In order to give a bit better idea what is involved in the application protocols, I'm going to show an example of SMTP, which is the mail protocol. (SMTP is ``simple mail transfer protocol.) We assume that a computer called TOPAZ.RUTGERS.EDU wants to send the following message.
Date: Sat, 27 Jun 87 13:26:31 EDT From: hedrick@topaz.rutgers.edu To: levy@red.rutgers.edu Subject: meeting Let's get together Monday at 1pm.First, note that the format of the message itself is described by an Internet standard (RFC 822). The standard specifies the fact that the message must be transmitted as net ASCII (i.e. it must be ASCII, with carriage return/linefeed to delimit lines). It also describes the general structure, as a group of header lines, then a blank line, and then the body of the message. Finally, it describes the syntax of the header lines in detail. Generally they consist of a keyword and then a value.
Note that the addressee is indicated as LEVY@RED.RUTGERS.EDU. Initially, addresses were simply ``person at machine''. However recent standards have made things more flexible. There are now provisions for systems to handle other systems' mail. This can allow automatic forwarding on behalf of computers not connected to the Internet. It can be used to direct mail for a number of systems to one central mail server. Indeed there is no requirement that an actual computer by the name of RED.RUTGERS.EDU even exist. The name servers could be set up so that you mail to department names, and each department's mail is routed automatically to an appropriate computer. It is also possible that the part before the @ is something other than a user name. It is possible for programs to be set up to process mail. There are also provisions to handle mailing lists, and generic names such as ``postmaster'' or ``operator''.
The way the message is to be sent to another system is described by RFC's 821 and 974. The program that is going to be doing the sending asks the name server several queries to determine where to route the message. The first query is to find out which machines handle mail for the name RED.RUTGERS.EDU. In this case, the server replies that RED.RUTGERS.EDU handles its own mail. The program then asks for the address of RED.RUTGERS.EDU, which is 128.6.4.2. Then the mail program opens a TCP connection to port 25 on 128.6.4.2. Port 25 is the well-known socket used for receiving mail. Once this connection is established, the mail program starts sending commands. Here is a typical conversation. Each line is labelled as to whether it is from TOPAZ or RED. Note that TOPAZ initiated the connection:
RED 220 RED.RUTGERS.EDU SMTP Service at 29 Jun 87 05:17:18 EDT TOPAZ HELO topaz.rutgers.edu RED 250 RED.RUTGERS.EDU - Hello, TOPAZ.RUTGERS.EDU TOPAZ MAIL From:<hedrick@topaz.rutgers.edu> RED 250 MAIL accepted TOPAZ RCPT To:<levy@red.rutgers.edu> RED 250 Recipient accepted TOPAZ DATA RED 354 Start mail input; end with <CRLF>.<CRLF> TOPAZ Date: Sat, 27 Jun 87 13:26:31 EDT TOPAZ From: hedrick@topaz.rutgers.edu TOPAZ To: levy@red.rutgers.edu TOPAZ Subject: meeting TOPAZ TOPAZ Let's get together Monday at 1pm. TOPAZ . RED 250 OK TOPAZ QUIT RED 221 RED.RUTGERS.EDU Service closing transmission channel
First, note that commands all use normal text. This is typical of the Internet standards. Many of the protocols use standard ASCII commands. This makes it easy to watch what is going on and to diagnose problems. For example, the mail program keeps a log of each conversation. If something goes wrong, the log file can simply be mailed to the postmaster. Since it is normal text, he can see what was going on. It also allows a human to interact directly with the mail server, for testing. (Some newer protocols are complex enough that this is not practical. The commands would have to have a syntax that would require a significant parser. Thus there is a tendency for newer protocols to use binary formats. Generally they are structured like C or Pascal record structures.) Second, note that the responses all begin with numbers. This is also typical of Internet protocols. The allowable responses are defined in the protocol. The numbers allow the user program to respond unambiguously. The rest of the response is text, which is normally for use by any human who may be watching or looking at a log. It has no effect on the operation of the programs. (However there is one point at which the protocol uses part of the text of the response.) The commands themselves simply allow the mail program on one end to tell the mail server the information it needs to know in order to deliver the message. In this case, the mail server could get the information by looking at the message itself. But for more complex cases, that would not be safe. Every session must begin with a HELO, which gives the name of the system that initiated the connection. Then the sender and recipients are specified. (There can be more than one RCPT command, if there are several recipients.) Finally the data itself is sent. Note that the text of the message is terminated by a line containing just a period. (If such a line appears in the message, the period is doubled.) After the message is accepted, the sender can send another message, or terminate the session as in the example above.
Generally, there is a pattern to the response numbers. The protocol defines the specific set of responses that can be sent as answers to any given command. However programs that don't want to analyze them in detail can just look at the first digit. In general, responses that begin with a 2 indicate success. Those that begin with 3 indicate that some further action is needed, as shown above. 4 and 5 indicate errors. 4 is a ``temporary'' error, such as a disk filling. The message should be saved, and tried again later. 5 is a permanent error, such as a non-existent recipient. The message should be returned to the sender with an error message.
(For more details about the protocols mentioned in this section, see RFC's 821/822 for mail, RFC 959 for file transfer, and RFC's 854/855 for remote logins. For the well-known port numbers, see the current edition of Assigned Numbers, and possibly RFC 814.)