Extending Networks

Three Levels of Extension

• Physical Layer– Repeaters

• Link Layer– Bridges– Switches

• Network– Routers: “Connecting networks”

(Physical Layer) 5-4-3 Rule

• For IEEE 802.3 LANs, there is a limit on the length of a segment, how to extend the limit?– Adding repeaters (Hubs) while respecting the

5-4-3 rule:• Any path should traverse at most 5 segments• Any path should traverse at most 4 repeaters (hubs)• Any path should traverse at most 3 populated

segments

(Link Layer) Bridges

• Filter network component– Back learning

• Internetworking component (between LANs of different standards)

DIX and 802.3 Frames

Preamble Dest. Source. Type Type CRC

DIX

Preamble Dest. Source. Length Type CRC

IEEE 802.3

8 6 6 2 Up to 1500 4

8 6 6 2 Up to 1500 4

How to make the difference?

Connecting Networks

• LANs are connected with point-to-point links

• Packets are “routed” using another level of addresses other than MAC addresses

• Paths may be multihop

Network Layer

Read Tanenbaum (Chapter 5)

Where in the OSI Reference Model ?

Transport Layer

Network Layer

Link Layer

Physical Layer

Session Layer

Fundamental Functions of a Network Layer

• Addressing

• Routing

• Congestion control– Note that not all network protocols provide

congestion control

Network Layer Protocols : two families

• Connectionless– Each piece of information is sent as an

independent entity. No state information is kept in hosts or routers

• Connection oriented– There exists a virtual circuit over which all

pieces of information will transit.

Connectionless vs Connection-orientedTanenbaum Figure 5.4

• Circuit setup

• Addressing

• State information

• Routing

• Effect of routing failures

• Congestion control

Routing

• Routing policy: – updates the routing table

• Routing mechanism:– decides how to route a packet

depending on some policy (i.e, chooses the output line)

Routing structure

modifies

reads

Quite elementary

Routing Policy: Updating the Routing Tables

• Manual

• Using routing Daemons such as:– RIP– OSPF– EGP– BGP

Routing Mechanism

• Takes the decision how to route

• For example, IP protocol implements in each Internet host a mechanism to route

Desirable Properties of Routing

• Correct

• Simple

• Fair

• Robust

• STABLE

• Optimal

Routing Mechanisms

• Flooding

• Hot-potatoe

• Shortest path

• Fixed routing

• Dynamic routing

Overview

• Three big classes of routing policies– Distance vector routing (DVR)– Link state routing (LSR)– Hierarchical Routing (HR)

• Application to Internet– RIP is a DVR– OSPF is an LSR– EGP, BGP (DVR)

Distance Vector

• A node – tells its neighbors only– its distance to EVERY NODE in the network

• Example:– Initial A(0,1,4,inf),– B(1,0,1,1)– C(4,1,0,2)– D(inf,1,2,0)

A

B

D

C

1 1

1

4 4

A receives update from B ….

Distance Vector (Problems!!!)

• When ? – Links go down

• What kind of problem ?– Count-to-infinity– Example

• Solution– path vector

A B C

1 1

Link-State Routing

• A node – tells its EVERY node– its distance to its NEIGHBORS

• How ?– Send link-state packets (LSP)– using controlled flooding– Use Dijkstra’s algorithm

A

B

D

C

1 1

1

4 4

A receives update from B ….

LANs

Internal and External Protocols

Internet Backbone

Autonomous Systems

Autonomous Systems

Autonomous Systems

Autonomous Systems

Exterior GatewayProtocol

Interior GatewayProtocol

RIP (RFC 1058)(Routing Information Protocol)

• Interior gateway protocol

• Distance vector protocol

• Uses split horizon to avoid count-to-infinity (Does not advertise a cost to a neighbor if it is a next hop for that destination)

• Exchange each 30s

• Time-out after 180 s.

OSPF (RFC 1247)(Open Shortest Path First)

• Interior gateway protocol

• Link state protocol

• Uses directly IP (while RIP uses UDP)

BGP (RFC 1267)(Exterior Gateway Protocol)

• Exterior gateway protocol

• (Exception: BGP uses TCP !!!!)

• Path Vector protocol (+ policy attributes)

• Topology may be loop-free (BGP guarantees loop-freeness)

The Internet Protocol (IP) RFC 791

Read 5.5

A Connectionless Network Layer: the Internet Protocol (IP)

• The Internet Protocol is found on every:– host that is connected to Internet– router on a LAN connected to the Internet– router on the backbone

Application

Transport

Network IP

Link Layer

Fundamental Idea of IP

• Routes between Networks, not between hosts

• This allows shorter routing tables

IP Header (Figure 3.1, p. 34)

310 15 16V HL TOS Total Length

16-bit identification Flgs13-bit frag. offset

TTL Protocol 16-bit Hdr Checksum

32-bit source IP address

32-bit destination IP address

Options (Variable 0 ---> ??)

Data (TCP segment, or UDP Dtg,or ICMP ….)

Internet Addresses

• Class A

• Class B

• Class C

• Class D

• Class E

0 netid hostid7bits 24 bits

1 netid hostid14 bits 16 bits

0

1 netid hostid21 bits 8 bits

1 0

1 Multicast group ID28 bits

1 1 0

1 Future use27 bits

1 1 1 0

Special Internet Addresses

• This host

• Local host

• Local broadcast

• Remote

• Loopback 127.X.X.X

000000000000…..00000000000000000032 bits

00000….0000000 hostid

1111111111111…..11111111111111111

11111111111…..111111111NetID

Internet Addresses (Cont’d)

• Unicast addresses (Classes A,B, and C)

• Multicast addresses (Class D)

• Dotted notation : the 32-address is divided in 4 groups of 8 bits (byte, octet). Each byte is expressed in base 10 separated by dots

Subnetting

• Why to do subnetting ?

• How is it done ?

• Notion of subnet mask

Subnetting (RFC 950)

1 netid hostid14 bits 16 bits

0Class B

netid hostidn bits m bits

Example : netid hostid5 bits 11 bits

Subnetmask

Idea : IP1 IP2& (Bitwise AND) &

Subnetmask Subnetmask

= R1 = R2

R1 = R2 if IP1 and IP2 are on the same subnet

R1 # R2 if IP1 and IP2 are on different subnets

IP Routing

• Based on a routing table with entries having– Destination IP addr (host ID or net ID)– Next hop router IP address– flags (hostid or netid, next hop or connected

interface…)– network interface

IP Routing Cont’d

• 1) Try to find a complete IP address match in the routing table

• 2) If 1) fails, then try to find a match with network id

• if 1) and 2) fail search for default router

Some IP Helpers

• ARP (Address Resolution Protocol) : RFC 826

• RARP : Reverse Address Protocol (RFC 903)

• BOOTP (RFC 951, 1048, 1084) replaced by

• DHCP (RFC 1541)

• ICMP (RFC 792)

Extensions to IP

• IPv6

• Mobile IP

Conclusion

• You must know:– The functions of the network layer– the difference between connectionless and

connection oriented network protocols (what they can do and cannot do)

– IP protocol and helpers (ICMP, DHCP, ARP)– the general features introduced by IPv6– globally what is Mobile IP.