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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'RFC4925' is defined on line 487, but no explicit reference was found in the text == Unused Reference: 'RFC4966' is defined on line 491, but no explicit reference was found in the text == Unused Reference: 'RFC5549' is defined on line 497, but no explicit reference was found in the text == Unused Reference: 'RFC5565' is defined on line 502, but no explicit reference was found in the text ** Obsolete normative reference: RFC 5549 (Obsoleted by RFC 8950) == Outdated reference: A later version (-09) exists of draft-ietf-dhc-dhcpv4-over-ipv6-06 Summary: 2 errors (**), 0 flaws (~~), 6 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Y. Cui 3 Internet-Draft J. Wu 4 Intended status: Informational P. Wu 5 Expires: October 28, 2013 Tsinghua University 6 O. Vautrin 7 Juniper Networks 8 Y. Lee 9 Comcast 10 April 26, 2013 12 Public IPv4 over IPv6 Access Network 13 draft-ietf-softwire-public-4over6-06 15 Abstract 17 When the service provider networks are upgraded to IPv6, end users 18 will continue to demand IPv4 connectivity. This document proposes a 19 mechanism for hosts or customer networks in IPv6 access network to 20 build bidirectional IPv4 communication with the IPv4 Internet. The 21 mechanism follows the hub and spokes softwire model, and uses IPv4- 22 over-IPv6 tunnel as basic method to traverse IPv6 network. The bi- 23 directionality of this IPv4 communication is achieved by explicitly 24 allocating public IPv4 addresses to end users, as well as maintaining 25 IPv4-IPv6 address binding on the border relay. This mechanism 26 features the allocation of full IPv4 address over IPv6 network, and 27 has been used in production for high-end IPv4 users, IPv6 transition 28 of ICPs, etc. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on October 28, 2013. 47 Copyright Notice 48 Copyright (c) 2013 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 3. Scenario and Use Cases . . . . . . . . . . . . . . . . . . . . 4 66 4. Public 4over6 Address Provisioning . . . . . . . . . . . . . . 5 67 4.1. Basic Provisioning Steps . . . . . . . . . . . . . . . . . 6 68 4.2. Public IPv4 Address Allocation . . . . . . . . . . . . . . 7 69 5. 4over6 CE Behavior . . . . . . . . . . . . . . . . . . . . . . 7 70 6. 4over6 BR Behavior . . . . . . . . . . . . . . . . . . . . . . 8 71 7. Fragmentation and reassembly . . . . . . . . . . . . . . . . . 9 72 8. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 73 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 74 10. Change Log from the -03 Version (RFC Editors please remove 75 this part) . . . . . . . . . . . . . . . . . . . . . . . . . . 9 76 11. Author List . . . . . . . . . . . . . . . . . . . . . . . . . 10 77 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 78 12.1. Normative References . . . . . . . . . . . . . . . . . . . 11 79 12.2. Informative References . . . . . . . . . . . . . . . . . . 12 81 1. Introduction 83 When deploying IPv6 networks, IPv4 connectivity is still a 84 functionality required by end users. It is used for IPv4 85 communication with IPv4-only part of the Internet during the IPv4- 86 IPv6 transition period. IPv4-over-IPv6 tunnel mechanisms are the 87 general solutions to provide this type of IPv4 services. 89 This document describes a mechanism for providing IPv4 connectivity 90 in this situation. The mechanism is similar to the Binding approach 91 of the Unified IPv4-in-IPv6 Softwire CPE effort that is documented in 92 [I-D.bfmk-softwire-unified-cpe] Section 2. Although the 93 functionality documented in the standard is similar, this document 94 describes existing practice that differs from the standard, but that 95 has been deployed in China Next Generation Internet (CNGI) - China 96 Education and Research Network 2 (CERNET2). 98 The purpose of this draft is to document the protocol that was 99 deployed, both for historical purposes and for the benefit of users 100 of that protocol in the field at the time of publication. Future 101 deployments with similar requirements should simply use the related 102 mechanism in [I-D.bfmk-softwire-unified-cpe]. 104 The advantage of IPv4-over-IPv6 tunnel mechanisms is the transparency 105 to the IPv6 infrastructure: since IPv4 is actually only needed on the 106 end user side as well as beyond the tunnel concentrator, most parts 107 and functionalities of the ISP network can remain IPv6 only. 108 Therefore, operators can run an IPv6-only infrastructure instead of a 109 fully dual-stack network, as well as save the IPv4 address resource 110 from being assigned all over the network. 112 While different IPv4-over-IPv6 mechanisms are developed for different 113 application scenarios, the mechanism proposed in this document 114 focuses on providing full end-to-end transparency to the user-side. 115 Therefore, carrier-side address translation should be avoided and 116 public IPv4 addresses should be provisioned to end users. Further 117 more, full address is preferred to port-restricted address. With 118 full address provisioned, user-side address translation is not 119 necessarily needed either. This means minimal changes to the user 120 side: operating system could support the mechanism smoothly, while 121 transparency on upper-layer applications is guaranteed. For many 122 ISPs which are actually capable of provisioning full IPv4 addresses, 123 the mechanism provide a pure, suitable solution. 125 Another focus of this mechanism is deployment and operation 126 flexibility. This mechanism allows IPv4 addressing and IPv6 127 addressing schemes to be independent of each other: end user IPv4 128 address is not embedded in its IPv6 address. The IPv6 infrastructure 129 in the middle is not involved with the IPv4-over-IPv6 mechanism, so 130 no special network planning is required; the service can be provided 131 in on-demand style; the IPv4 address resources can be managed in a 132 flat, centralized manner rather than distributed to customer sites 133 with IPv6. The tradeoff is per-subscriber binding state maintenance 134 on the border relay. 136 The mechanism follows hub and spokes softwire model, and uses IPv4- 137 over-IPv6 tunnel between end host or CPE and border relay as basic 138 data plane method. Full IPv4 addresses are allocated from the ISP to 139 the end host or CPE over IPv6 network. Simultaneously, the binding 140 between the allocated IPv4 address and the end user's IPv6 address 141 are maintained on the border relay for encapsulation usage. 143 2. Terminology 145 Public 4over6: Public 4over6 is a per-subscriber stateful, IPv4-over- 146 IPv6 tunnel mechanism proposed by this document. Public 4over6 147 supports bidirectional communication between IPv4 Internet and IPv4 148 hosts or customer networks in IPv6 access network, by leveraging 149 IPv4-in-IPv6 tunnel and public IPv4 address allocation over IPv6. 151 4over6 Customer Edge (CE): A device functioning as a Customer Edge 152 equipment in Public 4over6 environment. The 4over6 CE can be either 153 a dual-stack capable host, or a dual-stack CPE device, both of which 154 have a tunnel interface to support IPv4-in-IPv6 encapsulation. In 155 the former case, the host supports both IPv4 and IPv6 stack but is 156 provisioned with IPv6 only. In the latter case, the CPE has an IPv6 157 interface connecting to ISP network, and an IPv4 or dual-stack 158 interface connecting to customer network; hosts in the customer 159 network can be IPv4-only or dual-stack. 161 4over6 Border Relay (BR): A router functioning as the border relay in 162 Public 4over6 environment. 4over6 BR is the IPv4-in-IPv6 tunnel 163 concentrator located in IPv6 ISP network. It is a dual-stack router 164 which connects to both the IPv6 ISP network and IPv4 Internet. The 165 4over6 BR also works as a DHCPv4 over IPv6 166 [I-D.ietf-dhc-dhcpv4-over-ipv6] server/relay for assigning public 167 IPv4 address to 4over6 CEs. 169 3. Scenario and Use Cases 171 The general scenario of Public 4over6 is shown in Figure 1. Users in 172 an IPv6 network take IPv6 as their native service. Some users are 173 end hosts which face the ISP network directly, while the others are 174 customer LAN networks behind CPEs, such as a home LAN, an enterprise 175 network, etc. The ISP network is IPv6-only rather than dual-stack, 176 which means the ISP cannot provide native IPv4 service to users. 178 However, it is acceptable that some router(s) on the carrier side 179 becomes dual-stack and connects to IPv4 Internet. So if network 180 users require IPv4 connectivity, the dual-stack router(s) will work 181 as their "entrance". 183 +-------------------------+ 184 | IPv6 ISP Network | 185 +------+ | 186 |4over6|Host | 187 | CE |=================+-------+ +-----------+ 188 +------+ |4over6 | | IPv4 | 189 +---------+ | IPv4-in-IPv6 | BR |---| Internet | 190 |Customer | +------+ | | | | 191 |IPv4 LAN |--|4over6|=================+-------+ +-----------+ 192 | Network | | CE |CPE | 193 +---------+ +------+ | 194 | | 195 +-------------------------+ 197 Figure 1 Public 4over6 scenario 199 Public 4over6 can be applicable in several use cases. If an ISP 200 which switches to IPv6 still has plenty of IPv4 address resource, it 201 can deploy Public 4over6 to provide transparent IPv4 service for all 202 its customers. If the ISP does not have so much IPv4 addresses, it 203 can deploy Dual-Stack Lite [RFC6333] as the basic IPv4-over-IPv6 204 service. Along with DS-Lite, Public 4over6 can be deployed as a 205 value-added service, overcoming the service degradation caused by the 206 CGN. The two mechanisms can be integrated, because the IPv4-in-IPv6 207 tunnel functions are the same; the difference is that DS-Lite employs 208 a CGN while Public 4over6 employs an IPv4 provisioning process. A 209 typical case of the high-end users that could use Public 4over6 is 210 IPv4 application server. Full, public IPv4 address brings 211 significant convenience in this case, which is important to IPv6 212 transition for ICPs. The DNS registration can be direct using 213 dedicated address; the access of the application service can be 214 straightforward, with no translation involved; there will be no need 215 to hold the "pinhole" for incoming traffic, and no well-known port 216 collision will come up. 218 4. Public 4over6 Address Provisioning 220 4.1. Basic Provisioning Steps 221 The following figure shows the basic provisioning steps for Public 222 4over6. 224 4over6 DHCPv6 4over6 DHCPv4 225 CE Server BR Server 226 |Assign IPv6 Addr/Pref +| | | 227 | BR's IPv6 Addr Info | | | 228 |<----------------------| | | 229 | DHCPv6/Other | | | 230 WAN | | 231 IPv6 Configure | | 232 | | | 233 | Assign Public IPv4 Addr(DHCPv4-over-v6/Static Conf) | 234 |<-------------------------------------|<-------------| 235 | | IPv4-IPv6 | 236 | | Binding SYN | 237 Tunnel | 238 IPv4 Configure Binding Update 239 | | 240 | IPv4-in-IPv6 Tunnel | 241 |<------------------------------------>| 242 | | 244 Figure 2 Public 4over6 Address Provisioning 246 The main steps are: 248 o Provision IPv6 address/prefix to 4over6 CE, along with the 249 information of 4over6 BR's IPv6 address, by DHCPv6 or other means. 251 o 4over6 CE configures its WAN interface with globally unique IPv6 252 address which is a result of IPv6 provisioning, including DHCPv6, 253 SLAAC or manual configuration. 255 o Provision IPv4 address to 4over6 CE, by DHCPv4 over IPv6 or static 256 configuration. 258 o Synchronize the IPv4-IPv6 address binding between DHCPv4 server 259 and 4over6 BR, simultaneously with DHCPv4 provisioning. 261 o 4over6 CE configures its tunnel interface, as a result of IPv4 262 provisioning. 264 o 4over6 BR updates the IPv4-IPv6 address binding table, as a result 265 of address binding synchronization. 267 4.2. Public IPv4 Address Allocation 269 Usually each CE is provisioned with one public IPv4 address. However 270 it is possible that a CE would require an IPv4 prefix. The key 271 problem here is the mechanism for IPv4 address provisioning over IPv6 272 network. 274 There are two possibilities here: DHCPv4 over IPv6, and static 275 configuration. Public 4over6 supports both these methods. DHCPv4 276 over IPv6 enables DHCPv4 message to be transported in IPv6 rather 277 than IPv4; therefore, the DHCPv4 process can be performed over an 278 IPv6 network, between BR and CE. [I-D.ietf-dhc-dhcpv4-over-ipv6] 279 describes the DHCP protocol extensions to support that. As to static 280 configuration, 4over6 users and the ISP operators must negotiate 281 beforehand to authorize the IPv4 address(es). Then the tunnel 282 interface and the address binding are configured by the user and the 283 ISP respectively. 285 While regular users would probably take DHCPv4 over IPv6, the manual 286 configuration is usually seen in two cases: application server, which 287 requires a stable IPv4 address, and enterprise network, which usually 288 requires an IPv4 prefix rather than one single address (Note that 289 DHCPv4 does not support prefix allocation). 291 5. 4over6 CE Behavior 293 A CE must be provisioned with IPv6 before Public 4over6. It must 294 also learn the BR's IPv6 address beforehand. This IPv6 address can 295 be configured using a variety of methods, ranging from an out-of-band 296 mechanism, manual configuration, or DHCPv6 option. In order to 297 guarantee interoperability, the CE element ought to implement the 298 AFTR-Name DHCPv6 option defined in [RFC6334]. 300 A CE supports DHCPv4 over IPv6[I-D.ietf-dhc-dhcpv4-over-ipv6], to 301 dynamically require IPv4 address over IPv6 and assign it to the IPv4- 302 in-IPv6 tunnel interface. The CE considers the BR as DHCPv4-over- 303 IPv6 server/relay for public IPv4 address allocation, whose IPv6 304 address is learned by the CE as described above. 306 A CE also supports static configuration of the tunnel interface. In 307 the case of prefix provisioning, Well-Known IPv4 Address defined in 308 section 5.7 of [RFC6333] should be assigned to the tunnel interface, 309 rather than using an address from the prefix. If the CE has multiple 310 IPv6 addresses on its WAN interface, it uses the same IPv6 address 311 for DHCPv4 over IPv6/negotiation of manual configuration, and for 312 data plane encapsulation. 314 A CE performs IPv4-in-IPv6 encapsulation and decapsulation on the 315 tunnel interface. When sending out an IPv4 packet, it performs the 316 encapsulation, using the IPv6 address of the 4over6 BR as the IPv6 317 destination address, and its own IPv6 address as the IPv6 source 318 address. The decapsulation on 4over6 CE is simple. When receiving 319 an IPv4-in-IPv6 packet, the CE just removes the IPv6 header, and 320 either hands it to upper layer or forward it to customer network 321 according to the IPv4 destination address. 323 A CE runs a regular IPv4 NAPT for its customer network when it is 324 provisioned with one single IPv4 address. In that case, the assigned 325 IPv4 address of the tunnel interface would be the external IPv4 326 address of the NAPT. Then the CE performs IPv4 private-to-public 327 translation before encapsulation of IPv4 packets from the customer 328 network, and IPv4 public-to-private translation after decapsulation 329 of IPv4-in-IPv6 packets. 331 IPv4 NAPT is not necessarily when the CE is provisioned with an IPv4 332 prefix. In this case, the detailed customer network planning is out 333 of scope. 335 4over6 CE supports backward compatibility with DS-Lite. A CE may 336 employ Well-Known IPv4 Address for B4 [RFC6333] and switch to Dual- 337 Stack Lite for IPv4 communications, if it can't get a public IPv4 338 address from the DHCPv4 server (maybe because the DHCPv4 over IPv6 339 process fails or the DHCPv4 server refuses to allocate a public IPv4 340 address to it, etc.). 342 6. 4over6 BR Behavior 344 4over6 BR maintains the bindings between the CE IPv6 address and CE 345 IPv4 address (prefixes). The bindings are used to provide correct 346 encapsulation destination address for inbound IPv4 packets, as well 347 as validate the IPv6-IPv4 source of the outbound IPv4-in-IPv6 348 packets. 350 The BR is bound to synchronize the binding information with the IPv4 351 address provisioning process. For static configuration, the BR 352 configures the binding right after negotiation with the customer. As 353 for DHCPv4-over-IPv6, there are multiple possibilities which are 354 deployment-specific: 356 o The BR can be collocated with the DHCPv4-over-IPv6 server. Then 357 the synchronization happens within the BR. It installs a binding 358 when send out an ACK for a DHCP lease, and delete it when the 359 lease expires or a DHCP RELEASE is received. 361 o The BR can play the role of TRA as described in 362 [I-D.ietf-dhc-dhcpv4-over-ipv6], and snoop for the DHCPv4 ACK and 363 Release messages, as well as keep a timer for each binding 364 according to the DHCP lease time. 366 On the IPv6 side, the BR decapsulates IPv4-in-IPv6 packets coming 367 from 4over6 CEs. It removes the IPv6 header of every IPv4-in-IPv6 368 packet and forwards it to the IPv4 Internet. Before the 369 decapsulation, the BR must check the inner IPv4 source address 370 against the outer IPv6 source address, by matching such a binding 371 entry in the binding table. If no binding is found, the BR silently 372 drops the packet. On the IPv4 side, the BR encapsulates the IPv4 373 packets destined to 4over6 CEs. When performing the IPv4-in-IPv6 374 encapsulation, the BR uses its own IPv6 address as the IPv6 source 375 address, uses the IPv4 destination address in the packet to look up 376 IPv6 destination address in the address binding table. After the 377 encapsulation, the BR sends the IPv6 packet on its IPv6 interface to 378 reach a CE. 380 The BR supports hairpinning of traffic between two CEs, by performing 381 de-capsulation and re-encapsulation of packets. 383 7. Fragmentation and reassembly 385 The same considerations as described in section 5.3 and section 6.3 386 of [RFC6333] are to be taken into account. 388 8. DNS 390 The procedure described in Section 5.5 and Section 6.4 of [RFC6333] 391 is to be followed. 393 9. Security Considerations 395 The 4over6 BR should implement methods to limit service only to 396 registered customers. The first step is to allocate IPv4 addresses 397 only to registered customers. One simple solution is to filter on 398 the IPv6 source addresses of incoming DHCP packets and only respond 399 to the ones which have registered IPv6 source address. The BR can 400 also perform authentication during DHCP, for example, based on the 401 MAC address of the CEs. As to data packets, the BR can implement an 402 IPv6 ingress filter on the tunnel interface to accept only the IPv6 403 address range defined in the filter, as well as check the IPv4-IPv6 404 source address binding by looking up the binding table. 406 10. Change Log from the -03 Version (RFC Editors please remove this 407 part) 409 1. Change the Intended Status to Informational, and reword some text 410 to not use RFC2119 language. 412 2. Specify the feature of Public 4over6 and circumstances requiring 413 the mechanism in Abstract. 415 3. Explain the motivation of IPv4-over-IPv6 for Public 4over6 in 416 section 1. 418 4. Explain the relationship between Public 4over6 and Unified CPE, 419 as well as the purpose of this doc. 421 5. Clarify that customer network behind the 4over6 CE could be IPv4- 422 only or dual-stack in section 3. 424 6. Explain how to integrate Public 4over6 and DS-lite as a typical 425 use case in section 4 and section 5. 427 7. Clarify that IPv6 address/prefix can both be supported by 4over6 428 CEs in section 5. 430 8. Improve the preciseness of the texts. 432 9. Remove the text that describes the BR not participating the 433 DHCPv4-over-IPv6 process. 435 10. Updating the references. 437 11. Author List 439 The following are extended authors who contribute to the effort: 441 Huiling Zhao 442 China Telecom 443 Room 502, No.118, Xizhimennei Street 444 Beijing 100035 445 P.R.China 447 Phone: +86-10-58552002 448 Email: zhaohl@ctbri.com.cn 450 Chongfeng Xie 451 China Telecom 452 Room 708, No.118, Xizhimennei Street 453 Beijing 100035 454 P.R.China 456 Phone: +86-10-58552116 457 Email: xiechf@ctbri.com.cn 458 Qiong Sun 459 China Telecom 460 Room 708, No.118, Xizhimennei Street 461 Beijing 100035 462 P.R.China 464 Phone: +86-10-58552936 465 Email: sunqiong@ctbri.com.cn 467 Qi Sun 468 Tsinghua University 469 Beijing 100084 470 P.R.China 472 Phone: +86-10-62785822 473 Email: sunqi@csnet1.cs.tsinghua.edu.cn 475 Chris Metz 476 Cisco Systems 477 3700 Cisco Way 478 San Jose, CA 95134 479 USA 481 Email: chmetz@cisco.com 483 12. References 485 12.1. Normative References 487 [RFC4925] Li, X., Dawkins, S., Ward, D., and 488 A. Durand, "Softwire Problem 489 Statement", RFC 4925, July 2007. 491 [RFC4966] Aoun, C. and E. Davies, "Reasons to 492 Move the Network Address Translator 493 - Protocol Translator (NAT-PT) to 494 Historic Status", RFC 4966, 495 July 2007. 497 [RFC5549] Le Faucheur, F. and E. Rosen, 498 "Advertising IPv4 Network Layer 499 Reachability Information with an 500 IPv6 Next Hop", RFC 5549, May 2009. 502 [RFC5565] Wu, J., Cui, Y., Metz, C., and E. 503 Rosen, "Softwire Mesh Framework", 504 RFC 5565, June 2009. 506 [RFC6333] Durand, A., Droms, R., Woodyatt, J., 507 and Y. Lee, "Dual-Stack Lite 508 Broadband Deployments Following IPv4 509 Exhaustion", RFC 6333, August 2011. 511 [RFC6334] Hankins, D. and T. Mrugalski, 512 "Dynamic Host Configuration Protocol 513 for IPv6 (DHCPv6) Option for Dual- 514 Stack Lite", RFC 6334, August 2011. 516 12.2. Informative References 518 [I-D.bfmk-softwire-unified-cpe] Boucadair, M. and I. Farrer, 519 "Unified IPv4-in-IPv6 Softwire CPE", 520 draft-bfmk-softwire-unified-cpe-02 521 (work in progress), January 2013. 523 [I-D.ietf-dhc-dhcpv4-over-ipv6] Cui, Y., Wu, P., Wu, J., and T. 524 Lemon, "DHCPv4 over IPv6 Transport", 525 draft-ietf-dhc-dhcpv4-over-ipv6-06 526 (work in progress), March 2013. 528 Authors' Addresses 530 Yong Cui 531 Tsinghua University 532 Department of Computer Science, Tsinghua University 533 Beijing 100084 534 P.R.China 536 Phone: +86-10-6260-3059 537 EMail: yong@csnet1.cs.tsinghua.edu.cn 539 Jianping Wu 540 Tsinghua University 541 Department of Computer Science, Tsinghua University 542 Beijing 100084 543 P.R.China 545 Phone: +86-10-6278-5983 546 EMail: jianping@cernet.edu.cn 547 Peng Wu 548 Tsinghua University 549 Department of Computer Science, Tsinghua University 550 Beijing 100084 551 P.R.China 553 Phone: +86-10-6278-5822 554 EMail: pengwu.thu@gmail.com 556 Olivier Vautrin 557 Juniper Networks 558 1194 N Mathilda Avenue 559 Sunnyvale, CA 94089 560 USA 562 EMail: Olivier@juniper.net 564 Yiu L. Lee 565 Comcast 566 One Comcast Center 567 Philadelphia, PA 19103 568 USA 570 EMail: yiu_lee@cable.comcast.com