Network layer u3

NETWORK LAYER
TOPICS:
NETWORK LAYER
NETWORK LAYER DESIGN ISSUES
ROUTING ALGORITHMS,
M Indraja,Assistant Prof,RGUKT-Srikakulam. 1

What is network layer ?
It is responsible for the source to destination delivery of a packets across multiple networks.
Routing:-
It Provide a mechanism to transmit data over independent networks that are linked
together.
Logical addressing:-
Adds Logical addresses of sender and Receiver.
The network layer also defines how to fragment a packet into smaller packets to
accommodate different media.

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:
In this store and forward packet switching can be used for packet(message from
source to destination).
In this 1st design issues explore about the
 How routing was happening in the network ?
 How to store the packet in a router ? and
 Analysis to forward the packet by host to host.
 So, here we discuss about the ISP and different host networks.

Store-and-forward packet switching: (conti..)
 In this one of the major components of the network is ISP’s (Internet Services
Providers) equipment (routers connected by transmission lines).
 Host H1 is directly connected to one of the ISP’s routers. Host H2 is on a LAN, it
might be an office Ethernet, with a router “F” owned and operated by the
customer’s. The “F” is outside the oval because is does not belong to the ISP.

 The host with a packet to send transmits it to the nearest router, either on its own
LAN or over a point-to-point link to the ISP.
 The packet is stored there until it has fully arrived and the link has finished its
processing by verifying the checksum.
 Then it is forwarded to the next router along the path until it reaches the
destination host, where it is delivered. This mechanism is store-and-forward
packet switching.

Services Provided to the Transport Layer:
It provides services at network layer transport layer interface
It is the interface between carrier and customer i.e, the subnet boundary,
Its job is to deliver packets given to it by customers.
The network layer services are designed with the following goals:
 The Services should be independent of router technology.
 The transport layer should be shielded from number, type, topology of routers.
 The network addresses made available to the transport layer should use a uniform numbering plan,
even across LANs and WANs

Implementation of Connectionless Service:
If connectionless service is offered, packets are injected into the network individually and routed
independently of each other.
No advance setup is needed. In this context, the packets are frequently called datagrams (in analogy
with telegrams) and the network is called a datagram network.

 In this the process p1 has a long message for p2.So, in hands the message to the transport layer with
instructions to deliver it to process p2 on host h2. The transport layer sends data to network layer within
operating system.
 The above diagram and table representing the message is four times longer than the maximum packet size,
so the network layer has to break it into four packets, 1,2,3 and 4 and send each of the them in turn to
router A using some point-to-point protocol, for example, PPP.
 At this point the ISP takes over. Every router has an internal table telling it where to send packets for each
of the possible destinations. Each table entry is a pair consisting of a destination and the outgoing line to
use for that destination. Only directly connected lines can be used.

Implementation of Connection-Oriented Service:
 The implementation of connection-oriented service, we need a virtual circuit
network.
 A connection is established, a route from the source machine to the destination
machine is chosen as part of the connection setup and stored in tables inside the
routers.
 The router should maintain a traffic flow (it may like a telephone system works) .
 When the connection is released, the VC is also terminated. By connection-
oriented service, each packet carries a identifier telling which vc it belongs to.

If Host h1 has established connection
1 with host h2.The connection
remembered as the first entry in the
each of the routing tables.
So, the first line of A’s table says that
if a packet bearing connection
identifier 1 comes in from H1, it is to
be sent to router C and given
connection identifier 1.
 Similarly, the first entry at C routes
the packet to E, also with connection
identifier 1.

 let us consider what happens if H3 also wants to establish a connection to H2. It chooses
connection identifier 1 (because it is initiating the connection and this is its only
connection) and tells the network to establish the virtual circuit.
 This leads to the second row in the tables. Note that we have a conflict here because
although A can easily distinguish connection 1 packets from H1 from connection 1
packets from H3, C cannot do this. For this reason, A assigns a different connection
identifier to the outgoing traffic for the second connection.
 Avoiding conflicts of this kind is why routers need the ability to replace connection
identifiers in outgoing packets. This process is called label switching.
 Ex: A connection-oriented network service is MPLS (MultiProtocol Label Switching).It
is used within ISP networks in the Internet, with IP packets wrapped in an MPLS header
having a 20-bit connection identifier or label.

Comparison of Virtual-Circuit and Datagram Networks:
Both virtual circuits and datagrams have their supporters and their detractors.

Routing Algorithms:
 It is the process of selecting best paths in a network.
 Routing is performed for All kind of Networks.
Technically:
 It is the process of determining, selecting the best outgoing path that a packet has to take in a network.
Two Types of Routings:
 They are Static (Non Adaptive)
 Dynamic (Adaptive)

Static and Dynamic
Non Adaptive:
Routing decisions computed in advance, off-line and downloaded
Adaptive:
To changes in topology and Get Information (locally, from adjacent routers, from all routers)

Shortest Path Algorithm:
 Routing algorithms with a simple technique for computing optimal paths given a complete picture of the
network.
 The idea is to build a graph of the network, with each node of the graph representing a router.
 Each edge of the graph representing a communication line or link.
 When ever a shortest path is discovered, the table is updated. Such that it may be useful for the future.
 The easiest way to find out the shortest path is to count the no.of hops.
 The shortest path may be calculated by adding up the labels next to the nodes.These lables may be the
function of: distance, bandwidth, average traffic, communication cost, mean queue length and other factors.
 ABC is clearly much longer than ABE.

Flooding :
This adapts the technique in which every incoming packet is sent on every outgoing line except from which it arrived.
One problem with this is that packets may go in loop and as a result of which a node may receive duplicate packets.
These problems can be overcome with the help of sequence numbers, hop count and spanning tree.
Every packet is sent out on every outgoing line except the one it arrived at.

Distances Vector Routing:
 A distance vector routing algorithm operates by having each router maintain a table (i.e., a vector) giving the best known distance to
each destination and which link to use to get there. These tables are updated by exchanging information with the neighbors. Eventually,
every router knows the best link to reach each destination.
 The distance vector routing algorithm is sometimes called by other names, most commonly the distributed Bellman-Ford routing
algorithm, after the researchers who developed it (Bellman, 1957; and Ford and Fulkerson, 1962). It was the original ARPANET routing
algorithm and was also used in the Internet under the name RIP (Routing Information Protocol)
 In distance vector routing, each router maintains a routing table indexed by, and containing one entry for each router in the network. This
entry has two parts:
1. The preferred outgoing line to use for that destination and an estimate of the distance to that destination.
2. The distance might be measured as the number of hops or using another metric, as we discussed for computing shortest paths

Network layer u3

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