1. Open Shortest Path First Chang-Keun Park pck1982@postech.ac.kr
Good afternoon, I’m changkeun park
In this talk, I will talk about Open Shortest Path First Protocol

2. Content History Routing protocols taxonomy Terminology
OSPF Router Types
Features of OSPF
Types of Link
Link State Advertisements (LSAs)
Operation of OSPF
Dijkstra Algorithm
OSPF Packet Types
OSPF v3
Problems
Presentation outline is as follows

3. History Development began 1987 by IETF
OSPFv2 established in 1991 with RFC 1247
Goal: a link state protocol more efficient and scalable than RIP
Latest revision is RFC 2328, April 1998
OSPFv3 with IPv6 support is defined in RFC 2740

4. Routing protocols taxonomy
This figure shows routing protocols taxonomy
RIP and OSPF can be used to update routing tables insides autonomous system
RIP is simple protocol based on distance vector routing
OSPF is protocol based on link state routing
BGP can be used to update routing tables between autonomous systems

5. Terminology (1/3) Link: Interface on a router
Link state: Description of an interface and of its relationship to its neighboring routers
Metric: cost assigned for passing through a network
RIP uses metric based on hop-count
OSPF uses metric based on type of services (Throughput, delay)
This slides introduce some terminology about OSPF
Link is interface on a router
Link state is descripiton of an interface and of its relation ship to its neghboring routers such as metric, network type and ip address
Metric is cost assigned for passing through network
RIP uses metric based on hop-count because RIP treats all networks as same cost
OSPF uses metric based on types of service. So, networks have different cost in terms of metric, such as maximum throughput and minimum delay.

6. Terminology (2/3)
Area: collection of networks, hosts, and routers all contained within an autonomous system
All networks inside an area must be connected
Router inside an area floods the area with routing information
OSPF divides an autonomous system into several areas for routing efficiency

7. Terminology (3/3)
On broadcast media, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR)
All other routers in the LAN, establish adjacencies only with DR and BDR
send Link State Advertisements (LSAs) only to DR/BDR
help reduce routing update traffic
BDR becomes DR, only if the DR fails

8. OSPF Router Types Area Border Router (ABR) Backbone Router (BR)
Routers with interfaces attached to other areas
Backbone Router (BR)
Routers inside backbone (area 0)
Autonomous System Boundary Router (ASBR)
Routers that have at least one interface connected to an external internetwork (another autonomous system)
This figure shows an autonomous system and its areas
This is composed of 1 AS and 3 Areas.
ABR is routers with interfaces attached to other areas.
It summarize information about the area and send it to other area
Backbone router is routers inside backbone
Every Area is connected to the backbone
ASBR is router that have at least one interface connected to an external internetwork
ASBR distributes information about other autonomous system into the current system

9. Features OSPF is link-state routing protocol OSPF has fast convergence
RIP, IGRP and EIGRP are distance-vector routing protocol
OSPF has fast convergence
RIP and IGRP hold-down timers can cause slow convergence
Cisco’s OSPF metric is based on bandwidth
RIP is based on hop count
IGRP/EIGRP is based on bandwidth, delay, reliability, load
OSPF only sends out changes when they occur.
RIP sends entire routing table every 30 seconds, IGRP every 90 seconds
OSPF uses the concept of areas to implement hierarchical routing
This slide explains features of OSPF

10. Types of Link (1/4)
In OSPF, there are four kinds of links, Point to point, Transient, Stub and virtual

11. Types of Link (2/4) Point-to-Point Link
Connects two routers without any other host or router
Telephone line or T-line
Point-to-Point Link Connects two routers without any other host or router
Examples of this type is two routers connected by Telephone line or T-line

12. Types of Link (3/4) Transient Link
A network with several routers attached to it
LAN and WAN with two or more router
LAN and WAN with two or more router is this type

13. Types of Link (4/4) Stub Link Virtual Link
A network that is connected to only one router
Virtual Link
When the link between two routers is broken, the administration create a virtual link between them using longer path

14. Examples of an Internet
This figure shows small internet with seven networks and six routers
It is composed of 2 point to point networks, 2 transient networks and three stub networks

15. Graphical representation
This figure shows previous internet graphically.

16. Link State Advertisements (1/6)
To share information about neighbors, each router distributes link state advertisements (LSAs)
LSA announces states of entity links
To share information about neighbors, each router distributes link state advertisements which we call LSAs
LSA announces states of entity links.
According to type of entity, we can define five LSAs such as …

17. Link State Advertisements (2/6)
Router Link
Generated by each router for each area it belongs to
Flooded inside area
Describes the states of the links in the area to which this router belongs

18. Link State Advertisements (3/6)
Network Link
Defining the links of a network
Generated by the Designated router on every multi-access network
Designated Router (DR)
Act as a central point of contact
Each router sends LSAs only to DR/BDR (Backup DR)
DR forwards LSAs to all other routers in network
Network link advertisment defines the links of a networks. A designated router distributes this type of LSA packets

19. Link State Advertisements (4/6)
Summary link to network
Originated by the ABR
Describes links state between ABR and Internal Routers of the Local Area
Flood throughout the backbone and to other ABRs.

20. Link State Advertisements (5/6)
Summary link to AS boundary router
Originated by the ABR
Flood throughout the backbone area to the other ABRs
Describes the reachability to the ASBRs

21. Link State Advertisements (6/6)
External link
Originated by the ASBR
Describes destination networks external to the Autonomous System
Used to know which networks are available outside the autonomous system

22. OSPF Operations 1 – Flooding of link-state information
3 – SPF Algorithm
5 – Routing Table
2 – Building a Topological Database
4 – SPF Tree
This figure shows OSPF operations
First, every router floods link-state information
And then, each router build topological database by using link-state information
And then , each router make shortest path first tree using SPF algorithm
And then, each router make routing table using shortest path tree

23. Link State Database
Every router in an area receives the router link and network link LSAs from every other router and forms Link State database
Tabular representation of the topology in an area
Show the relationship between each router and its neighbors including the metrics

24. Dijkstra Algorithm
calculating the shortest path between two points on a network using a graph made up of nodes and edges
divide the nodes into two set
tentative and permanent
Dijkstra Algorithm calculate the shortest path between two points on a network using a graph made up of nodes and edges
To calculate its routing table, each router applies the Dijkstra algorithm to its link state database
This algorithm devides the nodes into two set, tentative and permanent.
It chooses nodes, makes them tentative, and examine them, and if they pass the criteria, makes them permanets
This figure shows algorithm steps
First, start with local node, the root of the tree. And assign cost of 0 this node and make it the first permanent node
And then, examine each neighbor node of the node that was last permanent node
And then assign cumulative coast to each node and make it tentative
And among the list of tentative nodes, find the node with smallest cumulative cost and make it permanent. If node can be reached from more than one direction
Select the direction with shortest cumulative cost
Repeate…

25. Shortest Path Calculation
This figure shows calculation of shortest path
White bax is tentative node and yellow box is permanent node
Start with root node,
make it permanent, and add its neighbors, and make them tentative nodes Among tentative nodes, choose smallest value, we choose N1 node.
And then, Make N1 permanent and add its neighbors and make them tentative nodes Among tentative nodes, choose smallest value, we choose C node.
Make C permanent and add its neighbors and make them tentative nodes.Among tentative nodes, choose smallest value, we choose B node

26. Shortest Path Calculation
Make B permanent and add its neighbors and make them tentative nodes. Among tentative nodes, choose smallest value, we choose N2 node.
Make N2 permanent and add its neighbors. Among tentative nodes, choose smallest value, we choose D node.
Make D permanent and add its neighbors and make them tentative nodes. Among tentative nodes, choose smallest value, we choose E node.
Make E permanent and add its neighbors and make them tentative nodes Howerver Node N3 can be reached from 2 directions. we choose 10 because it is smallest value. Among tentative nodes, choose smallest value, we choose N3 node.

27. Shortest Path Calculation
Make N3 permanent and add its neighbors and make them tentative nodes. Among tentative nodes, choose smallest value, we choose F node.
Make F permanent and add its neighbors and make them tentative nodes. Among tentative nodes, choose smallest value, we choose N4 node.
Make N4 permanent and add its neighbors Among tentative nodes, choose smallest value, we choose N5 node.
Make N5 permanent. And algorithm is terminated.

28. Routing Table
Each router uses the shortest path tree method to construct its routing table
Showing the cost of reaching each network in the area
To find the cost of reaching networks outside of the area, the routers use the summary link to network, the summary link to boundary router, and the external link advertisements.

29. Link state routing table for router A
Network Cost Next Router
N1 5
N2 7 C
N D
N B
N D
Link state routing table for router A is as follows
If packet wanna go N5 network, next router is D and link cost is 15.

30. OSPF packet header This figure shows OSPF packet header.
There are, OSPF protocol version, packet type, packet length, source router IP address, area identification, checksum, authentication type used in the area, and authentication data

31. Types of OSPF packets There is 5 kinds of OSPF packet type, such as ….
As I said, Link state update packet has 5 kinds of link state advertisement type such as…
Link state update type is Link state advertisement, which has 5 types such as …

32. Hello packet
Used for creating neighborhood relationships and to test the reachability of neighbors before flooding link state information
Hello packet is used for creating neighborhood relationships and to test the reachability of neighbors before flooding link state information
In Hello packet, There are many fields such as network mask, Hello message interval, E flag, T flag, priority, Dead interval, designated router IP address, backup designated router IP address and neighbor IP address
Hello interval – how often Hellos are sent (10 sec by default)
E flags check stub link type, T flags checks multiple metrics support.
Dead interval is time (without hellos) before neighboring router is considered down, default value is 40se

33. Database description packet
Used when a router is connected to the system for the first time or after a failure
In DD packet, There are many fields such as common header, E flag, B flag, I flag, M flag, MS flag, Message sequence number and serveral LSA header
If E flags set 1, It means that advertisement router is Autonomous system border router.
E 플래그 : 광고 라우터가 자율 시스템 경계 라우터이면 1
B 플래그 : 광고 라우터가 지역 경계 라우터이면 1
I 플래그 : 첫 번째 메시지 일 때 1
M 플래그 : 마지막 메시지가 아닐 경우에 1
M/S (Mster/Slave) 플래그 : 패킷의 시작점
메시지 순서 번호(message sequence number)
링크 상태 헤더(link state header)

34. Link state request packet
Used to request link state information about a specific route or routers
In Link state request packet, There are many fields such as

35. Link state update packet
used by a router to advertise the states of its links
In DD packet, There are many fields such as Number of link state advertisements and server LSAs

36. Link state acknowledgment packet
Used by a router to acknowledge LSAs to make flooding of LSAs reliable
Packet field is composed of common header and link state header

37. OSPFv3 OSPF for IPv6 Based on OSPFv2, with enhancements Similarities
Basic packet types
Mechanisms for neighbor discovery and adjacency formation
Nearly identical LSA types
Differences
OSPFv3 runs over a link, rather than a subnet
OSPFv3 uses standard authentication mechanism supporting IPv6
OSPF is for IPv6, OSPF is based on OSPFv2
Most of the algorithms from OSPF for IPv4 have preserved in OSPF for IPv6.
However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6
They have same packet types and They have same mechanisms. They have nearly identical LSA types
Difference, OSPFv3 runs over link, .. And OSPF v3 uses standard authentication mechanism supporing IPv6

38. Problems
Scalability
OSPF is very processor intensive
OSPF maintains multiple copies of routing information, increasing the amount of memory needed
OSPF is not as easy to learn as some other protocols
Link cost is traffic carried: can cause route oscillation
There are some problems in OSPF
First, scalability, OSPF is not suitable to large scale network.
Because OSPF is very processor intensive and maintains