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Understanding TCP Connection Management & Error Control

Learn about TCP functions, connection management, error, flow, and congestion control. Explore TCP segment structure, security, and how to secure TCP connections.

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Understanding TCP Connection Management & Error Control

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  1. IT 424 Networks2 Chapter 3: Transport Layer Part 2 : Transmission Control Protocol (TCP) Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross

  2. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  3. Explain the functions of TCP and its connection management scheme 1 Describe the three way handshaking process 2 Differentiate between error control, flow control and congestion control 3 Describe the flow and congestion control in TCP and the slow start mechanism 4 Describe the different fields in TCP packets and explain their roles Describe how to secure TCP connections 5 6 Learning Outcomes

  4. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  5. Transport Layer Protocols - TCP application transport network data link physical application transport network data link physical logical end-end transport Recap: Transport-Layer Services in The Internet • Reliable, in-order delivery: Transport Control Protocol (TCP) • Unreliable, unordered delivery: User Datagram Protocol (UDP) • Services not available: • Delay guarantees • Bandwidth guarantees

  6. Transport Layer Protocols - TCP TCP VS. UDP Functions • UDP: fragmentation and reassembly, Multiplexing/ Demultiplexing, error-detection using checksum • TCP: All UDP functionalities + reliable data transfer (connection management, error-control, flow control, congestion control)

  7. Transport Layer Protocols - TCP TCP: Overview RFCs: 793, 1323, 2581 • TCP creates reliable service on top of unreliable channel • Point-to-point: • One sender, one receiver • Reliable, in-order byte stream • Error control • Flow control • Congestion control • Full duplex data: • Bi-directional data flow in same connection • Connection-oriented: • Handshaking (exchange of control messages), initiate sender, receiver state before data exchange • Send & receive buffers

  8. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  9. TCP Connection Management TCP Connection Management • TCP sender, receiver establish “connection” before exchanging data segments • Initialize TCP variables: • Seq. #s • Buffers, flow control info (e.g. Rcvwindow) • Client: connection initiator • Server: contacted by client

  10. TCP Connection Management client server Initiate Specify seq# SYN Specify seq # Allocates Buffer SYNACK ACK TCP Connection Management (cont) • A. Three way handshake: • Step 1: client host initiates by sending TCP SYN segment to server • specifies initial seq # • no data • Step 2: server host receives SYN, replies with SYNACK segment • server allocates buffers • specifies server initial seq. # • Step 3: client receives SYNACK, replies with ACK segment, which may contain data

  11. TCP Connection Management TCP Connection Management (cont) B. Closing a connection: Client closes socket: Step 1:client end system sends TCP FIN control segment to server Step 2:server receives FIN, replies with ACK. Closes connection, sends FIN. Step 3:client receives FIN, replies with ACK, Enters “timed wait” Step 4:server, receives ACK. Connection closed. client server closing FIN ACK closing FIN ACK timed wait closed

  12. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  13. TCP Error Control TCP Reliable Data Transfer • Aim: • no bit will be received corrupted and no packet will be lost • Approach: • Reactive: resend lost or corrupted packet (error control ) • Proactive: Prevent packet loss at the receiver buffer (flow control) and at the router buffers (congestion control)

  14. TCP Error Control Error-Control Aim: no bit will be corrupted and no packet will be lost Why we need Error-Control? • Underlying channel may flip bits in packets (bit-error). Network layer does not provide a guaranteed service. • Packets may be lost if routed to a wrong address or if dropped because of congestion Q. How to provide a reliable transmission between end users?

  15. TCP Error Control Error Control Solution • Procedure • A method for detecting bit errors • A way for detecting lost packets • A feedback from the receiver • Retransmission of lost or corrupted packets • A mechanism that is based on retransmission when detecting an error is called ARQ: Automatic Repeat reQuest

  16. TCP Error Control - ARQ ARQ: Automatic Repeat reQuest • Bit-error detection: e.g. checksum • Packet-loss detection: sequence numbers • Receiver feedback: acknowledgement (ACK) or negative ack. (NAK) • Retransmission: • stop-and-wait; or • pipelined protocols using sliding window • (Taught previously in Networks_1, please refer to networks_1 slides for more details)

  17. TCP Error Control TCP Error Control Host B Host A • TCP creates reliable service on top of IP’s unreliable service • Pipelined segments • Cumulative acks • Single retransmission timer • Retransmissions triggered by: • Timeout events • Duplicate acks (fast retransmit) Seq=92, 8 bytes of data timeout ACK=100 X Seq=92, 8 bytes of data ACK=100 lost ACK scenario

  18. TCP Error Control Seq=100, 20 bytes of data timeout TCP Error Control – Retransmission Scenarios Host B Host B Host A Host A SendBase=92 Seq=92, 8 bytes of data Seq=92, 8 bytes of data Seq=100, 20 bytes of data timeout ACK=100 ACK=100 X ACK=120 ACK=120 Seq=92, 8 bytes of data SendBase=100 Seq=120, 15 bytes of data SendBase=120 ACK=120 SendBase=120 premature timeout cumulative ACK

  19. TCP Error Control – Fast Retransmit TCP Fast Retransmit TCP fast retransmit • Time-out period often relatively long: • Long delay before resending lost packet • Detect lost segments via duplicate acks. • Sender often sends many segments back-to-back • If segment is lost, there will likely be many duplicate acks. If sender receives 3 acks for same data (+original) (“Triple duplicate acks”), resend unacked segment with smallest seq # • Likely that unacked segment lost, so don’t wait for timeout

  20. TCP Error Control – Fast Retransmit TCP Fast Retransmit Host B Host A Seq=92, 8 bytes of data Seq=100, 20 bytes of data X Seq=120, 10 bytes of data ACK=100 ACK=100 timeout ACK=100 ACK=100 Seq=100, 20 bytes of data fast retransmit after sender receipt of triple duplicate ACK

  21. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  22. TCP Flow & Congestion Control - Flow Control TCP Flow Control • Prevent packet loss at receiver buffer • Sender won’t overflow receiver’s buffer by transmitting too much, too fast • Speed-matching service: matching the send rate to the receiving app’s drain rate • Approach: control number of sent packet (Sender window) based upon the receiver buffer receiver side of TCP connection has a receive buffer: app process may be slow at reading from buffer

  23. TCP Flow & Congestion Control - Flow Control TCP Flow Control: How It Works? • Receiver send the spare buffer-size to sender • Receiver advertises spare room by including value of RcvWindow in segments • Spare room in buffer • = RcvWindow • Sender limits unACKed data to RcvWindow • Guarantees receiver buffer doesn’t overflow

  24. TCP Flow & Congestion Control - Flow Control Flow Control: Sender Window Size Expanding the sender window Shrinking the sender window • The sender window size is controlled by the receiver window (the number of empty bytes in the receiver buffer) • Closing the sender window – if the receiver buffer is totally full; sender should stop

  25. TCP Flow & Congestion Control - Congestion Control Congestion Control • Congestion: • Informally: “too many sources sending too much data too fast for network to handle” • Consequences: • Lost packets (buffer overflow at routers) • Long delays (queueing in router buffers) • Two approaches: • end-to-end • network-assisted congestion control • Different from flow control!

  26. TCP Flow & Congestion Control - Congestion Control TCP Congestion Control • End- to-end congestion control: • No explicit feedback from network • Congestion inferred from end-system observed by loss, delay,.. • Basic idea: ask sender to slow down (or stop altogether) when there is congestion Procedure • Sender notes the receivers advertised window • AND a second window is defined, the Congestion Window • The sender can send up to the lowest of the two • Sender alters the Congestion Window according to the way the network is currently performing • The Congestion Window will keep increasing until segments timeout and then start reducing

  27. TCP Flow & Congestion Control - Slow Start TCP Slow Start Mechanism Host B Host A • When connection begins, increase rate exponentially until first loss event: • Initially cwnd = 1 MSS (maximum segment size) • Double cwnd every RTT • Done by incrementing cwnd for every ACK received • Summary:initial rate is slow but ramps up exponentially fast one segment RTT two segments four segments time

  28. Transport Layer Protocol - TCP TCP Connection Management TCP Error Control Overview TCP Flow & Congestion Control TCP Segment Structure & Security

  29. TCP Segment Structure & Security TCP Segment Structure

  30. TCP Segment Structure & Security Securing TCP TCP & UDP • No encryption • Clear text passwords sent into socket traverse Internet in clear text SSL • Provides encrypted TCP connection • Data integrity • End-point authentication SSL is at application layer • Applications use SSL libraries, which “talk” to TCP SSL socket API • Clear text passwords sent into socket traverse Internet encrypted

  31. Conclusion Conclusion TCP service: • Connection-oriented: setup required between client and server processes • Error control: between sending and receiving process • Flow control: sender won’t overwhelm receiver • Congestion control: throttle sender when network overloaded • Does not provide: timing, minimum bandwidth guarantees UDP service: • Unreliable data transfer between sending and receiving processes • Does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

  32. References References • Computer Networking: A Top-Down Approach Featuring the Internet by James Kurose and Keith Ross, Addison Wesley, 2012 (chapter 3 ) • Transmission Control Protocol (RFC793) • TCP Extensions for High Performance (RFC1323) • TCP Congestion Control (RFC2581)

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