Presentation on theme: "Winter 2008CS244a Handout #61 CS244a: An Introduction to Computer Networks Handout 6: The Transport Layer, Transmission Control Protocol (TCP), and User."— Presentation transcript:
Winter 2008CS244a Handout #61 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
Winter 2008CS244a Handout #62 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
Winter 2008CS244a Handout #63 The Transport Layer What is the transport layer for? What characteristics might it have? Reliable delivery Flow control …
Winter 2008CS244a Handout #64 Review of the transport layer NickDave Leland.Stanford.edu Athena.MIT.edu Network Layer Link Layer Application Layer Transport Layer O.S. HeaderDataHeaderData HD HD HD HDHD HD
Winter 2008CS244a Handout #65 Layering: The OSI Model Session Network Link Physical Application Presentation Transport Network Link Network Transport Session Presentation Application Network Link Physical Peer-layer communication layer-to-layer communication Router
Winter 2008CS244a Handout #66 Layering: Our FTP Example Network Link Transport Application Presentation Session Transport Network Link Physical The 7-layer OSI Model The 4-layer Internet model Application FTP ASCII/Binary IP TCP Ethernet
Winter 2008CS244a Handout #67 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.
Winter 2008CS244a Handout #68 TCP is connection-oriented Connection Setup 3-way handshake (Active) Client (Passive) Server Syn Syn + Ack Ack Connection Close/Teardown 2 x 2-way handshake (Active) Client (Passive) Server Fin (Data +) Ack Fin Ack
Winter 2008CS244a Handout #69 TCP supports a “stream of bytes” service Byte 0Byte 1 Byte 2Byte 3 Byte 0Byte 1Byte 2Byte 3 Host A Host B Byte 80
Winter 2008CS244a Handout #610 …which is emulated using TCP “segments” Byte 0Byte 1 Byte 2Byte 3 Byte 0Byte 1Byte 2Byte 3 Host A Host B Byte 80 TCP Data Byte 80 Segment sent when: 1.Segment full (MSS bytes), 2.Not full, but times out, or 3.“Pushed” by application.
Winter 2008CS244a Handout #611 The TCP Segment Format IP Hdr IP Data TCP HdrTCP Data Src portDst port Sequence # Ack Sequence # HLEN 4 RSVD 6 URGACK PSH RSTSYNFIN Flags Window Size ChecksumUrg Pointer (TCP Options) TCP Data TCP Header and Data + IP Addresses Src/dst port numbers and IP addresses uniquely identify socket
Winter 2008CS244a Handout #612 Sequence Numbers Host A Host B TCP Data TCP HDR TCP HDR ISN (initial sequence number) Sequence number = 1 st byte Ack sequence number = next expected byte
Winter 2008CS244a Handout #613 Initial Sequence Numbers Connection Setup 3-way handshake (Active) Client (Passive) Server Syn +ISN A Syn + Ack +ISN B Ack
Winter 2008CS244a Handout #614 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?
Winter 2008CS244a Handout #615 TCP Sliding Window Window Size Outstanding Un-ack’d data Data OK to send Data not OK to send yet Data ACK’d 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”.
Winter 2008CS244a Handout #616 TCP Sliding Window Host A Host B ACK Window Size Round-trip time (1) RTT > Window size ACK Window Size Round-trip time (2) RTT = Window size ACK Window Size ???
Winter 2008CS244a Handout #617 TCP: Retransmission and Timeouts Host A Host B ACK Round-trip time (RTT) ACK Retransmission TimeOut (RTO) Estimated RTT Data1Data2 Guard Band TCP uses an adaptive retransmission timeout value: Congestion Changes in Routing RTT changes frequently
Winter 2008CS244a Handout #618 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: 1. EstimatedRTT k = EstimatedRTT k-1 + (1 - ) SampleRTT 2. RTO = 2 * EstimatedRTT Characteristics of the original algorithm: Variance is assumed to be fixed. But in practice, variance increases as congestion increases. Determined empirically
Winter 2008CS244a Handout #619 TCP: Retransmission and Timeouts There will be some (unknown) distribution of RTTs. We are trying to estimate an RTO to minimize the probability of a false timeout. RTT Probability mean variance Load (Amount of traffic arriving to router) Average Queueing Delay Variance grows rapidly with load Router queues grow when there is more traffic, until they become unstable. As load grows, variance of delay grows rapidly.
Winter 2008CS244a Handout #620 TCP: Retransmission and Timeouts Newer Algorithm includes estimate of variance in RTT: Difference = SampleRTT - EstimatedRTT EstimatedRTT k = EstimatedRTT k-1 + ( *Difference) Deviation = Deviation + *( |Difference| - Deviation ) RTO = * EstimatedRTT + * Deviation 1 4 Same as before
Winter 2008CS244a Handout #621 TCP: Retransmission and Timeouts Karn’s Algorithm Retransmission Wrong RTT Sample Host AHost B Retransmission Wrong RTT Sample Host AHost B Problem: How can we estimate RTT when packets are retransmitted? Solution: On retransmission, don’t update estimated RTT (and double RTO).
Winter 2008CS244a Handout #622 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.
Winter 2008CS244a Handout #623 User-Datagram Protocol (UDP) App A1A1 A2A2 B1B1 B2B2 UDP OS IP Like TCP, UDP uses port number to demultiplex packets
Winter 2008CS244a Handout #624 SRC port DST port checksumlength DATA User-Datagram Protocol (UDP) Packet format 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. By default, only covers the header.