COMPUTER NETWORKS CHAPTER 3 NETWORK LAYER NOTES CSE 3RD year sem 1

CS503PC: COMPUTER NETWORKS
UNIT - III
Network Layer: Design issues, Routing algorithms: shortest path routing,
Flooding, Hierarchical routing, Broadcast, Multicast, distance vector routing,
Congestion Control Algorithms, Quality of Service, Internetworking, The
Network layer in the internet.
1

UNIT - III
The Network Layer
Routing Algorithms, Congestion Control Algorithms, QoS

Network Layer Design Issues
• Store-and-forward packet switching
• Services provided to transport layer
• Implementation of connectionless service
• Implementation of connection-oriented service
• Comparison of virtual-circuit and datagram networks

Store-and-Forward Packet Switching
The environment of the network layer protocols.
ISP’s equipment

Services Provided to the Transport Layer
1. Services independent of router technology.
2. Transport layer shielded from number, type, topology of routers.
3. Network addresses available to transport layer use uniform
numbering plan
– even across LANs and WANs

Implementation of Connectionless Service
Routing within a datagram network
ISP’s equipment
A’s table (initially) A’s table (later) C’s Table E’s Table

Implementation of Connection-Oriented Service
Routing within a virtual-circuit network
ISP’s equipment
A’s table C’s T
able E’s Table

Comparison of Virtual-Circuit and Datagram Networks
Comparison of datagram and virtual-circuit networks

Routing Algorithms
• Routing is a process of transferring the packets from source m/c to destination machine, while
routing algorithms (RA) are s/w responsible for deciding which outgoing line an incoming
packet should be transmitted on.
• The purpose of RA is to decide which route is to be followed by a packet on the basis of
following parameters:
• Availability of channels (paths/links/routes)
• Link transmission delay
• Traffic intensity, and
• Capacity (bandwidth) of the link

Routing Algorithms
• On the basis of nature of information used in the algorithm, Routing Algorithms are categorized into
two classes:
• Non-Adaptive: These do not base their routing decisions on the measurements/estimates
of current traffic or topology, instead the choice of route to get from node ‘I’ to node ‘j’ is
computed in advance or off-line, and hence also called as ‘Static Routing’ or ‘Pre-
determined Routing’.
• Adaptive: It attempts to change their routing decisions to reflect changes in topology and
the current traffic.
•
•
•
• On the basis of scope of information used in the algorithm, Routing Algorithms are classified into
three types:
Centralized Routing: The global algorithm uses information collected from the entire
subnet in an attempt to make optimal decision.
Isolated Routing: The local algorithm runs separately on each IMP(Interface Message
Processor) and uses information only available there e.g., queue length.
DistributedRouting: These use information availablelocally as well as information
available at their neighbors.

Routing Algorithms
Properties of the R.A.
 Simplicity: The algorithm should not use very complex features.
 Correctness: The algorithm should clearly say about the start and end of the route searching.
 Robustness: The algorithms should either be capable of correcting smaller mistakes or displaying
message to the user to correct errors (Detection and Corrections)
 Stability: In the event of failure of one or few IMPs, the total system should not be crashed or down.
 Fairness: Nothing should be ambiguous, everything should be stated clearly and the algorithms should
not lead to congestion.
 Optimality: The algorithms should ensure to minimize the mean packet delay time as well as maximize
the total throughput of the network (maximum number if message transmission with minimum delay).

Routing Algorithms
• Optimality principle
• Shortest path algorithm
• Flooding
• Distance vector routing
• Link state routing
• Routing in ad hoc networks
• Broadcast routing
• Multicast routing
• Anycast routing
• Routing for mobile hosts
• Routing in ad hoc networks

The Optimality Principle
• If Path from A to B is Optimal then, If any Router exist between A & B i.e C
then path from A to C and C to B will be Optimal
A
B
A
B
C
Optimal Path
Optimal Path Optimal Path

The Optimality Principle
(a) A network. (b) A sink tree for router B.
• The goal of routing algorithms is to discover and use the sink trees for all routers.

Shortest Path Algorithm (1)
This algorithm finds the shortest path between any two given nodes on the basis
of any of the following (metrics/functions):
• Number of hops
• Distance
• Avg. traffic
In general, the labels of the arcs can be function of distance, bandwidth, average traffic,
communication cost, mean queuing delay or transmission delay. Many algorithms may be designed
using these parameters.
One such algorithms is designed by Dijkstra (1959) to determine the shortest path between two
nodes.
• Communication cost
• bandwidth
• Geographicaldistance
• Mean queuing delay, etc.,.

Steps of Dijkstra’s SPA algorithm to determine the shortest path between two nodes:
1. Initially, no path is known. So all the nodes are labeled as at an infinite distance from source
node.
2. As the algorithm proceeds, the labels of the nodes changes accordingly reflecting a better path
from the given source to the given sink.
3. Start from a node, and examine all adjacent node(s) to it. If the sum of labels of nodes and
distance from working node to the node being examined is less than the label on that node, then
we have a shortest path, and the node is re-labeled. In a similar fashion, all the adjacent nodes to
the working node are inspected and the tentative labels are changed. If possible the entire
graph is searched for tentatively labeled nodes with the smallest value, the node is made the
permanent node. With the progress of the algorithm, all permanent nodes are encircled, so the
shortest path could be reconstructed.

The first five steps used in computing the shortest path from A to
D. The arrows indicate the working node
L(B) = min(∞, 0+2)= 2
L(G) = min(∞, 0+6)= 6
L(C) = min(∞, 2+7)=9
L(E) = min(∞, 2+2)= 4
L(F) = min(∞, 4+2)= 6
L(G) = min(∞, 4+1)= 5

. . .
Dijkstra’s algorithm to compute the shortest path through a graph.

. . .
. . .

. . .

Shortest Path Algorithm
Disadvantages:
• Total traffic is routed via the calculated single
path, which may lead to congestion.
• Sometimes, there exists some more paths that
are equally good, but packets can’t be routed
through these paths to reduce congestion.
Solution:
To overcome these problems, a new algorithm
called Multipath Algorithm, was designed by
Evan (1975).

Other Routing Algorithms
a) Baran’s Hot Potato Algorithm (1964): Isolated routing, also called as Shortest
Queue Algorithm. As soon as a node receives a packet, it tries to get rid of it by
putting/forwarding it to the line having shortest queue.
b) Flooding: Each incoming packet is forwarded to every outgoing line, thereby
ensuring shortest path and shortest transmission delay. But, operative for very
low traffic condition.
c) Centralized Routing using RCC: All IMPs in the network periodically sends
information to Routing Control Centre (RCC), regarding their queue length,
delay offered, list of
neighbors that are up, etc. Based on these
information, RCC computes the routing table
global
and
distributes to all IMPs. It was used in TYMNET in 1971.

(a) A network.
(b) Input from A, I, H, K, and the new routing table for J.
Distance Vector Routing

COMPUTER NETWORKS CHAPTER 3 NETWORK LAYER NOTES CSE 3RD year sem 1

1. The Count-to-Infinity Problems
2. Delay metric was queue length, it did not take line bandwidth into account,
when choosing routes. Initially all the lines were 50 kbps, and hence no
problem, but later some were upgraded to 230 kbps, and others to 1.544 Mbps.

Link State Routing (IS-IS, OSPF)
1. Discover neighbors, learn network addresses (Hello).
2. Set/Measure distance/cost metric to each neighbor (Echo).
3. Construct packet telling all it has learned.
4. Send packet to, receive packets from other routers (trickiest
part, flooding(to check flooding, packet seq. no. and age are
used)).
5. Compute shortest path to every other router (Dijkstra’s
Algorithm).

Learning about the Neighbors (1)
Nine routers and a broadcast LAN.

Learning about the Neighbors (2)
A graph model of previous slide.

Building Link State Packets
(a) A network. (b) The link state packets for this network.

Distributing the Link State Packets
The packet buffer for router B in previous slide

Hierarchical Routing
Hierarchical routing.

Broadcast Routing
Reverse path forwarding. (a) A network. (b) A sink tree.
(c) The tree built by reverse path forwarding.

Multicast Routing (1)
(a) A network. (b) A spanning tree for the leftmost router. (c) A
multicast tree for group 1. (d) A multicast tree for group 2.

Multicast Routing (2)
(a) Core-based tree for group 1.
(b) Sending to group 1.

Anycast Routing
(a) Anycast routes to group 1.
(b) Topology seen by the routing protocol.

Routing for Mobile Hosts
Packet routing for mobile hosts

Routing in Ad Hoc Networks
(a) Range of A’s broadcast.
(b) After B and D receive it.
(c) After C, F, and G receive it.
(d) After E, H, and I receive it.
The shaded nodes are
new recipients. The
dashed lines show
possible reverse routes.
The solid lines show the
discovered route.

Congestion Control Algorithms (1)
• Approaches to congestion control
• Traffic-aware routing
• Admission control
• Traffic throttling
• Load shedding

Congestion Control Algorithms (2)
When too much traffic is offered, congestion sets in and
performance degrades sharply.

Approaches to Congestion Control
Timescales of approaches to congestion control

Traffic-Aware Routing
A network in which the East and West parts
are connected by two links.

Traffic Throttling (1)
(a) A congested network. (b) The portion of the network that is
not congested. A virtual circuit from A to B is also shown.

Traffic Throttling (2)
Explicit congestion notification

Load Shedding (1)
A choke packet that affects only the source..

Load Shedding (2)
A choke packet that affects each hop it passes through.

Quality of Service
• Application requirements
• Traffic shaping
• Packet scheduling
• Admission control
• Integrated services
• Differentiated services

Application Requirements (1)
How stringent the quality-of-service requirements are.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

Categories of QoS and Examples
1. Constant bit rate
• Telephony
2. Real-time variable bit rate
• Compressed videoconferencing
3. Non-real-time variable bit rate
• Watching a movie on demand
4. Available bit rate
• File transfer

Traffic Shaping (1)
(a) Shaping packets. (b) A leaky bucket. (c) A token bucket

Traffic Shaping (2)
(a) Traffic from a host. Output shaped by a token bucket of rate
200 Mbps and capacity (b) 9600 KB, (c) 0 KB.

Traffic Shaping (3)
Token bucket level for shaping with rate 200 Mbps and capacity
(d) 16000 KB, (e) 9600 KB, and (f) 0KB..

Packet Scheduling (1)
Kinds of resources can potentially be
reserved for different flows:
1. Bandwidth.
2. Buffer space.
3. CPU cycles.

Round-robin Fair Queuing

(a) Weighted Fair Queueing.
(b) Finishing times for the packets.

Admission Control (1)
An example flow specification

Admission Control (2)
Bandwidth and delay guarantees with token buckets and WFQ.

Integrated Services (1)
(a) A network. (b) The multicast spanning tree for host 1.
(c) The multicast spanning tree for host 2.

Integrated Services (2)
(a) Host 3 requests a channel to host 1. (b) Host 3 then
requests a second channel, to host 2.
(c) Host 5 requests a channel to host 1.

Differentiated Services (1)
Expedited packets experience a traffic-free network

Differentiated Services (2)
A possible implementation of assured forwarding

Internetworking
• How networks differ
• How networks can be connected
• Tunneling
• Internetwork routing
• Packet fragmentation

How Networks Differ
Some of the many ways networks can differ

How Networks Can Be Connected
(a) A packet crossing different networks.
(b) Network and link layer protocol processing.

Tunneling (1)
Tunneling a packet from Paris to London.

Tunneling (2)
Tunneling a car from France to England

Packet Fragmentation (1)
Packet size issues:
1. Hardware
2. Operating system
3. Protocols
4. Compliance with (inter)national standard.
5. Reduce error-induced retransmissions
6. Prevent packet occupying channel too long.

(a) Transparent fragmentation.
(b) Nontransparent fragmentation

Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.

Fragmentation when the elementary data size is 1 byte
(b) Fragments after passing through a network
with maximum packet size of 8 payload bytes plus header.

Fragmentation when the elementary data size is 1 byte
(c) Fragments after passing through a size 5 gateway.

Path MTU Discovery

The Network Layer Principles (1)
1. Make sure it works
2. Keep it simple
3. Make clear choices
4. Exploit modularity
5. Expect heterogeneity
. . .

The Network Layer Principles (2)
. . .
6. Avoid static options and parameters
7. Look for good design (not perfect)
8. Strict sending, tolerant receiving
9. Think about scalability
10.Consider performance and cost

The Network Layer in the Internet (1)
• The IP Version 4 Protocol
• IP Addresses
• IP Version 6
• Internet Control Protocols
• Label Switching and MPLS
• OSPF—An Interior Gateway Routing Protocol
• BGP—The Exterior Gateway Routing Protocol
• Internet Multicasting
• Mobile IP

The Network Layer in the Internet (2)
The Internet is an interconnected collection of many networks.

The IP Version 4 Protocol (1)
The IPv4 (Internet Protocol) header.

The IP Version 4 Protocol (2)
Some of the IP options.

IP Addresses (1)
An IP prefix.

IP Addresses (2)
Splitting an IP prefix into separate networks with subnetting.

IP Addresses (3)
A set of IP address assignments

IP Addresses (4)
Aggregation of IP prefixes

IP Addresses (5)
Longest matching prefix routing at the New York router.

IP Addresses (6)
IP address formats

IP Addresses (7)
Special IP addresses

IP Addresses (8)
Placement and operation of a NAT box.

IP Version 6 Goals
• Support billions of hosts
• Reduce routing table size
• Simplify protocol
• Better security
• Attention to type of service
• Aid multicasting
• Roaming host without changing address
• Allow future protocol evolution
• Permit coexistence of old, new protocols. . .

IP Version 6 (1)
The IPv6 fixed header (required).

IP Version 6 (2)
IPv6 extension headers

IP Version 6 (3)
The hop-by-hop extension header for
large datagrams (jumbograms).

IP Version 6 (4)
The extension header for routing.

Internet Control Protocols (1)
The principal ICMP message types.

Internet Control Protocols (2)
Two switched Ethernet LANs joined by a router

Label Switching and MPLS (1)
Transmitting a TCP segment using IP, MPLS, and PPP.

Label Switching and MPLS (2)
Forwarding an IP packet through an MPLS network

OSPF—An Interior Gateway
Routing Protocol (1)
An autonomous system

A graph representation of the previous slide.

The relation between ASes, backbones, and areas in OSPF.

The five types of OSPF messages

BGP—The Exterior Gateway
Examples of routing constraints:
1. No commercial traffic for educat. network
2. Never put Iraq on route starting at Pentagon
3. Choose cheaper network
4. Choose better performing network
5. Don’t go from Apple to Google to Apple

Routing policies between four Autonomous Systems

Propagation of BGP route advertisements

Mobile IP
Goals
1. Mobile host use home IP address anywhere.
2. No software changes to fixed hosts
3. No changes to router software, tables
4. Packets for mobile hosts – restrict detours
5. No overhead for mobile host at home.

COMPUTER NETWORKS CHAPTER 3 NETWORK LAYER NOTES CSE 3RD year sem 1

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