注意：除了STAT，LIST和UIDL的響應(yīng)外，其它命令的響應(yīng)均為"+OK"和 "-ERR"。響應(yīng)后的所有文本將被客戶(hù)略去。
9. POP3會(huì)話(huà)實(shí)例
S: <等待連接到TCP端口110>
C: <打開(kāi)連接>
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: <服務(wù)器發(fā)送信件1>
S: .
C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S: <服務(wù)器發(fā)送信件2>
S: .
C: DELE 2
S: +OK message 2 deleted
C: QUIT
S: +OK dewey POP3 server signing off (maildrop empty)
C: <關(guān)閉連接>
S: <等待下一次連接>
10. 消息格式
　　在會(huì)話(huà)過(guò)程中的消息格式都假定與Internet文本消息格式標(biāo)準(zhǔn)一致。應(yīng)該注意的是，由于各個(gè)服務(wù)器對(duì)于換行符的處理不同，因此計(jì)數(shù)不一定相同。通常，在"確認(rèn)"狀態(tài)中，服務(wù)器能夠以八進(jìn)制計(jì)算信件的大小。例如，如果在打開(kāi)儲(chǔ)存郵件時(shí)服務(wù)器內(nèi)部認(rèn)定換行符代表一個(gè)字符，一般服務(wù)器在計(jì)算它時(shí)作為兩個(gè)字符計(jì)。注意，以終止符開(kāi)始的消息行不被計(jì)數(shù)兩次，因?yàn)榭蛻?hù)將在接收到多行響應(yīng)后刪除所有字節(jié)填充。
11. 安全性考慮
　　可以推測(cè)，使用APOP命令可以提供會(huì)話(huà)期間的保護(hù)。相應(yīng)的，同時(shí)實(shí)現(xiàn)PASS和APOP命令的服務(wù)器只允許用戶(hù)以一種方式訪問(wèn)；也就是說(shuō)要么使用USER/PASS組合，要么使用APOP命令，不能同時(shí)使用兩個(gè)。
而且，注意隨著共享密鑰長(zhǎng)度的增加，解讀的難度也就上升了。服務(wù)器要提供用戶(hù)名時(shí)不給出任何響應(yīng)，不給出任何暗示此用戶(hù)名是否正確。而口令卻在網(wǎng)絡(luò)上顯式傳送；使用RETR和TOP命令在網(wǎng)絡(luò)上顯式傳送信件。
12. 參考資料
   [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", RFC 1321,
       MIT Laboratory for Computer Science, April 1992.

   [RFC1730] Crispin, M., "Internet Message Access Protocol - Version
       4", RFC 1730, University of Washington, December 1994.

   [RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734,
       Carnegie Mellon, December 1994.

14. 致謝
   The POP family has a long and checkered history.  Although primarily
   a minor revision to RFC 1460, 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. 作者地址
   John G. Myers
   Carnegie-Mellon University
   5000 Forbes Ave
   Pittsburgh, PA 15213

   EMail: jgm+@cmu.edu

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

   EMail: mrose@dbc.mtview.ca.us
Appendix A. Differences from RFC 1725
   This memo is a revision to RFC 1725, a Draft Standard.  It makes the
   following changes from that document:

      - clarifies that command keywords are case insensitive.

      - specifies that servers must send "+OK" and "-ERR" in
        upper case.

      - specifies that the initial greeting is a positive response,
        instead of any string which should be a positive response.

      - clarifies behavior for unimplemented commands.

      - makes the USER and PASS commands optional.

      - clarified the set of possible responses to the USER command.

      - reverses the order of the examples in the USER and PASS
        commands, to reduce confusion.

      - clarifies that the PASS command may only be given immediately
        after a successful USER command.

      - clarified the persistence requirements of UIDs and added some
        implementation notes.

      - specifies a UID length limitation of one to 70 octets.

      - specifies a status indicator length limitation
        of 512 octets, including the CRLF.

      - clarifies that LIST with no arguments on an empty mailbox
        returns success.

      - adds a reference from the LIST command to the Message Format
        section

      - clarifies the behavior of QUIT upon failure

      - clarifies the security section to not imply the use of the
        USER command with the APOP command.

      - adds references to RFCs 1730 and 1734

      - clarifies the method by which a UA may enter mail into the
        transport system.

      - clarifies that the second argument to the TOP command is a
        number of lines.

      - changes the suggestion in the Security Considerations section
        for a server to not accept both PASS and APOP for a given user
        from a "must" to a "should".

      - adds a section on scaling and operational considerations

Appendix B. Command Index
       APOP .......................................................   15
       DELE .......................................................    8
       LIST .......................................................    6
       NOOP .......................................................    9
       PASS .......................................................   14
       QUIT .......................................................    5
       QUIT .......................................................   10
       RETR .......................................................    8
       RSET .......................................................    9
       STAT .......................................................    6
       TOP ........................................................   11
       UIDL .......................................................   12
       USER .......................................................   13



RFC1939  Post Office Protocol - Version 3                                   RFC1939-POP3協(xié)議

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