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'SUBMISSION') (Obsoleted by RFC 6409) Summary: 3 errors (**), 0 flaws (~~), 2 warnings (==), 11 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Draft: Deployment Considerations for 3 lemonade-compliant Mobile Email R. Gellens 4 Document: draft-ietf-lemonade-deployments-05.txt Qualcomm 5 Expires: August 2007 February 2007 7 Deployment Considerations for lemonade-compliant Mobile Email 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six 22 months and may be updated, replaced, or obsoleted by other documents 23 at any time. It is inappropriate to use Internet-Drafts as 24 reference material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt The list of 28 Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Copyright Notice 33 Copyright (C) The IETF Trust (2006). All Rights Reserved. 35 Abstract 37 This document discusses deployment issues and describes requirements 38 for successful deployment of mobile email which are implicit in the 39 IETF lemonade documents. 41 Gellens [Page 1] Expires August 2007 42 Table of Contents 44 1 Conventions Used in this Document . . . . . . . . . . . . . . 2 45 2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 46 3 Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 47 4 TCP Connections . . . . . . . . . . . . . . . . . . . . . . 3 48 4.1 Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 4 49 4.2 Maintenance during temporary transport loss . . . . . . 5 50 5 Dormancy . . . . . . . . . . . . . . . . . . . . . . . . . . 5 51 6 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 5 52 6.1 Firewall Traversal . . . . . . . . . . . . . . . . . . . 6 53 7 NATs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 54 8 Security Considerations . . . . . . . . . . . . . . . . . . . 7 55 9 IANA Considerations . . . . . . . . . . . . . . . . . . . . 9 56 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 57 11 Normative References . . . . . . . . . . . . . . . . . . . . 9 58 12 Informative References . . . . . . . . . . . . . . . . . . . 10 59 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . 10 60 Appendix A: Changes from Previous Version . . . . . . . . . . 10 61 Intellectual Property Statement . . . . . . . . . . . . . . . 11 62 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11 64 1 Conventions Used in this Document 66 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 67 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 68 document are to be interpreted as described in [KEYWORDS]. 70 2 Introduction 72 The IETF lemonade group has developed a set of extensions to IMAP 73 and Message Submission, along with a profile document which 74 restricts server behavior and describes client usage [PROFILE]. 76 Successful deployment of lemonade-compliant mobile email requires 77 various functionality which is generally assumed and hence often not 78 covered in email RFCs. This document describes some of these 79 additional considerations, with a focus on those which have been 80 reported to be problematic. 82 3 Ports 84 Both IMAP and Message Submission have been assigned well-known ports 85 [IANA] which MUST be available. IMAP uses port 143. Message 86 Submission uses port 587. It is REQUIRED that the client be able to 87 contact the server on these ports. Hence the client and server 89 Gellens [Page 2] Expires August 2007 90 systems, as well as any intermediary systems, MUST allow 91 communication on these ports. 93 Historically, MUAs have used port 25 for message submission, and 94 [SUBMISSION] does accommodate this. However, it has become 95 increasingly common for ISPs and organizations to restrict outbound 96 port 25. Additionally, hotels and other public accommodations 97 sometimes intercept port 25 connections, regardless of the 98 destination host, resulting in users unexpectedly submitting 99 potentially sensitive communications to unknown and untrusted 100 third-party servers. Typically, users are not aware of such 101 interception. (Such interception violates [FIREWALLS] and has many 102 negative consequences.) 104 Due to endemic security vulnerabilities in widely-deployed SMTP 105 servers, organizations often employ application-level firewalls 106 which intercept SMTP and permit only a limited subset of the 107 protocol. New extensions are therefore more difficult to deploy on 108 port 25. Since lemonade requires support for several [SUBMISSION] 109 extensions, it is extremely important that lemonade clients use, and 110 lemonade servers listen on, port 587 by default. 112 In addition to communications between the client and server systems, 113 lemonade requires that the Message Submission server be able to 114 establish a TCP connection to the IMAP server (for 115 forward-without-download). This uses port 143 by default. 117 Messaging clients sometimes use protocols to store, retrieve, and 118 update configuration and preference data. Functionality such as 119 setting a new device to use the configuration and preference data of 120 another device, or having a device inherit default configuration 121 data from a user account, an organization, or other source, is 122 likely to be even more useful with small mobile devices. One such 123 protocol which was developed for this purpose is [ACAP]. It is 124 therefore RECOMMENDED that clients be able to contact servers on 125 this port (674). 127 Note that systems which do not support [TCP] on arbitrary ports are 128 not full Internet clients. As a result, such systems use gateways 129 to the Internet which necessarily result in 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 [TCP] is REQUIRED of 137 both client and server systems. 139 Gellens [Page 3] Expires August 2007 140 The requirements and advice in [HOST-REQUIREMENTS] SHOULD be 141 followed. 143 For environments that do not support [TCP] but do support HTTP, 144 email can be offered by deploying webmail. Webmail is a common term 145 for email over the web, where a server speaks HTTP to the client and 146 an email protocol (often IMAP) to the mail store. Its functionality 147 is necessarily limited by the capabilities of the web client, the 148 webmail server, the protocols used between the webmail server and 149 the client (HTTP and a markup language such as HTML), and between 150 the webmail server and the mail store. However, if HTTP is all that 151 is available, the environment is by definition limited and thus 152 functionality offered to the user must also be limited. 154 4.1 Lifetime 156 The duration of the TCP connections between the client and server 157 systems for both IMAP and Message Submission can be arbitrarily 158 long. The client system, the server, as well as all intermediate 159 systems MUST NOT terminate these TCP connections simply because of 160 their duration. 162 The only permissible timeouts on TCP connections occur at the IMAP 163 and Message Submission application level: if no data is received 164 within a period of time, either side MAY terminate the connection as 165 permitted by the protocol (see [SUBMISSION] or [IMAP]). Such 166 timeouts MUST only be enforced by the server or client, not an 167 intermediary system. Since IMAP permits unsolicited notifications 168 of state changes, it is reasonable for clients to remain connected 169 for extended periods with no data being exchanged. 171 It has been reported that some mobile carrier network infrastructure 172 elements impose time restrictions of their own on TCP connections 173 other than HTTP. Such behavior is harmful to mobile email and all 174 other TCP-based protocols. It is unclear how widespread such 175 reported behavior is, or if it is an accidental consequence of an 176 attempt at optimizing for HTTP traffic or a deliberate choice. 177 Either way, such a barrier to TCP connections is a significant risk 178 to the increasing usage of IETF protocols on mobile networks. Note 179 that TCP is designed to be more efficient when it is used to 180 transfer data over time. Prohibiting such connections thus imposes 181 hidden costs on an operator's network, forcing clients to use TCP in 182 inefficient ways. 184 One way in which carriers can inadvertently force TCP connections 185 closed, resulting in users wasting packets by reopening them, is 186 described in Section 7. 188 Gellens [Page 4] Expires August 2007 189 4.2 Maintenance during temporary transport loss 191 TCP is designed to withstand temporary loss of lower-level 192 connectivity. Such transient loss is not uncommon in mobile systems 193 (for example, due to handoffs, fade, etc.). The TCP connection 194 SHOULD be able to survive temporary lower-level loss when the IP 195 address of the client does not change (for example, short-duration 196 loss of the mobile device's traffic channel or periods of high 197 packet loss). Thus, the TCP/IP stack on the client, the server, and 198 all intermediate systems SHOULD maintain the TCP connection during 199 transient loss of connectivity. 201 To this end, client and server systems SHOULD NOT set the TCP 202 keep-alive socket option, and SHOULD NOT close a connection based on 203 ICMP host unreachable messages. 205 5 Dormancy 207 Cellular data channels are connection-oriented (they are brought up 208 or down to establish or tear down connections); it costs network 209 resources to establish connections. 211 Some mobile devices and networks support dormant mode, in which the 212 traffic channel is brought down during idle periods, yet the PPP or 213 equivalent level remains active, and the mobile retains its IP 214 address. 216 Maintenance of TCP connections during dormancy SHOULD be supported 217 by the client, server, and any intermediate systems. Thus, as 218 stated in 4.2 above, client and server systems SHOULD NOT set the 219 TCP keep-alive socket option, and SHOULD NOT close a connection 220 based on ICMP host unreachable messages. 222 Sending packets just to keep the session active causes unnecessary 223 channel establishment and timeout; with a long-idle TCP connection, 224 this would periodically bring up the channel and then let it idle 225 until it times out, again and again. 227 6 Firewalls 229 New services must necessarily have their traffic pass through 230 firewalls in order to be usable by corporate employees or 231 organization members connecting externally, such as when using 232 mobile devices. Firewalls exist to block traffic, yet exceptions 233 must be made for services to be used. There is a body of best 234 practices based on long experience in this area. Numerous 235 techniques exist to help organizations balance protecting themselves 237 Gellens [Page 5] Expires August 2007 238 and providing services to their members, employees, and/or 239 customers. (Describing, or even enumerating, such techniques and 240 practices is beyond the scope of this document, but section 8 does 241 mention some.) 243 It is critical that protocol design and architecture permit such 244 practices, and not constrain them. One key way in which the design 245 of a new service can aid its secure deployment is to maintain the 246 one-to-one association of services and port numbers. 248 One or more firewalls might exist in the path between the client and 249 server systems, as well as between the Message Submission and IMAP 250 servers. Proper deployment REQUIRES that TCP connections be 251 possible from the client system to the IMAP and Message Submission 252 ports on the servers, as well as from the Message Submission server 253 to the IMAP server. This may require configuring firewalls to 254 permit such usage. 256 Firewalls deployed in the network path MUST conform to [FIREWALLS]. 258 Application proxies, which are a not uncommon mechanism, are 259 discussed in [PROXIES]. 261 6.1 Firewall Traversal 263 An often-heard complaint from those attempting to deploy new 264 services within an organization is that the group responsible for 265 maintaining the firewall is unable or unwilling to open the required 266 ports. The group which owns the firewall, being charged with 267 organizational network security, is often reluctant to open firewall 268 ports without an understanding of the benefits and the security 269 implications of the new service. 271 The group wishing to deploy a new service is often tempted to bypass 272 the procedure and internal politics necessary to open the firewall 273 ports. A tempting kludge is to tunnel the new service over an 274 existing service that is already permitted to pass through the 275 firewall, typically HTTP on port 80 or sometimes SMTP on port 25. 276 Some of the downsides to this are discussed in [KLUDGE]. 278 Such bypass can appear to be immediately successful, since the new 279 service seems to deploy. However, assuming the network security 280 group is competent, when they become aware of the kludge, their 281 response is generally to block the violation of organizational 282 security policy. It is difficult to design an application-level 283 proxy/firewall which can provide such access control without 284 violating the transparency requirements of firewalls, as described 285 in [FIREWALLS]. Collateral damage is common in these circumstances. 286 The new service (which initially appeared to have been successfully 288 Gellens [Page 6] Expires August 2007 289 deployed) as well as those existing services which were leveraged to 290 tunnel the new service, becomes subject to arbitrary and 291 unpredictable failures. This encourages an adversarial relationship 292 between the two groups, which hinders attempts at resolution. 294 Even more serious is what happens if a vulnerability is discovered 295 in the new service. Until the vulnerability is corrected, the 296 network security group must disable both the new service and the 297 (typically mission-critical) existing service on which it is 298 layered. 300 An often-repeated truism is that any computer which is connected to 301 a network is insecure. Security and usefulness are both 302 considerations, with organizations making choices about achieving 303 acceptable measures in both areas. Deploying new services typically 304 requires deciding to permit access to the ports used by the service, 305 with appropriate protections. While the delay necessary to review 306 the implications of a new service may be frustrating, in the long 307 run it is likely to be less expensive than a kludge. 309 7 NATs 311 Many NAT boxes place lifetime limits on state, which has the effect 312 of aging out long-idle TCP connections. Since memory is relatively 313 cheap, there's little benefit in arbitrary timeouts. Instead, the 314 oldest unused connection can be recycled if memory or other 315 resources (such as IP addresses) become exhausted, allowing 316 connections to stay stay up forever when resources are available. 318 Any NAT boxes which are deployed between client and server systems 319 SHOULD be configured to have extremely long connection lifetimes. 320 Unlimited lifetimes are RECOMMENDED. 322 Note that IMAP and message submission clients will automatically 323 re-open TCP connections as needed, but it saves time, packets, and 324 processing to avoid the need to do so. Re-opening IMAP and message 325 submission connections generally incurs costs for authentication, 326 TLS negotiation, and server processing, as well as resetting of TCP 327 behavior such as windows. It is also ridiculously wasteful to force 328 clients to send NOOP commands just to maintain NAT state, especially 329 since this can defeat dormancy mode. 331 8 Security Considerations 333 Gellens [Page 7] Expires August 2007 334 There are numerous security considerations whenever an organization 335 chooses to make any of its services available via the Internet. 336 This includes email from mobile clients. 338 Sites concerned about email security should perform a threat 339 analysis, get relevant defenses and/or insurance in place and then 340 make a conscious decision to open up this service. As discussed in 341 section 6.1, piggybacking email traffic on the HTTP port in an 342 attempt to avoid making a firewall configuration change to 343 explicitly permit mobile email connections would bypass this 344 important step and reduces the overall security of the system. 346 Organizations might wish to purchase a messaging server which comes 347 with some indemnity and/or a messaging server which is used "on the 348 edge" by the organization that sells the server. 350 This document does not attempt to catalogue either the various risks 351 an organization might face or the numerous techniques which can be 352 used to protect against the risks. However, to help illustrate the 353 deployment considerations, a very small sample of some of the risks 354 and countermeasures appear below. 356 Some organizations are concerned that permitting direct access to 357 their mail servers via the Internet increases their vulnerability, 358 since a successful exploit against a mail server can potentially 359 expose all mail and authentication credentials stored on that 360 server, and can serve as an injection point for spam. In addition, 361 there are concerns over eavesdropping or modification of mail data 362 and authentication credentials. 364 A large number of approaches exist which can mitigate the risks 365 while allowing access to mail services via mobile clients. 367 Placing servers inside one or more DMZs (demilitarized zones, also 368 called perimeter networks) can protect the rest of the network from 369 a compromised server. An additional way to reduce the risk is to 370 store authentication credentials on a system which is not accessible 371 from the Internet, and which the servers within the DMZ can access 372 only by sending the credentials as received from the client and 373 receiving an authorized/not authorized response. Such isolation 374 reduces the ability of a compromised server to serve as a base for 375 attacking other network hosts. 377 Many additional techniques for further isolation exist, such as 378 having the DMZ IMAP server have no mail store of its own. When a 379 client connects to such a server, the DMZ IMAP server might contact 380 the authentication server and receive a ticket, which it passes to 381 the mail store in order to access the client's mail. In this way a 382 compromised IMAP server cannot be used to access the mail or 384 Gellens [Page 8] Expires August 2007 385 credentials for other users. 387 It is important to realize that simply throwing an extra box in 388 front of the mail servers, such as a gateway which may use HTTP or 389 any of a number of synchronization protocols to communicate with 390 clients, does not itself change the security aspects. By adding 391 such a gateway, the overall security of the system, and the 392 vulnerability of the mail servers, may remain unchanged or may be 393 significantly worsened. Isolation and indirection can be used to 394 protect against specific risks, but to be effective, such steps need 395 to be done after a threat analysis, and with understanding of the 396 issues involved. 398 Organizations SHOULD deploy servers which support the use of TLS for 399 all connections and which can be optionally configured to require 400 TLS. When TLS is used, it SHOULD be via the STARTTLS extensions 401 rather than the alternate port method. TLS can be an effective 402 measure to protect against specific threats, including eavesdropping 403 and alteration, of the traffic between the end-points. However, 404 just because TLS is deployed does not mean the system is "secure." 406 Attempts at bypassing current firewall policy when deploying new 407 services have serious risks, as discussed in section 6.1. 409 It's rare for a new service to not have associated security 410 considerations. Making email available to an organization's members 411 using mobile devices can offer significant benefits. 413 9 IANA Considerations 415 None. 417 10 Acknowledgments 419 Chris Newman and Phil Karn suggested very helpful text. Brian Ross 420 and Dave Cridland reviewed drafts and provided excellent 421 suggestions. 423 11 Normative References 425 [FIREWALLS] "Behavior of and Requirements for Internet Firewalls", 426 N. Freed, RFC 2979, October 2000. 428 Gellens [Page 9] Expires August 2007 430 [HOST-REQUIREMENTS] "Requirements for Internet Hosts -- 431 Communication Layers", R. Braden, RFC 1122, October 1989. 433 [KEYWORDS] "Key words for use in RFCs to Indicate Requirement 434 Levels", S. Bradner, RFC 2119, BCP 14, March 1997. 436 [IANA] IANA Port Number Registry, 437 439 [TCP] "Transmission Control Protocol", J. Postel, RFC 793, STD 7, 440 September 1981. 442 12 Informative References 444 [ACAP] "ACAP -- Application Configuration Access Protocol", C. 445 Newman, J.G. Myers, RFC 2244, November 1997. 447 [IMAP] "Internet Message Access Protocol -- Version 4rev1", M. 448 Crispin, RFC 3501, March 2003. 450 [KLUDGE] "On the use of HTTP as a Substrate", K. Moore, BCP 56, 451 February 2002. 453 [PROFILE] "Lemonade Profile", S. Maes, A. Melnikov, RFC 4550, June 454 2006. 456 [PROXIES] "Classical versus Transparent IP Proxies", M. Chatel, RFC 457 1919, March 1996. 459 [SUBMISSION] "Message Submission for Mail", R. Gellens, J. Klensin, 460 RFC 4409, April 2006. 462 13 Author's Address 464 Randall Gellens 465 QUALCOMM Incorporated 466 5775 Morehouse Drive 467 San Diego, CA 92121 468 randy@qualcomm.com 470 Appendix A: Changes from Previous Version 472 THIS SECTION TO BE REMOVED PRIOR TO PUBLICATION. 474 Gellens [Page 10] Expires August 2007 475 Changes made from version -04 to -05 as a result of IETF Last Call: 476 o Fixed some typos. 477 o Made first use of TCP into a reference. 479 Changes made from version -03 to -04 as a result of WG Last Call: 480 o New boilerplate text 481 o Wording tweaks from lemonade list (e.g., expanding contractions) 482 o Explcitly state that support for TCP is REQUIRED 483 o Correct reference in timeout text from PROFILE to IMAP 484 o Add RFC number to KEYWORDS reference (nit checker doesn't like 485 BCP number only) 486 o Move HOST-REQUIREMENTS reference to normative from informative 487 o Add TCP reference (since TCP support is REQUIRED) 488 o Add IMAP reference (for port number) 489 o Update PROFILE reference to RFC (from RFC Ed pub queue) 491 Intellectual Property Statement 493 The IETF takes no position regarding the validity or scope of any 494 Intellectual Property Rights or other rights that might be claimed 495 to pertain to the implementation or use of the technology described 496 in this document or the extent to which any license under such 497 rights might or might not be available; nor does it represent that 498 it has made any independent effort to identify any such rights. 499 Information on the procedures with respect to rights in RFC 500 documents can be found in BCP 78 and BCP 79. 502 Copies of IPR disclosures made to the IETF Secretariat and any 503 assurances of licenses to be made available, or the result of an 504 attempt made to obtain a general license or permission for the use 505 of such proprietary rights by implementers or users of this 506 specification can be obtained from the IETF on-line IPR repository 507 at http://www.ietf.org/ipr. 509 The IETF invites any interested party to bring to its attention any 510 copyrights, patents or patent applications, or other proprietary 511 rights that may cover technology that may be required to implement 512 this standard. Please address the information to the IETF at 513 ietf-ipr@ietf.org. 515 Full Copyright Statement 517 Copyright (C) The IETF Trust (2007). 519 Gellens [Page 11] Expires August 2007 520 This document is subject to the rights, licenses and restrictions 521 contained in BCP 78, and except as set forth therein, the authors 522 retain all their rights. 524 This document and the information contained herein are provided on 525 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 526 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 527 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 528 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 529 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 530 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 531 FOR A PARTICULAR PURPOSE. 533 Gellens [Page 12] Expires August 2007