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Transcript of McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Chapter 23 Congestion Control and Quality of...
McGraw-Hill ©The McGraw-Hill Companies, Inc., 2004
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.
NoteNote::
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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.
NoteNote::
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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.
NoteNote::
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Figure 23.18 Token bucket
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The token bucket allows bursty traffic at a regulated maximum rate.
NoteNote::
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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.
NoteNote::
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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.
NoteNote::
