C H A P T E R 15 TRA N SM I SSI O N C O N T RO L P RO T O C O L (T C P ) SUM M A RY

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1 C H A P T E R 15 TRA N SM I SSI O N C O N T RO L P RO T O C O L (T C P ) SUM M A RY Transmission C ontrol Protocol (T C P) is one of the transport layer protocols in the T C P/IP protocol suite. T C P provides process-to-process, full-duplex, and connection-oriented service. T he unit of data transfer between two devices using T C P software is called a segment; it has 20 to 60 bytes of header, followed by data from the application program. A T C P connection consists of three phases: connection establishment, data transfer, and connection termination. C onnection establishment requires three-way handshaking; connection termination requires three- or four-way handshaking. T C P software is normally implemented as a finite state machine (FSM ). T C P uses flow control, implemented as a sliding window mechanism, to avoid overwhelming a receiver with data. T he T C P window size is determined by the receiver-advertised window size (rwnd) or the congestion window size (cwnd), whichever is smaller. T he window can be opened or closed by the receiver, but should not be shrunk. T he bytes of data being transferred in each connection are numbered by T C P. T he numbering starts with a randomly generated number. T CP uses error control to provide a reliable service. Error control is handled by checksums, acknowledgment, and time-outs. C orrupted and lost segments are eventually retransmitted and duplicate segments are discarded. Data may arrive out of order and temporarily stored by the receiving T CP, but T CP guarantees that no out-of-order segment is delivered to the process. In modern implementations, a retransmission occurs if the retransmission timer expires or three duplicate A CK segments have arrived. T C P uses congestion control to avoid and detect congestion in the network. T he slow start (exponential increase), congestion avoidance (additive increase), and congestion detection (multiplicative decrease) strategies are used for congestion control. I n the slow start algorithm the size of the congestion window increases exponentially until it reaches a threshold. I n the congestion avoidance algorithm the size of the congestion window increases additively until congestion is detected. D ifferent T C P implementations react differently to congestion detection. If detection is by time-out, a new slow start phase starts. If detection is by three duplicate A C K s, a new congestion avoidance phase starts. T C P uses four timers (retransmission, persistence, keepalive, and time-wait) in its operation. In T C P, there can be only be one RT T measurement in progress at any time. T C P does not consider the RT T of a retransmitted segment in its calculation of an RT T. T C P uses options to provide more services. T he maximum segment size option is used in connection setup to define the largest allowable T C P segment. T he value of M SS is determined during connection establishment and does not change during the connection. T he window scale factor is a multiplier that increases the window size. T he timestamp option shows how much time it takes for data to travel between sender and receiver. One application of the timestamp option is in the calculation of round-trip time (RT T ). A nother application is for PAW S. R ecent implementations of

2 498 PA R T 3 T RA N SP O R T L A Y E R T C P use two more options, SA C K -permitted option and SA C K option. T hese two options allow the selective acknowledgment of the received segments by the receiver PR A C T IC E SE T E xercises 1. C ompare the T C P header and the U D P header. L ist the fields in the T C P header that are not part of the U D P header. G ive the reason for each missing field. 2. A n IP datagram is carrying a T C P segment destined for address T he destination port address is corrupted and it arrives at destination H ow does the receiving T C P react to this error? 3. One IC M P message, discussed in C hapter 9, reports a destination port unreachable error. H ow can T C P detect the error in the destination port? 4. U D P is a message-oriented protocol. T C P is a byte-oriented protocol. If an application needs to protect the boundaries of its message, which protocol should be used, U D P or T C P? 5. W hat is the maximum size of the T C P header? W hat is the minimum size of the T C P header? 6. If the value of H L EN is 0111, how many bytes of option are included in the segment? 7. Show the entries for the header of a T C P segment that carries a message from an FT P client to an FT P server. Fill the checksum field with 0s. C hoose an appropriate ephemeral port number and the correct well-known port number. T he length of data is 40 bytes. 8. W hat can you say about the T C P segment in which the value of the control field is one of the following: a b c d e f T he following is a dump of a T C P header in hexadecimal format. ( FF ) 16 a. W hat is the source port number? b. W hat is the destination port number? c. W hat the sequence number? d. W hat is the acknowledgment number? e. W hat is the length of the header? f. W hat is the type of the segment? g. W hat is the window size?

3 C H A P T E R 15 TRA N SM I SSI O N C O N T RO L P RO T O C O L (T C P ) T he control field in a T C P segment is 6 bits. We can have 64 different combinations of bits. L ist some combinations that are valid. 11. To make the initial sequence number a random number, most systems start the counter at 1 during bootstrap and increment the counter by 64,000 every half second. H ow long does it take for the counter to wrap around? 12. In a T C P connection, the initial sequence number at the client site is 2,171. T he client opens the connection, sends only one segment carrying 1,000 bytes of data, and closes the connection. W hat is the value of the sequence number in each of the following segments sent by the client? a. T he SY N segment? b. T he data segment? c. T he FIN segment? 13. In a connection, the value of cwnd is 3000 and the value of rwnd is T he host has sent 2,000 bytes, which have not been acknowledged. H ow many more bytes can be sent? 14. T C P opens a connection using an initial sequence number (ISN ) of 14,534. T he other party opens the connection with an ISN of 21,732. a. Show the three T C P segments during the connection establishment. b. Show the contents of the segments during the data transmission if the initiator sends a segment containing the message H ello dear customer and the other party answers with a segment containing H i there seller. c. Show the contents of the segments during the connection termination. 15. A client uses T C P to send data to a server. T he data consist of 16 bytes. C alculate the efficiency of this transmission at the T C P level (ratio of useful bytes to total bytes). C alculate the efficiency of transmission at the IP level. A ssume no options for the I P header. C alculate the efficiency of transmission at the data link layer. A ssume no options for the IP header and use E thernet at the data link layer. 16. T C P is sending data at 1 megabyte per second. If the sequence number starts with 7,000, how long does it take before the sequence number goes back to zero? 17. A T C P connection is using a window size of 10,000 bytes and the previous acknowledgment number was 22,001. It receives a segment with acknowledgment number 24,001 and window size advertisement of 12,000. D raw a diagram to show the situation of the window before and after. 18. A window holds bytes 2001 to T he next byte to be sent is D raw a figure to show the situation of the window after the following two events. a. A n A C K segment with the acknowledgment number 2500 and window size advertisement 4000 is received. b. A segment carrying 1,000 bytes is sent. 19. A T C P connection is in the E STA BL ISH E D state. T he following events occur one after another: a. A FIN segment is received. b. T he application sends a close message. W hat is the state of the connection after each event? W hat is the action after each event?

4 500 PA R T 3 T RA N SP O R T L A Y E R 20. A T C P connection is in the E STA BL ISH E D state. T he following events occur one after another: a. T he application sends a close message. b. A n A C K segment is received. W hat is the state of the connection after each event? W hat is the action after each event? 21. A host has no data to send. It receives the following segments at the times shown (hour:minute:second:milliseconds after midnight). Show the acknowledgments sent by the host. a. Segment 1 received at 0:0:0:000. b. Segment 2 received at 0:0:0:027. c. Segment 3 received at 0:0:0:400. d. Segment 4 received at 0:0:1:200. e. Segment 5 received at 0:0:1: A host sends five packets and receives three acknowledgments. T he time is shown as hour:minute:seconds. a. Segment 1 was sent at 0:0:00. b. Segment 2 was sent at 0:0:05. c. A C K for segments 1 and 2 received at 0:0:07. d. Segment 3 was sent at 0:0:20. e. Segment 4 was sent at 0:0:22. f. Segment 5 was sent at 0:0:27. g. A C K for segments 1 and 2 received at 0:0:45. h. A C K for segment 3 received at 0:0:65. C alculate the values of RT T M, RT T S, RT T D, and RT O if the original RT O is 6 seconds. D id the sender miss the retransmission of any segment? Show which segments should have been retransmitted and when. R ewrite the events including the retransmission time. 23. Show the contents of a SA CK option to be sent if a host has received bytes 2001 to 3000 in order. Bytes 4001 to 6000 are out of order, and bytes 3501 to 4000 are duplicate. 24. Show a congestion control diagram like Figure using the following scenario. A ssume a maximum window size of 64 segments. a. T hree duplicate A C K s are received after the fourth RT T. b. A time-out occurs after the sixth RT T. 25. Show the transition diagrams (F SM s) for simultaneous-close scenario (See F igure 15.19). 26. Show the transition diagrams (FSM s) for denying-a-connection scenario (See Figure 15.20). 27. Show the transition diagrams (FSM s) for aborting-a-connection scenario (See Figure 15.21). 28. In a send window, S f = 401 and S n = 701. If window size is 1,000 bytes, show the send window before and after the station receives an A CK segment with ackn o = 601

5 C H A P T E R 15 TRA N SM I SSI O N C O N T RO L P RO T O C O L (T C P ) 501 and rwnd = 700. I gnore congestion control. D oes this situation means shrinking the window? 29. D raw a figure similar to Figure for the following scenario (ignore error control and congestion control): a. Time 1: T he client sends a SY N segment with seqn o = 301. b. Time 2: T he server sets its buffer size to 2,000 bytes. c. Time 3: T he server acknowledges the SY N segment. d. Time 4: T he client sends a segment of 300 bytes in the SY N + A C K segment. e. Time 5: T he client sends a segment of 400 bytes. f. Time 6: T he server process pulls 400 bytes. g. Time 7: T he server sends an A C K. h. Time 8: T he client sends a segment of 300 bytes. i. Time 9: T he server process pulls 300 bytes. j. Time 10: T he server sends an A C K. 30. Show time-line diagram for the following scenario (similar to Figure 15.29). a. T he client sends a segment carrying bytes 1401 to 1700, which arrives at the sender site. b. T he server sends a segment carrying bytes 2001 to 2100 and acknowledging the first segment from the client, which arrives. c. T he client sends a segment carrying bytes 1701 to 1900 and acknowledging the segment received, but the segment is lost. d. T he client sends a segment carrying bytes 1901 to 2100, but the segment is lost. e. Time out occurs at the client site. f. T he client resends a segment in response to time-out, this packet arrived. g. T he server sends an acknowledgment after A C K -delaying timer expires. h. A nother time-out occurs at the client site. i. T he client resends a segment in response to time-out, which arrives at the sender. j. T he server sends an acknowledgment after A C K -delaying timer expires. 31. R edraw the time-line diagram of Figure that allow the server to delay acknowledgements and send one A C K for each full cwnd window worth of data. R esearch A ctivities 32. We have not given all the rules about the transition diagram and T C P states. To be complete, we should show the next state for any state with the arrival of any type of segment. T C P should know what action to take if any of the segment types arrive when it is in any of the states. W hat are some of these rules? 33. W hat is the half-open case in T C P? 34. W hat is the half-duplex close case in T C P? 35. T he tcpdump command in U N IX or L IN U X can be used to print the headers of packets of a network interface. U se tcpdump to see the segments sent and received.