Network Working Group J. Postel Request for Comments: DRAFT ISI Replaces: RFC 788, 780, 772 August 1982
SIMPLE MAIL TRANSFER PROTOCOL
1. INTRODUCTION
The objective of Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. Appendices A, B, C, and D describe the use of SMTP with various transport services. A Glossary provides the definitions of terms as used in this document.
An important feature of SMTP is its capability to relay mail across transport service environments. A transport service provides an interprocess communication environment (IPCE). An IPCE may cover one network, several networks, or a subset of a network. It is important to realize that transport systems (or IPCEs) are not one-to-one with networks. A process can communicate directly with another process through any mutually known IPCE. Mail is an application or use of interprocess communication. Mail can be communicated between processes 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.
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2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as the result of a user mail request, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP. The receiver-SMTP may be either the ultimate destination or an intermediate. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply. The dialog is purposely lock-step, one-at-a-time.
The SMTP provides mechanisms for the transmission of mail; directly from the sending user's host to the receiving user's host when the
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two host are connected to the same transport service, or via one or more relay SMTP-servers when the source and destination hosts are not connected to the same transport service.
To be able to provide the relay capability the SMTP-server must be supplied with the name of the ultimate destination host as well as the destination mailbox name.
The argument to the MAIL command is a reverse-path, which specifies who the mail is from. The argument to the RCPT command is a forward-path, which specifies who the mail is to. The forward-path is a source route, while the reverse-path is a return route (which may be used to return a message to the sender when an error occurs with a relayed message).
When the same message is sent to multiple recipients the SMTP encourages the transmission of only one copy of the data for all the recipients at the same destination host.
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 4 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 SMTP implementations must take case to preserve the case of user names as they appear in mailbox arguments. Host names are not case sensitive.
Commands and replies are composed of characters from the ASCII character set [1]. When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero.
When specifying the general form of a command or reply, an argument (or special symbol) will be denoted by a meta-linguistic variable (or constant), for example, "<string>" or "<reverse-path>". Here the angle brackets indicate these are meta-linguistic variables. However, some arguments use the angle brackets literally. For example, an actual reverse-path is enclosed in angle brackets, i.e., "<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the angle brackets are actually transmitted in the command or reply).
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3. THE SMTP PROCEDURES
This section presents the procedures used in SMTP in several parts. First comes the basic mail procedure defined as a mail transaction. Following this are descriptions of forwarding mail, verifying mailbox names and expanding mailing lists, sending to terminals instead of or in combination with mailboxes, and the opening and closing exchanges. At the end of this section are comments on relaying, a note on mail domains, and a discussion of changing roles. Throughout this section are examples of partial command and reply sequences, several complete scenarios are presented in Appendix F.
3.1. MAIL
There are three steps to SMTP mail transactions. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. Then a DATA command gives the mail data. And finally, the end of mail data indicator confirms the transaction.
The first step in the procedure is the MAIL command. The <reverse-path> contains the source mailbox.
MAIL <SP> FROM:<reverse-path> <CRLF>
This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. It gives the reverse-path which can be used to report errors. If accepted, the receiver-SMTP returns a 250 OK reply.
The <reverse-path> can contain more than just a mailbox. The <reverse-path> is a reverse source routing list of hosts and source mailbox. The first host in the <reverse-path> should be the host sending this command.
The second step in the procedure is the RCPT command.
RCPT <SP> TO:<forward-path> <CRLF>
This command gives a forward-path identifying one recipient. If accepted, the receiver-SMTP returns a 250 OK reply, and stores the forward-path. If the recipient is unknown the receiver-SMTP returns a 550 Failure reply. This second step of the procedure can be repeated any number of times.
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The <forward-path> can contain more than just a mailbox. The <forward-path> is a source routing list of hosts and the destination mailbox. The first host in the <forward-path> should be the host receiving this command.
The third step in the procedure is the DATA command.
DATA <CRLF>
If accepted, the receiver-SMTP returns a 354 Intermediate reply and considers all succeeding lines to be the message text. When the end of text is received and stored the SMTP-receiver sends a 250 OK reply.
Since the mail data is sent on the transmission channel the end of the mail data must be indicated so that the command and reply dialog can be resumed. SMTP indicates the end of the mail data by sending a line containing only a period. A transparency procedure is used to prevent this from interfering with the user's text (see Section 4.5.2).
Please note that the mail data includes the memo header items such as Date, Subject, To, Cc, From [2].
The end of mail data indicator also confirms the mail transaction and tells the receiver-SMTP to now process the stored recipients and mail data. If accepted, the receiver-SMTP returns a 250 OK reply. The DATA command should fail only if the mail transaction was incomplete (for example, no recipients), or if resources are not available.
The above procedure is an example of a mail transaction. These commands must be used only in the order discussed above. Example 1 (below) illustrates the use of these commands in a mail transaction.
This SMTP example shows mail sent by Smith at host Alpha.ARPA, to Jones, Green, and Brown at host Beta.ARPA. Here we assume that host Alpha contacts host Beta directly.
S: MAIL FROM:<Smith@Alpha.ARPA> R: 250 OK
S: RCPT TO:<Jones@Beta.ARPA> R: 250 OK
S: RCPT TO:<Green@Beta.ARPA> R: 550 No such user here
S: RCPT TO:<Brown@Beta.ARPA> R: 250 OK
S: DATA R: 354 Start mail input; end with <CRLF>.<CRLF> S: Blah blah blah... S: ...etc. etc. etc. S: <CRLF>.<CRLF> R: 250 OK
The mail has now been accepted for Jones and Brown. Green did not have a mailbox at host Beta.
There are some cases where the destination information in the <forward-path> is incorrect, but the receiver-SMTP knows the correct destination. In such cases, one of the following replies should be used to allow the sender to contact the correct destination.
251 User not local; will forward to <forward-path>
This reply indicates that the receiver-SMTP knows the user's mailbox is on another host and indicates the correct forward-path to use in the future. Note that either the host or user or both may be different. The receiver takes responsibility for delivering the message.
551 User not local; please try <forward-path>
This reply indicates that the receiver-SMTP knows the user's mailbox is on another host and indicates the correct forward-path to use. Note that either the host or user or both may be different. The receiver refuses to accept mail for this user, and the sender must either redirect the mail according to the information provided or return an error response to the originating user.
SMTP provides as additional features, commands to verify a user name or expand a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. For the VRFY command, the string is a user name, and the response may include the full name of the user and must include the mailbox of the user. For the EXPN command, the string identifies a mailing list, and the multiline response may include the full name of the users and must give the mailboxes on the mailing list.
"User name" is a fuzzy term and used purposely. If a host implements the VRFY or EXPN commands then at least local mailboxes must be recognized as "user names". If a host chooses to recognize other strings as "user names" that is allowed.
In some hosts the distinction between a mailing list and an alias for a single mailbox is a bit fuzzy, since a common data structure may hold both types of entries, and it is possible to have mailing lists of one mailbox. If a request is made to verify a mailing list a positive response can be given if on receipt of a message so addressed it will be delivered to everyone on the list, otherwise an error should be reported (e.g., "550 That is a mailing list, not a user"). If a request is made to expand a user name a positive response can be formed by returning a list containing one name, or an error can be reported (e.g., "550 That is a user name, not a mailing list").
In the case of a multiline reply (normal for EXPN) exactly one mailbox is to be specified on each line of the reply. In the case of an ambiguous request, for example, "VRFY Smith", where there are two Smith's the response must be "553 User ambiguous".
The case of verifying a user name is straightforward as shown in example 3.
The character string arguments of the VRFY and EXPN commands cannot be further restricted due to the variety of implementations of the user name and mailbox list concepts. On some systems it may be appropriate for the argument of the EXPN command to be a file name for a file containing a mailing list, but again there is a variety of file naming conventions in the Internet.
The VRFY and EXPN commands are not included in the minimum implementation (Section 4.5.1), and are not required to work across relays when they are implemented.
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3.4. SENDING AND MAILING
The main purpose of SMTP is to deliver messages to user's mailboxes. A very similar service provided by some hosts is to deliver messages to user's terminals (provided the user is active on the host). The delivery to the user's mailbox is called "mailing", the delivery to the user's terminal is called "sending". Because in many hosts the implementation of sending is nearly identical to the implementation of mailing these two functions are combined in SMTP. However the sending commands are not included in the required minimum implementation (Section 4.5.1). Users should have the ability to control the writing of messages on their terminals. Most hosts permit the users to accept or refuse such messages.
The following three command are defined to support the sending options. These are used in the mail transaction instead of the MAIL command and inform the receiver-SMTP of the special semantics of this transaction:
SEND <SP> FROM:<reverse-path> <CRLF>
The SEND command requires that the mail data be delivered to the user's terminal. If the user is not active (or not accepting terminal messages) on the host a 450 reply may returned to a RCPT command. The mail transaction is successful if the message is delivered the terminal.
SOML <SP> FROM:<reverse-path> <CRLF>
The Send Or MaiL command requires that the mail data be delivered to the user's terminal if the user is active (and accepting terminal messages) on the host. If the user is not active (or not accepting terminal messages) then the mail data is entered into the user's mailbox. The mail transaction is successful if the message is delivered either to the terminal or the mailbox.
SAML <SP> FROM:<reverse-path> <CRLF>
The Send And MaiL command requires that the mail data be delivered to the user's terminal if the user is active (and accepting terminal messages) on the host. In any case the mail data is entered into the user's mailbox. The mail transaction is successful if the message is delivered the mailbox.
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The same reply codes that are used for the MAIL commands are used for these commands.
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3.5. OPENING AND CLOSING
At the time the transmission channel is opened there is an exchange to ensure that the hosts are communicating with the hosts they think they are.
The following two commands are used in transmission channel opening and closing:
HELO <SP> <domain> <CRLF>
QUIT <CRLF>
In the HELO command the host sending the command identifies itself; the command may be interpreted as saying "Hello, I am <domain>".
The forward-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. The mailbox is an absolute address, and the route is information about how to get there. The two concepts should not be confused.
Conceptually the elements of the forward-path are moved to the reverse-path as the message is relayed from one server-SMTP to another. The reverse-path is a reverse source route, (i.e., a source route from the current location of the message to the originator of the message). When a server-SMTP deletes its identifier from the forward-path and inserts it into the reverse-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 server-SMTP is known by different names in different environments.
If when the message arrives at an SMTP the first element of the forward-path is not the identifier of that SMTP the element is not deleted from the forward-path and is used to determine the next SMTP to send the message to. In any case, the SMTP adds its own identifier to the reverse-path.
Using source routing the receiver-SMTP receives mail to be relayed to another server-SMTP The receiver-SMTP may accept or reject the task of relaying the mail in the same way it accepts or rejects mail for a local user. The receiver-SMTP transforms the command arguments by moving its own identifier from the forward-path to the beginning of the reverse-path. The receiver-SMTP then becomes a sender-SMTP, establishes a transmission channel to the next SMTP in the forward-path, and sends it the mail.
The first host in the reverse-path should be the host sending the SMTP commands, and the first host in the forward-path should be the host receiving the SMTP commands.
Notice that the forward-path and reverse-path appear in the SMTP commands and replies, but not necessarily in the message. That is, there is no need for these paths and especially this syntax to appear in the "To:" , "From:", "CC:", etc. fields of the message header.
If a server-SMTP has accepted the task of relaying the mail and
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later finds that the forward-path is incorrect or that the mail cannot be delivered for whatever reason, then it must construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path).
This notification message must be from the server-SMTP at this host. Of course, server-SMTPs should not send notification messages about problems with notification messages. One way to prevent loops in error reporting is to specify a null reverse-path in the MAIL command of a notification message. When such a message is relayed it is permissible to leave the reverse-path null. A MAIL command with a null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail notification message is shown in example 7. This notification is in response to a message originated by JOE at HOSTW and sent via HOSTX to HOSTY with instructions to relay it on to HOSTZ. What we see in the example is the transaction between HOSTY and HOSTX, which is the first step in the return of the notification message.
S: MAIL FROM:<> R: 250 ok S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA> R: 250 ok S: DATA R: 354 send the mail data, end with . S: Date: 23 Oct 81 11:22:33 S: From: SMTP@HOSTY.ARPA S: To: JOE@HOSTW.ARPA S: Subject: Mail System Problem S: S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost. S: HOSTZ.ARPA said this: S: "550 No Such User" S: . R: 250 ok
Domains are a recently introduced concept in the ARPA Internet mail system. The use of domains changes the address space from a flat global space of simple character string host names to a hierarchically structured rooted tree of global addresses. The host name is replaced by a domain and host designator which is a sequence of domain element strings separated by periods with the understanding that the domain elements are ordered from the most specific to the most general.
For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
Whenever domain names are used in SMTP only the official names are used, the use of nicknames or aliases is not allowed.
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3.8. CHANGING ROLES
The TURN command may be used to reverse the roles of the two programs communicating over the transmission channel.
If program-A is currently the sender-SMTP and it sends the TURN command and receives an ok reply (250) then program-A becomes the receiver-SMTP.
If program-B is currently the receiver-SMTP and it receives the TURN command and sends an ok reply (250) then program-B becomes the sender-SMTP.
To refuse to change roles the receiver sends the 502 reply.
Please note that this command is optional. It would not normally be used in situations where the transmission channel is TCP. However, when the cost of establishing the transmission channel is high, this command may be quite useful. For example, this command may be useful in supporting be mail exchange using the public switched telephone system as a transmission channel, especially if some hosts poll other hosts for mail exchanges.
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4. THE SMTP SPECIFICATIONS
4.1. SMTP COMMANDS
4.1.1. COMMAND SEMANTICS
The SMTP commands define the mail transfer or the mail system function requested by the user. SMTP 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 SMTP commands are discussed below. The SMTP replies are discussed in the Section 4.2.
A mail transaction involves several data objects which are communicated as arguments to different commands. The reverse-path is the argument of the MAIL command, the forward-path is the argument of the RCPT command, and the mail data is the argument of the DATA command. These arguments or data objects must be transmitted and held pending the confirmation communicated by the end of mail data indication which finalizes the transaction. The model for this is that distinct buffers are provided to hold the types of data objects, that is, there is a reverse-path buffer, a forward-path buffer, and a mail data buffer. Specific commands cause information to be appended to a specific buffer, or cause one or more buffers to be cleared.
HELLO (HELO)
This command is used to identify the sender-SMTP to the receiver-SMTP. The argument field contains the host name of the sender-SMTP.
The receiver-SMTP identifies itself to the sender-SMTP in the connection greeting reply, and in the response to this command.
This command and an OK reply to it confirm that both the sender-SMTP and the receiver-SMTP are in the initial state, that is, there is no transaction in progress and all state tables and buffers are cleared.
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MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail data is delivered to one or more mailboxes. The argument field contains a reverse-path.
The reverse-path consists of an optional list of hosts and the sender mailbox. When the list of hosts is present, it is a "reverse" source route 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 a source route 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 IPCE to which it is relaying the mail rather than the IPCE from which the mail came (if they are different). In some types of error reporting messages (for example, undeliverable mail notifications) the reverse-path may be null (see Example 7).
This command clears the reverse-path buffer, the forward-path buffer, and the mail data buffer; and inserts the reverse-path information from this command into the reverse-path buffer.
RECIPIENT (RCPT)
This command is used to identify an individual recipient of the mail data; multiple recipients are specified by multiple use of this command.
The forward-path consists of an optional list of hosts and a required destination mailbox. When the list of hosts is present, it is a source route and indicates that the mail must be relayed to the next host on the list. If the receiver-SMTP does not implement the relay function it may user the same reply it would for an unknown local user (550).
When mail is relayed, the relay host must remove itself from the beginning forward-path and put itself at the beginning of the reverse-path. When mail reaches its ultimate destination (the forward-path contains only a destination mailbox), the receiver-SMTP inserts it into the destination mailbox in accordance with its host mail conventions.
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For example, mail received at relay host A with arguments
This command causes its forward-path argument to be appended to the forward-path buffer.
DATA (DATA)
The receiver treats the lines following the command as mail data from the sender. This command causes the mail data from this command to be appended to the mail data buffer. The mail data may contain any of the 128 ASCII character codes.
The mail data is terminated by a line containing only a period, that is the character sequence "<CRLF>.<CRLF>" (see Section 4.5.2 on Transparency). This is the end of mail data indication.
The end of mail data indication requires that the receiver must now process the stored mail transaction information. This processing consumes the information in the reverse-path buffer, the forward-path buffer, and the mail data buffer, and on the completion of this command these buffers are cleared. If the processing is successful the receiver must send an OK reply. If the processing fails completely the receiver must send a failure reply.
When the receiver-SMTP accepts a message either for relaying or for final delivery it inserts at the beginning of the mail data a time stamp line. The time stamp line indicates the identity of the host that sent the message, and the identity of the host that received the message (and is inserting this time stamp), and the date and time the message was received. Relayed messages will have multiple time stamp lines.
When the receiver-SMTP makes the "final delivery" of a message it inserts at the beginning of the mail data a
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return path line. The return path line preserves the information in the <reverse-path> from the MAIL command. Here, final delivery means the message leaves the SMTP world. Normally, this would mean it has been delivered to the destination user, but in some cases it may be further processed and transmitted by another mail system.
It is possible for the mailbox in the return path be different from the actual sender's mailbox, for example, if error responses are to be delivered a special error handling mailbox rather than the message senders.
The preceding two paragraphs imply that the final mail data will begin with a return path line, followed by one or more time stamp lines. These lines will be followed by the mail data header and body [2]. See Example 8.
Special mention is needed of the response and further action required when the processing following the end of mail data indication is partially successful. This could arise if after accepting several recipients and the mail data, the receiver-SMTP finds that the mail data can be successfully delivered to some of the recipients, but it cannot be to others (for example, due to mailbox space allocation problems). In such a situation, the response to the DATA command must be an OK reply. But, the receiver-SMTP must compose and send an "undeliverable mail" notification message to the originator of the message. Either a single notification which lists all of the recipients that failed to get the message, or separate notification messages must be sent for each failed recipient (see Example 7). All undeliverable mail notification messages are sent using the MAIL command (even if they result from processing a SEND, SOML, or SAML command).
Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA> Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST Date: 27 Oct 81 15:01:01 PST From: JOE@ABC.ARPA Subject: Improved Mailing System Installed To: SAM@JKL.ARPA This is to inform you that ...
This command is used to initiate a mail transaction in which the mail data is delivered to one or more terminals. The argument field contains a reverse-path. This command is successful if the message is delivered to a terminal.
The reverse-path consists of an optional list of hosts and the sender mailbox. When the list of hosts is present, it is a "reverse" source route 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 a source route 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 IPCE to which it is relaying the mail rather than the IPCE from which the mail came (if they are different).
This command clears the reverse-path buffer, the forward-path buffer, and the mail data buffer; and inserts the reverse-path information from this command into the reverse-path buffer.
SEND OR MAIL (SOML)
This command is used to initiate a mail transaction in which the mail data is delivered to one or more terminals or
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mailboxes. For each recipient the mail data is delivered to the recipient's terminal if the recipient is active on the host (and accepting terminal messages), otherwise to the recipient's mailbox. The argument field contains a reverse-path. This command is successful if the message is delivered to a terminal or the mailbox.
The reverse-path consists of an optional list of hosts and the sender mailbox. When the list of hosts is present, it is a "reverse" source route 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 a source route 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 IPCE to which it is relaying the mail rather than the IPCE from which the mail came (if they are different).
This command clears the reverse-path buffer, the forward-path buffer, and the mail data buffer; and inserts the reverse-path information from this command into the reverse-path buffer.
SEND AND MAIL (SAML)
This command is used to initiate a mail transaction in which the mail data is delivered to one or more terminals and mailboxes. For each recipient the mail data is delivered to the recipient's terminal if the recipient is active on the host (and accepting terminal messages), and for all recipients to the recipient's mailbox. The argument field contains a reverse-path. This command is successful if the message is delivered to the mailbox.
The reverse-path consists of an optional list of hosts and the sender mailbox. When the list of hosts is present, it is a "reverse" source route 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 a source route 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 IPCE to which it is relaying the mail rather than the IPCE from which the mail came (if they are different).
This command clears the reverse-path buffer, the
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forward-path buffer, and the mail data buffer; and inserts the reverse-path information from this command into the reverse-path buffer.
RESET (RSET)
This command specifies that the current mail transaction is to be aborted. Any stored sender, recipients, and mail data must be discarded, and all buffers and state tables cleared. The receiver must send an OK reply.
VERIFY (VRFY)
This command asks the receiver to confirm that the argument identifies a user. If it is a user name, the full name of the user (if known) and the fully specified mailbox are returned.
This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer.
EXPAND (EXPN)
This command asks the receiver to confirm that the argument identifies a mailing list, and if so, to return the membership of that list. The full name of the users (if known) and the fully specified mailboxes are returned in a multiline reply.
This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer.
HELP (HELP)
This command causes the receiver to send helpful information to the sender of the HELP command. The command may take an argument (e.g., any command name) and return more specific information as a response.
This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer.
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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.
This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer.
QUIT (QUIT)
This command specifies that the receiver must send an OK reply, and then close the transmission channel.
The receiver should not close the transmission channel until it receives and replies to a QUIT command (even if there was an error). The sender should not close the transmission channel until it send a QUIT command and receives the reply (even if there was an error response to a previous command). If the connection is closed prematurely the receiver should act as if a RSET command had been received (canceling any pending transaction, but not undoing any previously completed transaction), the sender should act as if the command or transaction in progress had received a temporary error (4xx).
TURN (TURN)
This command specifies that the receiver must either (1) send an OK reply and then take on the role of the sender-SMTP, or (2) send a refusal reply and retain the role of the receiver-SMTP.
If program-A is currently the sender-SMTP and it sends the TURN command and receives an OK reply (250) then program-A becomes the receiver-SMTP. Program-A is then in the initial state as if the transmission channel just opened, and it then sends the 220 service ready greeting.
If program-B is currently the receiver-SMTP and it receives the TURN command and sends an OK reply (250) then program-B becomes the sender-SMTP. Program-B is then in the initial state as if the transmission channel just opened, and it then expects to receive the 220 service ready greeting.
To refuse to change roles the receiver sends the 502 reply.
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There are restrictions on the order in which these command may be used.
The first command in a session must be the HELO command. The HELO command may be used later in a session as well. If the HELO command argument is not acceptable a 501 failure reply must be returned and the receiver-SMTP must stay in the same state.
The NOOP, HELP, EXPN, and VRFY commands can be used at any time during a session.
The MAIL, SEND, SOML, or SAML commands begin a mail transaction. Once started a mail transaction consists of one of the transaction beginning commands, one or more RCPT commands, and a DATA command, in that order. A mail transaction may be aborted by the RSET command. There may be zero or more transactions in a session.
If the transaction beginning command argument is not acceptable a 501 failure reply must be returned and the receiver-SMTP must stay in the same state. If the commands in a transaction are out of order a 503 failure reply must be returned and the receiver-SMTP must stay in the same state.
The last command in a session must be the QUIT command. The QUIT command can not be used at any other time in a session.
4.1.2. COMMAND SYNTAX
The commands consist of a command code followed by an argument field. 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 "TO" or "to" for the forward-path. Command codes and the argument fields are separated by one or more spaces. However, within the reverse-path and forward-path arguments case is important. In particular, in some hosts the user "smith" is different from the user "Smith".
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The argument field consists of a variable length character string ending with the character sequence <CRLF>. The receiver is to take no action until this sequence is received.
Square brackets denote an optional argument field. If the option is not taken, the appropriate default is implied.
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The following are the SMTP commands:
HELO <SP> <domain> <CRLF>
MAIL <SP> FROM:<reverse-path> <CRLF>
RCPT <SP> TO:<forward-path> <CRLF>
DATA <CRLF>
RSET <CRLF>
SEND <SP> FROM:<reverse-path> <CRLF>
SOML <SP> FROM:<reverse-path> <CRLF>
SAML <SP> FROM:<reverse-path> <CRLF>
VRFY <SP> <string> <CRLF>
EXPN <SP> <string> <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
QUIT <CRLF>
TURN <CRLF>
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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.
<CR> ::= the carriage return character (ASCII code 13)
<LF> ::= the line feed character (ASCII code 10)
<SP> ::= the space character (ASCII code 32)
<snum> ::= one, two, or three digits representing a decimal integer value in the range 0 through 255
<a> ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case
<c> ::= any one of the 128 ASCII characters, but not any <special> or <SP>
<d> ::= any one of the ten digits 0 through 9
<q> ::= any one of the 128 ASCII characters except <CR>, <LF>, quote ("), or backslash (\)
<x> ::= any one of the 128 ASCII characters (no exceptions)
<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "." | "," | ";" | ":" | "@" """ | the control characters (ASCII codes 0 through 31 inclusive and 127)
Note that the backslash, "\", is a quote character, which is used to indicate that the next character is to be used literally (instead of its normal interpretation). For 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. Note that the name elements of domains are the official names -- no use of nicknames or aliases is allowed.
Sometimes a host is not known to the translation function and communication is blocked. To bypass this barrier two 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., "[123.255.37.2]", which indicates a 32-bit ARPA Internet Address in four 8-bit fields.
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The time stamp line and the return path line are formally defined as follows:
Replies to SMTP commands are devised to ensure the synchronization of requests and actions in the process of mail transfer, and to guarantee that the sender-SMTP always knows the state of the receiver-SMTP. Every command must generate exactly one reply.
The details of the command-reply sequence are made explicit in Section 5.3 on Sequencing and Section 5.4 State Diagrams.
An SMTP 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-SMTP need not 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 and context dependent, so there are likely to be varying texts for each reply code. A discussion of the theory of reply codes is given in Appendix E. Formally, a reply is defined to be the sequence: a three-digit code, <SP>, one line of text, and <CRLF>, or a multiline reply (as defined in Appendix E). Only the EXPN and HELP commands are expected to result in multiline replies in normal circumstances, however multiline replies are allowed for any command.
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4.2.1. REPLY CODES BY FUNCTION GROUPS
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] 220 <domain> Service ready 221 <domain> Service closing transmission channel 421 <domain> Service not available, closing transmission channel [This may be a reply to any command if the service knows it must shut down] 250 Requested mail action okay, completed 251 User not local; will forward to <forward-path> 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: error in processing 551 User not local; please try <forward-path> 452 Requested action not taken: insufficient system storage 552 Requested mail action aborted: exceeded storage allocation 553 Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect] 354 Start mail input; end with <CRLF>.<CRLF> 554 Transaction failed
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4.2.2. NUMERIC ORDER LIST OF REPLY CODES
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] 220 <domain> Service ready 221 <domain> Service closing transmission channel 250 Requested mail action okay, completed 251 User not local; will forward to <forward-path> 354 Start mail input; end with <CRLF>.<CRLF> 421 <domain> 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] 551 User not local; please try <forward-path> 552 Requested mail action aborted: exceeded storage allocation 553 Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect] 554 Transaction failed
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4.3. SEQUENCING OF COMMANDS AND REPLIES
The communication between the sender and receiver is intended to be an alternating dialogue, controlled by the sender. As such, the sender issues a command and the receiver responds with a reply. The sender must wait for this response before sending further commands.
One important reply is the connection greeting. Normally, a receiver will send a 220 "Service ready" reply when the connection is completed. The sender should wait for this greeting message before sending any commands.
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.ARPA <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. The prefixes used before the possible replies are "P" for preliminary (not used in SMTP), "I" for intermediate, "S" for success, "F" for failure, and "E" for error. The 421 reply (service not available, closing transmission channel) may be given to any command if the SMTP-receiver knows it must shut down. This listing forms the basis for the State Diagrams in Section 4.4.
Following are state diagrams for a simple-minded SMTP implementation. Only the first digit of the reply codes is used. There is one state diagram for each group of SMTP 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 SMTP commands:
1,3 +---+ ----------->| E | | +---+ | +---+ cmd +---+ 2 +---+ | B |---------->| W |---------->| S | +---+ +---+ +---+ | | 4,5 +---+ ----------->| F | +---+
+---+ DATA +---+ 1,2 +---+ | B |---------->| W |-------------------->| E | +---+ +---+ ------------>+---+ 3| |4,5 | | | | -------------- ----- | | | | +---+ | ---------- -------->| S | | | | | +---+ | | ------------ | | | | V 1,3| |2 | +---+ data +---+ --------------->+---+ | |---------->| W | | F | +---+ +---+-------------------->+---+ 4,5
Note that the "data" here is a series of lines sent from the sender to the receiver with no response expected until the last line is sent.
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4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum implementation is required for all receivers:
COMMANDS -- HELO MAIL RCPT DATA RSET NOOP QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character sequence "<CRLF>.<CRLF>" ends 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-SMTP 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-SMTP it checks 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.
The mail data may contain any of the 128 ASCII characters. All characters are to be delivered to the recipient's mailbox including format effectors and other control characters. If the transmission channel provides an 8-bit byte (octets) data stream, the 7-bit ASCII codes are transmitted right justified in the octets with the high order bits cleared to zero.
In some systems it may be necessary to transform the data as it is received and stored. This may be necessary for hosts that use a different character set than ASCII as their local character set, or that store data in records rather than
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strings. If such transforms are necessary, they must be reversible -- especially if such transforms are applied to mail being relayed.
4.5.3. SIZES
There are several objects that have required minimum maximum sizes. That is, every implementation must be able to receive objects of at least these sizes, but must not send objects larger than these sizes.
**************************************************** * * * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION * * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH * * OF THESE OBJECTS SHOULD BE USED. * * * ****************************************************
user
The maximum total length of a user name is 64 characters.
domain
The maximum total length of a domain name or number is 64 characters.
path
The maximum total length of a reverse-path or forward-path is 256 characters (including the punctuation and element separators).
command line
The maximum total length of a command line including the command word and the <CRLF> is 512 characters.
reply line
The maximum total length of a reply line including the reply code and the <CRLF> is 512 characters.
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text line
The maximum total length of a text line including the <CRLF> is 1000 characters (but not counting the leading dot duplicated for transparency).
recipients buffer
The maximum total number of recipients that must be buffered is 100 recipients.
**************************************************** * * * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION * * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH * * OF THESE OBJECTS SHOULD BE USED. * * * ****************************************************
Errors due to exceeding these limits may be reported by using the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
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APPENDIX A
TCP Transport service
The Transmission Control Protocol [3] is used in the ARPA Internet, and in any network following the US DoD standards for internetwork protocols.
Connection Establishment
The SMTP 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 25 (31 octal), that is L=25.
Data Transfer
The TCP connection supports the transmission of 8-bit bytes. The SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero.
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APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [4] (implemented by the Network Control Program) may be used in the ARPANET.
Connection Establishment
The SMTP transmission channel is established via NCP between the sender process socket U and receiver process socket L. The Initial Connection Protocol [5] 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 25 (31 octal), that is L=25.
Data Transfer
The NCP data connections are established in 8-bit byte mode. The SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero.
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APPENDIX C
NITS
The Network Independent Transport Service [6] may be used.
Connection Establishment
The SMTP transmission channel is established via NITS between 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 SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero.
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APPENDIX D
X.25 Transport service
It may be possible to use the X.25 service [7] as provided by the Public Data Networks directly, however, it is suggested that a reliable end-to-end protocol such as TCP be used on top of X.25 connections.
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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-SMTP will be able to determine its next action (proceed as planned, redo, retrench, etc.) by simply examining this first digit. A sender-SMTP 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-SMTP should send another command specifying whether to continue or abort the action.
[Note: SMTP does not have any commands that allow this type of reply, and so does not have the continue or abort commands.]
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-SMTP 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
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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- SMTPs) must agree on the interpretation. Each reply in this category might have a different time value, but the sender-SMTP 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 and 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-SMTP 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-SMTP 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
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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. 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-SMTP any new information will return a 250 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-SMTP knows when it can stop reading the reply. This requires a special format to indicate a multiple line reply.
The format for multiline 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
In many cases the sender-SMTP then simply needs to search for the reply code followed by <SP> at the beginning of a line, and ignore all preceding lines. In a few cases, there is important data for the sender in the reply "text". The sender will know these cases from the current context.
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APPENDIX F
Scenarios
This section presents complete scenarios of several types of SMTP sessions.
A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host USC-ISIF, to Jones, Green, and Brown at host BBN-UNIX. Here we assume that host USC-ISIF contacts host BBN-UNIX directly. The mail is accepted for Jones and Brown. Green does not have a mailbox at host BBN-UNIX.
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready S: HELO MIT-AI.ARPA R: 250 USC-ISIE.ARPA
S: MAIL FROM:<JQP@MIT-AI.ARPA> R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA> R: 250 OK
S: DATA R: 354 Start mail input; end with <CRLF>.<CRLF> S: Date: 2 Nov 81 22:33:44 S: From: John Q. Public <JQP@MIT-AI.ARPA> S: Subject: The Next Meeting of the Board S: To: Jones@BBN-Vax.ARPA S: S: Bill: S: The next meeting of the board of directors will be S: on Tuesday. S: John. S: . R: 250 OK
S: QUIT R: 221 USC-ISIE.ARPA Service closing transmission channel
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Step 2 -- Relay Host to Destination Host
R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready S: HELO USC-ISIE.ARPA R: 250 BBN-VAX.ARPA
S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA> R: 250 OK
S: RCPT TO:<Jones@BBN-VAX.ARPA> R: 250 OK
S: DATA R: 354 Start mail input; end with <CRLF>.<CRLF> S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ; 2 Nov 81 22:40:10 UT S: Date: 2 Nov 81 22:33:44 S: From: John Q. Public <JQP@MIT-AI.ARPA> S: Subject: The Next Meeting of the Board S: To: Jones@BBN-Vax.ARPA S: S: Bill: S: The next meeting of the board of directors will be S: on Tuesday. S: John. S: . R: 250 OK
S: QUIT R: 221 USC-ISIE.ARPA Service closing transmission channel
First the user's name is verified, then an attempt is made to send to the user's terminal. When that fails, the messages is mailed to the user's mailbox.
First each of two mailing lists are expanded in separate sessions with different hosts. Then the message is sent to everyone that appeared on either list (but no duplicates) via a relay host.
Note that a real implementation must handle many recipients as specified in Section 4.5.3.
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GLOSSARY
ASCII
American Standard Code for Information Interchange [1].
command
A request for a mail service action sent by the sender-SMTP to the receiver-SMTP.
domain
The hierarchially structured global character string address of a host computer in the mail system.
end of mail data indication
A special sequence of characters that indicates the end of the mail data. In particular, the five characters carriage return, line feed, period, carriage return, line feed, in that order.
host
A computer in the internetwork environment on which mailboxes or SMTP processes reside.
line
A a sequence of ASCII characters ending with a <CRLF>.
mail data
A sequence of ASCII characters of arbitrary length, which conforms to the standard set in the Standard for the Format of ARPA Internet Text Messages (RFC 822 [2]).
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@domain". Additionally, the "container" in which mail is stored.
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receiver-SMTP process
A process which transfers mail in cooperation with a sender-SMTP process. It waits for a connection to be established via the transport service. It receives SMTP commands from the sender-SMTP, sends replies, and performs the specified operations.
reply
A reply is an acknowledgment (positive or negative) sent from receiver to sender via the transmission channel in response to a 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-SMTP process
A process which transfers mail in cooperation with a receiver-SMTP process. A local language may be used in the user interface command/reply dialogue. The sender-SMTP initiates the transport service connection. It initiates SMTP commands, receives replies, and governs the transfer of mail.
session
The set of exchanges that occur while the transmission channel is open.
transaction
The set of exchanges required for one message to be transmitted for one or more recipients.
transmission channel
A full-duplex communication path between a sender-SMTP and a receiver-SMTP for the exchange of commands, replies, and mail text.
transport service
Any reliable stream-oriented data communication services. For example, NCP, TCP, NITS.
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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 sequence of printing characters.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
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REFERENCES
[1] 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.
[2] RFC 822
Crocker, D., "Standard for the Format of ARPA Internet Text Messages," RFC 822, Department of Electrical Engineering, University of Delaware, August 1982.
[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet Program Protocol Specification", RFC 793, USC/Information Sciences Institute, NTIS AD Number A111091, September 1981. Also in: Feinler, E. and J. Postel, eds., "Internet Protocol Transition Workbook", SRI International, Menlo Park, California, March 1982.
[4] 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.
[5] 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.
[6] 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.
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[7] 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.
