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Routing: Internet Routing Protocols and Algorithms
Research · September 2015
DOI: 10.13140/RG.2.1.4341.1680
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Routing: Internet Routing Protocols and Algorithms
Jannatul Ferdaus & Rabia Salihi
Asian University for Women
Abstract:
Routing is the process of finding the best path for transferring the IP Packets from source
to destination. There are several practical uses of routing in networking such as routing for
Telephone networks, routing for Internet, and routing for VoIP communication. In this paper, we
focus on Internet routing architecture including some popular routing protocols such as Routing
Information Protocol (RIP), Open Shortest Path First Protocol (OSPF), and Border Gateway
Protocol (BGP) and two algorithms which are Link State Routing Algorithm and Distance
Vector Routing Algorithm. We also include a brief explanation of router architecture and
unicast, multicast, and broadcast routing.
Introduction:
Suppose you want to send a letter to your friend who is in another city or country.
Basically, you‟d choose a service that takes the shortest path (takes less time/delay) or you‟d
choose a path that is less expensive but takes more time. Depending on your situation, you‟d take
a path that is best for you. The same as in everyday life, communication devices as well have to
deal with choosing a path or route to send information from one node (device) to another.
Routing, also referred to as forwarding network in the past, is the process to find the best path
(usually shortest path) from a source to destination. Just as a person would travel through roads
with different traffic regulations, and taking different steps and using different kinds of maps.
Routing can have different processes depending on different rules (protocols) and steps
(algorithm) having different maps (tables or trees). In short, the points to be taken into
consideration in routing is which route to take, is the route the shortest and therefore in case the
route faces some problem how to solve it.
Depending on different factors, there are different types of routing. Basically, there are
two types of routing: source routing and hop-to-hop routing. In source routing, the source
collects all the information about the route and then sends the message (packet) containing the
full information [4]. However, in hop-to-hop routing the source is not responsible for all the
2
information about getting from source to destination. Instead it can send the packet to next hop
(node) and the next hope will take care of the hop after that and so on [4]. The most important
terms in the routing process are the routing table, routing protocols and routing algorithms.
Routing table has the information about different routes and based on routing protocols each
node and router will decide which route to take which differs based on different algorithms or
steps. In the process of routing, router plays a main role.
Autonomous System: Fig 1
Autonomous Systems:
An autonomous system or domain is
consisted of several networks and routers
under the authority of a single administration
[7]. Each domain uses Interior gateway
protocol (IGP) to route packets through the
routers inside the domain (for example:
Routing packets for Local Area Networks
(LAN)) and inter-domain routing protocol to route packets to other domain (for example:
Routing packets for Wide Area Network (WAN) [7].
Autonomous System Number:
Each autonomous system is uniquely identified through autonomous system number
(ASN). An ASN is the 16 bit number. There can be 65,536 possible values for numbering
Autonomous Systems. Within these possible values, there are different ranges of ASN for
different types of use such as: ASN for internet routing, ASN for identifying non-routed
networks etc.
ASN range
0
Use
Reserved ASN. Used for identifying nonrouted networks.
1 - 64,511 (except 23,456)
Internet routing
3
23,456
Pool transition
64,512 - 65,534
Private Use
65,535
Reserved
Router Architecture:
Routers are computers [11] having both hardware and software components. It has CPU,
RAM, ROM, NVRAM, Flash Memory, and Operating System and the four main components are
input ports, switching fabric, output port and routing processor. Router is an intermediate device
in the process of routing. Back to our example of traveling, a person might need a vehicle for
going from one place to another. Routing also needs a router to deliver a packet from one node to
another across a network and therefore the router plays a similar role as a vehicle. Basically, a
router connects networks together and is responsible for delivering a packet in a timely and
efficient manner. Therefore, the efficiency of the delivery depends on router which chooses the
best path [15]. In short, a router is responsible to choose the best path and also to forward the
packets towards their destinations.
Routing Table:
Router has a routing table which contains the information about all the routes. It is
initially the memory where the physical addresses of all neighboring routers and therefore the
node to node address of
frames are saved for
delivery. It is a router‟s
memory which records
the physical addresses of
its neighbor routers and
destination address of
frames for node to node
delivery. There are two
types of routing table.
Static routing table is
updated manually while
Routing Table-Fig 2
4
dynamic routing table updates automatically. In the routing process the routing table is analyzed
for choosing the shortest (best) path.
Routing Protocols:
A routing protocol is the software part in the routing process which is assigned to an
interface. It is part of the router‟s operating system which is used for creating routing tables and
updating them. Just as there are different traffic laws and regulations when a person drives on
different road (routes) anywhere, a router also has terms based on which it decides the
information it should keep and the routes it should take with the help of the routing table. A
routing protocol sets the standards for the exchange of information between nodes. It determines
how the errors should be checked and how the sender should show that it has finished sending
and for the receiver if it has received the message (packet). Basically, it is an “agreed upon
format for transmitting data” between two nodes or devices on a network. Depending on being
simple, reliable or faster, each routing protocol has it‟s own advantages and disadvantages [11].
Different routing protocols use different routing algorithms. The most popular routing protocols
are Distance Vector, Link State and Path Vector routing protocol where the first two are intradomain and the last one is for inter-domain routing. An intra-domain routing connects the nodes
and devices within a network or autonomous system and inter-domain routing connects different
networks or autonomous systems. There are basically two types of routing which will be
discussed in this paper: uni-casting and multicasting.
Unicasting:
First of all, in unicasting there is only one sender and one receiver. Therefore, the router uses
only one of its interfaces for forwarding a packet towards its next destination [12].
Multicasting:
In multicasting at least one sender sends the message (packet) to multiple receivers which is
called a multicast group. Suppose, for example, Google Chrome needs to be updated. As an
update alert, the browser‟s administrators will send a message to all its users. It will multicast the
message to a group of receivers and for that the router will use several of its interfaces for
forwarding the message [12]. The basic requirements in this type of routing is to find the
„optimal‟ (shortest path) where one copy of the packet should be received by the members while
the non-members should not receive any and that the packet should not visit a router more than
5
once which will create loops[12]. Multicasting is usually used for streaming media such as
online video news or transfer of audio or video in a live lecture to a set of participants and
Internet television applications [9]. Just as in unicasting tables are analyzed to find the best path,
in multicasting trees are used which are of two types: source-based trees and group-shared trees.
Broadcasting:
Broadcasting is when one sender sends a packet to everyone on the network. Therefore, if there
are N nodes in a network, there will be N receivers. In broadcasting, when a sender duplicates N
copies of the packet and then send it to all the nodes which is also called source duplication. On
the other hand, send one copy of the packet to the next node and therefore that node makes
another copy and sends it to all the nodes it is directly connected to. This type of duplicate
creation is called in-network duplication [11].
Internet Routing Architecture:
The definition of Internet is network of networks or connection between different
networks. It means there will be connection or network between millions of computers or hosts
through many links and intermediate nodes such as switch, routers. However, it is really difficult
to manage routing with so many nodes, so there is a two level hierarchy of internet routing which
are routing inside a domain or autonomous system and inter-domain routing system [10].
Internet Routing
Intra-domain / Intra-AS routing
Inter-domain Routing.
Intra-domain / Intra-AS routing: (Routing Protocols):
Usually in intra-domain routing, the routers use the interior gateway protocols for routing
the packets. The two most common routing protocols are Routing Information Protocol (RIP),
Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System to Intermediate
System (IS-IS), and Open Shortest Path First (OSPF) protocol. RIP and EIGRP are the
implemented routing protocol of distance vector routing protocol. IS-IS and OSPF are based on
Link State Routing Protocol [13].
6
Distance Vector Routing Protocol:
Distance vector routing protocol uses routing table which has both distance and direction.
The router table always includes the minimum distance for packet transferring using hop count
as the metric, so the minimum cost is the minimum number of hops or routers for transferring the
packets from source to destination [2]. Each node periodically shares its routing table with its
immediate neighbors [1].
Usually routers create and update their routing table using some algorithm. One of the most
common algorithms is bellman-ford algorithm [2].
Bellman-ford Algorithm:
Bellman ford is an asynchronous, iterative, self termination, and distributed algorithm.
Asynchronous: In this algorithm, there is no serial for updating routing table, all the
routers can update their routing table at the same time.
Iterative and self termination: The algorithm continues or iterates until all the routers
have same topology. When all the routers have same topology, it stops the iteration
automatically. There is no need of any specific signal for its termination which means the
algorithm is self terminating.
Distributed: After updating routing table, each node distributes it to its immediate
neighbors.
The routers find the shortest path between two nodes through the Bellman ford equation.
Dx (y) = minv {c(x, v) + dv(y)}
Hence, Dx (y) is the least-cost path from node x to node y. c(x, v) is the cost and dv(y) is
the distance.
The use of bellman ford equation is
given below with an example of three
nodes: x, y, z through updating, and sharing
their routing table in three iterations [10].
7
Iteration 0 / Initial Iteration:
Routing table for Node x
x
x
y
0
Routing table for Node y
Z
x
2
7
x
y
∞
∞
∞
y
z
∞
∞
∞
z
∞
2
∞
y
Routing table for Node z
Z
X
y
z
∞
∞
x
∞
∞
∞
0
1
y
∞
∞
∞
∞
∞
z
1
0
7
Initially, each node does not its routing table with one another, so the cost for neighbor nodes is
considered as infinity. Each node shares their initial routing table to their neighbor nodes. Hence
x share its table with y and z. y share its table with x and z. z share its table with y and x.
Iteration 1 / Second Iteration:
Second Iteration for node x:
After Iteration 0, the node x has routing table of y and z. Now node x will update its routing table
using the table of node y and z and using bellman-ford equation.
Dx(x) = 0
Dx (y) = min{c(x, y) + Dy (y), c(x, z) + Dz (y)} = min {2 + 0, 7 + 1} = 2
Dx (z) = min{c(x, y) + Dy (z), c(x, z) + Dz (z)} = min {2 + 1, 7 + 0} = 3
Routing table for Node x
x
X
0
y
Routing table for Node y
z
x
2
3
x
Y
∞
∞
∞
y
Z
∞
∞
∞
z
∞
2
∞
y
Z
x
y
Z
∞
∞
x
∞
∞
∞
0
1
y
∞
∞
∞
∞
∞
z
1
0
After the calculation, node x has the new updated routing table:
Routing table for Node x
Routing table for Node z
7
8
x
y
z
x
0
2
3
y
2
0
1
z
7
1
0
Node x will send this updated routing table to node y and node z.
Second Iteration for node y:
After Iteration 0, the node y has routing table of x and z. Now node y will update its routing table
using the table of node x and z and using bellman-ford equation.
Dy (x) = min{c(y, x) + Dx (x), c(y, z) + Dz (x)} = min {2 + 0, 1 + 7} = 2
Dy(y) = 0
Dy (z) = min{c(y, z) + Dz (z), c(y, x) + Dx (z)} = min {1 + 0, 2 + 7} = 1
Routing table for Node y
x
x
∞
y
z
2
∞
y
Routing table for Node x
z
x
∞
∞
X
0
1
Y
∞
∞
Z
y
Routing table for Node z
z
x
y
z
2
7
x
∞
∞
∞
∞
∞
∞
y
∞
∞
∞
∞
∞
∞
z
1
0
0
After the calculation, node y has the new routing table:
7
Routing table for Node y
x
y
z
x
0
2
7
y
2
0
1
z
7
1
0
Node y will not send this routing table to its neighbors since
there is no change in the cost of y (2, 0, 1) in previous and new table.
9
Second Iteration for node z:
After Iteration 0, the node z has routing table of y and x. Now node z will update its routing table
using the table of node y and x and using bellman-ford equation.
Dz (x) = min{c (z, x) + Dx (x), c (z, y) + Dy (x)} = min {7 + 0, 1 + 2} = 3
Dz (y) = min{c (z, y) + Dy (y), c (z, x) + Dx (y)} = min {1 + 0, 7 + 2} = 1
Dz (z) = 0
Routing table for Node z
x
y
Routing table for Node x
z
x
x
∞
∞
∞
x
y
∞
∞
∞
y
1
0
z
z
3
0
y
Routing table for Node y
z
x
2
7
x
∞
∞
∞
y
∞
∞
∞
z
y
∞
2
∞
z
∞
∞
0
1
∞
∞
After the calculation, node z has the new updated routing
Routing table for Node z
table:
x
y
z
x
0
2
7
y
2
0
1
z
3
1
0
Node z will send this updated routing table to node y and node x.
Routing Table of node x,y,and z after second iteration:
Routing table for Node x
Routing table for Node y
Routing table for Node z
10
x
y
z
x
y
z
x
y
z
x
0
2
3
x
0
2
7
x
0
2
7
y
2
0
1
y
2
0
1
y
2
0
1
z
7
1
0
z
7
1
0
z
3
1
0
Since node x, y, and z has different routing table, the algorithm will continue.
Iteration 2 / Third Iteration:
Third Iteration for node x:
After Iteration 1, the node
Old Routing table for Node x
x has updated routing table
Routing table for Node z
of z. Now node x will
x
compare its own table with
the table of node z and
create an updated table
with minimum cost.
y
z
x
y
z
x
0
2
3
x
0
2
7
y
2
0
1
y
2
0
1
z
7
1
0
z
3
1
0
Node x will now have the updated routing table.
Routing table for Node x
x
y
z
x
0
2
3
y
2
0
1
z
3
1
0
Third Iteration for node y:
After Iteration 1, the node y has updated routing table of z and x. Now node y will compare its
own table with the table of node z and x and create an updated table with minimum cost.
Old Routing table for Node y
Routing table for Node z
Routing table for Node x
11
x
y
z
x
y
z
x
y
z
x
0
2
7
X
0
2
7
x
0
2
3
y
2
0
1
Y
2
0
1
y
2
0
1
z
7
1
0
Z
3
1
0
z
7
1
0
Node x will now have the updated routing table.
Routing table for Node y
x
y
z
x
0
2
3
y
2
0
1
z
3
1
0
Third Iteration for node z:
After Iteration 1, the node z has updated routing table of x. Now node z will compare its own
table with the table of node x and create an updated table with minimum cost.
Old Routing table for Node z
x
y
Routing table for Node x
z
x
y
z
x
0
2
7
x
0
2
3
y
2
0
1
y
2
0
1
z
3
1
0
z
7
1
0
Node z will now have the updated routing table.
12
Routing table for Node z
x
y
z
x
0
2
3
y
2
0
1
z
3
1
0
It seems after third iteration, node x, y, z have the similar routing table, so the iteration will stop
here.
Final Routing table for Node
y
x
y
Final Routing table for Node
Final Routing table for Node
z
x
z
x
y
z
x
y
z
x
0
2
3
x
0
2
3
x
0
2
3
y
2
0
1
y
2
0
1
y
2
0
1
z
3
1
0
z
3
1
0
z
3
1
0
Figure 3: Summarized Table for Bellman ford Algorithm[10]
13
Routing Information Protocol:
Routing Information protocol is an implemented version of distance vector routing
protocol which is used inside an autonomous system [1]. This protocol is usually used for Local
Area Network (LAN). RIP uses hop count as the cost or metric during creating and updating
routing table. The maximum number of hop for RIP is 15. It means if there is 16 hop
requirements for packet transfer, the cost is infinity. Every 30s each router sends it updated
routing table to its immediate neighbors [14]. The maximum receiving time for routing table is
180s. If the receiver router does not get any updated routing table from sender router within
180s, it considers the sender router as down and consider cost for that router as 16 or infinity [6].
When one router sends its updated routing table to its neighbor routers, the neighbor
routers in turn pass the tables to the next neighbor routers and it continues until the state of
network convergence (a state when all the routers of an AS has the same routing topology)[14].
There are two version of RIP: RIP Version 1 and RIP Version 2.
Table 2: Difference between RIP Version1 and RIP Version 2[5]:
RIP Version 1
It is a Distance Vector Routing Protocol.
RIP Version 2
It is a hybrid (Distance vector + Link State)
routing protocol.
It is a classful routing protocol
It is a classless routing protocol.
It does not support VLSM (Variable Length
It supports VLSM.
Subnet Masking)
RIPV1 uses broadcasting.
RIPV2 uses multicasting.
It does not support authentication.
RIPV2 supports the authentication which helps
to confirm that the updated routing tables are
from authorized sources.
14
RIP Routing Table: Fig 4
Routing table in RIP:
RIP routing table usually has three columns. The
first column contains destination ip address, the second
column is about metric/cost which is hop count, and the
third column contains next node ip address for
transferring the packet.
[1]
Link State Routing Protocol:
Link State routing protocol is an intra-domain routing protocol which is used for
networks in several areas under one domain. In link state routing, the routers create Link State
Packet (LSP) periodically after 1-2 hours.The LSP contains info of node identity, list of links,
sequence number, and age. After creating LSP, the routers send it to all other routers. The router
creates shortest path tree using Dijkstra algorithm. The router calculate routing table from
shortest path tree.
Dijkstra Algorithm:
This algorithm creates a shortest path
tree from a graph. In this algorithm, one
node is considered as root node. All the
other nodes can be reached from root node.
The figure is a graph where u is the
root node. All other nodes can be reached
from root u using shortest path through
dijkstra algorithm.
Figure 6: Steps of Dijkstra Algorithm:
Dijkstra Algorithm graph: Fig 5
15
Hence, D(v) is the cost of least cost path from source node to destination. P(V) is the previous
node or neighbor node. N‟ is the subset of nodes.
Steps of the algorithm:
1. Initially, the root node finds the least cost path from root to other nodes which are
directly attached with the root. If other nodes are not directly attached, the cost is set to
infinity.
2. Then the root node finds the neighbor node which has the minimum cost and adds it in
N‟.
3. Step 2 is continued until the last node is added to N‟[10].
Open Shortest Path First Routing Protocol (OSPF):
OSPF is one of the link state routing protocols. In OSPF, an autonomous system is
divided into several areas.
Each area has set of networks,
routers, and hosts. There is a
backbone area with a
backbone router in an
autonomous system which
acts as the primary area. The
rest of the areas are secondary
Autonomous System Areas
areas. Each secondary area has one area border router to connect to the backbone router. There is
a boundary router in AS to connect it to other AS.The cost or metric for OSPF is minimum delay
and maximum bandwidth [1].
Inter-Domain routing: (Routing Protocols):
Inter-domain routing or routing between two or more autonomous system follows the
most common protocol named Border Gateway Protocol version 4 [9]. This protocol is the
implemented version of Path Vector Routing Protocol.
16
Table: Difference between RIP and OSPF.
RIP
OSPF
Implemented version of Distance Vector
Implemented Version of Link State Routing
Routing Protocol.
Protocol.
Bellman ford Algorithm
Dijkstra Algorithm
Router sends updated routing table to
Router generate new LSP periodically (in 1-2
neighbors every 30 sec.
hours)
Metric/Cost: number of hops. Max hop is 15.
Metric/Cost: Minimum Delay, Maximum
Throughput. Administrator can assign the cost.
Routing table is sent to neighbor nodes.
Routing table is sent to all nodes (not just
neighbors)
Path Vector Routing Protocol:
In path vector routing, each autonomous system has a speaker node which acts on behalf
of the whole AS. The speaker node creates a routing table and sends it to other speaker nodes of
immediate neighbor autonomous
Initial Routing Tables
systems. In fact, the speaker
node does not send any cost or
metric in path vector routing
table; rather it sends the path for
routing the packets.
In path vector routing,
initially each speaker nodes only
know the path of its own AS, so
initially the speaker node does
not know how to reach to other
AS[1]. When a speaker node
shares its routing table with its
Updated Routing Table after sharing routing table by speaker nodes
immediate neighbor Autonomous Systems‟ speaker node, the neighbor speaker node update their
routing tables by adding nodes that are not in its routing table and adding its own autonomous
17
system and the autonomous system that send the routing table[1].
Border Gateway Protocol:
Border Gateway protocol is a common protocol for packet transferring through internet.
It is an implemented version of path vector routing protocol. In BGP, the connection between the
speaker nodes of different AS is called a session. The connection or session is set up for sending
and receiving the routing table. The TCP level services are used for BGP session.
There are two types of session in BGP: External BGP session and Internal BGP session[1].
External BGP Session:
Interior and Exterior BGP session
This session is used for
exchanging routing
information between two
speaker nodes of two
different Autonomous
systems.
Internal BGP Session:
The I-BGP session is used
for exchanging routing
information between the routers inside an autonomous system[1].
Routing Table for BGP: In this protocol, the speaker nodes have minimum two column routing
table. The first column includes the
destination AS and the second column
includes the path/Intermediate AS to
reach the destination.
Conclusion:
Conclusion:
Routing is mainly the role of network layer in OSI model for packet transferring using
least cost. Routers are the intermediate nodes which route or direct the best path having least cost
(least number of hop, minimum delay, and maximum bandwidth) for packet transmission with
the help of its routing table. Routing table is created and updated based on some protocols. For
the packet transmission in LAN networks, some common intra-domain routing protocols are
used. Besides, some inter-domain routing protocols are used for routing in WAN or internet.
18
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