📄 rfc937.txt
字号:
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).
Butler, et. al. [Page 4]
RFC 937 February 1985
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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RFC 937 February 1985
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
Butler, et. al. [Page 6]
RFC 937 February 1985
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
Butler, et. al. [Page 7]
RFC 937 February 1985
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.
Butler, et. al. [Page 8]
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