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Bài giảng Computer Networks 1 (Mạng máy tính 1) - Lecture 5: Network Layer (Cont’) - Phạm Trần Vũ

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Computer Networks 1  
(Mạng Máy Tính 1)  
Lectured by: Dr. Phạm Trần Vũ  
1
Lecture 5: Network Layer (cont’)  
Reference:  
Chapter 5 - “Computer Networks”,  
Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.  
2
Contents  
ꢀ The network layer design issues  
ꢀ Routing algorithms  
ꢀ Congestion control algorithms  
ꢀ Quality of services  
ꢀ Internetworking  
ꢀ The network layer in the Internet  
3
Congestion Control Algorithms  
• General Principles of Congestion Control  
• Congestion Prevention Policies  
• Congestion cuit Subnets  
• Congestion Control in Datagram Subnets  
• Load Shedding  
• Jitter Control  
4
Network Congestion  
When too much traffic is offered, congestion sets  
in and performance degrades sharply.  
5
General Principles of Congestion  
Control  
ꢀ Open loop solutions  
ꢁ
ꢁ
ꢁ
Solve the problems by good design  
Prevent ng  
Make decision without regard to state of the  
network  
ꢀ Closed loop solutions  
ꢁ
Using feedback loop  
6
Closed Loop Solutions –  
Three Part Feedback Loop  
ꢀ Monitor the system  
ꢁ
detect when and where congestion occurs.  
ꢀ Pass inforn can be  
taken.  
ꢀ Adjust system operation to correct the  
problem.  
7
Open Loop Solutions - Congestion  
Prevention Policies  
Policies that affect congestion.  
8
Congestion Control in Virtual-Circuit  
Subnets  
(a) A congen subnet,  
eliminates congestion and a virtual circuit from  
A to B.  
9
Congestion Control in Datagram  
Subnets  
ꢀ
Warning bit  
ꢁ
ꢁ
ꢁ
Routers use a bit in the packet’s header to signal the  
warning state.  
The receivthe packet’s  
header to the ACK message  
The source, on receiving ACK with warning bit will adjust  
transmission rate accordingly  
ꢀ
Choke Packets  
ꢁ
The router sends choke packet directly to the source  
host  
10  
Hop-by-Hop  
Choke Packets  
(a) A choke packet  
that affects only the  
source.  
(b) A choke packet  
that affects each  
hop it pas
through.  
Load Shedding  
ꢀ When routers are so heavily loaded with  
packets that they can’t handle any more,  
they just throw them away  
ꢀ Packets can be selected randomly or by  
using some selection strategy  
12  
Random Early Detection  
ꢀ It is more effective to detect and prevent  
congestion from happening  
ꢀ Routers md on their  
queues, if they predict that congestion is  
about to happen, they start to drop packets  
13  
Jitter Control  
Jitter: variation in packet arrival times  
(a) High jitter. (b) Low jitter.  
14  
Quality of Service  
• Requirements  
• Techniques for Achieving Good Quality of  
Service  
• Integrated Services  
• Differentiated Services  
• Label Switching and MPLS  
15  
Requirements  
How stringent the quality-of-service requirements are.  
16  
Techniques for Good QoS  
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Overprovisioning  
Buffering  
Traffic shaping  
The leak bucket algorithm  
Token bucket algorithm  
Resource reservation  
Admission control  
Proportional routing  
Packet scheduling  
17  
Buffering  
Smoothing the output stream by buffering packets.  
18  
The Leaky Bucket Algorithm  
(a) A leaky bucket with water. (b) a leaky bucket with packets.  
19  
The Token Bucket Algorithm  
20  
(a) Before. (b) After.  
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