1 TCP: Reliable Transport Service

2 Transmission Control Protocol (TCP) Major transport protocol used in Internet Heavily used Completely reliable transfer

3 TCP Features Connection-oriented service Point-to-point Full-duplex communication Stream interface Stream divided into segments for transmission Each segment encapsulated in IP datagram Uses protocol ports to identify applications

4 TCP Feature Summary TCP provides a completely reliable (no data duplication or loss), connection-oriented, full- duplex stream transport service that allows two application programs to form a connection, send data in either direction, and then terminate the connection.

5 End-to-End Protocol

6 Apparent Contradiction IP offers best-effort (unreliable) delivery TCP uses IP TCP provides completely reliable transfer How is this possible?

7 Achieving Reliability Reliable connection startup Reliable data transmission Graceful connection shutdown

8 Reliable Data Transmission Positive acknowledgment –Receiver returns short message when data arrives –Called acknowledgment Retransmission –Sender starts timer whenever message is transmitted –If timer expires before acknowledgment arrives, sender retransmits message

9 Illustration Of Retransmission

10 How Long Should TCP Wait Before Retransmitting? Time for acknowledgment to arrive depends on –Distance to destination –Current traffic conditions Multiple connections can be open simultaneously Traffic conditions change rapidly

11 Important Point The delay required for data to reach a destination and an acknowledgment to return depends on traffic in the internet as well as the distance to the destination. Because it allows multiple application programs to communicate with multiple destinations concurrently, TCP must handle a variety of delays that can change rapidly.

12 Solving The Retransmission Problem Keep estimate of round trip time on each connection Use current estimate to set retransmission timer Known as adaptive retransmission Key to TCP’s success

13 Illustration Of Adaptive Retransmission Timeout depends on current round-trip estimate

14 TCP Flow Control Receiver –Advertises available buffer space –Called window Sender –Can send up to entire window before ack arrives

15 Window Advertisement Each acknowledgment carries new window information –Called window advertisement –Can be zero (called closed window) Interpretation: have received up through X, and can take Y more octets

16 Illustration Of Window Advertisement A TCP acknowledgement specifies the sequence number of the next octet that the receiver expects to receive

17 Startup And Shutdown Connection startup –Must be reliable Connection shutdown –Must be graceful Difficult

18 Why Startup And Shutdown Are Difficult Segments can be –Lost –Duplicated –Delayed –Delivered out of order –Either side can crash –Either side can reboot Need to avoid duplicate shutdown message from affecting later connection

19 TCP’s Solution For Startup/Shutdown Uses three-message exchange Known as 3-way handshake Necessary and sufficient for –Unambiguous, reliable startup –Unambiguous, graceful shutdown SYN used for startup FIN used for shutdown

20 Connection Establishment – Three- way Handshake Active participant (client) Passive participant (server) SYN, SequenceNum = x ACK, Acknowledgment =y+1 Acknowledgment =x+1 SYN+ACK, SequenceNum=y,

21 Closing a TCP Connection

22 TCP Segment Format All TCP segments have same format –Data –Acknowledgment –SYN (startup) –FIN (shutdown) Segment divided into two parts –Header –Payload area (zero or more bytes of data)

23 TCP Segment Format (continued) Header contains –Protocol port numbers to identify Sending application Receiving application –Code bits to specify items such as SYN FIN ACK –Fields for window advertisement, acknowledgment, etc.

24 Illustration Of TCP Segment Sequence number specifies where in stream outgoing data belongs Acknowledgment number refers to incoming data Few segments contain options

25 Checksum = 1s complement sum of the 1s complement of 4-bit quantities. 1s complement of 1111, 0000, 1100, 0101, 1000 is 0000, 1111, 0011, 1010, s complement sum: = = (carry) = = = (carry) = 0101 Ans: Checksum = 0101 Checksum - One Example

26 Flow Control And Congestion Receiver advertises window that specifies how many additional bytes it can accept Window size of zero means sender must not send normal data (ACKs and urgent data allowed) Sender chooses effective window smaller than receiver’s advertised window if congestion detected –Allowed_window = min(receiver_advertisement, congestion_window) –Congestion window is reduced by half for every loss – multiplicative decrease

27 TCP Congestion Control When CongWin is below Threshold, sender in slow- start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS (Maximum Segment Size). When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. Additive Increase Multiplicative Decrease (AIMD)

28 TCP Congestion Control Q: When should the exponential increase switch to linear? A: When CongestionWindow gets to 1/2 of its value before timeout. Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of CongWin just before loss event From Tanenbaum’s book, fourth edition, pp. 550