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'NAMESPACE' Summary: 14 errors (**), 0 flaws (~~), 8 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group B. Leiba 2 Internet Draft IBM T.J. Watson Research Center 3 Document: draft-leiba-imap-implement-guide-02.txt September 1997 4 Expires February 1998 6 IMAP4 Implementation Recommendations 8 Status of this Document 10 This document provides information for the Internet community. This 11 document does not specify an Internet standard of any kind. 12 Distribution of this document is unlimited. 14 This document is an Internet Draft. Internet Drafts are working 15 documents of the Internet Engineering Task Force (IETF), its Areas, 16 and its Working Groups. Note that other groups may also distribute 17 working documents as Internet Drafts. 19 Internet Drafts are draft documents valid for a maximum of six 20 months. Internet Drafts may be updated, replaced, or obsoleted by 21 other documents at any time. It is not appropriate to use Internet 22 Drafts as reference material or to cite them other than as a "working 23 draft" or "work in progress". 25 To learn the current status of any Internet-Draft, please check the 26 1id-abstracts.txt listing contained in the Internet-Drafts Shadow 27 Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or 28 munnari.oz.au. 30 A revised version of this draft document will be submitted to the RFC 31 editor. Discussion and suggestions for improvement are requested. 32 This document will expire by the end of February 1998. 34 1. Abstract 36 The IMAP4 specification [RFC-2060] describes a rich protocol for use 37 in building clients and servers for storage, retrieval, and 38 manipulation of electronic mail. Because the protocol is so rich and 39 has so many implementation choices, there are often trade-offs that 40 must be made and issues that must be considered when designing such 41 clients and servers. This document attempts to outline these issues 42 and to make recommendations in order to make the end products as 43 interoperable as possible. 45 Internet DRAFT Implementation Recommendations September 1997 47 2. Conventions used in this document 49 In examples, "C:" indicates lines sent by a client that is connected 50 to a server. "S:" indicates lines sent by the server to the client. 52 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 53 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 54 document are to be interpreted as described in [RFC-2119]. 56 3. Interoperability Issues and Recommendations 58 3.1. Accessibility 60 This section describes the issues related to access to servers and 61 server resources. Concerns here include data sharing and maintenance 62 of client/server connections. 64 3.1.1. Multiple Accesses of the Same Mailbox 66 One strong point of IMAP4 is that, unlike POP3, it allows for 67 multiple simultaneous access to a single mailbox. A user can, thus, 68 read mail from a client at home while the client in the office is 69 still connected; or the help desk staff can all work out of the same 70 inbox, all seeing the same pool of questions. An important point 71 about this capability, though is that NO SERVER IS GUARANTEED TO 72 SUPPORT THIS. If you are selecting an IMAP server and this facility 73 is important to you, be sure that the server you choose to install, 74 in the configuration you choose to use, supports it. 76 If you are designing a client, you MUST NOT assume that you can 77 access the same mailbox more than once at a time. That means 78 1. you must handle gracefully the failure of a SELECT command if 79 the server refuses the second SELECT, 80 2. you must handle reasonably the severing of your connection (see 81 "Severed Connections", below) if the server chooses to allow the 82 second SELECT by forcing the first off, 83 3. you must avoid making multiple connections to the same mailbox 84 in your own client (for load balancing or other such reasons), 85 and 86 4. you must avoid using the STATUS command on a mailbox that you 87 have selected (with some server implementations the STATUS 88 command has the same problems with multiple access as do the 89 SELECT and EXAMINE commands). 91 A further note about STATUS: The STATUS command is sometimes used to 92 check a non-selected mailbox for new mail. This mechanism MUST NOT 93 be used to check for new mail in the selected mailbox; section 5.2 of 95 Internet DRAFT Implementation Recommendations September 1997 97 [RFC-2060] specifically forbids this in its last paragraph. Further, 98 since STATUS takes a mailbox name it is an independent operation, not 99 operating on the selected mailbox. Because of this, the information 100 it returns is not necessarily in synchronization with the selected 101 mailbox state. 103 3.1.2. Severed Connections 105 The client/server connection may be severed for one of three reasons: 106 the client severs the connection, the server severs the connection, 107 or the connection is severed by outside forces beyond the control of 108 the client and the server (a telephone line drops, for example). 109 Clients and servers must both deal with these situations. 111 When the client wants to sever a connection, it's usually because it 112 has finished the work it needed to do on that connection. The client 113 SHOULD send a LOGOUT command, wait for the tagged response, and then 114 close the socket. But note that, while this is what's intended in 115 the protocol design, there isn't universal agreement here. Some 116 contend that sending the LOGOUT and waiting for the two responses 117 (untagged BYE and tagged OK) is wasteful and unnecessary, and that 118 the client can simply close the socket. The server should interpret 119 the closed socket as a log out by the client. The counterargument is 120 that it's useful from the standpoint of cleanup, problem 121 determination, and the like, to have an explicit client log out, 122 because otherwise there is no way for the server to tell the 123 difference between "closed socket because of log out" and "closed 124 socket because communication was disrupted". If there is a 125 client/server interaction problem, a client which routinely 126 terminates a session by breaking the connection without a LOGOUT will 127 make it much more difficult to determine the problem. 129 Because of this disagreement, server designers must be aware that 130 some clients might close the socket without sending a LOGOUT. In any 131 case, whether or not a LOGOUT was sent, the server SHOULD NOT 132 implicitly expunge any messages from the selected mailbox. If a 133 client wants the server to do so, it MUST send a CLOSE or EXPUNGE 134 command explicitly. 136 When the server wants to sever a connection it's usually due to an 137 inactivity timeout or is because a situation has arisen that has 138 changed the state of the mail store in a way that the server can not 139 communicate to the client. The server SHOULD send an untagged BYE 140 response to the client and then close the socket. Sending an 141 untagged BYE response before severing allows the server to send a 142 human-readable explanation of the problem to the client, which the 143 client may then log, display to the user, or both (see section 7.1.5 144 of [RFC-2060]). 146 Internet DRAFT Implementation Recommendations September 1997 148 3.2. Scaling 150 IMAP4 has many features that allow for scalability, as mail stores 151 become larger and more numerous. Large numbers of users, mailboxes, 152 and messages, and very large messages require thought to handle 153 efficiently. This document will not address the administrative 154 issues involved in large numbers of users, but we will look at the 155 other items. 157 3.2.1. Flood Control 159 There are three situations when a client can make a request that will 160 result in a very large response - too large for the client reasonably 161 to deal with: there are a great many mailboxes available, there are a 162 great many messages in the selected mailbox, or there is a very large 163 message part. The danger here is that the end user will be stuck 164 waiting while the server sends (and the client processes) an enormous 165 response. In all of these cases there are things a client can do to 166 reduce that danger. 168 There is also the case where a client can flood a server, by sending 169 an arbitratily long command. We’ll discuss that issue, too, in this 170 section. 172 3.2.1.1. Listing Mailboxes 174 Some servers present Usenet newsgroups to IMAP users. Newsgroups, 175 and other such hierarchical mailbox structures, can be very numerous 176 but may have only a few entries at the top level of hierarchy. Also, 177 some servers are built against mail stores that can, unbeknownst to 178 the server, have circular hierarchies - that is, it’s possible for 179 "a/b/c/d" to resolve to the same file structure as "a", which would 180 then mean that "a/b/c/d/b" is the same as "a/b", and the hierarchy 181 will never end. The LIST response in this case will be unlimited. 183 Clients that will have trouble with this are those that use 184 C: 001 LIST "" * 185 to determine the mailbox list. Because of this, clients SHOULD NOT 186 use an unqualified "*" that way in the LIST command. A safer 187 approach is to list each level of hierarchy individually, allowing 188 the user to traverse the tree one limb at a time, thus: 190 C: 001 LIST "" % 191 S: * LIST () "/" Banana 192 S: * LIST ...etc... 193 S: 001 OK done 195 Internet DRAFT Implementation Recommendations September 1997 197 and then 198 C: 002 LIST "" Banana/% 199 S: * LIST () "/" Banana/Apple 200 S: * LIST ...etc... 201 S: 002 OK done 203 Using this technique the client's user interface can give the user 204 full flexibility without choking on the voluminous reply to "LIST *". 205 Of course, it is still possible that the reply to 206 C: 005 LIST "" alt.fan.celebrity.% 207 may be thousands of entries long, and there is, unfortunately, 208 nothing the client can do to protect itself from that. This has not 209 yet been a notable problem. 211 Servers that may export circular hierarchies (any server that 212 directly presents a UNIX file system, for instance) SHOULD limit the 213 hierarchy depth to prevent unlimited LIST responses. A suggested 214 depth limit is 20 hierarchy levels. 216 3.2.1.2. Fetching the List of Messages 218 When a client selects a mailbox, it is given a count, in the untagged 219 EXISTS response, of the messages in the mailbox. This number can be 220 very large. In such a case it might be unwise to use 221 C: 004 FETCH 1:* ALL 222 to populate the user's view of the mailbox. One good method to avoid 223 problems with this is to batch the requests, thus: 225 C: 004 FETCH 1:50 ALL 226 S: * 1 FETCH ...etc... 227 S: 004 OK done 228 C: 005 FETCH 51:100 ALL 229 S: * 51 FETCH ...etc... 230 S: 005 OK done 231 C: 006 FETCH 101:150 ALL 232 ...etc... 234 Using this method, another command, such as "FETCH 6 BODY[1]" can be 235 inserted as necessary, and the client will not have its access to the 236 server blocked by a storm of FETCH replies. (Such a method could be 237 reversed to fetch the LAST 50 messages first, then the 50 prior to 238 that, and so on.) 240 As a smart extension of this, a well designed client, prepared for 241 very large mailboxes, will not automatically fetch data for all 242 messages AT ALL. Rather, the client will populate the user’s view 243 only as the user sees it, possibly pre-fetching selected information, 244 and only fetching other information as the user scrolls to it. For 246 Internet DRAFT Implementation Recommendations September 1997 248 example, to select only those messages beginning with the first 249 unseen one: 251 C: 003 SELECT INBOX 252 S: * 10000 EXISTS 253 S: * 80 RECENT 254 S: * FLAGS (\Answered \Flagged \Deleted \Draft \Seen) 255 S: * OK [UIDVALIDITY 824708485] UID validity status 256 S: * OK [UNSEEN 9921] First unseen message 257 S: 003 OK [READ-WRITE] SELECT completed 258 C: 004 FETCH 9921:* ALL 259 ... etc... 261 If the server does not return an OK [UNSEEN] response, the client may 262 use SEARCH UNSEEN to obtain that value. 264 This mechanism is good as a default presentation method, but only 265 works well if the default message order is acceptable. A client may 266 want to present various sort orders to the user (by subject, by date 267 sent, by sender, and so on) and in that case (lacking a SORT 268 extension on the server side) the client WILL have to retrieve all 269 message descriptors. A client that provides this service SHOULD NOT 270 do it by default and SHOULD inform the user of the costs of choosing 271 this option for large mailboxes. 273 3.2.1.3. Fetching a Large Body Part 275 The issue here is similar to the one for a list of messages. In the 276 BODYSTRUCTURE response the client knows the size, in bytes, of the 277 body part it plans to fetch. Suppose this is a 70 MB video clip. 278 The client can use partial fetches to retrieve the body part in 279 pieces, avoiding the problem of an uninterruptible 70 MB literal 280 coming back from the server: 282 C: 022 FETCH 3 BODY[1]<0.20000> 283 S: * 3 FETCH (FLAGS(\Seen) BODY[1]<0> {20000} 284 S: ...data...) 285 S: 022 OK done 286 C: 023 FETCH 3 BODY[1]<20001.20000> 287 S: * 3 FETCH (BODY[1]<20001> {20000} 288 S: ...data...) 289 S: 023 OK done 290 C: 024 FETCH 3 BODY[1]<40001.20000> 291 ...etc... 293 Internet DRAFT Implementation Recommendations September 1997 295 3.2.1.4. BODYSTRUCTURE vs. Entire Messages 297 Because FETCH BODYSTRUCTURE is necessary in order to determine the 298 number of body parts, and, thus, whether a message has "attachments", 299 clients often use FETCH FULL as their normal method of populating the 300 user's view of a mailbox. The benefit is that the client can display 301 a paperclip icon or some such indication along with the normal 302 message summary. However, this comes at a significant cost with some 303 server configurations. The parsing needed to generate the FETCH 304 BODYSTRUCTURE response may be time-consuming compared with that 305 needed for FETCH ENVELOPE. The client developer should consider this 306 issue when deciding whether the ability to add a paperclip icon is 307 worth the tradeoff in performance, especially with large mailboxes. 309 Some clients, rather than using FETCH BODYSTRUCTURE, use FETCH BODY[] 310 (or the equivalent FETCH RFC822) to retrieve the entire message. 311 They then do the MIME parsing in the client. This may give the 312 client slightly more flexibility in some areas (access, for instance, 313 to header fields that aren't returned in the BODYSTRUCTURE and 314 ENVELOPE responses), but it can cause severe performance problems by 315 forcing the transfer of all body parts when the user might only want 316 to see some of them - a user logged on by modem and reading a small 317 text message with a large ZIP file attached may prefer to read the 318 text only and save the ZIP file for later. Therefore, a client 319 SHOULD NOT normally retrieve entire messages and SHOULD retrieve 320 message body parts selectively. 322 3.2.1.5. Long Command Lines 324 A client can wind up building a very long command line in an effort 325 to try to be efficient about requesting information from a server. 326 This can typically happen when a client builds a message set from 327 selected messages and doesn’t recognise that contiguous blocks of 328 messages may be group in a range. Suppose a user selects all 10,000 329 messages in a large mailbox and then unselects message 287. The 330 client could build that message set as "1:286,288:10000", but a 331 client that doesn’t handle that might try to enumerate each message 332 individually and build "1,2,3,4, [and so on] ,9999,10000". Adding 333 that to the fetch command results in a command line that’s almost 334 49,000 octets long, and, clearly, one can construct a command line 335 that’s even longer. 337 A client SHOULD limit the length of the command lines it generates to 338 approximately 1000 octets (including all quoted strings but not 339 including literals). If the client is unable to group things into 340 ranges so that the command line is within that length, it SHOULD 341 split the request into multiple commands. The client SHOULD use 342 literals instead of long quoted strings, in order to keep the command 344 Internet DRAFT Implementation Recommendations September 1997 346 length down. 348 For its part, a server SHOULD allow for a command line of at least 349 8000 octets. This provides plenty of leeway for accepting reasonable 350 length commands from clients. The server SHOULD send a BAD response 351 to a command that does not end within the server’s maximum accepted 352 command length. 354 3.2.2. Subscriptions 356 The client isn't the only entity that can get flooded: the end user, 357 too, may need some flood control. The IMAP4 protocol provides such 358 control in the form of subscriptions. Most servers support the 359 SUBSCRIBE, UNSUBSCRIBE, and LSUB commands, and many users choose to 360 narrow down a large list of available mailboxes by subscribing to the 361 ones that they usually want to see. Clients, with this in mind, 362 SHOULD give the user a way to see only subscribed mailboxes. A 363 client that never uses the LSUB command takes a significant usability 364 feature away from the user. Of course, the client would not want to 365 hide the LIST command completely; the user needs to be able to go 366 both ways. 368 3.2.3. Searching 370 IMAP SEARCH commands can become particularly troublesome (that is, 371 slow) on mailboxes containing a large number of messages. So let's 372 put a few things in perspective in that regard. 374 The flag searches SHOULD be fast. The flag searches (ALL, [UN]SEEN, 375 [UN]ANSWERED, [UN]DELETED, [UN]DRAFT, [UN]FLAGGED, NEW, OLD, RECENT) 376 are known to be used by clients for the client's own use (for 377 instance, some clients use "SEARCH UNSEEN" to find unseen mail and 378 "SEARCH DELETED" to warn the user before expunging messages). 380 Other searches, particularly the text searches (HEADER, TEXT, BODY) 381 are initiated by the user, rather than by the client itself, and 382 somewhat slower performance can be tolerated, since the user is aware 383 that the search is being done (and is probably aware that it might be 384 time-consuming). 386 The client MAY allow other commands to be sent to the server while a 387 SEARCH is in progress, but at the time of this writing there is 388 little or no server support for parallel processing of multiple 389 commands in the same session (and see "Multiple Accesses of the Same 390 Mailbox" above for a description of the dangers of trying to work 391 around this by doing your SEARCH in another session). 393 Internet DRAFT Implementation Recommendations September 1997 395 Another word about text searches: some servers, built on database 396 back-ends with indexed search capabilities, may return search results 397 that do not match the IMAP spec's "case-insensitive substring" 398 requirements. While these servers are in violation of the protocol, 399 there is little harm in the violation as long as the search results 400 are used only to response to a user's request. Still, developers of 401 such servers should be aware that they ARE violating the protocol, 402 should think carefully about that behaviour, and MUST be certain that 403 their servers respond accurately to the flag searches for the reasons 404 outlined above. 406 3.3 Avoiding Invalid Requests 408 IMAP4 provides ways for a server to tell a client in advance what is 409 and isn’t permitted in some circumstances. Clients SHOULD use these 410 features to avoid sending requests that a well designed client would 411 know to be invalid. This section explains this in more detail. 413 3.3.1. The CAPABILITY Command 415 All IMAP4 clients SHOULD use the CAPABILITY command to determine what 416 version of IMAP and what optional features a server supports. The 417 client SHOULD NOT send IMAP4rev1 commands and arguments to a server 418 that does not advertize IMAP4rev1 in its CAPABILITY response. 419 Similarly, the client SHOULD NOT send IMAP4 commands that no longer 420 exist in IMAP4rev1 to a server that does not advertize IMAP4 in its 421 CAPABILITY response. An IMAP4rev1 server is NOT required to support 422 obsolete IMAP4 or IMAP2bis commands (though some do; do not let this 423 fact lull you into thinking that it’s valid to send such commands to 424 an IMAP4rev1 server). 426 A client SHOULD NOT send commands to probe for the existance of 427 certain extensions. All standard and standards-track extensions 428 include CAPABILITY tokens indicating their presense. All private and 429 experimental extensions SHOULD do the same, and clients that take 430 advantage of them SHOULD use the CAPABILITY response to determine 431 whether they may be used or not. 433 3.3.2. Don’t Do What the Server Says You Can’t 435 In many cases, the server, in response to a command, will tell the 436 client something about what can and can’t be done with a particular 437 mailbox. The client SHOULD pay attention to this information and 438 SHOULD NOT try to do things that it’s been told it can’t do. 439 Examples: 440 * Do not try to SELECT a mailbox that has the \Noselect flag set. 442 Internet DRAFT Implementation Recommendations September 1997 444 * Do not try to CREATE a sub-mailbox in a mailbox that has the 445 \Noinferiors flag set. 446 * Do not respond to a failing COPY or APPEND command by trying to 447 CREATE the target mailbox if the server does not respond with a 448 [TRYCREATE] response code. 449 * Do not try to expunge a mailbox that has been selected with the 450 [READ-ONLY] response code. 452 3.4. Miscellaneous Protocol Considerations 454 We describe here a number of important protocol-related issues, the 455 misunderstanding of which has caused significant interoperability 456 problems in IMAP4 implementations. One general item is that every 457 implementer should be certain to take note of and to understand 458 section 2.2.2 and the preamble to section 7 of the IMAP4rev1 spec 459 [RFC-2060]. 461 3.4.1. Well Formed Protocol 463 We cannot stress enough the importance of adhering strictly to the 464 protocol grammar. The specification of the protocol is quite rigid; 465 do not assume that you can insert blank space for "readability" if 466 none is called for. Keep in mind that there are parsers out there 467 that will crash if there are protocol errors. There are clients that 468 will report every parser burp to the user. And in any case, 469 information that cannot be parsed is information that is lost. Be 470 careful in your protocol generation. And see "A Word About Testing", 471 below. 473 In particular, note that the string in the INTERNALDATE response is 474 NOT an RFC-822 date string - that is, it is not in the same format as 475 the first string in the ENVELOPE response. Since most clients will, 476 in fact, accept an RFC-822 date string in the INTERNALDATE response, 477 it’s easy to miss this in your interoperability testing. But it will 478 cause a problem with some client, so be sure to generate the correct 479 string for this field. 481 3.4.2. Special Characters 483 Certain characters, currently the double-quote and the backslash, may 484 not be sent as-is inside a quoted string. These characters MUST be 485 preceded by the escape character if they are in a quoted string, or 486 else the string must be sent as a literal. Both clients and servers 487 MUST handle this, both on output (they must send these characters 488 properly) and on input (they must be able to receive escaped 490 Internet DRAFT Implementation Recommendations September 1997 492 characters in quoted strings). Example: 494 C: 001 LIST "" % 495 S: * LIST () "" INBOX 496 S: * LIST () "\\" TEST 497 S: * LIST () "\\" {12} 498 S: "My" mailbox 499 S: 001 OK done 500 C: 002 LIST "" "\"My\" mailbox\\%" 501 S: * LIST () "\\" {17} 502 S: "My" mailbox\Junk 503 S: 002 OK done 505 Note that in the example the server sent the hierarchy delimiter as 506 an escaped character in the quoted string and sent the mailbox name 507 containing imbedded double-quotes as a literal. The client used only 508 quoted strings, escaping both the backslash and the double-quote 509 characters. 511 The CR and LF characters may be sent ONLY in literals; they are not 512 allowed, even if escaped, inside quoted strings. 514 3.4.3. UIDs and UIDVALIDITY 516 Servers that support existing back-end mail stores often have no good 517 place to save UIDs for messages. Often the existing mail store will 518 not have the concept of UIDs in the sense that IMAP has: strictly 519 increasing, never re-issued, 32-bit integers. Some servers solve 520 this by storing the UIDs in a place that's accessible to end users, 521 allowing for the possibility that the users will delete them. Others 522 solve it by re-assigning UIDs every time a mailbox is selected. 524 The server SHOULD maintain UIDs permanently for all messages if it 525 can. If that's not possible, the server MUST change the UIDVALIDITY 526 value for the mailbox whenever any of the UIDs may have become 527 invalid. Clients MUST recognize that the UIDVALIDITY has changed and 528 MUST respond to that condition by throwing away any information that 529 they have saved about UIDs in that mailbox. There have been many 530 problems in this area when clients have failed to do this; in the 531 worst case it will result in loss of mail when a client deletes the 532 wrong piece of mail by using a stale UID. 534 It seems to be a common myth that "the UIDVALIDITY and the UID, taken 535 together, form a 64-bit identifier that uniquely identifies a message 536 on a server". This is absolutely NOT TRUE. There is no assurance 537 that the UIDVALIDITY values of two mailboxes be different, so the 538 UIDVALIDITY in no way identifies a mailbox. The ONLY purpose of 539 UIDVALIDITY is, as its name indicates, to give the client a way to 541 Internet DRAFT Implementation Recommendations September 1997 543 check the validity of the UIDs it has cached. While it is a valid 544 implementation choice to put these values together to make a 64-bit 545 identifier for the message, the important concept here is that UIDs 546 are not unique between mailboxes; they are only unique WITHIN a given 547 mailbox. 549 Some server implementations have toyed with making UIDs unique across 550 the entire server. This is inadvisable, in that it limits the life 551 of UIDs unnecessarily. The UID is a 32-bit number and will run out 552 in reasonably finite time if it's global across the server. If you 553 assign UIDs sequentially in one mailbox, you will not have to start 554 re-using them until you have had, at one time or another, 2**32 555 different messages in that mailbox. In the global case, you will 556 have to reuse them once you have had, at one time or another, 2**32 557 different messages in the entire mail store. Suppose your server has 558 around 8000 users registered (2**13). That gives an average of 2**19 559 UIDs per user. Suppose each user gets 32 messages (2**5) per day. 560 That gives you 2**14 days (16000+ days = about 45 years) before you 561 run out. That may seem like enough, but multiply the usage just a 562 little (a lot of spam, a lot of mailing list subscriptions, more 563 users) and you limit yourself too much. 565 What's worse is that if you have to wrap the UIDs, and, thus, you 566 have to change UIDVALIDITY and invalidate the UIDs in the mailbox, 567 you have to do it for EVERY mailbox in the system, since they all 568 share the same UID pool. If you assign UIDs per mailbox and you have 569 a problem, you only have to kill the UIDs for that one mailbox. 571 Under extreme circumstances (and this is extreme, indeed), the server 572 may have to invalidate UIDs while a mailbox is in use by a client - 573 that is, the UIDs that the client knows about in its active mailbox 574 are no longer valid. In that case, the server MUST immediately 575 change the UIDVALIDITY and MUST communicate this to the client. The 576 server MAY do this by sending an unsolicited UIDVALIDITY message, in 577 the same form as in response to the SELECT command. Clients MUST be 578 prepared to handle such a message and the possibly coincident failure 579 of the command in process. For example: 581 C: 032 UID STORE 382 +Flags.silent \Deleted 582 S: * OK [UIDVALIDITY 12345] New UIDVALIDITY value! 583 S: 032 NO UID command rejeced because UIDVALIDITY changed! 584 C: ...invalidates local information and re-fetches... 585 C: 033 FETCH 1:* UID 586 ...etc... 588 At the time of the writing of this document, the only server known to 589 do this does so only under the following condition: the client 590 selects INBOX, but there is not yet a physical INBOX file created. 591 Nonetheless, the SELECT succeeds, exporting an empty INBOX with a 593 Internet DRAFT Implementation Recommendations September 1997 595 temporary UIDVALIDITY of 1. While the INBOX remains selected, mail 596 is delivered to the user, which creates the real INBOX file and 597 assigns a permanent UIDVALIDITY (that is likely not to be 1). The 598 server reports the change of UIDVALIDITY, but as there were no 599 messages before, so no UIDs have actually changed, all the client 600 must do is accept the change in UIDVALIDITY. 602 Alternatively, a server may force the client to re-select the 603 mailbox, at which time it will obtain a new UIDVALIDITY value. To do 604 this, the server closes this client session (see "Severed 605 Connections" above) and the client then reconnects and gets back in 606 synch. Clients MUST be prepared for either of these behaviours. 608 We do not know of, nor do we anticipate the future existance of, a 609 server that changes UIDVALIDITY while there are existing messages, 610 but clients MUST be prepared to handle this eventuality. 612 3.4.4. FETCH Responses 614 When a client asks for certain information in a FETCH command, the 615 server MAY return the requested information in any order, not 616 necessarily in the order that it was requested. Further, the server 617 MAY return the information in separate FETCH responses and MAY also 618 return information that was not explicitly requested (to reflect to 619 the client changes in the state of the subject message). Some 620 examples: 622 C: 001 FETCH 1 UID FLAGS INTERNALDATE 623 S: * 5 FETCH (FLAGS (\Deleted)) 624 S: * 1 FETCH (FLAGS (\Seen) INTERNALDATE "..." UID 345) 625 S: 001 OK done 626 (In this case, the responses are in a different order. Also, the 627 server returned a flag update for message 5, which wasn't part of the 628 client's request.) 630 C: 002 FETCH 2 UID FLAGS INTERNALDATE 631 S: * 2 FETCH (INTERNALDATE "...") 632 S: * 2 FETCH (UID 399) 633 S: * 2 FETCH (FLAGS ()) 634 S: 002 OK done 635 (In this case, the responses are in a different order and were 636 returned in separate responses.) 638 C: 003 FETCH 2 BODY[1] 639 S: * 2 FETCH (FLAGS (\Seen) BODY[1] {14} 640 S: Hello world! 641 S: ) 642 S: 003 OK done 644 Internet DRAFT Implementation Recommendations September 1997 646 (In this case, the FLAGS response was added by the server, since 647 fetching the body part caused the server to set the \Seen flag.) 649 Because of this characteristic a client MUST be ready to receive any 650 FETCH response at any time and should use that information to update 651 its local information about the message to which the FETCH response 652 refers. A client MUST NOT assume that any FETCH responses will come 653 in any particular order, or even that any will come at all. If after 654 receiving the tagged response for a FETCH command the client finds 655 that it did not get all of the information requested, the client 656 SHOULD send a NOOP command to the server to ensure that the server 657 has an opportunity to send any pending EXPUNGE responses to the 658 client (see [RFC-2180]). 660 3.4.5. RFC822.SIZE 662 Some back-end mail stores keep the mail in a canonical form, rather 663 than retaining the original MIME format of the messages. This means 664 that the server must reassemble the message to produce a MIME stream 665 when a client does a fetch such as RFC822 or BODY[], requesting the 666 entire message. It also may mean that the server has no convenient 667 way to know the RFC822.SIZE of the message. Often, such a server 668 will actually have to build the MIME stream to compute the size, only 669 to throw the stream away and report the size to the client. 671 When this is the case, some servers have chosen to estimate the size, 672 rather than to compute it precisely. Such an estimate allows the 673 client to display an approximate size to the user and to use the 674 estimate in flood control considerations (q.v.), but requires that 675 the client not use the size for things such as allocation of buffers, 676 because those buffers might then be too small to hold the actual MIME 677 stream. Instead, a client SHOULD use the size that's returned in the 678 literal when you fetch the data. 680 The protocol requires that the RFC822.SIZE value returned by the 681 server be EXACT. Estimating the size is a protocol violation, and 682 server designers must be aware that, despite the performance savings 683 they might realize in using an estimate, this practice will cause 684 some clients to fail in various ways. If possible, the server SHOULD 685 compute the RFC822.SIZE for a particular message once, and then save 686 it for later retrieval. If that's not possible, the server MUST 687 compute the value exactly every time. Incorrect estimates do cause 688 severe interoperability problems with some clients. 690 3.4.6. Expunged Messages 692 If the server allows multiple connections to the same mailbox, it is 694 Internet DRAFT Implementation Recommendations September 1997 696 often possible for messages to be expunged in one client unbeknownst 697 to another client. Since the server is not allowed to tell the 698 client about these expunged messages in response to a FETCH command, 699 the server may have to deal with the issue of how to return 700 information about an expunged message. There was extensive 701 discussion about this issue, and the results of that discussion are 702 summarized in [RFC-2180]. See that reference for a detailed 703 explanation and for recommendations. 705 3.4.7. The Namespace Issue 707 Namespaces are a very muddy area in IMAP4 implementation right now 708 (see [NAMESPACE] for a proposal to clear the water a bit). Until the 709 issue is resolved, the important thing for client developers to 710 understand is that some servers provide access through IMAP to more 711 than just the user's personal mailboxes, and, in fact, the user's 712 personal mailboxes may be "hidden" somewhere in the user's default 713 hierarchy. The client, therefore, SHOULD provide a setting wherein 714 the user can specify a prefix to be used when accessing mailboxes. 715 If the user's mailboxes are all in "~/mail/", for instance, then the 716 user can put that string in the prefix. The client would then put 717 the prefix in front of any name pattern in the LIST and LSUB 718 commands: 719 C: 001 LIST "" ~/mail/% 720 (See also "Reference Names in the LIST Command" below.) 722 3.4.8. Creating Special-Use Mailboxes 724 It may seem at first that this is part of the namespace issue; it is 725 not, and is only indirectly related to it. A number of clients like 726 to create special-use mailboxes with particular names. Most 727 commonly, clients with a "trash folder" model of message deletion 728 want to create a mailbox with the name "Trash" or "Deleted". Some 729 clients want to create a "Drafts" mailbox, an "Outbox" mailbox, or a 730 "Sent Mail" mailbox. And so on. There are two major 731 interoperability problems with this practice: 732 1. different clients may use different names for mailboxes with 733 similar functions (such as "Trash" and "Deleted"), or may manage the 734 same mailboxes in different ways, causing problems if a user switches 735 between clients and 736 2. there is no guarantee that the server will allow the creation of 737 the desired mailbox. 739 The client developer is, therefore, well advised to consider 740 carefully the creation of any special-use mailboxes on the server, 741 and, further, the client MUST NOT require such mailbox creation - 742 that is, if you do decide to do this, you MUST handle gracefully the 744 Internet DRAFT Implementation Recommendations September 1997 746 failure of the CREATE command and behave reasonably when your 747 special-use mailboxes do not exist and can not be created. 749 In addition, the client developer SHOULD provide a convenient way for 750 the user to select the names for any special-use mailboxes, allowing 751 the user to make these names the same in all clients s/he uses and to 752 put them where s/he wants them. 754 3.4.9. Reference Names in the LIST Command 756 Many implementers of both clients and servers are confused by the 757 "reference name" on the LIST command. The reference name is intended 758 to be used in much the way a "cd" (change directory) command is used 759 on Unix, PC DOS, Windows, and OS/2 systems. That is, the mailbox 760 name is interpreted in much the same way as a file of that name would 761 be found if one had done a "cd" command into the directory specified 762 by the reference name. For example, in Unix we have the following: 764 > cd /u/jones/junk 765 > vi banana [file is "/u/jones/junk/banana"] 766 > vi stuff/banana [file is "/u/jones/junk/stuff/banana"] 767 > vi /etc/hosts [file is "/etc/hosts"] 769 The interoperability problems with this, in practice, are several. 770 First, while some IMAP servers are built on Unix or PC file systems, 771 many others are not, and the file system semantics do not make sense 772 in those configurations. Second, while some IMAP servers expose the 773 underlying file system to the clients, others allow access only to 774 the user's personal mailboxes, or to some other limited set of files, 775 making such file-system-like semantics less meaningful. Third, 776 because the IMAP spec leaves the interpretation of the reference name 777 as "implementation-dependent", the various server implementations 778 handle it in vastly differing ways, and fourth, many implementers 779 simply do not understand it and misuse it, do not use it, or ignore 780 it as a result. 782 The following statement gets somewhat into the religious issues that 783 we've tried to avoid scrupulously here; so be it: because of the 784 confusion around the reference name, its use by a client is a 785 dangerous thing, prone to result in interoperability problems. There 786 are servers that interpret it as originally intended; there are 787 servers that ignore it completely; there are servers that simply 788 prepend it to the mailbox name (with or without inserting a hierarchy 789 delimiter in between). Because a client can't know which of these 790 four behaviours to expect, a client SHOULD NOT use a reference name 791 itself, expecting a particular server behavior. However, a client 792 SHOULD permit a USER, by configuration, to use a reference name. 794 Internet DRAFT Implementation Recommendations September 1997 796 There is in no way universal agreement about the use or non-use of 797 the reference name. The last words here are, "Be aware." 799 3.4.10. Deleting Mailboxes 801 The protocol does not guarantee that a client may delete a mailbox 802 that is not empty, though on some servers it is permissible and is, 803 in fact, much faster than the alternative or deleting all the 804 messages from the client. If the client chooses to try to take 805 advantage of this possibility it MUST be prepared to use the other 806 method in the even that the more convenient one fails. Further, a 807 client SHOULD NOT try to delete the mailbox that it has selected, but 808 should first close that mailbox; some servers do not permit the 809 deletion of the selected mailbox. 811 Example: 812 [User tells the client to delete mailbox BANANA, which is 813 currently selected...] 814 C: 008 CLOSE 815 S: 008 OK done 816 C: 009 DELETE BANANA 817 S: 009 NO Delete failed; mailbox is not empty. 818 C: 010 SELECT BANANA 819 S: * ... untagged SELECT responses 820 S: 010 OK done 821 C: 011 STORE 1:* +FLAGS.SILENT \DELETED 822 S: 011 OK done 823 C: 012 CLOSE 824 S: 012 OK done 825 C: 013 DELETE BANANA 826 S: 013 OK done 828 3.5. A Word About Testing 830 Since the whole point of IMAP is interoperability, and since 831 interoperability can not be tested in a vacuum, the final 832 recommendation of this treatise is, "Test against EVERYTHING." Test 833 your client against every server you can get an account on. Test 834 your server with every client you can get your hands on. Many 835 clients make limited test versions available on the Web for the 836 downloading. Many server owners will give serious client developers 837 guest accounts for testing. Contact them and ask. NEVER assume that 838 because your client works with one or two servers, or because your 839 server does fine with one or two clients, you will interoperate well 840 in general. 842 In particular, in addition to everything else, be sure to test 844 Internet DRAFT Implementation Recommendations September 1997 846 against the reference implementations: the PINE client, the 847 University of Washington server, and the Cyrus server. 849 See the following URLs on the web for more information here: 850 IMAP Products and Sources: http://www.imap.org/products.html 851 IMC MailConnect: http://www.imc.org/imc-mailconnect 853 4. Security Considerations 855 This document describes behaviour of clients and servers that use the 856 IMAP4 protocol, and as such, has the same security considerations as 857 described in [RFC-2060]. 859 5. References 861 [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version 862 4rev1", RFC 2060, University of Washington, December 1996. 864 [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate 865 Requirement Levels", RFC 2119, Harvard University, March 1997. 867 [RFC-2180], Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC 868 2180, Microsoft, July 1997. 870 [NAMESPACE], Gahrns, M. & Newman, C., "IMAP4 Namespace", draft 871 document , June 1997. 873 6. Acknowledgments 875 To be completed... 877 7. Author's Address 879 Barry Leiba 880 IBM T.J. Watson Research Center 881 30 Saw Mill River Road 882 Hawthorne, NY 10532 884 Phone: 1-914-784-7941 885 Email: leiba@watson.ibm.com