Network Working Group                                         S. Dorner
Request for Comments: 1339                                   P. Resnick
                                    U. of Illinois at Urbana-Champaign
                                                             June 1992


                    Remote Mail Checking Protocol

Status of this Memo

  This memo defines an Experimental Protocol for the Internet
  community.  Discussion and suggestions for improvement are requested.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Abstract

  This RFC defines a protocol to provide a mail checking service to be
  used between a client and server pair. Typically, a small program on
  a client workstation would use the protocol to query a server in
  order to find out whether new mail has arrived for a specified user.

Intent

  This RFC defines a simple, low-overhead protocol for checking the
  status of a maildrop on a host. It is primarily intended for use in
  adjunct with "remote mail" servers such as those implementing the
  Post Office Protocol (RFC 1225). Remote mail clients must poll their
  servers to discover the arrival of mail. Using one of the remote mail
  protocols for periodic checking can be quite impractical and
  expensive for the server since either a constant connection between
  client and server must be maintained or repeated and expensive user
  validations must be done. Furthermore, users on less capable
  computers may not wish to devote the memory required to have a full
  implementation of the client polling for mail.  Thus, we feel that an
  easy to implement and inexpensive to use polling scheme would be of
  benefit both to mail servers and their clients.

Protocol Overview

  To avoid connection overhead, the Remote Mail Checking Protocol is
  based on the User Datagram Protocol (UDP), using UDP port 50 decimal
  (62 octal) for the server. The protocol provides for both non-
  authenticated and authenticated polling. Non-authenticated polling is
  simplest for both client and server. Authenticated polling provides a
  small increment of privacy, at the cost of more complexity in both
  client and server (but still far less than polling with one of the



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RFC 1339             Remote Mail Checking Protocol             June 1992


  remote mail protocols).

Non-Authenticated Protocol

  In the non-authenticated version of the protocol, the server will
  listen on port 50 for maildrop check requests for users with
  maildrops on the machine. A client will send a single UDP datagram
  from a randomly chosen unreserved UDP port to UDP port 50 on the
  server. The datagram will contain a 32-bit (four-octet) number which
  is set to all zeros (0), followed by a case-sensitive ASCII string of
  a username on the server system. The server will find the maildrop on
  the system for that user and determine the amount of time that has
  passed since the last message in the maildrop was appended, as well
  as the amount of time that has passed since the maildrop was last
  accessed for reading. The server will then send back a single UDP
  datagram containing three 32-bit numbers in network byte order to the
  originating port on the client. Again, the first will be zero (0),
  the second will contain the number of seconds plus one since the last
  addition to the specified user's maildrop and the third will contain
  the number of seconds plus one since the last read on the user's
  maildrop. If the username provided does not exist, if the maildrop is
  not on the system or if the maildrop is empty, the server will send
  back zero (0) in the last two numbers for its reply. The client will
  consider the maildrop to contain new mail if the number of seconds
  since the last read access is greater than or equal to the number of
  seconds since the last addition access of the maildrop and either
  number is non-zero, old mail if the number of seconds since the last
  read access is less than or equal to the number of seconds since the
  last addition access of the maildrop and either number is non-zero,
  and empty if both numbers are zero.

Authenticated Protocol

  The authenticated protocol operates identically to the non-
  authenticated protocol with the exception of the first interaction
  between the server and the client. After the client has sent its
  initial request containing the requested username, the server will
  send back a single UDP packet containing three 32-bit numbers. The
  first number will be a bit-mask instead of the normal 32-bits of
  zero. The bit-mask will indicate a request for authentication. Each
  bit in the mask represents a type of authentication that the server
  accepts. The bits (with the least significant bit numbered 0, and the
  most significant bit 31) are defined as follows:








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     0     Cleartext password The password for the maildrop, not
           NULL-terminated.

     1-23  Reserved for future use

     24-31 Implementation-dependent. Implementors wishing to
           experiment may use these.

  For each type of authentication that the server accepts, the
  corresponding bit will be set to one. All other bits will be set to
  zero.  The last two 32-bit numbers in the reply will be set to zero.
  If the client supports authentication, it will send back a 32-bit
  mask with the bit representing the kind of authentication it is using
  set to one, followed by the data used for authentication. The client
  is free to use any of the types of authentication indicated by the
  authentication request from the server. If the client does not
  support authentication and it receives an authentication request, it
  SHOULD stop sending requests (though this behavior is not required).

  Once a valid authentication is received by the server for a
  particular maildrop, the server considers the IP address and UDP port
  of the client along with that maildrop to be an authenticated
  address/port/maildrop triple. From then on, normal non-authenticated
  transactions take place between the server and the client as
  described above. Should a datagram come from an authenticated
  address/port pair with a different username, or if some amount of
  time has elapsed since the last request (which is implementation
  dependent), the server should remove the address/port/maildrop triple
  from its list of authenticated triples and send another
  authentication request. Since the time required for an authenticated
  triple to become unauthenticated is implementation dependent, clients
  should be prepared to send an authentication reply to containing the
  server whenever it is requested.

Server Implementation Notes

  Servers which implement either the authenticated or non-authenticated
  protocol may decide that they do not wish to reveal the actual amount
  of time that has passed since the last update or read from a
  maildrop. (See the "Security Considerations" section below for
  reasons some feel this is problematic.) In this case, a server may
  instead reply with the following:

                  First 32 bits     Second 32 bits     Third 32 bits
     New mail           0                 0                  1
     Old mail           0                 1                  0
     No mail            0                 0                  0




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RFC 1339             Remote Mail Checking Protocol             June 1992


  These values will appear to the client as correctly representing new,
  old or no mail respectively but will give no indication of the actual
  times that the changes took place.

  Servers implementing the non-authenticated protocol MUST provide some
  mechanism by which users on the system can give permission for their
  maildrops to accessed by the protocol. See the "Security
  Considerations" section below for specifics.

Client Implementation Notes

  Clients MUST not send more than one poll (and one authentication) per
  minute. In particular, lack of server response should not result in
  retransmission.

  Since the last two numbers in an authentication request from a server
  are always 0 as are the last two numbers in a response for an empty
  or non-existent maildrop, clients that do not support authentication
  need not examine the first number in the server datagram at all
  (though they are encouraged to do so for the sake of proper reporting
  to the user).

  Clients can turn the modification interval into absolute time, and
  track the changing of this absolute time in order to discern the
  arrival of new mail (as opposed to the mere existence of unread
  mail). However, such clients should bear three things in mind.
  First, network delays and clock vagaries may result in small
  inconsistencies in times. A "slop factor" of several seconds is
  encouraged. Second, the reading of mail often entails modification of
  the maildrop; the relationship of the access and modification
  intervals should always be consulted. Third, the special results of
  (1,0) and (0,1) are most properly handled as special cases.

  Clients need not recall whether or not they are authenticated (though
  they must use a consistent port if they receive any authentication
  requests for a given maildrop). It is sufficient to issue requests
  when desired, and to respond to any authentication requests that
  appear.

Security Considerations

  The are two security considerations for the protocol. The first is
  one mainly of privacy. Some sites and individual users consider it
  problematic to have information about mail arrival available freely.
  This can be a simple privacy issue for individuals or a security
  issue for highly secure sites. The authenticated version of the
  protocol allows sites to have a reasonable amount of security in that
  only people with passwords can access this information. The protocol



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RFC 1339             Remote Mail Checking Protocol             June 1992


  currently only uses cleartext passwords, but can be simply modified
  to use other authentication formats. The scheme mentioned in "Server
  Implementation Notes" of using only (0,1) and (1,0) in the responses
  also may limit access to some types of information.  Implementations
  that do not use the authenticated scheme MUST have a mechanism by
  which a user can give consent to have this information made
  available; the default for the unauthenticated implementation should
  be that a user's maildrop cannot be accessed until consent of the
  user is given. (For example, UNIX server implementations may wish to
  make use of the "owner execute" bit to indicate whether a particular
  maildrop allows use of the unauthenticated protocol. If this is done,
  a single "stat" call can be used to gather all information required
  to respond to a poll.) Servers which do not implement authentication
  should simply return a zero-filled datagram for maildrops which don't
  have permission.

  The other security consideration involves unknown maildrops and
  usernames. Some site administrators consider it a security risk give
  out any information which would reveal the existence or non-existence
  of a certain username or maildrop on the system. For this reason, we
  have chosen to have the server send back a zero-filled datagram as
  the response to either a request for an unknown username or a
  maildrop that does not exist or is empty. In this way, potential
  security violations are limited, since there is no way to tell the
  difference between an empty maildrop and non-existent maildrop, and
  also no way to tell if the user exists on the system or not. If
  greater security is desired, the protocol should probably not be run
  in the first place.

Authors' Addresses

  Steve Dorner
  Digital Computer Laboratory
  University of Illinois at Urbana-Champaign
  1304 West Springfield Avenue
  Urbana, Illinois 61801

  Phone: (217) 244-1765
  EMail: [email protected]

  Pete Resnick
  The Beckman Institute
  University of Illinois at Urbana-Champaign
  405 North Mathews Avenue
  Urbana, Illinois 61801

  Phone: (217) 244-1265
  EMail: [email protected]



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