simple for an end site which is connected to one 329 service provider: it uses address space assigned by its service 330 provider and that address space is a small piece of a larger block 331 allocated to the service provider. No explicit route is needed for 332 the end site - the service provider advertises a single aggregate 333 route for the larger block; this advertisement provides reachability 334 and routeability for all of the customers numbered in the block. 336 There are two, more complex, situations that reduce the effectiveness 337 of aggregation: 339 o An organization which is multi-homed. Because a multi-homed 340 organization must be advertised into the system by each of its 341 service providers, it is often not feasible to aggregate its 342 routing information into the address space of any one of those 343 providers. Note that the organization still may receive its 344 address assignment out of a service provider's address space 345 (which has other advantages), but a route to the organization's 346 prefix is, in the most general case, explicitly advertised by all 347 of its service providers. For this reason, the global routing 348 cost for a multi-homed organization is generally the same as it 349 was prior to the adoption of CIDR. A more detailed consideration 350 of multi-homing practices can be found in [RFC4116]. 352 o An organization which changes service provider but does not 353 renumber. This has the effect of "punching a hole" in one of the 354 original service provider's aggregated route advertisements. CIDR 355 handles this situation by requiring the newer service provider to 356 advertise a specific advertisement for the re-homed organization; 357 this advertisement is preferred over provider aggregates because 358 it is a longer match. To maintain efficiency of aggregation, it 359 is recommended that an organization which changes service 360 providers plan to eventually migrate its network into a an prefix 361 assigned from its new provider's address space. To this end, it 362 is recommended that mechanisms to facilitate such migration, such 363 as dynamic host address assignment using [RFC2131]) be deployed 364 wherever possible, and that additional protocol work be done to 365 develop improved technology for renumbering. 367 Note that some aggregation efficiency gain can still be had for 368 multi-homed sites (and, in general, for any site composed of 369 multiple, logical IPv4 networks) - by allocating a contiguous power- 370 of-two block address space to the site (as opposed to multiple, 371 independent prefixes) the site's routing information may be 372 aggregated into a single prefix. Also, since the routing cost 373 associated with assigning a multi-homed site out of a service 374 provider's address space is no greater than the old method of 375 sequential number assignment by a central authority, it makes sense 376 to assign all end-site address space out of blocks allocated to 377 service providers. 379 It is also worthwhile to mention that since aggregation may occur at 380 multiple levels in the system, it may still be possible to aggregate 381 these anomalous routes at higher levels of whatever hierarchy may be 382 present. For example, if a site is multi-homed to two relatively 383 small providers that both obtain connectivity and address space from 384 the same large provider, then aggregation by the large provider of 385 routes from the smaller networks will include all routes to the 386 multi-homed site. The feasibility of this sort of second-level 387 aggregation depends on whether topological hierarchy exists between a 388 site, its directly-connected providers, and other providers to which 389 they are connected; it may be practical in some regions of the global 390 Internet but not in others. 392 Note: in the discussion and examples which follow, prefix notation is 393 used to represent routing destinations. This is used for 394 illustration only and does not require that routing protocols use 395 this representation in their updates. 397 4.2. Distributed assignment of address space 399 In the early days of the Internet, IPv4 address space assignment was 400 performed by the central Network Information Center (NIC). Class 401 A/B/C network numbers were assigned in essentially arbitrary order, 402 roughly according to the size of the organizations that requested 403 them. All assignments were recorded centrally and no attempt was 404 made to assign network numbers in a manner that would allow routing 405 aggregation. 407 When CIDR was originally deployed, the central assignment authority 408 continued to exist but changed its procedures to assign large blocks 409 of "Class C" network numbers to each service provider. Each service 410 provider, in turn, assigned bitmask-oriented subsets of the 411 provider's address space to each customer. This worked reasonably 412 well as long as the number of service providers was relatively small 413 and relatively constant but did not scale well as the number of 414 service providers grew at a rapid rate. 416 As the Internet started to expand rapidly in the 1990s, it became 417 clear that a single, centralized address assignment authority was 418 problematic. This function began being de-centralized when address 419 space assignment for European Internet sites was delegated in bit- 420 aligned blocks of 16777216 addresses (what CIDR would later define as 421 a /8) to the RIPE NCC ([RIPE]), effectively making it the first of 422 the RIRs. Since then, address assignment has been formally 423 distributed as a hierarchical function with IANA, the RIRs, and the 424 service providers. Removing the bottleneck of a single organization 425 having responsibility for the global Internet address space greatly 426 improved the efficiency and response time for new assignments. 428 Hierarchical delegation of addresses in this manner implies that 429 sites with addresses assigned out of a given service provider are, 430 for routing purposes, part of that service provider and will be 431 routed via its infrastructure. This implies that routing information 432 about multi-homed organizations, i.e., organizations connected to 433 more than one network service provider, will still need to be known 434 by higher levels in the hierarchy. 436 A historical perspective on these issues is described in [RFC1518]. 437 Additional discussion may also be found in [RFC3221]. 439 5. Routing implementation considerations 441 With the change from classful network numbers to classless prefixes, 442 it is not possible to infer the network mask from the initial bit 443 pattern of an IPv4 address. This has implications for how routing 444 information is stored and propagated. Network masks or prefix 445 lengths must be explicitly carried in routing protocols. Interior 446 routing protocols such as OSPF [RFC2178], IS-IS [RFC1195], RIPv2 448 [RFC2453], and Cisco EIGRP, and the BGP4 exterior routing protocol 449 [RFC4271] all support this functionality, having been developed or 450 modified as part of the deployment of classless inter-domain routing 451 during the 1990s. 453 Older interior routing protocols, such as RIP [RFC1058], HELLO, and 454 Cisco IGRP, and older exterior routing protocols, such as EGP 455 [RFC904], do not support explicit carriage of prefix length/mask and 456 thus cannot be effectively used on the Internet in other than very 457 limited, stub configurations. While their use may be appropriate in 458 simple, legacy end-site configurations, they are considered obsolete 459 and should NOT be used in transit networks connected to the global 460 Internet. 462 Similarly, routing and forwarding tables in layer-3 network equipment 463 must be organized to store both prefix and prefix length or mask. 464 Equipment which organizes its routing/forwarding information 465 according to legacy class A/B/C network/subnet conventions cannot be 466 expected to work correctly on networks connected to the global 467 Internet; use of such equipment is not recommended. Fortunately, 468 very little such equipment is in use today. 470 5.1. Rules for route advertisement 472 1. Forwarding in the Internet is done on a longest-match basis. 473 This implies that destinations which are multi-homed relative to 474 a routing domain must always be explicitly announced into that 475 routing domain - they cannot be summarized (this makes intuitive 476 sense - if a network is multi-homed, all of its paths into a 477 routing domain which is "higher" in the hierarchy of networks 478 must be known to the "higher" network). 480 2. A router which generates an aggregate route for multiple, more- 481 specific routes must discard packets which match the aggregate 482 route but not any of the more-specific routes. In other words, 483 the "next hop" for the aggregate route should be the null 484 destination. This is necessary to prevent forwarding loops when 485 some addresses covered by the aggregate are not reachable. 487 Note that during failures, partial routing of traffic to a site which 488 takes its address space from one service provider but which is 489 actually reachable only through another (i.e., the case of a site 490 which has changed service providers) may occur because such traffic 491 will be forwarded along the path advertised by the aggregated route. 492 Rule #2 will prevent packet mis-delivery by causing such traffic to 493 be discarded by the advertiser of the aggregated route, but the 494 output of "traceroute" and other similar tools will suggest that a 495 problem exists within that network rather than in the network which 496 is no longer advertising the more-specific prefix. This may be 497 confusing to those trying to diagnose connectivity problems; see the 498 example in Section 6.2 for details. A solution to this perceived 499 "problem" is beyond the scope of this document - it lies with better 500 education of the user/operator community, not in routing technology. 502 An implementation following these rules should also be generalized, 503 so that an arbitrary network number and mask are accepted for all 504 routing destinations. The only outstanding constraint is that the 505 mask must be left contiguous. Note that the degenerate route to 506 prefix 0.0.0.0/0 is used as a default route and MUST be accepted by 507 all implementations. Further, to protect against accidental 508 advertisements of this route via the inter-domain protocol, this 509 route should only be advertised to another routing domain when a 510 router is explicitly configured to do so - never as a non-configured, 511 "default" option. 513 5.2. How the rules work 515 Rule #1 guarantees that the forwarding algorithm used is consistent 516 across routing protocols and implementations. Multi-homed networks 517 are always explicitly advertised by every service provider through 518 which they are routed even if they are a specific subset of one 519 service provider's aggregate (if they are not, they clearly must be 520 explicitly advertised). It may seem as if the "primary" service 521 provider could advertise the multi-homed site implicitly as part of 522 its aggregate, but longest-match forwarding causes this not to work. 523 More details are provided in [RFC4116]. 525 Rule #2 guarantees that no routing loops form due to aggregation. 526 Consider a site that has been assigned 192.168.64/19 by its "parent" 527 provider that has 192.168.0.0/16. The "parent" network will 528 advertise 192.168.0.0/16 to the "child" network. If the "child" 529 network were to lose internal connectivity to 192.168.65.0/24 (which 530 is part of its aggregate), traffic from the "parent" to the to the 531 "child" destined for 192.168.65.1 will follow the "child's" 532 advertised route. When that traffic gets to the "child", however, 533 the child *must not* follow the route 192.168.0.0/16 back up to the 534 "parent", since that would result in a forwarding loop. Rule #2 says 535 that the "child" may not follow a less-specific route for a 536 destination which matches one of its own aggregated routes 537 (typically, this is implemented by installing a "discard" or "null" 538 route for all aggregated prefixes which one network advertises to 539 another). Note that handling of the "default" route (0.0.0.0/0) is a 540 special case of this rule - a network must not follow the default to 541 destinations which are part of one of it's aggregated advertisements. 543 5.3. A note on prefix filter formats 545 Systems which process route announcements must be able to verify that 546 information which they receive is acceptable according to policy 547 rules. Implementations which filter route advertisements must allow 548 masks or prefix lengths in filter elements. Thus, filter elements 549 which formerly were specified as: 551 accept 172.16.0.0 552 accept 172.25.120.0.0 553 accept 172.31.0.0 554 deny 10.2.0.0 555 accept 10.0.0.0 557 now look something like: 559 accept 172.16.0.0/16 560 accept 172.25.0.0/16 561 accept 172.31.0.0/16 562 deny 10.2.0.0/16 563 accept 10.0.0.0/8 565 This is merely making explicit the network mask which was implied by 566 the class A/B/C classification of network numbers. It is also useful 567 to enhance filtering capability to allow the match of a prefix and 568 all more-specific prefixes with the same bit pattern; fortunately, 569 this functionality has been implemented by most vendors of equipment 570 used on the Internet. 572 5.4. Responsibility for and configuration of aggregation 574 Under normal circumstances, a routing domain (or "Autonomous System") 575 which has been allocated or assigned a set of prefixes has sole 576 responsibility for aggregation of those prefixes. In the usual case, 577 the AS will install configuration in one or more of its routers to 578 generate aggregate routes based on more-specific routes known to its 579 internal routing system; these aggregate routes are advertised into 580 the global routing system by the border routers for the routing 581 domain. The more-specific internal routes which overlap with the 582 aggregate routes should not be advertised globally. In some cases, 583 an AS may wish to delegate aggregation responsibility to another AS 584 (for example, a customer may wish for its service provider to 585 generate aggregated routing information on its behalf); in such 586 cases, aggregation is performed by a router in the second AS based on 587 the routes that it receives from the first combined with configured 588 policy information describing how those routes should be aggregated. 590 It should be mentioned that one provider may choose to perform 591 aggregation on the routes it receives from another without explicit 592 agreement; this is termed "proxy aggregation". This can be a useful 593 tool for reducing the amount of routing state that an AS must carry 594 and propagate to its customers and neighbors. However, proxy 595 aggregation can also create unintended consequences in traffic 596 engineering. Consider what happens if both AS 2 and 3 receive routes 597 from AS 1 but AS 2 performs proxy aggregation while AS 3 does not. 598 Other AS's which receive transit routing information from both AS 2 599 and AS 3 will see an inconsistent view of the routing information 600 originated by AS 1. This may cause an unexpected shift of traffic 601 toward AS 1 through AS 3 for AS 3's customers and any others 602 receiving transit routes from AS 3. Because proxy aggregation can 603 cause unanticipated consequences for parts of the Internet that have 604 no relationship with either the source of the aggregated routes or 605 the party providing aggregation, it should be used with extreme 606 caution. 608 Configuration of the routes to be combined into aggregates is an 609 implementation of routing policy and does require some manually- 610 maintained information. As an addition to the information that must 611 be maintained for a set of routeable prefixes, aggregation 612 configuration is typically just a line or two defining the range of 613 the block of IPv4 addresses to aggregate. A site performing its own 614 aggregation is doing so for address blocks that it has been assigned; 615 a site performing aggregation on behalf of another knows this 616 information based on an agreement to delegate aggregation. Assuming 617 that the best common practice for network administrators is to 618 exchange lists of prefixes to accept from each other, configuration 619 of aggregation information does not introduce significant additional 620 administrative overhead. 622 The generation of an aggregate route is usually specified either 623 statically or in response to learning an active dynamic route for a 624 prefix contained within the aggregate route. If such dynamic 625 aggregate route advertisement is done, care should be taken that 626 routes are not excessively added or withdrawn (known as "route 627 flapping"); in general, a dynamic aggregate route advertisement is 628 added when at least one component of the aggregate becomes reachable 629 and it is withdrawn only when all components become unreachable. 630 Properly configured, aggregated routes are more stable than non- 631 aggregated routes and thus improve global routing stability. 633 Implementation note: aggregation of the "Class D" (multicast) address 634 space is beyond the scope of this document. 636 5.5. Route propagation and routing protocol considerations 638 Prior to the original deployment of CIDR, common practice was to 639 propagate routes learned via exterior routing protocols (i.e. EGP or 640 BGP) through a site's interior routing protocol (typically, OSPF, 641 IS-IS, or RIP). This was done to ensure that consistent and correct 642 exit points were chosen for traffic destined to a destination learned 643 through those protocols. Four evolutionary effects -- the advent of 644 CIDR, explosive growth of global routing state, widespread adoption 645 of BGP4, and a requirement to propagate full path information -- have 646 combined to deprecate that practice. To ensure proper path 647 propagation and prevent inter-AS routing inconsistency (BGP4's loop 648 detection/prevention mechanism requires full path propagation), 649 transit networks must use internal BGP (iBGP) for carrying routes 650 learned from other providers both within and through their networks. 652 6. Example of new address assignments and routing 654 6.1. Address delegation 656 Consider the block of 524288 (2^19) addresses beginning with 657 10.24.0.0 and ending with 10.31.255.255 allocated to a single network 658 provider, "PA". This is equivalent in size to a block of 2048 legacy 659 "class C" network numbers (or /24s). A classless route to this block 660 would be described as 10.24.0.0 with mask of 255.248.0.0 the prefix 661 10.24.0.0/13. 663 Assume this service provider connects six sites in the following 664 order (significant because it demonstrates how temporary "holes" may 665 form in the service provider's address space): 667 o "C1" requiring fewer than 2048 addresses (/21 or 8 x /24) 669 o "C2" requiring fewer than 4096 addresses (/20 or 16 x /24) 671 o "C3" requiring fewer than 1024 addresses (/22 or 4 x /24) 673 o "C4" requiring fewer than 1024 addresses (/22 or 4 x /24) 675 o "C5" requiring fewer than 512 addresses (/23 or 2 x /24) 677 o "C6" requiring fewer than 512 addresses (/23 or 2 x /24) 679 In all cases, the number of IPv4 addresses "required" by each site is 680 assumed to allow for significant growth. The service provider 681 delegates its address space as follows: 683 o C1: assign 10.24.0 through 10.24.7. This block of networks is 684 described by the route 10.24.0.0/21 (mask 255.255.248.0) 686 o C2: assign 10.24.16 through 10.24.31. This block is described by 687 the route 10.24.16.0/20 (mask 255.255.240.0) 689 o C3: assign 10.24.8 through 10.24.11. This block is described by 690 the route 10.24.8.0/22 (mask 255.255.252.0) 692 o C4: assign 10.24.12 through 10.24.15. This block is described by 693 the route 10.24.12.0/22 (mask 255.255.252.0) 695 o C5: assign 10.24.32 and 10.24.33. This block is described by the 696 route 10.24.32.0/23 (mask 255.255.254.0) 698 o C6: assign 10.24.34 and 10.24.35. This block is described by the 699 route 10.24.34.0/23 (mask 255.255.254.0) 701 These six sites should be represented as six prefixes of varying size 702 within the provider's IGP. If, for some reason, the provider uses an 703 obsolete IGP that doesn't support classless routing or variable- 704 length subnets, then explicit routes for all /24s will have to be 705 carried. 707 To make this example more realistic, assume that C4 and C5 are multi- 708 homed through some other service provider, "PB". Further assume the 709 existence of a site "C7" which was originally connected to "RB" but 710 has moved to "PA". For this reason, it has a block of network 711 numbers which are assigned out "PB"'s block of (the next) 2048 x /24. 713 o C7: assign 10.32.0 through 10.32.15. This block is described by 714 the route 10.32.0.0/20 (mask 255.255.240.0) 716 For the multi-homed sites, assume that C4 is advertised as primary 717 via "RA" and secondary via "RB"; C5 is primary via "RB" and secondary 718 via "RA". In addition, assume that "RA" and "RB" are both connected 719 to the same transit service provider "BB". 721 Graphically, this topology looks something like this: 723 10.24.0.0 -- 10.24.7.0__ __10.32.0.0 - 10.32.15.0 724 C1: 10.24.0.0/21 \ / C7: 10.32.0.0/20 725 \ / 726 +----+ +----+ 727 10.24.16.0 - 10.24.31.0_ | | | | 728 C2: 10.24.16.0/20 \ | | _10.24.12.0 - 10.24.15.0__ | | 729 \| | / C4: 10.24.12.0/20 \ | | 730 | |/ \| | 731 10.24.8.0 - 10.24.11.0___/| PA |\ | PB | 732 C3: 10.24.8.0/22 | | \__10.24.32.0 - 10.24.33.0___| | 733 | | C5: 10.24.32.0/23 | | 734 | | | | 735 10.24.34.0 - 10.24.35.0__/| | | | 736 C6: 10.24.34.0/23 | | | | 737 +----+ +----+ 738 || || 739 routing advertisements: || || 740 || || 741 10.24.12.0/22 (C4) || 10.24.12.0/22 (C4) || 742 10.32.0.0/20 (C7) || 10.24.32.0/23 (C5) || 743 10.24.0.0/13 (PA) || 10.32.0.0/13 (PB) || 744 || || 745 VV VV 746 +---------- BACKBONE NETWORK BB ----------+ 748 6.2. Routing advertisements 750 To follow rule #1, PA will need to advertise the block of addresses 751 that it was given and C7. Since C4 is multi-homed and primary 752 through PA, it must also be advertised. C5 is multi-homed and 753 primary through PB. In principal (and in the example above), it need 754 not be advertised since longest match by PB will automatically select 755 PB as primary and the advertisement of PA's aggregate will be used as 756 a secondary. In actual practice, C5 will normally be advertised via 757 both providers. 759 Advertisements from "PA" to "BB" will be: 761 10.24.12.0/22 primary (advertises C4) 762 10.32.0.0/20 primary (advertises C7) 763 10.24.0.0/13 primary (advertises remainder of PA) 765 For PB, the advertisements must also include C4 and C5 as well as 766 it's block of addresses. 768 Advertisements from "PB" to "BB" will be: 770 10.24.12.0/22 secondary (advertises C4) 771 10.24.32.0/23 primary (advertises C5) 772 10.32.0.0/13 primary (advertises remainder of RB) 774 To illustrate the problem diagnosis issue mentioned in Section 5.1, 775 consider what happens if PA loses connectivity to C7 (the site which 776 is assigned out of PB's space). In a stateful protocol, PA will 777 announce to BB that 10.32.0.0/20 has become unreachable. Now, when 778 BB flushes this information out of its routing table, any future 779 traffic sent through it for this destination will be forwarded to PB 780 (where it will be dropped according to Rule #2) by virtue of PB's 781 less specific match 10.32.0.0/13. While this does not cause an 782 operational problem (C7 is unreachable in any case), it does create 783 some extra traffic across "BB" (and may also prove confusing to 784 someone trying to debug the outage with "traceroute"). A mechanism 785 to cache such unreachable state might be nice but is beyond the scope 786 of this document. 788 7. Domain Name Service considerations 790 One aspect of Internet services which was notably affected by the 791 move to CIDR was the mechanism used for address-to-name translation: 792 the IN-ADDR.ARPA zone of the domain system. Because this zone is 793 delegated on octet boundaries only, the move to an address assignment 794 plan which uses bitmask-oriented addressing caused some increase in 795 work for those who maintain parts of the IN-ADDR.ARPA zone. 797 A description of techniques to populate the IN-ADDR.ARPA zone when 798 using address blocks that do not align to octet boundaries is 799 described in [RFC2317]. 801 8. Transition to a long term solution 803 CIDR was designed to be a short-term solution to the problems of 804 routing state and address depletion on the IPv4 Internet. It does 805 not change the fundamental Internet routing or addressing 806 architectures. It is not expected to affect any plans for transition 807 to a more long-term solution except, perhaps, by delaying the urgency 808 of developing such a solution. 810 9. Analysis of CIDR's effect on global routing state 812 When CIDR was first proposed in the early 1990s, the original authors 813 made some observations about the growth rate of global routing state 814 and offered projections on how CIDR deployment would, hopefully, 815 reduce what appeared to be exponential growth to a more sustainable 816 rate. Since that deployment, an ongoing effort, called "The CIDR 817 Report" [CRPT] has attempted to quantify and track that growth rate. 818 What follows is a brief summary of the CIDR report as of March, 2005, 819 with an attempt to explain the various patterns of and change in 820 growth rate that have occurred since measurements of the size of 821 global routing state began in 1988. 823 Examining the graph of "Active BGP Table Entries" [CBGP] there appear 824 to be several different growth trends with distinct inflection points 825 reflecting changes in policy and practice. The trends and events 826 which are believed to have caused them were: 828 1. Exponential growth at the far left of the graph. This represents 829 the period of early expansion and commercialization of the former 830 research network, from the late 1980s through approximately 1994. 831 The major driver for this growth was a lack of aggregation 832 capability for transit providers, and the widespread use of 833 legacy Class C allocations for end sites. Each time a new site 834 was connected to the global Internet, one or more new routing 835 entries were generated. 837 2. Acceleration of the exponential trend in late 1993 and early 1994 838 as CIDR "supernet" blocks were first assigned by the NIC and 839 routed as separate legacy class-C networks by service provider. 841 3. A sharp drop in 1994 as BGP4 deployment by providers allowed 842 aggregation of the "supernet" blocks. Note that the periods of 843 largest declines in the number of routing table entries typically 844 correspond to the weeks following each meeting of the IETF CIDR 845 Deployment Working Group. 847 4. Roughly linear growth from mid-1994 to early 1999 as CIDR-based 848 address assignments were made and aggregated routes added 849 throughout the network. 851 5. A new period of exponential growth again from early 1999 until 852 2001 as the "high-tech bubble" fueled both rapid expansion of 853 Internet as well as a large increase in more-specific route 854 advertisements for multi-homing and traffic engineering. 856 6. Flattening of growth through 2001 caused by a combination of the 857 "dot-com bust", which caused many organizations to cease 858 operations, and the "CIDR police" [CPOL] work aimed at improving 859 aggregation efficiency. 861 7. Roughly linear growth through 2002 and 2003. This most likely 862 represents a resumption of the "normal" growth rate observed 863 before the "bubble" as well as an end to the "CIDR Police" 864 effort. 866 8. A more recent trend of exponential growth beginning in 2004. The 867 best explanation would seem to be an improvement of the global 868 economy driving increased expansion of the Internet and the 869 continued absence of the "CIDR Police" effort, which previously 870 served as an educational tool for new providers to improve 871 aggregation efficiency. There have also been some cases where 872 service providers have deliberately de-aggregated prefixes in an 873 attempt to mitigate security problems caused by conflicting route 874 advertisements (see Section 12). While this behavior may solve 875 the short-term problems seen by such providers, it is 876 fundamentally non-scalable and quite detrimental to the community 877 as a whole. In addition, there appear to be many providers 878 advertising both their allocated prefixes and all of the /24 879 components of them, probably due to a lack of consistent current 880 information about recommended routing configuration. 882 10. Conclusions and Recommendations 884 In 1992, when CIDR was first developed, there were serious problems 885 facing the continued growth of the Internet. Growth in routing state 886 complexity and the rapid increase in consumption of address space 887 made it appear that one or both problems would preclude continued 888 growth of the Internet within a few short years. 890 Deployment of CIDR, in combination with BGP4's support for carrying 891 classless prefix routes, alleviated the short-term crisis. It was 892 only through a concerted effort by both the equipment manufacturers 893 and the provider community that this was achieved. The threat (and, 894 perhaps in some cases, actual implementation of) charging networks 895 for advertising prefixes may have offered an additional incentive to 896 share the address space, and hence the associated costs of 897 advertising routes to service providers. 899 The IPv4 routing system architecture carries topology information 900 based on aggregate address advertisements and a collection of more- 901 specific advertisements that are associated with traffic engineering, 902 multi-homing and local configuration. As of March, 2005, the base 903 aggregate address load in the routing system has some 75,000 entries. 904 Approximately 85,000 additional entries are more specific entries of 905 this base "root" collection. There is reason to believe that many of 906 these additional entries exist to solve problems of regional or even 907 local scope and should not need to be globally propagated. 909 An obvious question to ask is whether CIDR can continue to be a 910 viable approach to keeping global routing state growth and address 911 space depletion at sustainable rates. Recent measurements indicate 912 that exponential growth has resumed but further analysis suggests 913 that this trend can be mitigated by a more active effort to educate 914 service providers on efficient aggregation strategies and proper 915 equipment configuration. Looking farther forward, there is a clear 916 need for better multi-homing technology that does not require global 917 routing state for each site and for methods of performing traffic 918 load balancing that do not require adding even more state. Without 919 such developments and in the absence of major architectural change, 920 aggregation is the only tool available for making routing scale in 921 the global Internet. 923 11. Status updates to CIDR documents 925 This memo renders obsolete and requests re-classification as Historic 926 the following RFCs describing CIDR usage and deployment: 928 o RFC 1467: Status of CIDR Deployment in the Internet 930 This Informational RFC described the status of CIDR deployment in 931 1993. As of 2005, CIDR has been thoroughly deployed, so this 932 status note only provides a historical data point. 934 o RFC 1481: IAB Recommendation for an Intermediate Strategy to 935 Address the Issue of Scaling 937 This very short Informational RFC described the IAB's endorsement 938 of the use of CIDR to address scaling issues. Because the goal of 939 RFC 1481 has been achieved, it is now only of historical value. 941 o RFC 1482: Aggregation Support in the NSFNET Policy-Based Routing 942 Database 944 This Informational RFC describes plans for support of route 945 aggregation, as specified by CIDR, on the NSFNET. Because the 946 NSFNET has long since ceased to exist and CIDR has been been 947 ubiquitously deployed, RFC 1482 now only has historical relevance. 949 o RFC 1517: Applicability Statement for the Implementation of 950 Classless Inter-Domain Routing (CIDR) 952 This Standards Track RFC described where CIDR was expected to be 953 required and where it was expected to be (strongly) recommended. 954 With the full deployment of CIDR on the Internet, situations where 955 CIDR is not required are of only historical interest. 957 o RFC 1518: An Architecture for IP Address Allocation with CIDR 959 This Standards Track RFC discussed routing and address aggregation 960 considerations at some length. Some of these issues are 961 summarized in this document in section Section 3.1. Because 962 address assignment policies and procedures now reside mainly with 963 the RIRs, it is not appropriate to try to document those practices 964 in a Standards Track RFC. In addition, [RFC3221] also describes 965 many of the same issues from point of view of the routing system. 967 o RFC 1520: Exchanging Routing Information Across Provider 968 Boundaries in the CIDR Environment 970 This Informational RFC described transition scenarios where CIDR 971 was not fully supported for exchanging route information between 972 providers. With the full deployment of CIDR on the Internet, such 973 scenarios are no longer operationally relevant. 975 o RFC 1817: CIDR and Classful Routing 977 This Informational RFC described the implications of CIDR 978 deployment in 1995; it notes that formerly-classful addresses were 979 to be allocated using CIDR mechanisms and describes the use of a 980 default route for non-CIDR-aware sites. With the full deployment 981 of CIDR on the Internet, such scenarios are no longer 982 operationally relevant. 984 o RFC 1878: Variable Length Subnet Table For IPv4 986 This Informational RFC provided a table of pre-calculated subnet 987 masks and address counts for each subnet size. With the 988 incorporation of a similar table into this document (see 989 Section 3.1), it is no longer necessary to document it in a 990 separate RFC. 992 o RFC 2036: Observations on the use of Components of the Class A 993 Address Space within the Internet 995 This Informational RFC described several operational issues 996 associated with the allocation of classless prefixes from 997 previously-classful address space. With the full deployment of 998 CIDR on the Internet and more than half a dozen years of 999 experience making classless prefix allocations out of historical 1000 "class A" address space, this RFC now has only historical value. 1002 12. Security Considerations 1004 The introduction of routing protocols which support classless 1005 prefixes and a move to a forwarding model that mandates that more- 1006 specific (longest-match) routes be preferred when they overlap with 1007 routes to less-specific prefixes introduces at least two security 1008 concerns: 1010 1. Traffic can be hijacked by advertising a prefix for a given 1011 destination that is more specific than the aggregate that is 1012 normally advertised for that destination. For example, assume a 1013 popular end system with address 192.168.17.100 that is connected 1014 to a service provider that advertises 192.168.16.0/20. A 1015 malicious network operator interested in intercepting traffic for 1016 this site might advertise, or at least attempt to advertise, 1017 192.168.17.0/24 into the global routing system. Because this 1018 prefix is more-specific than the "normal" prefix, traffic will be 1019 diverted away from the legitimate end system and to the network 1020 owned by the malicious operator. Prior to the advent of CIDR, it 1021 was possible to induce traffic from some parts of the network to 1022 follow a false advertisement that exactly matched a particular 1023 network number; CIDR makes this problem somewhat worse, since 1024 longest-match routing generally causes all traffic to prefer 1025 more-specific routes over less-specific routes. The remedy for 1026 the CIDR-based attack, though, is the same as for a pre-CIDR- 1027 based attack: establishment of trust relationships between 1028 providers, coupled with and strong route policy filters at 1029 provider borders. Unfortunately, the implementation of such 1030 filters is difficult in the highly de-centralized Internet. As a 1031 workaround, many providers do implement generic filters that set 1032 upper bounds, derived from RIR guidelines for the sizes of blocks 1033 that they allocate, on the lengths of prefixes that are accepted 1034 from other providers. It is worth noting that "spammers" have 1035 been observed using this sort of attack to temporarily hijack 1036 address space in order to hide the origin of the traffic ("spam" 1037 email messages) that they generate. 1039 2. Denial-of-service attacks can be launched against many parts of 1040 the Internet infrastructure by advertising a large number of 1041 routes into the system. Such an attack is intended to cause 1042 router failures by overflowing routing and forwarding tables. A 1043 good example of a non-malicious incident which caused this sort 1044 of failure was the infamous "AS 7007" event [7007] where a router 1045 mis-configuration by an operator caused a huge number of invalid 1046 routes to be propagated through the global routing system. 1047 Again, this sort of attack is not really new with CIDR; using 1048 legacy class A/B/C routes, it was possible to advertise a maximum 1049 of 16843008 unique network numbers into the global routing 1050 system, a number which is sufficient to cause problems for even 1051 the most modern routing equipment made in 2005. What is 1052 different is that the moderate complexity of correctly 1053 configuring routers in the presence of CIDR does tend to make 1054 accidental "attacks" of this sort more likely. Measures to 1055 prevent this sort of attack are much the same as those described 1056 above for the hijacking, with the addition that best common 1057 practice is to also configure a reasonable maximum number of 1058 prefixes that a border router will accept from its neighbors. 1060 Note that this is not intended to be an exhaustive analysis of the 1061 sorts of attacks that CIDR makes easier; a more comprehensive 1062 analysis of security vulnerabilities in the global routing system is 1063 beyond the scope of this document. 1065 13. IANA Considerations 1067 [RFC Editor: This section to be removed prior to publication.] 1069 There are no IANA Considerations raised in this document. 1071 14. Acknowledgments 1073 The authors wish to express appreciation to the other original 1074 authors of RFC1519 (Kannan Varadhan, Jessica Yu), to the ROAD group 1075 with whom many of the ideas behind CIDR were inspired and developed, 1076 and to the early reviewers of this re-spun version of the document 1077 (Barry Greene, Danny McPherson, Dave Meyer, Eliot Lear, Bill Norton, 1078 Ted Seely, Philip Smith, Pekka Savola) whose comments, corrections, 1079 and suggestions were invaluable. We would especially like to thank 1080 Geoff Huston for contributions well above and beyond the call of 1081 duty. 1083 15. Appendix A: Area Director Comments and Responses 1085 [RFC Editor: Please remove this section prior to publication] 1087 Review comments and responses: 1089 1. The document describes the interaction between the IANA and the 1090 RIRs in address allocation. Is this logically part of a 1091 standards-track document that is describing address aggregation? 1093 2. This part of the document is describing the current situation 1094 with respect to address distribution. It appears that the 1095 defining document here is 1096 http://www.aso.icann.org/docs/aso-001-2.pdf, which is entirely 1097 consistent with the document. 1099 3. As this is a description of the current situation, and as this 1100 are no "IANA Considerations" section then it is felt that it is 1101 clear that this is not to be interpreted as a direction to IANA. 1102 To further ensure that this is clear to future generations, 1103 we've also added a suitable caveat to section Section 3. 1105 4. The text describes interactions between RIRs and LIRs or ISPs. 1106 Is this description correct? 1108 5. In considering the entire RIR system this is indeed the case. 1109 While some RIRs use LIRs who, in turn, interact with ISPs, other 1110 RIRs interact directly with ISPs, or use a mixed mode of 1111 interaction with both LIRs and ISPS. 1113 6. The text references dynamic host address assignment [RFC2131] as 1114 a recommended technology, and suggests that additional protocol 1115 work be undertaken to develop improved technology for 1116 renumbering. The review suggested further document references 1117 and further elaboration in the text. 1119 7. While it would be possible to include a larger set of references 1120 and additional text on this topic, it is a matter where there is 1121 a distinct risk of the document losing focus here. The topic of 1122 this section is one of situations where there are constraints on 1123 aggregation, rather than a detailed examination of various 1124 mitigating steps. 1126 8. The example in Section 5 uses network 10 rather than the 1127 documentation prefix 192.0.2.0/24. 1129 9. The text is showing a practical example of aggregation using 1130 prefix sizes that would be encountered in an operational 1131 context. The documentation prefix is too small to encompass 1132 this example, and designated private address space was used in 1133 this example. 1135 10. The text shows an example of DNS delegations where the address 1136 blocks are smaller than a /24. Should the solution be reworded 1137 as a reference to RFC2137? 1139 11. The text describes the impact of CIDR on reverse delegations in 1140 the DNS and the methods used in the DNS to respond to this. It 1141 is considered to be an integral part of this document. 1143 12. Should the document refer to a graph of data by reference? 1145 13. The document is describing a sequence of trends in the state of 1146 inter-domain routing over the past years, and the graph is the 1147 most effective presentation of this material. 1149 16. References 1151 16.1. Normative References 1153 [RFC791] Postel, J., "Internet Protocol", RFC 791, September 1981. 1155 16.2. Informative References 1157 [7007] "NANOG mailing list discussion of the "AS 7007" incident", 1158 . 1161 [CBGP] "Graph: Active BGP Table Entries, 1988 to Present", 1162 . 1164 [CPOL] "CIDR Police - Please Pull Over and Show Us Your BGP", 1165 . 1167 [CRPT] "The CIDR Report", . 1169 [IANA] "Internet Assigned Numbers Authority", 1170 . 1172 [LWRD] "The Long and Winding Road", 1173 . 1175 [NRO] "Number Resource Organization", . 1177 [RFC904] Mills, D., "Exterior Gateway Protocol formal 1178 specification", RFC 904, April 1984. 1180 [RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058, 1181 June 1988. 1183 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1184 dual environments", RFC 1195, December 1990. 1186 [RFC1338] Fuller, V., Li, T., Varadhan, K., and J. Yu, 1187 "Supernetting: an Address Assignment and Aggregation 1188 Strategy", RFC 1338, June 1992. 1190 [RFC1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing 1191 and Addressing", RFC 1380, November 1992. 1193 [RFC1518] Rekhter, Y. and T. Li, "An Architecture for IP Address 1194 Allocation with CIDR", RFC 1518, September 1993. 1196 [RFC1519] Fuller, V., Li, T., Varadhan, K., and J. Yu, "Classless 1197 Inter-Domain Routing: an Address Assignment and 1198 Aggregation Strategy", RFC 1519, September 1993. 1200 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 1201 RFC 2131, March 1997. 1203 [RFC2178] Moy, J., "The OSPF Specification Version 2", RFC 2178, 1204 July 1997. 1206 [RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN- 1207 ADDR.ARPA delegation", RFC 2317, March 1998. 1209 [RFC2453] Malkin, G., "RIP Version 2", RFC 2453, November 1998. 1211 [RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson, 1212 "Using 31-Bit Prefixes on IPv4 Point-to-Point Links", 1213 RFC 3021, December 2000. 1215 [RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the 1216 Internet", RFC 3221, December 2001. 1218 [RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V. 1219 Gill, "IPv4 Multihoming Practices and Limitations", 1220 RFC 4116, July 2005. 1222 [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway 1223 Protocol 4 (BGP-4)", RFC 4271, January 2006. 1225 [RIPE] "RIPE Network Coordination Centre", . 1227 Authors' Addresses 1229 Vince Fuller 1230 170 W. Tasman Drive 1231 San Jose, CA 95134 1232 USA 1234 Email: vaf@cisco.com 1236 Tony Li 1237 Li Consulting 1239 Email: tony.li@tony.li 1241 Intellectual Property Statement 1243 The IETF takes no position regarding the validity or scope of any 1244 Intellectual Property Rights or other rights that might be claimed to 1245 pertain to the implementation or use of the technology described in 1246 this document or the extent to which any license under such rights 1247 might or might not be available; nor does it represent that it has 1248 made any independent effort to identify any such rights. 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