Router-assisted congestion control. Lecture 8 CS 653, Fall 2010

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1 Router-assisted congestion control Lecture 8 CS 653, Fall 2010

2 TCP congestion control performs poorly as bandwidth or delay increases Shown analytically in [Low01] and via simulations Avg. TCP Utilization 50 flows in both directions Buffer = BW x Delay RTT = 80 ms Avg. TCP Utilization Because TCP lacks fast response 50 flows in both directions Buffer = BW x Delay BW = 155 Mb/s Spare bandwidth is available TCP increases by 1 pkt/rtt even if spare bandwidth is huge When a TCP starts, it increases exponentially Too many drops Flows ramp up by 1 pkt/rtt, Bottleneck taking forever Bandwidth to grab (Mb/s) the large Round bandwidth Trip Delay (sec)

3 Proposed Solution: Decouple Congestion Control from Fairness High Utilization; Small Queues; Few Drops Bandwidth Allocation Policy

4 Proposed Solution: Decouple Congestion Control from Fairness Coupled because a single mechanism controls both Example: In TCP, Additive-Increase Multiplicative- Decrease (AIMD) controls both How does decoupling solve the problem? 1. To control congestion: use MIMD which shows fast response 2. To control fairness: use AIMD which converges to fairness

5 Characteristics ofsolution 1. Improved Congestion Control (in high bandwidthdelay & conventional environments): Small queues Almost no drops 2. Improved Fairness Flexible bandwidth allocation: min-max fairness, proportional fairness, differential bandwidth allocation, 3. Scalable (no per-flow state)

6 XCP: An explicit Control Protocol 1. Congestion Controller 2. Fairness Controller

7 How does XCP Work? Round Trip Round Time Trip Time Congestion Congestion Window Window Feedback = packet Congestion Header

8 How does XCP Work? Round Trip Time Congestion Window Feedback = packet

9 How does XCP Work? Congestion Window = Congestion Window + Feedback XCP extends ECN and CSFQ Routers compute feedback without any per-flow state

10 How Does an XCP Router Compute the Congestion Controller Congestion Goal: Matches input traffic to link capacity Controller & drains the queue Looks at aggregate traffic & queue Algorithm: Aggregate traffic changes by Δ Δ ~ Spare Bandwidth Δ ~ - Queue Size So, Δ = α d avg Spare - β Queue Feedback? Δ Fairness Controller Fairness Goal: Divides Δ between flows Controller to converge to fairness Looks at a flow s state in Congestion Header MIMD AIMD Algorithm: If Δ > 0 Divide Δ equally between flows If Δ < 0 Divide Δ between flows proportionally to their current rates

11 Getting the devil out of the details Congestion Controller Δ = α d avg Spare - β Queue Theorem: System converges to optimal utilization (i.e., stable) for any link bandwidth, delay, number of sources if: Fairness Controller Algorithm: If Δ > 0 Divide Δ equally between flows If Δ < 0 Divide Δ between flows proportionally to their current rates Need to estimate number of flows N 0 π < α < and β = α N = 1 T ( Cwnd pkt / RTT pkt ) pkts in T (Proof based on Nyquist Criterion) No Per-Flow State RTT pkt : Round Trip Time in header Cwnd pkt : Congestion Window in header T: Counting Interval

12 Implementation Implementation uses few multiplications & additions per packet Practical! Liars? Policing agents at edges of the network or statistical monitoring Easier to detect than in TCP Gradual Deployment XCP can co-exist with TCP and can be deployed gradually

13 Performance

14 Subset of Results S 1 Bottleneck S 2 R 1, R 2,, R n S n Similar behavior over:

15 XCP Remains Efficient as Bandwidth or Delay Increases Utilization as a function of Bandwidth Utilization as a function of Delay Avg. Utilization Avg. Utilization Bottleneck Bandwidth (Mb/s) Round Trip Delay (sec)

16 XCP Remains Efficient as Bandwidth or Delay Increases Utilization as a function of Bandwidth Utilization as a function of Delay Avg. Utilization XCP increases proportionally to spare bandwidth Avg. Utilization α and β chosen to make XCP robust to delay Bottleneck Bandwidth (Mb/s) Round Trip Delay (sec)

17 XCP Shows Faster Response than TCP Start 40 Flows Stop the 40 Flows Start 40 Flows Stop the 40 Flows XCP shows fast response!

18 XCP Deals Well with Short Web-Like Flows Average Utilization Average Queue Drops Arrivals of Short Flows/sec

19 XCP is Fairer than TCP Same RTT Different RTT Avg. Throughput Avg. Throughput Flow ID Flow ID (RTT is 40 ms 330 ms )

20 XCP XCP Summary Outperforms TCP Efficient for any bandwidth Efficient for any delay Scalable Benefits of Decoupling Use MIMD for congestion control which can grab/ release large bandwidth quickly Use AIMD for fairness which converges to fair bandwidth allocation

21 XCP Pros and Cons Long-lived flows: Works well Convergence to fair share rates, high link utilization, small queue, low loss Mix of flow lengths: Deviates from processor sharing Non-trivial convergence time Flow durations longer

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30 ATM ABR congestion control ABR: available bit rate: Elastic service If sender s path underloaded : sender should use available bandwidth If sender s path congested: sender throttled to minimum guaranteed rate RM (resource management) cells: Sent by sender, interspersed with data cells Bits in RM cell set by switches ( network-assisted ) NI bit: no increase in rate (mild congestion) CI bit: congestion indication RM cells returned to sender by receiver, with bits intact

31 ATM ABR congestion control Two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell sender send rate thus minimum supportable rate on path EFCI bit in data cells: set to 1 in congested switch if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell

32 ATM ERICA Switch Algorithm ERICA: Explicit rate indication for congestion avoidance goals: Utilization: allocate all available capacity to ABR flows Queueing delay: keep queue small Fairness: max-min sought only after utilization achieved (decoupled from utilization?) Stability, ie reaches steady-state, and robustness, ie graceful degradation, when no steady-state

33 ERICA: Setting explicit rate (ER) Initialization MaxAllocPrev = MaxAllocCur = FairShare End of avg ing interval Total ABR Cap. = Link Cap. - VBR Cap. Target ABR Cap. = Fraction*Tot. ABR Cap. Z = ABR Input rate FairShare = Target ABR Cap. / # Active VCs Goto Initialization During congestion VCShare = VCRate/Z If (Z > 1+±) ER = max(fairshare, VCShare) Else ER = max(maxallocprev, VCShare) MaxAllocCur = max (MaxAllocCur, ER) If (ER > FairShare and VCRate < FairShare) ER = FairShare

34 ABR vs. XCP or RCP? Similarities? Differences?

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