RFC relevance of response

As per Relevance of the word response, we have this rfc below:

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

Post Office Protocol - Version 3

Status of this

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

Table of

1. Introduction ................................................ 2
2. A Short Digression .......................................... 2
3. Basic Operation ............................................. 3
4. The AUTHORIZATION State ..................................... 4
QUIT Command ................................................ 5
5. The TRANSACTION State ....................................... 5
STAT Command ................................................ 6
LIST Command ................................................ 6
RETR Command ................................................ 8
DELE Command ................................................ 8
NOOP Command ................................................ 9
RSET Command ................................................ 9
6. The UPDATE State ............................................ 10
QUIT Command ................................................ 10
7. Optional POP3 Commands ...................................... 11
TOP Command ................................................. 11
UIDL Command ................................................ 12
USER Command ................................................ 13
PASS Command ................................................ 14
APOP Command ................................................ 15
8. Scaling and Operational Considerations ...................... 16
9. POP3 Command Summary ........................................ 18
10. Example POP3 Session ....................................... 19
11. Message Format ............................................. 19
12. References ................................................. 20
13. Security Considerations .................................... 20
14. Acknowledgements ........................................... 20
15. Authors' Addresses ......................................... 21
Appendix A. Differences from RFC 1725 .......................... 22

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RFC 1939 POP3 May 1996

Appendix B. Command Index ...................................... 23

1.

On certain types of smaller nodes in the Internet it is
impractical to maintain a message transport system (MTS).
example, a workstation may not have sufficient resources (cycles
disk space) in order to permit a SMTP server [RFC821] and
local mail delivery system to be kept resident and
running. Similarly, it may be expensive (or impossible) to keep
personal computer interconnected to an IP-style network for
amounts of time (the node is lacking the resource known
"connectivity").

Despite this, it is often very useful to be able to manage mail
these smaller nodes, and they often support a user agent (UA) to
the tasks of mail handling. To solve this problem, a node which
support an MTS entity offers a maildrop service to these less
nodes. The Post Office Protocol - Version 3 (POP3) is intended
permit a workstation to dynamically access a maildrop on a
host in a useful fashion. Usually, this means that the POP3
is used to allow a workstation to retrieve mail that the server
holding for it

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

For the remainder of this memo, the term "client host" refers to
host making use of the POP3 service, while the term "server host
refers to a host which offers the POP3 service

2. A Short

This memo does not specify how a client host enters mail into
transport system, although a method consistent with the philosophy
this memo is presented here

When the user agent on a client host wishes to enter a
into the transport system, it establishes an SMTP connection
its relay host and sends all mail to it. This relay host
be, but need not be, the POP3 server host for the client host.
course, the relay host must accept mail for delivery to
recipient addresses, that functionality is not required of
SMTP servers

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RFC 1939 POP3 May 1996

3. Basic

Initially, the server host starts the POP3 service by listening
TCP port 110. When a client host wishes to make use of the service
it establishes a TCP connection with the server host. When
connection is established, the POP3 server sends a greeting.
client and POP3 server then exchange commands and
(respectively) until the connection is closed or aborted

Commands in the POP3 consist of a case-insensitive keyword,
followed by one or more arguments. All commands are terminated by
CRLF pair. Keywords and arguments consist of printable
characters. Keywords and arguments are each separated by a
SPACE character. Keywords are three or four characters long.
argument may be up to 40 characters long

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

Responses to certain commands are multi-line. In these cases,
are clearly indicated below, after sending the first line of
response and a CRLF, any additional lines are sent, each
by a CRLF pair. When all lines of the response have been sent,
final line is sent, consisting of a termination octet (decimal
046, ".") and a CRLF pair. If any line of the multi-line
begins with the termination octet, the line is "byte-stuffed"
pre-pending the termination octet to that line of the response
Hence a multi-line response is terminated with the five
"CRLF.CRLF". When examining a multi-line response, the client
to see if the line begins with the termination octet. If so and
octets other than CRLF follow, the first octet of the line (
termination octet) is stripped away. If so and if CRLF
follows the termination character, then the response from the
server is ended and the line containing ".CRLF" is not
part of the multi-line response

A POP3 session progresses through a number of states during
lifetime. Once the TCP connection has been opened and the POP
server has sent the greeting, the session enters the
state. In this state, the client must identify itself to the POP
server. Once the client has successfully done this, the
acquires resources associated with the client's maildrop, and
session enters the TRANSACTION state. In this state, the
requests actions on the part of the POP3 server. When the client

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RFC 1939 POP3 May 1996

issued the QUIT command, the session enters the UPDATE state.
this state, the POP3 server releases any resources acquired
the TRANSACTION state and says goodbye. The TCP connection is
closed

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

A POP3 server MAY have an inactivity autologout timer. Such a
MUST be of at least 10 minutes' duration. The receipt of any
from the client during that interval should suffice to reset
autologout timer. When the timer expires, the session does NOT
the UPDATE state--the server should close the TCP connection
removing any messages or sending any response to the client

4. The AUTHORIZATION

Once the TCP connection has been opened by a POP3 client, the POP
server issues a one line greeting. This can be any
response. An example might be

S: +OK POP3 server

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

Once the POP3 server has determined through the use of
authentication command that the client should be given access to
appropriate maildrop, the POP3 server then acquires an exclusive
access lock on the maildrop, as necessary to prevent messages
being modified or removed before the session enters the UPDATE state
If the lock is successfully acquired, the POP3 server responds with
positive status indicator. The POP3 session now enters
TRANSACTION state, with no messages marked as deleted. If
maildrop cannot be opened for some reason (for example, a lock
not be acquired, the client is denied access to the

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RFC 1939 POP3 May 1996

maildrop, or the maildrop cannot be parsed), the POP3 server
with a negative status indicator. (If a lock was acquired but
POP3 server intends to respond with a negative status indicator,
POP3 server must release the lock prior to rejecting the command.)
After returning a negative status indicator, the server may close
connection. If the server does not close the connection, the
may either issue a new authentication command and start again, or
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
"1", the second is assigned "2", and so on, so that the nth
in a maildrop is assigned a message-number of "n". In POP3
and responses, all message-numbers and message sizes are expressed
base-10 (i.e., decimal).

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

Arguments:

Restrictions:

Possible Responses
+

Examples
C:
S: +OK dewey POP3 server signing

5. The TRANSACTION

Once the client has successfully identified itself to the POP3
and the POP3 server has locked and opened the appropriate maildrop
the POP3 session is now in the TRANSACTION state. The client may
issue any of the following POP3 commands repeatedly. After
command, the POP3 server issues a response. Eventually, the
issues the QUIT command and the POP3 session enters the UPDATE state

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RFC 1939 POP3 May 1996

Here are the POP3 commands valid in the TRANSACTION state

Arguments:

Restrictions
may only be given in the TRANSACTION

Discussion
The POP3 server issues a positive response with a
containing information for the maildrop. This line
called a "drop listing" for that maildrop

In order to simplify parsing, all POP3 servers
required to use a certain format for drop listings.
positive response consists of "+OK" followed by a
space, the number of messages in the maildrop, a
space, and the size of the maildrop in octets. This
makes no requirement on what follows the maildrop size
Minimal implementations should just end that line of
response with a CRLF pair. More advanced
may include other information

NOTE: This memo STRONGLY discourages
from supplying additional information in the
listing. Other, optional, facilities are
later on which permit the client to parse the
in the maildrop

Note that messages marked as deleted are not counted
either total

Possible Responses
+OK nn

Examples
C:
S: +OK 2 320

LIST [msg

Arguments
a message-number (optional), which, if present, may
refer to a message marked as

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RFC 1939 POP3 May 1996

Restrictions
may only be given in the TRANSACTION

Discussion
If an argument was given and the POP3 server issues
positive response with a line containing information
that message. This line is called a "scan listing"
that message

If no argument was given and the POP3 server issues
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
information for that message. This line is also called
"scan listing" for that message. If there are
messages in the maildrop, then the POP3 server
with no scan listings--it issues a positive
followed by a line containing a termination octet and
CRLF pair

In order to simplify parsing, all POP3 servers
required to use a certain format for scan listings.
scan listing consists of the message-number of
message, followed by a single space and the exact size
the message in octets. Methods for calculating the
size of the message are described in the "Message Format
section below. This memo makes no requirement on
follows the message size in the scan listing.
implementations should just end that line of the
with a CRLF pair. More advanced implementations
include other information, as parsed from the message

NOTE: This memo STRONGLY discourages
from supplying additional information in the
listing. Other, optional, facilities are
later on which permit the client to parse the
in the maildrop

Note that messages marked as deleted are not listed

Possible Responses
+OK scan listing
-ERR no such

Examples
C:
S: +OK 2 messages (320 octets
S: 1 120

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RFC 1939 POP3 May 1996

S: 2 200
S: .
...
C: LIST 2
S: +OK 2 200
...
C: LIST 3
S: -ERR no such message, only 2 messages in

RETR

Arguments
a message-number (required) which may NOT refer to
message marked as

Restrictions
may only be given in the TRANSACTION

Discussion
If the POP3 server issues a positive response, then
response given is multi-line. After the initial +OK,
POP3 server sends the message corresponding to the
message-number, being careful to byte-stuff the
character (as with all multi-line responses).

Possible Responses
+OK message
-ERR no such

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

DELE

Arguments
a message-number (required) which may NOT refer to
message marked as

Restrictions
may only be given in the TRANSACTION

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RFC 1939 POP3 May 1996

Discussion
The POP3 server marks the message as deleted. Any
reference to the message-number associated with the
in a POP3 command generates an error. The POP3 server
not actually delete the message until the POP3
enters the UPDATE state

Possible Responses
+OK message
-ERR no such

Examples
C: DELE 1
S: +OK message 1
...
C: DELE 2
S: -ERR message 2 already

Arguments:

Restrictions
may only be given in the TRANSACTION

Discussion
The POP3 server does nothing, it merely replies with
positive response

Possible Responses
+

Examples
C:
S: +

Arguments:

Restrictions
may only be given in the TRANSACTION

Discussion
If any messages have been marked as deleted by the POP
server, they are unmarked. The POP3 server then

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RFC 1939 POP3 May 1996

with a positive response

Possible Responses
+

Examples
C:
S: +OK maildrop has 2 messages (320 octets

6. The UPDATE

When the client issues the QUIT command from the TRANSACTION state
the POP3 session enters the UPDATE state. (Note that if the
issues the QUIT command from the AUTHORIZATION state, the POP
session terminates but does NOT enter the UPDATE state.)

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

Arguments:

Restrictions:

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

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

Possible Responses
+
-ERR some deleted messages not

Examples
C:
S: +OK dewey POP3 server signing off (maildrop empty
...
C:

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RFC 1939 POP3 May 1996

S: +OK dewey POP3 server signing off (2 messages left
...

7. Optional POP3

The POP3 commands discussed above must be supported by all
implementations of POP3 servers

The optional POP3 commands described below permit a POP3
greater freedom in message handling, while preserving a simple POP
server implementation

NOTE: This memo STRONGLY encourages implementations to
these commands in lieu of developing augmented drop and
listings. In short, the philosophy of this memo is to
intelligence in the part of the POP3 client and not the POP
server

TOP msg

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

Restrictions
may only be given in the TRANSACTION

Discussion
If the POP3 server issues a positive response, then
response given is multi-line. After the initial +OK,
POP3 server sends the headers of the message, the
line separating the headers from the body, and then
number of lines of the indicated message's body,
careful to byte-stuff the termination character (as
all multi-line responses).

Note that if the number of lines requested by the POP
client is greater than than the number of lines in
body, then the POP3 server sends the entire message

Possible Responses
+OK top of message
-ERR no such

Examples
C: TOP 1 10
S: +

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RFC 1939 POP3 May 1996

S: message, a blank line, and the first 10
of the body of the message
S: .
...
C: TOP 100 3
S: -ERR no such

UIDL [msg

Arguments
a message-number (optional), which, if present, may
refer to a message marked as

Restrictions
may only be given in the TRANSACTION state

Discussion
If an argument was given and the POP3 server issues a
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
response, then the response given is multi-line. After
initial +OK, for each message in the maildrop, the POP3
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
use a certain format for unique-id listings. A unique-
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-
string, consisting of one to 70 characters in the range 0x21
to 0x7E, which uniquely identifies a message within
maildrop and which persists across sessions.
persistence is required even if a session ends
entering the UPDATE state. The server should never reuse
unique-id in a given maildrop, for as long as the
using the unique-id exists

Note that messages marked as deleted are not listed

While it is generally preferable for server
to store arbitrarily assigned unique-ids in the maildrop

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RFC 1939 POP3 May 1996

this specification is intended to permit unique-ids to
calculated as a hash of the message. Clients should be
to handle a situation where two identical copies of
message in a maildrop have the same unique-id

Possible Responses
+OK unique-id listing
-ERR no such

Examples
C:
S: +
S: 1 whqtswO00WBw418f9t5
S: 2 QhdPYR:00WBw1Ph7x
S: .
...
C: UIDL 2
S: +OK 2 QhdPYR:00WBw1Ph7x
...
C: UIDL 3
S: -ERR no such message, only 2 messages in

USER

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

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

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

The server may return a positive response even though
such mailbox exists. The server may return a
response if mailbox exists, but does not permit

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RFC 1939 POP3 May 1996

password authentication

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

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

PASS

Arguments
a server/mailbox-specific password (required

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

Discussion
When the client issues the PASS command, the POP3
uses the argument pair from the USER and PASS commands
determine if the client should be given access to
appropriate maildrop

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

Possible Responses
+OK maildrop locked and
-ERR invalid
-ERR unable to lock

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

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RFC 1939 POP3 May 1996

APOP name

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

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

Discussion
Normally, each POP3 session starts with a USER/
exchange. This results in a server/user-id
password being sent in the clear on the network.
intermittent use of POP3, this may not introduce a
risk. However, many POP3 client implementations connect
the POP3 server on a regular basis -- to check for
mail. Further the interval of session initiation may be
the order of five minutes. Hence, the risk of
capture is greatly enhanced

An alternate method of authentication is required
provides for both origin authentication and
protection, but which does not involve sending a
in the clear over the network. The APOP command
this functionality

A POP3 server which implements the APOP command
include a timestamp in its banner greeting. The syntax
the timestamp corresponds to the `msg-id' in [RFC822],
MUST be different each time the POP3 server issues a
greeting. For example, on a UNIX implementation in which
separate UNIX process is used for each instance of a POP
server, the syntax of the timestamp might be

hostname

where `process-ID' is the decimal value of the process'
PID, clock is the decimal value of the system clock,
hostname is the fully-qualified domain-name
to the host where the POP3 server is running

The POP3 client makes note of this timestamp, and
issues the APOP command. The `name' parameter
identical semantics to the `name' parameter of the
command. The `digest' parameter is calculated by
the MD5 algorithm [RFC1321] to a string consisting of
timestamp (including angle-brackets) followed by a

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RFC 1939 POP3 May 1996

secret. This shared secret is a string known only to
POP3 client and server. Great care should be taken
prevent unauthorized disclosure of the secret, as
of the secret will allow any entity to
masquerade as the named user. The `digest'
itself is a 16-octet value which is sent in
format, using lower-case ASCII characters

When the POP3 server receives the APOP command, it
the digest provided. If the digest is correct, the POP
server issues a positive response, and the POP3
enters the TRANSACTION state. Otherwise, a
response is issued and the POP3 session remains in
AUTHORIZATION state

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

Possible Responses
+OK maildrop locked and
-ERR permission

Examples
S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us
C: APOP mrose c4c9334bac560ecc979e58001b3e22
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

<1896.697170952@dbc.mtview.ca.us>

which produces a digest value

c4c9334bac560ecc979e58001b3e22

8. Scaling and Operational

Since some of the optional features described above were added to
POP3 protocol, experience has accumulated in using them in large
scale commercial post office operations where most of the users
unrelated to each other. In these situations and others, users
vendors of POP3 clients have discovered that the combination of
the UIDL command and not issuing the DELE command can provide a
version of the "maildrop as semi-permanent repository"
normally associated with IMAP. Of course the other capabilities

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RFC 1939 POP3 May 1996

IMAP, such as polling an existing connection for newly
messages and supporting multiple folders on the server, are
present in POP3.

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

Consequently, it is recommended that operators of large-scale multi
user servers, especially ones in which the user's only access to
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
result in the user's inability to receive new ones into
maildrop. Sites which choose this option should be sure to
users of impending or current exhaustion of quota, perhaps
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
retention of messages on the server, both read and unread.
example, a site might delete unread messages from the server
60 days and delete read messages after 7 days. Such
deletions are outside the scope of the POP3 protocol and are
considered a protocol violation

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

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

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

One special case of a site policy is that messages may only
downloaded once from the server, and are deleted after this
been accomplished. This could be implemented in POP3

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RFC 1939 POP3 May 1996

software by the following mechanism: "following a POP3 login by
client which was ended by a QUIT, delete all messages
during the session with the RETR command". It is important not
delete messages in the event of abnormal connection
(ie, if no QUIT was received from the client) because the
may not have successfully received or stored the messages
Servers implementing a download-and-delete policy may also wish
disable or limit the optional TOP command, since it could be
as an alternate mechanism to download entire messages

9. POP3 Command

Minimal POP3 Commands

USER name valid in the AUTHORIZATION
PASS

STAT valid in the TRANSACTION
LIST [msg
RETR
DELE

Optional POP3 Commands

APOP name digest valid in the AUTHORIZATION

TOP msg n valid in the TRANSACTION
UIDL [msg

POP3 Replies

+
-

Note that with the exception of the STAT, LIST, and UIDL commands
the reply given by the POP3 server to any command is
only to "+OK" and "-ERR". Any text occurring after this
may be ignored by the client

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10. Example POP3

S: connection on TCP port 110>
C: connection
S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us
C: APOP mrose c4c9334bac560ecc979e58001b3e22
S: +OK mrose's maildrop has 2 messages (320 octets
C:
S: +OK 2 320
C:
S: +OK 2 messages (320 octets
S: 1 120
S: 2 200
S: .
C: RETR 1
S: +OK 120
S:
S: .
C: DELE 1
S: +OK message 1
C: RETR 2
S: +OK 200
S:
S: .
C: DELE 2
S: +OK message 2
C:
S: +OK dewey POP3 server signing off (maildrop empty
C: connection
S: connection

11. Message

All messages transmitted during a POP3 session are assumed to
to the standard for the format of Internet text messages [RFC822].

It is important to note that the octet count for a message on
server host may differ from the octet count assigned to that
due to local conventions for designating end-of-line. Usually
during the AUTHORIZATION state of the POP3 session, the POP3
can calculate the size of each message in octets when it opens
maildrop. For example, if the POP3 server host internally
end-of-line as a single character, then the POP3 server simply
each occurrence of this character in a message as two octets.
that lines in the message which start with the termination octet
not (and must not) be counted twice, since the POP3 client
remove all byte-stuffed termination characters when it receives
multi-line response

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RFC 1939 POP3 May 1996

12.

[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
821, USC/Information Sciences Institute, August 1982.

[RFC822] Crocker, D., "Standard for the Format of ARPA-Internet
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 -
4", RFC 1730, University of Washington, December 1994.

[RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734,
Carnegie Mellon, December 1994.

13. Security

It is conjectured that use of the APOP command provides
identification and replay protection for a POP3 session
Accordingly, a POP3 server which implements both the PASS and
commands should not allow both methods of access for a given user
that is, for a given mailbox name, either the USER/PASS
sequence or the APOP command is allowed, but not both

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

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

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

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

Otherwise, security issues are not discussed in this memo

14.

The POP family has a long and checkered history. Although
a minor revision to RFC 1460, POP3 is based on the ideas presented
RFCs 918, 937, and 1081.

In addition, Alfred Grimstad, Keith McCloghrie, and Neil
provided significant comments on the APOP command

Myers & Rose Standards Track [Page 20]

RFC 1939 POP3 May 1996

15. Authors'

John G.
Carnegie-Mellon
5000 Forbes
Pittsburgh, PA 15213

EMail: jgm+@cmu.

Marshall T.
Dover Beach Consulting, Inc
420 Whisman
Mountain View, CA 94043-2186

EMail: mrose@dbc.mtview.ca.

Myers & Rose Standards Track [Page 21]

RFC 1939 POP3 May 1996

Appendix A. Differences from RFC 1725

This memo is a revision to RFC 1725, a Draft Standard. It makes
following changes from that document

- clarifies that command keywords are case insensitive

- specifies that servers must send "+OK" and "-ERR"
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
commands, to reduce confusion

- clarifies that the PASS command may only be given
after a successful USER command

- clarified the persistence requirements of UIDs and added
implementation notes

- specifies a UID length limitation of one to 70 octets

- specifies a status indicator length
of 512 octets, including the CRLF

- clarifies that LIST with no arguments on an empty
returns success

- adds a reference from the LIST command to the Message

- clarifies the behavior of QUIT upon

- clarifies the security section to not imply the use of
USER command with the APOP command

- adds references to RFCs 1730 and 1734

- clarifies the method by which a UA may enter mail into
transport system

Myers & Rose Standards Track [Page 22]

RFC 1939 POP3 May 1996

- clarifies that the second argument to the TOP command is
number of lines

- changes the suggestion in the Security Considerations
for a server to not accept both PASS and APOP for a given
from a "must" to a "should".

- adds a section on scaling and operational

Appendix B. Command

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

Myers & Rose Standards Track [Page 23]

if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.

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