Internet DRAFT - draft-rose-pop3-whatever
draft-rose-pop3-whatever
Draft POP3 Apr 93
Post Office Protocol - Version 3
Sat Apr 3 16:22:22 1993
Marshall T. Rose
Dover Beach Consulting, Inc.
mrose@dbc.mtview.ca.us
Status of this Memo
This document is an Internet Draft. Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.
Internet Drafts are valid for a maximum of six months and may
be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as a "work in progress".
Overview
This memo is a PROPOSED revision to RFC 1460, a draft standard. It
makes the following changes from that document:
- removed text regarding "split-UA model", which didn't add anything
to the understanding of POP;
- clarified syntax of commands, keywords, and arguments;
- removed "across-session" semantics of "highest number accessed",
which no one implemented (or even pretended to);
- fixed typo in example of TOP command; and,
- added warning regarding length of shared secrets with APOP.
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.
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Similarly, it may be expensive (or impossible) to keep a personal
computer interconnected to an IP-style network for long amounts of
time (the node is lacking the resource known as "connectivity").
Despite this, it is often very useful to be able to manage mail on
these smaller nodes, and they often support a user agent (UA) to aid
the tasks of mail handling. To solve this problem, a node which can
support an MTS entity offers a maildrop service to these less endowed
nodes. The Post Office Protocol - Version 3 (POP3) is intended to
permit a workstation to dynamically access a maildrop on a server
host in a useful fashion. Usually, this means that the POP3 is used
to allow a workstation to retrieve mail that the server is holding
for it.
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 (this relay host could be, but need not be, the
POP3 server host for the client host).
If this method is followed, then the client host appears to the MTS
as a user agent, and should NOT be regarded as a "trusted" MTS entity
in any sense whatsoever.
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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 keyword possibly followed by an
argument. All commands are terminated by a CRLF pair. Keywords and
arguments consist of printable ASCII characters separated by whitespace
(one or more consecutive SPACE and/or TAB characters). Keywords are
4 characters long, whilst each argument may be upto 40 characters long.
Responses in the POP3 consist of a success indicator and a keyword
possibly followed by additional information. All responses are
terminated by a CRLF pair. There are currently two success
indicators: positive ("+OK") and negative ("-ERR").
Responses to certain commands are multi-line. In these cases, which
are clearly indicated below, after sending the first line of the
response and a CRLF, any additional lines are sent, each terminated
by a CRLF pair. When all lines of the response have been sent, a
final line is sent, consisting of a termination octet (decimal code
046, ".") and a CRLF pair. If any line of the multi-line response
begins with the termination octet, the line is "byte-stuffed" by
pre-pending the termination octet to that line of the response.
Hence a multi-line response is terminated with the five octets
"CRLF.CRLF". When examining a multi-line response, the client checks
to see if the line begins with the termination octet. If so and if
octets other than CRLF follow, the the first octet of the line (the
termination octet) is stripped away. If so and if CRLF immediately
follows the termination character, then the response from the POP
server is ended and the line containing ".CRLF" is not considered
part of the multi-line response.
A POP3 session progresses through a number of states during its
lifetime. Once the TCP connection has been opened and the POP3
server has sent the greeting, the session enters the AUTHORIZATION
state. In this state, the client must identify itself to the POP3
server. Once the client has successfully done this, the server
acquires resources associated with the client's maildrop, and the
session enters the TRANSACTION state. In this state, the client
requests actions on the part of the POP3 server. When the client has
finished its transactions, the session enters the UPDATE state. In
this state, the POP3 server releases any resources acquired during
the TRANSACTION state and says goodbye. The TCP connection is then
closed.
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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 string terminated
by CRLF. An example might be:
S. +OK POP3 server ready
Note that this greeting is a POP3 reply. The POP3 server should
always give a positive response as the greeting.
The POP3 session is now in the AUTHORIZATION state. The client must
now issue the USER command. If the POP3 server responds with a
positive success indicator ("+OK"), then the client may issue either
the PASS command to complete the authorization, or the QUIT command
to terminate the POP3 session. If the POP3 server responds with a
negative success indicator ("-ERR") to the USER command, then the
client may either issue a new USER command or may issue the QUIT
command.
When the client issues the PASS command, the POP3 server uses the
argument pair from the USER and PASS commands to determine if the
client should be given access to the appropriate maildrop. If so,
the POP3 server then acquires an exclusive-access lock on the
maildrop. If the lock is successfully acquired, the POP3 server
parses the maildrop into individual messages (read note below), sets
the "highest number accessed" to zero, and responds with a positive
success indicator. The POP3 session now enters the TRANSACTION state.
If the lock can not be acquired or the client should is denied access
to the appropriate maildrop or the maildrop can't be parsed for some
reason, the POP3 server responds with a negative success indicator.
(If a lock was acquired but the POP3 server intends to respond with a
negative success indicator, the POP3 server must release the lock
prior to rejecting the command.) At this point, the client may
either issue a new USER command and start again, or the client may
issue the QUIT command.
NOTE: Minimal implementations of the POP3 need only be
able to break a maildrop into its component messages;
they need NOT be able to parse individual messages.
More advanced implementations may wish to have this
capability, for reasons discussed later.
After the POP3 server has parsed the maildrop into individual
messages, it assigns a message-id to each message, and notes the size
of the message in octets. The first message in the maildrop is
assigned a message-id of "1", the second is assigned "2", and so on,
so that the n'th message in a maildrop is assigned a message-id of
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"n". In POP3 commands and responses, all message-id's and message
sizes are expressed in base-10 (i.e., decimal).
Here are summaries for the three POP3 commands discussed thus far:
USER name
Arguments: a string identifying a mailbox (required),
which is of signficance 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
Possible Responses:
+OK name is a valid mailbox
-ERR never heard of mailbox name
Examples:
C: USER mrose
S: +OK mrose is a real hoopy frood
...
C: USER frated
S: -ERR sorry, no mailbox for frated here
PASS string
Arguments: a server/mailbox-specific password (required)
Restrictions: may only be given in the AUTHORIZATION
state after a successful USER command
Possible Responses:
+OK maildrop locked and ready
-ERR invalid password
-ERR unable to lock maildrop
Examples:
C: USER mrose
S: +OK mrose is a real hoopy frood
C: PASS secret
S: +OK mrose's maildrop has 2 messages (320 octets)
...
C: USER mrose
S: +OK mrose is a real hoopy frood
C: PASS secret
S: -ERR maildrop already locked
QUIT
Arguments: none
Restrictions: none
Possible Responses:
+OK
Examples:
C: QUIT
S: +OK dewey POP3 server signing off
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5. The TRANSACTION State
Once the client has successfully identified itself to the POP3 server
and the POP3 server has locked and burst the appropriate maildrop,
the POP3 session is now in the TRANSACTION state. The client may now
issue any of the following POP3 commands repeatedly. After each
command, the POP3 server issues a response. Eventually, the client
issues the QUIT command and the POP3 session enters the UPDATE state.
Here are the POP3 commands valid in the TRANSACTION state:
STAT
Arguments: none
Restrictions: may only be given in the TRANSACTION state
Discussion:
The POP3 server issues a positive response with a line
containing information for the maildrop. This line is
called a "drop listing" for that maildrop.
In order to simplify parsing, all POP3 servers are
required to use a certain format for drop listings.
The first octets present must indicate the number of
messages in the maildrop. Following this is the size
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
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LIST [msg]
Arguments: a message-id (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 called a "scan listing" for that message.
In order to simplify parsing, all POP3 servers are
required to use a certain format for scan listings.
The first octets present must be the message-id of
the message. Following the message-id is the size of
the message in octets. This memo makes no requirement
on what follows the message size in the scan listing.
Minimal implementations should just end that line of
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
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Examples:
C: LIST
S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
...
C: LIST 2
S: +OK 2 200
...
C: LIST 3
S: -ERR no such message, only 2 messages in
maildrop
RETR msg
Arguments: a message-id (required) 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-id, being careful to byte-stuff the
termination character (as with all multi-line
responses).
If the number associated with this message is higher
than the "highest number accessed" in the maildrop, the
POP3 server updates the "highest number accessed" to
the number associated with this message.
Possible Responses:
+OK message follows
-ERR no such message
Examples:
C: RETR 1
S: +OK 120 octets
S: <the POP3 server sends the entire message here>
S: .
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DELE msg
Arguments: a message-id (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-id associated with the
message in a POP3 command generates an error. The POP3
server does not actually delete the message until the
POP3 session enters the UPDATE state.
If the number associated with this message is higher
than the "highest number accessed" in the maildrop,
the POP3 server updates the "highest number accessed"
to the number associated with this message.
Possible Responses:
+OK message deleted
-ERR no such message
Examples:
C: DELE 1
S: +OK message 1 deleted
...
C: DELE 2
S: -ERR message 2 already deleted
NOOP
Arguments: none
Restrictions: may only be given in the TRANSACTION state
Discussion:
The POP3 server does nothing, it merely replies with a
positive response.
Possible Responses:
+OK
Examples:
C: NOOP
S: +OK
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LAST
Arguments: none
Restrictions: may only be issued in the TRANSACTION state
Discussion:
The POP3 server issues a positive response with a line
containing the "highest number accessed". Zero is
returned in case no message in the maildrop has been
accessed during previous transactions. A client may
thereafter infer that messages, if any, numbered greater
than the response to the LAST command are messages not
yet accessed by the client.
Possible Response:
+OK nn
Examples:
C: STAT
S: +OK 4 320
C: LAST
S: +OK 1
C: RETR 3
S: +OK 120 octets
S: <the POP3 server sends the entire message
here>
S: .
C: LAST
S: +OK 3
C: DELE 2
S: +OK message 2 deleted
C: LAST
S: +OK 3
C: RSET
S: +OK
C: LAST
S: +OK 0
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. In addition, the
"highest number accessed" is also reset to zero.
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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.)
QUIT
Arguments: none
Restrictions: none
Discussion:
The POP3 server removes all messages marked as deleted
from the maildrop. It then releases the
exclusive-access lock on the maildrop and replies as
to the success of
these operations. The TCP connection is then closed.
Possible Responses:
+OK
Examples:
C: QUIT
S: +OK dewey POP3 server signing off (maildrop
empty)
...
C: QUIT
S: +OK dewey POP3 server signing off (2 messages
left)
...
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.
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NOTE: This memo STRONGLY encourages implementations to
support these commands in lieu of developing augmented
drop and scan listings. In short, the philosophy of
this memo is to put intelligence in the part of the
POP3 client and not the POP3 server.
TOP msg n
Arguments: a message-id (required) which may NOT refer to
to a message marked as deleted, and a number
Restrictions: may only be given in the TRANSACTION state
Discussion:
If the POP3 server issues a positive response, then
the response given is multi-line. After the initial
+OK, the POP3 server sends the headers of the message,
the blank line separating the headers from the body,
and then the number of lines indicated message's body,
being careful to byte-stuff the termination character
(as with all multi-line responses).
Note that if the number of lines requested by the POP3
client is greater than than the number of lines in the
body, then the POP3 server sends the entire message.
Possible Responses:
+OK top of message follows
-ERR no such message
Examples:
C: TOP 1 10
S: +OK
S: <the POP3 server sends the headers of the
message, a blank line, and the first 10 lines
of the body of the message>
S: .
...
C: TOP 100
S: -ERR no such message
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APOP name digest
Arguments: a string identifying a mailbox and a a digest
string (both required)
Restrictions: may only be given in the AUTHORIZATION
state after the POP3 greeting
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:
<process-ID.clock@hostname>
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.
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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 `tanstaaf'.
Hence, the MD5 algorithm is applied to the string
<1896.697170952@dbc.mtview.ca.us>tanstaaf
which produces a digest value of
c4c9334bac560ecc979e58001b3e22fb
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8. 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
LAST
RSET
QUIT valid in the UPDATE state
Optional POP3 Commands:
APOP name digest valid in the AUTHORIZATION state
TOP msg n valid in the TRANSACTION state
POP3 Replies:
+OK
-ERR
Note that with the exception of the STAT command, the reply given
by the POP3 server to any command is significant only to "+OK"
and "-ERR". Any text occurring after this reply may be ignored
by the client.
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9. Example POP3 Session
S: <wait for connection on TCP port 110>
C: <open connection>
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: <the POP3 server sends message 1>
S: .
C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S: <the POP3 server sends message 2>
S: .
C: DELE 2
S: +OK message 2 deleted
C: QUIT
S: +OK dewey POP3 server signing off (maildrop empty)
C: <close connection>
S: <wait for next connection>
10. 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 client
can calculate the size of each message in octets when it parses the
maildrop into messages. For example, if the POP3 server host
internally represents end-of-line as a single character, then the
POP3 server simply counts each occurrence of this character in a
message as two octets. Note that lines in the message which start
with the termination octet need not be counted twice, since the POP3
client will remove all byte-stuffed termination characters when it
receives a multi-line response.
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12. References
[RFC821] Postel, J., "Simple Mail Transfer Protocol", RFC 821,
USC/Information Sciences Institute, August, 1982.
[RFC822] Crocker, D., "Standard for the Format of ARPA-Internet
Text Messages", RFC 822, University of Delaware, August,
1982.
[RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", RFC 1320,
MIT Laboratory for Computer Science, April, 1992.
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 must not
allow both methods of access for a given user; that is, for a given
"USER name" either the PASS or 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.
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. Author's Address:
Marshall T. Rose
Dover Beach Consulting, Inc.
Mountain View, CA 94043-2186
Phone: +1 415 968 1052
Fax: +1 415 968 2510
EMail: mrose@dbc.mtview.ca.us
X.500: rose, dbc, us
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