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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Operations Working Group (v6ops) F. Gont 3 Internet-Draft SI6 Networks / UTN-FRH 4 Intended status: Informational N. Hilliard 5 Expires: January 2, 2016 INEX 6 G. Doering 7 SpaceNet AG 8 W. Liu 9 Huawei Technologies 10 W. Kumari 11 Google 12 July 1, 2015 14 Operational Implications of IPv6 Packets with Extension Headers 15 draft-gont-v6ops-ipv6-ehs-packet-drops-00 17 Abstract 19 This document summarizes the security and operational implications of 20 IPv6 extension headers, and attempts to analyze reasons why packets 21 with IPv6 extension headers may be dropped in the public Internet. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on January 2, 2016. 40 Copyright Notice 42 Copyright (c) 2015 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 2. Previous Work on IPv6 Extension Headers . . . . . . . . . . . 3 59 3. Security Implications . . . . . . . . . . . . . . . . . . . . 3 60 4. Operational Implications . . . . . . . . . . . . . . . . . . 5 61 4.1. Enforcing infrastructure ACLs . . . . . . . . . . . . . . 5 62 4.2. Route-Processor Protection . . . . . . . . . . . . . . . 5 63 4.3. DDoS Management and Customer Requests for Filtering . . . 5 64 4.4. ECMP and Hash-based Load-Sharing . . . . . . . . . . . . 6 65 4.5. Packet Forwarding Engine Constraints . . . . . . . . . . 6 66 5. A Possible Attack Vector . . . . . . . . . . . . . . . . . . 7 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 68 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 69 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 70 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 71 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 72 9.2. Informative References . . . . . . . . . . . . . . . . . 10 73 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 75 1. Introduction 77 IPv6 Extension Headers (EHs) allow for the extension of the IPv6 78 protocol, and provide support for core functionality such as IPv6 79 fragmentation. However, widespread implementation limitations 80 suggest that EHs present a challenge for IPv6 packet routing 81 equipment, and evidence exists to suggest that IPv6 with EHs may be 82 intentionally dropped on the public Internet in some network 83 deployments. 85 Discussions about the security and operational implications of IPv6 86 extension headers are a regular feature in IETF working groups and 87 other places. Often in these discussions, important security and 88 operational issues are overlooked. 90 This document tries to raise awareness about the security and 91 operational implications of IPv6 Extension Headers, and presents 92 reasons why some networks drop packets containing IPv6 Extension 93 Headers. 95 Section 2 of this document summarizes the work that has been done in 96 the area of IPv6 extension headers. Section 3 discusses the security 97 implications of IPv6 Extension Headers, while Section 4 discusses 98 their operational implications. 100 2. Previous Work on IPv6 Extension Headers 102 Some of the implications of IPv6 Extension Headers have been 103 discussed in IETF circles. For example, [I-D.taylor-v6ops-fragdrop] 104 discusses a rationale for which operators drop IPv6 fragments. 105 [I-D.wkumari-long-headers] discusses possible issues arising from 106 "long" IPv6 header chains. [RFC7045] clarifies how intermediate 107 nodes should deal with IPv6 extension headers. [RFC7112] discusses 108 the issues arising in a specific case where the IPv6 header chain is 109 fragmented into two or more fragments (and formally forbids such 110 case). [I-D.kampanakis-6man-ipv6-eh-parsing] describes how 111 inconsistencies in the way IPv6 packets with extension headers are 112 parsed by different implementations may result in evasion of security 113 controls, and presents guidelines for parsing IPv6 extension headers 114 with the goal of providing a common and consistent parsing 115 methodology for IPv6 implementations. [RFC6980] analyzes the 116 security implications of employing IPv6 fragmentation with Neighbor 117 Discovery for IPv6, and formally recommends against such usage. 118 Finally, [RFC7123] discusses how some popular RA-Guard 119 implementations are subject to evasion by means of IPv6 extension 120 headers. 122 Some preliminary measurements regarding the extent to which packet 123 containing IPv6 EHs are dropped in the public Internet have been 124 presented in [PMTUD-Blackholes], [Gont-IEPG88], [Gont-Chown-IEPG89], 125 and [Linkova-Gont-IEPG90]. [I-D.ietf-v6ops-ipv6-ehs-in-real-world] 126 presents more comprehensive results and documents the methodology for 127 obtaining the presented results. 129 3. Security Implications 131 The security implications of IPv6 Extension Headers generally fall 132 into one or more of these categories: 134 o Evasion of security controls 136 o DoS due to processing requirements 138 o DoS due to implementation errors 140 o Extension Header-specific issues 142 Unlike IPv4 packets where the upper-layer protocols can be trivially 143 found by means of the "IHL" ("Internet Header Length") IPv4 header 144 field, the structure of IPv6 packets is more flexible and complex. 146 Locating upper-layer protocol information requires that all IPv6 147 extension headers be examined. This has presented implementation 148 difficulties, and packet filtering mechanisms on several security 149 devices can be trivially evaded by inserting IPv6 Extension Headers 150 between the main IPv6 header and the upper layer protocol. [RFC7113] 151 describes this issue for the RA-Guard case, but the same techniques 152 can be employed to circumvent other IPv6 firewall and packet 153 filtering mechanisms. Additionally, implementation inconsistencies 154 in packet forwarding engines may result in evasion of security 155 controls [I-D.kampanakis-6man-ipv6-eh-parsing] [Atlasis2014] 156 [BH-EU-2014]. 158 As noted in Section 4, packets that use IPv6 Extension Headers may 159 have a negative performance impact on the handling devices. Unless 160 appropriate mitigations are put in place (e.g., packet filtering and/ 161 or rate-limiting), an attacker could simply send a large amount of 162 IPv6 traffic employing IPv6 Extension Headers with the purpose of 163 performing a Denial of Service (DoS) attack. 165 NOTE: In the most trivial case, a packet that includes a Hop-by- 166 Hop Options header will typically go through the slow forwarding 167 path, and be processed by the router's CPU. An implementation- 168 dependent case might be that in which a router that has been 169 configured to enforce an ACL based on upper-layer information 170 (e.g., upper layer protocol or TCP Destination Port), needs to 171 process the entire IPv6 header chain (in order to find the 172 required information) and this causes the packet to be processed 173 in the slow path [Cisco-EH-Cons]. We note that, for obvious 174 reasons, the aforementioned performance issues may also affect 175 other devices such as firewalls, Network Intrusion Detection 176 Systems (NIDS), etc. [Zack-FW-Benchmark]. The extent to which 177 these devices are affected will typically be implementation- 178 dependent. 180 IPv6 implementations, like all other software, tend to mature with 181 time and wide-scale deployment. While the IPv6 protocol itself has 182 existed for almost 20 years, serious bugs related to IPv6 Extension 183 Header processing continue to be discovered. Because there is 184 currently little operational reliance on IPv6 Extension headers, the 185 corresponding code paths are rarely exercised, and there is the 186 potential that bugs still remain to be discovered in some 187 implementations. 189 IPv6 Fragment Headers are employed to allow fragmentation of IPv6 190 packets. While many of the security implications of the 191 fragmentation / reassembly mechanism are known from the IPv4 world, 192 several related issues have crept into IPv6 implementations. These 193 range from denial of service attacks to information leakage, for 194 example [I-D.ietf-6man-predictable-fragment-id], [Bonica-NANOG58] and 195 [Atlasis2012]). 197 4. Operational Implications 199 Intermediate systems and middleboxes often need to process the entire 200 IPv6 extension header chain to find the layer-4 header. The 201 following subsections discuss some of reasons for which such layer-4 202 information may be needed by an intermediate systems or middlebox, 203 and why packets containing IPv6 extension headers may represent a 204 challenge in such scenarios. 206 4.1. Enforcing infrastructure ACLs 208 Generally speaking, infrastructure ACLs drop unwanted packets 209 destined to parts of a provider's infrastructure, because they are 210 not operationally needed and can be used for attacks of different 211 sorts against the router's control plane. Some traffic needs to be 212 differentiated depending on layer-3 or layer-4 criteria to achieve a 213 useful balance of protection and functionality, for example: 215 o Permit some amount of ICMP echo (ping) traffic towards the 216 router's addresses for troubleshooting. 218 o Permit BGP sessions on the shared network of an exchange point 219 (potentially differentiating between the amount of packets/seconds 220 permitted for established sessions and connection establishment), 221 but do not permit other traffic from the same peer IP addresses. 223 4.2. Route-Processor Protection 225 Most modern routers have a fast hardware-assisted forwarding plane 226 and a loosely coupled control plane, connected together with a link 227 that has much less capacity than the forwarding plane could handle. 228 Traffic differentiation cannot be done by the control plane side, 229 because this would overload the internal link connecting the 230 forwarding plane to the control plane. 232 4.3. DDoS Management and Customer Requests for Filtering 234 The case of customer DDoS protection and edge-to-core customer 235 protection filters is similar in nature to the infrastructure ACL 236 protection. Similar to iACL protection, layer-4 ACLs generally need 237 to be applied as close to the edge of the network as possible, even 238 though the intent is to protect the customer edge rather than the 239 provider core. Application of layer-4 DDoS protection to a network 240 edge is often automated using Flowspec [RFC5575]. 242 For example, a web site which normally only handled traffic on TCP 243 ports 80 and 443 could be subject to a volumetric DDoS attack using 244 NTP and DNS packets with randomised source IP address, thereby 245 rendering useless traditional [RFC5635] source-based real-time black 246 hole mechanisms. In this situation, DDoS protection ACLs could be 247 configured to block all UDP traffic at the network edge without 248 impairing the web server functionality in any way. Thus, being able 249 to filter out arbitrary protocols at the network edge can avoid DDoS- 250 related problems both in the provider network and on the customer 251 edge link. 253 4.4. ECMP and Hash-based Load-Sharing 255 In the case of ECMP (equal cost multi path) load sharing, the router 256 on the sending side of the link needs to make a decision regarding 257 which of the links to use for a given packet. Since round-robin 258 usage of the links is usually avoided in order to prevent packet 259 reordering, forwarding engines need to use a mechanism which will 260 consistently forward the same data streams down the same forwarding 261 paths. Most forwarding engines achieve this by calculating a simple 262 hash using an n-tuple gleaned from a combination of layer-2 through 263 to layer-4 packet header information. This n-tuple will typically 264 use the src/dst MAC address, src/dst IP address, and if possible 265 further layer-4 src/dst port information. As layer-4 port 266 information increases the entropy of the hash, it is highly desirable 267 to use it where possible. 269 4.5. Packet Forwarding Engine Constraints 271 Most modern routers use dedicated hardware (e.g. ASICs or NPUs) to 272 determine how to forward packets across their internal fabrics. One 273 of the common methods of handling next-hop lookup is to send a small 274 portion of the ingress packet to a lookup engine with specialised 275 hardware (e.g. Tertiary CAM or RLDRAM) to determine the packet's 276 next-hop. Technical constraints mean that there is a trade-off 277 between the amount of data sent to the lookup engine and the overall 278 performance of the lookup engine. If more data is sent, the lookup 279 engine can inspect further into the packet, but the overall 280 performance of the system will be reduced. If less data is sent, the 281 overall performance of the router will be increased but the packet 282 lookup engine may not be able to inspect far enough into a packet to 283 determine how it should be handled. 285 Note: For example, current high-end routers at the time of 286 authorship of this document can use up to 192 bytes of header 287 (Cisco ASR9000 Typhoon) or 384 bytes of header (Juniper MX Trio) 289 If a hardware forwarding engine on a modern router cannot make a 290 forwarding decision about a packet because critical information is 291 not sent to the look-up engine, then the router will normally drop 292 the packet. Historically, some packet forwarding engines punted 293 packets of this form to the control plane for more in-depth analysis, 294 but this is unfeasible on most current router architectures as a 295 result of the vast difference between the hardware forwarding 296 capacity of the router and the size of the management link which 297 connects the control plane to the forwarding plane. 299 If an IPv6 header chain is sufficiently long that its header exceeds 300 the packet look-up capacity of the router, then it may be dropped due 301 to hardware inability to determine how it should be handled. 303 5. A Possible Attack Vector 305 The widespread drop of IPv6 packets employing IPv6 Extension Headers 306 can, in some scenarios, be exploited for malicious purposes: if 307 packets employing IPv6 EHs are known to be dropped on the path from 308 system A to system B, an attacker could cause packets sent from A to 309 B to be dropped by sending a forged ICMPv6 Packet Too Big (PTB) 310 [RFC4443] error message to A (advertising an MTU smaller than 1280), 311 such that subsequent packets from A to B include a fragment header 312 (i.e., they result in atomic fragments [RFC6946]). 314 Possible scenarios where this attack vector could be exploited 315 include (but are not limited to): 317 o Communication between any two systems through to public network 318 (e.g., client from/to server or server from/to server), where 319 packets with IPv6 extension headers are dropped by some 320 intermediate router 322 o Communication between two BGP peers employing IPv6 transport, 323 where these BGP peers implement ACLs to drop IPv6 fragments (to 324 avoid control-plane attacks) 326 The aforementioned attack vector is exacerbated by the following 327 factors: 329 o The attacker does not need to forge the IPv6 Source Address of his 330 attack packets. Hence, deployment of simple BCP38 filters will 331 not help as a counter-measure. 333 o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6 334 payload need to be forged. While one could envision filtering 335 devices enforcing BCP38-style filters on the ICMPv6 payload, the 336 use of extension headers (by the attacker) could make this 337 difficult, if not impossible. 339 o Many implementations fail to perform validation checks on the 340 received ICMPv6 error messages, as recommended in Section 5.2 of 341 [RFC4443] and documented in [RFC5927]. It should be noted that in 342 some cases, such as when an ICMPv6 error message has (supposedly) 343 been elicited by a connection-less transport protocol (or some 344 other connection-less protocol being encapsulated in IPv6), it may 345 be virtually impossible to perform validation checks on the 346 received ICMPv6 error messages. And, because of IPv6 extension 347 headers, the ICMPv6 payload might not even contain any useful 348 information on which to perform validation checks. 350 o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big" 351 error messages, the Destination Cache [RFC4861] is usually updated 352 to reflect that any subsequent packets to such destination should 353 include a Fragment Header. This means that a single ICMPv6 354 "Packet Too Big" error message might affect multiple communication 355 instances (e.g. TCP connections) with such destination. 357 o A router or other middlebox cannot simply drop all incoming ICMPv6 358 Packet Too Big error messages, as this would create a PMTUD 359 blackhole. 361 Possible mitigations for this issue include: 363 o Filtering incoming ICMPv6 Packet Too Big error messages that 364 advertise a Next-Hop MTU smaller than 1280 bytes. 366 o Artificially reducing the MTU to 1280 bytes and filter incoming 367 ICMPv6 PTB error messages. 369 Both of these mitigations come at the expense of possibly preventing 370 communication through SIIT [RFC6145] that rely on IPv6 atomic 371 fragments (see [I-D.ietf-6man-deprecate-atomfrag-generation]), and 372 also implies that the filtering device has the ability to filter ICMP 373 PTB messages based on the contents of the MTU field. 375 [I-D.ietf-6man-deprecate-atomfrag-generation] has recently proposed 376 to deprecate the generation of IPv6 atomic fragments, and update SIIT 377 [RFC6145] such that it does not rely on ICMPv6 atomic fragments. 378 Thus, any of the above mitigations would eliminate the attack vector 379 without any interoperability implications. 381 6. IANA Considerations 383 There are no IANA registries within this document. The RFC-Editor 384 can remove this section before publication of this document as an 385 RFC. 387 7. Security Considerations 389 The security implications of IPv6 extension headers are discussed in 390 Section 3. A specific attack vector that could leverage the 391 widespread filtering of packets with IPv6 EHs (along with possible 392 countermeasures) is discussed in Section 5. This document does not 393 introduce any new security issues. 395 8. Acknowledgements 397 The authors would like to thank (in alphabetical order) [TBD] for 398 providing valuable comments on earlier versions of this document. 399 Additionally, the authors would like to thank participants of the 400 v6ops working group for their valuable input on the topics that led 401 to the publication of this document. 403 Fernando Gont would like to thank Fernando Gont would like to thank 404 Jan Zorz / Go6 Lab , and Jared Mauch / NTT 405 America, for providing access to systems and networks that were 406 employed to perform experiments and measurements involving packets 407 with IPv6 Extension Headers. Additionally, he would like to thank 408 SixXS for providing IPv6 connectivity. 410 9. References 412 9.1. Normative References 414 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 415 (IPv6) Specification", RFC 2460, December 1998. 417 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 418 Message Protocol (ICMPv6) for the Internet Protocol 419 Version 6 (IPv6) Specification", RFC 4443, March 2006. 421 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 422 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 423 September 2007. 425 [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation 426 Algorithm", RFC 6145, April 2011. 428 [RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC 429 6946, May 2013. 431 9.2. Informative References 433 [Atlasis2012] 434 Atlasis, A., "Attacking IPv6 Implementation Using 435 Fragmentation", BlackHat Europe 2012. Amsterdam, 436 Netherlands. March 14-16, 2012, 437 . 440 [Atlasis2014] 441 Atlasis, A., "A Novel Way of Abusing IPv6 Extension 442 Headers to Evade IPv6 Security Devices", May 2014, 443 . 446 [BH-EU-2014] 447 Atlasis, A., Rey, E., and R. Schaefer, "Evasion of High- 448 End IDPS Devices at the IPv6 Era", BlackHat Europe 2014, 449 2014, . 452 [Bonica-NANOG58] 453 Bonica, R., "IPv6 Extension Headers in the Real World 454 v2.0", NANOG 58. New Orleans, Louisiana, USA. June 3-5, 455 2013, . 458 [Cisco-EH-Cons] 459 Cisco, , "IPv6 Extension Headers Review and 460 Considerations", October 2006, 461 . 464 [Gont-Chown-IEPG89] 465 Gont, F. and T. Chown, "A Small Update on the Use of IPv6 466 Extension Headers", IEPG 89. London, UK. March 2, 2014, 467 . 470 [Gont-IEPG88] 471 Gont, F., "Fragmentation and Extension header Support in 472 the IPv6 Internet", IEPG 88. Vancouver, BC, Canada. 473 November 13, 2013, . 476 [I-D.ietf-6man-deprecate-atomfrag-generation] 477 Gont, F., LIU, S., and T. Anderson, "Deprecating the 478 Generation of IPv6 Atomic Fragments", draft-ietf-6man- 479 deprecate-atomfrag-generation-01 (work in progress), April 480 2015. 482 [I-D.ietf-6man-predictable-fragment-id] 483 Gont, F., "Security Implications of Predictable Fragment 484 Identification Values", draft-ietf-6man-predictable- 485 fragment-id-08 (work in progress), June 2015. 487 [I-D.ietf-v6ops-ipv6-ehs-in-real-world] 488 Gont, F., Linkova, J., Chown, T., and S. LIU, 489 "Observations on IPv6 EH Filtering in the Real World", 490 draft-ietf-v6ops-ipv6-ehs-in-real-world-00 (work in 491 progress), April 2015. 493 [I-D.kampanakis-6man-ipv6-eh-parsing] 494 Kampanakis, P., "Implementation Guidelines for parsing 495 IPv6 Extension Headers", draft-kampanakis-6man-ipv6-eh- 496 parsing-01 (work in progress), August 2014. 498 [I-D.taylor-v6ops-fragdrop] 499 Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo, 500 M., and T. Taylor, "Why Operators Filter Fragments and 501 What It Implies", draft-taylor-v6ops-fragdrop-02 (work in 502 progress), December 2013. 504 [I-D.wkumari-long-headers] 505 Kumari, W., Jaeggli, J., Bonica, R., and J. Linkova, 506 "Operational Issues Associated With Long IPv6 Header 507 Chains", draft-wkumari-long-headers-03 (work in progress), 508 June 2015. 510 [Linkova-Gont-IEPG90] 511 Linkova, J. and F. Gont, "IPv6 Extension Headers in the 512 Real World v2.0", IEPG 90. Toronto, ON, Canada. July 20, 513 2014, . 516 [PMTUD-Blackholes] 517 De Boer, M. and J. Bosma, "Discovering Path MTU black 518 holes on the Internet using RIPE Atlas", July 2012, 519 . 522 [RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J., 523 and D. McPherson, "Dissemination of Flow Specification 524 Rules", RFC 5575, August 2009. 526 [RFC5635] Kumari, W. and D. McPherson, "Remote Triggered Black Hole 527 Filtering with Unicast Reverse Path Forwarding (uRPF)", 528 RFC 5635, August 2009. 530 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. 532 [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation 533 with IPv6 Neighbor Discovery", RFC 6980, August 2013. 535 [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing 536 of IPv6 Extension Headers", RFC 7045, December 2013. 538 [RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of 539 Oversized IPv6 Header Chains", RFC 7112, January 2014. 541 [RFC7113] Gont, F., "Implementation Advice for IPv6 Router 542 Advertisement Guard (RA-Guard)", RFC 7113, February 2014. 544 [RFC7123] Gont, F. and W. Liu, "Security Implications of IPv6 on 545 IPv4 Networks", RFC 7123, February 2014. 547 [RIPE-Atlas] 548 RIPE, , "RIPE Atlas", . 550 [Zack-FW-Benchmark] 551 Zack, E., "Firewall Security Assessment and Benchmarking 552 IPv6 Firewall Load Tests", IPv6 Hackers Meeting #1, 553 Berlin, Germany. June 30, 2013, 554 . 558 Authors' Addresses 560 Fernando Gont 561 SI6 Networks / UTN-FRH 562 Evaristo Carriego 2644 563 Haedo, Provincia de Buenos Aires 1706 564 Argentina 566 Phone: +54 11 4650 8472 567 Email: fgont@si6networks.com 568 URI: http://www.si6networks.com 569 Nick Hilliard 570 INEX 571 4027 Kingswood Road 572 Dublin 24 573 IE 575 Email: nick@inex.ie 577 Gert Doering 578 SpaceNet AG 579 Joseph-Dollinger-Bogen 14 580 Muenchen D-80807 581 Germany 583 Email: gert@space.net 585 Will (Shucheng) Liu 586 Huawei Technologies 587 Bantian, Longgang District 588 Shenzhen 518129 589 P.R. China 591 Email: liushucheng@huawei.com 593 Warren Kumari 594 Google 595 1600 Amphitheatre Parkway 596 Mountain View, CA 94043 597 US 599 Email: warren@kumari.net