Routing in networking

1. Routing Concept
Sirak Kaewjamnong
Computer Network Tech and Security

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

3. 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 A Network B

4. 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. 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. 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. 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. 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. 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. 10
Hop Count
• A hop is defined as a passage through one router
R1 R2
R3
1 hop 1 hop
1 hop 1 hop
2 hops

11. 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. 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. 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
incoming
flooding

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

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

16. 16
Routing Algorithm: Source Routing
• 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. 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. 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>
routing table
W 0
X 0
Y 1
Z 2
routing table
W 1
X 0
Y 0
Z 1
routing table
W 2
X 1
Y 0
Z 0
W X Y Z
R1 R2 R3

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

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

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

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

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

24. 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……
R1 R2 R3
hop count to R3
2 1 initial
2 3 1st round
4 3 2nd round
4 5 3rd round
I J
R3 crashed

25. 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 R2 R3
I J R3 crashed
to R3
to R3

26. 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. 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. 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. 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. 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)
link state
W 0
X 0
link state
X 0
Y 0
link state
Y 0
Z 0
W X Y Z

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

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

33. 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. 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. 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. 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. 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. 38
Internet Routing Architecture
Autonomous
System
Autonomous
System
Autonomous
System
IGP
IGP
IGP
BGP4
BGP4
BGP4
IGP
IGP
IGP IGP
IGP
IGP
EGP/BGP EGP/BGP EGP/BGP EGP/BGP
EGP/BGP
EGP/BGP
Autonomous
System
Autonomous
System

39. 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. 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. 41
Gateway Protocol
• Interior 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. 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. 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. 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 171.64/16 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. 45
References
• http://www.cisco.com/en/US/products/hw/routers/p
s274/index.html
• http://www.cisco.com/en/US/products/hw/routers/p
s274/products_data_sheet09186a008010fba1.html
• Computer Networks with Internet Technology
By William Stallings Chapter 11 Interior Routing Protocols