1. Chapter 29 Structure of Computer Names Domain Names Within an Organization The DNS Client-Server Model The DNS Server Hierarchy Resolving a Name Optimization of DNS Performance

2. Introduction Computers are assigned symbolic names –Convenient for humans –Inconvenient for computers IP addresses require less computation IP addresses require less memory and time to transmit Symbolic names need to be translated into equivalent IP addresses

3. Introduction Domain names are used in the Web and for email The URL http://www.calpoly.edu contains the domain name calpoly.eduhttp://www.calpoly.edu Everytime you use a domain name, you use the internet’s domain name servers to translate it into an IP address

4. Structure of Computer Names Naming scheme used in the Internet is called DNS(Domain Name System) Each computer name consists of a sequence of alpha- numeric segments separated by periods –Ex. Mordred.cs.purdue.edu Domain names are arranged in a hierarchy –Most significant is on the right –Left most segment is the name of the computer –Other segments identify the group that owns the name

5. Each organization chooses structure of computer name Most significant segment is called the top level of the DNS

6. Structure of Computer Names Continued ICANN(Internet Corporation for Assigned Names and Numbers) is the organization that is responsible for domain names To obtain a domain name, each organization has to apply for a name under one of the top-level domains Once an organization has been assigned a domain, the suffix is reserved for that organization DNS allows organizations to use geographic registration –Ex., cnri.reston.va.us –Ex., ac.uk

7. Domain Names Within an Organization Once an organization owns a domain, the organization can decide whether to introduce hierarchical structure

8. DNS Server Client Model DNS is autonomous Organizations that have an Internet connection runs a domain server Software that translates computer names into addresses demonstrates the use of client-server interaction –Whenever an application program needs to translate a name, the application become a client of the naming system –Client places the name to be translated in a DNS request message and sends it to the DNS server –Server extracts the names, translates it, and returns the resulting address in a reply message

9. DNS Server Hierarchy DNS Servers are arranged in a hierarchy that matches the naming hierarchy Root server occupies the top of the hierarchy and is an authority for the top-level domains –Root server contains info about how to reach other servers Corporation can choose to place all its domain names in a single server, or it can choose to run several servers

11. Resolving a Name Software that performs the translation is called resolver software Each resolver is configured with the address of a local domain name server When a request specifies a name for which a server is an authority, the server answers directly, otherwise the server becomes a client of another server

12. Optimization of DNS Performance DNS that was just described is inefficient 2 primary optimizations –Replication each server is replicated –Caching each server maintains a cache of names

13. Summary DNS provides an automated mapping b/w computer names and equivalent IP addresses Names are allocated hierarchically, and segments in the name correspond to levels in the hierarchy The client sends a request to its local server, which either answers the request directly or contacts other servers. DNS servers use two performance optimization techniques: replication and caching

14. Chapter 30 Electronic Mail Paradigm Electronic Mailboxes and Addresses Electronic Mail Message Format Multipurpose Internet Mail Extensions Mail Transfer SMTP and POP Protocols

15. Chapter 30 Mail Exploders,lists, and forwarders Mailbox Gateways Mailbox Access

16. The Electronic Mail Paradigm Email was designed from the traditional office memo –It was built to allow a person to communicate with other people Electronic mail systems have evolved from the original design and are now permitted to more complex interactions

17. Electronic Mailboxes and Addresses Before email can be sent, the person must be assigned a mailbox Email address contains 2 parts –Mailbox@computer –Allows each computer system to assign mailbox identifiers independently –Sender’s computer uses the second part to select a destination –Recipient’s computer uses the first part to select a mailbox

18. Electronic Mail Message Format Message consists of ASCII text that is separated into 2 parts by a blank line –The first part is called the header –The second part is the body

20. Multipurpose Internet Mail Extensions Email was designed to handle only text MIME standard allows sender to encode non-text data for transmission –MIME software decodes the attached data automatically For transparent encoding and decoding, MIME adds two lines to an email header MIME-Version: 1.0 Content-Type: multipart/mixed;

21. Mail Transfer A user interacts with an email interface –Composing or reading Email system contains a mail transfer program –Handles the detail of sending message

22. Simple Mail Transfer Protocol When a transfer mail program contacts a server on a remote machine, it forms a TCP connection over which it communicates Once the connection is in place, SMTP allows the sender to identify itself, specify a recipient, and transfer an email

23. Optimizing For Multiple Recipients Most mail transfer programs are optimized to handle all recipients on a given remote computer at the same time Optimizing reduces –Network bandwidth –Delay required for all users to receive a copy of the message

24. Mail Exploders,Lists,Forwarders Mail exploder/forwarder-program that forwards copies of a message When an email message arrives, the mail exploder examines the destination address

25. Mail Gateways Mail gateway is a computer dedicated to forwarding email

26. Mailbox Access TCP/IP protocol called POP(Post Office Protocol) allows remote access to an electronic mailbox –The server with the mailbox uses the POP protocol –User runs email software that becomes a client of the POP server to access the mailbox

27. Mailbox Access

28. Email Client Example Email client connects to the SMTP server at openmail2.calpoly.edu Email client gets the address of the sender and recipient as well as the body of the message from the SMTP server SMTP server at recipient connects with the SMTP server at the sender

29. Summary Electronic mail uses the office memo paradigm Email address is separated into 2 parts MIME standard allows a sender to encode non- text data SMTP protocol is used to transfer a message Optimizing for multiple recipients reduces bandwidth and delay Mail exploders make it possible for a large group to communicate via email

30. Summary Mail gateways forward email POP protocol used to access a remote mailbox