pop.txt

早期freebsd实现

	      If the number associated with this message is higher than
	      the "highest number accessed" in the maildrop, the POP
	      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.
	    Discussion:

	      The POP 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 POP 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 POP 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 1

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

	      If any messages have been marked as deleted by the POP
	      server, they are unmarked.  The POP server then replies
	      with a positive response.  In addition, the "highest
	      number accessed" is also reset to the value determined
	      at the beginning of the POP session.

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



			   The UPDATE State

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

	QUIT
	    Arguments: none
	    Restrictions: none
	    Discussion:

	      The POP 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 POP server signing off (maildrop empty)
		  ...
		C:    QUIT
		S:    +OK dewey POP server signing off (2 messages left)
		  ...

			 Optional POP Commands

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

    The optional POP commands described below permit a POP client
    greater freedom in message handling, while preserving a simple POP
    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 POP client
	      and not the POP 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 POP server issues a positive response, then the
	      response given is multi-line.  After the initial +OK, the
	      POP 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
	      bit-stuff the termination character (as with all
	      multi-line responses).  

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

	    Possible Responses:
		+OK top of message follows
		-ERR no such message
	    Examples:
		C:    TOP 10
		S:    +OK
		S:    <the POP 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

	RPOP user
	    Arguments: a client specific user-id (required)
	    Restrictions: may only be given in the AUTHORIZATION state
		after a successful USER command; in addition, may
		only be given if the client used a reserved (privileged)
		TCP port to connect to the server.
	    Discussion:

	      The RPOP command may be used instead of the PASS command
	      to authenticate access to the maildrop.  In order for this
	      command to be successful, the POP client must use a
	      reserved TCP port (port < 1024) to connect to the server.
	      The POP server uses the argument pair from the USER and
	      RPOP commands to determine if the client should be given
	      access to the appropriate maildrop.  Unlike the PASS
	      command however, the POP server considers if the remote
	      user specified by the RPOP command who resides on the POP
	      client host is allowed to access the maildrop for the user
	      specified by the USER command (e.g., on Berkeley UNIX, the
	      .rhosts mechanism is used).  With the exception of this
	      differing in authentication, this command is identical to
	      the PASS command.  

	    Possible Responses:
		+OK maildrop locked and ready
		-ERR permission denied
	    Examples:
		C:    USER mrose
		S:    +OK mrose is a real hoopy frood
		C:    RPOP mrose
		S:    +OK mrose's maildrop has 2 messages (320 octets)

		       POP Command/Reply Summary

    Minimal POP 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

	QUIT			valid in the UPDATE state

    Optional POP Commands:
	RPOP user		valid in the AUTHORIZATION state

	TOP msg n		valid in the TRANSACTION state

    POP Replies:
	+OK
	-ERR

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

    S: <wait for connection on TCP port 109>
	...
    C: <open connection>
    S:    +OK dewey POP server ready (Comments to: PostMaster@UDel)
    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:    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 POP server sends message 1>
    S:	  .
    C:    DELE 1
    S:    +OK message 1 deleted
    C:    RETR 2
    S:    +OK 200 octets
    S:    <the POP server sends message 2>
    S:	  .
    C:    DELE 2
    S:    +OK message 2 deleted
    C:    QUIT
    S:    +OK dewey POP server signing off (maildrop empty)
    C:  <close connection>
    S:  <wait for next connection>



			    Message Format

    All messages transmitted during a POP session are assumed to
    conform to the standard for the format of ARPA 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 POP session, the POP
    client can calculate the size of each message in octets when it
    parses the maildrop into messages.  For example, if the POP server
    host internally represents end-of-line as a single character, then
    the POP 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 POP
    client will remove all bit-stuffed termination characters when it
    receives a multi-line response.



		    The POP and the Split-UA model

    The underlying paradigm in which the POP functions is that of a
    split-UA model.  The POP 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 (POP). 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 POP.  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.  To connect to
    the POP 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 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 POP server is started, and the PC retrieves
    mail from the server host. After the QUIT command is given to the
    POP 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 POP 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 POP
	      herein.

    In fact, some UAs present in the ARPA Internet already support the
    notion of posting directly to an SMTP server and retreiving mail
    directly from a POP server, even if the POP 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?

			      References

    [MZnet]	E.A. Stefferud, J.N. Sweet, T.P. 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]	J.B. Postel.
		"Simple Mail Transfer Protocol", USC/Information Sciences
		Institute. (August, 1982)

    [RFC822]	D.H. Crocker.
		"Standard for the Format of ARPA Internet Text
		Messages", University of Delaware.  (August, 1982)

    [RFC918]    J.K. Reynolds.
		"Post Office Protocol", USC/Information Sciences Institute.
		(October, 1984)

    [RFC923]    J.K. Reynolds, J.B. Postel.
		"Assigned Numbers", USC/Information Sciences Institute.
		(October, 1984)