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The Network Layer
The main design goals for Network Layer services:
(see the book “Computer Networks by
Tanenbaum” P345).
Two kinds of subnets:
1- Datagram subnet (Internet community). The
subnet job should be delivering packets only.
No connection setup or reservation is
required. (Internet community).
2- Virtual Circuit subnet (VC). There should be
one reserved path from source to destination
to provide quality of service. (telephone
companies).
Example of the first kind is the internet. Second
kind example is ATM networks.
Connection less services:
1- Each packet is inserted to the network
individually and also the routers deal with
each packet individually.
2- Packets are called Datagrams.
3- Each router holds a routing table which allow
packets to move from source to destination.
4- Routing tables are updated and modified
based on the network conditions by ROUTING
algorithms.
5- No bandwidth is wasted.
6- Congestion can occur and quality of service is
harder to guarantee.
Connection oriented services:
1- In this kind the route from source to
destination should be established before
any packet is sent.
2- The connection is called virtual circuit (VC)
and the subnet is called a virtual circuit
subnet.
3- Avoid choosing a new path for every
packet. The path from source to destination
is fixed and established at the beginning of
the connection.
4- Provide quality of service and can avoid
congestions easily.
5- Network resources maybe wasted if nobody
is using the reserved line.
The Optimality Principle:
 Sink Tree: Gives the optimal paths from
several sources to one destination (the sink).
 There may be more than one sink tree at the
same time.
 Router algorithms aims to find and use sink
trees for different routes.
 Using sink trees will allow packets to be
delivered in finite number of hops.
Shortest Path Routing:
 Different ways to measure the path length
(number of hops, distance, time, cost.. etc).
 To put a standard way of measuring the
path length we put weight for each link.
This weight could be distance, cost… etc. or
any number of them.
See example Figure 5-7 page 353-354
(Tanenbaum)
Flooding Routing:
 Packets are sent on every outgoing link.
 Problems: can send multiple unneeded
packets. Packets may take loops.
 Solutions: to reduce outgoing lines be
estimating the packet direction
(selective flooding). Keep track of which
packet has been flooded so routers
avoid sending them again.
 Example of flooding useful applications
are:
o Military applications.
o Distributed databases.
o Wireless networks.
o For performance comparison with
other routing algorithms.
Distance Vector Routing:
 Dynamic routing: takes new network
conditions into account.
 Each router creates a table of two entries
(Destination, Line, Distance) for all
destinations.
 Then each router sends its table to neighbor
routers.
 When the network conditions change, the
affected router update its distance vector
table and distribute it again.
Link State Routing:
 Widely used in today’s networks.
 The procedure used in Link State Routing is
as following:
o Discovering neighbors and learning
their network address. This is done by
sending a HELLO packet to each directly
connected (point-to-point) router. The
receiver replies with its information
(address).
o Calculating the delay of each neighbor.
To calculate the delay, each router
sends an ECHO packet and the receiver
should reply so that the sender can
calculate how long it takes the packet
to travel from the sender to the
receiver (round trip time). The sender
then calculate the delay by dividing this
time by 2  delay = round trip time /2.
o Creating a packet to tell all neighbors
about the new findings. This packet has
three entries: The sender address, a list
of neighbors with delays for each one
and age.
o Sending the packet to neighbors. For
that the FLOODING technique is used.
However to solve the problem of having
loops each routers make sure it does
not send a flooding packet twice by
checking the source router and packet
sequence number.
o Computing shortest path to every other
router using Sorters Path Algorithm
(explained before).