NETWORK LAYER 3
Switching: Packet Switching- Internet protocol -
IPV4
RARP, ICMP, DHCP IPAddressing Subnetting - IPV6, ARP,
3.1 PACKET SWITCHING
Amechanism that allows us to interconnect
links to form a large
network. A multi-input, multi-output device which transfers
an input to one or more
packets from
outputs.
A switch's primary job is to receive
incoming packets on one of its
links and to transmit them on some other link. This function is referred
as
switching and forwarding
Switching
Circuit Packet
Switching Switching9
Virtual circuit
Datagram
approach approach
Figure 3.1: Classification of Switching
Circuit switching states that dedicated communication path is
allable between two stations. A circuit network is heavily dependent on
the
e
number of channel available. Example -
T'elephone landline connection
3.2 cOMPUTER NETWORKS
In packet switching. the message gets broken into small data packets.
Each packet is sent with a header addresses. This header address tells it
where its final destination is, so it knows where to go.
Packet switching is done by
*Datagram approach (Connectionless
Virtual circuit approach(Connection oriented)
3.1.1 Datagram Approach
Every packet contains enough information to enable any switch
to decide how to get it to destination
Every packet contains the complete destination address
Switch maintains a forwarding table (sometimes called a routing
table) which is used to forward a packet.
Characteristics of Connectionless (Datagram) Network
A host can send a packet anywhere at any time., since any packet
that turns up at the switch can be immediately forwarded
(assuming a correctly populated forwarding table)
When a host sends a packet, it has no way of knowing if the
network is capable of delivering it or if the destination host is
even up and running.
A switch or link failure might not have any serious effect on
communication if it is possible to find an alternate route around
the failure and update the forwarding table accordingly.
3.1.2 Virtual Circuit Approach
Widely used technique for packet switching. Uses the concept of
virtual circuit (VC). Also called a connection-oriented model.
First set up a virtual connection from the source host to the
destination host and then send the data. VC approach has two-stage
procesS such as
Connection setup phase
Data Transfer phase
3.3
NETWORKLAYER
nnectiona setup phase
the
Establish "connection state" in each of the switches between
source and destination hosts.
T h e connection s t a t e f o r a single connection c o n s i s t s ol an c
in the "VC table" in each switch through which the connection
passeS.
o lo
2
Switch 1 Switch 2 2
Switch 2
2 Host B
Host A
Figure 3.2: Example for Connection setup phase
&One entry in the VC table on a single switch contains.
A Virtual Circuit Identifier (VCI) that uniquely identifies the
connection at this switch and that will be carried inside the header
of the that belong to this connection.
packets
on which packets for this VC arrive at the
An incoming interface
switch.
A n outgoing interface in which packets for this VC leave the switch.
A potentially different VCI that will be used for outgoing packets.
Table 3.1: VC Table for above diagramm
Incoming Incoming Outgoing Outgoing
S.N. Switch ID Interface VCI Interface VCI
2 5 1 11
Switch l
2. 3 11 2 7
Switch 2
7
3 Switch 3 1 4
3.4 OMPUTER NETWORKS
Two broad classes of approach to establishing connection state
Network Administrator will configure the state
The virtual circuit is permanent (PVC)
The network administrator can delete this
Can be thought of as a long-lived or administratively configured v
* A host can send
be established
messages into the network to cause the statetoto
This is referred as signaling and the resulting virtual circuit
is said to be switched (SVC)
A host may set up and delete such VC
a
dynamically without
the involvement of a network administrator.
Data Transfer Phase
For any packet that A wants to send to
B, A puts the VCI value 5 in
the header of the packet and sends it to
switch 1.
o
Switch 1
Switch 2 2
Switch 3
Host A 2
Host B
Figure 3.3: Example for Data Transfer
phase setup
Switch 1 receives any such
packet on interface 2, and it uses tnc
combination of the interface and the VCI in the packet header to find ue
appropriate VC table entry.
The table entry on switch 1 tells the switch to forward the packet out
of interface 1 and to put the VCI value 11 in the header.
NETWORK LAYER 3.5
Table 3.2: Difference between Datagram and Virtual circuit approac
Issues
SI No. Datagram network Virtual- circuit network
Circuit setup Not needed
Required
Addressing Each packet contains the full Each packet contains a short
source and destination address VC number
State infomation Routers do not hold state Each VC requires router table
infomation about connections space per connection
Routing Each packet is routed Route chosen when VC is
independently setup: all packets follow it
5 Effect of router failures None, except for packets lost All VCs that passed through
during the crash the failed router are terminated
6Qualityof service Difficult Easy if enough resources can
be allocated in advance for
each VC
Congestion control Difficult Easy if enough resources can
be allocated in advance for
each VC
Table 3.3: Difference between Circuit Switching and Packet Switching
SI.No. Circuit switching Packet switching
Guaranteed capacity No guarantees (best effort)
2. is wasted if data is bursty More efficient
Capacity
3 Before sending data establishes a path Send data immediately
4 All data in a single flow follow one path Different packets might follow different paths
5. No reordering; constant delay; Packets may be reordered, delayed or
no pkt drops dropped
