Unicast Routing Protocols
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A routing protocol is a combination of rules and
procedures that lets routers in the internet inform
each other of changes.
It allows routers to share whatever they know about
the internet or their neighborhood
Autonomous System(AS)
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An autonomous system is group of networks and
routers under the authority of a single administration.
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Routing inside an autonomous system is referred to
as intradomain routing. Routing between
autonomous systems is referred to as interdomain
routing.
Autonomous systems
Popular routing protocols
Popular protocols
Intradomain
 Routing Information Protocol(RIP)
 Open Shortest Path First(OSPF)
Interdomain
 Border Gateway Protocol(BGP)
Distance vector routing
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In distance vector routing, the least cost route
between any two nodes is the route with
minimum distance. In this protocol each node
maintains a vector (table) of minimum
distances to every node
Distance vector routing tables
Initialization of tables in distance vector routing
Process
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Initializing
Sharing
 Periodically
 Triggered
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Updating
update
Issues
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Two node loop Instability
 Define
infinity
 Split horizon
 Split horizon and poison reverse
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Three node instability
Routing Information Protocol
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The Routing Information Protocol (RIP) is an
intradomain routing protocol used inside an
autonomous system. It is a very simple protocol based
on distance vector routing.
Encapsulated in UDP user datagram
Example of a domain using RIP
RIP message format
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Command
 Request—1
 Response—2
Request messages
Timers in RIP
Periodic Timer
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Each router has periodic timer that is randomly set a
number between 25 and 35. It counts down ;when
zero is reached, the update message is sent, and
the timer is randomly set once again.
Expiration Timer
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Expiration timer governs the validity of a route.
Every route has its own expiration timer
When a router receives update information for a
route , the expiration timer is set to 180s
If timer goes off , the route is considered expired
and the hop count of the route is set to 16
Garbage collection timer
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When a route becomes invalid, the garbage
collection timer is set to 120s
Set the hop count value to 16
When the timer goes off , the route is purged from
the table
RIP version 2 format
Link State Routing
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In link state routing, if each node in the
domain has the entire topology of the domain,
the node can use Dijkstra’s algorithm to build
a routing table.
Building routing tables
1)
2)
3)
4)
Creation of the states of the links by each node called
Link state Packet(LSP)
Dissemination of LSP to every other router called
flooding
Formation of a shortest path tree for each node
Calculation of a routing table based on the shortest
path tree
Link state packet
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Node identity
The list of links
A sequence number
Age
Open shortest path first(OSPF)
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The Open Shortest Path First (OSPF) protocol
is an intradomain routing protocol based on
link state routing. Its domain is also an
autonomous system.
OSPF
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Divides an autonomous system into areas
An area is a collection of networks , hosts, and
routers all connected within in an autonomous
system.
Routers inside an area flood the area with routing
information.
Area Border routers
At the border of an area, special routers called
area border routers summarize the information
about the area and send it to other area.
Backbone
Is a special area in an autonomous system. All other
areas must be connected to backbone.
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Areas in an autonomous system
Metric
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The metric can be based on a type of service Minimum
delay
 Maximum throughput
 Hop count
A router can have multiple routing tables each based on a
different type of service.
Types of Links
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In OSPF a connection is called a link
Point to point
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Connects two routers without any other host or router in between
Transient Link
is a network with several routers attached to it. The data can enter through any
of the routers and leave through any router
Sub link
Is a network that is connected to only one router
Virtual link
When the link between two routers is broken , the administration may create a
virtual link between them using a longer path that probably goes through
several routers
Types of OSPF packets
OSPF common header
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Version: 8 bit field current version 2
Type: 8 bit field: 5 types
Message length: 16 bit: length of total message including header
Source router IP addess: 32 bit: IP address of router that sends the
packet
Area identification: 32 bit: defines the area within which routing
taking place
Checksum: used for error detection: excluding authentication type
and data field
Authentication type: 16 bit: 0 or none and 1 for password
Authentication: 64 bit: value of authentication data
Link sate packet update
Link state update packet
LSA general header
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Link Stage age
E flag: set to 1 means stub area. Means only one router
between backbone area
T flag: set 1 means router can handle multiple service
Type : any of the five
Advertising router: IP of router of LSA
Link state sequence number
Link state checksum: it is calculated using fletchers
algorithm except for age
Length: length of the whole packet
Hello Message
OSPF Common Header
24 bytes type 1
Network Mask
Hello interval All 0s
E
T
Priority
Dead Interval
Designated router IP address
Backup Designated router IP address
Neighbor IP Address(repeated)
Hello Message
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Network Mask: 32 bit defines network mask
Hello interval: 16 bit: defines no of seconds
between the messages
E flag: if set – means stub area
T flag: if set – router supports multiple service
Priority: defines the priority of the router
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Dead Interval: 32 bit: the number of seconds that
must pass before a router assumes that a neighbor
is dead
Designated IP address: 32 bit:
Backup Designated IP address: 32 bit:
Neighbor IP address
Database description message
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When hello message is received for the first time
from a router, the receiver router sends database
description message.
Only outline
Database description packet
OSPF Common Header
24 bytes type 2
All 0s
All 0s
E
B
All 0s
I
Message Sequence Number
LSA Header(20 Bytes) [repeated]
M
M/S
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E flag: advertising router is an autonomous
boundary router
B flag: advertising router is area boundary router
M flag: more flag, not the last message
M/S: master/Slave : origin of the packet
LSA Header:
Link state request packet
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Sent by a router that needs information about a
specific route or routes
Answered with a link state update packet
OSPF Common Header
24 bytes type 3
Link state type
Link state ID
Advertising router
Link state Acknowledgement Packet
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Every router must acknowledge the receipt of every
link state update packet.
OSPF Common Header
24 bytes type 5
LSA General Header
Path vector routing
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Path vector routing is similar to distance
vector routing. There is at least one node,
called the speaker node, in each AS that
creates a routing table and advertises it to
speaker nodes in the neighboring ASs..
Initial routing tables in path vector routing
Stabilized tables for four autonomous systems
Border Gateway Protocol(BGP)
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Border Gateway Protocol (BGP) is an
interdomain routing protocol using path vector
routing. It first appeared in 1989 and has gone
through four versions.
Types of AS
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Three types
Stub AS
Multihomed AS
Transient AS
BGP Sessions
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A session is a connection that is established between
two BGP.
Uses services of TCP
Types sessions
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External BGP
Internal BGP
E-BGP session is used to exchange information between
two speaker nodes
I-BGP session is used to exchange routing information
between two routers inside an autonomous system
Internal and external BGP sessions
Types of BGP messages
BGP packet header
Marker is for authentication
Open message
Keepalive
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Routers running BGP protocol exchange keepalive
message regularly to tell each other that they are
alive.
Consist only the common header
Keepalive message
Notification Message
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A notification message is sent by a router whenever
an error is detected or a route wants to close the
connection
Notification message
Error codes
Update message
Update Message
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Is used by a router to withdraw destination that
have been advertised previously.
Announce a route to a new destination
Encapsulation
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BGP messages are encapsulated in TCP segments
using the port number 179.