1. EEC-484/584 Computer Networks Lecture 6 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book)

2. 2 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Outline Quiz#1 result Introduction to transport layer Multiplexing/demultiplexing Reliable data transfer mechanisms Sliding window protocols (part I)

3. 3 EEC584 Quiz#1 Result Average: 80.6, high: 100 (6 students!), low: 47.5 Q1-38.7, Q2-13.5, Q3-19.3, Q4-9 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao

4. 4 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Transport Layer Our goals: Understand principles behind transport layer services: –multiplexing/ demultiplexing –reliable data transfer –flow control –congestion control Learn about transport layer protocols in the Internet: –UDP: connectionless transport –TCP: connection- oriented transport –TCP congestion control

5. 5 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Transport vs. Data Link Layer Similarities: deal with error control, sequencing, flow control Difference: operating environments Environment of the data link layer Environment of the transport layer

6. 6 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Transport vs. Network Layer Network layer: logical communication between hosts Transport layer: logical communication between processes –Relies on, enhances, network layer services

7. 7 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Internet Transport-Layer Protocols Reliable, in-order delivery (TCP) –congestion control –flow control –connection setup Unreliable, unordered delivery: UDP –no-frills extension of “best- effort” IP Services not available: –delay guarantees –bandwidth guarantees application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

8. 8 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Multiplexing/Demultiplexing application transport network link physical P1 application transport network link physical application transport network link physical P2 P3 P4 P1 host 1 host 2 host 3 = process= socket delivering received segments to correct socket Demultiplexing at rcv host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Multiplexing at send host:

9. 9 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao How Demultiplexing Works Host receives IP datagrams –Each datagram has source IP address, destination IP address –Each datagram carries 1 transport-layer segment –Each segment has source, destination port number Host uses IP addresses & port numbers to direct segment to appropriate socket source port #dest port # 32 bits application data (message) other header fields TCP/UDP segment format

10. 10 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Reliable Data Transfer characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)

11. 11 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Reliable Data Transfer Why ack is needed Why retransmission of data seg is needed Why seq number on data seg is needed Why seq number on ack is needed When do we transmit an ack? Do we retransmit ack? Basic RDT mechanisms: Acknowledgement, Retransmission, Sequence numbers