Application Protocols: ELECTRONIC MAIL (SMTP, POP) CSNB534 Semester 2, 2007/2008 Asma Shakil

Application Protocols An application protocol facilitates communication between applications using the services of lower-level OSI model layers. E.g. when you check your , browse a web site, play games or download files over the Internet, the s/w you run is using an application protocol for communication.

Why do we need to have an application protocol? For applications to interoperate, they must implement an application protocol. In other words, both applications must be talking the same language. It is not necessary that the implementation of a protocol support every feature of the protocol. must be in the same way or in the same language or on the same machine. However, protocol implementations must outwardly behave in the same wayu. When they cannot fulfill a request or support a feature They must communicate it effectively using a commonly understood process.

Application Protocol Specification The design of a network protocol is an evolutionary process that involves many contributors. When the protocol nears completion and is ready for applications to implement it, it is published as a Request For Comment (RFC) document. Each RFC is assigned a number for identification. E.g. RFC 1945 for HTTP/1.0 Finding RFC Documents

Application Protocol Implementation The most enjoyable part of n/w programming is putting the n/w theory (that you have learnt so far ) by writing real-life applications. In today’s class, we will look at implementing protocols.

Electronic Mail Among most widely used Internet services Two major components User interface Mail transfer software Paradigm: transfer is separate background activity. This is because a mail system must provide for instances when the remote machine is temporarily unreachable – done by spooling. The background mail transfer process become a client that attempts to form a TCP connection to the mail server on the destination machine.

Illustration Of System Components

Format Of Message Message consists of Header Blank line Body of message Headers have form keyword : information Standard given in RFC 2822

Protocol For Transfer Specifies interaction between transfer components Transfer client Transfer server Standard protocol is Simple Mail Transfer Protocol (SMTP)

SMTP Application-level protocol known as RFC2821 Uses TCP Commands and responses encoded in ASCII This protocol assumes that some other method is used to actually read the messages.

Example Of SMTP S: 220 Beta.GOV Simple Mail Transfer Service Ready // Upon connecting, the server will send a response code in greeting, to ensure that the client is talking to an SMTP service and not some other service using port 25. C: HELO Alpha.EDU // Protcol specification for telling the server who we really are. S: 250 Beta.GOV // Correct response is 250 which signals OK. C: MAIL // used to set the sender S: 250 OK C: RCPT // used to set the recipient S: 250 OK C: RCPT // recipient S: 550 No such user here C: RCPT // there can be more than one S: 250 OK C: DATA // to signal that the client is ready to send a message body S: 354 Start mail input; end with. // valid response code is 354 C:...sends body of mail message... C:...continues for as many lines as message contains C:. S: 250 OK C: QUIT // to indicate that the client has completed its transaction with the SMTP server. S: 221 Beta.GOV Service closing transmission channel

SMTP Client Implementation Code for SMTPClientDemo.SMTPClientDemo

POP The SMTP transfer scheme implies that a server must be ready to accept at all times. The scenario works well if the server runs on a computer that has a permanent Internet connection but not for one with intermittent connectivity (e.g. a laptop computer). To resolve this, a two-stage delivery process is adpoted. Each user is assigned a mailbox on a computer that is always on and has a permanent Internet connectivity. The computer runs a conventional SMTP server, which always remains ready to accept . The user connects to the Internet and then runs a protocol that retrieves messages from the permanent mailbox. The protocol transfers the messages to the user’s computer where they can be read. Two protocols used for this are  POP (Post Office Protocol)  IMAP(Internet Message Access Protocol)

Example of POP telnet pop.foo.bar 110 Trying Connected to pop.foo.bar. Escape character is '^]'. +OK QPOP (version 2.4) at pop.foo.bar starting. USER user.123 +OK Password required for foo.123. PASS foo +OK user.123 has 3 messages (1548 octets). STAT //Displays number of messages and space taken by mailspool +OK LIST //Displays space taken by each message +OK 3 messages (1548 octets) RETR 2 // Displays message message_number +OK 386 octets Return-Path: Received: (from by pop.foo.bar (8.8.8/8.8.8) id SAA29614 for user.123; Wed, 3 Dec :55: (EST) Date: Wed, 3 Dec :55: (EST) From: Pat Gunn Message-Id: X- Real-To: user.123 Subject: Hi! Content-Type: text/plain X-UIDL: f6aba8f9429ffcb4f343c6b061cd82bb Status: U Hi Pat! What's up?.

POP3 Client Implementation Code for POP3ClientDemo.POP3ClientDemo