Router-assisted congestion control

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
Avg. TCP Utilization
50 flows in both directions Buffer BW x
Delay RTT 80 ms
50 flows in both directions Buffer BW x
Delay BW 155 Mb/s

• Because TCP lacks fast response
• 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,
• taking forever to grab the large bandwidth

Bottleneck Bandwidth (Mb/s)
Round 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
5
Characteristics ofSolution

• Improved Congestion Control (in high
  bandwidth-delay conventional environments)
• Small queues
• Almost no drops
• Improved Fairness
• Flexible bandwidth allocation min-max
  fairness, proportional fairness, differential
  bandwidth allocation,
• Scalable (no per-flow state)

6
XCP An eXplicit Control Protocol

• Congestion Controller
• Fairness Controller

7
How does XCP Work?
Feedback 0.1 packet
8
How does XCP Work?
Feedback - 0.3 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 Feedback?
Congestion Controller
Fairness Controller
11
Getting the devil out of the details
Congestion Controller
Fairness Controller
No Per-Flow State
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
Similar behavior over
15
XCP Remains Efficient as Bandwidth or Delay
Increases
Utilization as a function of Delay
Utilization as a function of Bandwidth
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
Avg. Utilization
Bottleneck Bandwidth (Mb/s)
Round Trip Delay (sec)
17
XCP Shows Faster Response than TCP
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
20
XCP Summary

• XCP
• 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

22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
ATM ABR congestion control

• ABR available bit rate
• Elastic service
• If senders path underloaded
• sender should use available bandwidth
• If senders 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 avging interval
• Total ABR Cap. Link Cap. - VBR Cap.
• Target ABR Cap. FractionTot. ABR Cap.
• Z ABR Input rate
• FairShare Target ABR Cap. / Active VCs
• Goto Initialization

• During congestion
• VCShare VCRate/Z
• If (Z gt 1)
• ER max(FairShare, VCShare)
• Else
• ER max(MaxAllocPrev, VCShare)
• MaxAllocCur max(MaxAllocCur, ER)
• If (ER gt FairShare and VCRate lt FairShare)
• ER FairShare

34
ABR vs. XCP or RCP?

• Similarities?
• Differences?