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 RFCdefines 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 RFCdefines 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 (RFC1225). 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
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:
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
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
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.edu
