Bài giảng Computer Networks 1 (Mạng máy tính 1) - Lecture 5: Network Layer - Phạm Trần Vũ
Lecture 5: Network Layer
Reference:
Chapter 5 - “Computer Networks”,
Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.
Contents
ꢀ The network layer design issues
ꢀ Routing algorithms
ꢀ Congestion control algorithms
ꢀ Quality of services
ꢀ Internetworking
ꢀ The network layer in the Internet
3
Store-and-Forward Packet
Switching – Router
The environment of the network layer protocols.
5
Services Provided to the Transport
Layer
ꢀ Network layer provides services to the
transport layer
ꢀ Goals of n
ꢂ
Independent of router technology
ꢂ
ꢂ
Network addresses available to the transport
layer should be uniformed and even across
LANs and WANs
6
Two Classes of Services in the
Network Layer
ꢀ
ꢀ
Connection less service
ꢂ
ꢂ
ꢂ
Packets are called datagrams
The subnet is called a datagram subnet
Packets may arrive at the destination by multiple paths
Connection oriented service
ꢂ
ꢂ
ꢂ
The connection is called Virtual Circuit
All packets arrive at the destination by the same route
7
Implementation of Connectionless
Service
Routing within a diagram subnet.
8
Implementation of Connection-
Oriented Service
Routing within a virtual-circuit subnet.
9
Comparison of Virtual-Circuit and
Datagram Subnets
10
Routing Algorithms
ꢀ To route packets from a source to a
destination
ꢀ Distinction between routing and forwarding
ꢂ
Routing: make decision on which route to use
ꢂ
ꢀ Two class of algorithms
ꢂ
Nonadaptive (static)
Adaptive
ꢂ
11
Fairness vs Optimality
Conflict between fairness and optimality.
12
The Optimality Principle
ꢀ If router J is on the optimal path from router I
to router K, then the optimal path from J to K
is also on the same route.
(a) A subnet. ter B.
13
Shortest Path Routing
ꢀ
Use Dijkstra algorithm
15
Flooding
ꢀ Incoming packets are sent to every outgoing
lines
ꢀ Generate vast numbers of duplicates
ꢀ Alternatives for improvement
ꢂ
ꢂ
ꢂ
Tracking packets sent
Use TTL (time-to-live)
Selective flooding
ꢀ Not practical in most applications
16
Distance Vector Routing (1)
ꢀ Also known as Bellman-Ford and Ford-Fulkerson
algorithm
ꢀ Originally was used in ARPANET
ꢀ Used in Inte
outgoing line to get there
neigbors
ꢀ Then, all routers recalculate their tables
17
Distance Vector Routing (2)
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.
18
Distance Vector Routing (3)
The count-to-infinity problem
a) initially, all routers are down
b) Initially, all routers are up, then A is down
19
Link State Routing
Each router must do the following:
ꢀ
Discover its neighbors, learn their network
address.
ꢀ
Measure the delay or cost to each of its
neighbors.
ꢀ
ꢀ
ꢀ
Construct a packet telling all it has just learned.
Send this packet to all other routers.
Compute the shortest path to every other router.
20
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