Pop Protocol Essay Research Paper Network Working

Pop Protocol Essay, Research Paper

Network Working Group J. Myers

Request for Comments: 1725 Carnegie Mellon

Obsoletes: 1460 M. Rose

Category: Standards Track Dover Beach Consulting, Inc.

November 1994

Post Office Protocol – Version 3

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the “Internet

Official Protocol Standards” (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Overview

This memo is a 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

- clarified behavior on broken connection

- explicitly permitted an inactivity autologout timer

- clarified the requirements of the “exclusive-access lock”

- removed implementation-specific wording regarding the parsing of

the maildrop

- allowed servers to close the connection after a failed

authentication command

- removed the LAST command

- fixed typo in example of TOP command

- clarified that the second argument to the TOP command is non-

negative

- added the optional UIDL command

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- added warning regarding length of shared secrets with APOP

- added additional warnings to the security considerations section

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. 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).

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

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(respectively) until the connection is closed or aborted.

Commands in the POP3 consist of a keyword, possibly followed by one

or more arguments. All commands are terminated by a CRLF pair.

Keywords and arguments consist of printable ASCII characters.

Keywords and arguments are each separated by a single SPACE

character. Keywords are three or four characters long. Each argument

may be up to 40 characters long.

Responses in the POP3 consist of a status indicator and a keyword

possibly followed by additional information. All responses are

terminated by a CRLF pair. There are currently two status

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

issued the QUIT command, 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.

A POP3 server MAY have an inactivity autologout timer. Such a timer

MUST be of at least 10 minutes’ duration. The receipt of any command

from the client during that interval should suffice to reset the

autologout timer. When the timer expires, the session does NOT enter

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the UPDATE state–the server should close the TCP connection without

removing any messages or sending any response to the client.

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 identify and authenticate itself to the POP3 server. Two

possible mechanisms for doing this are described in this document,

the USER and PASS command combination and the APOP command. The APOP

command is described later in this document.

To authenticate using the USER and PASS command combination, the

client must first issue the USER command. If the POP3 server

responds with a positive status indicator (”+OK”), then the client

may issue either the PASS command to complete the authentication, or

the QUIT command to terminate the POP3 session. If the POP3 server

responds with a negative status indicator (”-ERR”) to the USER

command, then the client may either issue a new authentication

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.

Once the POP3 server has determined through the use of any

authentication command that the client should be given access to the

appropriate maildrop, the POP3 server then acquires an exclusive-

access lock on the maildrop, as necessary to prevent messages from

being modified or removed before the session enters the UPDATE state.

If the lock is successfully acquired, the POP3 server responds with a

positive status indicator. The POP3 session now enters the

TRANSACTION state, with no messages marked as deleted. If the the

maildrop cannot be opened for some reason (for example, a lock can

not be acquired, the client is denied access to the appropriate

maildrop, or the maildrop cannot be parsed), the POP3 server responds

with a negative status indicator. (If a lock was acquired but the

POP3 server intends to respond with a negative status indicator, the

POP3 server must release the lock prior to rejecting the command.)

After returning a negative status indicator, the server may close the

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connection. If the server does not close the connection, the client

may either issue a new authentication command and start again, or the

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 of

“1″, the second is assigned “2″, and so on, so that the n’th message

in a maildrop is assigned a message-number of “n”. In POP3 commands

and responses, all message-number’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

significance 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

Discussion:

Since the PASS command has exactly one argument, a POP3

server may treat spaces in the argument as part of the

password, instead of as argument separators.

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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

5. The TRANSACTION State

Once the client has successfully identified itself to the POP3 server

and the POP3 server has locked and opened 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

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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 required to

use a certain format for drop listings. The positive

response consists of “+OK” followed by a single space, the

number of messages in the maildrop, a single space, and 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

LIST [msg]

Arguments:

a message-number (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.

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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 also 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. A scan listing

consists of the message-number of the message, followed by

a single space and the exact 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

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-number (required) which may not refer to a

message marked as deleted

Restrictions:

may only be given in the TRANSACTION state

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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-number, being careful to byte-stuff the termination

character (as with all multi-line responses).

Possible Responses:

+OK message follows

-ERR no such message

Examples:

C: RETR 1

S: +OK 120 octets

S:

S: .

DELE msg

Arguments:

a message-number (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-number 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.

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

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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

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.

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.)

If a session terminates for some reason other than a client-issued

QUIT command, the POP3 session does NOT enter the UPDATE state and

MUST not remove any messages from the maildrop.

QUIT

Arguments: none

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Restrictions: none

Discussion:

The POP3 server removes all messages marked as deleted from

the maildrop. It then releases any exclusive-access lock

on the maildrop and replies as to the status 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.

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-number (required) which may NOT refer to to a

message marked as deleted, and a non-negative number

(required)

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

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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:

S: .

…

C: TOP 100 3

S: -ERR no such message

UIDL [msg]

Arguments:

a message-number (optionally) If a message-number is given,

it 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 “unique-id 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 “unique-id listing” for that message.

In order to simplify parsing, all POP3 servers are required to

use a certain format for unique-id listings. A unique-id

listing consists of the message-number of the message,

followed by a single space and the unique-id of the message.

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No information follows the unique-id in the unique-id listing.

The unique-id of a message is an arbitrary server-determined

string, consisting of characters in the range 0Ч21 to 0Ч7E,

which uniquely identifies a message within a maildrop and

which persists across sessions. The server should never reuse

an unique-id in a given maildrop, for as long as the entity

using the unique-id exists.

Note that messages marked as deleted are not listed.

Possible Responses:

+OK unique-id listing follows

-ERR no such message

Examples:

C: UIDL

S: +OK

S: 1 whqtswO00WBw418f9t5JxYwZ

S: 2 QhdPYR:00WBw1Ph7Ч7

S: .

…

C: UIDL 2

S: +OK 2 QhdPYR:00WBw1Ph7Ч7

…

C: UIDL 3

S: -ERR no such message, only 2 messages in maildrop

APOP name digest

Arguments:

a string identifying a mailbox and a MD5 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.

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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:

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.

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).

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Possible Responses:

+OK maildrop locked and ready

-ERR permission denied

Examples:

S: +OK POP3 server ready

C: APOP mrose c4c9334bac560ecc979e58001b3e22fb

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 string

tanstaaf

which produces a digest value of

c4c9334bac560ecc979e58001b3e22fb

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

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

UIDL [msg]

POP3 Replies:

+OK

-ERR

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Note that with the exception of the STAT, LIST, and UIDL commands,

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.

9. Example POP3 Session

S:

C:

S: +OK POP3 server ready

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:

S: .

C: DELE 1

S: +OK message 1 deleted

C: RETR 2

S: +OK 200 octets

S:

S: .

C: DELE 2

S: +OK message 2 deleted

C: QUIT

S: +OK dewey POP3 server signing off (maildrop empty)

C:

S:

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 server

can calculate the size of each message in octets when it opens the

maildrop. For example, if the POP3 server host internally represents

end-of-line as a single character, then the POP3 server simply counts

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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.

11. References

[RFC821] Postel, J., “Simple Mail Transfer Protocol”, STD 10, RFC

821, USC/Information Sciences Institute, August 1982.

[RFC822] Crocker, D., “Standard for the Format of ARPA-Internet Text

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.

12. 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.

Servers that answer -ERR to the USER command are giving potential

attackers clues about which names are valid

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

network.

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

network.

Otherwise, security issues are not discussed in this memo.

13. 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.

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14. Authors’ Addresses

John G. Myers

Carnegie-Mellon University

5000 Forbes Ave

Pittsburgh, PA 15213

EMail: jgm+@cmu.edu

Marshall T. Rose

Dover Beach Consulting, Inc.

420 Whisman Court

Mountain View, CA 94043-2186

EMail: mrose@dbc.mtview.ca.us

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.