Understanding Internet

cursefarmNetworking and Communications

Oct 24, 2013 (3 years and 7 months ago)

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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
-
oriented

Tanenbaum 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 Gateway

Protocol

Interior Gateway

Protocol

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)

31

0

15 16

V

HL

TOS

Total Length

16
-
bit identification

Flgs

13
-
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

hostid

7bits

24 bits

1

netid

hostid

14 bits

16 bits

0

1

netid

hostid

21 bits

8 bits

1

0

1

Multicast group ID

28 bits

1

1

0

1

Future use

27 bits

1

1

1

0

Special Internet Addresses


This host



Local host



Local broadcast



Remote



Loopback 127.X.X.X

000000000000…..000000000000000000

32 bits

00000….0000000

hostid

1111111111111…..11111111111111111

11111111111…..111111111

NetID

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

hostid

14 bits

16 bits

0

Class B

netid hostid

n bits m bits

Example :

netid hostid

5 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.