1 / 22

TCP Congestion Control

TCP Congestion Control. Dina Katabi & Sam Madden nms.csail.mit.edu/~dina. Sharing the Internet. Difficult because of: Size Billions of users, links, routers Heterogeneity bandwidth: 9.6Kb/s (then modem, now cellular), 10 Tb/s

Download Presentation

TCP Congestion Control

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. TCP Congestion Control Dina Katabi & Sam Madden nms.csail.mit.edu/~dina

  2. Sharing the Internet Difficult because of: • Size • Billions of users, links, routers • Heterogeneity • bandwidth: 9.6Kb/s (then modem, now cellular), 10 Tb/s • latency: 50us (LAN), 133ms (wired), 1s (satellite), 260s (Mars) How do you manage resources in a huge system like the Internet, where users with different interests share the same links?

  3. 10Mb/s S1 R1 2Mb/s D 100Mb/s S2 S1 S2 Congestion • Sources compete for link bandwidth, and buffer space • Why a problem? • Sources are unaware of current state of resource • Sources are unaware of each other • Manifestations: • Lost packets (buffer overflow at routers) • Long delays (queuing in router buffers) • In many situations will result in < 2 Mb/s of throughput for the above topology (congestion collapse)

  4. 10Mb/s S1 R1 2Mb/s D 100Mb/s S2 Objectives of Congestion Control Efficiency & Fairness • Efficiency • Maximize link utilization • Minimize queue size (i.e., delay) • Minimize packet drops • Many solutions! • (S1= 1 Mb/s, S2= 1 Mb/s) and (S1=1.5 Mb/s, S2=0.5 Mb/s) are both efficient. • Want Fairness

  5. S1 9Mb/s R1 D S2 S2 1= 1Mb/s 2= 7Mb/s 3=  Demands 1= 1Mb/s 2= 4Mb/s 3= 4Mb/s Max-min Fair Rates Fairness • Max-Min Fairness • At each bottleneck, user gets min(user’s demand, fair share) • User’s rate is the minimum max-min fair rate along the path

  6. TCP

  7. TCP • TCP provides reliability & congestion control • Reliable transmission ensures the receiver’s application receives the correct and complete data sent by the sender’s application • TCP recovers from lost packets, eliminates duplicates and ensures in-order packet delivery to the application • Reliability was discussed in 6.02 • Congestion control • Sender reacts to congestion and discovers its fair and efficient send rate

  8. TCP Cong. Cont. • Basic Idea: • Send a few packets. If a packet is dropped decrease rate. If no drops, increase rate • How does TCP detect drops? • Packets have sequence numbers • Receiver acks the next expected sequence number (i.e., if received 1, it acks 2 saying it is expecting 2 to arrive next. Note that TCP implementations ack bytes but for simplicity we talk about acking packets.)

  9. TCP controls throughput via the congestion window • Congestion window is the number of outstanding packets, i.e., number of packets sender can send without waiting for an ack • TCP is window-based: sources change their sending rate by modifying the window size: “cwnd” • Avg. Throughput = (Avg. cwnd) /(Avg. RTT) • Why not changing rate directly? • Window protocols are easy to implement (no need for accurate timers, i.e., works for slow machines an sensors)

  10. How much should TCP Increase/decrease? • Probe for the correct sending window • Additive Increase / Multiplicative Decrease (AIMD) • Every RTT: No loss: cwnd = cwnd + 1 A loss: cwnd = cwnd /2

  11. cwnd += 1 cwnd = 2 cwnd = 1 cwnd = 3 cwnd = 4 D A D D A A D D D A A A Additive Increase Src Dest Actually, On ack arrival: cwnd = cwnd + 1/cwnd On timeout: cwnd = cwnd /2

  12. (bx1+a,bx2+a) (bx1,bx2) AIMD Leads to Efficiency and Fairness fairness line (x1,x2) User 2: x2 Efficiency line (x1+x2 = BW) User 1: x1

  13. Timeout because of a packet loss correct cwnd Grab back Bandwidth Halve Congestion Window (and Rate) TCP AIMD Congestion Window Time Need the queue to absorb these saw-tooth oscillations

  14. “Slow Start” • Cold start a connection at startup or after a timeout • At the beginning of a connection, increase exponentially • On ack arrival: cwnd += 1 1 2 4 8 Src D D D A A D D D D A A A A A A A A A Dest

  15. AIMD Slow start stops at halve previous cwnd Exponential Increase Adding Slow Start Congestion Window Loss+Timeout Time

  16. Tweaking TCP Fast Retransmit • Timeouts are too slow • When packet is dropped, receiver still acks the next in-order packet • Use 3 duplicate ACKs to indicate a drop • Why 3? When this does not work? Fast Recovery • If there are still ACKs coming in then no need for slow-start • Divide cwnd by 2 after fast retransmit • Increment cwnd by 1/cwnd for each dupack

  17. Putting It Together Congestion Window Time

  18. Putting It Together Congestion Window Time Slow Start

  19. Putting It Together 3 dupacks (Fast Retransmit) Congestion Window Time

  20. Putting It Together Congestion Window Time Fast Recovery

  21. so, drop rate is: 1 drop every Throughput  is the packets sent divided by the time it took to send them From the two eq. TCP Steady-State Throughput as Function of Loss Rate W(t) 1 drop Wm Wm/2 RTT * Wm/2 time

  22. Reflections on TCP • The probing mechanism of TCP is based on causing congestion then detecting it • Assumes that all sources cooperate • Assumes flows are long enough • Too bursty • Vulnerable to non-congestion related loss (e.g. wireless errors) • Unfair to long RTT flows

More Related