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. 0 4 10 16 24 31 ... CHECKSUM URGENT POINTER OPTIONS (IF ANY) PADDING DATA HLEN RESERVED CODE BITS WINDOW ACKNOWLEDGMENT NUMBER SOURCE PORT DESTINATION PORT SEQUENCE NUMBER TCP Segment Format • TCP divides the data stream into segments for transmission:

4. TCP Segment Header Fields • Code bits - identify the contents of the segment: • Window - how much data the sender is willing to accept (flow control) • Urgent pointer - specifies the position in the segment where urgent data ends

5. Out of Band Data • TCP provides a mechanism to handle urgent data • Urgent data is received before octets already in the stream • Sender: • Sets urgent bit in segment header • Puts urgent data at the beginning of the data field • Sets urgent pointer to the end of the urgent data • Receiver: • Notified of the urgent data as soon as it arrives • Enters “urgent mode” until all urgent data has been consumed • Returns to “normal mode”

6. TCP Segment Header Fields (cont) • Checksum - used to verify the integrity of the segment • Computed over: • Segment header • Segment data • Pseudo header • 1’s complement addition algorithm • Options - signal various options • Maximum segment size

7. TCP - Acknowledgments and Retransmission • Uses cumulative acknowledgment scheme: • ACK 7 = “all octets up to but not including number 7 have been received correctly” • Advantages • ACKs are easy to generate and unambiguous • Lost acknowledgments do not force retransmission • Disadvantages • Sender does not receive information about all successful transmissions

8. TCP - Timeout and Retransmission • For each segment sent: • Start timer and wait for acknowledgment • Retransmit if timer expires • TCP uses an adaptive retransmission algorithm because internet delays are so variable • Round trip time of each connection is recomputed every time an acknowledgment arrives • Timeout value is adjusted accordingly

9. TCP - Timeout and Retransmission (cont) • How should the most recent round trip sample (RTS) effect the round trip time (RTT)? New RTT = (a * RTT) + ((1-a) * RTS) • a = 0 • a = 1 • How should RTT be used to compute timeout? Timeout = b * RTT • b = 1 • b > 1

10. Accurate Measurement ofRound Trip Samples • Simple answer: • Difference in time segment sent and time ACK received • But what about retransmissions? • Does ACK correspond to first or second copy of segment sent? • Assuming ACK is for the earliest transmission causes problems • Assuming ACK is for the latest transmission causes problems • TCP acknowledgments are ambiguous

11. Accurate Measurement ofRound Trip Samples • To avoid problems with ambiguous ACKs: • TCP should not update the RTT for retransmitted segments • Problem: if timeout value is too small all segments will cause retransmissions and RTT will never be updated • Timer back-off: if the timer expires and causes a retransmission TCP increases the timeout • New timeout = c * timeout

12. Karn’s Algorithm “When computing the round trip estimate, ignore samples that correspond to retransmitted segments, but use a backoff strategy, and retain the timeout value from a retransmitted packet for subsequent packets until a valid sample is obtained.”

13. Responding to HighVariance in Delay • Often a good estimate of the round trip time is not very useful because internet delays tend to have a high variance • Most TCP implementations estimate: • Average round trip time • Variance • Using the variance Timeout = b * RTT

14. Responding to Congestion • Endpoints cannot know the details of where in the internet congestion has occurred or why • Congestion will usually lengthen delays • TCP’s response to lengthened delays (retransmission) can cause congestion collapse • Instead, TCP must reduce transmission rates when congestion occurs

15. Responding to Congestion (cont) • Augment the TCP transmission window with a congestion window: Window=min(receiver advertisement,congestion window) • Multiplicative Decrease Congestion Avoidance: • Upon loss of a segment reduce the congestion window by half (down to a minimum of one segment) • Backoff the retransmission timer exponentially

16. Responding to Congestion (cont) • Slow-start (additive) recovery: • When starting traffic on a new connection or increasing traffic after a period of congestion • Start the congestion window at the size of a single segment and increase the congestion window by one segment each time an ACK arrives • Congestion avoidance phase (during recovery): • Once the congestion window reaches half of its original size before congestion occurred • Increase the size of the congestion window by 1 only if all segments in the window have been acknowledged

17. TCP Timeout and Retransmission - Summary • What do we get for all this stuff: • Slow-start increase • Multiplicitive decrease • Congestion avoidance • Measurement of RTT and variance • Exponential timer backoff • Dramatic improvement of TCP performance without adding significant computational overhead

18. Site 1 Network Site 2 Receive SYN&ACK Send SYN seq=x Send ACK y+1 Send SYN seq=y, ACK x+1 Receive ACK Establishing a TCP Connection • The 3-way handshake • Guarantee that both sides are ready for connection • Allows both sides to agree on initial sequence numbers Receive SYN

19. Closing a TCP Connection • Applications should close a connection when they have no more data to transmit • Connection can be closed in either one or both directions • Site 1 finishes transmitting data and waits for ACK from site 2 • Site 1 transmits a segment with the FIN bit set • Site 2 acknowledges the FIN segment • Site 2 notifies the application that no more data is coming • Data can still be transmitted from site 2 to site 1 • Site 1 will still receive and acknowledge data from site 2 • Eventually, site 2 will finish transmitting and close its connection • Both endpoints delete record of the connection

20. Closing a TCP Connection (cont) Site 1 Network Site 2 (app closes connection) Send FIN seq=x Receive FIN Send ACK x+1 (inform application) Receive ACK (app closes connection) Send FIN seq=y, ACK x+1 Receive FIN&ACK Send ACK y+1 Receive ACK

21. TCP Connection Reset • Applications normally close connections • Sometimes abnormal conditions arise that break a connection • Broken connections can be reset: • Site 1 sends a segment with the RST bit set • Site 2 receives segment and aborts the connection • Transfers in both directions cease immediately • Resources for the connection are released • Applications programs are informed

22. Forcing Data Delivery • TCP divides the stream of octets into segments for transmission • This improves efficiency since octets can be buffered until a good-sized segment can be sent • TCP provides a push operation for applications that want to force delivery of octets • Set PSH bit • Send segment

23. Reserved TCP Port Numbers • Like UDP: • Static port bindings for commonly used services • Ports 0-1024 are reserved • Dynamic port bindings • Port numbers over 1024 • Port numbers for services accessible by both UDP and TCP usually match • ECHO (7) • TIME (37)

24. Reserved TCP Port Numbers Port number: Service: 0 Reserved 7 Echo 17 Quote of the day 21 FTP 23 TELNET 25 SMTP 37 Time 79 Finger 80 HTTP 119 NNTP

25. TCP Performance • Silly Window Syndrome • Sender generates data quickly • Receiver reads incoming data one octet at a time Sender Receiver

26. TCP Performance (cont) • Silly Window Syndrome • Each ACK advertises a small amount of space • Each segment carries a small amount of data • Problems: • Poor use of network bandwidth • Unnecessary computational overhead

27. TCP Performance (cont) • Avoiding Silly Window Syndrome • Use heuristics at sender to avoid transmitting a small amount of data in each segment • Use heuristics at receiver to avoid sending small window advisements • Receive-side silly window avoidance • Monitor receive window size • Delay advertising an increase until a “significant” increase is possible • “Significant” = min(half the window, maximum segment size)

28. Receive-Side Silly Window Avoidance Example Receive 6 octets, send ACK 7 with window advisement of 0 Application reads one octet Application reads one octet Application reads one octet Send window advisement of 3, receive 3 octets

29. Receive-Side SillyWindow Avoidance • Two approaches: • Receiver can ACK received octets but does not advertise an increase in its window until the increase is significant • Receiver can not send ACKs when the window is not large enough to advertise • Advantages/disadvantages?

30. Send-Side SillyWindow Avoidance • Goal: avoid sending small segments • Application can generate data in small blocks • TCP must collect data sent by application into a single large segment (clump) for transmission • TCP must delay sending a segment until it contains a reasonable amount of data • How long should TCP wait before transmitting data?

31. Send-Side Silly Window Avoidance (cont) • The Nagle Algorithm: • Application generates data to be sent over a connection that has already transmitted some data • If all previous transmissions have been acknowledged send the data immediately • If any ACKs are still pending do not transmit until: • Maximum segment size is reached, or • An ACK arrives • Self-clocking - does not compute delays • Applies even if the application requests a push

32. TCP Summary • Provides reliable stream delivery service • Full duplex • Out-of-band for urgent data • Makes efficient use of the network • Piggybacking • Sliding windows • Efficiency • End-to-end flow control • Acknowledgment and retransmission • Congestion recovery/avoidance