1. Transport Layer Natawut Nupairoj, Ph.D. Department of Computer Engineering Chulalongkorn University

2. Outline Overview. Transport Layer Functions. UDP and TCP protocols.

3. Overview Core of OSI and TCP/IP protocols. Interface between lower-level and upper-level protocols (even application layers). Provide network-independent interface Application-to-application delivery.

4. Transport Layer

5. Network-Independent Interface

6. Example: TCP Layer

7. Transport Layer Functions Application-to-application delivery. Addressing For not just end-to-end, but for application-to- application. Service access point. Some transports: reliable layer over unreliable network layer.

8. Service Access Point: Port

9. Well-Known Port Addresses Port 80 – HTTP (web server). Port 25 – SMTP (mail server). Port 23 – Telnet. Port 22 – SSH. Port 110 – POP3.

10. Reliable Delivery Error control Corrupted packets. Sequence control Delay packets. Out-of-order delivery. Transmission speed mismatches. Loss control Lost packets. Duplicate control Duplicate packets.

11. Providing Reliability Assume unreliable network layer. Use flow control – variable-size sliding window protocol Acknowledgement. Window size is dynamic and adjustable in real- time.

12. UDP: User Datagram Protocol Connectionless transport protocol. Simple and efficient: no connection setup. no flow control. Unreliable. Discard when error occurs.

13. UDP Datagram Format

14. TCP: Transmission Control Protocol Features Connection-oriented. Reliable. Full duplex. Assume unreliable network protocol. Flow control and congestion control. Three-way handshake for connection establishment. Application sends/receives data as “stream”.

15. TCP Segment Format

16. TCP Flow Control Variable-size sliding window protocol: Sequence number: the first byte being sent. Acknowledgement: the next byte expected. Window size: control the transmission speed. 0 = receive all, but want to more data. Out of order segments ? Depending on the receiver: Kept or discard.

17. TCP Sliding Window Protocol

19. TCP Congestion Control Congestion - 2 causes: fast network, low-capacity receiver. slow network, high-capacity receiver. Using window size. Sender maintains 2 windows: receiving window. congestion window. The effective window is the minimum of the two window.

20. TCP Congestion Window After connection is established: congestion window size = one maximum segments (negotiate during connection). 64K threshold. Sender sends one maximum segment. If the ack comes back before timeout, double the congestion window size.

21. TCP Congestion Window Keep sending until: reach the threshold: increase the window size by one. reach the receiving window size: no more increasing. timeout: set threshold = half of the current congestion window size and start sending from one maximum segment again.