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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) == Missing Reference: 'MUST' is mentioned on line 175, but not defined ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) == Outdated reference: A later version (-07) exists of draft-ietf-rtgwg-dst-src-routing-00 -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Maintenance F. Baker 3 Internet-Draft Cisco Systems 4 Updates: 4861 (if approved) B. Carpenter 5 Intended status: Standards Track Univ. of Auckland 6 Expires: August 25, 2016 February 22, 2016 8 Routing packets from hosts in a multi-prefix network 9 draft-ietf-6man-multi-homed-host-05 11 Abstract 13 This document describes expected IPv6 host behavior in a network that 14 has more than one prefix, each allocated by an upstream network that 15 implements BCP 38 ingress filtering, when the host has multiple 16 routers to choose from. It also applies to other scenarios such as 17 the usage of stateful firewalls that effectively act as address-based 18 filters. This host behavior may interact with source address 19 selection in a given implementation, but logically follows it. Given 20 that the network or host is, or appears to be, multihomed with 21 multiple provider-allocated addresses, that the host has elected to 22 use a source address in a given prefix, and that some but not all 23 neighboring routers are advertising that prefix in their Router 24 Advertisement Prefix Information Options, this document specifies to 25 which router a host should present its transmission. It updates RFC 26 4861. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 25, 2016. 45 Copyright Notice 47 Copyright (c) 2016 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction and Applicability . . . . . . . . . . . . . . . 2 63 1.1. Host Model . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 65 2. Sending context expected by the host . . . . . . . . . . . . 5 66 2.1. Expectations the host has of the network . . . . . . . . 5 67 2.2. Expectations of multihomed networks . . . . . . . . . . . 7 68 3. Reasonable expectations of the host . . . . . . . . . . . . . 7 69 3.1. Interpreting Router Advertisements . . . . . . . . . . . 7 70 3.2. Default Router Selection . . . . . . . . . . . . . . . . 8 71 3.3. Source Address Selection . . . . . . . . . . . . . . . . 8 72 3.4. Redirects . . . . . . . . . . . . . . . . . . . . . . . . 9 73 3.5. History . . . . . . . . . . . . . . . . . . . . . . . . . 9 74 4. Residual issues . . . . . . . . . . . . . . . . . . . . . . . 9 75 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 76 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 77 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 78 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 79 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 80 8.2. Informative References . . . . . . . . . . . . . . . . . 11 81 Appendix A. Change Log (RFC Editor: please delete) . . . . . . . 12 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 84 1. Introduction and Applicability 86 This document describes the expected behavior of an IPv6 [RFC2460] 87 host in a network that has more than one prefix, each allocated by an 88 upstream network that implements BCP 38 [RFC2827] ingress filtering, 89 and in which the host is presented with a choice of routers. It 90 expects that the network will implement some form of egress routing, 91 so that packets sent to a host outside the local network from a given 92 ISP's prefix will go to that ISP. If the packet is sent to the wrong 93 egress, it is liable to be discarded by the BCP 38 filter. However, 94 the mechanics of egress routing once the packet leaves the host are 95 out of scope. The question here is how the host interacts with that 96 network. 98 Various aspects of this issue, and possible solution approaches, are 99 discussed in the document IPv6 Multihoming without Network Address 100 Translation [RFC7157]. 102 BCP 38 filtering by ISPs is not the only scenario where such behavior 103 is valuable. Implementations that combine existing recommendations, 104 such as [RFC6092] [RFC7084] can also result in such filtering. 105 Another case is when the connections to the upstream networks include 106 stateful firewalls, such that return packets in a stream will be 107 discarded if they do not return via the firewall that created state 108 for the outgoing packets. A similar cause of such discards is 109 unicast reverse path forwarding (uRPF) [RFC3704]. 111 In this document, the term "filter" is used for simplicity to cover 112 all such cases. In any case, one cannot assume the host to be aware 113 whether an ingress filter, a stateful firewall, or any other type of 114 filter is in place. Therefore, the only consistent solution is to 115 implement the features defined in this document. 117 Note that, apart from ensuring that a message with a given source 118 address is given to a first-hop router that appears to know about the 119 prefix in question, this specification is consistent with [RFC4861]. 120 Nevertheless, implementers of Sections 5.2, 6.2.3, 6.3.4 and 8 of RFC 121 4861 will need to extend their implementations accordingly. This 122 specification is fully consistent with [RFC6724] and implementers 123 will need to add support for its Rule 5.5. Hosts that do not support 124 these features may fail to communicate in the presence of filters as 125 described above. 127 1.1. Host Model 129 It could be argued that the proposal of this document, which is to 130 send messages using a source address in a given prefix to the router 131 that advertised the prefix in its RA, is a form of [RFC1122]'s Strong 132 ES (e.g. Host) Model, discussed in section 3.3.4.2 of that document. 133 In short, [RFC1122] identifies two basic models, in which the "strong 134 host" model models the host as a set of hosts in one chassis, each of 135 which uses a single address on a single interface, and always both 136 sends and receives on that interface, and the "weak host" model 137 treats the host as one system with zero or more addresses on every 138 interface, and capable of using any interface for any communication. 139 As noted there, neither model is completely satisfactory. For 140 example, a host with a link-local-only interface and a default route 141 pointing to the interface will necessarily send packets using any 142 address using that interface, and therefore be a de facto weak host. 143 If the router upstream from such a host implements BCP 38 Ingress 144 Filtering [RFC2827], such as by implementing uRPF on each interface, 145 the router might prevent communication by weak hosts. 147 +-----------------+ 148 | | 149 | MIF Router +---/--- other interfaces 150 | | 151 +---+---------+---+ 152 | | Two interfaces sharing a prefix 153 --+-+-- --+-+-- 154 | | 155 +--+---------+--+ 156 | MIF Host | 157 +---------------+ 159 Figure 1: Hypothetical MIF interconnection 161 The proposal also differs slightly from [RFC1122]'s language of the 162 Strong Host Model. The statement is that the packet will go to the 163 router that advertised a given prefix, but doesn't state what 164 interface that might happen on. Hence, if the router is a MIF router 165 and is using the same prefix on two or more LANs shared by the host 166 (as in Figure 1), the host might use each of those LANs and meet the 167 requirement. The Strong Host Model is not stated in those terms, but 168 in terms of the interface used, and would find a MIF router quite 169 confusing: 171 (A) A host [MUST] silently discard an incoming datagram whose 172 destination address does not correspond to the physical interface 173 through which it is received. 175 (B) A host [MUST] restrict itself to sending (non-source- routed) 176 IP datagrams only through the physical interface that corresponds 177 to the IP source address of the datagrams. 179 However, comparing the presumptive route lookup mechanisms in each 180 model, this proposal is indeed most similar to the Strong Host Model, 181 as is any source/destination routing paradigm. 183 Strong: route(src IP addr, dest IP addr, TOS) -> gateway 185 Weak: route(dest IP addr, TOS) -> gateway, interface 186 In the hypothetical MIF model suggested in Figure 1, the address 187 fails to identify a single interface, but it does identify a single 188 gateway. 190 1.2. Requirements Language 192 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 193 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 194 document are to be interpreted as described in [RFC2119]. 196 2. Sending context expected by the host 198 2.1. Expectations the host has of the network 200 A host receives prefixes in a Router Advertisement [RFC4861], which 201 goes on to identify whether they are usable by SLAAC [RFC4862] 202 [RFC4941] [RFC7217]. When no prefixes are usable for SLAAC, the 203 Router Advertisement would normally signal the availability of DHCPv6 204 [RFC3315] and the host would use it to configure its addresses. In 205 the latter case (or if both SLAAC and DHCPv6 are used on the same 206 link for some reason) it will generally be the case that the 207 configured addresses match one of the prefixes advertised in a Router 208 Advertisement that are supposed to be on-link for that link. 210 The simplest multihomed network implementation in which a host makes 211 choices among routers might be a LAN with one or more hosts on it and 212 two or more routers, one for each upstream network, or a host that is 213 served by disjoint networks on separate interfaces. In such a 214 network, especially the latter, there is not necessarily a routing 215 protocol, and the two routers may not even know that the other is a 216 router as opposed to a host, or may be configured to ignore its 217 presence. One might expect that the routers may or may not receive 218 each other's RAs and form an address in the other router's prefix 219 (which is not per [RFC4862], but is implemented by some stub router 220 implementations). However, all hosts in such a network might be 221 expected to create an address in each prefix so advertised. 223 +---------+ +---------+ +---------+ +---------+ 224 | ISP | | ISP | | ISP | | ISP | 225 +----+----+ +----+----+ +----+----+ +----+----+ 226 | | | | 227 | | | | 228 +----+----+ +----+----+ +----+----+ +----+----+ 229 | Router | | Router | | Router | | Router | 230 +----+----+ +----+----+ +----+----+ +----+----+ 231 | | | | 232 +------+------+ | +--------+ | 233 | +--+ Host +--+ 234 +----+----+ +--------+ 235 | Host | 236 +---------+ 237 Common LAN Case Disjoint LAN Case 238 (Multihomed Network) (Multihomed Host) 240 Figure 2: Two simple networks 242 If there is no routing protocol among those routers, there is no 243 mechanism by which packets can be deterministically forwarded between 244 the routers (as described in BCP 84 [RFC3704]) in order to avoid 245 filters. Even if there was routing, it would result in an indirect 246 route, rather than a direct route originating with the host; this is 247 not "wrong", but can be inefficient. Therefore the host would do 248 well to select the appropriate router itself. 250 Since the host derives fundamental default routing information from 251 the Router Advertisement, this implies that, in any network with 252 hosts using multiple prefixes, each prefix SHOULD be advertised via a 253 Prefix Information Option (PIO) [RFC4861] by one of the attached 254 routers, even if addresses are being assigned using DHCPv6. A router 255 that advertises a prefix indicates that it is able to appropriately 256 route packets with source addresses within that prefix, regardless of 257 the setting of the L and A flags in the PIO. 259 In some circumstances both L and A might be zero. If SLAAC is not 260 wanted (A=0) and there is no reason to announce an on-link prefix 261 (L=0), a PIO SHOULD be sent to inform hosts that the prefix is 262 source-routed by the router in question. Although this does not 263 violate the existing standard [RFC4861], such a PIO has not 264 previously been common, and it is possible that existing host 265 implementations simply ignore such a PIO or that a router 266 implementation rejects such a PIO as a configuration error. Newer 267 implementations that support this mechanism will need to be updated 268 accordingly: a host SHOULD NOT ignore a PIO simply because both L and 269 A flags are cleared; a router SHOULD be able to send such a PIO. 271 2.2. Expectations of multihomed networks 273 The direct implication of Section 2.1 is that, if the network uses a 274 routing protocol, the routing protocols used in multihomed networks 275 SHOULD implement source-prefix based egress routing, for example as 276 described in [I-D.ietf-rtgwg-dst-src-routing]. Network designs exist 277 that can usefully limit themselves to static routing (such as a 278 simple tree network), or may internally use no routers at all, such 279 as a single LAN with two CE routers, each of which leads to a 280 different upstream network. 282 3. Reasonable expectations of the host 284 3.1. Interpreting Router Advertisements 286 As described in [RFC4191] and [RFC4861], a Router Advertisement may 287 contain zero or more Prefix information Options (PIOs), or zero or 288 more Route Information Options (RIOs). In their original intent, 289 these indicate general information to a host: "the router whose 290 address is found in the source address field of this packet is one of 291 your default routers", "you might create an address in this prefix", 292 or "this router would be a good place to send traffic directed to a 293 given destination prefix". In a multi-homed network implementing 294 source/destination routing, the interpretation of default router or 295 an RIO has to be modified with the words "if the source address is in 296 one of the prefixes I advertise in a PIO". Additionally, the PIO 297 must be reinterpreted to also imply that the advertising router would 298 be a reasonable first hop for any packet using a source address in 299 any advertised prefix. 301 +---------+ | 302 ( ISP A ) - + Bob-A +--+ +-----+ 303 +-------+ / +---------+ +--+ | 304 | | / | | | 305 | Alice +--/--( The Internet ) | Bob | 306 | | \ | | | 307 +-------+ \ +---------+ +--+ | 308 ( ISP B ) - + Bob-B +--+ +-----+ 309 +---------+ | 311 Figure 3: PIOs, RIOs, and Default Routes 313 The implications bear consideration. Imagine, Figure 3, that hosts 314 Alice and Bob are in communication, Bob's network is multihomed, and 315 Bob's first hop routers are Bob-A (to upstream ISP A advertising 316 prefix PA) and Bob-B (to upstream network B and advertising prefix 317 PB). If Bob is responding to a message from Alice, his choice of 318 source address is forced to be the address Alice used as a 319 destination (which we may presume to have been in prefix PA). Hence, 320 Bob created or was assigned an address in PA, and can only reasonably 321 send traffic using it to Bob-A as a first hop router. If there were 322 several instances of Bob-A and one had advertised itself as a default 323 router or as having a route to Alice, that is the router Bob should 324 choose. If none of Bob-A have advertised that but Bob-B has, it is 325 irrelevant; Bob is using the address allocated in PA and courts a BCP 326 38 discard if he doesn't send the packet to Bob-A. 328 In the special case that Bob is initiating the conversation, an RIO 329 might, however, influence source address choice. Bob could 330 presumably use any address allocated to him, in this case his address 331 in PA or PB. If Bob-B has advertised an RIO for Alice's prefix and 332 Bob-A has not, Bob MAY take that fact into account in address 333 selection - choosing an address that would allow him to make use of 334 the RIO. 336 3.2. Default Router Selection 338 Default Router Selection is modified as follows: A host SHOULD select 339 default routers for each prefix it is assigned an address in. 340 Routers that have advertised the prefix in its Router Advertisement 341 message SHOULD be preferred over routers that do not advertise the 342 prefix. 344 As a result of doing so, when a host sends a packet using a source 345 address in one of those prefixes and has no history directing it 346 otherwise, it SHOULD send it to the indicated default router. In the 347 "simplest" network described in Section 2.1, that would get it to the 348 only router that is directly capable of getting it to the right ISP. 349 This will also apply in more complex networks, even when more than 350 one physical or virtual interface is involved. 352 In more complex cases, wherein routers advertise RAs for multiple 353 prefixes whether or not they have direct or isolated upstream 354 connectivity, the host is dependent on the routing system already. 355 If the host gives the packet to a router advertising its source 356 prefix, it should be able to depend on the router to do the right 357 thing. 359 3.3. Source Address Selection 361 There is an interaction with Default Address Selection [RFC6724]. A 362 host following the recommendation in the previous section will store 363 information about which next-hops advertised which prefixes. Rule 364 5.5 of RFC 6724 states that the source address used to send to a 365 given destination address should if possible be chosen from a prefix 366 known to be advertised by the next-hop router for that destination. 368 This selection rule would therefore be applicable in a host following 369 the recommendation in the previous section. 371 3.4. Redirects 373 There is potential for adverse interaction with any off-link Redirect 374 (Redirect for a destination that is not on-link) message sent by a 375 router in accordance with Section 8 of [RFC4861]. Hosts SHOULD apply 376 off-link redirects only for the specific pair of source and 377 destination addresses concerned, so the host's Destination Cache may 378 need to contain appropriate source-specific entries. 380 3.5. History 382 Some modern hosts maintain history, in terms of what has previously 383 worked or not worked for a given address or prefix and in some cases 384 the effective window and MSS values for TCP or other protocols. This 385 might include a next hop address for use when a packet is sent to the 386 indicated address. 388 When such a host makes a successful exchange with a remote 389 destination using a particular address pair, and the host has 390 previously received a PIO that matches the source address, then the 391 host SHOULD include the prefix in such history, whatever the setting 392 of the L and A flags in the PIO. On subsequent attempts to 393 communicate with that destination, if it has an address in that 394 prefix at that time, a host MAY use an address in the remembered 395 prefix for the session. 397 4. Residual issues 399 Consider a network where routers on a link run a routing protocol and 400 are configured with the same information. Thus, on each link all 401 routers advertise all prefixes on the link. The assumption that 402 packets will be forwarded to the appropriate egress by the local 403 routing system might cause at least one extra hop in the local 404 network (from the host to the wrong router, and from there to another 405 router on the same link). 407 In a slightly more complex situation such as the disjoint LAN case of 408 Figure 2, which happens to be one of the authors' home plus corporate 409 home-office configuration, the two upstream routers might be on 410 different LANs and therefore different subnets (e.g., the host is 411 itself multi-homed). In that case, there is no way for the "wrong" 412 router to detect the existence of the "right" router, or to route to 413 it. 415 In such a case it is particularly important that hosts take the 416 responsibility to memorize and select the best first-hop as described 417 in Section 3. 419 5. IANA Considerations 421 This memo asks the IANA for no new parameters. 423 6. Security Considerations 425 This document does not create any new security or privacy exposures. 426 It is intended to avoid connectivity issues in the presence of BCP 38 427 ingress filters or stateful firewalls combined with multihoming. 429 There might be a small privacy improvement, however: with the current 430 practice, a multihomed host that sends packets with the wrong address 431 to an upstream router or network discloses the prefix of one upstream 432 to the other upstream network. This practice reduces the probability 433 of that occurrence. 435 7. Acknowledgements 437 Comments were received from Jinmei Tatuya and Ole Troan, who have 438 suggested important text, plus Mikael Abrahamsson, Steven Barth, 439 Juliusz Chroboczek, Toerless Eckert, David Farmer, Dusan Mudric, 440 Tadahisa Okimoto, Pierre Pfister, Behcet Sarikaya, Mark Smith, Bob 441 Hinden, and James Woodyatt. 443 8. References 445 8.1. Normative References 447 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 448 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 449 RFC2119, March 1997, 450 . 452 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 453 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, 454 December 1998, . 456 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 457 More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, 458 November 2005, . 460 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 461 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 462 DOI 10.17487/RFC4861, September 2007, 463 . 465 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 466 "Default Address Selection for Internet Protocol Version 6 467 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 468 . 470 8.2. Informative References 472 [I-D.ietf-rtgwg-dst-src-routing] 473 Lamparter, D., "Destination/Source Routing", draft-ietf- 474 rtgwg-dst-src-routing-00 (work in progress), October 2015. 476 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 477 Communication Layers", STD 3, RFC 1122, DOI 10.17487/ 478 RFC1122, October 1989, 479 . 481 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 482 Defeating Denial of Service Attacks which employ IP Source 483 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 484 May 2000, . 486 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 487 C., and M. Carney, "Dynamic Host Configuration Protocol 488 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 489 2003, . 491 [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 492 Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 493 2004, . 495 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 496 Address Autoconfiguration", RFC 4862, DOI 10.17487/ 497 RFC4862, September 2007, 498 . 500 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 501 Extensions for Stateless Address Autoconfiguration in 502 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 503 . 505 [RFC6092] Woodyatt, J., Ed., "Recommended Simple Security 506 Capabilities in Customer Premises Equipment (CPE) for 507 Providing Residential IPv6 Internet Service", RFC 6092, 508 DOI 10.17487/RFC6092, January 2011, 509 . 511 [RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic 512 Requirements for IPv6 Customer Edge Routers", RFC 7084, 513 DOI 10.17487/RFC7084, November 2013, 514 . 516 [RFC7157] Troan, O., Ed., Miles, D., Matsushima, S., Okimoto, T., 517 and D. Wing, "IPv6 Multihoming without Network Address 518 Translation", RFC 7157, DOI 10.17487/RFC7157, March 2014, 519 . 521 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 522 Interface Identifiers with IPv6 Stateless Address 523 Autoconfiguration (SLAAC)", RFC 7217, DOI 10.17487/ 524 RFC7217, April 2014, 525 . 527 Appendix A. Change Log (RFC Editor: please delete) 529 Initial Version: 2015-08-05 531 Version 01: Update text on PIOs, added text on Redirects, and 532 clarified the concept of a "simple" network, 2015-08-13. 534 Version 02: Clarifications after WG discussions, 2015-08-19. 536 Version 03: More clarifications after more WG discussions, 537 especially adding stateful firewalls, uRPF, and more precise 538 discussion of RFC 4861, 2015-09-03. 540 Version 04: Responds to various comments including 542 * Questions regarding RFC 1122's strong and weak host models. 543 This model is, strictly speaking, neither, but is most similar 544 to the strong host model. 546 * Some wording errors. 548 * Requests for discussion of the handling of the RIO, PIO, and 549 Default Router List in an RA. 551 WG Versions 00-02: More clarifications after more WG discussions, 552 2015-11-03. 554 WG Version 03: A final clarification re uRPF, 2015-12-15. 556 Authors' Addresses 558 Fred Baker 559 Cisco Systems 560 Santa Barbara, California 93117 561 USA 563 Email: fred@cisco.com 565 Brian Carpenter 566 Department of Computer Science 567 University of Auckland 568 PB 92019 569 Auckland 1142 570 New Zealand 572 Email: brian.e.carpenter@gmail.com