Routing is the process of selecting best paths in a network. In the past, the term routing was also used to mean forwarding network traffic among networks. However this latter function is much better described as simply forwarding. Routing is performed for many kinds of networks, including the telephone network (circuit switching), electronic data networks (such as the Internet), and transportation networks. In packet switching networks, routing directs packet forwarding (the transit of logically addressed network packets from their source toward their ultimate destination) through intermediate nodes. Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls, or switches. General-purpose computers can also forward packets and perform routing, though they are not specialized hardware and may suffer from limited performance. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the router's memory, is very important for efficient routing. Most routing algorithms use only one network path at a time. Multipath routing techniques enable the use of multiple alternative paths.

1. Advance Computer Networks:
Default Routing And On demand Routing

2. Overview:
What is routing?
What is default routing?
Examples of default routing
What is On demand routing?
Examples of On Demand routing
Conclusion
Bibliography

3. Introduction:
 Process of selecting best paths in a
network.
 Routing is performed for many kinds
of networks, including the
telephone network (circuit switching
), electronic data networks (such as
the Internet), and
transportation networks.
 The routing process usually directs
forwarding on the basis of
routing tables which maintain a
record of the routes to various
network destinations.

4. What is default routing?
Is configured for unknown destinations.
Generally used on internet as destinations are unknown.
• Example: ID on gmail or yahoo, etc
Configured at end points.
It is last preferred routing.

5.  If there is no static route or learned route, the Network
Layer checks for a default route.

6. The Role of the Default Route
►Maintaining a list of all possible destinations
►Maintains memory required to store thousands of IP addresses,
each four octets long.
►Detailed information about networks within the corporate
network, so it can forward inbound packets correctly and only
single router is represented over internet.

7. Default router in IP
In IP, the default route is also known as the default gateway or
the gateway of last resort.
The default route is represented using a special IP address (0.0.0.0)
and subnet mask (0.0.0.0).
In IP, routers and end stations use a simple rule when selecting a
route from the routing table: the best match wins.
Eg: 192.14.17.0, 192.14.0.0, 192.0.0.0, and 0.0.0.0, and packet
arrived destined for 192.14.17.23, the route associated with
192.14.17.0 would be used.

8. Implementing the default routing.
IP routing table, the default route can be placed there statically, or
learned dynamically.
Typically entered statically by a network administrator in a router
that connects by a single path to the broader internetwork.

9. Example Implementing the default routing.

10. Introduction On Demand Routing Protocol
A protocol used to discover other Cisco devices on either broadcast
or non-broadcast media.
With the help of CDP, it is possible to find the device type, the IP
address, the Cisco IOS® version running on the neighbour Cisco
device, the capabilities of the neighbour device, and so on.
ODR is able to carry Variable Length Subnet Mask (VLSM)
information.

11. What is On Demand Routing Protocol?
The routes are created when required
The source has to discover a route to the destination.
The source and intermediate nodes have to maintain a route as long
as it is used.
Routes have to be repaired in case of topology changes.

12. On-Demand Routing Protocols
1. Ad hoc On-demand Distance Vector Routing
2. Dynamic Source Routing Protocol
3. Temporally Ordered Routing Algorithm
4. Associativity Based Routing

13. 1.Ad Hoc On-demand Distance Vector
Routing
Includes route discovery and route maintenance.
Minimizes the number of broadcasts by creating routes on-demand
Uses only symmetric links because the route reply packet follows
the reverse path of route request packet.
Uses hello messages to know its neighbors and to ensure symmetic
links.

14. Example.
source
RREQ
The node discards the packets having been seen
destination
The source broadcasts a route packet
The neighbors in turn broadcast the
packet till it reaches the destination
RREP
Reply packet follows the reverse path of route
request packet recorded in broadcast packetaa

15. Route Maintenance
If the source node moves, it reinitiates the route
discovery.
If intermediate node moves, its upstream node sends a
RREP to the source. The source restarts the route
discovery.

16. 2.Dynamic Source Routing Protocol
A node maintains route caches containing the routes it
knows.
Include route discovery and route maintenance.

17. Route discovery
The source sends a broadcast packet which contains
source address, destination address, request id and path.
If a host saw the packet before, discards it.
Otherwise, the route looks up its route caches to look
for a route to destination, If not find, appends its address
into the packet, rebroadcast,
 If finds a route in its route cache, sends a route reply
packet, which is sent to the source by route cache or the
route discovery.

18. 
source broadcasts a packet containing address of source and destination
The destination sends a reply packet to source.
destination
source
1
The node discards the packets having been seen
6
5
4
3
2
8
7
(1,4)
(1,2)
(1,3)
(1,3,5) (1,3,5,6)
(1,4,7)
The route looks up its route caches to look for a route to destination
If not find, appends its address into the packet

19. How to send a reply packet
 If the destination has a route to the source in its
route cache, use it
Else if symmetric links are supported, use the
reverse of route record
Else if symmetric links are not supported, the
destination initiates route discovery to source

20. Route maintenance
 Whenever a node transmits a data packet, a
route reply, or a route error, it must verify that
the next hop correctly receives the packet.
If not, the node must send a route error to the
node responsible for generating this route header
The source restart the route discovery

21. Add entries into route cache
 The Source and destination in route discovery
Intermediate hosts in route discovery
The hosts receiving any broadcast

22. 3.Temporally Order Routing
Algorithm
 Creating Routes: query/reply
• QRY packet is flooded through network
• UPD packet propagates back if route exist
Maintaining Routes: link-reversal
• UPD packets re-orient the route structure
Erasing Routes
• CLR packet is flood through network to erase invalid routes

23. The source broadcasts a QRY packet with height(D)=0, all others NULL
(0,0,0,3,a) (0,0,0,2,d)
a
QRY
Only the non-NULL node (destination) responds with a UPD packet.
f
e
d
c
QRY QRY
b
h
UPD
g
(-,-,-,-,d)
(-,-,-,-,b)
(-,-,-,-,c)
(-,,-,-,-e) (-,-,-,-,f)
(0,0,0,0,h)
(-,-,-,-,a)
(0,0,0,4,b)
(0,0,0,4,c)
(0,0,0,3,e) (0,0,0,2,f)
source
Dest.
((0-,,0-,,-0,,-1,g,g))
A node receiving a UPD sets its height to one more than UPD
Source receives a UPD with less height

24. TORA: Height metric
 Each node contains a quintuple
• Logical time of a link failure
• Unique ID of the node that defined the new
reference level
• Reflection indicator bit
• A propagation ordering parameter, height
• Unique ID of the node

25. Route Maintenance and Erasing
 No reaction necessary if all nodes still have downstream links.
A new reference level is defined if a node loses its last downstream
link.
Synchronized clock is important, accomplished via GPS or
algorithm such as Network Time Protocol.
CLR packet to be flooded to clear the invalid packet.

26. a
f
e
d
c
b
h
g
(0,0,0,0,h)
(0,0,0,4,b)
(0,0,0,4,c)
(0,0,0,3,e)
(0,0,0,2,f)
(0,0,0,3,a) (0,0,0,2,d)
Dest.
(0,0,0,1,g)
Link failure with no reaction

27. Re-establishing route after link failure
A new reference level is defined
f
e
d
(1,d,0,-2,s) ((10,,d0,,00,,-31,a,a))
c
b
h
g
(0,0,0,0,h)
(0,0,0,4,b)
(0,0,0,4,c)
(0,0,0,3,e)
(0,0,0,2,f)
(0,0,0,2,d)
0,0,0,4,s)
Dest.
(0,0,0,1,g)
(1,d,0,0,d)
UDP
s a
UDP

28. 4.Associativity Based Routing
 Each route keeps a associativity table
A high value of associativity tick indicates a low
state of node mobility
A route is selected based on associativity states of
nodes, finds the high value of associativity tick
(low mobility routes)

29. Associativity table
 All nodes generate periodic beacons
When a neighbor node receives a beacon, it increases
its associativity tick with respect to the sending node
in associativity table
Associativity ticks are reset when the neighbors of a
node or the node itself move out of proximity

30. Route Discovery
 The source broadcast a QRY message
Each intermediate node appends its address and
associativity ticks to QRY,
The destination can examine the associativity ticks to
select route. If the multiple paths have the same
overall degree of stability, select the minimum
number of hops

31. Route Erasing
 If the the route is no longer desired, the source may
not be aware of any route node changes because
partial reconstruction.
The source node initiates a route delete (RD)
broadcast to erase the invalid route.

32. Conclusion
Default Routing:
 Is configured for unknown destinations.
Generally used on internet as destinations are unknown.
Configured at end points.
It is last preferred routing.

33. Conclusion Contin….
On-Demand Routing:
In On-Demand Routing the routes are created when required.
The source and intermediate nodes have to maintain a route as
long as it is used.
The source has to discover a route to the destination.

34. Bibilography
 http://www.hill2dot0.com/wiki/index.php?title=Default_route
 http://www.cisco.com/c/en/us/tech/ip/on-demand-routing-odr/
index.html
 http://www.slideshare.net/kishoreeekumarrr/multi-static-routng-default-routing-
29830267?v=default&b=&from_search=2
 https://cs.wmich.edu/wsn/cs691_sp03/adhocrouting.ppt
 http://kiki.ee.ntu.edu.tw/mmnet1/adhoc/

36. Q & A?