File: rfc1460.txt

package info (click to toggle)
doc-rfc 20181229-2
  • links: PTS, VCS
  • area: non-free
  • in suites: buster
  • size: 570,944 kB
  • sloc: xml: 285,646; sh: 107; python: 90; perl: 42; makefile: 14
file content (955 lines) | stat: -rw-r--r-- 38,827 bytes parent folder | download | duplicates (7)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955






Network Working Group                                            M. Rose
Request for Comments: 1460                  Dover Beach Consulting, Inc.
Obsoletes: 1225                                                June 1993


                    Post Office Protocol - Version 3


Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Overview

   This memo is a revision to RFC 1225, a Draft Standard.  It makes the
   following changes from that document:

        - the RPOP facility is removed;

        - the optional APOP facility is added (which is in interoperable,
          operational use in at least three implementations);

        - a typo was corrected with respect to the interaction of LAST
          and RSET;

        - section numbers were added; and,

        - an acknowledgements section was added.

1. Introduction

   On certain types of smaller nodes in the Internet it is often
   impractical to maintain a message transport system (MTS).  For
   example, a workstation may not have sufficient resources (cycles,
   disk space) in order to permit a SMTP server [RFC821] and associated
   local mail delivery system to be kept resident and continuously
   running.

   Similarly, it may be expensive (or impossible) to keep a personal
   computer interconnected to an IP-style network for long amounts of
   time (the node is lacking the resource known as "connectivity").

   Despite this, it is often very useful to be able to manage mail on
   these smaller nodes, and they often support a user agent (UA) to aid



Rose                                                            [Page 1]

RFC 1460                          POP3                         June 1993


   the tasks of mail handling.  To solve this problem, a node which can
   support an MTS entity offers a maildrop service to these less endowed
   nodes.  The Post Office Protocol - Version 3 (POP3) is intended to
   permit a workstation to dynamically access a maildrop on a server
   host in a useful fashion.  Usually, this means that the POP3 is used
   to allow a workstation to retrieve mail that the server is holding
   for it.

   For the remainder of this memo, the term "client host" refers to a
   host making use of the POP3 service, while the term "server host"
   refers to a host which offers the POP3 service.

2. A Short Digression

   This memo does not specify how a client host enters mail into the
   transport system, although a method consistent with the philosophy of
   this memo is presented here:

      When the user agent on a client host wishes to enter a message
      into the transport system, it establishes an SMTP connection to
      its relay host (this relay host could be, but need not be, the
      POP3 server host for the client host).

   If this method is followed, then the client host appears to the MTS
   as a user agent, and should NOT be regarded as a "trusted" MTS entity
   in any sense whatsoever.  This concept, along with the role of the
   POP3 as a part of a split-UA model is discussed later in this memo.

3. Basic Operation

   Initially, the server host starts the POP3 service by listening on
   TCP port 110.  When a client host wishes to make use of the service,
   it establishes a TCP connection with the server host.  When the
   connection is established, the POP3 server sends a greeting.  The
   client and POP3 server then exchange commands and responses
   (respectively) until the connection is closed or aborted.

   Commands in the POP3 consist of a keyword possibly followed by an
   argument.  All commands are terminated by a CRLF pair.

   Responses in the POP3 consist of a success indicator and a keyword
   possibly followed by additional information.  All responses are
   terminated by a CRLF pair.  There are currently two success
   indicators: positive ("+OK") and negative ("-ERR").

   Responses to certain commands are multi-line.  In these cases, which
   are clearly indicated below, after sending the first line of the
   response and a CRLF, any additional lines are sent, each terminated



Rose                                                            [Page 2]

RFC 1460                          POP3                         June 1993


   by a CRLF pair.  When all lines of the response have been sent, a
   final line is sent, consisting of a termination octet (decimal code
   046, ".") and a CRLF pair.  If any line of the multi-line response
   begins with the termination octet, the line is "byte-stuffed" by
   pre-pending the termination octet to that line of the response.

   Hence a multi-line response is terminated with the five octets
   "CRLF.CRLF".  When examining a multi-line response, the client checks
   to see if the line begins with the termination octet.  If so and if
   octets other than CRLF follow, the the first octet of the line (the
   termination octet) is stripped away.  If so and if CRLF immediately
   follows the termination character, then the response from the POP
   server is ended and the line containing ".CRLF" is not considered
   part of the multi-line response.

   A POP3 session progresses through a number of states during its
   lifetime.  Once the TCP connection has been opened and the POP3
   server has sent the greeting, the session enters the AUTHORIZATION
   state.  In this state, the client must identify itself to the POP3
   server.  Once the client has successfully done this, the server
   acquires resources associated with the client's maildrop, and the
   session enters the TRANSACTION state.  In this state, the client
   requests actions on the part of the POP3 server.  When the client has
   finished its transactions, the session enters the UPDATE state.  In
   this state, the POP3 server releases any resources acquired during
   the TRANSACTION state and says goodbye.  The TCP connection is then
   closed.

4. The AUTHORIZATION State

   Once the TCP connection has been opened by a POP3 client, the POP3
   server issues a one line greeting.  This can be any string terminated
   by CRLF.  An example might be:

      S.  +OK POP3 server ready

   Note that this greeting is a POP3 reply.  The POP3 server should
   always give a positive response as the greeting.

   The POP3 session is now in the AUTHORIZATION state.  The client must
   now issue the USER command.  If the POP3 server responds with a
   positive success indicator ("+OK"), then the client may issue either
   the PASS command to complete the authorization, or the QUIT command
   to terminate the POP3 session.  If the POP3 server responds with a
   negative success indicator ("-ERR") to the USER command, then the
   client may either issue a new USER command or may issue the QUIT
   command.




Rose                                                            [Page 3]

RFC 1460                          POP3                         June 1993


   When the client issues the PASS command, the POP3 server uses the
   argument pair from the USER and PASS commands to determine if the
   client should be given access to the appropriate maildrop.  If so,
   the POP3 server then acquires an exclusive-access lock on the
   maildrop.  If the lock is successfully acquired, the POP3 server
   parses the maildrop into individual messages (read note below),
   determines the last message (if any) present in the maildrop that was
   referenced by the RETR command, and responds with a positive success
   indicator.  The POP3 session now enters the TRANSACTION state.  If
   the lock can not be acquired or the client should is denied access to
   the appropriate maildrop or the maildrop can't be parsed for some
   reason, the POP3 server responds with a negative success indicator.
   (If a lock was acquired but the POP3 server intends to respond with a
   negative success indicator, the POP3 server must release the lock
   prior to rejecting the command.)  At this point, the client may
   either issue a new USER command and start again, or the client may
   issue the QUIT command.

                 NOTE: Minimal implementations of the POP3 need only be
                 able to break a maildrop into its component messages;
                 they need NOT be able to parse individual messages.
                 More advanced implementations may wish to have this
                 capability, for reasons discussed later.

   After the POP3 server has parsed the maildrop into individual
   messages, it assigns a message-id to each message, and notes the size
   of the message in octets.  The first message in the maildrop is
   assigned a message-id of "1", the second is assigned "2", and so on,
   so that the n'th message in a maildrop is assigned a message-id of
   "n".  In POP3 commands and responses, all message-id's and message
   sizes are expressed in base-10 (i.e., decimal).

   It sets the "highest number accessed" to be that of the last message
   referenced by the RETR command.

   Here are summaries for the three POP3 commands discussed thus far:

           USER name
               Arguments: a server specific user-id (required)
               Restrictions: may only be given in the AUTHORIZATION
                   state after the POP3 greeting or after an
                   unsuccessful USER or PASS command
               Possible Responses:
                   +OK name is welcome here
                   -ERR never heard of name
               Examples:
                   C:    USER mrose
                   S:    +OK mrose is a real hoopy frood



Rose                                                            [Page 4]

RFC 1460                          POP3                         June 1993


                     ...
                   C:    USER frated
                   S:    -ERR sorry, frated doesn't get his mail here

           PASS string
               Arguments: a server/user-id specific password (required)
               Restrictions: may only be given in the AUTHORIZATION
                   state after a successful USER command
               Possible Responses:
                   +OK maildrop locked and ready
                   -ERR invalid password
                   -ERR unable to lock maildrop
               Examples:
                   C:    USER mrose
                   S:    +OK mrose is a real hoopy frood
                   C:    PASS secret
                   S:    +OK mrose's maildrop has 2 messages
                         (320 octets)
                     ...
                   C:    USER mrose
                   S:    +OK mrose is a real hoopy frood
                   C:    PASS secret
                   S:    -ERR unable to lock mrose's maildrop, file
                         already locked

           QUIT
               Arguments: none
               Restrictions: none
               Possible Responses:
                   +OK
               Examples:
                   C:    QUIT
                   S:    +OK dewey POP3 server signing off

5. The TRANSACTION State

   Once the client has successfully identified itself to the POP3 server
   and the POP3 server has locked and burst the appropriate maildrop,
   the POP3 session is now in the TRANSACTION state.  The client may now
   issue any of the following POP3 commands repeatedly.  After each
   command, the POP3 server issues a response.  Eventually, the client
   issues the QUIT command and the POP3 session enters the UPDATE state.

   Here are the POP3 commands valid in the TRANSACTION state:

           STAT
               Arguments: none
               Restrictions: may only be given in the TRANSACTION state.



Rose                                                            [Page 5]

RFC 1460                          POP3                         June 1993


               Discussion:

                 The POP3 server issues a positive response with a line
                 containing information for the maildrop.  This line is
                 called a "drop listing" for that maildrop.

                 In order to simplify parsing, all POP3 servers are
                 required to use a certain format for drop listings.
                 The first octets present must indicate the number of
                 messages in the maildrop.  Following this is the size
                 of the maildrop in octets.  This memo makes no
                 requirement on what follows the maildrop size.
                 Minimal implementations should just end that line of
                 the response with a CRLF pair.  More advanced
                 implementations may include other information.

                      NOTE: This memo STRONGLY discourages
                      implementations from supplying additional
                      information in the drop listing.  Other,
                      optional, facilities are discussed later on
                      which permit the client to parse the messages
                      in the maildrop.

                 Note that messages marked as deleted are not counted in
                 either total.

               Possible Responses:
                   +OK nn mm
               Examples:
                   C:    STAT
                   S:    +OK 2 320

           LIST [msg]
               Arguments: a message-id (optionally)  If a message-id is
                   given, it may NOT refer to a message marked as
                   deleted.
               Restrictions: may only be given in the TRANSACTION state.
               Discussion:

                 If an argument was given and the POP3 server issues a
                 positive response with a line containing information
                 for that message.  This line is called a "scan listing"
                 for that message.

                 If no argument was given and the POP3 server issues a
                 positive response, then the response given is
                 multi-line.  After the initial +OK, for each message
                 in the maildrop, the POP3 server responds with a line



Rose                                                            [Page 6]

RFC 1460                          POP3                         June 1993


                 containing information for that message.  This line
                 is called a "scan listing" for that message.

                 In order to simplify parsing, all POP3 servers are
                 required to use a certain format for scan listings.
                 The first octets present must be the message-id of
                 the message.  Following the message-id is the size of
                 the message in octets.  This memo makes no requirement
                 on what follows the message size in the scan listing.
                 Minimal implementations should just end that line of
                 the response with a CRLF pair.  More advanced
                 implementations may include other information, as
                 parsed from the message.

                      NOTE: This memo STRONGLY discourages
                      implementations from supplying additional
                      information in the scan listing.  Other, optional,
                      facilities are discussed later on which permit
                      the client to parse the messages in the maildrop.

                 Note that messages marked as deleted are not listed.

               Possible Responses:
                   +OK scan listing follows
                   -ERR no such message
               Examples:
                   C:    LIST
                   S:    +OK 2 messages (320 octets)
                   S:    1 120
                   S:    2 200
                   S:    .
                     ...
                   C:    LIST 2
                   S:    +OK 2 200
                     ...
                   C:    LIST 3
                   S:    -ERR no such message, only 2 messages in
                         maildrop

           RETR msg
               Arguments: a message-id (required)  This message-id may
                   NOT refer to a message marked as deleted.
               Restrictions: may only be given in the TRANSACTION state.
               Discussion:

                 If the POP3 server issues a positive response, then the
                 response given is multi-line.  After the initial +OK,
                 the POP3 server sends the message corresponding to the



Rose                                                            [Page 7]

RFC 1460                          POP3                         June 1993


                 given message-id, being careful to byte-stuff the
                 termination character (as with all multi-line
                 responses).

                 If the number associated with this message is higher
                 than the "highest number accessed" in the maildrop, the
                 POP3 server updates the "highest number accessed" to
                 the number associated with this message.

               Possible Responses:
                   +OK message follows
                   -ERR no such message
               Examples:
                   C:    RETR 1
                   S:    +OK 120 octets
                   S:    <the POP3 server sends the entire message here>
                   S:    .

           DELE msg
               Arguments: a message-id (required)  This message-id
                   may NOT refer to a message marked as deleted.
               Restrictions: may only be given in the TRANSACTION state.
               Discussion:

                 The POP3 server marks the message as deleted.  Any
                 future reference to the message-id associated with the
                 message in a POP3 command generates an error.  The POP3
                 server does not actually delete the message until the
                 POP3 session enters the UPDATE state.

                 If the number associated with this message is higher
                 than the "highest number accessed" in the maildrop,
                 the POP3 server updates the "highest number accessed"
                 to the number associated with this message.

               Possible Responses:
                   +OK message deleted
                   -ERR no such message
               Examples:
                   C:    DELE 1
                   S:    +OK message 1 deleted
                     ...
                   C:    DELE 2
                   S:    -ERR message 2 already deleted

           NOOP
               Arguments: none
               Restrictions: may only be given in the TRANSACTION state.



Rose                                                            [Page 8]

RFC 1460                          POP3                         June 1993


               Discussion:

                 The POP3 server does nothing, it merely replies with a
                 positive response.

               Possible Responses:
                   +OK
               Examples:
                   C:    NOOP
                   S:    +OK

           LAST
               Arguments: none
               Restrictions: may only be issued in the TRANSACTION state.
               Discussion:

                 The POP3 server issues a positive response with a line
                 containing the highest message number which accessed.
                 Zero is returned in case no message in the maildrop has
                 been accessed during previous transactions.  A client
                 may thereafter infer that messages, if any, numbered
                 greater than the response to the LAST command are
                 messages not yet accessed by the client.

             Possible Response:
                   +OK nn

             Examples:
                   C:      STAT
                   S:      +OK 4 320
                   C:      LAST
                   S:      +OK 1
                   C:      RETR 3
                   S:      +OK 120 octets
                   S:      <the POP3 server sends the entire message
                           here>
                   S:      .
                   C:      LAST
                   S:      +OK 3
                   C:      DELE 2
                   S:      +OK message 2 deleted
                   C:      LAST
                   S:      +OK 3
                   C:      RSET
                   S:      +OK
                   C:      LAST
                   S:      +OK 0




Rose                                                            [Page 9]

RFC 1460                          POP3                         June 1993


           RSET
               Arguments: none
               Restrictions: may only be given in the TRANSACTION
                   state.
               Discussion:

                 If any messages have been marked as deleted by the POP3
                 server, they are unmarked.  The POP3 server then
                 replies with a positive response.  In addition, the
                 "highest number accessed" is also reset to zero.

               Possible Responses:
                   +OK
               Examples:
                   C:    RSET
                   S:    +OK maildrop has 2 messages (320 octets)

6. The UPDATE State

   When the client issues the QUIT command from the TRANSACTION state,
   the POP3 session enters the UPDATE state.  (Note that if the client
   issues the QUIT command from the AUTHORIZATION state, the POP3
   session terminates but does NOT enter the UPDATE state.)

           QUIT
               Arguments: none
               Restrictions: none
               Discussion:

                 The POP3 server removes all messages marked as deleted
                 from the maildrop.  It then releases the
                 exclusive-access lock on the maildrop and replies as
                 to the success of these operations.  The TCP
                 connection is then closed.

               Possible Responses:
                   +OK
               Examples:
                   C:    QUIT
                   S:    +OK dewey POP3 server signing off (maildrop
                         empty)
                     ...
                   C:    QUIT
                   S:    +OK dewey POP3 server signing off (2 messages
                         left)
                     ...





Rose                                                           [Page 10]

RFC 1460                          POP3                         June 1993


7. Optional POP3 Commands

   The POP3 commands discussed above must be supported by all minimal
   implementations of POP3 servers.

   The optional POP3 commands described below permit a POP3 client
   greater freedom in message handling, while preserving a simple POP3
   server implementation.

                 NOTE: This memo STRONGLY encourages implementations to
                 support these commands in lieu of developing augmented
                 drop and scan listings.  In short, the philosophy of
                 this memo is to put intelligence in the part of the
                 POP3 client and not the POP3 server.

           TOP msg n
               Arguments: a message-id (required) and a number.  This
                   message-id may NOT refer to a message marked as
                   deleted.
               Restrictions: may only be given in the TRANSACTION state.
               Discussion:

                 If the POP3 server issues a positive response, then
                 the response given is multi-line.  After the initial
                 +OK, the POP3 server sends the headers of the message,
                 the blank line separating the headers from the body,
                 and then the number of lines indicated message's body,
                 being careful to byte-stuff the termination character
                 (as with all multi-line responses).

                 Note that if the number of lines requested by the POP3
                 client is greater than than the number of lines in the
                 body, then the POP3 server sends the entire message.

               Possible Responses:
                   +OK top of message follows
                   -ERR no such message
               Examples:
                   C:    TOP 10
                   S:    +OK
                   S:    <the POP3 server sends the headers of the
                          message, a blank line, and the first 10 lines
                          of the body of the message>
                   S:    .
                     ...
                   C:    TOP 100
                   S:    -ERR no such message




Rose                                                           [Page 11]

RFC 1460                          POP3                         June 1993


           APOP name digest
               Arguments: a server specific user-id and a digest string
                    (both required).
               Restrictions: may only be given in the AUTHORIZATION
                   state after the POP3 greeting
               Discussion:

                 Normally, each POP3 session starts with a USER/PASS
                 exchange.  This results in a server/user-id specific
                 password being sent in the clear on the network.  For
                 intermittent use of POP3, this may not introduce a
                 sizable risk.  However, many POP3 client
                 implementations connect to the POP3 server on a
                 regular basis -- to check for new mail.  Further the
                 interval of session initiation may be on the order of
                 five minutes.  Hence, the risk of password capture is
                 greatly enhanced.

                 An alternate method of authentication is required
                 which provides for both origin authentication and
                 replay protection, but which does not involve sending
                 a password in the clear over the network.  The APOP
                 command provides this functionality.

                 A POP3 server which implements the APOP command will
                 include a timestamp in its banner greeting.  The
                 syntax of the timestamp corresponds to the "msg-id"
                 in [RFC822], and MUST be different each time the POP3
                 server issues a banner greeting.  For example, on a
                 UNIX implementation in which a separate UNIX process
                 is used for each instance of a POP3 server, the
                 syntax of the timestamp might be:

                    <process-ID.clock@hostname>

                 where "process-ID" is the decimal value of the
                 process's PID, clock is the decimal value of the
                 system clock, and hostname is the fully-qualified
                 domain-name corresponding to the host where the POP3
                 server is running.

                 The POP3 client makes note of this timestamp, and
                 then issues the APOP command.  The "name" parameter
                 has identical semantics to the "name" parameter of
                 the USER command. The "digest" parameter is
                 calculated by applying the MD5 algorithm [RFC1321] to
                 a string consisting of the timestamp (including
                 angle-brackets) followed by a shared secret.  This



Rose                                                           [Page 12]

RFC 1460                          POP3                         June 1993


                 shared secret is a string known only to the POP3
                 client and server. Great care should be taken to
                 prevent unauthorized disclosure of the secret, as
                 knowledge of the secret will allow any entity to
                 successfully masquerade as the named user.  The
                 "digest" parameter itself is a 16-octet value which
                 is sent in hexadecimal format, using lower-case ASCII
                 characters.

                 When the POP3 server receives the APOP command, it
                 verifies the digest provided.  If the digest is
                 correct, the POP3 server issues a positive response,
                 and the POP3 session enters the TRANSACTION state.
                 Otherwise, a negative response is issued and the POP3
                 session remains in the AUTHORIZATION state.

               Possible Responses:
                   +OK maildrop locked and ready
                   -ERR permission denied
               Examples:
                   S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
                   C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
                   S: +OK maildrop has 1 message (369 octets)

                 In this example, the shared secret is the string "tanstaaf".
                 Hence, the MD5 algorithm is applied to the string

                    <1896.697170952@dbc.mtview.ca.us>tanstaaf

                 which produces a digest value of

                    c4c9334bac560ecc979e58001b3e22fb

8. POP3 Command Summary

       Minimal POP3 Commands:
           USER name               valid in the AUTHORIZATION state
           PASS string
           QUIT

           STAT                    valid in the TRANSACTION state
           LIST [msg]
           RETR msg
           DELE msg
           NOOP
           LAST
           RSET




Rose                                                           [Page 13]

RFC 1460                          POP3                         June 1993


           QUIT                    valid in the UPDATE state

       Optional POP3 Commands:
           APOP name digest        valid in the AUTHORIZATION state

           TOP msg n               valid in the TRANSACTION state

       POP3 Replies:
           +OK
           -ERR

       Note that with the exception of the STAT command, the reply given
       by the POP3 server to any command is significant only to "+OK"
       and "-ERR".  Any text occurring after this reply may be ignored
       by the client.

9. Example POP3 Session

    S: <wait for connection on TCP port 110>
        ...
    C: <open connection>
    S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
    C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
    S:    +OK mrose's maildrop has 2 messages (320 octets)
    C:    STAT
    S:    +OK 2 320
    C:    LIST
    S:    +OK 2 messages (320 octets)
    S:    1 120
    S:    2 200
    S:    .
    C:    RETR 1
    S:    +OK 120 octets
    S:    <the POP3 server sends message 1>
    S:    .
    C:    DELE 1
    S:    +OK message 1 deleted
    C:    RETR 2
    S:    +OK 200 octets
    S:    <the POP3 server sends message 2>
    S:    .
    C:    DELE 2
    S:    +OK message 2 deleted
    C:    QUIT
    S:    +OK dewey POP3 server signing off (maildrop empty)
    C:  <close connection>
    S:  <wait for next connection>




Rose                                                           [Page 14]

RFC 1460                          POP3                         June 1993


10. Message Format

   All messages transmitted during a POP3 session are assumed to conform
   to the standard for the format of Internet text messages [RFC822].

   It is important to note that the byte count for a message on the
   server host may differ from the octet count assigned to that message
   due to local conventions for designating end-of-line.  Usually,
   during the AUTHORIZATION state of the POP3 session, the POP3 client
   can calculate the size of each message in octets when it parses the
   maildrop into messages.  For example, if the POP3 server host
   internally represents end-of-line as a single character, then the
   POP3 server simply counts each occurrence of this character in a
   message as two octets.  Note that lines in the message which start
   with the termination octet need not be counted twice, since the POP3
   client will remove all byte-stuffed termination characters when it
   receives a multi-line response.

11. The POP and the Split-UA model

   The underlying paradigm in which the POP3 functions is that of a
   split-UA model.  The POP3 client host, being a remote PC based
   workstation, acts solely as a client to the message transport system.
   It does not provide delivery/authentication services to others.
   Hence, it is acting as a UA, on behalf of the person using the
   workstation.  Furthermore, the workstation uses SMTP to enter mail
   into the MTS.

   In this sense, we have two UA functions which interface to the
   message transport system: Posting (SMTP) and Retrieval (POP3).  The
   entity which supports this type of environment is called a split-UA
   (since the user agent is split between two hosts which must
   interoperate to provide these functions).

                 ASIDE:  Others might term this a remote-UA instead.
                 There are arguments supporting the use of both terms.

   This memo has explicitly referenced TCP as the underlying transport
   agent for the POP3.  This need not be the case.  In the MZnet split-
   UA, for example, personal micro-computer systems are used which do
   not have IP-style networking capability [MZnet].  To connect to the
   POP3 server host, a PC establishes a terminal connection using some
   simple protocol (PhoneNet).  A program on the PC drives the
   connection, first establishing a login session as a normal user.  The
   login shell for this pseudo-user is a program which drives the other
   half of the terminal protocol and communicates with one of two
   servers.  Although MZnet can support several PCs, a single pseudo-
   user login is present on the server host.  The user-id and password



Rose                                                           [Page 15]

RFC 1460                          POP3                         June 1993


   for this pseudo-user login is known to all members of MZnet.  Hence,
   the first action of the login shell, after starting the terminal
   protocol, is to demand a USER/PASS authorization pair from the PC.
   This second level of authorization is used to ascertain who is
   interacting with the MTS.  Although the server host is deemed to
   support a "trusted" MTS entity, PCs in MZnet are not.  Naturally, the
   USER/PASS authorization pair for a PC is known only to the owner of
   the PC (in theory, at least).

   After successfully verifying the identity of the client, a modified
   SMTP server is started, and the PC posts mail with the server host.
   After the QUIT command is given to the SMTP server and it terminates,
   a modified POP3 server is started, and the PC retrieves mail from the
   server host.  After the QUIT command is given to the POP3 server and
   it terminates, the login shell for the pseudo-user terminates the
   terminal protocol and logs the job out.  The PC then closes the
   terminal connection to the server host.

   The SMTP server used by MZnet is modified in the sense that it knows
   that it's talking to a user agent and not a "trusted" entity in the
   message transport system.  Hence, it does performs the validation
   activities normally performed by an entity in the MTS when it accepts
   a message from a UA.

   The POP3 server used by MZnet is modified in the sense that it does
   not require a USER/PASS combination before entering the TRANSACTION
   state.  The reason for this (of course) is that the PC has already
   identified itself during the second-level authorization step
   described above.

                 NOTE: Truth in advertising laws require that the author
                 of this memo state that MZnet has not actually been
                 fully implemented.  The concepts presented and proven
                 by the project led to the notion of the MZnet
                 split-slot model.  This notion has inspired the
                 split-UA concept described in this memo, led to the
                 author's interest in the POP, and heavily influenced
                 the the description of the POP3 herein.

   In fact, some UAs present in the Internet already support the notion
   of posting directly to an SMTP server and retrieving mail directly
   from a POP3 server, even if the POP3 server and client resided on the
   same host!

                 ASIDE: this discussion raises an issue which this memo
                 purposedly avoids: how does SMTP know that it's talking
                 to a "trusted" MTS entity?




Rose                                                           [Page 16]

RFC 1460                          POP3                         June 1993


12. References

   [MZnet]   Stefferud, E., Sweet, J., and T. Domae, "MZnet: Mail
             Service for Personal Micro-Computer Systems,:
             Proceedings, IFIP 6.5 International Conference on
             Computer Message Systems, Nottingham, U.K., May 1984.

   [RFC821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
             RFC 821, USC/Information Sciences Institute, August 1982.

   [RFC822]  Crocker, D., "Standard for the Format of ARPA-Internet
             Text Messages", STD 11, RFC 822, University of Delaware,
             August 1982.

   [RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", MIT
             Laboratory for Computer Science, April 1992.

13. Security Considerations

   It is conjectured that use of the APOP command provides origin
   identification and replay protection for a POP3 session.
   Accordingly, a POP3 server which implements both the PASS and APOP
   commands must not allow both methods of access for a given user; that
   is, for a given "USER name" either the PASS or APOP command is
   allowed, but not both.

   Otherwise, security issues are not discussed in this memo.

14. Acknowledgements

   The POP family has a long and checkered history.  Although primarily
   a minor revision to [RFC1225], POP3 is based on the ideas presented
   in RFCs 918, 937, and 1081.

   In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
   provided significant comments on the APOP command.

15. Author's Address

   Marshall T. Rose
   Dover Beach Consulting, Inc.
   Mountain View, CA  94043-2186

   Phone: +1 415 968 1052
   Fax:   +1 415 968 2510

   EMail: mrose@dbc.mtview.ca.us
   X.500: rose, dbc, us



Rose                                                           [Page 17]