The Transmission Control Protocol (TCP)

1. Application Services (Telnet, FTP, e-mail, WWW) Reliable Stream Transport (TCP) Unreliable Transport Service (UDP) Connectionless Packet Delivery Service (IP) The Transmission Control Protocol (TCP)

2. The Transmission Control Protocol (TCP) • TCP is a protocol that specifies: • How to distinguish among multiple destinations on a given machine • How to initiate and terminate a stream transfer • Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer • Procedures the computers use to ensure that the data arrives correctly

3. Distinguishing AmongMultiple Destinations • Like UDP, TCP uses protocol ports to identify the ultimate destination within a machine • Unlike UDP, TCP defines a connection as the fundamental abstraction for data transfer • Connection: • Virtual circuit • Identified by a pair of endpoints

4. Initiating Stream Transfers • Virtual Circuit - simulate a circuit switched network over a packet-switched network • Sender and receiver interact before transfer begins to set up the connection • Once the connection is established a stream transfer can begin • During the transfer, protocol software on the two machines communicate to ensure that data is delivered correctly • Upon completion of the transfer the connection is closed

5. Initiating Stream Transfers (cont) • An endpoint for a TCP connection is defined by a (host, port) pair • Host = the IP address of for a host • Port = a TCP port on that host • A TCP connection is defined by a pair of endpoints: • Port 1037 on www.whitehouse.gov and port 76 on viper.cs.virginia.edu: • (198.137.240.91, 1037) and (128.143.137.17, 76)

6. Initiating Stream Transfers (cont) • Multiple connections to same host: • Port 1037 on www.whitehouse.gov and port 76 on viper.cs.virginia.edu: • (198.137.240.91, 1037) and (128.143.137.17, 76) • Port 355 on falcon.cs.jmu.edu and port 801 on viper.cs.virginia.edu: • (134.126.10.30, 355) and (128.143.137.17, 801)

7. Initiating Stream Transfers (cont) • Multiple connections to same port: • Port 1037 on www.whitehouse.gov and port 444 on viper.cs.virginia.edu: • (198.137.240.91, 1037) and (128.143.137.17, 444) • Port 355 on falcon.cs.jmu.edu and port 444 on viper.cs.virginia.edu: • (134.126.10.30, 355) and (128.143.137.17, 444) • No ambiguity - connection identified by both endpoints

8. Initiating Stream Transfers (cont) • Both endpoints must agree to participate: • Passive open - one application program contacts its O.S. and indicates that it will accept an incoming connection • Active open - application program at other end contacts its O.S. and requests a connection • Both TCP software modules cooperate to establish a connection • Application programs can transmit data (TCP provides reliability)

9. TCP Data Stream Format • Data from application programs is a sequence of octets (with no type associated) • TCP divides data into segments for transmission • Usually, each segment travels across the internet in a single IP datagram

10. Review: Unreliable PacketDelivery Service • Benefits: • Fast • Cost-effective • Drawbacks: • Packet loss, corruption, delay, duplication, out-of-order delivery • Sender might transmit faster than receiver can receive • Dictates a set packet size

11. The Need for a Reliable Stream Delivery Service • Application-level programmers don’t want: • To have to provide reliability in each application • To be constrained by fixed-size packets • Application-level programmers do want: • Reliability • Unstructured, stream-oriented service • Full duplex virtual circuit connection • Buffered transfer

12. Sender Network Receiver Send packet 1 Receive packet 1 Send ACK 1 Receive ACK 1 Send packet 2 Receive packet 2 Send ACK 2 Receive ACK 2 Providing Reliability with Acknowledgments and Retransmissions

13. Packet Loss Sender Network Receiver Send packet 1 (Timeout) Send packet 1 Receive packet 1 Send ACK 1 Receive ACK 1

14. ACK Loss Sender Network Receiver Send packet 1 Receive packet 1 Send ACK 1 (Timeout) Send packet 1 Receive packet 1 (Discard) Send ACK 1 Receive ACK 1

15. ACK Delayed Sender Network Receiver Send packet 1 Receive packet 1 Send ACK 1 (Timeout) Send packet 1 Receive packet 1 Receive ACK 1 (Discard) Send ACK 1 Receive ACK 1 (Discard)

16. A Problem A simple positive acknowledgment protocol wastes a substantial amount of network bandwidth because it must delay sending a new packet until it receives an acknowledgment for the previous packet.

17. 1 2 3 ACK1 ACK2 ACK3 A Solution Sender Network Receiver

18. 10 9 1 2 3 4 5 6 7 8 8 10 9 1 2 3 4 5 6 7 Sliding Windows window All packets inside the window can be transmitted When an acknowledgment for packet 1 is received the window slides to the right and allows the transmission of packet 9:

19. window 9 1 2 3 4 5 6 7 8 10 1 2 3 4 5 6 7 8 10 9 Sliding Windows (cont) • Receiver must keep a sliding window as well: • After packet 1 is received and acknowledged:

20. 1 2 3 4 5 6 7 8 9 10 Sliding Windows (cont) • Conceptually partitions the packets into three classes: • Transmitted, received, and acknowledged packets (outside and left of sliding window) • Packets being transmitted (inside the window) • Packets waiting to be transmitted (outside and right of sliding window)

21. Sliding Windows (cont) • Only unacknowledged packets are retransmitted • Keep separate timer for each packet • Performance: • Depends on the window size and the speed of the underlying network • Note: sliding window protocol with window size = 1 is the same as the simple positive acknowledgment protocol we saw earlier

22. Sliding Windows: Performance • Tuning the sliding window protocol • Increase window size so that sender is transmitting packets as fast as the network can carry them • Eliminates network idle time • Increases throughput (as compared to the simple positive acknowledgment protocol)

23. Ideal Sliding Window Performance Sender Network Receiver

24. TCP - Efficient Transmission and Flow Control • Uses a specialized sliding window mechanism • Multiple segments can be sent before an acknowledgment arrives • Efficient transmission • Allows receiver to restrict transmission until it has sufficient buffer space to accommodate more data • Flow control

25. 1 2 3 4 5 6 7 8 9 10 TCP Sliding Windows • Operates at the octet level • Octets of the data stream are numbered sequentially Octets 1&2 have been sent and acknowledged Octets 7&8 can be sent Octets 3-6 sent but not acknowledged

26. TCP Sliding Windows • Four per (full duplex) connection: • A sends to B • A receives from B • B sends to A • B receives from A

27. TCP Sliding Windows - Flow Control • Each window can vary in size over time • Each ACK contains a window advisement • Specifies how many additional octets of data the receiver is willing to accept • Sender increases or decreases sending window sized based on the receiver’s advise • Provides end-to-endflow control

28. 1 2 3 4 5 6 7 8 9 10 14 10 10 14 14 11 12 13 TCP Sliding Windows - Flow Control Example • Sender transmits 3 octets • Receives an ACK 3 with window advisement = 3 • Sender transmits 1 octet 1 2 3 4 5 6 7 8 9 11 12 13 1 2 3 4 5 6 7 8 9 11 12 13

29. 1 2 3 4 5 6 7 8 9 11 12 13 14 14 10 10 14 10 1 2 3 4 5 6 7 8 9 11 12 13 TCP Sliding Windows - Flow Control Example (cont) • Receives an ACK 4 with window advisement = 7 • Sender transmits 4 octets • Receives an ACK 8 with window advisement = 4 1 2 3 4 5 6 7 8 9 11 12 13

30. Summary • TCP is a transport protocol that specifies: • How to distinguish among multiple destinations on a given machine • How to initiate and terminate a stream transfer • Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer • Procedures the computers use to ensure that the data arrives correctly