mcgraw-hill©the mcgraw-hill companies, inc., 2004 chapter 23 congestion control and quality of...

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Chapter 23

Congestion Controland

Quality of Service

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23.1 Data Traffic23.1 Data Traffic

Traffic Descriptor

Traffic Profiles

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Figure 23.1 Traffic descriptors

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Figure 23.2 Constant-bit-rate traffic

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Figure 23.3 Variable-bit-rate traffic

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Figure 23.4 Bursty traffic

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23.2 Congestion23.2 Congestion

Network Performance

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Figure 23.5 Incoming packet

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Figure 23.6 Packet delay and network load

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Figure 23.7 Throughput versus network load

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23.3 Congestion Control23.3 Congestion Control

Open LoopOpen Loop

Closed Loop

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23.4 Two Examples23.4 Two Examples

Congestion Control in TCP

Congestion Control in Frame Relay

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TCP assumes that the cause of a lost segment is due to congestion

in the network.

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If the cause of the lost segment is congestion, retransmission of the

segment does not remove the cause—it aggravates it.

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23.5 Quality of Service23.5 Quality of Service

Flow Characteristics

Flow Classes

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23.6 Techniques to Improve QoS23.6 Techniques to Improve QoS

Scheduling

Traffic Shaping

Resource Reservation

Admission Control

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Figure 23.12 Flow characteristics

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Figure 23.13 FIFO queue

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Figure 23.14 Priority queuing

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Figure 23.15 Weighted fair queuing

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Figure 23.16 Leaky bucket

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Figure 23.17 Leaky bucket implementation

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A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate. It may drop

the packets if the bucket is full.

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Figure 23.18 Token bucket

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The token bucket allows bursty traffic at a regulated maximum rate.

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23.7 Integrated Services23.7 Integrated Services

Signaling

Flow Specification

Admission

Service Classes

RSVP

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Integrated Services is a flow-based QoS model designed for IP.

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Figure 23.19 Path messages

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Figure 23.20 Resv messages

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Figure 23.21 Reservation merging

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Quality of service

QoS = provide the service the application needs

Requirements of common applications:

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QOS: techniques

No magic bullet: many techniques used Overprovisioning

Provide more than enough Router capacity, buffer space,bandwidth

Expensive Ex. Telephone system: dial tone nearly always there

Buffering Traffic shaping: leaky/token bucket Resource reservation Admission control Proportional routing Packet scheduling

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QOS: techniques Buffering

Buffer flow before delivering to application

Reduces jitter Increases delay

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QOS: techniques

Traffic shaping Smooth output traffic at server side Agreement between customer &

carrier Traffic pattern = SLA or Service Level Agreement

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QoS: techniques

Traffic shaping Main cause of congestion: bursty traffic Open loop solution:

force transmission at a predictable rate

Agreement between client and carrier Client: my traffic look like … Carrier: accepts and reserves resources

Carrier will monitor traffic generated by client Leaky bucket algorithm Token bucket algorithm

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QoS: techniques - leaky bucket

Approach: strict flow regulation

Transmit 1 packet /time unit

Finite queue

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Environment:

o Communication line supports: 25 MB/sec

o Limit set at: 2 MB/sec

o Bucket capacity: 1 MB

Example: Computer sends burst of 1 MB

o Completely stored in bucket

o Flow to network: 2 MB/sec for 500 msec

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Too rigid output pattern?

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QoS: techniques - token bucket

Approach: Bucket holds

tokens Token generated

at every clock tick Transmission of

packet destroys token

Bucket has limited capacity for tokens

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QoS: techniques - token bucket

Environment:

o Communication line supports: 25 MB/sec

o Limit set at: 2 MB/sec

o Bucket capacity?

Initially full

Max:

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Why not?

250 KB / 25 MB/sec = 10 msec

250 KB

500 KB

750 KB

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QoS: techniques - token bucket

Parameters:

o Communication line supports: M Bytes/sec

o token arrival rate: Bytes/sec

o Bucket capacity: C Bytes?

o Burst length: S sec

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25 MB/sec

2 MB / sec

250 KBS = C / ( M - )

C + S = M S

250 KB

500 KB

750 KB

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QoS: techniques - token bucket

Token bucket high peak = line transfer rate

Combined approach: Token bucket followed by leaky bucket Peak is limited by leaky bucket

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Resource reservation Regulate the shape of load is useless if

routers, lines are overloaded Scattering packets over various lines? Create route for a flow!! “connection oriented”

Bandwidth Never oversubscribe an output line

Buffer space Reserve some buffers per flow How to compute?

CPU cycles: mean delay T = 1 / ( - )

QoS: techniques

= mean processing capacity

= mean arrival rate

in packets/sec

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QoS: techniques Admission control

Only accept flows that can be handled!

Flow specification? How to transform into needed

resources?

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QoS: techniques

Proportional routing Split a flow over multiple paths How? Which info is available?

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QoS: techniques

Packet scheduling In router for each output line:

A queue for each router Round robin handling of queues Problem: favours large packets

Simulated byte-by byte round-robin Find finishing time for each packet Sort packets using increasing finishing times Send packets in that order Problem: same priority for all hosts

Weighted fair queueing Give some hosts more bytes per clock tick

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QoS: Techniques

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Network Layer

Design issues

Routing

Congestion

Quality of service

Internetworking

Internet Protocols

Requirements Techniques Integrated services Differentiated

services Label switching &

MPLS

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QoS: Integrated Services

Problem: Handle multicasts

From multiple senders To multiple destinations

Allow for dynamically changing groups Examples:

Video conferencing Television

RSVP: Resource reSerVation Protocol Multicast routing

Spanning tree Group address

example

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QoS: Integrated Services

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QoS: Integrated Services

RSVP: Resource reSerVation Protocol Multicast routing

Spanning tree Group address

Example Receivers can send reservation messages

up the tree to the sender To get better reception To eliminate congestion

Example Sharing of channels possible

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QoS: Integrated Services

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QoS: Integrated Services

Conclusion for flow-based algorithms: Good quality of service possible Downside

Advanced setup needed State/flow in routers Complex algorithms in routers Complex router-to-router exchanges

Few implementations Solution: class-based QoSDifferentiated services

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Network Layer

Design issues

Routing

Congestion

Quality of service

Internetworking

Internet Protocols

Requirements Techniques Integrated services Differentiated

services Label switching &

MPLS

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Differentiated Services

Class-based Offered by a set of routers forming an

administrative domain Administration defines

Classes + forwarding rules Customers sign up (&pay) for a class

IETF has standardised an architecture Network-independent service classes

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Differentiated Services

2 classes only: expedited regular

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Reserved bandwidth for expedited class: 10% packets 20% bandwidth Low delay

Implementation: weighted fair queueing

Expedited forwarding

Most packets!

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Differentiated Services

4 priority classes Each having its own

resources 3 discard

probabilities: Low – Medium - High

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Assured forwarding Processing steps for a packet:

Classify it Mark it (header field needed) Discard? Queue packet

To be executed at host or ingress router

Most packets!

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Network Layer

Design issues

Routing

Congestion

Quality of service

Internetworking

Internet Protocols

Requirements Techniques Integrated services Differentiated

services Label switching &

MPLS

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Label switching & MPLS

How to improve forwarding in a router? Based on address: slow lookup Based on label: used a index in a table

Fast! Connections?

Various names: Label switching Tag switching

Standardization by IETF: MPLSMultiProtocol Label Switching

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Label switching & MPLS

Extra layer between Datalink Network

Belongs to … layer?

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Local signific

ance only

Fields in header Label QoS service classes S hierarchy of labels TTL

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Label switching & MPLS

Data-driven First packet (without a

flow) on router triggers construction of a FEC

Router Requests next router to assign a label; this router recursively does the same

Used on ATM networks

Control-driven Booting router creates

FECs for all the hosts for which it is the destination

Forwards the labels to its neighbours

Other variants exist

Construction of forwarding tables

FEC forwarding equivalence class group of flows under a single label

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Network Layer

Design issues Routing Congestion Quality of service Internetworking Internet Protocols

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Figure 23.22 Reservation styles

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23.8 Differentiated Services23.8 Differentiated Services

An Alternative to An Alternative to Integrated ServicesIntegrated Services

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Differentiated Services is a class-based QoS model designed for IP.

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