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RFC 937
Network Working Group M. Butler
Request for Comments: 937 J. Postel
D. Chase
J. Goldberger
J. K. Reynolds
Obsoletes: RFC 918 ISI
February 1985
POST OFFICE PROTOCOL - VERSION 2
Status of this Memo
This RFC suggests a simple method for workstations to dynamically
access mail from a mailbox server. This RFC specifies a proposed
protocol for the ARPA-Internet community, and requests discussion and
suggestions for improvement. This memo is a revision of RFC 918.
Distribution of this memo is unlimited.
Introduction
The intent of the Post Office Protocol Version 2 (POP2) is to allow a
user's workstation to access mail from a mailbox server. It is
expected that mail will be posted from the workstation to the mailbox
server via the Simple Mail Transfer Protocol (SMTP). For further
information see RFC-821 [1] and RFC-822 [2].
This protocol assumes a reliable data stream such as provided by TCP
or any similar protocol. When TCP is used, the POP2 server listens
on port 109 [4].
System Model and Philosophy
While we view the workstation as an Internet host in the sense that
it implements IP, we do not expect the workstation to contain the
user's mailbox. We expect the mailbox to be on a server machine.
We believe it is important for the mailbox to be on an "always up"
machine and that a workstation may be frequently powered down, or
otherwise unavailable as an SMTP server.
POP2 is designed for an environment of workstations and servers on a
low-delay, high-throughput, local networks (such as Ethernets). POP2
may be useful in other environments as well, but if the environment
is substantially different, a different division of labor between the
client and server may be appropriate, and a different protocol
required.
Suppose the user's real name is John Smith, the user's machine is
called FIDO, and that the mailbox server is called DOG-HOUSE. Then
Butler, et. al. [Page 1]
RFC 937 February 1985
Post Office Protocol
we expect the user's mail to be addressed to JSmith@DOG-HOUSE.ARPA
(not JSmith@FIDO.ARPA).
That is, the destination of the mail is the mailbox on the server
machine. The POP2 protocol and the workstation are merely a
mechanism for viewing the messages in the mailbox.
The user is not tied to any particular workstation for accessing his
mail. The workstation does not appear as any part of the mailbox
address.
This is a very simple protocol. This is not a user interface. We
expect that there is a program in the workstation that is friendly to
the user. This protocol is not "user friendly". One basic rule of
this protocol is "if anything goes wrong close the connection".
Another basic rule is to have few options.
POP2 does not parse messages in any way. It does not analyze message
headers (Date:, From:, To:, Cc:, or Subject:). POP2 simply transmits
whole messages from a mailbox server to a client workstation.
The Protocol
The POP2 protocol is a sequence of commands and replies. The design
draws from many previous protocols of the ARPA-Internet community.
The server must be listening for a connection. When a connection
is opened the server sends a greeting message and waits for
commands. When commands are received the server acts on them and
responds with replies.
The client opens a connection, waits for the greeting, then sends
the HELO command with the user name and password arguments to
establish authorization to access mailboxes. The server returns
the number of messages in the default mailbox.
The client may read the default mailbox associated with the user
name or may select another mailbox by using the FOLD command. The
server returns the number of messages in the mailbox selected.
The client begins a message reading transaction with a READ
command. The read command may optionally indicate which message
number to read, the default is the current message (incremented
when a message is read and set to one when a new folder is
selected). The server returns the number of characters in the
message.
Butler, et. al. [Page 2]
RFC 937 February 1985
Post Office Protocol
The client asks for the content of the message to be sent with the
RETR command. The server sends the message data.
When all the data has been received the client sends an
acknowledgment command. This is one of ACKS, ACKD, and NACK.
ACKS means "I've received the message successfully and please
keep it in the mailbox".
ACKD means "I've received the message successfully and please
delete it from the mailbox".
NACK means "I did not receive the message and please keep it in
the mailbox".
In the case of ACKS or ACKD the server increments the current
message indicator. In the case of NACK the current message
indicator stays the same.
In all cases the server returns the number of characters in the
(now) current message.
The client terminates the session with the QUIT command. The
server returns an ok.
Butler, et. al. [Page 3]
RFC 937 February 1985
Post Office Protocol
The Normal Scenario
Client Server
------ ------
Wait for Connection
Open Connection -->
<-- + POP2 Server Ready
Wait for Command
HELO Fred Secret -->
<-- #13 messages for you
Wait for Command
READ 13 -->
<-- =537 characters in that message
Wait for Command
RETR -->
<-- (send the message data)
Wait for Command
ACKS -->
<-- =0 no more messages
Wait for Command
QUIT -->
<-- + OK
Close connection --> <-- Close connection
Wait for Connection (go back to start)
Conventions
Arguments
These arguments have system specific definitions.
user - A login account name.
password - The password for the login account.
mailbox - A mailbox name (also called a mail folder).
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Post Office Protocol
Default Mailboxes
TOPS-20
MAIL.TXT.1 - from login directory
UNIX
both
/usr/spool/mail/user
and
/usr/user/Mail/inbox/*
where "user" is the user value supplied in the HELO command.
End of Line
End of Line is Carriage Return (CR) followed by Line Feed (LF).
This sequence is indicated by "CRLF" in this document. This end
of line convention must be used for commands and replies.
Message Length
The reply to the READ command or an acknowledgment command (ACKS,
ACKD, NACK) is the length (a character count) of the next message
to be transmitted. This includes all the characters in the data
transmitted. CRLF counts as two characters. A length of zero
means the message does not exist or is empty. A request to
transmit a message of zero length will result in the server
closing the connection. The message is transmitted in the
standard internet format described in RFC-822 [2] and NVT-ASCII.
This may be different from the storage format and may make
computing the message length from the stored message non-trivial.
Message Numbers
The reply to the HELO and FOLD commands is a count of the number
of messages in a the selected mailbox. The READ command has a
message number as an optional argument. These numbers are
decimal, start at one, and computed with respect to the current
mailbox. That is, the first message in a mailbox is message
number 1.
Numbers
All numbers in this memo and protocol are decimal.
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Post Office Protocol
Quoting
In a few cases, there may be a need to have a special character in
an argument (user, password, or mailbox) that is not allowed by
the syntax. For example, a space in a password. To allow for
this, a quoting convention is defined. Unfortunately, such
quoting conventions "use up" another otherwise uninteresting
character. In this protocol the back slash "\" is used as the
quote character. To include a space in an argument the two
character sequence "back-slash, space" is transmitted. To include
a back-slash in an argument the two character sequence
"back-slash, back-slash" is transmitted. This quoting convention
is used in the command arguments only, it is not used in the mail
data transmitted in response to a RETR command.
Reply Strings
The first character is required to be as specified (i.e.,
"+", "-", "=", "#"). The optional strings that follow can be
whatever the implementer thinks is appropriate.
Definitions of Commands and Replies
Summary of Commands and Replies
Commands Replies
-------- -------
HELO user password + OK
FOLD mailbox - Error
READ [n] #xxx
RETR =yyy
ACKS
ACKD
NACK
QUIT
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Post Office Protocol
Commands
HELO user password
The Hello command identifies the user to the server and carries
the password authenticating this user. This information is
used by the server to control access to the mailboxes. The
Hello command is the "HELO" keyword, followed by the user
argument, followed by the password argument, followed by CRLF.
Possible responses:
"#nnn"
where nnn is the number of messages in the default
mailbox,"
"- error report" and Close the connection.
FOLD mailbox
The Folder command selects another mailbox or mail folder. The
server must check that the user is permitted read access to
this mailbox. If the mailbox is empty or does not exist, the
number of messages reported is zero. The Folder command is the
"FOLD" keyword, followed by the mailbox argument, followed by
CRLF.
Possible responses:
"#nnn"
where nnn is the number of messages in this mailbox.
READ [nnn]
The Read command begins a message reading transaction. If the
Read command is given without an argument the current message
is implied (the current message indicator is incremented by
the ACKS or ACKD commands). If an argument is used with the
Read command it is the message number to be read, and this
command sets the current message indicator to that value. The
server returns the count of characters in the message to be
transmitted. If there is no message to be read, the count of
zero is returned. If the message was previously deleted with
the ACKD command, the count of zero is returned. The Read
command is followed by the RETR command, the READ command, the
FOLD command, or the QUIT command. Do not attempt to RETR a
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Post Office Protocol
message of zero characters. The Read command is the "READ"
keyword, optionally followed by the message number argument,
followed by CRLF.
Possible responses:
"=ccc"
where ccc is the number of characters in this message.
RETR
The Retrieve command confirms that the client is ready to
receive the mail data. It must be followed by an
acknowledgment command. The server will close the connection
if asked to transmit a message of zero characters (i.e.,
transmit a non-existent message). The message is transmitted
according to the Internet mail format standard RFC-822 [2] in
NVT-ASCII. The Retrieve command is the "RETR" keyword,
followed by CRLF.
Possible responses:
the message data
Close the connection
ACKS
The Acknowledge and Save command confirms that the client has
received and accepted the message. The ACKS command ends the
message reading transaction. The message is kept in the
mailbox. The current message indicator is incremented. The
server returns the count of characters in the now current
message to be transmitted. If there is no message to be read
or the message is marked deleted, the count of zero is
returned. The Acknowledge and Save command is the "ACKS"
keyword, followed by CRLF.
Possible responses:
"=ccc"
where ccc is the number of characters in the next
message.
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Post Office Protocol
ACKD
The Acknowledge and Delete command confirms that the client has
received and accepted the message. The ACKD command ends the
message reading transaction. If the user is authorized to have
write access to the mailbox, the message is deleted from the
mailbox. Actually, the message is only marked for deletion.
The actual change is made when the mailbox is released at the
end of the session or when the client selects another mailbox
with the FOLD command. The messages are not renumbered until
the mailbox is released. If the user does not have write
access to the mailbox no change is made to the mailbox. The
response is the same whether or not the message was actually
deleted. The current message indicator is incremented. The
server returns the count of characters in the now current
message to be transmitted. If there is no message to be read
or the message is marked deleted, the count of zero is
returned. The Acknowledge and Delete command is the "ACKD"
keyword, followed by CRLF.
Possible responses:
"=ccc"
where ccc is the number of characters in the next
message.
NACK
The Negative Acknowledge command reports that the client did
not receive the message. The NACK command ends the message
reading transaction. The message is kept in the mailbox. The
current message indicator remains the same. The server returns
the count of characters in the current message. Since the
count to be returned is for the message just transmitted it the
message must exist and not be marked deleted, and the count
must be positive (non-zero). The Negative Acknowledge command
is the "NACK" keyword, followed by CRLF.
Possible responses:
"=ccc"
where ccc is the number of characters in this message.
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Post Office Protocol
QUIT
The Quit command indicates the client is done with the session.
The server sends an OK response and then closes the connection.
The Quit command is the "QUIT" keyword, followed by CRLF.
Possible responses:
"+ OK" and Close the connection
Replies
Greeting
The greeting is sent by the server as soon as the connection is
established. The greeting is a plus sign, followed by the
protocol name ("POP2"), followed by the server host name,
optionally followed by text, and ending with a CRLF.
+
The success or plus sign response indicates successful
completion of the operation specified in the command. The
success response is a plus sign, optionally followed by text,
and ending with a CRLF.
-
The failure or minus sign response indicates the failure of the
operation specified in the command. The failure response is a
minus sign, optionally followed by text, and ending with a
CRLF.
=
The length or equal sign response tells the length in
characters of the message referenced by the command. The
length response is a equal sign, followed by a number,
optionally followed by text, and ending with a CRLF.
#
The count or number sign response tells the number of messages
in a folder or mailbox referenced by the command. The count
response is a number sign, followed by a number, optionally
followed by text, and ending with a CRLF.
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Post Office Protocol
Timeouts
In any protocol of this type there have to be timeouts. Neither
side wants to get stuck waiting forever for the other side
(particularly is the other side has gone crazy or crashed).
The client expects a reply to a command fairly quickly and so
should have a short timeout for this. This timeout is called T1.
For some servers, it may take some processing to compute the
number of messages in a mailbox, or the length of a message, or
to reformat a stored message for transmission, so this time out
has to allow for such processing time. Also care must be taken
not to timeout waiting for the completion of a RETR reply while
a long message is in fact being transfered.
The server expects the session to progress with some but not
excessive delay between commands and so should have a long timeout
waiting for the next command. This time out is T2.
One model of use of this protocol is that any number of
different types of clients can be built with different ways of
interacting with the human user and the server, but still
expecting the client to open the connection to the server,
present a sequence of commands, and close the connection,
without waiting for intervention by the human user. With such
client implementations, it is reasonable for the server to have
a fairly small value for timeout T2.
On the other hand, one could easily have the client be very
human user directed with the user making decisions between
commands. This would cause arbitrary delays between client
commands to the server, and require the value of timeout T2 to
be quite large.
Implementation Discussion
Comments on a Server on TOPS-20
On TOPS-20, a mailbox is a single file. New messages are appended
to the file. There is a separator line between messages.
The tricky part of implementing a POP2 server on TOPS-20 is to
provide for deleting messages. This only has to be done for the
mailboxes (files) for which the user has write access. The
problem is to avoid both (1) preventing other users from accessing
or updating the mailbox for long periods, and (2) accidentally
deleting a message the user has not seen.
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RFC 937 February 1985
Post Office Protocol
One suggestion is as follows:
When a mailbox is first selected, if the user has write access,
rename the mailbox file to some temporary name. Thus new
messages will be placed in a new instance of the mailbox file.
Conduct all POP2 operation on the temporary mailbox file
(including deleting messages). When the POP2 session is over
or another mailbox is selected, prepend any messages left
undeleted in the temporary file to the new instance of the
mailbox file.
Sizes
The maximum length of a command line is 512 characters (including
the command word and the CRLF).
The maximum length of a reply line is 512 characters (including
the success indicator (+, -, =, #) and the CRLF).
The maximum length of a text line is 1000 characters (including
CRLF).
ISI has developed a POP2 server for TOPS-20 and for Berkeley 4.2
Unix, and a POP2 client for an IBM-PC and for Berkeley 4.2 Unix.
Extensions Not Supported
POP2 does not examine the internal data of messages. In particular,
the server does not parse message headers.
The server doesn't have any state information (i.e., it doesn't know
from one session to the next what has happened). For example, the
server doesn't know which messages were received since the last time
the user used POP2, so it can't send just the "new" messages.
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Post Office Protocol
Examples
Example 1:
Client Server
------ ------
Wait for connection
Open connection -->
<-- + POP2 USC-ISIF.ARPA Server
HELO POSTEL SECRET -->
<-- #2 messages in your mailbox
READ -->
<-- =537 characters in message 1
RETR -->
<-- [data of message 1]
ACKD -->
<-- =234 characters in message 2
RETR -->
<-- [data of message 2]
ACKD -->
<-- =0 no more messages
QUIT -->
<-- + OK, bye, bye
Close connection --> <-- Close connection
Go back to start
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RFC 937 February 1985
Post Office Protocol
Example 2:
Client Server
------ ------
Wait for connection
Open connection -->
<-- + POP2 ISI-VAXA.ARPA server here
HELO smith secret -->
<-- #35 messages
FOLD /usr/spool/mail/smith -->
<-- #27 messages
READ 27 -->
<-- =10123 characters in that message
RETR -->
<-- [data of message 27]
ACKS -->
<-- =0 no more messages
QUIT -->
<-- + bye, call again sometime.
Close connection --> <-- Close connection
Go back to start
Example 3:
Client Server
------ ------
Wait for connection
Open connection -->
<-- + POP2 ISI-VAXA.ARPA server here
HELO Jones secret -->
<-- #0 messages
READ -->
<-- Close connection
Close connection -->
Go back to start
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Post Office Protocol
Formal Syntax
<digit> = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<letter> = A | B | C | ... | Z
a | b | c | ... | z
<punct> = ! | " | # | $ | % | & | ' | ( | ) | * |
+ | , | - | / | : | < | = | > | ? | @ |
[ | ] | ^ | _ | ` | { | | | } | ~
<quote> = \
<any> = any one of the 128 ASCII codes
<CR> = carriage return, code 10
<LF> = line feed, code 13
<SP> = space, code 32
<CRLF> = <CR> <LF>
<print> = <letter> | <digit> | <punct> | <quote> <any>
<char> = <print> | <SP>
<word> = <print> | <print> <word>
<string> = <char> | <char> <string>
<ld> = <letter> | <digit>
<ldh> = <letter> | <digit> | -
<ldhs> = <ldh> | <ldh> <ldhs>
<name> = <letter> [ [ <ldhs> ] <ld> ]
<host> = <name> | <name> . <host>
<user> = <word>
<password> = <word>
<mailbox> = <string>
<number> = <digit> | <digit> <number>
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Post Office Protocol
<helo> = HELO <SP> <user> <SP> <password> <CRLF>
<fold> = FOLD <SP> <mailbox> <CRLF>
<read> = READ [<SP> <number>] <CRLF>
<retr> = RETR <CRLF>
<acks> = ACKS <CRLF>
<ackd> = ACKD <CRLF>
<nack> = NACK <CRLF>
<quit> = QUIT <CRLF>
<ok> = + [<SP> <string>] <CRLF>
<err> = - [<SP> <string>] <CRLF>
<count> = # <number> [<SP> <string>] <CRLF>
<greet> = + <SP> POP2 <SP> <host> [<SP> <string>] <CRLF>
<length> = = <number> [<SP> <string>] <CRLF>
<command> = <helo> | <fold> | <read> | <retr> |
<acks> | <ackd> | <nack> | <quit>
<reply> = <ok> | <err> | <count> | <length> | <greet>
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Post Office Protocol
Client State Diagram
| ^ + BYE
| Open | -----
| Greet | Close
V ----- |
+-------+ QUIT +-------+
| CALL |-------------->| EXIT |
+-------+ +-------+
| ^
| Greet |
| ----- |
| HELO |
+---->+ | |
#NNN ^ | | #NNN |
---- | V V ---- |
FOLD | +-------+ QUIT |
+<---| NMBR |--------------------->+
+-------+ ^
^ | |
| | #NNN |
| | ---- |
=CCC | | READ |
---- | | |
FOLD | | =CCC |
| V ---- |
=CCC +--->+-------+ QUIT |
---- ^ | SIZE |--------------------->+
READ +<---+-------+
^ |
| | =CCC
data | | ----
---- | | RETR
ack | |
| V
+-------+
| XFER |
+-------+
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Post Office Protocol
Server State Diagram
+<----------------------+ Close
| | -----
Listen | | Close
V |
+-------+ +-------+
| LSTN | | DONE |
+-------+ +-------+
| ^
| Open |
| ----- |
| Greet |
| |
| QUIT |
V ----- |
+-------+ + BYE |
| AUTH |--------------------->+
+-------+ ^
| |
| HELO |
| ---- |
| #NNN |
| |
| QUIT |
V ----- |
FOLD +--->+-------+ + BYE |
---- ^ | MBOX |--------------------->+
#NNN +<---+-------+ ^
^ | |
| | READ |
FOLD | | ---- |
---- | | =CCC |
#NNN | | QUIT |
| V ----- |
READ +--->+-------+ + BYE |
---- ^ | ITEM |--------------------->+
=CCC +<---+-------+
^ |
| | RETR
ack | | ----
---- | | data
=CCC | |
| V
+-------+
| NEXT |
+-------+
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Post Office Protocol
Combined Flow Diagram
+----+
|CALL|<------------------------------------------------------------+
|LSTN| ^
+----+ |
| Greet |
| |
| +----------------------------------------------------->+ |
| ^ QUIT | |
V | V |
+----+ +----+ +----+ |
|CALL| HELO |NMBR| |EXIT| |
|AUTH|------->|AUTH| |AUTH| |
+----+ +----+ +----+ |
| #NNN + Bye | |
| | |
| +------------------------------------>+ | |
| ^ QUIT | | |
V | V | |
+--->+----+ +----+ +----+ | |
FOLD ^ |NMBR| READ |SIZE| |EXIT| | |
---- | |MBOX|------->|MBOX| |MBOX| | |
#NNN +<---+----+ +----+ +----+ | |
^ | =CCC + Bye | | |
| | | | |
FOLD +<--------+ | +------------------->+ | | |
---- ^ | ^ QUIT | | | |
#NNN | V | V | | |
+--->+-----+ +----+ +----+ | | |
READ ^ |SIZE | RETR |XFER| |EXIT| | | |
---- | | ITEM|------->|ITEM| |ITEM| | | |
=CCC +<---+-----+ +----+ +----+ | | |
^ | data | | | |
| | | | | |
=CCC | V + Bye | | | |
+----+ +----+ | | | |
|SIZE| Ack |XFER| | | | |
|NEXT|<-------|NEXT| | | | |
+----+ +----+ | | | |
| | | |
| | | |
V V V |
+-------+ |
| EXIT |-->+
| DONE |
+-------+
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Post Office Protocol
Client Decision Table
| STATE |
-------+----------------------------------|
INPUT | CALL | NMBR | SIZE | XFER | EXIT |
-------+----------------------------------|
Greet | 2 | 1 | 1 | 1 | 6 |
-------+----------------------------------|
#NNN | 1 | 3 | 1 | 1 | 6 |
-------+----------------------------------|
=CCC | 1 | 1 | 4 | 1 | 6 |
-------+----------------------------------|
data | 1 | 1 | 1 | 5 | 6 |
-------+----------------------------------|
+ Bye | 1 | 1 | 1 | 1 | 6 |
-------+----------------------------------|
Close | 1 | 1 | 1 | 1 | 6 |
-------+----------------------------------|
other | 1 | 1 | 1 | 1 | 6 |
-------+----------------------------------|
Timeout| 1 | 1 | 1 | 1 | 6 |
-------+----------------------------------|
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Post Office Protocol
Actions:
1. This is garbage. Send "QUIT", and go to EXIT state.
2. (a) If the greeting is right then send "HELO"
and go to NMBR state,
(b) Else send "QUIT" and go to EXIT state.
3. (a) If user wants this folder and NNN > 0
then send "READ" and go to SIZE state,
(b) If user wants a this folder and NNN = 0
then send "QUIT" and go to EXIT state,
(c) If user wants a different folder
then send "FOLD" and go to NMBR state.
4. (a) If user wants this message and CCC > 0
then send "RETR" and go to XFER state,
(b) If user wants a this message and CCC = 0
then send "QUIT" and go to EXIT state,
(c) If user wants a different message
then send "READ" and go to SIZE state.
5. (a) If user wants this message kept
then send "ACKS" and go to SIZE state,
(b) If user wants a this message deleted
then send "ACKD" and go to SIZE state,
(c) If user wants a this message again
then send "NACK" and go to SIZE state.
6. Close the connection.
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RFC 937 February 1985
Post Office Protocol
Server Decision Table
| STATE
-------+-----------------------------------------
INPUT | LSTN | AUTH | MBOX | ITEM | NEXT | DONE |
-------+-----------------------------------------|
Open | 2 | 1 | 1 | 1 | 1 | 1 |
-------+-----------------------------------------|
HELO | 1 | 3 | 1 | 1 | 1 | 1 |
-------+-----------------------------------------|
FOLD | 1 | 1 | 5 | 5 | 1 | 1 |
-------+-----------------------------------------|
READ | 1 | 1 | 6 | 6 | 1 | 1 |
-------+-----------------------------------------|
RETR | 1 | 1 | 1 | 7 | 1 | 1 |
-------+-----------------------------------------|
ACKS | 1 | 1 | 1 | 1 | 8 | 1 |
-------+-----------------------------------------|
ACKD | 1 | 1 | 1 | 1 | 8 | 1 |
-------+-----------------------------------------|
NACK | 1 | 1 | 1 | 1 | 8 | 1 |
-------+-----------------------------------------|
QUIT | 1 | 4 | 4 | 4 | 1 | 1 |
-------+-----------------------------------------|
Close | 1 | 1 | 1 | 1 | 1 | 9 |
-------+-----------------------------------------|
other | 1 | 1 | 1 | 1 | 1 | 1 |
-------+-----------------------------------------|
Timeout| | 1 | 1 | 1 | 1 | 1 |
-------+-----------------------------------------|
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Post Office Protocol
Actions:
1. This is garbage. Send "- error", and Close the connection.
2. Send the greeting. Go to AUTH state.
3. (a) If authorized user then send "#NNN" and go tp MBOX state,
(b) Else send "- error" and Close the connection.
4. Send "+ Bye" and go to DONE state.
5. Send "+NNN" and go to MBOX state.
6. Send "=CCC" and go to ITEM state.
7. If message exists then send the data and got to NEXT state,
Else Close the connection.
8. Do what ACKS/ACKD/NACK require and go to ITEM state.
9. Close the connection.
Butler, et. al. [Page 23]
RFC 937 February 1985
Post Office Protocol
Acknowledgment
We would like to acknowledge the helpful comments that we received on
the first version of POP described in RFC 918, and the draft of POP2
distributed to interested parties.
References
[1] Postel, J., "Simple Mail Transfer Protocol", RFC 821,
USC/Information Sciences Institute, August 1982.
[2] Crocker, D., "Standard for the Format of ARPA-Internet Text
Messages", RFC 822, University of Delaware, August 1982.
[3] Reynolds, J.K., "Post Office Protocol", RFC 918, USC/Information
Sciences Institute, October 1984.
[4] Reynolds, J.K., and J. Postel, "Assigned Numbers", RFC 923,
USC/Information Sciences Institute, October 1984.
Butler, et. al. [Page 24]
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