Chapter 4 Network Layer

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

Routing Algorithm ClassificationGlobal Routing

Algorithm

Computes least cost path 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)

Decentralized Routing Algorithm

No node has complete 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)

Distance Vector Algorithm

Distributed: Receives info from the neighbors Performs calculation Distributes the results to neighborsIterative: Process continues until no more

information is exchanged between neighbours.

Asynchronous:Does not require all the nodes to

operate in lockstep with each other

Distance Vector Algorithm

Bellman-Ford Equation Definedx(y) := cost of least-cost path from x to y

Then

dx(y) = min {c(x,v) + dv(y) }

where min is taken over all neighbors v of x

v

Bellman-Ford example

u

yx

wv

z2

2

13

1

1

2

53

5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

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

B-F equation says:

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)

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0

from

cost to

x y z

xyz

∞ ∞

∞ ∞ ∞

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

32

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 3

from

cost to

x y z

xyz

∞ ∞

∞ ∞ ∞

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

Example

5

v

z

2

x

y

1

7 10

Using DV algorithm show the table entries at node z?

Solution

Cost to z y x v

z 0 3 2 5 y 3 0 1 7 x 2 1 0 7

v 5 7 7 0

From

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.

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

From router A to subnets:

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.

RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor /link declared deadRIP modifies local routing table Propagates this information by

sending this advertisements to its neighboring routers.

Called as Expiration timer.

Network span limited by maximum hop count of 15Note that actual number of networks can be > 15!

Slow convergenceDV Algorithm

Limitations of RIP

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.

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

IPv4 Datagram Format

ver length

data (variable length,

a TCP or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offset

protocol

32 bit destination IP address

Options (if any) e.g. timestamp,record routetaken etcFind other use?

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 datagramso Referred as

Fragmentso “Reassembled” only at

final destinationo IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

IP Fragmentation & Reassembly Identification Number, Flag and Offset fields in the datagram

header

Identification Numbero Sending Host stamps the datagram with an identification numbero Destination receives datagram

o Examines the identification number

Flag Bito All fragments set to 1o Last fragment set to 0

Offset Fieldo Use to specify where fragment fits in the within the original IP

datagram

IP Fragmentation & Reassembly

Example Datagram of 4000 bytes

o 20 bytes of IP header, 3980 bytes of payloado MTU is 1500 bytes

Fragment Bytes ID Offset Flag

1st Fragment 1480 bytes 777 0 (beginning) 1 (there is more)

2nd Fragment 1480 bytes 777 185 1 (there is more)

3rd Fragment 1020 bytes 777 370 0 (Last )

Hierarchical Routing

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

All routers run the same routing Algorithm?

… not true in practice

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 intra-AS 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.

1d

1c

AS11a 1b

3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

Interconnected ASs

1b, 1c, 2a and 3a are Gateway Routers

Inter-AS tasks Suppose router in AS1

receives datagram destined outside of AS1: Router should forward

packet to gateway router, but which one?

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 tableJob of inter-AS routing protocol!

3b

1d

3a1c

2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

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)

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

x…

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.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3cx… …

Learn from inter-AS protocol that network

x is reachable via multiple gateways

Use routing infofrom 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

Example: Choosing among multiple ASs

Steps in adding an outside AS destination in a routers forwarding Table