1. Sirak Kaewjamnong Computer Network Tech and Security
Routing Concept
Sirak Kaewjamnong
Computer Network Tech and Security

2. Agenda Basic concepts Routing components Classes of routing protocol
Internet routing protocol

3. What’s Routing Routing - path finding from one end to the other
Routing occurs at layer 3
Bridging occurs at layer 2
Physical link
Data link layer
Network layer
Transport layer
Session layer
Presentation layer
Application layer
Physical link
Data link layer
Network layer
Transport layer
Session layer
Presentation layer
Application layer
Physical link
Data link layer
Network layer
Network B
Network A

4. IP Routing IP performs:
search for a matching host address
search for a matching network address
search for a default entry
Routing done by IP router, when it searches the routing table and decide which interface to end a packet out.
incoming
which interface ?

5. Routing Tables
Routing is carried out in a router by consulting routing table.
No unique format for routing tables, typically table contains:
address of a destination
IP address of next hop router
network interface to be used
subnet mask for the this interface
distance to the destination

6. Routing Component Three important routing elements :
algorithm
database
protocol
Algorithm : can be differentiate based on several key characteristics
Database : table in routers or routing table
Protocol: the way information for routing to be gathered and distributed

7. Routing Algorithm Design goals Optimality - compute the best route
Simplicity/low overhead - efficient with a minimum software and utilization overhead
Robustness/stability- perform correctly in the face of unusual circumstances
Rapid convergence- responds quickly when the network changes
Flexibility- accurate adapt to a variety of network

8. Routing Protocols
Routing protocol : protocol to exchange of information between routers about the current state of the network
Routing protocol jobs
create routing table entries
keep routing table up-to-date
compute the best choice for the next hop router

9. Routing Metrics
How do we decide that one route is better than another?
Solution : using a metric as a measurement to compare routes
Metrics may be distance, throughput, delay, error rate, and cost.
Today, IP supports Delay, Throughput, Reliability and Cost (DTRC)

10. Hop Count A hop is defined as a passage through one router 2 hops

11. Routing Algorithm Types
Static V.S. Dynamic
Source routing V.S. Hop-by-hop
Centralize V.S. Distributed
Distance vector V.S. Link state

12. Routing Algorithm: Static Route
Manually configuration routing table
Can’t react dynamically to network change such as router’s crash
Work well with small network or simple topology
Unix hosts use command route to add an entry
point to point
connection
route to this
way only, no need
for update

13. Routing Algorithm: Static Technique
Flooding
Every incoming packet is sent out every outgoing
Retransmit on all outgoing at each node
Simple technique, require no network information
Generate vast numbers of duplicate packet
flooding
incoming

14. Routing Algorithm: Dynamic Route
Network protocol adjusts automatically for topology or traffic changes
Unix hosts run routing daemon routed or gated

15. Routing Algorithm: Dynamic Route operation
Routing protocol maintains and distributes routing information
Routing
Table
Routing
Table
Routing
Protocol
Routing
Protocol
Update Routing Information

16. Routing Algorithm: Source Routing
Source will determine the entire route
Routers only act as store-forward devices
Hop-by-hop
Routers determine the path based on theirs own calculation

17. Routing Algorithm: Distance Vector
Distance means routing metric
Vector means destination
Flood routing table only to its neighbors
RIP is an example
Also known as Bellmann-Ford algorithm or Ford-Fulkerson algorithm

18. Distance Vector Algorithm
Using hop count as a metric
Each router periodically sends a copy of its routing table to neighbors
send <network X, hopcount Y> R1 R2 R3 W X Z Y
routing table W X Y Z
routing table W X Y Z
routing table W X Y Z

19. Distance Vector Routing Update
Step by step from router to router
Slow convergence
recompute R1’s
routing table
recompute R2’s
routing table
recompute R3’s
routing table
topology
change R1 R2 R3
R2 sends out
the updated table
R3 sends out
the updated table

20. Distance Vector: Broadcast (I)
The first round
I, 1 hop
J, 1 hop R1
J, 1 hop
K, 1 hop
M, 1 hop
N, 1 hop J I
I, 1 hop
K, 1 hop
L, 1 hop R3 K R2
N, 1 hop
O, 1 hop N L M R5
L, 1 hop
M, 1 hop
O 1 hop O R4

21. Distance Vector: Broadcast (II)
The second round
I, 1 hop
J, 1 hop
K, 2 hops
L, 2 hops
M, 2 hops
N, 2 hops
J, 1 hop
K, 1 hop
M, 1 hop
N, 1 hop
I, 2 hops
L, 2 hops
O, 2 hops R1
N, 1 hop
O, 1 hop
J, 2 hops
K, 2 hops
M, 1 hop
L, 2 hops J I R3 K R2 N
I, 1 hop
K, 1 hop
L, 1 hop
J, 2 hops
M, 2 hops
N, 2 hops
O, 2 hops L M R5 O R4
L, 1 hop
M, 1 hop
O, 1 hop
I, 2 hops
K, 2 hops
J, 2 hops
N, 2 hops

22. Distance Vector: Broadcast (III)
The third round
I, 1 hop
J, 1 hop
K, 2 hops
L, 2 hops
M, 2 hops
N, 2 hops
O, 3 hops
J, 1 hop
K, 1 hop
M, 1 hop
N, 1 hop
I, 2 hops
L, 2 hops
O, 2 hops R1
N, 1 hop
O, 1 hop
J, 2 hops
K, 2 hops
M, 1 hop
L, 2 hops
I, 3 hops J I R3 K R2 N
I, 1 hop
K, 1 hop
L, 1 hop
J, 2 hops
M, 2 hops
N, 2 hops
O, 2 hops L M R5 O R4
L, 1 hop
M, 1 hop
O, 1 hop
I, 2 hops
K, 2 hops
J, 2 hops
N, 2 hops

23. Distance Vector: Crashed Recovery
New complete route of R1 R1 J I R3 R2 K
R1 routing table
hop via
N/A R2 R3 R5 L N
net I J K L M N O M R5 O
net I J K L M N O
hop via
N/A R2 R2 R2 R4

24. Count to Infinity R2 does not hear any thing from R3
R1 says : don’t worry, I can reach R3 in 2 hops, R2 update hop count to 3
R1 sees R2’s update, then update itself to 4 and so on……
R3 crashed R1 I R2 J R3
initial
st round
nd round
rd round
hop count to R3

25. Split Horizon Solve by set distance “16” as infinity
No destination can be more than 15 hops away from any other
Distance to X is not reported on the line that packet for X are sent
Actually, it reports with infinity R1 I R2 J R3
R3 crashed
to R3
to R3

26. Dijkstra’s Shortest Path First Algorithm
Routers send out update messages whenever the state of a link changes. Hence the name: “Link State” algorithm.
Each router calculates lowest cost path to all others, starting from itself.
At each step of the algorithm, router adds the next shortest (i.e. lowest-cost) path to the tree.
Finds spanning tree routed on source router.

27. Open Shortest Path First (OSPF)
RIP limited in large internets
OSPF preferred interior routing protocol for TCP/IP based internets
Link state routing used

28. Routing Algorithm: Link State
Flood routing information to all nodes
Each router finds who is up and flood this information to the entire routers
Use the link state to build a shortest path map to everybody
OSPF is an example
Also known as Shortest Path First (SPF) algorithm

29. Flooding Packet sent by source router to every neighbor
Incoming packet resent to all outgoing links except source link
Duplicate packets already transmitted are discarded
Prevent incessant retransmission
All possible routes tried so packet will get through if route exists
Highly robust
At least one packet follows minimum delay route
Reach all routers quickly
All nodes connected to source are visited
All routers get information to build routing table
High traffic load

30. Link State Overview Using cost as a metric
Exchange its connection and cost to its neighbors
Each router compute the set of optimum path to all destination (Shortest Path First) W X Y Z
link state W X
link state X Y
link state Y Z

31. Link State Concept
Each router initially begins with directly connected network
Determine full knowledge of distant routers and theirs connection R1
exchange link
state packets R2 R4
build topological
database
Routing
Table R3
compute SPF
update routing
table

32. Link State Routing Update
Send information to other routers
Fast convergence R1 R4
topology
change R2 R3

33. OSPF Overview Router maintains descriptions of state of local links
Transmits updated state information to all routers it knows about
Router receiving update must acknowledge
Lots of traffic generated
Each router maintains database
Directed graph

34. Link Costs Cost of each hop in each direction is called routing metric
OSPF provides flexible metric scheme based on type of service (TOS)
Normal (TOS) 0
Minimize monetary cost (TOS 2)
Maximize reliability (TOS 4)
Maximize throughput (TOS 8)
Minimize delay (TOS 16)

35. Areas Make large internets more manageable
Configure as backbone and multiple areas
Area – Collection of contiguous networks and hosts plus routers connected to any included network
Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas

36. Operation of Areas
Each are runs a separate copy of the link state algorithm
Topological database and graph of just that area
Link state information broadcast to other routers in area
Reduces traffic
Intra-area routing relies solely on local link state information

37. Comparison Distance Vector Link State
pass a copy of pass links state update
whole routing table
add metric from calculate the shortest path
router to router to other routers
frequent periodic update: event updated: fast
slow convergence convergence

38. Internet Routing Architecture
Autonomous
System
IGP
EGP/BGP
IGP
EGP/BGP
IGP
Autonomous
System
BGP4
BGP4
Autonomous
System
IGP
IGP
BGP4
Autonomous
System
EGP/BGP
EGP/BGP
EGP/BGP
EGP/BGP
IGP
IGP
IGP
IGP
Autonomous
System

39. Routing in the Internet
The Internet uses hierarchical routing
The Internet is split into Autonomous Systems (AS’s)
Within an AS, the administrator chooses an Interior Gateway Protocol (IGP)
Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).
Between AS’s, the Internet uses an Exterior Gateway Protocol
AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)

40. Autonomous System
AS is a collectionof LANs and WANs and the interconnectting routers which under the control of one management authority
The same AS runs the same Interior Gateway Protocol
Why setting up AS? - establish a direct link to each other rather than route through the core Internet
How to select AS? - register and get the AS number from IAB

41. Gateway Protocol Interior gateway protocol Exterior gateway protocol
exchange routing information between routers within a single AS
RIP, RIP II, OSPF
Exterior gateway protocol
collect network-reachablity information for the AS
EGP, BGP

42. Interior Routing Protocols
RIP
Uses distributed Bellman-Ford algorithm.
Updates sent every 30 seconds.
No authentication.
Originally in BSD UNIX.
OSPF
Link-state updates sent (using flooding) as and when required.
Every router runs Dijkstra’s algorithm.
Authenticated updates.
Autonomous system may be partitioned into “areas”.

43. Exterior Routing Protocols
Problems:
Topology: The Internet is a complex mesh of different AS’s with very little structure.
Autonomy of AS’s: Each AS defines link costs in different ways, so not possible to find lowest cost paths.
Trust: Some AS’s can’t trust others to advertise good routes (e.g. two competing backbone providers), or to protect the privacy of their traffic (e.g. two warring nations).
Policies: Different AS’s have different objectives (e.g. route over fewest hops; use one provider rather than another).

44. Border Gateway Protocol (BGP-4)
BGP is not a link-state or distance-vector routing protocol.
BGP advertises complete paths (a list of AS’s).
Example of path advertisement:
“The network /16 can be reached via the path {AS1, AS5, AS13}”.
Paths with loops are detected locally and ignored.
Local policies pick the preferred path among options.
When a link/router fails, the path is “withdrawn”.

45. References
Computer Networks with Internet Technology By William Stallings Chapter 11 Interior Routing Protocols