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Transmission Control Protocol (TCP)
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- 2. Agenda Introduction Features ConnectionEstablishment and Termination Flow Control Error Control Congestion Control 2 of 35
- 3. TCP Introduction TCP: TCP seeksto deliver a byte stream from end-toend, in order, reliably. ◦ ◦ ◦ ◦ ◦ ◦ ◦ Process-to-Process communication Full Duplex communication Connection-oriented service Stream-oriented protocol (stream of bytes) Segmentation (datagrams in IP layer) Utilizes buffers at both ends (flow & error control) Reliable (ACKs) 3 of 35
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- 5. TCP Features Numbering system ◦Byte Number TCP numbers all the data bytes that are transmitted in a connection Numbering does not necessarily start from 0 TCP generates a random number between 0 and 232-1 for numbering first byte Eg. Random no. = 3423 (first byte) Total bytes = 5000 Numbering range = 3423-8422 5 of 35
- 6. TCP Features ◦ Sequencenumber Bytes are grouped into “segments” Sequence number for each segment is the number of the first byte carried in that segment 3423 (3423-4422) 4423 (4423-5422) 5423 (5423-6422) 6423 (6423-7422) 7423 (7423-8422) 6 of 35
- 7. TCP Features ◦ Acknowledgementnumber Defines the number of the next byte that the party expects to receive It is cumulative ACK = 5487 It means it has received all bytes from beginning up to 5486 (beginning may not be 0) 7 of 35
- 8. TCP Features Flow control ◦Receiver of the data controls the amount of data that are to be sent by the sender ◦ Numbering system allows TCP to use byte-oriented flow control Error control ◦ Considers a segment as a unit of data for error detection Congestion control 8 of 35
- 9. Connection Establishment Establishes avirtual path between the source and destination How TCP is connection-oriented while using IP (connection-less)? ◦ Connection is virtual ◦ TCP uses the services of IP to deliver individual segments, but it controls the connection itself ◦ IP is unaware of retransmission, out-of-order segments 9 of 35
- 10. Connection Establishment Three wayHandshaking 10 of 35
- 11. Connection Establishment SYN: SYN+ACK: ACK ◦ Itis for synchronization of sequence numbers ◦ It consumes 1 sequence number ◦ Carries no real data ◦ SYN segment for communication in other direction and ACK for the received SYN ◦ It consumes 1 sequence number ◦ Just an ACK segment ◦ Does not consume any sequence number 11 of 35
- 12. Connection Termination Three wayHandshaking 12 of 35
- 13. Connection Termination FIN: ◦ Itconsumes 1 sequence number ◦ May or may not carry real data FIN+ACK: ◦ FIN segment to announce closing of connection in other direction and ACK for the received FIN ◦ It consumes 1 sequence number ACK ◦ Just an ACK segment ◦ Does not consume any sequence number 13 of 35
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- 15. Flow Control TCP usessliding window to handle flow control The size of the window is determined by the lesser of two values: rwnd or cwnd rwnd: it is the number of bytes the receiver can accept before its buffer overflows cwnd: it is the value determined by the network to avoid congestion The receiver controls most of the aspects 15 of 35
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- 17. Error Control Includes mechanismsfor detecting corrupted segments, lost segments, out-of-order segments and duplicated segments Achieved through the use of three simple tools: ◦ Checksum ◦ Acknowledgement ◦ Retransmission 17 of 35
- 18. Checksum Each segment includesa checksum field, used to check for corrupted segment TCP uses a 16-bit checksum Corrupted segment is discarded by the destination and is considered lost 18 of 35
- 19. Acknowledgement Confirm the receiptof data segments Control segments that carry no data but consume a sequence number are also acknowledged ACK segments are never acknowledged 19 of 35
- 20. Retransmission A segment isretransmitted on two occasions: ◦ When a retransmission timer expires ◦ When the sender receives three duplicate ACKs There is no retransmission for ACK segments Retransmission after RTO: ◦ TCP maintains one RTO timer for all outstanding (sent, but not acknowledged) segments ◦ When the timer matures, the earliest outstanding segment is retransmitted ◦ Value of RTO is dynamic and is updated based on RTT 20 of 35
- 21. Fast Retransmission Let thevalue of RTO be very large One segment is lost and receiver receives so many out-of-order segments that they cannot be saved (buffer size) When the sender receives 4 ACKs with same value (1 original and 3 duplicates), even though the timer has not matured the fast retransmission requires that the segment be resent immediately 21 of 35
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- 23. Fast Retransmission When thesender receives retransmitted ACK, it knows that the four segments are safe and sound because ACK is cumulative 23 of 35
- 24. Adaptive Retransmission TCP attemptsto determine the approximate round-trip time between the devices, and adjusts it over time to compensate for increases or decreases in the average delay. TCP aims for an average RTT value for the connection. This average should respond to consistent movement up or down in the RTT without overreacting to a few very slow or fast acknowledgments. TCP re-estimates RTT after every successful transmission (not retransmission). 24 of 35
- 25. Adaptive Retransmission The RTTcalculation uses a smoothing formula: ◦ New RTT = (a * Old RTT) + ( (1-a) * Newest RTT Measurement) Where “a” (alpha) is a smoothing factor between 0 and 1. Higher values of “a" (closer to 1) provide better smoothing and avoiding sudden changes as a result of one very fast or very slow RTT measurement 25 of 35
- 26. Adaptive Retransmission Conversely, thisalso slows down how quickly TCP reacts to more sustained changes in round-trip time. Lower values of alpha (closer to 0) make the RTT change more quickly in reaction to changes in measured RTT, but can cause “over-reaction” when RTTs fluctuate wildly. Adaptive retransmission is a key for TCP success since it allows TCP to run in fast networks as well as slow networks. 26 of 35
- 27. Congestion Control TCP’s generalpolicy for handling congestion is based on three phases: ◦ Slow Start: Exponential Increase ◦ Congestion Avoidance: Additive Increase ◦ Congestion Detection: Multiplicative Decrease 27 of 35
- 28. Slow Start Size ofcongestion window (cwnd) starts with 1 max. segment size (MSS), determined during conn. establishment 28 of 35
- 29. Slow Start Slow startcannot continue indefinitely Sender keeps a track of a variable named ssthresh, when the size of window, in bytes, reaches this threshold, slow start stops In most cases the value of ssthresh is 65,535 bytes 29 of 35
- 30. Congestion Avoidance When theslow start phase stops, the additive phase begins Each time the whole window of segments is acknowledged, the size of the congestion window is increased by 1 30 of 35
- 31. Congestion Detection If congestionoccurs, the congestion window size must be decreased Sender can guess congestion by need to retransmit a segment Retransmission can occur in one of two cases: ◦ Time-out ◦ 3 ACKs are received 31 of 35
- 32. Congestion Detection If detectionis by time-out: ◦ It sets the value of the threshold to one half of current window size ◦ It sets cwnd to the size of one segment ◦ It starts the slow-start phase again If detection is by 3 ACKs: ◦ It sets the value of the threshold to one half of current window size ◦ It sets cwnd to the value of the threshold ◦ It starts the congestion avoidance phase 32 of 35
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