1 / 22

Communication Networks

Communication Networks. Recitation 9. Fairness & TCP Congestion Control. Max-Min Fairness. The intuition: to maximize the bandwidth allocated to the session with the minimum allocation

jtopper
Télécharger la présentation

Communication Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Communication Networks Recitation 9 Netcomm 2005

  2. Fairness & TCP Congestion Control Netcomm 2005

  3. Max-Min Fairness • The intuition: to maximize the bandwidth allocated to the session with the minimum allocation • More formally: to maximize the allocation of each session i under constrain that an increase in i’s allocation doesn’t cause a decrease in some other session allocation with the same or smaller rate than i Netcomm 2005

  4. Example of max-min fair flow Session 2 Session 3 Session 1 Capacity=1 Capacity=1 Session 0 Maximal fair flow division will be to give for the sessions 0,1,2 a flow rate of 1/3 and for the session 3 a flow rate of 2/3 Netcomm 2005

  5. TCP Overview • Full duplex • Flow control: keep sender from overrunning receiver • Congestion control: keep sender from overrunning network • Connection-oriented • Byte-stream • app writes bytes • TCP sends segments • app reads bytes • Reliable data transfer Netcomm 2005

  6. 32 bits URG: urgent data (generally not used) counting by bytes of data (not segments!) source port # dest port # sequence number ACK: ACK # valid acknowledgement number head len not used rcvr window size U A P R S F PSH: push data now (generally not used) # bytes rcvr willing to accept checksum ptr urgent data Options (variable length) RST, SYN, FIN: connection management (reset, setupteardown commands) application data (variable length) Also in UDP TCP Segment Structure Netcomm 2005

  7. Connection Establishment Active participant Passive participant (client) (server) SYN, SequenceNum = x , y 1 + SYN + ACK, SequenceNum = x Acknowledgment = ACK, Acknowledgment = y + 1 Netcomm 2005

  8. Connection Termination Active participant Passive participant (server) (client) FIN, SequenceNum = x 1 + x Acknowledgment = y FIN, SequenceNum= Acknowledgment = y + 1 Netcomm 2005

  9. Internet Congestion Collapse • In the late 80s, the Internet suffered a congestion collapse Data + Retransmissions Host Congestion! Packet drops!! Host Collapse Host More Drops!!! Netcomm 2005

  10. Detecting Congestion • Packet drops indicate congestion • Is that really true? • Why does it work? Packet Dst Src Drop Ack Timeout! No Ack = Congestion! Netcomm 2005

  11. Controlling Congestion – The Effect of Window Size • Note that sender’s window is equal to the number of sender packets in flight (in the network). Source Destination Window A Window’s worth of packets X acks X more packets Netcomm 2005

  12. Controlling Congestion • Reduce window  less packets in the network • Increase window  more packets in the network • Idea: Concept of a congestion window– window is smaller when congestion is larger and vice versa Netcomm 2005

  13. Additive Increase, Multiplicative Decrease • Each time a packet drop occurs, slash window size in half (multiplicative decrease) • Multiplicative decrease is necessary to avoid congestion • When no losses are observed, gradually increase window size (additive increase) Netcomm 2005

  14. Additive Increase Src D D A A D D D A A A D A Dest Netcomm 2005

  15. Leads to the TCP “sawtooth” Timeouts Rate halved Could take a long time to get started! t Netcomm 2005

  16. “Slow Start” Designed to cold-start connection quickly at startup or if a connection has been halted (e.g. window dropped to zero,or window full, but ACK is lost). How it works: increase cwnd by 1 for each ACK received. 1 2 4 8 Src D D D A A D D D D A A A A A Dest Netcomm 2005

  17. Slow Start Timeouts Rate halved Slow start in operation until it reaches half of previous cwnd. Exponential “slow start” t Why is it called slow-start? Because TCP originally had no congestion control mechanism. The source would just start by sending a whole window’s worth of data. Netcomm 2005

  18. Fast Retransmit & Recovery Sender Receiver • Upon 3 duplicate ACKs, TCP retransmits. • Do not enter slow-start. Packet 1 Packet 2 ACK 1 Packet 3 ACK 2 Packet 4 ACK 2 Packet 5 Packet 6 ACK 2 ACK 2 Retransmit packet 3 ACK 6 Netcomm 2005

  19. Self Clocking • Each packet’s transmission is “clocked” by an ACK – no bursts develop … … Netcomm 2005

  20. TCP Vegas • Uses congestion avoidance instead of congestion control • Reno: Congestion control React to congestion after it occurs • Vegas: Congestion avoidance Predict and avoid congestion before it occurs Netcomm 2005

  21. Observation • Packet accumulation in the network can be inferred by monitoring RTT and sending rate cwnd Overloaded Router Sending Rate Bottleneck Link Sending Rate = cwnd / RTT Netcomm 2005

  22. TCP Vegas Congestion Control • BaseRTT is the minimum of all measured RTTs (commonly the RTT of the first packet) • If not overflowing the connection, then ExpectRate = CongestionWindow/BaseRTT • Source calculates ActualRate once per RTT • Source compares ActualRate with ExpectRate Diff = ExpectedRate - ActualRate if Diff < a increase CongestionWindow linearly else if Diff > b decrease CongestionWindow linearly else leave CongestionWindow unchanged Netcomm 2005

More Related