Network Layer Two - Local and Wide Area Networks - Lecture Slides, Slides of Social Work

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Introduction to LAN/WAN

Network Layer (part II)

Topics

The Network Layer

• Introduction

• Routing (5.2)

• The Internet

‹

IP, IP addresses ‹

ARP (5.5.4)

‹

OSPF (5.5.5)

‹

BGP (5.5.6)

• Congestion Control (5.3)

Internet Structure

)

Backbones: high bandwidth lines, fast routers

)

Regional networks attached to backbones

)

LANs (universities, companies, ISP, etc)connected to regional network

Internet Protocol (IP)

)

IP concerned with routing (best effort)

)

Interesting options:

• security, strict source routing, loose source routing,

record route, timestamp

IP Addresses

)

IPv4: 32 bit addresses: 2

32

addresses (e.g.

ccc4.wpi.edu

has IP address 130.215.36.158)

)

Addresses controlled by ICANN

)

Previously

classfull

addressing (A, B, C, etc)

• Class A: fix first bit at 0, 2

7

networks, 2

24

hosts

• Class B: start with 10, 2

14

networks, 2

16

hosts, etc

• Class C: start with 110, 2

29

networks, 2

8

hosts

IP Addresses

)

Disproportional demands for address classes:

• Few organizations have up to 2

24

hosts (class A)

• Most want class B (

16

hosts)

• Quick depletion of some classes (class B) while others

remained un-used (class A)

)

Now Classless InterDomain Routing (CIDR)pronounced “

cydar”

• Basically allow variable network address (subnet mask)

and host address part

• Indicate length by adding subnet mask– Example: 223.1.1.0/24 means first 24 bits should be

treated as network address (subnet mask)

)

CIDR was temporary solution => IPv6 (128 bits)

Network to Data Link Address

Translation

Internet hosts use IP

Data link layer does not understand IP

• Ethernet uses 48-bit address– ex: ifconfig gives

00:10:4B:9E:B3:E

Q: How do IP addresses get mapped ontodata link layer addresses, such as Ethernet?

A: The Address Resolution Protocol (ARP)

Example 1

Host 1 wants to sends message to Host 2, say “

mary@eagle.cs.uni.edu

Docsity.com

Address Resolutioning

Host 1 broadcasts packet on LAN with IPaddress

asking “Who owns

IP address

Each machine checks its IP address.

Host 2 responds w/Ethernet address (E2)

Address Resolution Protocol

(ARP)

Host 1 data-link can then encapsulate IPpacket in frame addressed to E2 and dump

Ethernet board on Host 2 recognizes, stripsframe header and sends up to IP layer

ARP Optimizations

)

Send to H2 again?

• cache requests (time out in case of new card)

)

Many times, H1 requires ack from H

• send H1 IP + Ethernet (

, E1)

• H2 caches and uses if needed

)

Hosts broadcast mapping when boot

• host looks for its own IP address

‹

should get no answer, else don’t boot

• other Ethernet hosts all cache answer

Solutions

Solution 1

• CS router configured to respond to ARP

requests for

• Host 1 makes an ARP cache entry of

, E3)

‹

sends all traffic to Host 4 to CS router

• Called

Proxy ARP

Solution 2

• Host 1 knows Host 4 is on different subnet

‹

sends to CS router

• CS router doesn’t need to know about remote

networks

Either way ...

Host 1 packs IP into Enet frame to E

CS router receives frame, removes packet

• sees

to

Sends ARP packet onto FDDI

• learns

is at F

Puts packet into payload of FDDI frame andput on ring

EE router receives frame, removes packet ...

Routing on the Internet

Internet made up of Autonomous Systems(AS)

Standard for routing inside AS

• Interior Gateway Protocol–

OSPF

Standard for routing outside AS

• Exterior Gateway Protocol–

BGP

ASes, Backbones and Areas