Chapter 4 - Internetworking

Networks are built with different technology We want to be able to connect these differing

networks This chapter is about the problems of

interconnecting different networks. Basically layer 3 – the Network layer Routers are the main network device

Main Issues

IP or Internet protocol Finding efficient, loop-free paths through the

constituent networks Internet problems: address space, large

routing tables, scaleable networks Multicast

What is an Internetwork?

Look over figures 4.1and 4.2 They are graphically describing the issue of

interconnecting differing networks and a simple way of looking at the overall changing of protocols

IP service model: best effort

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8

IP Packet Structure

Note in particular: source address destination address TTL (time-to-live) Protocol (higher level-TCP, UDP etc.)

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

IP Addresses

Examples: www.cnn.com 64.236.16.20

Registered to billionaire Ted Turner, one big site!! www.nvc.cs.vt.edu 208.22.18.79

CS department site, a small to medium site www.somuchdata.com hosted at 216.40.247.57

Registered to me, William May (used to advertise a book I wrote three years ago), a very small site!

IP Addresses and Classes

Class A, B and C addresses Shown in figure 4.6 For most medium to large organizations like

companies, universities, government offices etc. Class A is way too big and Class C is too small

Class B is where the crunch came

Network Host

7 24

0(a)

Network Host

14 16

1 0(b)

Network Host

21 8

1 1 0(c)

Datagram forwarding in IP

Every IP datagram contains the destination IP address

The “network” part of the address uniquely identifies a single physical network

All hosts that share the same network part of the address, are connected to the same physical network and can communicate with each other by sending frames over that network

So just how does the datagram get forwarded?

Two parts Routers forward the datagram based on the

network part of the address At the end there has to be a (local) table of

address pairs – that is a table that maps IP addresses into physical addresses ARP (Address Resolution Protocol)

And how does a sender determine where to send the datagram?

A host needs the address of the first router to send a datagram on its way

Called the default router DHCP is one common way of learning which

is your default router

DHCPrelay

DHCPserver

Other networks

Unicast to server

Broadcast

Host

Virtual Networks and Tunnels

Figure 4.12 explains it best, another level of encapsulation

IP header ,Destination = 2.x

IP payload

IP header ,Destination = 10.0.0.1

IP header ,Destination = 2.x

IP payload

IP header ,Destination = 2.x

IP payload

Network 1 R1 Internetwork Network 2R2

10.0.0.1

Routing

Routing table – generally contains mappings from network numbers to next hops (which are labeled as output ports on the router)

“Routing is the process by which forwarding tables are built.” (page 281)

Routing Protocols

Routers talking to other routers (very roughly-experts would shoot me for this

analogy!) the router analog of the spanning tree procedure for switches

Routing Protocols

We will look at several: Static routes (work fine for small internetworks) RIP (most widely used, simple but can be used on

reasonably large internetworks) OSPF (scaleable, good for campus-sized

internetworks) BGP (used by ISPs, very complicated, we will

only discuss in passing)

RIP

The ICND book has a great presentation of this protocol

I recommend going over that presentation, it is mainly pictures of how the routing table changes

Distance vector – RIP counts “hops”

4

3

6

21

9

1

1D

A

FE

B

C

D

G

A

F

E

B

C

A

C

1

2

3

B

D

4

5

6

Address of net 2

Distance to net 2

Command Must be zero

Family of net 2 Address of net 2

Family of net 1 Address of net 1

Address of net 1

Distance to net 1

Version

0 8 16 31

OSPF

Link State Figure 4.17 (next slide) show the basics of

how the routers talk to each other

X A

C B D

(a)

X A

C B D

(b)

X A

C B D

(c)

X A

C B D

(d)

D

A

B

C

5 3

211

10

OSPF Hierarchy

Hierarchy is one of the main tools to make systems more scaleable

OSPF allows more hierarchy to be imposed by partitioning a domain into areas

This means: a router within a domain does not necessarily need to know how to reach every network within that domain – cuts down on the information that has to be stored and processed

R9 R7R8

R5R6

R4

Area 1Area 0

Area 3

Area 2

R1 R3

R2

Authentication

Version Type Message length

Checksum Authentication type

SourceAddr

AreaId

0 8 16 31

LS Age Options Type=1

0 Flags 0 Number of links

Link type Num_TOS Metric

Link state ID

Advertising router

LS sequence number

Link ID

Link data

Optional TOS information

More links

LS checksum Length

The Internet

1990 form in figure 4.23 Today’s is shown next

Backbone service provider

Peeringpoint

Peeringpoint

Large corporation

Large corporation

Smallcorporation

“Consumer ” ISP

“Consumer ” ISP

“ Consumer ” ISP

Exhaustion of IP Addresses

~4,000,000,000 (232 addresses) are not enough!

Class B particularly bad off Subnetting and CIDR (Classless InterDomain

Routing) are a temporary solution

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

111111111111111111111111 00000000

Network number Host IDSubnet ID

Subnet mask: 255.255.255.128Subnet number: 128.96.34.0

128.96.34.15128.96.34.1

H1

R1

128.96.34.130 Subnet mask: 255.255.255.128Subnet number: 128.96.34.128

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0

H3

BGP

Additional level of hierarchy: AS or Autonomous Systems

Page 322 explains three major problems with interdomain routing and the Internet. BGP is designed to help with these

A very hard protocol, the book only skims over it

Border gateway(advertises path to

11000000000001)

Regional network

Corporation X(11000000000001000001)

Corporation Y(11000000000001000000)

R1

Autonomous system 1R2

R3

Autonomous system 2R4

R5 R6

Backbone network(AS 1)

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23

IPv6

Aimed at solving several of the problems that we have seen

The book describes the packet at a high level but doesn’t focus on what IPv6 will do for the Internet of the future

Multicast

Motivation: there are applications that want to send a packet to more than one destination host

Forcing the source to send a separate packet to each destination host wastes resources big time!

Multicast (a work still in progress) is aimed at making this procedure more efficient by not forcing separate packets on the sender

AR1 R2

B

R3 R4 R5

CR6 R7

Source

AR1 R2

B

R3 R4 R5

CR6 R7

Source

AR1

R2

B

R3 R4 R5

CR6 R7

Source

AR1 R2

B

R3 R4 R5

CR6 R7