NWG/RFC# 743 KLH 30-Dec-77 08:39 42759
Network Working Group K. Harrenstien
Request for Comments: 743 SRI-KL
NIC: 42758 30 December 1977
FTP extension: XRSQ/XRCP
This RFC describes an extension to FTP which allows the user of an ITS
FTP server (i.e. on MIT-(AI/ML/MC/DMS)) to mail the text of a message to
several recipients simultaneously; such message transmission is far more
efficient than the current practice of sending the text again and again
for each additional recipient at a site.
Within this extension, there are two basic ways of sending a single text
to several recipients. In one, all recipients are specified first, and
then the text is sent; in the other, the order is reversed and the text
is sent first, followed by the recipients. Both schemes are necessary
becaue neither by itself is optimal for all systems, as will be
explained later. To select a particular scheme, the XRSQ command is
used; to specify recipients after a scheme is chosen, XRCP commands are
given; and to furnish text, the usual MAIL or MLFL commands apply.
Scheme Selection: XRSQ
XRSQ is the means by which a user program can test for implementation
of XRSQ/XRCP, select a particular scheme, reset its state thereof,
and even do some rudimentary negotiation. Its format is like that of
the TYPE command, as follows:
XRSQ [ ]
= a single character. The following are defined:
R Recipients first. If not implemented, T must be.
T Text first. If this is not implemented, R must be.
? Request for preference. Must always be implemented.
No argument means a "selection" of none of the schemes (the
default).
Replies:
200 OK, we'll use specified scheme.
215 This is the scheme I prefer.
501 I understand XRSQ but can't use that scheme.
5xx Command unrecognized or unimplemented.
See Appendix A for more about the choice of reply codes.
Three aspects of XRSQ need to be pointed out here. The first is that
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an XRSQ with no argument must always return a 200 reply and restore
the default state of having no scheme selected. Any other reply
implies that XRSQ and hence XRCP are not understood or cannot be
performed correctly.
The second is that the use of "?" as a asks the FTP server
to return a 215 reply in which the server specifies a "preferred"
scheme. The format of this reply is simple:
215 [ ]
Any other reply (e.g. 4xx or 5xx) implies that XRSQ and XRCP are
not implemented, because "?" must always be implemented if XRSQ
is.
The third important thing about XRSQ is that it always has the side
effect of resetting all schemes to their initial state. This reset
must be done no matter what the reply will be - 200, 215, or 501.
The actions necessary for a reset will be explained when discussing
how each scheme actually works.
Message Text Specification: MAIL/MLFL
Regardless of which scheme (if any) has been selected, a MAIL or MLFL
with a non-null argument will behave exactly as before; this
extension has no effect on them. However, such normal MAIL/MLFL
commands do have the same side effect as XRSQ; they "reset" the
current scheme to its initial state.
It is only when the argument is null (e.g. MAIL or MLFL)
that the particular scheme being used is important, because rather
than producing an error (as most servers currently do), the server
will accept message text for this "null" specification; what it does
with it depends on which scheme is in effect, and will be described
in "Scheme Mechanics".
Recipient specification: XRCP
In order to specify recipient names and receive some acknowledgement
(or refusal) for each name, the following new command is also
defined:
XRCP
Reply for no scheme:
507 No scheme specified yet; use XRSQ.
Replies for scheme T are identical to those for MAIL/MLFL.
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Replies for scheme R (recipients first):
200 OK, name stored.
440 Recipient table full, this name not stored.
450 Recipient name rejected. (Permanent!)
520 Recipient name rejected.
4xx Temporary error, try this name again later.
5xx Permanent error, report to sender.
See Appendix A for more about the choice of reply codes.
Note that use of this command is an error if no scheme has been
selected yet; an XRSQ must have been given if XRCP is to be
used.
Scheme mechanics: XRSQ R (Recipients first)
In the recipients-first scheme, XRCP is used to specify names which
the FTP server stores in a list or table. Normally the reply for
each XRCP will be either a 200 for acceptance, or a 4xx/5xx code for
rejection; 450 and all 5xx codes are permanent rejections (e.g. user
not known) which should be reported to the human sender, whereas 4xx
codes in general connote some temporary error that may be rectified
later. None of the 4xx/5xx replies impinge on previous or succeeding
XRCP commands, except for 440 which indicates that no further XRCP's
will succeed unless a message is sent to the already stored
recipients or a reset is done.
Sending message text to stored recipients is done by giving a MAIL or
MLFL command with no argument; that is, just MAIL or
MLFL. Transmission of the message text is exactly the same as
for normal MAIL/MLFL; however, a positive acknowledgement at the end
of transmission means that the message has been sent to ALL
recipients that were remembered with XRCP, and a failure code means
that it should be considered to have failed for ALL of these
specified recipients. This applies regardless of the actual error
code; and whether the reply signifies success or failure, all stored
recipient names are flushed and forgotten - in other words, things
are reset to their initial state. This purging of the recipient name
list must also be done as the "reset" side effect of any use of XRSQ.
A 440 reply to an XRCP can thus be handled by using a MAIL/MLFL to
specify the message for currently stored recipients, and then sending
more XRCP's and another MAIL/MLFL, as many times as necessary; for
example, if a server only had room for 10 names this would result in
a 50-recipient message being sent 5 times, to 10 different recipients
each time.
If a user attempts to specify message text (MAIL/MLFL with no
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argument) before any successful XRCP's have been given, this should
be treated exactly as a "normal" MAIL/MLFL with a null recipient
would be; most servers will return an error of some type, such as
"450 Null recipient".
See Appendix B for an example using XRSQ R.
Scheme mechanics: XRSQ T (Text first)
In the text-first scheme, MAIL/MLFL with no argument is used to
specify message text, which the server stores away. Succeeding
XRCP's are then treated as if they were MAIL/MLFL commands, except
that none of the text transfer manipulations are done; the stored
message text is sent to the specified recipient, and a reply code is
returned identical to that which an actual MAIL/MLFL would invoke.
(Note ANY 2xx code indicates success.)
The stored message text is not forgotten until the next MAIL/MLFL or
XRSQ, which will either replace it with new text or flush it
entirely. Any use of XRSQ will reset this scheme by flushing stored
text, as will any use of MAIL/MLFL with a non-null argument.
If an XRCP is seen before any message text has been stored, the user
in effect is trying to send a null message; some servers might allow
this, others would return an error code.
See Appendix C for an example using XRSQ T.
Why two schemes anyway?
Because neither by itself is optimal for all systems. XRSQ R allows
more of a "bulk" mailing, because everything is saved up and then
mailed simultaneously; this is very useful for systems such as ITS
where the FTP server does not itself write mail directly, but hands
it on to a central mailer demon of great power; the more information
(e.g. recipients) associated with a single "hand-off", the more
efficiently mail can be delivered.
By contrast, XRSQ T is geared to FTP servers which want to deliver
mail directly, in one-by-one incremental fashion. This way they can
return an individual success/failure reply code for each recipient
given which may depend on variable file system factors such as
exceeding disk allocation, mailbox access conflicts, and so forth; if
they tried to emulate XRSQ R's bulk mailing, they would have to
ensure that a success reply to the MAIL/MLFL indeed meant that it had
been delivered to ALL recipients specified - not just some.
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Stray notes:
* Because this is after all an extension of FTP protocol, one must be
prepared to deal with sites which don't recognize either XRSQ or
XRCP. "XRSQ" and "XRSQ ?" are explicitly designed as tests to see
whether either scheme is implemented; XRCP is not, and a failure
return of the "unimplemented" variety could be confused with "No
scheme selected yet", or even with "Recipient unknown". Be safe, be
sure, use XRSQ!
* There is no way to indicate in a positive response to "XRSQ ?" that
the preferred "scheme" for a server is that of the default state;
i.e. none of the multi-recipient schemes. The rationale is that in
this case, it would be pointless to implement XRSQ/XRCP at all, and
the response would therefore be negative.
* One reason that the use of MAIL/MLFL is restricted to null
arguments with this multi-recipient extension is the ambiguity that
would result if a non-null argument were allowed; for example, if
XRSQ R was in effect and some XRCP's had been given, and a MAIL
FOO was done, there would be no way to distinguish a failure
reply for mailbox "FOO" from a global failure for all recipients
specified. A similar situation exists for XRSQ T; it would not be
clear whether the text was stored and the mailbox failed, or vice
versa, or both.
* "Resets" are done by all XRSQ's and "normal" MAIL/MLFL's to avoid
confusion and overly complicated implementation. The XRSQ command
implies a change or uncertainty of status, and the latter commands
would otherwise have to use some independent mechanisms to avoid
clobbering the data bases (e.g. message text storage area) used by
the T/R schemes. However, once a scheme is selected, it remains "in
effect" just as a "TYPE A" or "BYTE 8" remains selected. The
recommended way for doing a reset, without changing the current
selection, is with "XRSQ ?". Remember that "XRSQ" alone reverts to
the no-scheme state.
* It is permissible to intersperse other FTP commands among the
XRSQ/XRCP/MAIL sequences.
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Appendix A - on FTP reply codes
On FTP reply codes
The choice of appropriate reply codes for new or experimental
commands is difficult because there have been three possible
"official" sets of codes which one may draw on, and it is not clear
which of them might be in use at any particular site; these are (1)
Old FTP, (2) New FTP, (3) Revised New FTP. In my choice of code
assignments, I have for the most part ignored these and used RFC 691,
"One More Try on the FTP", by Brian Harvey. My motivation for this
is the simple observation that I know of no site which implements
"new FTP", and RFC 691 incorporates much of the "new FTP" reply code
logic into the framework of "old FTP". The only sharp conflict is
treated by allowing 450 to have its "old" meaning, equivalent to 520
- permanent failure. Note that when testing to see whether a site
understands a FTP command, a reply of 5xx (specifically, 500) will
generally indicate, for all sets of codes, that the command is
unrecognized.
By the way, I recommend RFC 691 as required reading for FTP
implementors; maybe if enough people get together this mess can be
straightened out.
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Appendix B - Example of XRSQ R
Example of XRSQ R (Recipients first)
This is an example of how XRSQ R is used; first the user must
establish that the server in fact implements XRSQ:
U: XRSQ
S: 200 OK, no scheme selected.
An XRSQ with a null argument always returns a 200 if implemented,
selecting the "scheme" of null, i.e. none of them. If XRSQ were not
implemented, a code of 4xx or 5xx would be returned.
U: XRSQ R
S: 200 OK, using that scheme
All's well; now the recipients can be specified.
U: XRCP Foo
S: 200 OK
U: XRCP Raboof
S: 520 Who's that? No such user here.
U: XRCP bar
S: 200 OK
Well, two out of three ain't bad. Note that the demise of "Raboof"
has no effect on the storage of "Foo" or "bar". Now to furnish the
message text, by giving a MAIL or MLFL with no argument:
U: MAIL
S: 350 Type mail, ended by .
U: Blah blah blah blah....etc etc etc
U: .
S: 256 Mail sent.
The text has now been sent to both "Foo" and "bar".
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Appendix C - Example of XRSQ T
Example of XRSQ T (Text first)
Using the same message as the previous example:
U: XRSQ ?
S: 215 T Text first, please.
XRSQ is indeed implemented, and the server says that it prefers "T",
but that needn't stop the user from trying something else:
U: XRSQ R
S: 501 Sorry, I really can't do that.
Oh well. It's possible that it could have understood "R" also, but
in general it's best to use the "preferred" scheme, since the server
knows which is most efficient for its particular site. Anyway:
U: XRSQ T
S: 200 OK, using that scheme.
Scheme "T" is now selected, and the text must be sent:
U: MAIL
S: 350 Type mail, ended by .
U: Blah blah blah blah....etc etc etc
U: .
S: 256 Mail stored.
Now recipients can be specified:
U: XRCP Foo
S: 256 Stored mail sent.
U: XRCP Raboof
S: 520 Who's that? No such user here.
U: XRCP bar
S: 256 Stored mail sent.
Again, the text has now been sent to both "Foo" and "bar", and still
remains stored. A new message can be sent with another MAIL/XRCP...
sequence, but the fastidious or paranoid could chose to do:
U: XRSQ ?
S: 215 T Text first, please.
Which resets things without altering the scheme in effect.
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RFC, FYI, BCP