MAIL TRANSFER PROTOCOL
Suzanne Sluizer
and
Jonathan B. Postel
RFC 780
May 1981
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291
(213) 822-1511
May 1981 RFC 780 Mail Transfer Protocol TABLE OF CONTENTS 1. INTRODUCTION .................................................. 1 2. THE MTP MODEL ................................................. 2 3. BASIC MAIL .................................................... 4 3.1. Forwarding ............................................... 5 3.2. Source Routing ........................................... 6 4. MULTI-RECIPIENT MAIL .......................................... 8 4.1. Scheme Selection: MRSQ ................................... 8 4.2. Message Text Specification: MAIL ......................... 9 4.3. Recipient Specification: MRCP ........................... 10 4.4. Scheme Mechanics: Recipients First ...................... 10 4.5. Scheme Mechanics: Text First ............................ 12 4.6. Discussion .............................................. 12 5. SPECIFICATIONS ............................................... 16 5.1. MTP Commands ............................................ 16 5.1.1. Command Semantics ..................................... 16 5.1.2. Command Syntax ........................................ 18 5.2. MTP Replies ............................................. 22 5.2.1. Reply Codes by Function Group ......................... 23 5.2.2. Reply Codes in Numeric Order .......................... 24 5.3. Sequencing of Commands and Replies ...................... 25 5.4. State Diagrams .......................................... 28 5.5. Details ................................................. 30 5.5.1. Minimum Implementation ................................ 30 5.5.2. Transparency .......................................... 30 5.5.3. Sizes ................................................. 30 APPENDIX A: TCP ................................................. 32 APPENDIX B: NCP ................................................. 33 APPENDIX C: NITS ................................................ 34 APPENDIX D: X.25 ................................................ 35 APPENDIX E: Theory of Reply Codes ............................... 36 GLOSSARY ......................................................... 39 REFERENCES ....................................................... 42
Network Working Group S. Sluizer
Request for Comments: 780 J. Postel
ISI
Replaces: RFC 772 May 1981
MAIL TRANSFER PROTOCOL
1. INTRODUCTION
The objective of Mail Transfer Protocol (MTP) is to transfer mail
reliably and efficiently.
MTP is designed to be independent of the particular transmission
subsystem and requires only a reliable ordered data stream channel.
Appendices describe the use of MTP with various transport services.
A Glossary provides the definitions of terms as used in this
document.
An important feature of MTP is its capability to relay mail from one
transport environment to another. A transport service provides an
interprocess communication environment (IPCE). An IPCE may cover one
network, several networks, or a subset of a network. A process can
communicate directly with another process anywhere in its own IPCE.
Mail is a special case of interprocess communication. Mail can be
communicated between proceses in different IPCEs by relaying through
a process connected to two (or more) IPCEs. More specifically, mail
can be relayed between hosts on different transport systems by a host
on both transport systems. It is important to realize that transport
systems (or IPCEs) are not one-to-one with networks.
Sluizer & Postel [Page 1]
May 1981 RFC 780 Mail Transfer Protocol 2. THE MTP MODEL The MTP design is based on the following model of communication: at the initiation of the user, the sender-MTP establishes the full-duplex transmission channel. MTP commands are generated by the sender-MTP and sent to the receiver-MTP. MTP replies are sent from the receiver-MTP to the sender-MTP in response to the commands. In the simplest case, once the transmission channel is established the MTP-sender sends a MAIL command indicating the sender and receiver of the mail. If the MTP-receiver can accept the mail it responds with a go ahead reply. Then the MTP-sender sends the mail data, terminating with a special sequence. If the MTP-receiver successfully processes the mail it responds with an OK reply. ------------------------------------------------------------- +----------+ +----------+ +------+ | | | | | User |<-->| | MTP | | +------+ | Sender- |Commands/Replies| Receiver-| +------+ | MTP |<-------------->| MTP | +------+ | File |<-->| | and Mail | |<-->| File | |System| | | | | |System| +------+ +----------+ +----------+ +------+ Sender-MTP Receiver-MTP Model for MTP Use Figure 1 ------------------------------------------------------------- The MTP provides mechanisms for the transmission of mail; directly from the sending user's host to the receiving user's host when the two host are connected to the same transport service, or via one or more relay MTP-servers when the source and destination hosts are not connected to the same transport service. To be able to provide the relay capability the MTP-server must be supplied with the name of the ultimate destination host as well as the destination mailbox name. [Page 2] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol The arguments to the MAIL command are a FROM path and a TO path. The TO path is a source route while the FROM path is a return route (which may be used to return a message to the sender when an error occurs with a relayed message). The preceding discussion has outlined the transmission of one copy of one message from a source to a destination host and the possibility of relaying messages between different transport services. The MTP additionally supports the transmission of one copy of a message addressed to multiple recipients. In order for mail to be successfully transmitted the destination users must be known at the destination receiver-MTP and the mail data must be correctly received and stored. In the single recipient case discussed above the positive response to the MAIL command indicated the recipient was known, and the final OK response indicated the mail was received and stored. To support multi-recipient mail, MTP provides two procedures: Text-First, and Recipients-First. In the text-first scheme the mail data is sent and acknowledged, then each recipient identification is sent and acknowledged (or refused) separately. In the recipients-first scheme the recipients are negotiated first, then the text is sent and acknowledged (for all recipients at once). The choice of scheme is up to the MTP-receiver, and depends on the way mail is handled in the destination host. The multi-recipient mail procedures are optional and the determination of which scheme to use is negotiated. The use of the multi-recipient schemes is strongly encouraged by the economy they provide in transmission and processing. The mail commands and replies have a rigid syntax. Replies also have a numeric code. In the following, examples appear which use actual commands and replies. The complete lists of commands and replies appears in Section 5 on specifications. Commands and replies are not case sensitive. That is, a command or reply word may be upper case, lower case, or any mixture of upper and lower case. Note that this is not true of mailbox user names. For some hosts the user name is case sensitive, and MTP implementations must take case to preserve the case of user names as they appear in mailbox arguments. Sluizer & Postel [Page 3]
May 1981 RFC 780 Mail Transfer Protocol 3. BASIC MAIL The basic command for transmitting mail is MAIL. This command causes the transmitted data to be entered into the recipient's mailbox, or accepted for relaying to the destination host. The mail text is also sent on the transmission channel. This requires that the end of the text be signalled so that the command and reply dialog can be resumed. MTP signals the end of the mail text by sending a line containing only a period. A transparency procedure is used to prevent this interfering with the users text (see Section 5.5.2). MAIL <SP> FROM:<sender-path> <SP> TO:<receiver-path> <CRLF> The <sender-path> contains the source mailbox; the <receiver-path> contains the destination mailbox. If accepted, the receiver-MTP returns a 354 reply and considers all succeeding lines to be the message text. The message text is terminated by a line containing only a period, upon which a 250 completion reply is returned. Various errors are possible. Actually the <sender-path> and <receiver-path> are more than just the mailboxes, they may be source routes. The <receiver-path> is a source routing list of hosts and destination mailbox; the <sender-path> is a reverse source routing list of hosts and source mailbox. [Page 4] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol ------------------------------------------------------------- Example of MAIL (Basic Mail) This MAIL command specifies the mail is sent by Waldo at host A, and is to be delivered to Foo at host Y. Here we assume that host A contacts host Y directly. S: MAIL FROM:<waldo@A> TO:<Foo@Y> <CRLF> R: 354 Start mail input; end with <CRLF>.<CRLF> S: Blah blah blah blah....etc. etc. etc. S: <CRLF>.<CRLF> R: 250 Mail sent The mail text has now been sent to "Foo". Example 1 ------------------------------------------------------------- 3.1. FORWARDING There are two possible preliminary replies that a receiver may use to indicate that it is accepting mail for a user whose mailbox is not at that host. 151 User not local; will forward to <user>@<host> This reply indicates that the receiver-MTP knows the user's mailbox is on another host and will take responsibility for forwarding the mail to that host. This reply is only sent when the sender would not expect the mail to be forwarded. That is, when <receiver-path> as given in the command indicates mail relaying, this reply will not be used. This reply could be used for an organization with several hosts when each has a list of many of the users on the hosts. A host can accept mail for any user on its list and forward it to the correct host. 152 User Unknown; mail will be forwarded by the operator This reply indicates that the host does not recognize the user name, but that it will accept the mail and have the operator attempt to deliver it. This is useful if the user name is misspelled, but may be a disservice if the mail is really undeliverable. Sluizer & Postel [Page 5]
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If forwarding by the operator is unacceptable or if the
sending-user would prefer to send the mail directly to the
recipient's actual host, the action may be aborted.
The MTP-sender must accept or reject the proposal in the
preliminary reply by sending a continue (CONT) or abort (ABRT)
command. In the case of the continue, the next reply from the
MTP-receiver will be any of the replies expected for the MAIL
command, most likely "354 Start mail input, ...". In the case of
the abort, the next reply from the MTP-receiver will be "201
Command okay, action aborted".
3.2. SOURCE ROUTING
The receiver-path may be a source route of the form
"@ONE,@TWO,JOE@THREE", where ONE, TWO, and THREE are hosts. This
form is used to emphasize the distinction between an address and a
route.
At some distant future time it might be necessary to expand the
mailbox format to include a region identifier, such as
"user@host@region". If this occured the MTP path convention
could be expanded to
"host@region,host@region,...user@host@region". For example,
"ONE@R1,TWO@R2,JOE@THREE@R3".
The mailbox is an absolute address, and the route is information
about how to get there. The two concepts should not be confused.
The elements of the receiver-path are to be moved to the
sender-path as the message is relayed from one MTP to another. The
sender-path is a reverse source route, that is, a source route to
the originator of the message. When an MTP deletes its identifier
from the receiver-path and inserts it into the sender-path, it
must use the name it is known by in the environment it is sending
into, not the environment the mail came from, in case the MTP is
known be different names in different environments.
When source routing is used the receiver-MTP will receive mail to
be relayed to another MTP. The receiver-MTP may accept the task
of relaying the mail or reject it in the same way it accepts or
reject mail for a local user. It does not use the 151 "User not
local" or 152 "User unknown" preliminary replies. Once the
receiver-MTP accepts the relaying task it receives the mail text
and transforms the command arguments by removing its own
identifier from the receiver-path and inserting it in the
[Page 6] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol beginning of the sender-path. The receiver-MTP then becomes a sender-MTP and establishes a transmission channel to the next MTP in the receiver-path and sends it the mail. If an MTP has accepted the task of relaying the mail and later finds that the receiver-path is incorrect or that the mail cannot be delivered for whatever reason, then it must construct a notification message and send it to the originator of the undeliverable mail as indicated by the sender-path. This notification message must be from the MTP at this host. That is, the sender-path of the notification message itself will be "MTP@<host>", and in the notification message header the From field will be "MTP at <host>". Of course, MTPs should not send notification messages about problems with notification messages. Sluizer & Postel [Page 7]
May 1981 RFC 780 Mail Transfer Protocol 4. MULTI-RECIPIENT MAIL There are two MTP commands which allow the text of a message to be mailed to several recipients simultaneously; such message transmission is far more efficient than the practice of sending the text again and again for each additional recipient at a host. In one scheme, all recipients are specified first, and then the text is sent. In the other scheme, the order is reversed and the text is sent first, followed by the recipients. The sender-MTP suggests the scheme it would prefer, but receiver-MTP controls which scheme is actually used. To select a particular scheme, the MRSQ command is used; to specify recipients after a scheme is chosen, MRCP commands are given; and to furnish text, the MAIL command is used. Both schemes are necessary because neither by itself is optimal for all systems. MRSQ 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 MTP-receiver does not itself write mail directly, but hands it on to a central mailer demon. The more information (e.g., recipients) associated with a single "hand-off", the more efficiently mail can be delivered. By contrast, MRSQ T is geared to receiver-MTPs which want to deliver mail directly, in one-by-one incremental fashion. For each given recipient this scheme returns an individual success/failure reply code which may depend on variable mail system factors such as exceeding disk allocation, mailbox access conflicts, and so forth. If these receiver-MTPs tried to emulate MRSQ Rs bulk mailing, they would have to ensure that a success reply to the MAIL indeed meant that it had been delivered to ALL recipients specified -- not just some. 4.1. SCHEME SELECTION: MRSQ MRSQ is the means by which a sender-MTP can test for MRSQ/MRCP implementation, select a particular scheme, reset its state, and even do some rudimentary negotiation. Its format is as follows: MRSQ [<SP> <scheme>] <CRLF> <scheme> is a single character. The following are defined: R Recipients first. If this is not implemented, T must be. T Text first. If this is not implemented, R must be. ? Request for preference. This must always be implemented. [Page 8] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol No argument means a "selection" of none of the schemes (the default). Possible replies are: 200 OK, use the specified scheme 215 <scheme> This is the scheme I prefer 504 I understand MRSQ but can't use that scheme 5xx Command unrecognized or unimplemented There are three aspects of MRSQ. The first is that an MRSQ with no argument must always return a 200 reply and restore the default state of having no scheme selected. Any other reply implies that MRSQ and hence MRCP are not understood or cannot be performed correctly. The second is that the use of "?" as a <scheme> asks the MTP receiver to return a 215 reply in which the receiver specifies a "preferred" scheme. The format of this reply is simple: 215 <SP> <scheme> [<SP> <string>] <CRLF> Any other reply (e.g., 4xx or 5xx) implies that MRSQ and MRCP are not implemented, because "?" must always be implemented if MRSQ is. The third important point about MRSQ is that it always has the side effect of reseting all schemes to their initial state. This reset must be done no matter what the reply will be -- 200, 215, or 504. The actions necessary for a reset will be explained when discussing how each scheme actually works. Note that the receiver gets to choose which scheme is used. The sender must be prepared to do either. 4.2. MESSAGE TEXT SPECIFICATION: MAIL Regardless of which scheme (if any) has been selected, a MAIL command with a non-null receiver-path argument will behave exactly as before; the MRSQ/MRCP commands have no effect on it. However, a normal MAIL command does have the same side effect as MRSQ; it "resets" all schemes to their initial state. It is only when the receiver-path argument is null that the particular scheme chosen is important. MAIL FROM:<sender-path> <CRLF> Sluizer & Postel [Page 9]
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Rather than producing an error, the receiver will accept message
text for this "null" recipient specification. What it does with
it depends on which scheme is in effect, and will be described in
the section on Scheme Mechanics.
4.3. RECIPIENT SPECIFICATION: MRCP
In order to specify recipient names (i.e., mailboxes) and receive
some acknowledgment (or refusal) for each name, the following
command is used:
MRCP <SP> TO:<receiver-path> <CRLF>
Reply for no scheme:
503 No scheme specified yet; use MRSQ
Replies for scheme T are identical to those for MAIL.
Replies for scheme R (recipients first):
200 OK, name stored
452 Recipient table full, this name not stored
550 Recipient name rejected
4xx Temporary error, try this name again later
5xx Permanent error, report to sender
Note that use of this command is an error if no scheme has been
selected yet; an MRSQ <scheme> must have been given if MRCP is to
be used.
4.4. SCHEME MECHANICS: MRSQ R (RECIPIENTS-FIRST)
In the recipients-first scheme, MRCP is used to specify names
which the MTP receiver stores in a list or table. Normally the
reply for each MRCP will be either a 200 for acceptance or a
4xx/5xx rejection code. All 5xx codes are permanent rejections
(e.g., user not known) which should be reported to the human user,
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 MRCP commands, except for 452 which
indicates that no further MRCPs 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
command with no receiver-path argument; that is, just MAIL <SP>
FROM: <sender-path> <CRLF>. Transmission of the message text is
exactly the same as for normal MAIL. However, a positive
acknowledgment at the end of transmission means the message has
been sent to ALL recipients that were remembered with MRCP, and a
[Page 10] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol 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. Regardless of what the reply signifies, 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 MRSQ (or MAIL with a non-null receiver-path argument). A 452 reply (out of storage space) to an MRCP can be handled by using MAIL to specify the message for currently stored recipients, and then sending more MRCPs and another MAIL, as many times as necessary. For example, if a receiver 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 sender attempts to specify message text (MAIL with no receiver-path argument) before any successful MRCPs have been given, this should be treated exactly as a "normal" MAIL with a null recipient would be; some receivers return an error, such as "550 Null recipient". ------------------------------------------------------------- Example of MRSQ R (Recipients First) First the sender must establish that the receiver implements MRSQ. S: MRSQ <CRLF> R: 200 OK, no scheme selected An MRSQ with a null argument always returns a 200 if implemented, selecting the default "scheme", i.e., none of them. If MRSQ were not implemented, a code of 4xx or 5xx would be returned. S: MRSQ R <CRLF> R: 200 OK, using that scheme All is well; now the recipients can be specified. S: MRCP TO:<Foo@Y> <CRLF> R: 200 OK Sluizer & Postel [Page 11]
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Mail Transfer Protocol
S: MRCP TO:<Raboof@Y> <CRLF>
R: 550 No such user here
S: MRCP TO:<bar@Y> <CRLF>
R: 200 OK
S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
R: 200 OK
Note that the failure of "Raboof" has no effect on the storage
of mail for "Foo", "bar" or the mail to be relayed to "fubar@Z"
through host "X". Now the message text is furnished, by giving
a MAIL command with no receiver-path argument.
S: MAIL FROM:<waldo@A><CRLF>
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah blah....etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 Mail sent
The mail text has now been sent to "Foo" and "bar" as well as
relayed to "fubar@Z".
Example 2
-------------------------------------------------------------
4.5. SCHEME MECHANICS: MRSQ T (TEXT-FIRST)
In the text-first scheme, MAIL with no receiver-path argument is
used to specify message text, which the receiver stores away.
Succeeding MRCPs are then treated as if they were MAIL 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
would invoke. (Note that any 2xx code indicates success.)
The stored message text is not forgotten until the next MAIL or
MRSQ, which will either replace it with new text or flush it
entirely. Any use of MRSQ will reset this scheme by flushing
stored text, as will any use of MAIL with a non-null receiver-path
argument.
If an MRCP is seen before any message text has been stored, the
sender in effect is trying to send a null message; some receivers
might allow this, others would return an error code.
[Page 12] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol ------------------------------------------------------------- Example of MRSQ T (Text First) First the sender must establish that the receiver implements MRSQ. S: MRSQ ? <CRLF> R: 215 T Text first, please MRSQ is indeed implemented, and the receiver says that it prefers "T", but that needn't stop the sender from trying something else. S: MRSQ R <CRLF> R: 504 Sorry, I really can't do that It's possible that it could have understood "R" also, but in general it's best to use the "preferred" scheme, since the receiver knows which is most efficient for its particular site. S: MRSQ T <CRLF> R: 200 OK, using that scheme Scheme "T" is now selected, and the message text is sent by giving a mail command with no receiver-path argument. S: MAIL FROM:<WALDO@A><CRLF> R: 354 Start mail input; end with <CRLF>.<CRLF> S: Blah blah blah blah....etc. etc. etc. S: <CRLF>.<CRLF> R: 250 Mail stored Now recipients can be specified. S: MRCP TO:<Foo@Y> <CRLF> R: 250 Stored mail sent S: MRCP TO:<Raboof@Y> <CRLF> R: 550 No such user here S: MRCP TO:<bar@Y> <CRLF> R: 250 Stored mail sent S: MRCP TO:<@Y,@X,fubar@Z> <CRLF> R: 250 Mail accepted for relaying Sluizer & Postel [Page 13]
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The text has now been sent to "Foo" and "bar" at host "Y" and
will be relayed to "fubar@Z" through host "X", and still
remains stored. A new message can be sent with another
MAIL/MRCP ... sequence, but a careful sender would reset the
state using the exchange below.
S: MRSQ ? <CRLF>
R: 215 T Text first, please
Which resets the state without altering the scheme in effect.
Example 3
-------------------------------------------------------------
4.6. DISCUSSION
Because these commands are not required in the minimum
implementation of MTP, one must be prepared to deal with sites
which don't recognize either MRSQ or MRCP. "MRSQ" and "MRSQ ?"
are explicitly designed as tests to see whether either scheme is
implemented. MRCP is not designed as a test, and a failure return
of the "unimplemented" variety could be confused with "No scheme
selected yet", or even with "Recipient unknown".
There is no way to indicate in a positive response to "MRSQ ?"
that the preferred "scheme" for a receiver 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 MRSQ/MRCP
at all, and the response would therefore be negative.
One reason that the use of MAIL is restricted to null
receiver-path arguments with this multi-recipient extension is the
ambiguity that would result if a non-null receiver-path argument
were allowed. For example, if MRSQ R was in effect and some MRCPs
had been given, and a MAIL FROM:<X@Y> TO:<FOO@Z><CRLF> 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 MRSQ T; it would not be clear whether
the text was stored and the mailbox failed, or vice versa, or
both.
"Resets" of all schemes are done by all MRSQs and "normal" MAILs
to avoid confusion and overly complicated implementation. The
MRSQ command implies a change or uncertainty of status, and the
MAIL command would otherwise have to use some independent
[Page 14] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol 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. The recommended way for doing a reset, without changing the current selection, is with "MRSQ ?". Remember that "MRSQ" alone reverts to the no-scheme state. Sluizer & Postel [Page 15]
May 1981 RFC 780 Mail Transfer Protocol 5. SPECIFICATIONS 5.1. MTP COMMANDS 5.1.1. COMMAND SEMANTICS The MTP commands define the mail transfer or the mail system function requested by the user. MTP commands are character strings terminated by <CRLF>. The command codes themselves are alphabetic characters terminated by <SP> if parameters follow and <CRLF> otherwise. The syntax of mailboxes must conform to receiver site conventions. The MTP commands are discussed below. MTP replies are discussed in the Section 5.2. MAIL (MAIL) This command is used to send mail over the transmission channel. The argument field contains a sender-path sequence and optional receiver-path sequence. The sender-path sequence consists of an optional list of hosts and the sender mailbox. When the list of hosts is present, it is "reverse" source routing information and indicates that the mail was relayed through each host on the list (the first host in the list was the most recent relay). This list is used as source routing to return non-delivery notices to the sender. As each relay host adds itself to the beginning of the list, it must use its name as known in the network to which it is relaying the mail rather than the network from which the mail came (if they are different). If the receiver-path sequence is present, it consists of an optional list of hosts and a destination mailbox. When the list of hosts is present, it is source routing information and indicates that the mail must be relayed to the first host on the list. The receiver treats the lines following the command as mail text from the sender. The mail text is terminated by the character sequence "<CRLF>.<CRLF>", (see Section 5.5.2 on Transparency). As mail is relayed along the receiver-path sequence, each relay host must remove itself from the path sequence and put itself at the beginning of the sender-path sequence. When mail reaches its ultimate destination (the receiver-path [Page 16] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol sequence has only a destination mailbox), the receiver-MTP inserts it into the destination mailbox in accordance with its host mail conventions. (For example, "MAIL FROM:<X@Y> TO:<@A,@B,C@D> <CRLF>" will eventually be relayed as "MAIL FROM:<@A,X@Y> TO:<@B,C@D> <CRLF>.) If the receiver-path sequence is empty, the mail is destined for a printer or other designated place for host general delivery mail (if allowed at this host). The mail may be marked as sent from the sender as specified in the sender-path sequence field. MAIL RECIPIENT SCHEME QUESTION (MRSQ) This MTP command is used to select a scheme for the transmission of mail to several users at the same host. The schemes are recipients-first, or text-first. MAIL RECIPIENT (MRCP) This command is used to identify the individual recipients of the mail in the transmission of mail for multiple users at one host. HELP (HELP) This command causes the receiver to send helpful information regarding its implementation status over the transmission channel to the receiver. The command may take an argument (e.g., any command name) and return more specific information as a response. QUIT (QUIT) This command specifies that the receiver must close the transmission channel. NOOP (NOOP) This command does not affect any parameters or previously entered commands. It specifies no action other than that the receiver send an OK reply. Sluizer & Postel [Page 17]
May 1981 RFC 780
Mail Transfer Protocol
CONTINUE (CONT)
This command specifies that the previously specified action
is to be continued. This is sent only following a
preliminary reply.
ABORT (ABRT)
This command specifies that the previously specified action
is to be aborted. This is sent only following a preliminary
reply. It specifies no further action other than that the
receiver send an OK reply.
5.1.2. COMMAND SYNTAX
The commands begin with a command code followed by an argument
field. The command codes are four alphabetic characters.
Upper and lower case alphabetic characters are to be treated
identically. Thus any of the following may represent the mail
command:
MAIL Mail mail MaIl mAIl
This also applies to any symbols representing parameter values,
such as R or r for RECIPIENT first. The command codes and the
argument fields are separated by one or more spaces.
But, note that in the sender-path and receiver-path arguments
case is important. In particular, in some hosts the user "foo"
is different from the user "Foo".
The argument field consists of a variable length character
string ending with the character sequence <CRLF>. It should be
noted that the receiver is to take no action until the end of
the line is received.
Square brackets denote an optional argument field. If the
option is not taken, the appropriate default is implied. All
characters are in the ASCII characters set.
[Page 18] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol The following are the MTP commands: MAIL <SP> FROM:<sender-path> [<SP> TO:<receiver-path>] <CRLF> MRSQ [<SP> <scheme>] <CRLF> MRCP <SP> TO:<receiver-path> <CRLF> HELP [<SP> <string>] <CRLF> QUIT <CRLF> NOOP <CRLF> CONT <CRLF> ABRT <CRLF> Sluizer & Postel [Page 19]
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The syntax of the above argument fields (using BNF notation
where applicable) is given below. The "..." notation indicates
that a field may be repeated one or more times.
<sender-path> ::= <path>
<receiver-path> ::= <path>
<scheme> ::= "R" | "T" | "?"
<string> ::= <char> | <char> <string>
<path> ::= "<" ["@" <host> "," ...] <mailbox> ">"
<host> ::= <a> <string> | "#" <number> | "[" <dotnum> "]"
<mailbox> ::= <user> "@" <host>
<user> ::= <string>
<char> ::= <c> | '\' <c> | '\' <s>
<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
<number> ::= <d> | <d> <number>
<snum> ::= three digits representing an integer value in the
range 0 through 255
<specials> ::= '<', '>', '(', ')', '\', ',', ';', ':', '@',
'"', and the control characters (ASCII codes 0 through 37
octal inclusive and 177 octal)
<a> ::= any one of the 26 letters A through Z in either case
<c> ::= any one of the 128 ASCII characters except
<specials>
<d> ::= any one of the ten digits 0 through 9
<s> ::= any one of <specials>
Note that the backslash, '\', is a quote character, which is
used to indicate that the next character is to be used
literally instead of with its normal interpretation. For
[Page 20] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol example, "Joe\,Smith" could be used to indicate a single nine character user field with comma being the fourth character of the field. Hosts are generally known by names which are translated to addresses in each host. Sometimes a host is not known to the translation function and communication is blocked. To bypass this barrier numeric forms are also allowed for host "names". One form is a decimal integer prefixed by a pound sign, "#", which indicates the number is the address of the host. Another form is four small decimal integers separated by dots and enclosed by brackets, e.g., "[123.255.37.321]", which indicates a 32 bit ARPA Internet Address in four eight bit fields. Sluizer & Postel [Page 21]
May 1981 RFC 780 Mail Transfer Protocol 5.2. MTP REPLIES Replies to MTP commands are devised to ensure the synchronization of requests and actions in the process of mail transfer, and to guarantee that the sender-MTP always knows the state of the receiver-MTP. Every command must generate exactly one reply. Additionally, some commands must occur sequentially, such as MRSQ T->MAIL->MRCP or MRSQ R->MRCP->MAIL. The details of the command-reply sequence are made explicit in the Sections 5.3 and 5.4 on Sequencing and State Diagrams. An MTP reply consists of a three digit number (transmitted as three alphanumeric characters) followed by some text. The number is intended for use by automata to determine what state to enter next; the text is meant for the human user. It is intended that the three digits contain enough encoded information that the sender-MTP will not need to examine the text and may either discard it or pass it on to the user, as appropriate. In particular, the text may be receiver-dependent, so there are likely to be varying texts for each reply code. Further explanation of the assignment of reply codes is given in the Appendix E on the Theory of Reply Codes. Formally, a reply is defined to be the sequence: a three-digit code, <SP>, one line of text, and <CRLF>. [Page 22] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol 5.2.1. REPLY CODES BY FUNCTION GROUPS 200 Command okay 201 Command okay, action aborted 500 Syntax error, command unrecognized [This may include errors such as command line too long] 501 Syntax error in parameters or arguments 502 Command not implemented 503 Bad sequence of commands 504 Command parameter not implemented 211 System status, or system help reply 214 Help message [Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user] 215 <scheme> is the preferred scheme 120 <host> Service ready in nnn minutes 220 <host> Service ready for new user 221 <host> Service closing transmission channel 421 <host> Service not available, closing transmission channel [This may be a reply to any command if the service knows it must shut down] 151 User not local; will forward to <user>@<host> 152 User unknown; mail will be forwarded by the operator 250 Requested mail action okay, completed 450 Requested mail action not taken: mailbox unavailable [E.g., mailbox busy] 550 Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access] 451 Requested action aborted: local error in processing 452 Requested action not taken: insufficient system storage 552 Requested mail action aborted: exceeded storage allocation [For current mailbox location] 553 Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect] 354 Start mail input; end with <CRLF>.<CRLF> Sluizer & Postel [Page 23]
May 1981 RFC 780
Mail Transfer Protocol
5.2.2. NUMERIC ORDER LIST OF REPLY CODES
120 <host> Service ready in nnn minutes
151 User not local; will forward to <user>@<host>
152 User unknown; mail will be forwarded by the operator
200 Command okay
201 Command okay, action aborted
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
215 <scheme> is the preferred scheme
220 <host> Service ready for new user
221 <host> Service closing transmission channel
250 Requested mail action okay, completed
354 Start mail input; end with <CRLF>.<CRLF>
421 <host> Service not available, closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
552 Requested mail action aborted: exceeded storage allocation
[For current mailbox location]
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
[Page 24] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol 5.3. SEQUENCING OF COMMANDS AND REPLIES The communication between the sender and receiver is intended to be an alternating dialogue. As such, the sender issues an MTP command and the receiver responds with a prompt primary reply. The sender should wait for this response before sending further commands. The preliminary (1xx) and intermediate (3xx) replies indicate that further commands and information are required to complete the required action. The preliminary replies require either a continue or abort command to proceed; the intermediate replies require action dependent further commands. One important reply is the connection greetings. Under normal circumstances, a receiver will send a 220 "Awaiting input" reply when the connection is completed. The sender should wait for this greeting message before sending any commands. If the receiver is unable to accept input right away, it should send a 120 "Expected delay" reply immediately. The sender can then indicate it is willing to wait via a continue command, or not via the abort command. The receiver will respond to the abort with a 201 reply, and to the continue with the 220 reply when ready. Note: all the greeting type replies have the official name of the server host as the first word following the reply code. For example, 220 <SP> USC-ISIF <SP> Service ready <CRLF> The table below lists alternative success and failure replies for each command. These must be strictly adhered to; a receiver may substitute text in the replies, but the meaning and action implied by the code numbers and by the specific command reply sequence cannot be altered. COMMAND-REPLY SEQUENCES Each command is listed with its possible replies. Preliminary replies are listed first with their succeeding replies indented under them, then success and failure completion, and finally intermediary replies with the remaining commands from the sequence following. The prefixes used before the possible replies are "P" for preliminary, "I" for intermediate, "S" for success, "F" for failure, and "E" for error. The 421 reply Sluizer & Postel [Page 25]
May 1981 RFC 780 Mail Transfer Protocol (service not available, closing transmission channel) may be given to any command if the MTP-receiver knows it must shut down. This listing forms the basis for the State Diagrams, in Section 5.4. CONNECTION ESTABLISHMENT P: 120 -> CONT -> S: 220 F: 421 ABRT S: 201 F: 421 S: 220 F: 421 MAIL P: 151 -> CONT -> I: 354 -> text -> S: 250 152 F: 451,552,450, 550,452,553 ABRT -> S: 201 F: 451,552,450,550,452,553 I: 354 -> text -> S: 250 F: 451,552,450,550,452,553 F: 451, 552, 450, 550, 452, 553 E: 500, 501, 502, 421 MRSQ S: 200, 215 E: 500, 501, 502, 504, 421 MRCP P: 151 -> CONT -> S: 200, 215, 250 152 F: 451,552,450,550,452,553 ABRT -> S: 201 F: 451,552,450,550,452,553 S: 200, 215, 250 F: 451, 552, 450, 550, 452, 553 E: 500, 501, 502, 503, 421 [Page 26] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol QUIT S: 221 E: 500, 421 HELP S: 211, 214 E: 500, 501, 502, 504, 421 NOOP S: 200 E: 500, 421 CONT S: depends on previous command F: depends on previous command E: 500, 501, 502, 504, 421 ABRT S: 201, E: 500, 501, 502, 504, 421 Sluizer & Postel [Page 27]
May 1981 RFC 780
Mail Transfer Protocol
5.4. STATE DIAGRAMS
Following are state diagrams for a very simple minded MTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of MTP commands.
The command groupings were determined by constructing a model for
each command and then collecting together the commands with
structurally identical models.
For each command there are three possible outcomes: "success"
(S), "failure" (F), and "error" (E). In the state diagrams below
we use the symbol B for "begin", and the symbol W for "wait for
reply".
First, the diagram that represents most of the MTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
HELP, MRCP, MRSQ, NOOP, QUIT, ABRT.
[Page 28] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol A more complex diagram models the MAIL command: ABRT +---+ 1,3 CONT ---- ------------->| W |------- | | +---+ | | |1 4,5| |2 V +---+ cmd -->+---+ 2 | | +---+ | B |---------->| W |-------------------->| E | +---+ +---+ ------------>+---+ 3| |4,5 | | | | | | | | -------------- ------ | | | | | | | ---->+---+ | ----------------------->| S | | | | | | +---+ | | -------- | | | | | | V 2| |1,3 | | +---+ text +---+ | ------->+---+ | |---------->| W | --------------->| F | +---+ +---+-------------------->+---+ 4,5 Note that the "text" here is a series of lines sent from the sender to the receiver with no response expected until the last line is sent. Sluizer & Postel [Page 29]
May 1981 RFC 780
Mail Transfer Protocol
5.5. DETAILS
5.5.1. MINIMUM IMPLEMENTATION
In order to make MTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- MAIL
QUIT
NOOP
5.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the the mail text and cannot be
sent by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.
1. Before sending a line of mail text the sender-MTP checks the
first character of the line. If it is a period, one additional
period is inserted at the beginning of the line.
2. When a line of mail text is received by the receiver-MTP it
checks the the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.
5.5.3. SIZES
There are several objects that ought to have defined maximum
sizes.
user
The maximum total length of a user name is 40 characters.
host
The maximum total length of a host name or number is 20
characters.
[Page 30] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol path The maximum total length of a sender-path or receiver-path is 100 characters. command line The maximum total length of a command line including the command word and the <CRLF> is 200 characters. reply line The maximum total length of a reply line including the reply code and the <CRLF> is 65 characters. text line The maximum total length of a text line including the the <CRLF> is 1000 characters. To the maximum extent possible implementation techniques which impose no limits at all to the length of these objects should be used. Sluizer & Postel [Page 31]
May 1981 RFC 780 Mail Transfer Protocol APPENDIX A TCP Transport service The Transmission Control Protocol [1] is used in the ARPA Internet, and in any network following the US DoD standards for internetwork protocols. Connection Establishment The MTP transmission channel is a TCP connection established between the sender process port U and the receiver process port L. This single full duplex connection is used as the transmission channel. This protocol is assigned the service port 57 (71 octal), that is L=57. Data Transfer The TCP connection supports the transmission of 8-bit bytes. The MTP data is 7-bit ASCII characters. Each character is transmitted as a 8-bit byte with the high-order bit cleared to zero. [Page 32] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol APPENDIX B NCP Transport service The ARPANET Host-to-Host Protocol [2] (implemented by the Network Control Program) may be used in the ARPANET. Connection Establishment The MTP transmission channel is established via NCP between the the sender process socket U and receiver process socket L. The Initial Connection Protocol [3] is followed resulting in a pair of simplex connections. This pair of connections is used as the transmission channel. This protocol is assigned the contact socket 57 (71 octal), that is L=57. Data Transfer The NCP data connections are established in 8-bit byte mode. The MTP data is 7-bit ASCII characters. Each character is transmitted as a 8-bit byte with the high-order bit cleared to zero. Sluizer & Postel [Page 33]
May 1981 RFC 780 Mail Transfer Protocol APPENDIX C NITS The Network Independent Transport Service [4] may be used. Connection Establishment The MTP transmission channel is established via NITS between the the sender process and receiver process. The sender process executes the CONNECT primitive, and the waiting receiver process executes the ACCEPT primitive. Data Transfer The NITS connection supports the transmission of 8-bit bytes. The MTP data is 7-bit ASCII characters. Each character is transmitted as a 8-bit byte with the high-order bit cleared to zero. [Page 34] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol APPENDIX D X.25 Transport service It may be possible to use the X.25 service [5] as provided by the Public Data Networks directly, but there are indications that it is too error prone to qualify as a reliable channel. It is suggested that a reliable end-to-end protocol such as TCP be used on top of X.25 connections. Sluizer & Postel [Page 35]
May 1981 RFC 780
Mail Transfer Protocol
APPENDIX E
Theory of Reply Codes
The three digits of the reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated sender-MTP will be able to
determine its next action (proceed as planned, redo, retrench,
etc.) by simply examining this first digit. A sender-MTP that
wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit, reserving the third digit for the finest gradation of
information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action
is being held in abeyance, pending confirmation of the
information in this reply. The sender-MTP should send
another command specifying whether to continue or abort
the action.
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The sender-MTP should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not occur. However, the error condition is temporary and
the action may be requested again. The sender should
return to the beginning of the command sequence (if any).
It is difficult to assign a meaning to "transient" when
two different sites (receiver- and sender- MTPs) must
agree on the interpretation. Each reply in this category
[Page 36] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol might have a different time value, but the sender-MTP is encouraged to try again. A rule of thumb to determine if a reply fits into the 4yz or the 5yz category (see below) is that replies are 4yz if they can be repeated without any change in command form or in properties of the sender or receiver. (E.g., the command is repeated identically; the receiver does not put up a new implementation). 5yz Permanent Negative Completion reply The command was not accepted and the requested action did not occur. The sender-MTP is discouraged from repeating the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct the sender-MTP to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status.) The second digit encodes responses in specific categories: x0z Syntax -- These replies refer to syntax errors, syntactically correct commands that don't fit any functional category, and unimplemented or superfluous commands. x1z Information -- These are replies to requests for information, such as status or help. x2z Connections -- These are replies referring to the transmission channel. x3z Unspecified as yet. x4z Unspecified as yet. x5z Mail system -- These replies indicate the status of the receiver mail system vis-a-vis the requested transfer or other mail system action. The third digit gives a finer gradation of meaning in each category specified by the second digit. The list of replies illustrates this. Each reply text is recommended rather than mandatory, and may even change according to the command with which it is associated. On the other hand, the reply codes must strictly follow the specifications in this section. Sluizer & Postel [Page 37]
May 1981 RFC 780
Mail Transfer Protocol
Receiver implementations should not invent new codes for
slightly different situations from the ones described here, but
rather adapt codes already defined.
For example, a command such as NOOP whose successful execution
does not offer the sender-MTP any new information will return a
200 reply. The response is 502 when the command requests an
unimplemented non-site-specific action. A refinement of that
is the 504 reply for a command that is implemented, but that
requests an unimplemented parameter.
The reply text may be longer than a single line; in these cases
the complete text must be marked so the sender-MTP knows when it
can stop reading the reply. This requires a special format to
indicate a multiple line reply.
The format for multi-line replies requires that every line,
except the last, begin with the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
The sender-MTP then simply needs to search for the reply code
followed by <SP> at the beginning of a line, and ignore all
preceding lines.
[Page 38] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol GLOSSARY ASCII American Standard Code for Information Interchange [6]. command A request for a mail service action sent by the sender-MTP to the receiver-MTP. host A computer in the internetwork environment on which mailboxes or MTP processes reside. line A line of text ending with a <CRLF>. mail A sequence of ASCII characters of arbitrary length, which conforms to the standard set in RFC 733 (Standard for the Format of ARPA Network Text Messages [7]). mailbox A character string (address) which identifies a user to whom mail is to be sent. Mailbox normally consists of the host and user specifications. The standard mailbox naming convention is defined to be "user@host". Additionally, the "container" in which mail is stored. receiver-MTP process A process which transfers mail in cooperation with a sender-MTP process. It waits for a connection to be established via the transport service. It receives MTP commands from the sender-MTP, sends replies, and governs the transfer of mail. Sluizer & Postel [Page 39]
May 1981 RFC 780
Mail Transfer Protocol
reply
A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
MTP command. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes are
for use by programs and the text is usually intended for human
users.
sender-MTP process
A process which transfers mail in cooperation with a receiver-MTP
process. A local language may be used in the user interface
command/reply dialogue. The sender-MTP initiates the transport
service connection. It initiates MTP commands, receives replies,
and governs the transfer of mail.
transmission channel
A full-duplex communication path between a sender-MTP and a
receiver-MTP for the exchange of commands, replies, and mail text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
word
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
<CRLF>
The characters carriage return and line feed (in that order).
[Page 40] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol <SP> The space character. Sluizer & Postel [Page 41]
May 1981 RFC 780 Mail Transfer Protocol REFERENCES [1] TCP Postel, J., ed., "DOD Standard Transmission Control Protocol", IEN 129, RFC 761, USC/Information Sciences Institute, NTIS ADA082609, January 1980. Appears in: Computer Communication Review, Special Interest Group on Data Communications, ACM, V.10, N.4, October 1980. [2] NCP McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246, January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for the Defense Communications Agency by SRI International, Menlo Park, California, Revised January 1978. [3] Initial Connection Protocol Postel, J., "Official Initial Connection Protocol", NIC 7101, 11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for the Defense Communications Agency by SRI International, Menlo Park, California, Revised January 1978. [4] NITS PSS/SG3, "A Network Independent Transport Service", Study Group 3, The Post Office PSS Users Group, February 1980. Available from the DCPU, National Physical Laboratory, Teddington, UK. [5] X.25 CCITT, "Recommendation X.25 - Interface Between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks," CCITT Orange Book, Vol. VIII.2, International Telephone and Telegraph Consultative Committee, Geneva, 1976. [Page 42] Sluizer & Postel
RFC 780 May 1981 Mail Transfer Protocol [6] ASCII ASCII, "USA Code for Information Interchange", United States of America Standards Institute, X3.4, 1968. Also in: Feinler, E. and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for the Defense Communications Agency by SRI International, Menlo Park, California, Revised January 1978. [7] RFC 733 Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for the Format of ARPA Network Text Messages," RFC 733, NIC 41952, November 1977. Also in: Feinler, E. and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for the Defense Communications Agency by SRI International, Menlo Park, California, Revised January 1978. Sluizer & Postel [Page 43]
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