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====== POP3 FOR USE OVER STANAG 5066 TRANSPORT (HF-POP3) ======

This section of this Annex presents an adaptation of the POP3 protocol, as described in RFC
1939, for use over the transport system described in the main body of and mandatory annexes
to this STANAG. Large parts of the RFC are reproduced here, however there are differences
so read carefully.

===== Introduction =====

It will in general be impractical to maintain a message transport system (MTS) over HF radio
to allow immediate delivery of messages to a node when that node relies on HF radio for
connectivity. It is still necessary to be able to manage the transfer of mail to these smaller
nodes. 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), as
modified in this Annex, is intended to permit a workstation to dynamically access a maildrop
on a server host over HF radio. Usually, this means that the POP3 protocol is used to allow a
node to retrieve mail that the server is holding for it. This annex presents a revision of the
POP3 protocol which is intended to provide efficient operation over HF radio links. This
revised protocol will be referred to as “HF-POP” in this annex.

For the remainder of this annex, the term "client host" refers to a host making use of the HFPOP
service, while the term "server host" refers to a host which offers the HF-POP service.

This annex does not specify how a client host enters mail into the transport system, although
a method consistent with the philosophy of this annex is presented here:

When the user agent on a client host wishes to enter a message into the transport system, it
establishes an HMTP connection to its relay host and sends all mail to it.

===== Basic Operation =====

Initially, the server host starts the HF-POP3 service by listening for incoming connections
over the HF subnet. When a client host wishes to make use of the service, it establishes a
connection with the server host. When the connection is established, the HF-POP3 server
sends a greeting. The client and POP3 server then exchange commands and responses
(respectively) until the connection is closed or aborted.

Commands in the HF-POP consist of a case-insensitive 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 HF-POP consist of a status indicator and a keyword possibly followed by
additional information. All responses are terminated by a CRLF pair. Responses may be up
to 512 characters long, including the terminating CRLF. There are two status indicators:
positive ("+OK") and negative ("-ERR"). Servers shall send the "+OK" and "-ERR" in upper
case.

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 prepending
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 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 HF-POP3 session progresses through a number of states during its lifetime. Once the
SnAP connection has been opened and the HF-POP3 server has sent the greeting, the session
enters the AUTHORIZATION state. In this state, the client must identify itself to the HFPOP3
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 HF-POP3 server. When the client has
issued the QUIT command, the session enters the UPDATE state. In this state, the HF-POP3
server releases any resources acquired during the TRANSACTION state and says goodbye.
The connection is then closed.

A server MUST respond to an unrecognized, unimplemented, or syntactically invalid
command by responding with a negative status indicator. A server MUST respond to a
command issued when the session is in an incorrect state by responding with a negative status
indicator. There is no general method for a client to distinguish between a server which does
not implement an optional command and a server which is unwilling or unable to process the
command.

A HF-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 the UPDATE state--the server should close the connection without removing any
messages or sending any response to the client.

===== The AUTHORIZATION State =====

Once the connection has been opened by a HF-POP3 client, the HF-POP3 server shall issue a
one line greeting as follows:

  S: +OK HF-POP3 (STANAG 5066) server ready <process-ID.clock@hostname>

(Responses which do not conform to this format shall be interpreted by the HF_POP3 client
as negative responses.) The HF-POP3 session is now in the AUTHORIZATION state. The
client must now identify and authenticate itself to the HF-POP3 server. The APOP command
shall be used for this purpose.

==== The APOP command ====

__Usage:__
  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 or after
an unsuccessful USER or PASS command

__Discussion:__
  
In the context of this standard, the APOP command provides both origin
authentication and replay protection. An HF-POP3 servers shall 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:

  <process-ID.clock@hostname>

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 session enters the TRANSACTION state and the
POP3 server issues a positive response including the LIST response for the maildrop,
as shown below. 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).

__Possible Responses:__
  S: +OK maildrop locked and ready; maildrop has 1 message (320 octets)
  S: 1 120
  S: 2 200
  S: .
  S: -ERR permission denied
  
__Examples:__
  S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
  C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
  S: +OK maildrop has 1 message (369 octets)
  S: 1 369
  
In this example, the shared secret is the string ''tanstaaf''. Hence, the MD5
algorithm is applied to the string
  
  <1896.697170952@dbc.mtview.ca.us>tanstaaf
  
which produces a digest value of ''c4c9334bac560ecc979e58001b3e22fb''

Once the POP3 server has determined through the use of the APOP command that the client
should be given access to the appropriate maildrop, the HF-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 plus the response to the LIST
command with no arguments (described in the following section on the TRANSACTION
state). The HF-POP3 session now enters the TRANSACTION state, with no messages
marked as deleted. If 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 HF-POP3 server responds with a negative status indicator. (If a lock
was acquired but the HF-POP3 server intends to respond with a negative status indicator, the
HF-POP3 server must release the lock prior to rejecting the command.) After returning a
negative status indicator, the server may close the 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 HF-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 nth
message in a maildrop is assigned a message-number of "n". In HF-POP3 commands and
responses, all message-numbers and message sizes are expressed in base-10 (i.e., decimal).

==== The QUIT command ====

Here is the summary for the QUIT command when used in the AUTHORIZATION state:

  QUIT
  
__Arguments:__ none
  
__Restrictions:__ none
  
__Possible Responses:__
  +OK

__Examples:__
  C: QUIT
  S: +OK dewey POP3 server signing off

===== 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. Note that this transition occurs after the APOP line from the client is
processed and before the server generates the UIDL response. 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:

==== The LIST Command ====

__Usage:__
  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. 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. If there are no messages in the maildrop, then the POP3 server
responds with no scan listings--it issues a positive response followed by a line containing a
termination octet and a CRLF pair.

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, followed by a CRLF pair. Methods for
calculating the exact size of the message are described in the "Message Format" section of
RFC 1939.

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

==== The RETR Command ====

__Usage:__
  RETR nn

__Arguments:__
a message-number (optional) which may NOT refer to a message marked as deleted

__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. If
an argument was given, 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). If no argument was given,
the server sends all messages in the drop which are not marked as deleted. Use UIDL
to identify boundary between messages? If so, need to include the UIDL response as
part of the response to APOP.

__Possible Responses:__
  +OK message nn follows
  +OK m messages follow
  -ERR no such message
  
__Examples:__
  C: RETR 1
  S: +OK 120 octets
  S: <the POP3 server sends the entire message here>
  S: .

==== The DELE Command ====

__Usage:__
  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

==== The RSET Command ====

__Usage:__
  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)

==== The STAT Command ====

__Usage:__
  STAT
  
__Arguments:__ none
  
__Restrictions:__
  may only be given in the TRANSACTION state
  
__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 are 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, the size of the maildrop in octets, and the line shall
be terminated with a CRLF pair.

Note that messages marked as deleted are not counted in either total.

__Possible Responses:__
  +OK nn mm
  
__Examples:__
  C: STAT
  S: +OK 2 320

==== The UIDL Command (OPTIONAL) ====

__Usage:__
  UIDL [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 "unique-id listing" for
that message.
  
If no argument was given, then the response given is multi-line. After the initial
+OK, for each message in the maildrop, the HF-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. 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 one
to 70 characters in the range 0x21 to 0x7E, which uniquely identifies a message
within a maildrop and which persists across sessions. This persistence is required
even if a session ends without entering the UPDATE state. 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.
  
While it is generally preferable for server implementations to store arbitrarily
assigned unique-ids in the maildrop, this specification is intended to permit uniqueids
to be calculated as a hash of the message. Clients should be able to handle a
situation where two identical copies of a message in a maildrop have the same
unique-id.
  
__Possible Responses:__
  +OK unique-id listing follows
  -ERR no such message
  
__Examples:__
  C: UIDL
  S: +OK
  S: 1 whqtswO00WBw418f9t5JxYwZ
  S: 2 QhdPYR:00WBw1Ph7x7
  S: .
  ...
  C: UIDL 2
  S: +OK 2 QhdPYR:00WBw1Ph7x7
  ...
  C: UIDL 3
  S: -ERR no such message, only 2 messages in maildrop

===== 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.

==== The QUIT Command ====

__Usage:__
  QUIT
  
__Arguments:__ none
  
__Restrictions:__ none
  
__Discussion:__
The POP3 server removes all messages marked as deleted from the maildrop and
replies as to the status of this operation. If there is an error, such as a resource
shortage, encountered while removing messages, the maildrop may result in having
some or none of the messages marked as deleted be removed. In no case may the
server remove any messages not marked as deleted.

Whether the removal was successful or not, the server then releases any exclusiveaccess
lock on the maildrop and closes the connection

__Possible Responses:__
  +OK
  -ERR some deleted messages not removed

__Examples:__
  C: QUIT
  S: +OK dewey POP3 server signing off (maildrop empty)
  ...
  C: QUIT
  S: +OK dewey POP3 server signing off (2 messages left)
  ...
  
==== The TOP Command ====

__Usage:__
  TOP msg n
  
__Arguments:__
a message-number (required) which may NOT refer to to a message marked as
deleted, and a non-negative number of lines (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
line separating the headers from the body, and then the number of lines of the
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: <the POP3 server sends the headers of the message, a blank line, and the first 10
  lines of the body of the message>
  S: .
  ...
  C: TOP 100 3
  S: -ERR no such message

===== Scaling and Operational Considerations =====

The HF-POP3 protocol implements a special case of a site policy in order to prevent users
from accumulating large mail queues on the server system. These large queues can result in a
full disk and system crashes; this problem is frequently observed in POP3 servers. Messages
may only be downloaded once from the server, and shall be deleted after this has been
accomplished. This could be implemented in HF-POP3 server software by the following
mechanism: "following a POP3 login by a client which was ended by a QUIT, delete all
messages downloaded during the session with the RETR command". It is important not to
delete messages in the event of abnormal connection termination (ie, if no QUIT was
received from the client) because the client may not have successfully received or stored the
messages. Servers implementing a download-and-delete policy may also wish to disable or
limit the optional TOP command, since it could be used as an alternate mechanism to
download entire messages.