imapext-2007
view docs/rfc/rfc1732.txt @ 0:ada5e610ab86
imap-2007e
| author | yuuji@gentei.org |
|---|---|
| date | Mon, 14 Sep 2009 15:17:45 +0900 |
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7 Network Working Group M. Crispin
8 Request for Comments: 1732 University of Washington
9 Category: Informational December 1994
12 IMAP4 COMPATIBILITY WITH IMAP2 AND IMAP2BIS
15 Status of this Memo
17 This memo provides information for the Internet community. This memo
18 does not specify an Internet standard of any kind. Distribution of
19 this memo is unlimited.
21 Introduction
23 This is a summary of hints and recommendations to enable an IMAP4
24 implementation to interoperate with implementations that conform to
25 earlier specifications. None of these hints and recommendations are
26 required by the IMAP4 specification; implementors must decide for
27 themselves whether they want their implementation to fail if it
28 encounters old software.
30 IMAP4 has been designed to be upwards compatible with earlier
31 specifications. For the most part, IMAP4 facilities that were not in
32 earlier specifications should be invisible to clients unless the
33 client asks for the facility.
35 In some cases, older servers may support some of the capabilities
36 listed as being "new in IMAP4" as experimental extensions to the
37 IMAP2 protocol described in RFC 1176.
39 This information may not be complete; it reflects current knowledge
40 of server and client implementations as well as "folklore" acquired
41 in the evolution of the protocol.
58 Crispin [Page 1]
60 RFC 1732 IMAP4 - Compatibility December 1994
63 IMAP4 client interoperability with old servers
65 In general, a client should be able to discover whether an IMAP2
66 server supports a facility by trial-and-error; if an attempt to use a
67 facility generates a BAD response, the client can assume that the
68 server does not support the facility.
70 A quick way to check whether a server implementation supports the
71 IMAP4 specification is to try the CAPABILITY command. An OK response
72 that includes the IMAP4 capability value indicates a server that
73 supports IMAP4; a BAD response or one without the IMAP4 capability
74 value indicates an older server.
76 The following is a list of facilities that are only in IMAP4, and
77 suggestions for how new clients might interoperate with old servers:
79 CAPABILITY command
80 A BAD response to this command indicates that the server
81 implements IMAP2 (or IMAP2bis) and not IMAP4.
83 AUTHENTICATE command.
84 Use the LOGIN command.
86 LSUB and LIST commands
87 Try the RFC 1176 FIND command.
89 * in a sequence
90 Use the number of messages in the mailbox from the EXISTS
91 unsolicited response.
93 SEARCH extensions (character set, additional criteria)
94 Reformulate the search request using only the searching
95 options listed in search_old in the IMAP4 grammar. This may
96 entail doing multiple searches to achieve the desired
97 results.
99 BODYSTRUCTURE fetch data item
100 Try to fetch the non-extensible BODY data item.
102 body section number 0
103 Fetch the entire message and extract the header.
105 RFC822.HEADER.LINES and RFC822.HEADER.LINES.NOT fetch data items
106 Use RFC822.HEADER and remove the unwanted information.
108 BODY.PEEK[section], RFC822.PEEK, and RFC822.TEXT.PEEK fetch data
109 items Use the corresponding non-PEEK versions and manually
110 clear the \Seen flag as necessary.
114 Crispin [Page 2]
116 RFC 1732 IMAP4 - Compatibility December 1994
119 UID fetch data item and the UID commands
120 No equivalent capabilitity exists in older servers.
122 FLAGS.SILENT, +FLAGS.SILENT, and -FLAGS.SILENT store data items
123 Use the corresponding non-SILENT versions and ignore the
124 untagged FETCH responses which com eback.
127 The following IMAP4 facilities were introduced in the experimental
128 IMAP2bis revisions to RFC-1176, and may be present in a server that
129 does not support the CAPABILITY command:
131 CREATE, DELETE, and RENAME commands
132 To test whether these commands are present, try a CREATE
133 INBOX command. If the response is NO, these commands are
134 supported by the server. If the response is BAD, they are
135 not. Older servers without the CREATE capability may sup-
136 port implicit creation of a mailbox by a COPY command with a
137 non-existant name as the destination.
139 APPEND command
140 To test whether this command is present, try to append a
141 zero-length stream to a mailbox name that is known not to
142 exist (or at least, highly unlikely to exist) on the remote
143 system.
145 SUBSCRIBE and UNSUBSCRIBE commands
146 Try the form of these commands with the optional MAILBOX
147 keyword.
149 EXAMINE command
150 Use the SELECT command instead.
152 flags and internal date argument to APPEND command
153 Try the APPEND without any flag list and internal date argu-
154 ments.
156 BODY, BODY[section], and FULL fetch data items
157 Use RFC822.TEXT and ALL instead. Server does not support
158 MIME.
160 PARTIAL command
161 Use the appropriate FETCH command and ignore the unwanted
162 data.
165 IMAP4 client implementations must accept all responses and data for-
166 mats documented in the IMAP4 specification, including those labeled
170 Crispin [Page 3]
172 RFC 1732 IMAP4 - Compatibility December 1994
175 as obsolete. This includes the COPY and STORE unsolicited responses
176 and the old format of dates and times. In particular, client imple-
177 mentations must not treat a date/time as a fixed format string; nor
178 may they assume that the time begins at a particular octet.
180 IMAP4 client implementations must not depend upon the presence of any
181 server extensions that are not in the base IMAP4 specification.
183 The experimental IMAP2bis version specified that the TRYCREATE spe-
184 cial information token is sent as a separate unsolicited OK response
185 instead of inside the NO response.
187 The FIND BBOARDS, FIND ALL.BBOARDS, and BBOARD commands of RFC 1176
188 are removed from IMAP4. There is no equivalent to the bboard com-
189 mands, which provided a separate namespace with implicit restrictions
190 on what may be done in that namespace.
192 Older server implementations may automatically create the destination
193 mailbox on COPY if that mailbox does not already exist. This was how
194 a new mailbox was created in older specifications. If the server
195 does not support the CREATE command (see above for how to test for
196 this), it will probably create a mailbox on COPY.
198 Older server implementations may not preserve flags or internal dates
199 on COPY. Some server implementations may not permit the preservation
200 of certain flags on COPY or their setting with APPEND as site policy.
226 Crispin [Page 4]
228 RFC 1732 IMAP4 - Compatibility December 1994
231 IMAP4 server interoperability with old clients
233 In general, there should be no interoperation problem between a
234 server conforming to the IMAP4 specification and a well-written
235 client that conforms to an earlier specification. Known problems are
236 noted below:
238 Poor wording in the description of the CHECK command in earlier
239 specifications implied that a CHECK command is the way to get the
240 current number of messages in the mailbox. This is incorrect. A
241 CHECK command does not necessarily result in an EXISTS response.
242 Clients must remember the most recent EXISTS value sent from the
243 server, and should not generate unnecessary CHECK commands.
245 An incompatibility exists with COPY in IMAP4. COPY in IMAP4
246 servers does not automatically create the destination mailbox if
247 that mailbox does not already exist. This may cause problems with
248 old clients that expect automatic mailbox creation in COPY.
250 The PREAUTH unsolicited response is new in IMAP4. It is highly
251 unlikely that an old client would ever see this response.
253 The format of dates and times has changed due to the impending end
254 of the century. Clients that fail to accept a four-digit year or
255 a signed four-digit timezone value will not work properly with
256 IMAP4.
258 An incompatibility exists with the use of "\" in quoted strings.
259 This is best avoided by using literals instead of quoted strings
260 if "\" or <"> is embedded in the string.
262 Security Considerations
264 Security issues are not discussed in this memo.
266 Author's Address:
268 Mark R. Crispin
269 Networks and Distributed Computing, JE-30
270 University of Washington
271 Seattle, WA 98195
273 Phone: (206) 543-5762
275 EMail: MRC@CAC.Washington.EDU
282 Crispin [Page 5]