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CS244a: An Introduction to Computer Networks

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

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CS244a: An Introduction to Computer Networks

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

  2. Outline • The Transport Layer • The TCP Protocol • TCP Characteristics • TCP Connection setup • TCP Segments • TCP Sequence Numbers • TCP Sliding Window • Timeouts and Retransmission • (Congestion Control and Avoidance) • The UDP Protocol CS244a Handout #6

  3. The Transport Layer • What is the transport layer for? • What characteristics might it have? • Reliable delivery • Flow control • … CS244a Handout #6

  4. Application Layer Transport Layer O.S. O.S. Link Layer Network Layer D D D D D D H H H H H H Data Data Header Header Review of the transport layer Athena.MIT.edu Leland.Stanford.edu Nick Dave CS244a Handout #6

  5. Layering: The OSI Model layer-to-layer communication Application Application 7 7 Presentation Presentation 6 6 Session Session 5 5 Peer-layer communication Transport Transport Router Router 4 4 Network Network Network Network 3 3 Link Link Link Link 2 2 Physical Physical Physical Physical 1 1 CS244a Handout #6

  6. FTP ASCII/Binary TCP IP Ethernet Layering: Our FTP Example Application Application Presentation Transport Session Transport Network Network Link Link Physical The 4-layer Internet model The 7-layer OSI Model CS244a Handout #6

  7. TCP Characteristics • TCP is connection-oriented. • 3-way handshake used for connection setup. • 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. CS244a Handout #6

  8. TCP is connection-oriented (Active) Client (Passive) Server (Active) Client (Passive) Server Syn Fin Syn + Ack (Data +) Ack Ack Fin Ack Connection Setup 3-way handshake Connection Close/Teardown 2 x 2-way handshake CS244a Handout #6

  9. 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 CS244a Handout #6

  10. …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 CS244a Handout #6

  11. The TCP Segment Format IP Data IP Hdr TCP Data TCP Hdr 0 15 31 Src port Dst port Sequence # Src/dst port numbers and IP addresses uniquely identify socket Ack Sequence # TCP Header and Data + IP Addresses Flags Window Size RSVD 6 HLEN 4 SYN PSH URG RST FIN ACK Checksum Urg Pointer (TCP Options) TCP Data CS244a Handout #6

  12. 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 CS244a Handout #6

  13. Initial Sequence Numbers (Active) Client (Passive) Server Syn +ISNA Syn + Ack +ISNB Ack Connection Setup 3-way handshake CS244a Handout #6

  14. 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? CS244a Handout #6

  15. TCP Sliding Window Window Size Data ACK’d Outstanding Un-ack’d data Data OK to send Data not OK to send yet • Window is meaningful to the sender. • Current window size is “advertised” by receiver • (usually 4k – 8k Bytes when connection set-up). • TCP’s Retransmission policy is “Go Back N”. CS244a Handout #6

  16. 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 CS244a Handout #6

  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 CS244a Handout #6

  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: • EstimatedRTTk=  EstimatedRTTk-1 + (1 - ) SampleRTT • RTO = 2 * EstimatedRTT • Characteristics of the original algorithm: • Variance is assumed to be fixed. • But in practice, variance increases as congestion increases. Determined empirically CS244a Handout #6

  19. Average Queueing Delay Variance grows rapidly with load Load (Amount of traffic arriving to router) TCP: Retransmission and Timeouts • Router queues grow when there is more traffic, until they become unstable. • As load grows, variance of delay grows rapidly. • There will be some (unknown) distribution of RTTs. • We are trying to estimate an RTO to minimize the probability of a false timeout. Probability variance RTT mean CS244a Handout #6

  20. TCP: Retransmission and Timeouts • Newer Algorithm includes estimate of variance in RTT: • Difference = SampleRTT - EstimatedRTT • EstimatedRTTk = EstimatedRTTk-1 + (*Difference) • Deviation = Deviation + *( |Difference| - Deviation ) • RTO =  * EstimatedRTT +  * Deviation •   1 •   4 Same as before CS244a Handout #6

  21. 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). CS244a Handout #6

  22. User Datagram Protocol (UDP) Characteristics • UDP is a connectionless datagram service. • There is no connection establishment: packets may show up at any time. • UDP packets are self-contained. • UDP is unreliable: • No acknowledgements to indicate delivery of data. • Checksums cover the header, and only optionally cover the data. • Contains no mechanism to detect missing or mis-sequenced packets. • No mechanism for automatic retransmission. • No mechanism for flow control, and so can over-run the receiver. CS244a Handout #6

  23. User-Datagram Protocol (UDP) A1 A2 B1 B2 App App App App OS UDP Like TCP, UDP uses port number to demultiplex packets IP CS244a Handout #6

  24. User-Datagram Protocol (UDP)Packet format SRC port DST port By default, only covers the header. checksum length DATA • Why do we have UDP? • It is used by applications that don’t need reliable delivery, or • Applications that have their own special needs, such as streaming of real-time audio/video. CS244a Handout #6

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