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TCP - Part II                1
What is Flow/Congestion/Error Control ?• Flow Control:       Algorithms to prevent that the sender                      ov...
Acknowledgements in TCP• TCP receivers use acknowledgments (ACKs) to confirm the  receipt of data to the sender• Acknowled...
Sequence Numbers and Acknowledgments in TCP•   TCP uses sequence numbers to keep    track of transmitted and acknowledged ...
Sequence Numbers and Acknowledgments in TCP• An acknowledgment is a confirmation  of delivery of data• When a TCP receiver...
unacknowledged data with a smaller sequence number                                                                        ...
Cumulative Acknowledgements• With cumulative ACKs, the receiver can only acknowledge a  segment if all previous segments h...
Rules for sending Acknowledgments• TCP has rules that influence the transmission of  acknowledgments• Rule 1: Delayed Ackn...
Observing Delayed Acknowledgements                                   3.                         send echo of character    ...
Observing Delayed Acknowledgements• This is the output of typing 3 (three) characters :   Time 44.062449: Argon  Neon:   ...
Why 3 segments per character?                                                    character• We would expect four segments ...
Delayed Acknowledgement• TCP delays transmission of ACKs for up to 200ms• Goal: Avoid to send ACK packets that do not carr...
• Because of delayed ACKs, an ACK is often observed for                                                                   ...
Observing Nagle’s Rule• This is the output of typing 7 characters :  Time 16.401963:    Argon  Tenet:   Push, SeqNo 1:2(1...
Observing Nagle’s Rule• Observation: Transmission of  segments follows a different  pattern, i.e., there are only two  seg...
Observing Nagle’s Rule• Observations:    – Argon never has multiple unacknowledged segments outstanding    – There are few...
Nagle’s Rule• Only one 1-byte segment can be in transmission (Here: Since  no data is sent from B to A, we also see delaye...
TCP Flow Control                   18
TCP Flow Control•   TCP uses a version of the sliding window flow control,    where       • Sending acknowledgements is se...
Window Management in TCP• The receiver is returning two parameters to the sender                       window size        ...
Sliding Window Flow Control• Sliding Window Protocol is performed at the byte level:                                    Ad...
Sliding Window: “Window Closes”• Transmission of a single byte (with SeqNo = 6) and acknowledgement isreceived (AckNo = 5,...
Sliding Window: “Window Closes”• Transmission of a single byte (with SeqNo = 6) and acknowledgement isreceived (AckNo = 5,...
Sliding Window: “Window Opens”• Acknowledgement is received that enlarges the window to the right(AckNo = 5, Win=6):• A re...
Sliding Window: “Window Opens”• Acknowledgement is received that enlarges the window to the right(AckNo = 5, Win=6):• A re...
Sliding Window: “Window Shrinks”• Acknowledgement is received that reduces the window from the right(AckNo = 5, Win=3):• S...
Sliding Window: “Window Shrinks”• Acknowledgement is received that reduces the window from the right(AckNo = 5, Win=3):   ...
Sliding Window: Example                                                                    Receiver                       ...
TCP Error Control                    29
Error Control in TCP• TCP maintains a Retransmission Timer for each  connection:   – The timer is started during a transmi...
TCP Retransmission Timer• Retransmission Timer:   – The setting of the retransmission timer is crucial for     efficiency ...
Round-Trip Time Measurements• The retransmission mechanism of TCP is adaptive• The retransmission timers are set based on ...
Round-Trip Time Measurements• Retransmission timer is set to a Retransmission Timeout  (RTO) value.• RTO is calculated bas...
Round-Trip Time Measurements• Initial value:• Value of RTO before first RTT measurement: RTO = 3 sec• For the first RTT me...
Karn’s Algorithm• If an ACK for a retransmitted                                          segment  segment is received, the...
Measuring TCP Retransmission Timers ftp session from ellington to satchmo       ellington.cs.virginia.edu   satchmo.cs.vir...
Exponential Backoff• Scenario: File transfer  between two machines.                   600  Disconnect cable.              ...
TCP Congestion Control                         38
TCP Congestion Control• TCP has a mechanism for congestion control. The  mechanism is implemented at the sender• The windo...
TCP Congestion Control• TCP congestion control is governed by two parameters:   – Congestion Window (cwnd)   – Slow-start ...
Summary of TCP congestion control Initially:     cwnd = 1;     ssthresh =            advertised window size; New Ack recei...
Slow Start•   Initial value:              Set cwnd = 1                » Note: Unit is a segment size. TCP actually is base...
Slow Start Example• The congestion  window size grows                     segment 1                         cwnd = 1  very...
Congestion Avoidance• Congestion avoidance phase is started if cwnd has reached  the slow-start threshold value• If cwnd ≥...
Example of Slow Start/Congestion AvoidanceAssume that ssthresh = 8                            cwnd = 1                    ...
Responses to Congestion• So, TCP assumes there is congestion if it detects a packet  loss• A TCP sender can detect lost pa...
Summary of TCP congestion control Initially:     cwnd = 1;     ssthresh =            advertised window size; New Ack recei...
Slow Start / Congestion Avoidance•    A typical plot of cwnd for a TCP connection (MSS = 1500    bytes) with TCP Tahoe:   ...
Flavors of TCP Congestion Control• TCP Tahoe (1988, FreeBSD 4.3 Tahoe)   – Slow Start   – Congestion Avoidance   – Fast Re...
Acknowledgments in TCP• Receiver sends ACK to sender   – ACK is used for flow control,     error control, and congestion  ...
Acknowledgments in TCP• Receiver sends ACK to sender   – ACK is used for flow control,     error control, and congestion  ...
Fast Retransmit• If three or more duplicate  ACKs are received in a row,  the TCP sender believes that  a segment has been...
Fast Recovery•   Fast recovery avoids slow start after    a fast retransmit•   Intuition: Duplicate ACKs indicate    that ...
TCP Reno• Duplicate ACKs:     • Fast retransmit     • Fast recovery    Fast Recovery avoids slow start• Timeout:     • Re...
TCP Tahoe and TCP Reno(for single segment losses) cwnd                               Taho                                 ...
TCP New Reno•   When multiple packets are dropped, Reno has problems•   Partial ACK:     – Occurs when multiple packets ar...
SACK• SACK = Selective acknowledgment• Issue: Reno and New Reno retransmit at most 1 lost packet per round trip  time• Sel...
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Module14 tcp2 v11

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Transcript of "Module14 tcp2 v11"

  1. 1. TCP - Part II 1
  2. 2. What is Flow/Congestion/Error Control ?• Flow Control: Algorithms to prevent that the sender overruns the receiver with information• Error Control: Algorithms to recover or conceal the effects from packet losses• Congestion Control: Algorithms to prevent that the sender overloads the network The goal of each of the control mechanisms are different. In TCP, the implementation of these algorithms is combined 2
  3. 3. Acknowledgements in TCP• TCP receivers use acknowledgments (ACKs) to confirm the receipt of data to the sender• Acknowledgment can be added (“piggybacked”) to a data segment that carries data in the opposite direction• ACK information is included in the the TCP header• Acknowledgements are used for flow control, error control, and congestion control Data for B A Data for A ACK B ACK 3
  4. 4. Sequence Numbers and Acknowledgments in TCP• TCP uses sequence numbers to keep track of transmitted and acknowledged data• Each transmitted byte of payload data is associated with a sequence number• Sequence numbers count bytes and not segments• Sequence number of first byte in payload is written in SeqNo field• Sequence numbers wrap when they reach 232-1• The sequence number of the first sequence number (Initial sequence number) is negotiated during connection setup 4
  5. 5. Sequence Numbers and Acknowledgments in TCP• An acknowledgment is a confirmation of delivery of data• When a TCP receiver wants to acknowledge data, it – writes a sequence number in the AckNo field, and – sets the ACK flagIMPORTANT: An acknowledgment confirms receipt for all unacknowledged data that has a smaller sequence number than given in the AckNo fieldExample: AckNo=5 confirms delivery for 1,2,3,4 (but not 5). 5
  6. 6. unacknowledged data with a smaller sequence number 100 ACK o= 9 0 SeqN s 10 by te 6 An acknowledgment confirms the receipt of all • TCP has cumulative acknowledgements: o= 8 0 SeqN sCumulative Acknowledgements te 10 by 0 o= 7 SeqN s 70 te ACK 10 by o= 6 0 SeqN s te 10 by o= 5 0 SeqN s te 10 by o= 4 0 SeqN s 40 te ACK 10 by o=30 SeqN s te 10 by o=20 SeqN s 20 te 10 by ACK o=10 SeqN s 10 e 10 byt ACK o=0 Se q N s te 10 by B A
  7. 7. Cumulative Acknowledgements• With cumulative ACKs, the receiver can only acknowledge a segment if all previous segments have been received• With cumulative ACKs, receiver cannot selectively acknowledge blocks of segments: e.g., ACK for S0-S3 and S5-S7 (but not for S4)• Note: The use of cumulative ACKs imposes constraints on the retransmission schemes: – In case of an error, the sender may need to retransmit all data that has not been acknowledged 7
  8. 8. Rules for sending Acknowledgments• TCP has rules that influence the transmission of acknowledgments• Rule 1: Delayed Acknowledgments – Goal: Avoid sending ACK segments that do not carry data – Implementation: Delay the transmission of (some) ACKs• Rule 2: Nagle’s rule – Goal: Reduce transmission of small segments Implementation: A sender cannot send multiple segments with a 1-byte payload (i.e., it must wait for an ACK) 8
  9. 9. Observing Delayed Acknowledgements 3. send echo of character and/or output 2. interpret character 1. send character Host with Host with Telnet client Telnet server• Remote terminal applications (e.g., Telnet) send characters to a server. The server interprets the character and sends the output at the server to the client.• For each character typed, you see three packets: 1. Client  Server: Send typed character 2. Server  Client: Echo of character (or user output) and acknowledgement for first packet 3. Client  Server: Acknowledgement for second packet 9
  10. 10. Observing Delayed Acknowledgements• This is the output of typing 3 (three) characters : Time 44.062449: Argon  Neon: Push, SeqNo 0:1(1), AckNo 1 Time 44.063317: Neon  Argon: Push, SeqNo 1:2(1), AckNo 1 Time 44.182705: Argon  Neon: No Data, AckNo 2 Time 48.946471: Argon  Neon: Push, SeqNo 1:2(1), AckNo 2 Time 48.947326: Neon  Argon: Push, SeqNo 2:3(1), AckNo 2 Time 48.982786: Argon  Neon: No Data, AckNo 3 Time 55.116581: Argon  Neon: Push, SeqNo 2:3(1) AckNo 3 Time 55.117497: Neon  Argon: Push, SeqNo 3:4(1) AckNo 3 Time 55.183694: Argon  Neon: No Data, AckNo 4 10
  11. 11. Why 3 segments per character? character• We would expect four segments per character: ACK of character echo of character ACK of echoed character character• But we only see three segments per character: ACK and echo of character ACK of echoed character• This is due to delayed acknowledgements 11
  12. 12. Delayed Acknowledgement• TCP delays transmission of ACKs for up to 200ms• Goal: Avoid to send ACK packets that do not carry data. – The hope is that, within the delay, the receiver will have data ready to be sent to the receiver. Then, the ACK can be piggybacked with a data segment In Example: – Delayed ACK explains why the “ACK of character” and the “echo of character” are sent in the same segment – The duration of delayed ACKs can be observed in the example when Argon sends ACKsExceptions:• ACK should be sent for every second full sized segment• Delayed ACK is not used when packets arrive out of order 12
  13. 13. • Because of delayed ACKs, an ACK is often observed for 90 ACK o= 8 0 SeqN s 10 by te 13 o= 7 0 SeqN s 70 te 10 by 0 ACK o= 6 SeqN s te 10 byDelayed Acknowledgement 50 ACK o= 5 0 Max. Delay for an ACK SeqN s te 10 by o= 4 0 SeqN s 40 te ACK 10 by o=30 SeqN s every other segment te 10 by o=20 SeqN s 20 te 10 by ACK o=10 SeqN s e 10 byt o=0 Se q N s te 10 by B A
  14. 14. Observing Nagle’s Rule• This is the output of typing 7 characters : Time 16.401963: Argon  Tenet: Push, SeqNo 1:2(1), AckNo 2 Time 16.481929: Tenet  Argon: Push, SeqNo 2:3(1) , AckNo 2 Time 16.482154: Argon  Tenet: Push, SeqNo 2:3(1) , AckNo 3 Time 16.559447: Tenet  Argon: Push, SeqNo 3:4(1), AckNo 3 Time 16.559684: Argon  Tenet: Push, SeqNo 3:4(1), AckNo 4 Time 16.640508: Tenet  Argon: Push, SeqNo 4:5(1) AckNo 4 Time 16.640761: Argon  Tenet: Push, SeqNo 4:8(4) AckNo 5 Time 16.728402: Tenet  Argon: Push, SeqNo 5:9(4) AckNo 8 14
  15. 15. Observing Nagle’s Rule• Observation: Transmission of segments follows a different pattern, i.e., there are only two segments per character typed• Delayed acknowledgment does not kick in at Argon• The reason is that there is always data at Argon ready to sent when the ACK arrives• Why is Argon not sending the data (typed character) as soon as it is available? 15
  16. 16. Observing Nagle’s Rule• Observations: – Argon never has multiple unacknowledged segments outstanding – There are fewer transmissions than there are characters.• This is due to Nagle’s Rule: – Each TCP connection can have only one small (1-byte) segment outstanding that has not been acknowledged• Implementation: Send one byte and buffer all subsequent bytes until acknowledgement is received.Then send all buffered bytes in a single segment. (Only enforced if byte is arriving from application one byte at a time)• Goal of Nagle’s Rule: Reduce the amount of small segments.• The algorithm can be disabled. 16
  17. 17. Nagle’s Rule• Only one 1-byte segment can be in transmission (Here: Since no data is sent from B to A, we also see delayed ACKs) Typed characters A SeqN SeqN SeqN o=1, 4 o=5 o=0, 1 , 5 byt byt byt e e e B 1 5 10 ACK ACK ACK Delayed ACK Delayed ACK 17 Delayed ACK
  18. 18. TCP Flow Control 18
  19. 19. TCP Flow Control• TCP uses a version of the sliding window flow control, where • Sending acknowledgements is separated from setting the window size at sender • Acknowledgements do not automatically increase the window size• During connection establishment, both ends of a TCP connection set the initial size of the sliding window 19
  20. 20. Window Management in TCP• The receiver is returning two parameters to the sender window size AckNo (win) 32 bits 16 bits• The interpretation is: • I am ready to receive new data with SeqNo= AckNo, AckNo+1, …., AckNo+Win-1• Receiver can acknowledge data without opening the window• Receiver can change the window size without acknowledging data 20
  21. 21. Sliding Window Flow Control• Sliding Window Protocol is performed at the byte level: Advertised window 1 2 3 4 5 6 7 8 9 10 11 sent and sent but not acknowledged acknowledged can be sent USABLE WINDOW cant sent•Here: Sender can transmit sequence numbers 6,7,8. 21
  22. 22. Sliding Window: “Window Closes”• Transmission of a single byte (with SeqNo = 6) and acknowledgement isreceived (AckNo = 5, Win=4): 22
  23. 23. Sliding Window: “Window Closes”• Transmission of a single byte (with SeqNo = 6) and acknowledgement isreceived (AckNo = 5, Win=4): 1 2 3 4 5 6 7 8 9 10 11 Transmit Byte 6 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 4 is received 1 2 3 4 5 6 7 8 9 10 11 23
  24. 24. Sliding Window: “Window Opens”• Acknowledgement is received that enlarges the window to the right(AckNo = 5, Win=6):• A receiver opens a window when TCP buffer empties (meaning that datais delivered to the application). 24
  25. 25. Sliding Window: “Window Opens”• Acknowledgement is received that enlarges the window to the right(AckNo = 5, Win=6):• A receiver opens a window when TCP buffer empties (meaning that datais delivered to the application). 25
  26. 26. Sliding Window: “Window Shrinks”• Acknowledgement is received that reduces the window from the right(AckNo = 5, Win=3):• Shrinking a window should not be used 26
  27. 27. Sliding Window: “Window Shrinks”• Acknowledgement is received that reduces the window from the right(AckNo = 5, Win=3): 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 3 is received 1 2 3 4 5 6 7 8 9 10 11• Shrinking a window should not be done 27
  28. 28. Sliding Window: Example Receiver Buffer Sender 0 4K sends 2K of data 2K SeqNo=0 2K Sender AckNo=2048 Win=2048 sends 2K of data 2K SeqNo=2048 Sender blocked 4K AckNo=4096 Win=0 3K AckNo=4096 Win=1024 28
  29. 29. TCP Error Control 29
  30. 30. Error Control in TCP• TCP maintains a Retransmission Timer for each connection: – The timer is started during a transmission. A timeout causes a retransmission• TCP couples error control and congestion control (i.e., it assumes that errors are caused by congestion) – Retransmission mechanism is part of congestion control algorithm• Here: How to set the timeout value of the retransmission timer? 30
  31. 31. TCP Retransmission Timer• Retransmission Timer: – The setting of the retransmission timer is crucial for efficiency – Timeout value too small  results in unnecessary retransmissions – Timeout value too large  long waiting time before a retransmission can be issued – A problem is that the delays in the network are not fixed – Therefore, the retransmission timers must be adaptive 31
  32. 32. Round-Trip Time Measurements• The retransmission mechanism of TCP is adaptive• The retransmission timers are set based on round-trip time (RTT) measurements that TCP performs Segment 1 RTT #1 ACK for Segment 1The RTT is based on time difference Segment 2between segment transmission and Segment 3 RTT #2ACK +3 ACK for Segment 2But: Segment 4 TCP does not ACK each Segment 5 segment RTT #3 ACK for Segment 4 Each connection has only one ACK for Segment 5 timer 32
  33. 33. Round-Trip Time Measurements• Retransmission timer is set to a Retransmission Timeout (RTO) value.• RTO is calculated based on the RTT measurements.• The RTT measurements are smoothed by the following estimators srtt and rttvar: srttn+1 = α RTT + (1- α ) srttn rttvarn+1 = β ( | RTT - srttn | ) + (1- β ) rttvarn RTOn+1 = srttn+1 + 4 rttvarn+1• The gains are set to α =1/4 and β =1/8 33
  34. 34. Round-Trip Time Measurements• Initial value:• Value of RTO before first RTT measurement: RTO = 3 sec• For the first RTT measurement srtt1 = RTT rttvar1 = RTT / 2 RTO1 = srtt1 + 4 rttvar1 34
  35. 35. Karn’s Algorithm• If an ACK for a retransmitted segment segment is received, the sender retransmission Timeout ! cannot tell if the ACK belongs to of segment RTT ? the original or the retransmission. RTT ? ACKKarn’s Algorithm: Don’t update srtt on any segments that have been retransmitted. Each time when TCP retransmits, it sets: RTOn+1 = min ( 2 RTOn, 64) (exponential backoff) 35
  36. 36. Measuring TCP Retransmission Timers ftp session from ellington to satchmo ellington.cs.virginia.edu satchmo.cs.virginia.edu•Transfer file from ellington to satchmo• Unplug Ethernet cable in the middle of file transfer 36
  37. 37. Exponential Backoff• Scenario: File transfer between two machines. 600 Disconnect cable. 500• The interval between retransmission attempts in 400 seconds is: Seconds 1.03, 3, 6, 12, 24, 48, 64, 300 64, 64, 64, 64, 64, 64.• Time between retrans- 200 missions is doubled each time (Exponential Backoff 100 Algorithm)• Timer is not increased 0 beyond 64 seconds 0 2 4 6 8 10 12• TCP gives up after 13th Transmission Attempts attempt and 9 minutes. 37
  38. 38. TCP Congestion Control 38
  39. 39. TCP Congestion Control• TCP has a mechanism for congestion control. The mechanism is implemented at the sender• The window size at the sender is set as follows: Send Window = MIN (flow control window, congestion window)where – flow control window is advertised by the receiver – congestion window is adjusted based on feedback from the network 39
  40. 40. TCP Congestion Control• TCP congestion control is governed by two parameters: – Congestion Window (cwnd) – Slow-start threshhold Value (ssthresh) Initial value is advertised window• Congestion control works in two modes: – slow start (cwnd < ssthresh) – congestion avoidance (cwnd ≥ ssthresh 40
  41. 41. Summary of TCP congestion control Initially: cwnd = 1; ssthresh = advertised window size; New Ack received: if (cwnd < ssthresh) /* Slow Start*/ cwnd = cwnd + 1; else /* Congestion Avoidance */ cwnd = cwnd + 1/cwnd; Timeout: /* Multiplicative decrease */ ssthresh = cwnd/2; cwnd = 1; 41
  42. 42. Slow Start• Initial value: Set cwnd = 1 » Note: Unit is a segment size. TCP actually is based on bytes and increments by 1 MSS (maximum segment size)• Each time an ACK is received by the sender, the congestion window is increased by 1 segment: cwnd = cwnd + 1 » If an ACK acknowledges two segments, cwnd is still increased by only 1 segment. » Even if ACK acknowledges a segment that is smaller than MSS bytes long, cwnd is increased by 1. » Note: Generally, a TCP receiver sends an ACK for every other segment.• Does Slow Start increment slowly? Not really. In fact, the increase of cwnd is exponential 42
  43. 43. Slow Start Example• The congestion window size grows segment 1 cwnd = 1 very rapidly ent 1 ACK for segm – For every ACK, we cwnd = 2 segment 2 segment 3 increase cwnd by ents 2 ACK for segm 1 irrespective of ACK for segm ents 3 cwnd = 4 the number of segment 4 segment 5 segments ACK’ed segment 6• TCP slows down the ents 4 ACK for segm increase of cwnd ACK for segm ents 5 ents 6 ACK for segm when cwnd = 7 cwnd > ssthresh 43
  44. 44. Congestion Avoidance• Congestion avoidance phase is started if cwnd has reached the slow-start threshold value• If cwnd ≥ ssthresh then each time an ACK is received, increment cwnd as follows: • cwnd = cwnd + 1/ cwnd• So cwnd is increased by one only if all cwnd segments have been acknowledged. 44
  45. 45. Example of Slow Start/Congestion AvoidanceAssume that ssthresh = 8 cwnd = 1 cwnd = 2 cwnd = 4 14 12 cwnd = 8 Cwnd (in segments) 10 ssthresh 8 6 4 cwnd = 9 2 0 0 2 4 6 t= t= t= t= Roundtrip times cwnd = 10 45
  46. 46. Responses to Congestion• So, TCP assumes there is congestion if it detects a packet loss• A TCP sender can detect lost packets via: • Timeout of a retransmission timer • Receipt of a duplicate ACK• TCP interprets a Timeout as a binary congestion signal. When a timeout occurs, the sender performs: – cwnd is reset to one: cwnd = 1 – ssthresh is set to half the current size of the congestion window: ssthressh = cwnd / 2 – and slow-start is entered 46
  47. 47. Summary of TCP congestion control Initially: cwnd = 1; ssthresh = advertised window size; New Ack received: if (cwnd < ssthresh) /* Slow Start*/ cwnd = cwnd + 1; else /* Congestion Avoidance */ cwnd = cwnd + 1/cwnd; Timeout: /* Multiplicative decrease */ ssthresh = cwnd/2; cwnd = 1; 47
  48. 48. Slow Start / Congestion Avoidance• A typical plot of cwnd for a TCP connection (MSS = 1500 bytes) with TCP Tahoe: 48
  49. 49. Flavors of TCP Congestion Control• TCP Tahoe (1988, FreeBSD 4.3 Tahoe) – Slow Start – Congestion Avoidance – Fast Retransmit• TCP Reno (1990, FreeBSD 4.3 Reno) – Fast Recovery• New Reno (1996)• SACK (1996)• RED (Floyd and Jacobson 1993) 49
  50. 50. Acknowledgments in TCP• Receiver sends ACK to sender – ACK is used for flow control, error control, and congestion control• ACK number sent is the next sequence number expected Lost segment 50
  51. 51. Acknowledgments in TCP• Receiver sends ACK to sender – ACK is used for flow control, error control, and congestion control• ACK number sent is the next sequence number expected Out-of-order arrivals 51
  52. 52. Fast Retransmit• If three or more duplicate ACKs are received in a row, the TCP sender believes that a segment has been lost.• Then TCP performs a retransmission of what seems to be the missing segment, without waiting for a timeout to happen.• Enter slow start: ssthresh = cwnd/2 cwnd = 1 52
  53. 53. Fast Recovery• Fast recovery avoids slow start after a fast retransmit• Intuition: Duplicate ACKs indicate that data is getting through• After three duplicate ACKs set: – Retransmit packet that is presumed lost – ssthresh = cwnd/2 – cwnd = ssthresh+3 – (note the order of operations) – Increment cwnd by one for each additional duplicate ACK• When ACK arrives that acknowledges “new data” (here: AckNo=6148), set: cwnd=ssthresh enter congestion avoidance 53
  54. 54. TCP Reno• Duplicate ACKs: • Fast retransmit • Fast recovery  Fast Recovery avoids slow start• Timeout: • Retransmit • Slow Start• TCP Reno improves upon TCP Tahoe when a single packet is dropped in a round-trip time. 54
  55. 55. TCP Tahoe and TCP Reno(for single segment losses) cwnd Taho time Reno cwnd time 55
  56. 56. TCP New Reno• When multiple packets are dropped, Reno has problems• Partial ACK: – Occurs when multiple packets are lost – A partial ACK acknowledges some, but not all packets that are outstanding at the start of a fast recovery, takes sender out of fast recovery Sender has to wait until timeout occurs• New Reno: – Partial ACK does not take sender out of fast recovery – Partial ACK causes retransmission of the segment following the acknowledged segment• New Reno can deal with multiple lost segments without going to slow start 56
  57. 57. SACK• SACK = Selective acknowledgment• Issue: Reno and New Reno retransmit at most 1 lost packet per round trip time• Selective acknowledgments: The receiver can acknowledge non- continuous blocks of data (SACK 0-1023, 1024-2047)• Multiple blocks can be sent in a single segment.• TCP SACK: – Enters fast recovery upon 3 duplicate ACKs – Sender keeps track of SACKs and infers if segments are lost. Sender retransmits the next segment from the list of segments that are deemed lost. 57
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