Pop Protocol Essay, Research Paper

Network Working Group J. Myers

Request for Remarks: 1725 Carnegie Mellon

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Obsoletes: 1460 M. Rose

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November 1994

Post Office Protocol & # 8211 ; Version 3

Status of this Memo

This papers specifies an Internet criterions path protocol for the

Internet community, and petitions treatment and suggestions for

betterments. Please refer to the current edition of the & # 8220 ; Internet

Official Protocol Standards & # 8221 ; ( STD 1 ) for the standardisation province

and position of this protocol. Distribution of this memo is limitless.

Overview

This memo is a alteration to RFC 1460, a Draft Standard. It makes the

following alterations from that papers:

– removed text sing & # 8220 ; split-UA theoretical account & # 8221 ; , which didn & # 8217 ; t add

anything to the apprehension of POP

– clarified sentence structure of bids, keywords, and statements

– clarified behaviour on broken connexion

– explicitly permitted an inaction autologout timer

– clarified the demands of the & # 8220 ; exclusive-access lock & # 8221 ;

– removed implementation-specific diction sing the parsing of

the maildrop

– allowed waiters to shut the connexion after a failed

hallmark bid

– removed the LAST bid

– fixed misprint in illustration of TOP bid

– clarified that the 2nd statement to the TOP bid is non-

negative

– added the optional UIDL bid

Myers & A ; Rose [ Page 1 ]

RFC 1725 POP3 November 1994

– added warning sing length of shared secrets with APOP

– added extra warnings to the security considerations subdivision

1. Introduction

On certain types of smaller nodes in the Internet it is frequently

impractical to keep a message conveyance system ( MTS ) . For

illustration, a workstation may non hold sufficient resources ( rhythms,

disc infinite ) in order to allow a SMTP waiter [ RFC821 ] and associated

local mail bringing system to be kept resident and continuously

running. Similarly, it may be expensive ( or impossible ) to maintain a

personal computing machine interconnected to an IP-style web for long

sums of clip ( the node is missing the resource known as

& # 8220 ; connectivity & # 8221 ; ) .

Despite this, it is frequently really utile to be able to pull off mail on

these smaller nodes, and they frequently support a user agent ( UA ) to help

the undertakings of mail handling. To work out this job, a node which can

support an MTS entity offers a maildrop service to these less endowed

nodes. The Post Office Protocol & # 8211 ; Version 3 ( POP3 ) is intended to

license a workstation to dynamically entree a maildrop on a waiter

host in a utile manner. Normally, this means that the POP3 is used

to let a workstation to recover mail that the waiter is keeping

for it.

For the balance of this memo, the term & # 8220 ; client host & # 8221 ; refers to a

host devising usage of the POP3 service, while the term & # 8220 ; server host & # 8221 ;

refers to a host which offers the POP3 service.

2. A Short Digression

This memo does non stipulate how a client host enters mail into the

conveyance system, although a method consistent with the doctrine of

this memo is presented here:

When the user agent on a client host wants to come in a message

into the conveyance system, it establishes an SMTP connexion to

its relay host ( this relay host could be, but need non be, the

POP3 waiter host for the client host ) .

3. Basic Operation

Initially, the waiter host starts the POP3 service by listening on

TCP port 110. When a client host wants to do usage of the service,

it establishes a TCP connexion with the waiter host. When the

connexion is established, the POP3 waiter sends a salutation. The

client and POP3 server so interchange bids and responses

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RFC 1725 POP3 November 1994

( severally ) until the connexion is closed or aborted.

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

or more statements. All bids are terminated by a CRLF brace.

Keywords and statements consist of printable ASCII characters.

Keywords and statements are each separated by a individual Space

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

may be up to 40 characters long.

Responses in the POP3 consist of a position index and a keyword

perchance followed by extra information. All responses are

terminated by a CRLF brace. There are presently two position

indexs: positive ( & # 8221 ; +OK & # 8221 ; ) and negative ( & # 8221 ; -ERR & # 8221 ; ) .

Responses to certain bids are multi-line. In these instances, which

are clearly indicated below, after directing the first line of the

response and a CRLF, any extra lines are sent, each terminated

by a CRLF brace. When all lines of the response have been sent, a

concluding line is sent, dwelling of a expiration eight ( denary codification

046, & # 8220 ; . & # 8221 ; ) and a CRLF brace. If any line of the multi-line response

Begins with the expiration eight, the line is & # 8220 ; byte-stuffed & # 8221 ; by

pre-pending the expiration eight to that line of the response.

Therefore a multi-line response is terminated with the five eights

& # 8220 ; CRLF.CRLF & # 8221 ; . When analyzing a multi-line response, the client cheques

to see if the line begins with the expiration eight. If so and if

eights other than CRLF follow, the the first eight of the line ( the

expiration eight ) is stripped off. If so and if CRLF instantly

follows the expiration character, so the response from the POP

waiter is ended and the line incorporating & # 8220 ; .CRLF & # 8221 ; is non considered

portion of the multi-line response.

A POP3 session progresses through a figure of provinces during its

life-time. Once the TCP connexion has been opened and the POP3

waiter has sent the salutation, the session enters the Mandate

province. In this province, the client must place itself to the POP3

waiter. Once the client has successfully done this, the waiter

acquires resources associated with the client & # 8217 ; s maildrop, and the

session enters the TRANSACTION province. In this province, the client

petitions actions on the portion of the POP3 waiter. When the client has

issued the QUIT bid, the session enters the UPDATE province. In

this province, the POP3 waiter releases any resources acquired during

the TRANSACTION province and says adieu. The TCP connexion is so

closed.

A POP3 waiter MAY have an inaction autologout timer. Such a timer

MUST be of at least 10 proceedingss & # 8217 ; continuance. The reception of any bid

from the client during that interval should do to reset the

autologout timer. When the timer expires, the session does NOT come in

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RFC 1725 POP3 November 1994

the UPDATE province & # 8211 ; the waiter should shut the TCP connexion without

taking any messages or directing any response to the client.

4. The AUTHORIZATION State

Once the TCP connexion has been opened by a POP3 client, the POP3

server issues a one line salutation. This can be any twine terminated

by CRLF. An illustration might be:

Second: +OK POP3 waiter ready

Note that this salutation is a POP3 answer. The POP3 waiter should

ever give a positive response as the salutation.

The POP3 session is now in the AUTHORIZATION province. The client must

now place and authenticate itself to the POP3 waiter. Two

possible mechanisms for making this are described in this papers,

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

bid is described subsequently in this papers.

To authenticate utilizing the USER and PASS bid combination, the

client must first publish the USER bid. If the POP3 waiter

responds with a positive position index ( & # 8221 ; +OK & # 8221 ; ) , so the client

may publish either the PASS bid to finish the hallmark, or

the QUIT bid to end the POP3 session. If the POP3 waiter

responds with a negative position index ( & # 8221 ; -ERR & # 8221 ; ) to the USER

bid, so the client may either publish a new hallmark

bid or may publish the QUIT bid.

When the client issues the PASS bid, the POP3 waiter uses the

statement brace from the USER and PASS commands to find if the

client should be given entree to the appropriate maildrop.

Once the POP3 waiter has determined through the usage of any

hallmark bid that the client should be given entree to the

appropriate maildrop, the POP3 waiter so acquires an exclusive-

entree lock on the maildrop, as necessary to forestall messages from

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

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

positive position index. The POP3 session now enters the

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

maildrop can non be opened for some ground ( for illustration, a lock can

non be acquired, the client is denied entree to the appropriate

maildrop, or the maildrop can non be parsed ) , the POP3 waiter responds

with a negative position index. ( If a lock was acquired but the

POP3 waiter intends to react with a negative position index, the

POP3 waiter must let go of the lock prior to rejecting the bid. )

After returning a negative position index, the waiter may shut the

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RFC 1725 POP3 November 1994

connexion. If the waiter does non shut the connexion, the client

may either publish a new hallmark bid and start once more, or the

client may publish the QUIT bid.

After the POP3 waiter has opened the maildrop, it assigns a message-

figure to each message, and notes the size of each message in eights.

The first message in the maildrop is assigned a message-number of

& # 8220 ; 1 & # 8243 ; , the 2nd is assigned & # 8220 ; 2 & # 8243 ; , and so on, so that the n & # 8217 ; th message

in a maildrop is assigned a message-number of & # 8220 ; n & # 8221 ; . In POP3 bids

and responses, all message-number & # 8217 ; s and message sizes are expressed

in base-10 ( i.e. , decimal ) .

Here are sum-ups for the three POP3 bids discussed therefore far:

User name

Arguments:

a twine placing a letter box ( required ) , which is of

significance ONLY to the waiter

Restrictions:

may merely be given in the AUTHORIZATION province after the POP3

salutation or after an unsuccessful User or PASS bid

Possible Responses:

+OK name is a valid letter box

-ERR ne’er heard of letter box name

Examples:

Degree centigrades: USER mrose

Second: +OK mrose is a existent hoopy frood

& # 8230 ;

Degree centigrades: USER frated

Second: -ERR sorry, no letter box for frated here

PASS twine

Arguments:

a server/mailbox-specific watchword ( required )

Restrictions:

may merely be given in the AUTHORIZATION province after a

successful USER bid

Discussion:

Since the PASS bid has precisely one statement, a POP3

waiter may handle infinites in the statement as portion of the

watchword, alternatively of as statement centrifuges.

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RFC 1725 POP3 November 1994

Possible Responses:

+OK maildrop locked and ready

-ERR invalid watchword

-ERR unable to lock maildrop

Examples:

Degree centigrades: USER mrose

Second: +OK mrose is a existent hoopy frood

Degree centigrades: PASS secret

Second: +OK mrose & # 8217 ; s maildrop has 2 messages ( 320 eights )

& # 8230 ;

Degree centigrades: USER mrose

Second: +OK mrose is a existent hoopy frood

Degree centigrades: PASS secret

Second: -ERR maildrop already locked

QUIT

Arguments: none

Restrictions: none

Possible Responses:

+OK

Examples:

Degree centigrades: Discontinue

Second: +OK Dewey POP3 server subscribing off

5. The TRANSACTION State

Once the client has successfully identified itself to the POP3 waiter

and the POP3 waiter has locked and opened the appropriate maildrop,

the POP3 session is now in the TRANSACTION province. The client may now

issue any of the undermentioned POP3 bids repeatedly. After each

bid, the POP3 waiter issues a response. Finally, the client

issues the QUIT bid and the POP3 session enters the UPDATE province.

Here are the POP3 commands valid in the TRANSACTION province:

STAT

Arguments: none

Restrictions:

may merely be given in the TRANSACTION province

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RFC 1725 POP3 November 1994

Discussion:

The POP3 waiter issues a positive response with a line

incorporating information for the maildrop. This line is

called a & # 8220 ; bead listing & # 8221 ; for that maildrop.

In order to simplify parsing, all POP3 waiters required to

utilize a certain format for bead listings. The positive

response consists of & # 8220 ; +OK & # 8221 ; followed by a individual infinite, the

figure of messages in the maildrop, a individual infinite, and the

size of the maildrop in eights. This memo makes no

demand on what follows the maildrop size. Minimal

executions should merely stop that line of the response

with a CRLF brace. More advanced executions may

include other information.

Note: This memo STRONGLY discourages executions

from providing extra information in the bead

listing. Other, optional, installations are discussed

subsequently on which permit the client to parse the messages

in the maildrop.

Note that messages marked as deleted are non counted in

either sum.

Possible Responses:

+OK nn millimeter

Examples:

Degree centigrades: STAT

Second: +OK 2 320

LIST [ monosodium glutamate ]

Arguments:

a message-number ( optional ) , which, if present, may NOT

refer to a message marked as deleted

Restrictions:

may merely be given in the TRANSACTION province

Discussion:

If an statement was given and the POP3 waiter issues a

positive response with a line incorporating information for

that message. This line is called a & # 8220 ; scan listing & # 8221 ; for

that message.

If no statement was given and the POP3 waiter issues a

positive response, so the response given is multi-line.

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RFC 1725 POP3 November 1994

After the initial +OK, for each message in the maildrop,

the POP3 waiter responds with a line incorporating information

for that message. This line is besides called a & # 8220 ; scan

naming & # 8221 ; for that message.

In order to simplify parsing, all POP3 waiters are required

to utilize a certain format for scan listings. A scan listing

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

a individual infinite and the exact size of the message in eights.

This memo makes no demand on what follows the message

size in the scan listing. Minimal executions should

merely stop that line of the response with a CRLF brace. More

advanced executions may include other information, as

parsed from the message.

Note: This memo STRONGLY discourages executions

from providing extra information in the scan

listing. Other, optional, installations are discussed

subsequently on which permit the client to parse the messages

in the maildrop.

Note that messages marked as deleted are non listed.

Possible Responses:

+OK scan listing follows

-ERR no such message

Examples:

Degree centigrades: List

Second: +OK 2 messages ( 320 eights )

Second: 1 120

Second: 2 200

Second: .

& gt ;

& # 8230 ;

Degree centigrades: List 2

Second: +OK 2 200

& # 8230 ;

Degree centigrades: List 3

Second: -ERR no such message, merely 2 messages in maildrop

RETR monosodium glutamate

Arguments:

a message-number ( required ) which may non mention to a

message marked as deleted

Restrictions:

may merely be given in the TRANSACTION province

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RFC 1725 POP3 November 1994

Discussion:

If the POP3 waiter issues a positive response, so the

response given is multi-line. After the initial +OK, the

POP3 waiter sends the message matching to the given

message-number, being careful to byte-stuff the expiration

character ( as with all multi-line responses ) .

Possible Responses:

+OK message follows

-ERR no such message

Examples:

Degree centigrades: RETR 1

Second: +OK 120 eights

Second:

Second: .

DELE monosodium glutamate

Arguments:

a message-number ( required ) which may non mention to a

message marked as deleted

Restrictions:

may merely be given in the TRANSACTION province

Discussion:

The POP3 waiter marks the message as deleted. Any hereafter

mention to the message-number associated with the message

in a POP3 bid generates an mistake. The POP3 waiter does

non really cancel the message until the POP3 session

enters the UPDATE province.

Possible Responses:

+OK message deleted

-ERR no such message

Examples:

Degree centigrades: DELE 1

Second: +OK message 1 deleted

& # 8230 ;

Degree centigrades: DELE 2

Second: -ERR message 2 already deleted

NOOP

Arguments: none

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RFC 1725 POP3 November 1994

Restrictions:

may merely be given in the TRANSACTION province

Discussion:

The POP3 waiter does nil, it simply replies with a

positive response.

Possible Responses:

+OK

Examples:

Degree centigrades: NOOP

Second: +OK

RSET

Arguments: none

Restrictions:

may merely be given in the TRANSACTION province

Discussion:

If any messages have been marked as deleted by the POP3

waiter, they are unmarked. The POP3 waiter so answers

with a positive response.

Possible Responses:

+OK

Examples:

Degree centigrades: RSET

Second: +OK maildrop has 2 messages ( 320 eights )

6. The UPDATE State

When the client issues the QUIT bid from the TRANSACTION province,

the POP3 session enters the UPDATE province. ( Note that if the client

issues the QUIT bid from the AUTHORIZATION province, the POP3

session terminates but does NOT come in the UPDATE province. )

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

QUIT bid, the POP3 session does NOT come in the UPDATE province and

MUST non take any messages from the maildrop.

QUIT

Arguments: none

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RFC 1725 POP3 November 1994

Restrictions: none

Discussion:

The POP3 waiter removes all messages marked as deleted from

the maildrop. It so releases any exclusive-access lock

on the maildrop and answers as to the position of these

operations. The TCP connexion is so closed.

Possible Responses:

+OK

Examples:

Degree centigrades: Discontinue

Second: +OK Dewey POP3 server subscribing off ( maildrop empty )

& # 8230 ;

Degree centigrades: Discontinue

Second: +OK Dewey POP3 server subscribing off ( 2 messages left )

& # 8230 ;

7. Optional POP3 Commands

The POP3 bids discussed above must be supported by all minimum

executions of POP3 waiters.

The optional POP3 bids described below license a POP3 client

greater freedom in message handling, while continuing a simple POP3

server execution.

Note: This memo STRONGLY encourages executions to back up

these bids in stead of developing augmented bead and scan

listings. In short, the doctrine of this memo is to set

intelligence in the portion of the POP3 client and non the POP3

waiter.

Top monosodium glutamate N

Arguments:

a message-number ( required ) which may NOT mention to to a

message marked as deleted, and a non-negative figure

( required )

Restrictions:

may merely be given in the TRANSACTION province

Discussion:

If the POP3 waiter issues a positive response, so the

response given is multi-line. After the initial +OK, the

POP3 waiter sends the headings of the message, the space

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RFC 1725 POP3 November 1994

line dividing the headings from the organic structure, and so the

figure of lines indicated message & # 8217 ; s organic structure, being careful to

byte-stuff the expiration character ( as with all multi-

line responses ) .

Note that if the figure of lines requested by the POP3

client is greater than than the figure of lines in the

organic structure, so the POP3 waiter sends the full message.

Possible Responses:

+OK top of message follows

-ERR no such message

Examples:

Degree centigrades: Top 1 10

Second: +OK

Second:

Second: .

& # 8230 ;

Degree centigrades: Top 100 3

Second: -ERR no such message

UIDL [ monosodium glutamate ]

Arguments:

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

it may NOT mention to a message marked as deleted.

Restrictions:

may merely be given in the TRANSACTION province.

Discussion:

If an statement was given and the POP3 waiter issues a positive

response with a line incorporating information for that message.

This line is called a & # 8220 ; unique-id listing & # 8221 ; for that message.

If no statement was given and the POP3 waiter issues a positive

response, so the response given is multi-line. After the

initial +OK, for each message in the maildrop, the POP3 waiter

responds with a line incorporating information for that message.

This line is called a & # 8220 ; unique-id listing & # 8221 ; for that message.

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

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

naming consists of the message-number of the message,

followed by a individual infinite and the unique-id of the message.

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RFC 1725 POP3 November 1994

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

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

twine, dwelling of characters in the scope 0 & # 215 ; 21 to 0 & # 215 ; 7E,

which unambiguously identifies a message within a maildrop and

which persists across Sessionss. The waiter should ne’er recycle

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

utilizing the unique-id exists.

Note that messages marked as deleted are non listed.

Possible Responses:

+OK unique-id listing follows

-ERR no such message

Examples:

Degree centigrades: UIDL

Second: +OK

Second: 1 whqtswO00WBw418f9t5JxYwZ

Second: 2 QhdPYR:00WBw1Ph7 & # 215 ; 7

Second: .

& # 8230 ;

Degree centigrades: UIDL 2

Second: +OK 2 QhdPYR:00WBw1Ph7 & # 215 ; 7

& # 8230 ;

Degree centigrades: UIDL 3

Second: -ERR no such message, merely 2 messages in maildrop

APOP name digest

Arguments:

a twine placing a letter box and a MD5 digest twine

( both required )

Restrictions:

may merely be given in the AUTHORIZATION province after the POP3

recognizing

Discussion:

Normally, each POP3 session starts with a USER/PASS

exchange. This consequences in a server/user-id particular

watchword being sent in the clear on the web. For

intermittent usage of POP3, this may non present a ample

hazard. However, many POP3 client executions connect to

the POP3 waiter on a regular footing & # 8212 ; to look into for new

mail. Further the interval of session induction may be on

the order of five proceedingss. Hence, the hazard of watchword

gaining control is greatly enhanced.

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RFC 1725 POP3 November 1994

An alternate method of hallmark is required which

provides for both origin hallmark and rematch

protection, but which does non affect directing a watchword

in the clear over the web. The APOP bid provides

this functionality.

A POP3 waiter which implements the APOP bid will

include a timestamp in its streamer salutation. The sentence structure of

the timestamp corresponds to the `msg-id & # 8217 ; in [ RFC822 ] , and

MUST be different each clip the POP3 waiter issues a streamer

salutation. For illustration, on a UNIX execution in which a

separate UNIX procedure is used for each case of a POP3

waiter, the sentence structure of the timestamp might be:

where `process-ID & # 8217 ; is the denary value of the procedure & # 8217 ; s

PID, clock is the denary value of the system clock, and

hostname is the fully-qualified domain-name corresponding

to the host where the POP3 waiter is running.

The POP3 client makes note of this timestamp, and so

issues the APOP bid. The `name & # 8217 ; parametric quantity has

indistinguishable semantics to the `name & # 8217 ; parametric quantity of the USER

bid. The `digest & # 8217 ; parametric quantity is calculated by using

the MD5 algorithm [ RFC1321 ] to a threading consisting of the

timestamp ( including angle-brackets ) followed by a shared

secret. This shared secret is a threading known merely to the

POP3 client and waiter. Great attention should be taken to

prevent unauthorised revelation of the secret, as cognition

of the secret will let any entity to successfully

mask as the named user. The `digest & # 8217 ; parametric quantity

itself is a 16-octet value which is sent in hexadecimal

format, utilizing lower-case ASCII characters.

When the POP3 waiter receives the APOP bid, it verifies

the digest provided. If the digest is right, the POP3

server issues a positive response, and the POP3 session

enters the TRANSACTION province. Otherwise, a negative

response is issued and the POP3 session remains in the

AUTHORIZATION province.

Note that as the length of the shared secret additions, so

does the trouble of deducing it. As such, shared

secrets should be long strings ( well longer than

the 8-character illustration shown below ) .

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RFC 1725 POP3 November 1994

Possible Responses:

+OK maildrop locked and ready

-ERR permission denied

Examples:

Second: +OK POP3 waiter ready

Degree centigrades: APOP mrose c4c9334bac560ecc979e58001b3e22fb

Second: +OK maildrop has 1 message ( 369 eights )

In this illustration, the shared secret is the threading `tan-

staaf & # 8217 ; . Hence, the MD5 algorithm is applied to the twine

tanstaaf

which produces a digest value of

c4c9334bac560ecc979e58001b3e22fb

8. POP3 Command Summary

Minimal POP3 Commands:

USER name valid in the AUTHORIZATION province

PASS twine

QUIT

STAT valid in the TRANSACTION province

LIST [ monosodium glutamate ]

RETR monosodium glutamate

DELE monosodium glutamate

NOOP

RSET

QUIT valid in the UPDATE province

Optional POP3 Commands:

APOP name digest valid in the AUTHORIZATION province

Top monosodium glutamate N valid in the TRANSACTION province

UIDL [ monosodium glutamate ]

POP3 Answers:

+OK

-ERR

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RFC 1725 POP3 November 1994

Note that with the exclusion of the STAT, LIST, and UIDL bids,

the answer given by the POP3 waiter to any bid is important merely

to & # 8220 ; +OK & # 8221 ; and & # 8220 ; -ERR & # 8221 ; . Any text occurring after this answer may be

ignored by the client.

9. Example POP3 Session

Second:

Degree centigrades:

Second: +OK POP3 waiter ready

Degree centigrades: APOP mrose c4c9334bac560ecc979e58001b3e22fb

Second: +OK mrose & # 8217 ; s maildrop has 2 messages ( 320 eights )

Degree centigrades: STAT

Second: +OK 2 320

Degree centigrades: List

Second: +OK 2 messages ( 320 eights )

Second: 1 120

Second: 2 200

Second: .

Degree centigrades: RETR 1

Second: +OK 120 eights

Second:

Second: .

Degree centigrades: DELE 1

Second: +OK message 1 deleted

Degree centigrades: RETR 2

Second: +OK 200 eights

Second:

Second: .

Degree centigrades: DELE 2

Second: +OK message 2 deleted

Degree centigrades: Discontinue

Second: +OK Dewey POP3 server subscribing off ( maildrop empty )

Degree centigrades:

Second:

10. Message Format

All messages transmitted during a POP3 session are assumed to conform

to the criterion for the format of Internet text messages [ RFC822 ] .

It is of import to observe that the eight count for a message on the

server host may differ from the eight count assigned to that message

due to local conventions for denominating end-of-line. Normally,

during the AUTHORIZATION province of the POP3 session, the POP3 waiter

can cipher the size of each message in eights when it opens the

maildrop. For illustration, if the POP3 waiter host internally represents

end-of-line as a individual character, so the POP3 waiter merely counts

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RFC 1725 POP3 November 1994

each happening of this character in a message as two eights. Note

that lines in the message which start with the expiration eight demand

non be counted twice, since the POP3 client will take all byte-

stuffed expiration characters when it receives a multi-line

response.

11. Mentions

[ RFC821 ] Postel, J. , & # 8220 ; Simple Mail Transfer Protocol & # 8221 ; , STD 10, RFC

821, USC/Information Sciences Institute, August 1982.

[ RFC822 ] Crocker, D. , & # 8220 ; Standard for the Format of ARPA-Internet Text

Messages & # 8221 ; , STD 11, RFC 822, University of Delaware, August 1982.

[ RFC1321 ] Rivest, R. & # 8220 ; The MD5 Message-Digest Algorithm & # 8221 ; , RFC 1321,

MIT Laboratory for Computer Science, April, 1992.

12. Security Considerations

It is conjectured that usage of the APOP bid provides beginning

designation and rematch protection for a POP3 session.

Consequently, a POP3 waiter which implements both the PASS and APOP

bids must non let both methods of entree for a given user ; that

is, for a given & # 8220 ; USER name & # 8221 ; either the PASS or APOP bid is

allowed, but non both.

Further, note that as the length of the shared secret additions, so

does the trouble of deducing it.

Waiters that answer -ERR to the USER bid are giving possible

aggressors hints about which names are valid

Use of the PASS bid sends watchwords in the clear over the

web.

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

web.

Otherwise, security issues are non discussed in this memo.

13. Recognitions

The POP household has a long and checked history. Although chiefly

a minor alteration to RFC 1460, POP3 is based on the thoughts presented in

RFCs 918, 937, and 1081.

In add-on, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff

provided important remarks on the APOP bid.

Myers & A ; Rose [ Page 17 ]

RFC 1725 POP3 November 1994

14. Authors & # 8217 ; Addresss

John G. Myers

Carnegie-Mellon University

5000 Forbes Ave

Pittsburgh, PA 15213

Electronic mail: jgm+ @ cmu.edu

Marshall T. Rose

Dover Beach Consulting, Inc.

420 Whisman Court

Mountain View, CA 94043-2186

Electronic mail: mrose @ dbc.mtview.ca.us

Myers & A ; Rose [ Page 18 ]

.

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