Lecture 14: Congestion Control › ... › lectures › 222A-wi15-l14.pdf · Lecture 14 Overview...

CSE 222A: Computer Communication Networks Alex C. Snoeren

Lecture 14:Congestion Control

Thanks: Amin Vahdat, Dina Katabi

Lecture 14 Overview

●  TCP congestion control review

●  XCP Overview

2 CSE 222A – Lecture 14: Congestion Control

Congestion Control Overview●  Challenge: how do we efficiently share network

resources among billions of hosts? ◆  Today: TCP

Hosts adjust rate based on packet losses ◆  Alternative solutions

Fair queuing, RED (router support) Vegas, packet pair (add functionality to TCP) Rate control, credits


Queuing Disciplines●  How to distribute buffers among users/flows

◆  When buffer overflows, which packet to drop?

●  Simple solution: FIFO ◆  First in, first out ◆  If packet comes along with no available buffer space, drop it


Fair Queuing●  Goals:

◆  Allocate resources equally among all users/flows ◆  Low delay for interactive users ◆  Protection against misbehaving users

●  Approach: simulate general processor sharing (from OS world) ◆  Bitwise round robin ◆  Need to compute number of competing flows at each instant


TCP Congestion Problems●  Original TCP sent full window of data

●  When links become loaded, queues fill up, leading to: ◆  Congestion collapse: when round-trip time exceeds retransmit

interval this can create a stable condition in which every packet is being retransmitted many times

◆  Synchronized behavior: network oscillates between loaded and unloaded

»  Feedback loop


Jacobson’s Solution●  Transport protocols should obey conservation of

packets ◆  Use ACKs to clock injection of new packets

●  Modify retransmission timer to adapt to variations in delay

●  Infer network bandwidth from packet loss ◆  Drops è congestion è reduce rate ◆  No drops è no congestion è increase rate

●  Limit send rate based on minimum of congestion window and advertised window


Tracking Bottleneck Bandwidth●  Throughput = window size/RTT

●  Multiplicative decrease ◆  Timeout è dropped packet è cut window size in half

●  Additive increase ◆  ACK arrives è no drop è increase window size by one

packet/window


TCP “Sawtooth”●  Oscillates around bottleneck bandwidth

◆  Adjusts to changes in competing traffic

0

2

4

6

8

10

12

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18

window (in segs)

round-trip times

Additive Increase/Multiplicative Decrease


Slow Start●  How do we find bottleneck bandwidth? ●  Cannot use ACKs to clock without reaching equilibrium

◆  Start by sending a single packet Start slow to avoid overwhelming network

◆  Multiplicative increase until get packet loss Quickly find bottleneck Cut rate by half

◆  Shift into linear increase/multiplicative decrease


Slow Start●  Quickly find the bottleneck bandwidth

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8

window (in segs)

round-trip times

Slow Start


Slow Start Problems●  Slow start usually overshoots bottleneck

◆  Leading to many lost packets in window ◆  Can lose up to half of window size

●  Bursty traffic source ◆  Will cause bursty losses for other flows

●  Short flows ◆  Can spend entire time in slow start

Especially for large bottleneck bandwidth

●  Consider repeated connections to the same server ◆  E.g., for web connections


ACK Pacing in TCP

●  ACKs open up slots in the congestion/advertised window ◆  Bottleneck link determines rate to send ◆  ACK indicates one packet has left the network


Problems with ACK Pacing●  ACK compression

◆  Variations in queuing delays on return path changes spacing between ACKs

◆  Example: ACK waits for single long packet ◆  Worse with bursty cross-traffic

●  What happens after a timeout? ◆  Potentially, no ACKs to time packet transmissions

●  Congestion avoidance ◆  Slow start back to last successful rate ◆  Back to linear increase/multiplicative increase at this point


Two TCP Connections●  Reach equilibrium independent of initial bandwidth

(assuming equal RTTs)

0

2

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

window (in segs)

round-trip times


TCP “Friendliness”●  Problem: many different TCP implementations ●  If cut back more slowly after drops è grab bigger

share ●  If add more quickly after ACKs è grab bigger share ●  Incentive to cause congestion collapse

◆  Many TCP “accelerators” ◆  Easy to improve perf at expense of network

●  Solutions? ◆  Per-flow fair queuing at router


Example: In TCP, Additive-Increase Multiplicative-Decrease (AIMD) controls both

Coupled because a single mechanism controls both

XCP argues decoupling solves the problem:

1.  To control congestion: use MIMD which shows fast response

2.  To control fairness: use AIMD which converges to fairness

Fairness vs. Efficiency


XCP Advantages●  Tighter Congestion Control

◆  Small queues ◆  Almost no drops

●  Scalable (no per-flow state)

●  Flexible fairness definitions ◆  Max/min ◆  Proportional ◆  Differential bandwidth ◆  Etc.


1.  Congestion Controller 2.  Fairness Controller

XCP Overview


Feedback

Round Trip Time

Congestion Window

Congestion Header

Feedback

Round Trip Time

Congestion Window

Feedback = + 0.1 packet

XCP Example


Feedback = + 0.1 packet

Round Trip Time

Congestion Window

Feedback = - 0.3 packet

XCP Example


Congestion Window = Congestion Window + Feedback

Routers compute feedback without any per-flow state

XCP Example


Congestion Controller Fairness Controller Goal: Divides Δ between flows to converge to fairness Looks at a flow’s state in Congestion Header

Algorithm:

If Δ > 0 ⇒ Divide Δ equally between flows If Δ < 0 ⇒ Divide Δ between flows proportionally to their current rates

MIMD AIMD

Goal: Matches input traffic to link capacity & drains the queue Looks at aggregate traffic & queue Algorithm: Aggregate traffic changes by Δ Δ ~ Spare Bandwidth Δ  ~ - Queue Size So, Δ = α davg Spare - β Queue

Feedback Computation


Bottleneck S1 S2

R1, R2, …, Rn

Sn

Dumbbell Topology


Bottleneck Bandwidth (Mb/s)

Avg.

Util

izat

ion

Round Trip Delay (sec)

Avg.

Util

izat

ion

Efficiency


Start

40 Flows Stop the 40 Flows

Response to Dynamics


Average Utilization

Average Queue

Drops

Short Flows


Different RTT Same RTT

Avg.

Thr

ough

put

Avg.

Thr

ough

put

Fairness


For Next Class… ●  Read and review DCTCP paper

●  We’ll be in touch regarding Checkpoint meetings ◆  Email 1-2 page summary to Bhanuif you haven’t already

29 29 CSE 222A – Lecture 14: Congestion Control

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