1 Applications Computer Networks

2 Where are we?

3 Important Note Although the ISO OSI Reference Model defines a session and presentation layer, they are often integrated into some other layer in practice. In many cases they are simply not used.

4 Separation of Duties Network Transfer bits/bytes Operates at the application ’ s request Applications What data to transmit When to transmit data Where to transmit data to Meaning of bits/bytes

5 Client/Server

6 Characteristics Client Actively initiates contacts with server Uses different source port for each connection Server Passively awaits connections from clients Uses the same source port for all clients

7 Use of Protocol Ports Each service given unique port number, P Server Inform Operating System it is using port P Waits for requests to arrive Client Forms request Sends request to port P on server computer

8 Protocol Ports Key Idea Each application is assigned a unique port identifier. A server must specify the identifier when it begins execution (application startup). A client must specify the identifier when it requests the network software to contact the server. Protocol software on the server computer uses the identifier to direct incoming requests to the correct server application.

9 Recall: Application Multiplexing

10 Example: IP address and TCP port pairs

11 Interacting with Protocol Software Client or server uses transport protocols Protocol software resides in OS Applications outside the OS Mechanism to bridge the two Application Programming Interface (API) The book: TCP/IP Illustrated, Volume 2 - Stevens

12 Application Programming Interface Part of the operating system Permits applications to use protocols Defines Operations allowed Arguments for each operation

13 Example API: Sockets Originally designed for BSD UNIX To use with TCP/IP protocols Now industry standard available on many operating systems e.g. winsock

14 Sample Socket Procedure

15 Example Application: DNS Internet communication requires IP addresses Humans prefer to use computer names Automated system available to translate names to addresses Known as Domain Name System (DNS) Base specs in RFC 1034 and RFC 1035 The book: DNS and Bind - Albitz and Liu

16 DNS Functionality Given Name of a computer Return Computer ’ s internet (IP) address Method Distributed lookup Client contact server(s) as necessary

17 Domain Name Syntax Alphanumeric segments separated by dots Examples: aharp.is-net.depaul.edu Most significant part on the right

18 Obtaining a Domain Name Organization Chooses a desired name Must be unique Registers with central authority Placed under one top-level domain Names subject to international law Trademarks Copyright

19 Top-Level Domains.comcommercial organization.edueducational institution.govgovernment organization.milmilitary group.netmajor network provider.orgorganization other than above.arpatemporary ARPA domain (still used).intinternational organization country codeA country (e.g..uk or.tw)

20 Example Name Structure First level is.com Second level is company name Third level is division within a company Fourth level either company subdivision individual computer

21 DNS Illustrated

22 Key Concept The number of segments in a domain name corresponds to the naming hierarchy. There is no universal standard for this hierarchy; each organization can choose its own naming convention. Furthermore, names within an organization do not need to follow a uniform pattern; individual groups within the organization can choose a hierarchical structure that is appropriate for that group.

23 DNS Client/Server Interaction Client known as resolver Multiple DNS servers used Arranged in a hierarchy Each server corresponds to contiguous part of naming hierarchy

24 Inter-Server Links All domain name servers are linked together to form a unified system. Each server knows how to reach a root server and how to reach servers that are authorities for names further down the hierarchy.

25 DNS Hierarchy Illustrated

26 DNS Record Types Stored with each name Resolver must specify the type in lookup request Type A (address): IP address for name Type MX (mail exchanger): IP address of mail server for the name Type CNAME (Computer Name): alias to another name

27 Example Application: Internet address text string which specifies destination mailbox e.g. Mail message format header identifies sender, receiver, contents format is keyword: information blank line body

28 MIME Original restricted to ASCII text Users desire to send image files audio clips compiled (binary) programs Solution Multi-purpose Internet Mail Exchange (MIME) Defined in RFC 1521 and RFC 1522

29 MIME [continued] Allows transmission of binary data multimedia files (video/audio clips) multiple types in a single message mixed formats Backward compatible

30 MIME Encoding Sender inserts additional header lines encodes binary data in (printable) ASCII Sent like standard message Receiver interprets header lines Extracts and decodes parts Separate standards for content and encoding

31 MIME Example Header lines added MIME-Version: 1.0 Content-Type: Multipart/Mixed; Boundary=xxxsep Specifies Using MIME version 1.0 Line xxxsep appears before each message part

32 Simple Mail Transfer Protocol (SMTP) Runs on top of TCP Used between Mail transfer program on sender ’ s computer Mail server on recipient ’ s computer Specifies how Client interacts with server Recipients specified Message is transferred Defined in RFC 821

33 Mail Transfer Illustrated

34 Computers Without Mail Servers (SMTP) Typically small, personal computer not continuously connected to the network To receive , user must establish mailbox on large computer (server) access mailbox as necessary Post Office Protocol (POP) often used defined in RFC 1939

35 POP Illustrated

36 Example Application: Remote Login w/ TELNET Provides interactive access to computer from a remote site Text-oriented interface User invokes client specifies remote computer Client forms TCP connection to server passes keystrokes over connection displays output on screen Defined in RFC 854

37 Example Application: File Transfer Protocol (FTP) Complete file copy Uses TCP Supports binary or text file transfers Large set of commands Until 1995, it was the major source of packets on the Internet Defined in RFC 959

38 FTP Illustrated

39 Example Application: World Wide Web (WWW) Web pages can contain text, images, imbedded objects and links standard authoring format is HTML links use URL tags transferred using HTTP See for all the details

40 HyperText Markup Language Document is free-format Embedded tags give display format Tags (often appear in pairs) Paragraph and Line break Headings, DePaul

41 Uniform Resource Locator (URL) Symbolic representation Embedded in HTML document Browser hides text of link from user associates link with item on page makes items selectable

42 URLs Illustrated

43 HyperText Transfer Protocol (HTTP) Web server makes web pages available Server uses port 80 by default Web client (browser) requests pages Creates a TCP connection to server HTTP sits on top of TCP HTTP v1.1 defined in RFC 2068 major enhancement over v1.0: single TCP connection for multiple HTTP requests

44 Browser Operation

45 File Sharing and Peer to Peer Becoming a killer app e.g. Napster Besides the copyright issues Dramatic change in traffic patterns Lots of traffic engineering trying to be done You haven't seen anything yet

46 How to hide your app Tunneling Port hopping Encryption Chunking Conclusion: Network traffic will conform to the rules of the network administrator, but the data will not change!