Lecture 6: Sun: 8/5/1435 Distributed Applications Lecturer/ Kawther Abas CS- 492 : Distributed system & Parallel Processing

Electronic Mail 4 Most heavily used application on any network 4 Simple Mail Transfer Protocol (SMTP) –TCP/IP –Delivery of simple text messages 4 Multi-purpose Internet Mail Extension (MIME) –Delivery of other types of data –Voice, images, video clips

SMTP 4 RFC Not concerned with format of messages or data –Covered in RFC 822 (see later) 4 SMTP uses info written on envelope of mail –Message header 4 Does not look at contents –Message body 4 Except: –Standardize message character set to 7 bit ASCII –Add log info to start of message Shows path taken

Basic Operation 4 Mail created by user agent program (mail client) –Message consists of: Header containing recipient’s address and other info Body containing user data 4 Messages queued and sent as input to SMTP sender program –Typically a server process (daemon on UNIX)

SMTP Mail Flow

Operation Phases 4 Connection setup 4 Exchange of command-response pairs 4 Connection termination

Multipurpose Internet Mail Extension (MIME) 4 Extension to RFC822 4 SMTP can not transmit executables –Uuencode and other schemes are available Not standardized 4 Can not transmit text including international characters (e.g. â, å, ä, è, é, ê, ë) –Need 8 bit ASCII 4 Servers may reject mail over certain size 4 Translation between ASCII and EBCDIC not standard 4 SMTP gateways to X.400 can not handle none text data in X.400 messages 4 Some SMTP implementations do not adhere to standard –CRLF, truncate or wrap long lines, removal of white space, etc.

Overview of MIME 4 Five new message header fields –MIME version –Content type –Content transfer encoding –Content Id –Content Description 4 Number of content formats defines 4 Transfer encoding defined

Hypertext Transfer Protocol-HTTP 4 Underlying protocol of the World Wide Web 4 Not a protocol for transferring hypertext –For transmitting information with efficiency necessary for hypertext jumps 4 Can transfer plain text, hypertext, audio, images, and Internet accessible information

HTTP Overview 4 Transaction oriented client/server protocol 4 Usually between Web browser (clinet) and Web server 4 Uses TCP connections 4 Stateless –Each transaction treated independently –Each new TCP connection for each transaction –Terminate connection when transaction complete

Examples of HTTP Operation

Example of Distributed Network Management Configuration

Mobile Code Applications(1) 4 They are still immature and lack of suitable methodologies. 4 The benefits of mobile code are appealing.(Mobile agent systems, Internet agents) 4 It is important to distinguish the implementation process and the paradigm.

Mobile Code Applications(2) Benefits: 1. Service Customization- The interfaces or services are are not statically defined. 2. Supporting more flexible software deployment and maintenance. 3. Automatic software upgrade without human interaction on the client side.

Mobile Code Applications(3) Benefits: 4. Autonomy of application components. - heterogeneous communication infrastructure complexes the network and requires autonomic design. Two solutions: Put more operations on the server Or put more operations on the clients

Mobile Code Applications(4) Benefits: 5. The fault tolerance is much improved in Mobile code than the conventional client-server system. 6. A new protocol is needed to enhance the code migration among distributed systems. 7. Date management flexibility and protocol encapsulation.