rurfid.ru.ac.bd

Network Layer and Routing

NL responsibility

• Main responsibilities

– Host to Host delivery across different network – Finding path to destination.

• Services provided to TL, receive from DL

– Connectionless and connection oriented services

– Identifying source and destination uniquely and thereby use NL address. Fragments TL data if necessary. Uses packet switching (store and forward) with datagram approach.

– In the router NL finds the appropriate interface through which it will reach the destination.

– At the destination matches the address and checks corrupt

packets, and assembly fragmented units and then sent to TL

Design Issues

• The services should be independent of the router technology.

• The transport layer should be shielded

from the number, type, and topology of the routers present.

• The network addresses made available to

the transport layer should use a uniform

numbering plan, even across LANs and

WANs.

Switching

• Store and Forward means?

• How is a packet sent from one node to another node?

– Circuit switching: connection oriented – Packet switching:

– VC approach – call set up before message transfer: packet switching connection oriented – Datagram approach – independent packet, does

not need call setup phase: packet switching

connecionless

Connectionless service

Connection oriented service

Homework

• Comparison between datagram and

virtual circuit networks

Internet Protocol

• Network layer of datagram-oriented network such as internet has three major components:

– Network protocol, e.g IP – Path determination

– Error reporting

Transport Layer:TCP, UDP

Link Layer Routing Protocols:

• path selection

•RIP, OSPF, BGP

Routing Table

IP Protocols:

• addressing conventions

•Datagram format

•Packet handling conventions ICMP protocol:

• error reporting

•Router “signaling”

IPv4 header

• header has 20bytes fixed part and a variable length optional part.

• IHL – header length in 32 bit words. minimum IHL = 5 words = 20 bytes.

maximum is 15 words.

• types of service – 6 bit field. voice, text etc. different classes of service.

• total length – 65,535 bytes

Version IHL(4) Type of Service Total length

Identification ^D_{F MF} Fragment offset

TTL protocol Header checksum

Source Add Destination Add

options ( 0 or more words)

 

16 bits 16 bits

• Identification – identifies the fragment which datagram it belongs to.

• unused 1 bit field.

• DF stands for don’t fragment. 1 bit.

• MF stands for more fragment – all fragments except the last one has this bit set. 1 bit

• Fragment offset (13 bits) – where in the current datagram this fragment belongs. All fragments except the last one in a datagram must be a multiple of 8 bytes. there is a maximum 8192 fragments per datagram.

• TTL – counter to limit packet lifetimes in seconds. max. value 255. it must be decremented on each hop, and supposed to be decremented when

queued for long time in a router. when it hits 0 the packet is discarded and a warning message is sent back to source.

• Protocol – TCP or UDP etc.

• checksum – for header only.

• source and destination address – IP addresses.

• Options – security, strict source routing, loose source routing, record routes, timestamp etc.

IPv4 addressing

• IPv4 is 32 bits long

• Usually written in dotted decimal notation, eg.

193.32.216.9

• In the globak internet

each interface must have a unique IP address.

• Network address and host address.

• Subnet mask.

• Meaning of 223.1.1.0/24

Classful addressing

0 network Host

10 network Host

110 network Host

1110 network Mutlicast address

11110 network For future use

1.0.0.0 –

127.255.255.255 128.0.0.0 – 191.255.255.255

192.0.0.0 – 223.255.255.255 224.0.0.0 – 239.255.255.255 240.0.0.0 – 255.255.255.255

192.68.1.20/255.255.255.0 or 192.168.1.20/24 network number or address – 192.68.1.0 host number of address - 20

Special IP

• 0.0.0.0 – this host; used when the machine is booting.

• IP address with 0 as the network number refer to the current network.

• IP with all 1s allows broadcasting on local network, typically a LAN.

• IP address with all 1s in host field allows broadcasting on the remote network that matches the network number.

• 127.x.x.x - loopback

Internet structure

• At the NL internet can be viewed as a

collection of subnets or Autonomous systems

(AS) that are interconnected.

Internet working

• TL takes data streams and breaks them up into datagrams. Datagrams can be up to 64Kbytes each, but usually not more than 1500 bytes. If necessary it is

fragmented.

• When all pieces finally reaches the

destination they are reassembled by NL to original datagram. It is then handed over to TL which inserts it into receiving

process.

Transporting a datagram from src to dest

• Addressing and routing

– Key fields of IP datagram

Misc fields

Source IP address

Desti.IP address

Data

Dest. Net Next

router Nhops

223.1.1.0/24 - 1

223.1.2.0/24 223.1.1.4 2 223.1.3.0/24 223.1.1.4 2

Routing table in A A

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.27

223.1.2.1 223.1.2.2

B

E

Hub

Dest. Net Next

router Nhops interface 223.1.1.0/24 - 1 223.1.1.4 223.1.2.0/24 - 1 223.1.2.9

223.1.3.0/24 1 223.1.3.27

Routing table in router

• How is this table created in the router?

A campus network

main rout

er routers hosts

APE

CSE

Subnets

• Splitting a network into several subnetworks.

– Reduced network traffic

• Routers create broadcast domains. The smaller broadcast domains you create, the less network traffic on that network segment.

– Optimized network performance

• This is a result of reduced network traffic

– Simplified management

• It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic

network.

– Facilitated spanning of large geographical

distances

• Internet corporation for Assigned Names and Numbers (ICANN) – manages IP numbers

and DNS root servers

• ICANN appointed some regional authority called Internet registry who are now

responsible for IP address assignment for a particular region

– North america: American registry for internet – Reseaux IP Europeans

– Asia pacific Network Information center (APNIC)

Distance Vector Algorithm

• It is iterative, asynchronous and distrbuted.

• Each node keeps a distance table. Node X’s distance table entry, D

^X

(Y,Z) is the sum of the cost of the direct one hop link between X and Z, plus neighbor Z’s

currently known least cost path from itself to Y.

D

^X

(Y,Z) = c(X,Z)+min

_w

{D

^Z

(Y,W)}

After the algorithm converged

C

E A

B

D

D

^E

() A B C

A 1 14 5

B 7 8 5

C 6 9 4

D 4 11 2

Destination

Cost to destination via

7

1

2

2 8

1

X Z Y

D^x Y Z

Y 2 ∞

Z ∞ 7

D^y X Z

X 2 ∞

Z ∞ 1

D^z X Y

X 7 ∞

Y ∞ 1

D^x Y Z

Y 2 8

Z 3 7

D^y X Z

X 2 8

Z 9 1

D^z X Y

X 7 3

Y 9 1

D^x Y Z

Y 2 8

Z 3 7

D^y X Z

X 2 1+3

Z 2+3 1

D^z X Y

X 7 3

Y 9 1

2

7

1

Link cost changes and failure

D^y X Z

X 4 6

D^z X Y X 50 5

X Z

Y 4

50 1 1

D^y X Z

X 1 6

D^z X Y X 50 5

t₀ t₁ t₂

D^y X Z

X 1 6

D^z X Y X 50 2

D^y X Z

X 1 3

D^z X Y X 50 2

• When link-cost decreases

• When link-cost increases

X Z

Y 4

50 60 1

D^y X Z

X 4 6

D^z X Y X 50 5

D^y X Z X 60 6 D^z X Y X 50 5

t₀ t₁ t₂

D^y X Z X 60 6 D^z X Y X 50 7

D^y X Z X 60 8 D^z X Y X 50 7

D^y X Z X 60 8 D^z X Y X 50 9

• Routing loop

• Count to infinity problem

t₃

CIDR

• Classless InterDomain Routing