1. Computer Networks Chapter 2: Applications and Layered Architectures

2. Introduction: ● Variety of services provided and possibly many more in the future. ● 70s proprietary networking protocols. ● Divide-and-conquer...subdivide the task => Instead of complex monolithic problem, smaller more manageable ones: Layering ● Changing one function does not make everything else obsolete.

3. HTTP example ● HyperText Transfer Protocol (HTTP) ● Client queries DNS ● Client sets up a TCP connection ● Document is requested to server ● Document is delivered HTTP server HTTP client Request Res pon se

4. HTTP example: HTTP server HTTP client TCP GET 80, # #, 80 STATUS Port 80 Ephemeral Port #

5. HTTP running on TCP 1. Users selects document 2. Network software of client locates the server host and establishes two-way connection. 3. HTTP client sends message requesting GET/infocom/index.html HTTP/1.0 document. 4. HTTP daemon listening on TCP port 80 interprets message. 5. HTTP daemon sends a result code and HTTP/1.1 200 OK SERVER: Apache/1.2.5 a description of the information. Content-Length:414 Content- Type: text/html 6. HTTP daemon reads the file and sends the requested file through the TCP port. 7. HTTP daemon disconnects the connection 8. Text is displayed by client browser, which interprets the HTML format.

6. DNS Query 1. Application requests name to address translation. 2. Resolver composes query message. Header: OPCODE=SQUERY Question: QNAME=tesla.comm.toronto.edu., QCLASS=IN, QTYPE=A 3. Resolver sends UDP datagram encapsu- lating the query message. 4. DNS server looks up address and prepa- Header: OPCODE=SQUERY, res response. RESPONSE, AA Question: QNAME=tesla.comm.toronto.edu., QCLASS=IN, QTYPE=A Answer: tesla.comm.toronto.edu. 86400 IN A 128.100.11.56 5. DNS sends UDP datagram encapsulating the response message.

7. OSI Reference model ● Open Systems Interconnection put in place by the International Organization for StandardizationISO ● It is a reference model to facilitate creation of computer communication standards. ● Partitions overall communication into functions carried by layers. ● Each layer conducts peer-to-peer communication ● The packet exchanged between the equivalent layer is called a PDU which contains a header and user info SDU.

8. OSI reference model ● To reach it's peer layer, each layer communicates with next layer through SAP uniquely identified. ● Except for last layer none of intermediate layer makes use of SDUs. ● Services provided by underlying layer is either connection-oriented/connectionless, confirmed/ unconfirmed. ● Possibly segmentation (numbering) re- assembly and blocking/unblocking.

9. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Electrical and/or Optical Signals Application A Application B Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Communication Network Seven-Layer Model

10. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application A Application B data ah ph sh th nh dh bits dt Figure 2.9

11. TCP/IP: Overview ● To the contrary of OSI model, the TCP/IP suite of protocoles has emerged from real networks ● 4 layers: Application, TCP/UDP, IP, Network interface. ● Reliable and un-acknowledged services are both possible: TCP, UDP ● TCP: end-to-end packet transmission ● IP: transfer of packet through multiple networks ● Network Interface: Allows us to run over multiple networks (X.25, ATM,...)

12. Application Transport Internet Network Interface Application Transport Internet Network Interface Internet Network Interface Network 1 Network 2 Machine A Machine B Router/Gateway TCP/IP over networks

13. HTT P SMTP RTP TCP UDP IP Network Interface 1 Network Interface 3 Network Interface 2 DNS Hour-shaped glass

14. Net Interface IP TCP HTTP Net Interface IP Net Interface IP TCP HTTP EthernetPPP Router router (1,1) s (1,2) w (2,1) (1,3) r (2,2) PPP Ethernet (a) (b) Server PC TCP/IP: Example

15. Berkeley API for Sockets programming ● You need to read about the Sockets programming part using Berkeley API.

16. Networking utilities ● Telnet ● FTP ● Ping ● Traceroute ● Netstat ● TCPDump