Unicast Routing Protocols 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) An autonomous system is group of networks and

routers under the authority of a single administration.

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 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 Initializing Sharing

Periodically Triggered update

Updating

Issues Two node loop Instability

Define infinity Split horizon Split horizon and poison reverse

Three node instability

Routing Information Protocol 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

Command Request—1 Response—2

Request messages

Timers in RIP

Periodic Timer 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 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 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 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 tables1) Creation of the states of the links by each

node called Link state Packet(LSP)2) Dissemination of LSP to every other router

called flooding3) Formation of a shortest path tree for each

node4) Calculation of a routing table based on the

shortest path tree

Link state packet Node identity The list of links A sequence number Age

Open shortest path first(OSPF) 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 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.

BackboneIs a special area in an autonomous

system. All other areas must be connected to backbone.

Areas in an autonomous system

Metric 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 In OSPF a connection is called a link

Point to point Connects two routers without any other host or router in between

Transient Linkis a network with several routers attached to it. The data can enter

through any of the routers and leave through any router

Sub linkIs a network that is connected to only one router

Virtual linkWhen 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

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

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 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

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 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 M M/S

Message Sequence Number

LSA Header(20 Bytes) [repeated]

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 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

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 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) 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 Three types Stub AS Multihomed AS Transient AS

BGP Sessions A session is a connection that is

established between two BGP. Uses services of TCP

Types sessions 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 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 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 Is used by a router to withdraw

destination that have been advertised previously.

Announce a route to a new destination

Encapsulation BGP messages are encapsulated in TCP

segments using the port number 179.