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Network Working Group                                         S. DornerRequest for Comments: 1339                                   P. Resnick                                     U. of Illinois at Urbana-Champaign                                                              June 1992Remote Mail Checking ProtocolStatus 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 theDorner & Resnick                                                [Page 1]

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:Dorner & Resnick                                                [Page 2]

RFC 1339             Remote Mail Checking Protocol             June 1992      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                  0Dorner & Resnick                                                [Page 3]

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 protocolDorner & Resnick                                                [Page 4]

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: s-dorner@uiuc.edu   Pete Resnick   The Beckman Institute   University of Illinois at Urbana-Champaign   405 North Mathews Avenue   Urbana, Illinois 61801   Phone: (217) 244-1265   EMail: resnick@cogsci.uiuc.eduDorner & Resnick                                                [Page 5]

RFC 1339             Remote Mail Checking Protocol             June 1992Dorner & Resnick                                                [Page 6]