📄 rfc937.txt
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RFC 937 February 1985
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.
Butler, et. al. [Page 9]
RFC 937 February 1985
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.
Butler, et. al. [Page 10]
RFC 937 February 1985
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.
Butler, et. al. [Page 11]
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.
Butler, et. al. [Page 12]
RFC 937 February 1985
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
Butler, et. al. [Page 13]
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
Butler, et. al. [Page 14]
RFC 937 February 1985
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>
Butler, et. al. [Page 15]
RFC 937 February 1985
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>
Butler, et. al. [Page 16]
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