Remove RFCs from the trunk, since we don't distribute them anyways. All of the removed RFCs are listed in the design-notes.html file, with the exception of NNTP (RFC977). Also add a link to the "LAN Mail Protocols" document to the design-notes.html file.

svn path=/trunk/; revision=4013

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diff --git a/RFC/rfc1081.txt b/RFC/rfc1081.txt
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-
-
-
-
-
-
-Network Working Group                                            M. Rose
-Request for Comments: 1081                                           TWG
-                                                           November 1988
-
-                    Post Office Protocol - Version 3
-
-
-Status of this Memo
-
-   This memo suggests a simple method for workstations to dynamically
-   access mail from a mailbox server.  This RFC specifies a proposed
-   protocol for the Internet community, and requests discussion and
-   suggestions for improvements.  Distribution of this memo is
-   unlimited.
-
-   This memo is based on RFC 918 (since revised as RFC 937).  Although
-   similar in form to the original Post Office Protocol (POP) proposed
-   for the Internet community, the protocol discussed in this memo is
-   similar in spirit to the ideas investigated by the MZnet project at
-   the University of California, Irvine.
-
-   Further, substantial work was done on examining POP in a PC-based
-   environment.  This work, which resulted in additional functionality
-   in this protocol, was performed by the ACIS Networking Systems Group
-   at Stanford University.  The author gratefully acknowledges their
-   interest.
-
-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 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
-   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.
-
-
-
-
-Rose                                                            [Page 1]
-
-RFC 1081                          POP3                     November 1988
-
-
-   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.
-
-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.
-
-   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
-   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
-
-
-
-Rose                                                            [Page 2]
-
-RFC 1081                          POP3                     November 1988
-
-
-   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.
-
-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 dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)
-
-   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.
-
-   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
-
-
-
-Rose                                                            [Page 3]
-
-RFC 1081                          POP3                     November 1988
-
-
-   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
-                     ...
-                   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
-
-
-
-Rose                                                            [Page 4]
-
-RFC 1081                          POP3                     November 1988
-
-
-                   -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
-
-
-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.
-               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
-
-
-
-Rose                                                            [Page 5]
-
-RFC 1081                          POP3                     November 1988
-
-
-                 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
-                 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
-
-
-
-Rose                                                            [Page 6]
-
-RFC 1081                          POP3                     November 1988
-
-
-                 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
-                 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.
-
-
-
-
-
-Rose                                                            [Page 7]
-
-RFC 1081                          POP3                     November 1988
-
-
-               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.
-               Discussion:
-
-                 The POP3 server does nothing, it merely replies with a
-                 positive response.
-
-               Possible Responses:
-                   +OK
-
-
-
-
-
-Rose                                                            [Page 8]
-
-RFC 1081                          POP3                     November 1988
-
-
-               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 1
-
-           RSET
-               Arguments: none
-               Restrictions: may only be given in the TRANSACTION
-                   state.
-               Discussion:
-
-                 If any messages have been marked as deleted by the POP3
-
-
-
-Rose                                                            [Page 9]
-
-RFC 1081                          POP3                     November 1988
-
-
-                 server, they are unmarked.  The POP3 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 POP3 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 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)
-                     ...
-
-
-Optional POP3 Commands
-
-   The POP3 commands discussed above must be supported by all minimal
-   implementations of POP3 servers.
-
-
-
-Rose                                                           [Page 10]
-
-RFC 1081                          POP3                     November 1988
-
-
-   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
-
-           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
-
-
-
-Rose                                                           [Page 11]
-
-RFC 1081                          POP3                     November 1988
-
-
-                   (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 POP3
-                 client must use a reserved TCP port (port < 1024) to
-                 connect tothe server.  The POP3 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 POP3 server considers if the remote user
-                 specified by the RPOP command who resides on the POP3
-                 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.
-
-                 Note that the use of this feature has allowed much wider
-                 penetration into numerous hosts on local networks (and
-                 sometimes remote networks) by those who gain illegal
-                 access to computers by guessing passwords or otherwise
-                 breaking into the system.
-
-               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)
-
-       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 12]
-
-RFC 1081                          POP3                     November 1988
-
-
-           QUIT                    valid in the UPDATE state
-
-       Optional POP3 Commands:
-           RPOP user               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.
-
-Example POP3 Session
-
-    S: <wait for connection on TCP port 110>
-        ...
-    C: <open connection>
-    S:    +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)
-    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 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
-
-
-
-
-
-Rose                                                           [Page 13]
-
-RFC 1081                          POP3                     November 1988
-
-
-    S:    +OK dewey POP3 server signing off (maildrop empty)
-    C:  <close connection>
-    S:  <wait for next connection>
-
-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.
-
-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.  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
-
-
-
-Rose                                                           [Page 14]
-
-RFC 1081                          POP3                     November 1988
-
-
-   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 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 POP server, even if the POP server and client resided on the
-   same host!
-
-                 ASIDE: this discussion raises an issue which this memo
-
-
-
-Rose                                                           [Page 15]
-
-RFC 1081                          POP3                     November 1988
-
-
-                 purposedly avoids: how does SMTP know that it's talking
-                 to a "trusted" MTS entity?
-
-References
-
-     [MZnet]   Stefferud, E., J. Sweet, 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",
-               USC/Information Sciences Institute, August 1982.
-
-     [RFC822]  Crocker, D., "Standard for the Format of ARPA-Internet
-               Text Messages", University of Delaware, August 1982.
-
-     [RFC937]  Butler, M., J. Postel, D. Chase, J. Goldberger, and J.
-               Reynolds, "Post Office Protocol - Version 2", RFC 937,
-               USC/Information Sciences Institute, February 1985.
-
-     [RFC1010] Reynolds, J., and J. Postel, "Assigned Numbers", RFC
-               1010, USC/Information Sciences Institute, May 1987.
-
-Author's Address:
-
-
-   Marshall Rose
-   The Wollongong Group
-   1129 San Antonio Rd.
-   Palo Alto, California 94303
-
-   Phone: (415) 962-7100
-
-   Email: MRose@TWG.COM
-
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-Rose                                                           [Page 16]
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