Network Working Group                                           J. Myers
Request for Comments: 1939                               Carnegie Mellon
STD: 53                                                          M. Rose
Obsoletes: 1725                             Dover Beach Consulting, Inc.
Category: Standards Track                                       May 1996


                    Post Office Protocol - Version 3                    

Status of this Memo                                                     

   This document specifies an Internet standards track protocol for the 
   Internet community, and requests discussion and suggestions for      
   improvements.  Please refer to the current edition of the "Internet  
   Official Protocol Standards" (STD 1) for the standardization state   
   and status of this protocol.  Distribution of this memo is unlimited.

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 
   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 protocol
   is used to allow a workstation to retrieve mail that the server is   
   holding for it.                                                      

   POP3 is not intended to provide extensive manipulation operations of 
   mail on the server; normally, mail is downloaded and then deleted.  A
   more advanced (and complex) protocol, IMAP4, is discussed in         
   [RFC1730].                                                           

   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 and sends all mail to it.  This relay host could   
      be, but need not be, the POP3 server host for the client host.  Of
      course, the relay host must accept mail for delivery to arbitrary 
      recipient addresses, that functionality is not required of all    
      SMTP servers.                                                     

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 case-insensitive keyword, possibly 
   followed by one or more arguments.  All commands are terminated by a 
   CRLF pair.  Keywords and arguments consist of printable ASCII        
   characters.  Keywords and arguments are each separated by a single   
   SPACE character.  Keywords are three or four characters long. Each   
   argument may be up to 40 characters long.                            

   Responses in the POP3 consist of a status indicator and a keyword    
   possibly followed by additional information.  All responses are      
   terminated by a CRLF pair.  Responses may be up to 512 characters    
   long, including the terminating CRLF.  There are currently two status
   indicators: positive ("+OK") and negative ("-ERR").  Servers MUST    
   send the "+OK" and "-ERR" in upper case.                             

   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 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
   issued the QUIT command, 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.                                                              

   A server MUST respond to an unrecognized, unimplemented, or          
   syntactically invalid command by responding with a negative status   
   indicator.  A server MUST respond to a command issued when the       
   session is in an incorrect state by responding with a negative status
   indicator.  There is no general method for a client to distinguish   
   between a server which does not implement an optional command and a  
   server which is unwilling or unable to process the command.          

   A POP3 server MAY have an inactivity autologout timer.  Such a timer 
   MUST be of at least 10 minutes' duration.  The receipt of any command
   from the client during that interval should suffice to reset the     
   autologout timer.  When the timer expires, the session does NOT enter
   the UPDATE state--the server should close the TCP connection without 
   removing any messages or sending any response to the client.         

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 positive         
   response.  An example might be:                                      

      S:  +OK POP3 server ready                                         

   The POP3 session is now in the AUTHORIZATION state.  The client must 
   now identify and authenticate itself to the POP3 server.  Two        
   possible mechanisms for doing this are described in this document,   
   the USER and PASS command combination and the APOP command.  Both    
   mechanisms are described later in this document.  Additional         
   authentication mechanisms are described in [RFC1734].  While there is
   no single authentication mechanism that is required of all POP3      
   servers, a POP3 server must of course support at least one           
   authentication mechanism.                                            

   Once the POP3 server has determined through the use of any           
   authentication command that the client should be given access to the 
   appropriate maildrop, the POP3 server then acquires an exclusive-    
   access lock on the maildrop, as necessary to prevent messages from   
   being modified or removed before the session enters the UPDATE state.
   If the lock is successfully acquired, the POP3 server responds with a
   positive status indicator.  The POP3 session now enters the          
   TRANSACTION state, with no messages marked as deleted.  If the       
   maildrop cannot be opened for some reason (for example, a lock can   
   not be acquired, the client is denied access to the appropriate      
   maildrop, or the maildrop cannot be parsed), the POP3 server responds
   with a negative status indicator.  (If a lock was acquired but the   
   POP3 server intends to respond with a negative status indicator, the 
   POP3 server must release the lock prior to rejecting the command.)   
   After returning a negative status indicator, the server may close the
   connection.  If the server does not close the connection, the client 
   may either issue a new authentication command and start again, or the
   client may issue the QUIT command.                                   

   After the POP3 server has opened the maildrop, it assigns a message- 
   number to each message, and notes the size of each message in octets.
   The first message in the maildrop is assigned a message-number of    
   "1", the second is assigned "2", and so on, so that the nth message  
   in a maildrop is assigned a message-number of "n".  In POP3 commands 
   and responses, all message-numbers and message sizes are expressed in
   base-10 (i.e., decimal).                                             

   Here is the summary for the QUIT command when used in the            
   AUTHORIZATION state:                                                 

      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 opened 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   
             positive response consists of "+OK" followed by a single   
             space, the number of messages in the maildrop, a single    
             space, and 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-number (optional), which, if present, 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 also called a  
             "scan listing" for that message.  If there are no          
             messages in the maildrop, then the POP3 server responds    
             with no scan listings--it issues a positive response       
             followed by a line containing a termination octet and a    
             CRLF pair.                                                 

             In order to simplify parsing, all POP3 servers are         
             required to use a certain format for scan listings.  A     
             scan listing consists of the message-number of the         
             message, followed by a single space and the exact size of  
             the message in octets.  Methods for calculating the exact  
             size of the message are described in the "Message Format"  
             section below.  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-number (required) which 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-number, being careful to byte-stuff the termination
             character (as with all multi-line responses).              

         Possible Responses:                                            
             +OK message follows                                        
             -ERR no such message                                       

         Examples:                                                      
             C: RETR 1                                                  
             S: +OK 120 octets                                          
             S:          
             S: .                                                       


      DELE msg                                                          

         Arguments:                                                     
             a message-number (required) which 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-number 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.                                   

         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                                                        

         Examples:                                                      
             C: NOOP                                                    
             S: +OK                                                     


      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.                                  

         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.)             

   If a session terminates for some reason other than a client-issued   
   QUIT command, the POP3 session does NOT enter the UPDATE state and   
   MUST not remove any messages from the maildrop.                      

      QUIT                                                              

         Arguments: none                                                

         Restrictions: none                                             

         Discussion:                                                    
             The POP3 server removes all messages marked as deleted     
             from the maildrop and replies as to the status of this     
             operation.  If there is an error, such as a resource       
             shortage, encountered while removing messages, the         
             maildrop may result in having some or none of the messages 
             marked as deleted be removed.  In no case may the server   
             remove any messages not marked as deleted.                 

             Whether the removal was successful or not, the server      
             then releases any exclusive-access lock on the maildrop    
             and closes the TCP connection.                             

         Possible Responses:                                            
             +OK                                                        
             -ERR some deleted messages not removed                     

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

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-number (required) which may NOT refer to to a    
             message marked as deleted, and a non-negative number       
             of lines (required)                                        

         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 of the 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 1 10                                                
             S: +OK                                                     
             S:                              
             S: .                                                       
                ...                                                     
             C: TOP 100 3                                               
             S: -ERR no such message                                    


      UIDL [msg]                                                        

      Arguments:                                                        
          a message-number (optional), which, if present, 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 "unique-id 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 "unique-id listing" for that message.   

          In order to simplify parsing, all POP3 servers are required to
          use a certain format for unique-id listings.  A unique-id     
          listing consists of the message-number of the message,        
          followed by a single space and the unique-id of the message.  
          No information follows the unique-id in the unique-id listing.

          The unique-id of a message is an arbitrary server-determined  
          string, consisting of one to 70 characters in the range 0x21  
          to 0x7E, which uniquely identifies a message within a         
          maildrop and which persists across sessions.  This            
          persistence is required even if a session ends without        
          entering the UPDATE state.  The server should never reuse an  
          unique-id in a given maildrop, for as long as the entity      
          using the unique-id exists.                                   

          Note that messages marked as deleted are not listed.          

          While it is generally preferable for server implementations   
          to store arbitrarily assigned unique-ids in the maildrop,     
          this specification is intended to permit unique-ids to be     
          calculated as a hash of the message.  Clients should be able  
          to handle a situation where two identical copies of a         
          message in a maildrop have the same unique-id.                

      Possible Responses:                                               
          +OK unique-id listing follows                                 
          -ERR no such message                                          

      Examples:                                                         
          C: UIDL                                                       
          S: +OK                                                        
          S: 1 whqtswO00WBw418f9t5JxYwZ                                 
          S: 2 QhdPYR:00WBw1Ph7x7                                       
          S: .                                                          
             ...                                                        
          C: UIDL 2                                                     
          S: +OK 2 QhdPYR:00WBw1Ph7x7                                   
             ...                                                        
          C: UIDL 3                                                     
          S: -ERR no such message, only 2 messages in maildrop          


      USER name                                                         

         Arguments:                                                     
             a string identifying a mailbox (required), which is of     
             significance ONLY to the server                            

         Restrictions:                                                  
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command     

         Discussion:                                                    
             To authenticate using the USER and PASS command            
             combination, the client must first issue the USER          
             command.  If the POP3 server responds with a positive      
             status indicator ("+OK"), then the client may issue        
             either the PASS command to complete the authentication,    
             or the QUIT command to terminate the POP3 session.  If     
             the POP3 server responds with a negative status indicator  
             ("-ERR") to the USER command, then the client may either   
             issue a new authentication command or may issue the QUIT   
             command.                                                   

             The server may return a positive response even though no   
             such mailbox exists.  The server may return a negative     
             response if mailbox exists, but does not permit plaintext  
             password authentication.                                   

         Possible Responses:                                            
             +OK name is a valid mailbox                                
             -ERR never heard of mailbox name                           

         Examples:                                                      
             C: USER frated                                             
             S: -ERR sorry, no mailbox for frated here                  
                ...                                                     
             C: USER mrose                                              
             S: +OK mrose is a real hoopy frood                         


      PASS string                                                       

         Arguments:                                                     
             a server/mailbox-specific password (required)              

         Restrictions:                                                  
             may only be given in the AUTHORIZATION state immediately   
             after a successful USER command                            

         Discussion:                                                    
             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.                                      

             Since the PASS command has exactly one argument, a POP3    
             server may treat spaces in the argument as part of the     
             password, instead of as argument separators.               

         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: -ERR maildrop already locked                            
               ...                                                      
             C: USER mrose                                              
             S: +OK mrose is a real hoopy frood                         
             C: PASS secret                                             
             S: +OK mrose's maildrop has 2 messages (320 octets)        


      APOP name digest                                                  

         Arguments:                                                     
             a string identifying a mailbox and a MD5 digest string     
             (both required)                                            

         Restrictions:                                                  
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command     

         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:              

                                             

             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 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.                                       

             Note that as the length of the shared secret increases, so 
             does the difficulty of deriving it.  As such, shared       
             secrets should be long strings (considerably longer than   
             the 8-character example shown below).                      

         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  `tan-
             staaf'.  Hence, the MD5 algorithm is applied to the string 

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

             which produces a digest value of                           

                c4c9334bac560ecc979e58001b3e22fb                        

8. Scaling and Operational Considerations                               

   Since some of the optional features described above were added to the
   POP3 protocol, experience has accumulated in using them in large-    
   scale commercial post office operations where most of the users are  
   unrelated to each other.  In these situations and others, users and  
   vendors of POP3 clients have discovered that the combination of using
   the UIDL command and not issuing the DELE command can provide a weak 
   version of the "maildrop as semi-permanent repository" functionality 
   normally associated with IMAP.  Of course the other capabilities of  
   IMAP, such as polling an existing connection for newly arrived       
   messages and supporting multiple folders on the server, are not      
   present in POP3.                                                     

   When these facilities are used in this way by casual users, there has
   been a tendency for already-read messages to accumulate on the server
   without bound.  This is clearly an undesirable behavior pattern from 
   the standpoint of the server operator.  This situation is aggravated 
   by the fact that the limited capabilities of the POP3 do not permit  
   efficient handling of maildrops which have hundreds or thousands of  
   messages.                                                            

   Consequently, it is recommended that operators of large-scale multi- 
   user servers, especially ones in which the user's only access to the 
   maildrop is via POP3, consider such options as:                      

   *  Imposing a per-user maildrop storage quota or the like.           

      A disadvantage to this option is that accumulation of messages may
      result in the user's inability to receive new ones into the       
      maildrop.  Sites which choose this option should be sure to inform
      users of impending or current exhaustion of quota, perhaps by     
      inserting an appropriate message into the user's maildrop.        

   *  Enforce a site policy regarding mail retention on the server.     

      Sites are free to establish local policy regarding the storage and
      retention of messages on the server, both read and unread.  For   
      example, a site might delete unread messages from the server after
      60 days and delete read messages after 7 days.  Such message      
      deletions are outside the scope of the POP3 protocol and are not  
      considered a protocol violation.                                  

      Server operators enforcing message deletion policies should take  
      care to make all users aware of the policies in force.            

      Clients must not assume that a site policy will automate message  
      deletions, and should continue to explicitly delete messages using
      the DELE command when appropriate.                                

      It should be noted that enforcing site message deletion policies  
      may be confusing to the user community, since their POP3 client   
      may contain configuration options to leave mail on the server     
      which will not in fact be supported by the server.                

      One special case of a site policy is that messages may only be    
      downloaded once from the server, and are deleted after this has   
      been accomplished.  This could be implemented in POP3 server      
      software by the following mechanism: "following a POP3 login by a 
      client which was ended by a QUIT, delete all messages downloaded  
      during the session with the RETR command".  It is important not to
      delete messages in the event of abnormal connection termination   
      (ie, if no QUIT was received from the client) because the client  
      may not have successfully received or stored the messages.        
      Servers implementing a download-and-delete policy may also wish to
      disable or limit the optional TOP command, since it could be used 
      as an alternate mechanism to download entire messages.            

9. 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                                                           
         RSET                                                           
         QUIT                                                           

      Optional POP3 Commands:                                           

         APOP name digest        valid in the AUTHORIZATION state       

         TOP msg n               valid in the TRANSACTION state         
         UIDL [msg]                                                     

      POP3 Replies:                                                     

         +OK                                                            
         -ERR                                                           

      Note that with the exception of the STAT, LIST, and UIDL commands,
      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.                                     

10. Example POP3 Session                                                

      S:                           
      C:                                               
      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:                               
      S:    .                                                           
      C:    DELE 1                                                      
      S:    +OK message 1 deleted                                       
      C:    RETR 2                                                      
      S:    +OK 200 octets                                              
      S:                               
      S:    .                                                           
      C:    DELE 2                                                      
      S:    +OK message 2 deleted                                       
      C:    QUIT                                                        
      S:    +OK dewey POP3 server signing off (maildrop empty)          
      C:                                              
      S:                                      

11. 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 octet 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 server  
   can calculate the size of each message in octets when it opens the   
   maildrop.  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 (and must not) be counted twice, since the POP3 client will      
   remove all byte-stuffed termination characters when it receives a    
   multi-line response.                                                 

12. References                                                          

   [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.                                  

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 should not allow both methods of access for a given user;   
   that is, for a given mailbox name, either the USER/PASS command      
   sequence or the APOP command is allowed, but not both.               

   Further, note that as the length of the shared secret increases, so  
   does the difficulty of deriving it.                                  

   Servers that answer -ERR to the USER command are giving potential    
   attackers clues about which names are valid.                         

   Use of the PASS command sends passwords in the clear over the        
   network.                                                             

   Use of the RETR and TOP commands sends mail in the clear over the    
   network.                                                             

   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 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. Authors' Addresses                                                  

   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