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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft: Deployment Considerations for 2 lemonade-compliant Mobile Email R. Gellens 3 Document: draft-ietf-lemonade-deployments-07.txt Qualcomm 4 Expires: November 2007 May 2007 6 Deployment Considerations for lemonade-compliant Mobile Email 8 Status of this Memo 10 By submitting this Internet-Draft, each author represents that any 11 applicable patent or other IPR claims of which he or she is aware 12 have been or will be disclosed, and any of which he or she becomes 13 aware will be disclosed, in accordance with Section 6 of BCP 79. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six 21 months and may be updated, replaced, or obsoleted by other documents 22 at any time. It is inappropriate to use Internet-Drafts as 23 reference material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt The list of 27 Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 Copyright Notice 32 Copyright (C) The IETF Trust (2007). All Rights Reserved. 34 Abstract 36 This document discusses deployment issues and describes requirements 37 for successful deployment of mobile email which are implicit in the 38 IETF lemonade documents. 40 Gellens [Page 1] Expires November 2007 41 Table of Contents 43 1 Conventions Used in this Document . . . . . . . . . . . . . . 2 44 2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 45 3 Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 46 4 TCP Connections . . . . . . . . . . . . . . . . . . . . . . 3 47 4.1 Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 4 48 4.2 Maintenance during temporary transport loss . . . . . . 5 49 5 Dormancy . . . . . . . . . . . . . . . . . . . . . . . . . . 5 50 6 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 6 51 6.1 Firewall Traversal . . . . . . . . . . . . . . . . . . . 7 52 7 NATs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 53 8 Security Considerations . . . . . . . . . . . . . . . . . . . 8 54 9 IANA Considerations . . . . . . . . . . . . . . . . . . . . 10 55 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 56 11 Normative References . . . . . . . . . . . . . . . . . . . . 10 57 12 Informative References . . . . . . . . . . . . . . . . . . . 10 58 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . 11 59 Appendix A: Changes from Previous Version . . . . . . . . . . 11 60 Intellectual Property Statement . . . . . . . . . . . . . . . 12 61 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 12 63 1 Conventions Used in this Document 65 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 66 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 67 document are to be interpreted as described in [KEYWORDS]. 69 2 Introduction 71 The IETF lemonade group has developed a set of extensions to IMAP 72 and Message Submission, along with a profile document which 73 restricts server behavior and describes client usage [PROFILE]. 75 Successful deployment of lemonade-compliant mobile email requires 76 various functionality which is generally assumed and hence often not 77 covered in email RFCs. This document describes some of these 78 additional considerations, with a focus on those which have been 79 reported to be problematic. 81 3 Ports 83 Both IMAP and Message Submission have been assigned well-known ports 84 [IANA] which MUST be available. IMAP uses port 143. Message 85 Submission uses port 587. It is REQUIRED that the client be able to 86 contact the server on these ports. Hence the client and server 88 Gellens [Page 2] Expires November 2007 89 systems, as well as any intermediary systems, MUST allow 90 communication on these ports. 92 Historically, MUAs have used port 25 for message submission, and 93 [SUBMISSION] does accommodate this. However, it has become 94 increasingly common for ISPs and organizations to restrict outbound 95 port 25. Additionally, hotels and other public accommodations 96 sometimes intercept port 25 connections, regardless of the 97 destination host, resulting in users unexpectedly submitting 98 potentially sensitive communications to unknown and untrusted 99 third-party servers. Typically, users are not aware of such 100 interception. (Such interception violates [FIREWALLS] and has many 101 negative consequences.) 103 Due to endemic security vulnerabilities in widely-deployed SMTP 104 servers, organizations often employ application-level firewalls 105 which intercept SMTP and permit only a limited subset of the 106 protocol. New extensions are therefore more difficult to deploy on 107 port 25. Since lemonade requires support for several [SUBMISSION] 108 extensions, it is extremely important that lemonade clients use, and 109 lemonade servers listen on, port 587 by default. 111 In addition to communications between the client and server systems, 112 lemonade requires that the Message Submission server be able to 113 establish a TCP connection to the IMAP server (for 114 forward-without-download). This uses port 143 by default. 116 Messaging clients sometimes use protocols to store, retrieve, and 117 update configuration and preference data. Functionality such as 118 setting a new device to use the configuration and preference data of 119 another device, or having a device inherit default configuration 120 data from a user account, an organization, or other source, is 121 likely to be even more useful with small mobile devices. One such 122 protocol which was developed for this purpose is [ACAP]. It is 123 therefore RECOMMENDED that clients be able to contact servers on 124 this port (674). 126 Note that systems which do not support application use of [TCP] on 127 arbitrary ports are not full Internet clients. As a result, such 128 systems use gateways to the Internet which necessarily result in 129 data integrity problems. 131 4 TCP Connections 133 Both IMAP and Message Submission use [TCP]. Hence the client system 134 MUST be able to establish and maintain TCP connections to these 135 servers. The Message Submission server MUST be able to initiate a 136 connection to the IMAP server. Support for application use of [TCP] 138 Gellens [Page 3] Expires November 2007 139 is REQUIRED of both client and server systems. 141 The requirements and advice in [HOST-REQUIREMENTS] SHOULD be 142 followed. 144 Note that, for environments that do not support application use of 145 [TCP] but do so for HTTP, email can be offered by deploying webmail. 146 Webmail is a common term for email over the web, where a server 147 speaks HTTP to the client and an email protocol (often IMAP) to the 148 mail store. Its functionality is necessarily limited by the 149 capabilities of the web client, the webmail server, the protocols 150 used between the webmail server and the client (HTTP and a markup 151 language such as HTML), and between the webmail server and the mail 152 store. However, if HTTP is all that is available to an application, 153 the environment is by definition limited and thus functionality 154 offered to the user must also be limited, and can't be lemonade 155 compliant. 157 4.1 Lifetime 159 The duration of the TCP connections between the client and server 160 systems for both IMAP and Message Submission can be arbitrarily 161 long. The client system, the server, as well as all intermediate 162 systems MUST NOT terminate these TCP connections simply because of 163 their duration (that is, just because of how long they have been 164 open). 166 Lemonade depends on idle timers being enforced only at the 167 application level (IMAP and Message Submission): if no data is 168 received within a period of time, either side MAY terminate the 169 connection as permitted by the protocol (see [SUBMISSION] or 170 [IMAP]). Since IMAP permits unsolicited notifications of state 171 changes, it is reasonable for clients to remain connected for 172 extended periods with no data being exchanged. Being forced to send 173 data just to keep the connection alive can prevent or hinder 174 optimizations such as dormancy mode (see section 5). 176 Two hours is a fairly common configuration timeout at middleboxes. 177 That is, there are a number of sites at which TCP connections are 178 torn down by the network two hours after data was last sent in 179 either direction. Thus, lemonade clients and servers SHOULD make 180 sure that, in the absence of specific configuration otherwise, the 181 TCP connection not remain idle for two hours. 183 It has been reported that some networks impose duration time 184 restrictions of their own on TCP connections other than HTTP. Such 185 behavior is harmful to email and all other TCP-based protocols. It 186 is unclear how widespread such reported behavior is, or if it is an 188 Gellens [Page 4] Expires November 2007 189 accidental consequence of an attempt at optimizing for HTTP traffic, 190 implementation limitations in firewalls, NATs or other devices, or a 191 deliberate choice. Either way, such a barrier to TCP connections is 192 a significant risk to the increasing usage of IETF protocols on such 193 networks. Note that TCP is designed to be more efficient when it is 194 used to transfer data over time. Prohibiting such connections thus 195 imposes hidden costs on an operator's network, forcing clients to 196 use TCP in inefficient ways. One way in which carriers can 197 inadvertently force TCP connections closed, resulting in users 198 wasting packets by reopening them, is described in Section 7 200 Note that systems remain able to terminate TCP connections at any 201 time based on local decisions, for example, to prevent overload 202 during a denial-of-service attack. These mechanisms are permitted 203 to take idle time into consideration and are not affected by these 204 requirements. 206 4.2 Maintenance during temporary transport loss 208 TCP is designed to withstand temporary loss of lower-level 209 connectivity. Such transient loss is not uncommon in mobile systems 210 (for example, due to handoffs, fade, etc.). The TCP connection 211 SHOULD be able to survive temporary lower-level loss when the IP 212 address of the client does not change (for example, short-duration 213 loss of the mobile device's traffic channel or periods of high 214 packet loss). Thus, the TCP/IP stack on the client, the server, and 215 all intermediate systems SHOULD maintain the TCP connection during 216 transient loss of connectivity. 218 To this end, client and server systems SHOULD NOT set the TCP 219 keep-alive socket option, and SHOULD NOT close a connection based on 220 ICMP "soft" errors, such as host unreachable messages. 222 5 Dormancy 224 Cellular data channels are connection-oriented (they are brought up 225 or down to establish or tear down connections); it costs network 226 resources to establish connections. Generally speaking, mobile 227 device battery charges last longer when the traffic channel is used 228 less. 230 Some mobile devices and networks support dormant mode, in which the 231 traffic channel is brought down during idle periods, yet the PPP or 232 equivalent level remains active, and the mobile retains its IP 233 address. 235 Gellens [Page 5] Expires November 2007 236 Maintenance of TCP connections during dormancy SHOULD be supported 237 by the client, server, and any intermediate systems. Thus, as 238 stated in 4.2 above, client and server systems SHOULD NOT set the 239 TCP keep-alive socket option, and SHOULD NOT close a connection 240 based on ICMP host unreachable messages. 242 Sending packets just to keep the session active causes unnecessary 243 channel establishment and timeout; with a long-idle TCP connection, 244 this would periodically bring up the channel and then let it idle 245 until it times out, again and again. 247 6 Firewalls 249 New services must necessarily have their traffic pass through 250 firewalls in order to be usable by corporate employees or 251 organization members connecting externally, such as when using 252 mobile devices. Firewalls exist to block traffic, yet exceptions 253 must be made for services to be used. There is a body of best 254 practices based on long experience in this area. Numerous 255 techniques exist to help organizations balance protecting themselves 256 and providing services to their members, employees, and/or 257 customers. (Describing, or even enumerating, such techniques and 258 practices is beyond the scope of this document, but section 8 does 259 mention some.) 261 It is critical that protocol design and architecture permit such 262 practices, and not constrain them. One key way in which the design 263 of a new service can aid its secure deployment is to maintain the 264 one-to-one association of services and port numbers. 266 One or more firewalls might exist in the path between the client and 267 server systems, as well as between the Message Submission and IMAP 268 servers. Proper deployment REQUIRES that TCP connections be 269 possible from the client system to the IMAP and Message Submission 270 ports on the servers, as well as from the Message Submission server 271 to the IMAP server. This may require configuring firewalls to 272 permit such usage. 274 Firewalls deployed in the network path MUST NOT damage protocol 275 traffic. In particular, both message submission and IMAP 276 connections from the client MUST be permitted. Firewalls MUST NOT 277 partially block extensions to these protocols, such as by allowing 278 one side of an extension negotiation, as doing so results in the two 279 sides being out of synch, with later failures. See [FIREWALLS] for 280 more discussion. 282 Gellens [Page 6] Expires November 2007 283 Application proxies, which are a not uncommon mechanism, are 284 discussed in [PROXIES]. 286 6.1 Firewall Traversal 288 An often-heard complaint from those attempting to deploy new 289 services within an organization is that the group responsible for 290 maintaining the firewall is unable or unwilling to open the required 291 ports. The group which owns the firewall, being charged with 292 organizational network security, is often reluctant to open firewall 293 ports without an understanding of the benefits and the security 294 implications of the new service. 296 The group wishing to deploy a new service is often tempted to bypass 297 the procedure and internal politics necessary to open the firewall 298 ports. A tempting kludge is to tunnel the new service over an 299 existing service that is already permitted to pass through the 300 firewall, typically HTTP on port 80 or sometimes SMTP on port 25. 301 Some of the downsides to this are discussed in [KLUDGE]. 303 Such bypass can appear to be immediately successful, since the new 304 service seems to deploy. However, assuming the network security 305 group is competent, when they become aware of the kludge, their 306 response is generally to block the violation of organizational 307 security policy. It is difficult to design an application-level 308 proxy/firewall which can provide such access control without 309 violating the transparency requirements of firewalls, as described 310 in [FIREWALLS]. Collateral damage is common in these circumstances. 311 The new service (which initially appeared to have been successfully 312 deployed) as well as those existing services which were leveraged to 313 tunnel the new service, becomes subject to arbitrary and 314 unpredictable failures. This encourages an adversarial relationship 315 between the two groups, which hinders attempts at resolution. 317 Even more serious is what happens if a vulnerability is discovered 318 in the new service. Until the vulnerability is corrected, the 319 network security group must disable both the new service and the 320 (typically mission-critical) existing service on which it is 321 layered. 323 An often-repeated truism is that any computer which is connected to 324 a network is insecure. Security and usefulness are both 325 considerations, with organizations making choices about achieving 326 acceptable measures in both areas. Deploying new services typically 327 requires deciding to permit access to the ports used by the service, 328 with appropriate protections. While the delay necessary to review 329 the implications of a new service may be frustrating, in the long 330 run it is likely to be less expensive than a kludge. 332 Gellens [Page 7] Expires November 2007 333 7 NATs 335 Many NAT boxes place lifetime limits on state, which has the effect 336 of aging out long-idle TCP connections. Since memory is relatively 337 cheap, there's little benefit in arbitrary timeouts. Instead, the 338 oldest unused connection can be recycled if memory or other 339 resources (such as IP addresses) become exhausted, allowing 340 connections to stay stay up forever when resources are available. 342 Any NAT boxes which are deployed between client and server systems 343 SHOULD be configured to have extremely long connection lifetimes. 344 Unlimited lifetimes are RECOMMENDED. 346 Note that IMAP and message submission clients will automatically 347 re-open TCP connections as needed, but it saves time, packets, and 348 processing to avoid the need to do so. Re-opening IMAP and message 349 submission connections generally incurs costs for authentication, 350 TLS negotiation, and server processing, as well as resetting of TCP 351 behavior such as windows. It is also ridiculously wasteful to force 352 clients to send NOOP commands just to maintain NAT state, especially 353 since this can defeat dormancy mode. 355 8 Security Considerations 357 There are numerous security considerations whenever an organization 358 chooses to make any of its services available via the Internet. 359 This includes email from mobile clients. 361 Sites concerned about email security should perform a threat 362 analysis, get relevant protections in place and then make a 363 conscious decision to open up this service. As discussed in section 364 6.1, piggybacking email traffic on the HTTP port in an attempt to 365 avoid making a firewall configuration change to explicitly permit 366 mobile email connections would bypass this important step and 367 reduces the overall security of the system. 369 Organizations might wish to purchase a messaging server which comes 370 with some indemnity and/or a messaging server which is used "on the 371 edge" by the organization that sells the server. 373 This document does not attempt to catalogue either the various risks 374 an organization might face or the numerous techniques which can be 375 used to protect against the risks. However, to help illustrate the 376 deployment considerations, a very small sample of some of the risks 377 and countermeasures appear below. 379 Gellens [Page 8] Expires November 2007 380 Some organizations are concerned that permitting direct access to 381 their mail servers via the Internet increases their vulnerability, 382 since a successful exploit against a mail server can potentially 383 expose all mail and authentication credentials stored on that 384 server, and can serve as an injection point for spam. In addition, 385 there are concerns over eavesdropping or modification of mail data 386 and authentication credentials. 388 A large number of approaches exist which can mitigate the risks 389 while allowing access to mail services via mobile clients. 391 Placing servers inside one or more DMZs (demilitarized zones, also 392 called perimeter networks) can protect the rest of the network from 393 a compromised server. An additional way to reduce the risk is to 394 store authentication credentials on a system which is not accessible 395 from the Internet, and which the servers within the DMZ can access 396 only by sending the credentials as received from the client and 397 receiving an authorized/not authorized response. Such isolation 398 reduces the ability of a compromised server to serve as a base for 399 attacking other network hosts. 401 Many additional techniques for further isolation exist, such as 402 having the DMZ IMAP server have no mail store of its own. When a 403 client connects to such a server, the DMZ IMAP server might contact 404 the authentication server and receive a ticket, which it passes to 405 the mail store in order to access the client's mail. In this way a 406 compromised IMAP server cannot be used to access the mail or 407 credentials for other users. 409 It is important to realize that simply throwing an extra box in 410 front of the mail servers, such as a gateway which may use HTTP or 411 any of a number of synchronization protocols to communicate with 412 clients, does not itself change the security aspects. By adding 413 such a gateway, the overall security of the system, and the 414 vulnerability of the mail servers, may remain unchanged or may be 415 significantly worsened. Isolation and indirection can be used to 416 protect against specific risks, but to be effective, such steps need 417 to be done after a threat analysis, and with understanding of the 418 issues involved. 420 Organizations SHOULD deploy servers which support the use of TLS for 421 all connections and which can be optionally configured to require 422 TLS. When TLS is used, it SHOULD be via the STARTTLS extensions 423 rather than the alternate port method. TLS can be an effective 424 measure to protect against specific threats, including eavesdropping 425 and alteration, of the traffic between the end-points. However, 426 just because TLS is deployed does not mean the system is "secure." 428 Gellens [Page 9] Expires November 2007 429 Attempts at bypassing current firewall policy when deploying new 430 services have serious risks, as discussed in section 6.1. 432 It's rare for a new service to not have associated security 433 considerations. Making email available to an organization's members 434 using mobile devices can offer significant benefits. 436 9 IANA Considerations 438 None. 440 10 Acknowledgments 442 Chris Newman and Phil Karn suggested very helpful text. Brian Ross 443 and Dave Cridland reviewed drafts and provided excellent 444 suggestions. 446 11 Normative References 448 [HOST-REQUIREMENTS] "Requirements for Internet Hosts -- 449 Communication Layers", R. Braden, RFC 1122, October 1989. 451 [KEYWORDS] "Key words for use in RFCs to Indicate Requirement 452 Levels", S. Bradner, RFC 2119, BCP 14, March 1997. 454 [IANA] IANA Port Number Registry, 455 457 [TCP] "Transmission Control Protocol", J. Postel, RFC 793, STD 7, 458 September 1981. 460 12 Informative References 462 [ACAP] "ACAP -- Application Configuration Access Protocol", C. 463 Newman, J.G. Myers, RFC 2244, November 1997. 465 [FIREWALLS] "Behavior of and Requirements for Internet Firewalls", 466 N. Freed, RFC 2979, October 2000. 468 [IMAP] "Internet Message Access Protocol -- Version 4rev1", M. 469 Crispin, RFC 3501, March 2003. 471 Gellens [Page 10] Expires November 2007 473 [KLUDGE] "On the use of HTTP as a Substrate", K. Moore, BCP 56, 474 February 2002. 476 [PROFILE] "Lemonade Profile", S. Maes, A. Melnikov, RFC 4550, June 477 2006. 479 [PROXIES] "Classical versus Transparent IP Proxies", M. Chatel, RFC 480 1919, March 1996. 482 [SUBMISSION] "Message Submission for Mail", R. Gellens, J. Klensin, 483 RFC 4409, April 2006. 485 13 Author's Address 487 Randall Gellens 488 QUALCOMM Incorporated 489 5775 Morehouse Drive 490 San Diego, CA 92121 491 randy@qualcomm.com 493 Appendix A: Changes from Previous Version 495 THIS SECTION TO BE REMOVED PRIOR TO PUBLICATION. 497 Changes made from version -06 to -07 as a result of IESG DISCUSS by 498 Cullen Jennings: 499 o Removed prohibition on NATs closing connections just because of 500 how long they have been open, without taking any other factor 501 into account, since products made by Cullen's employer may 502 violate this. Replaced with discussion on the issue and 503 recommendation that lemonade clients not allow connections to 504 remain idle for two hours, unless they have specific 505 configuration instructions to do so (e.g., in networks where 506 NATs and firewalls allow this, and the IMAP server permits it) 507 o Changed explicit note to permit all systems to terminate TCP 508 connections at any time based on local decisions 509 o Removed word "insurance" 511 Changes made from version -05 to -06 as a result of IESG DISCUSSes 512 by Jari Arkko, Lars Eggert, Cullen Jennings, and Magnus Westerlund: 513 o Clarified that "support" for TCP means availability of TCP to 514 applications, as opposed to deployment or use of TCP within a 515 network 516 o Clarified that an HTTP-only environment can offer webmail, which 517 may be email, but isn't lemonade email 518 o Added explicit note that end systems remain able to terminate 519 TCP connections at any time based on local decisions 521 Gellens [Page 11] Expires November 2007 522 o Made [FIREWALLS] an informative, not normative reference by 523 restating requirement 524 o Additional clarifications to make draft easier to read from a 525 non-Applications Area viewpoint 527 Changes made from version -04 to -05 as a result of IETF Last Call: 528 o Fixed some typos. 529 o Made first use of TCP into a reference. 531 Changes made from version -03 to -04 as a result of WG Last Call: 532 o New boilerplate text 533 o Wording tweaks from lemonade list (e.g., expanding contractions) 534 o Explcitly state that support for TCP is REQUIRED 535 o Correct reference in timeout text from PROFILE to IMAP 536 o Add RFC number to KEYWORDS reference (nit checker doesn't like 537 BCP number only) 538 o Move HOST-REQUIREMENTS reference to normative from informative 539 o Add TCP reference (since TCP support is REQUIRED) 540 o Add IMAP reference (for port number) 541 o Update PROFILE reference to RFC (from RFC Ed pub queue) 543 Intellectual Property Statement 545 The IETF takes no position regarding the validity or scope of any 546 Intellectual Property Rights or other rights that might be claimed 547 to pertain to the implementation or use of the technology described 548 in this document or the extent to which any license under such 549 rights might or might not be available; nor does it represent that 550 it has made any independent effort to identify any such rights. 551 Information on the procedures with respect to rights in RFC 552 documents can be found in BCP 78 and BCP 79. 554 Copies of IPR disclosures made to the IETF Secretariat and any 555 assurances of licenses to be made available, or the result of an 556 attempt made to obtain a general license or permission for the use 557 of such proprietary rights by implementers or users of this 558 specification can be obtained from the IETF on-line IPR repository 559 at http://www.ietf.org/ipr. 561 The IETF invites any interested party to bring to its attention any 562 copyrights, patents or patent applications, or other proprietary 563 rights that may cover technology that may be required to implement 564 this standard. Please address the information to the IETF at 565 ietf-ipr@ietf.org. 567 Gellens [Page 12] Expires November 2007 568 Full Copyright Statement 570 Copyright (C) The IETF Trust (2007). 572 This document is subject to the rights, licenses and restrictions 573 contained in BCP 78, and except as set forth therein, the authors 574 retain all their rights. 576 This document and the information contained herein are provided on 577 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 578 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 579 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 580 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 581 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 582 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 583 FOR A PARTICULAR PURPOSE. 585 Gellens [Page 13] Expires November 2007