1. TCP/IP Networking Review Covered Subjects:  Packet Switched Network Structure  Issues of PSNs  Ports & IP Numbers  Delivery Services  Domain Name Servers  Client Server Programming

2. Packet Switched Network  Packet switched network ◦ means that data is carried in discrete certain- sized packets ◦ Analogy: Postal service IP Network A B D C

3. Issues 1. Addressing ◦ Unique address for every host (IPv4 or IPv6 address) 2. Packet size ◦ Max size = 2 16 -1 = 65535 bytes 3. Packet forwarding ◦ How to deliver the packet to the destination? ◦ Simply give the packet to the default router

4. Analogy  Is this how you send your letters to your friends?  What is missing on the address parts? ◦ The names of the sender & receiver. ◦ It is the people (applications) that communicate. From: Bob To: Alice I will write home everyday. Love. Bob I will write home everyday. Love. Bob

5. Ports & IPs IP Network AB Bob Alice From: Bob @ A To: Alice @ B Port # IP Address

6. What is next?  How do you want the packet delivered?  Do you want the ACK of delivery? ◦ reliable delivery or not Delivery services: 1. Best effort (unreliable)  UDP 2. Reliable  TCP UDP: connectionless, unreliable, unordered, datagram services TCP: connection-oriented, reliable, ordered, byte-stream

7. IP stack & interaction Socket Layer UDP (32-bit) (128-bit) IPv4 IPv6 TCP ports sockets IP Network AB Browser Web Server designated port

8. Domain Name Server  How do you identify the server? www.anadolu.edu.tr DNS resolver IP address  What about port #? ◦ designated  port 80 – web server port 21 – FTP servers port 23 for Telnet ….

9. Next question  Who initiates a transaction? ◦ Browser -- called the client  Client is the active entity that initiates the transaction/contact  Server is the passive entity that waits for requests from clients & sends replies in response.  This is why we call this the «client & server» programming.