The document describes routing algorithms and protocols at the network layer. It discusses global routing algorithms that use complete network knowledge versus decentralized algorithms that do not. It also covers the distance vector routing algorithm and how it works iteratively to share routing information between neighbors. Specific distance vector protocols discussed include RIP, which uses hop count as its metric. The document also introduces the concepts of hierarchical routing and autonomous systems to allow scaling of routing in a decentralized manner across large networks.

1. Chapter 4
Network Layer
Computer Networking: A
Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

2. Routing Algorithm Classification
Global Routing Algorithm
Decentralized Routing
Algorithm
 Computes least cost path
 No node has complete




using complete global
knowledge about the
network.
Takes connectivity between
all nodes and all link costs
as input.
All routers have complete
topology, link cost
information
Also called “Link State”
Algorithms
Open Shortest Path First
Protocol (OSPF)




information about the cost
of all links.
In the beginning knowledge
of its own directly attached
links.
Computes least cost path by
an iterative process of
calculation and exchange of
information.
Also called Distance Vector
(DV) Algorithm
Routing Information Protocol
(RIP)

3. Distance Vector Algorithm
Distributed:
 Receives info from the neighbors
 Performs calculation
 Distributes the results to neighbors
Iterative:
 Process continues until no more information
is exchanged between neighbours.
 Asynchronous:
 Does not require all the nodes to operate
in lockstep with each other

4. Distance Vector Algorithm
Bellman-Ford Equation
Define
dx(y) := cost of least-cost path from x to y
Then
dx(y) = min {c(x,v) + dv(y) }
v
where min is taken over all neighbors v of x

5. Bellman-Ford example
5
2
u
v
2
1
x
3
w
3
1
5
z
1
y
Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
2
B-F equation says:
du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z),c(u,w) + dw(z) }
= min {2 + 5, 1+ 3, 5 + 3}
=4
Node that achieves minimum is next hop in shortest
path ➜ forwarding table

6. Distance Vector Algorithm
Basic idea:
 From time-to-time, each node sends its own
distance vector estimate to neighbors
 When a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)}
for each node y ∊ N
 All the nodes continue to exchange their distance
vectors until the estimate Dx(y) converges to the
actual least cost dx(y)

7. Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
node x table
cost to
x y z
= min{2+1 , 7+0} = 3
cost to
x y z
from
from
x 0 2 7
y ∞∞ ∞
z ∞∞ ∞
node y table
cost to
x y z
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
x 0 2 3
y 2 0 1
z 7 1 0
x ∞ ∞ ∞
y 2 0 1
z ∞∞ ∞
node z table
cost to
x y z
from
from
x
x ∞∞ ∞
y ∞∞ ∞
z 71 0
time
2
y
7
1
z

8. Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
node x table
cost to
x y z
x ∞∞ ∞
y ∞∞ ∞
z 71 0
from
from
from
from
x 0 2 7
y 2 0 1
z 7 1 0
cost to
x y z
x 0 2 7
y 2 0 1
z 3 1 0
x 0 2 3
y 2 0 1
z 3 1 0
cost to
x y z
x 0 2 3
y 2 0 1
z 3 1 0
cost to
x y z
from
from
from
x ∞ ∞ ∞
y 2 0 1
z ∞∞ ∞
node z table
cost to
x y z
x 0 2 3
y 2 0 1
z 7 1 0
= min{2+1 , 7+0} = 3
cost to
x y z
cost to
x y z
from
from
x 0 2 7
y ∞∞ ∞
z ∞∞ ∞
node y table
cost to
x y z
cost to
x y z
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
x 0 2 3
y 2 0 1
z 3 1 0
time
x
2
y
7
1
z

9. Example
v
5
z
10
7
2
1
y
x
Using DV algorithm show the table entries at node z?

10. From
Solution
z
z 0
y 3
x 2
v 5
Cost to
y x
3 2
0 1
1
0
7 7
v
5
7
7
0

11. RIP ( Routing Information Protocol)
 Distance Vector Algorithm
 Hop Count as cost metric
 Each link has cost 1
 Cost are actually from source router to a destination subnet
 RIP uses the term hop, which is the number of subnets
traversed along the shortest path from source router to
destination subnet.
u
v
A
z
C
B
w
x
D
y
From router A to subnets:
destination hops
u
1
v
2
w
2
x
3
y
3
z
2

12. RIP ( Routing Information Protocol)
 The maximum cost of a path is limited to 15

Limits the use of RIP to networks that have fewer
than 15 hops.
 Neighboring routers exchange information
after every 30 sec via RIP Response message
(also called RIP advertisements)

Known as Periodic timer.
 A Router can also request information about
its neighbor’s cost to a given destination using
RIP’s Request message.

13. RIP: Link Failure and Recovery
 If no advertisement heard after 180
sec --> neighbor /link declared dead
 RIP modifies local routing table
 Propagates this information by
sending this advertisements to its
neighboring routers.
 Called as Expiration timer.

14. Limitations of RIP
Network span limited by
maximum hop count of 15
Note that actual number of
networks can be > 15!
Slow convergence
DV Algorithm

15. RIP Implementation
 Router send RIP request and Response message over
UDP port no 520.
 RIP routing tables managed by application-level process
called routed.
 Routed executes RIP i.e. maintains routing information
and exchanges messages with other routed processes in
neighbors.
routed
routed
Transprt
(UDP)
network
(IP)
link
physical
Transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical

16. IPv4 Datagram Format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
time to
header
protocol
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
a TCP
or UDP segment)
e.g. timestamp,
record route
taken etc
Find other use?

17. IP Fragmentation & Reassembly
 Network links have MTU
(Maximum Transmission Unit)
- largest possible link-level
data that can be encapsulated
in a frame.
o Different link between
sender and destination
• Use different MTUs
 Large IP datagram is divided
(“fragmented”)
o One datagram becomes
several datagrams
o Referred as Fragments
o “Reassembled” only at final
destination
o IP header bits used to
identify, order related
fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly

18. IP Fragmentation & Reassembly
 Identification Number, Flag and Offset fields in the datagram
header
 Identification Number
o
o
Sending Host stamps the datagram with an identification number
Destination receives datagram
o Examines the identification number
 Flag Bit
o
o
All fragments set to 1
Last fragment set to 0
 Offset Field
o
Use to specify where fragment fits in the within the original IP
datagram

19. IP Fragmentation & Reassembly
Example
 Datagram of 4000 bytes
o 20 bytes of IP header, 3980 bytes of payload
o MTU is 1500 bytes
Fragment
Bytes
ID
Offset
0 (beginning)
Flag
1st Fragment
1480 bytes
777
1 (there is more)
2nd Fragment
1480 bytes
777
185
1 (there is more)
3rd Fragment
1020 bytes
777
370
0 (Last )

20. Hierarchical Routing
 All routers run the same routing
Algorithm?
… not true in practice
Scale: With millions of
destinations:
 Can’t store all destinations
in routing tables
 Exchange of Routing
updates would swamp links!
Administrative Autonomy
 Internet = network of
networks
 Each network administrator
may want to control routing
in its own network

21. Hierarchical Routing
 Routers are organized
into “Autonomous
Systems” (ASs).



Routers in same
administrative domain
The routing algorithm
within an autonomous
system is called an
“intra-AS” routing
protocol
Routers in different AS
can run different intraAS routing protocol
Gateway Routers
 One or more of the
routers in an AS will
have the added task of
forwarding packets to
destinations outside
the AS.
1c
1a
1d
1b
AS1

22. Interconnected ASs
3c
3b
3a
2a
AS3
1c
1a
1d
2c
2b
AS2
1b
AS1
1b, 1c, 2a and 3a are Gateway Routers

23. Inter-AS tasks
AS1 must:
1. Learn which destinations are
reachable through AS2, which
through AS3
2. Propagate this reachability
information to all routers in
AS1.

Configure Forwarding table
Job of inter-AS routing protocol!
 Suppose router in AS1
receives datagram destined
outside of AS1:
 Router should forward
packet to gateway
router, but which one?
3c
3a
3b AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2c
AS2
2b

24. Example: Setting forwarding table in router 1d
 Suppose AS1 learns (via inter-AS protocol) that
network x reachable via AS3 (gateway 1c) but not via
AS2.
 Inter-AS protocol propagates reachability info to all
internal routers.
 Router 1d determines from intra-AS routing info that
its interface I is on the least cost path to 1c.
 installs forwarding table entry (x,I)
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b AS1
2c
2b
AS2

25. Example: Choosing among multiple ASs
 Now suppose AS1 learns from inter-AS protocol
that a network x is reachable from AS3 and from
AS2.
 To configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
 Hot Potato Routing: Send packet towards closest of
two routers.
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b
2c
AS2
AS1
2b

26. Example: Choosing among multiple ASs
Learn from inter-AS
protocol that network
x is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to
determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the
gateway
that has the
smallest least cost
Determine the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
Steps in adding an outside AS destination in a routers forwarding Table