Network Congestion

Network CongestionNetwork Congestion

Gabriel Nell

UC Berkeley

OutlineOutline

Background: what is congestion?Congestion control

– End-to-end– Router-based

Economic insightsOverview of TCP-friendly congestion

control mechanisms

CongestionCongestion

The state of sustained network overload Congestion collapse

– Traffic dominated by overhead such as packet retransmissions

Current internet– Dominated by best-effort traffic– TCP for guaranteed delivery; Congestion-aware– UDP for streaming applications; Congestion-unaware

Controlling CongestionControlling Congestion

End-hosts– Advantages: cheap, scalable– Disadvantage: requires cooperation

Routers– Advantages: can be more aggressive, has a

complete picture of network traffic– Disadvantages: expensive, algorithms difficult

to implement in hardware

TCP Congestion ControlTCP Congestion Control

Implemented at end hosts Relies on feedback

– Implicit: packet drops indicate congestion– Explicit: ECN flags in header

Congestion controlled by changing window size: additive increase, multiplicative decrease (AIMD)

Problem: delays in adapting to network conditions may cause oscillations

Router-Based ControlRouter-Based Control

Scheduling– Determines service order– Should be easy to implement, provide fairness

and protection, and perform wellScheduling algorithms

– FIFO (first in, first out)– Round-robin / weighted round-robin– Weighted fair queuing

Router-Based ControlRouter-Based Control

Buffer Management– Absorbs bursts– Shared/per-flow– Introduce delay

Queue Management– Manage queue length, decide what packets to drop– RED effective, but difficult to parameterize for variable

conditions

Economic InsightsEconomic Insights

Tragedy of the Commons– Network resources a public good– Negative externality

Solution: Internalize costs– Congestion pricing: cover fixed costs, charge

extra under congestion conditions– Charge by willingness to pay

Economic InsightsEconomic Insights

New problem - customers prefer:– Flat rates– Constant performance, even if variable

performance is better on average

TCP-friendly Congestion TCP-friendly Congestion ControlControl

TCP-friendly: long-term throughput does not exceed that of TCP under the same conditions

Motivation: want to stream data such as audio and video without degrading overall network performance

For convenience, consider long-lasting streams

Congestion Control SchemesCongestion Control Schemes

Window-based vs. Rate-basedUnicast vs. MulticastEnd-to-end vs. Router-supported

Single-rate Vs. Multi-rateSingle-rate Vs. Multi-rate

Meaningful when considering multicastSingle-rate sends data to each client at the

same rateMultirate sends data to each client at

whatever rate is best for that client

Single-rate ProtocolsSingle-rate Protocols

Rate-based ApproachesRate-based Approaches

RAP – Rate Adaptation Protocol– Simple AIMD behavior

LDA+ – Loss-Delay Based Adaption Algorithm– Dynamic AIMD based on RTCP feedback

TFRC – TCP-Friendly Rate Control Protocol– Adjusts sending rate based on complex TCP equation

TEAR – TCP Emulation at Receivers– Uses a congestion window to determine rate, but

averages over larger timescales

Window-based ApproachesWindow-based Approaches

RLA – Random Listening Algorithm– Tracks number n of congested receivers, window is

decreased if a random number is 1/n MTCP – Multicast TCP

– Arrange receivers in a tree, children report congestion to parents.

– Root receives aggregate info, sends only as much data as smallest window

NCA – Nominee-Based Congestion Avoidance– Selects bottleneck as representative receiver, uses TCP-

style congestion control algorithm

Multi-rate ProtocolsMulti-rate Protocols

Rate-based ApproachesRate-based Approaches

RLC – Receiver-Driven Layered Congestion Control– Bandwidth consumed by each layer increases

exponentially– Subscription to additional layers comes at

particular times, which also increase exponentially; however congestion causes immediate layer drops

Rate-based ApproachesRate-based Approaches

FLID-DL – Fair Layered Increase/Decrease with Dynamic Layering– Encodes data with digital fountain– Bandwidth consumed by a layer decreases over

timeLTS/TFRP – Layered Transmission

Scheme/TCP-Friendly Transport Protocol– Use simple TCP rate equation to decide

subscription level

MLDA and RainbowMLDA and Rainbow

MLDA – Multicast Loss-Delay Based Adaption Algorithm (rate-based)– Same as LDA+, but performs rate calculation at

receiver

Rainbow (window-based)– Encode data with digital fountain– Receivers individually request packets based on

individual windows

ConclusionConclusion

Congestion is an important and complex problem

Many solutions of varying effectiveness and complexity for various applications

Areas of future research:– Methods of comparing protocols– Improve definitions of fairness, friendliness– Improve models of TCP traffic

Questions?Questions?