1. EEC-484/584 Computer Networks Lecture 12 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 2007EEC-484/584: Computer NetworksWenbing Zhao Outline Reminder: –Nov. 14 (Wed): CSU IS&T data center tour –Nov. 12 (Mon): no class, in honor of the Veterans Day Quiz#3 results Introduction to transport layer Multiplexing/demultiplexing Reliable data transfer mechanisms Sliding window protocols (part I)

3. 3 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao EEC 484 Quiz#3 Results High: 98, low: 75, average: 89.3 Q1: 46.7/50, Q2: 18.5/20, Q3: 9.2/10, Q4: 15/20

4. 4 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao EEC 584 Quiz#3 Results High: 100, low: 75, average: 84.4 Q1: 40.7/50, Q2: 19/20, Q3: 8.5/10, Q4: 16.2/20

5. 5 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Transport Layer Our goals: Understand principles behind transport layer services: –multiplexing/demult iplexing –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

6. 6 Fall Semester 2007EEC-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

7. 7 Fall Semester 2007EEC-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

8. 8 Fall Semester 2007EEC-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

9. 9 Fall Semester 2007EEC-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:

10. 10 Fall Semester 2007EEC-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

11. 11 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Reliable Data Transfer characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)

12. 12 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Reliable Data Transfer Basic reliable data transfer mechanisms –Acknowledgement –Retransmission –Sequence numbers

13. 13 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Sliding Window Protocols Full-duplex: Use same connection for data in both directions (A  B and B  A) Interleave data and ack packets –B piggybacks its ack for A ’ s packet onto B ’ s next packet –Savings of header in separate ack packet If B sends data infrequently, use timeout to determine when B should send ack in separate ack packet

14. 14 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Sliding Window Protocols Each packet contains sequence number in ranges 0..2 n -1 (for n-bit sequence numbers) Sending window – list of consecutive sequence numbers of packets that sender is permitted to send 0 1 2 3 4 5 6 7 1 st outstanding packet Last packet sent 0 1 2 3 4 5 6 7 3 ack 0 1 2 3 4 5 6 7 A new packet sent (if send window allows) Sent window enlarges when more packet is sent 0 1 2 3 4 5 6 7 Sent window shrinks when the ack corresponding to the 1 st outstanding packet Is received

15. 15 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Sliding Window Protocols When new packet arrives from application layer, it is given next highest sequence number, and upper edge of window is incremented When ack arrives from receiver, lower edge of window is incremented Within sending window, packets sent but not acked –Sender must keep those packets for possible retransmission –If max window size = w, need w buffers

16. 16 Fall Semester 2007EEC-484/584: Computer NetworksWenbing Zhao Sliding Window Protocols Receiving window – list of consecutive sequence numbers of packets that receiver is permitted to accept When packet with (seq num = lower edge of window) arrives –Packet is passed to higher layer –Ack is generated –Window slid down by 1 (remains same size as was initially) 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 3