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CS492B Project #2 TCP Tutorial #1

CS492B Project #2 TCP Tutorial #1. 2003. 9. 15 Jin Hyun Ju. CS244a: An Introduction to Computer Networks. Handout 6: The Transport Layer, User Datagram Protocol (UDP) , and Transmission Control Protocol (TCP). Nick McKeown Professor of Electrical Engineering

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CS492B Project #2 TCP Tutorial #1

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  1. CS492B Project #2TCP Tutorial #1 2003. 9. 15 Jin Hyun Ju

  2. CS244a: An Introduction to Computer Networks Handout 6: The Transport Layer, User Datagram Protocol (UDP) , and Transmission Control Protocol (TCP) Nick McKeown Professor of Electrical Engineering and Computer Science, Stanford University nickm@stanford.edu http://www.stanford.edu/~nickm

  3. Outline • The TCP Protocol • TCP Characteristics • TCP Connection setup • TCP Segments • TCP Sequence Numbers • TCP Sliding Window • ACK Policy • Timeouts and Retransmission • (Congestion Control and Avoidance)

  4. TCP Characteristics • TCP is connection-oriented. • TCP provides a stream-of-bytes service. • TCP is reliable: • Acknowledgements indicate delivery of data. • Checksums are used to detect corrupted data. • Sequence numbers detect missing, or mis-sequenced data. • Corrupted data is retransmitted after a timeout. • Mis-sequenced data is re-sequenced. • (Window-based) Flow control prevents over-run of receiver. • TCP uses congestion control to share network capacity among users. We’ll study this in the next lecture.

  5. TCP is connection-oriented (Active) Client (Passive) Server (Active) Client (Passive) Server Syn Fin Syn + Ack (Data +) Ack Ack Fin TIME_WAIT Ack CLOSED Connection Setup 3-way handshake Connection Close/Teardown 2 x 2-way handshake

  6. TCP supports a “stream of bytes” service Host A Byte 0 Byte 1 Byte 2 Byte 3 Byte 80 Host B Byte 0 Byte 1 Byte 2 Byte 3 Byte 80

  7. …which is emulated using TCP “segments” Host A Byte 0 Byte 1 Byte 2 Byte 3 Byte 80 • Segment sent when: • Segment full (MSS bytes), • Not full, but times out, or • “Pushed” by application. TCP Data TCP Data Host B Byte 0 Byte 1 Byte 2 Byte 3 Byte 80

  8. The TCP Segment Format TCP Data TCP Hdr 0 15 31 Src port Dst port Sequence # Ack Sequence # Flags Window Size RSVD 6 HLEN 4 SYN PSH URG RST FIN ACK Checksum Urg Pointer (TCP Options) TCP Data

  9. Sequence Numbers Host A ISN (initial sequence number) Sequence number = 1st byte TCP HDR TCP Data Ack sequence number = next expected byte TCP HDR TCP Data Host B

  10. Initial Sequence Numbers (Active) Client (Passive) Server Syn +ISNA Syn + Ack +ISNB Ack Connection Setup 3-way handshake

  11. TCP Window • Pipelining • Retransmission • Flow control • (Congestion control)

  12. TCP Sliding Window • How much data can a TCP sender have outstanding in the network? • How much data should TCP retransmit when an error occurs? Just selectively repeat the missing data? • How does the TCP sender avoid over-running the receiver’s buffers?

  13. TCP Sliding Window Window Size Data ACK’d Outstanding Un-ack’d data Data OK to send Data not OK to send yet ACK • Retransmission policy is “Go Back N”. • Current window size is “advertised” by receiver • (usually 4k – 8k Bytes when connection set-up).

  14. Round-trip time Window Size Window Size ??? ACK (2) RTT = Window size TCP Sliding Window Round-trip time Window Size Host A Host B ACK ACK (1) RTT > Window size

  15. Delayed ACK • Data types in TCP segment: • Bulk data (FTP, Email, News, HTTP) : 90% • Interactive data (Telnet, Rlogin) : 10% • one key-stroke generates one segment, carrying only 1 byte. • 90% of Telnet & Rlogin packets carry less than 10 bytes of user data • TCP can be silly without any optimization!

  16. Delayed ACK Event in-order segment arrival, no gaps, everything else already ACKed in-order segment arrival, no gaps, one delayed ACK pending out-of-order segment arrival higher-than-expect seq. # gap detected arrival of segment that partially or completely fills gap TCP Receiver action delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK immediately send single cumulative ACK send duplicate ACK, indicating seq. # of next expected byte immediate ACK if segment starts at lower end of gap

  17. TCP: Retransmission and Timeouts Round-trip time (RTT) Retransmission TimeOut (RTO) Guard Band Host A Estimated RTT Data1 Data2 ACK ACK Host B • TCP uses an adaptive retransmission timeout value: • Congestion • Changes in Routing RTT changes frequently

  18. TCP: Retransmission and Timeouts • Picking the RTO is important: • Pick a values that’s too big and it will wait too long to retransmit a packet, • Pick a value too small, and it will unnecessarily retransmit packets. • The original algorithm for picking RTO: • EstimatedRTT =  EstimatedRTT + (1 - ) SampleRTT • RTO = 2 * EstimatedRTT • Characteristics of the original algorithm: • Variance is assumed to be fixed. • But in practice, variance increases as congestion increases.

  19. TCP: Retransmission and Timeouts • Newer Algorithm includes estimate of variance in RTT: • Difference = SampleRTT - EstimatedRTT • EstimatedRTT = EstimatedRTT + (*Difference) • Deviation = Deviation + *( |Difference| - Deviation ) • RTO =  * EstimatedRTT +  * Deviation •   1 •   4

  20. TCP: Retransmission and TimeoutsKarn’s Algorithm Host A Host B Host A Host B Retransmission Retransmission Wrong RTT Sample Wrong RTT Sample Problem: How can we estimate RTT when packets are retransmitted? Solution: On retransmission, don’t update estimated RTT (and double RTO).

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