NETWORK LAYER IN PRACTICE: IP AND ATM

refereeoppositeNetworking and Communications

Oct 30, 2013 (3 years and 5 months ago)

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Weiqiang Sun

NETWORK LAYER IN PRACTICE: IP AND
ATM

Weiqiang Sun

The TCP/IP protocol suite

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Transmission Control Protocol / Internet Protocol


Developed by DARPA to connect universities and research labs

Applications

Transport

Network

Link

Telnet, FTP, email etc

TCP, UDP

IP, ICMP, IGMP

Device drivers, interface cards

TCP: Transmission Control Protocol

UDP: User Datagram Protocol

IP: Internet Protocol

The Internet Layered model

Weiqiang Sun

Internet sub
-
layer


A sub
-
layer between the transport and network layers is required when
various incompatible networks are joined together


This sub
-
layer is used at gateways between the different networks


In the internet this function is accomplished using the Internet Protocol (IP)

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IP

DLC Layer

Link 1

DLC Layer

Link 1

DLC Layer

Link 1

On a gateway connecting different types of networks,

IP is the protocol to realize inter
-
operability

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Internetworking with TCP/IP

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application

transport

IP

Data link and
lower layer

application

transport

IP

Data link and
lower layer

IP

Ethernet

Token
ring

Token ring

Ethernet

Ethernet

IP

protocol

IP

protocol

TCP/UDP protocol

FTP, HTTP, SMTP…

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The TCP/IP suite

5

PING

telnet &
rlogin

FTP

SMTP

X

Trace
route

DNS

TFTP

BOOTP

SNMP

RPC

TCP

UDP

ICMP

IP

IGMP

DATA
LINK

ARP

RARP

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IP addresses


32 bit address written as four decimal numbers


One per byte of address (202.120.39.134)


IP Address classes

6

10

Net ID

Host

ID

0

Net ID

Host

ID

8

32

16

32

110

Net ID

Host

ID

16

32

Class A Address

Class B Address

Class C Address

11100000

16

32

Class D Address,
(For multicast only)


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IPv4
address classes

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Routing a packet in the network

Routing table in 2

Dest

Next

Hop

Out
Intf

1

1

1

3

1

1

4

5

2

5

5

2

6

5

2

8

1

2

5

3

6

4

1

2

1

2

3

1

3

2

1

2

3

4

1

2

3

1

2

3

Routing table in 5

Dest

Next

Hop

Out
Intf

1

3

2

2

2

1

3

3

2

4

3

2

6

6

3

Routing table in 3

Dest

Next

Hop

Out
Intf

1

1

1

2

5

4

6

6

3

4

4

2

5

5

4

Routing table in 4

Dest

Next

Hop

Out
Intf

1

1

1

2

1

1

3

3

2

5

3

2

6

6

3

4

Weiqiang Sun

Host name


Each host has a unique name


Domain name system (DNS): a distributed
database that provides a mapping between IP
addresses and host names


E.g., 202.120.39.134


FRONT.SJTU.EDU.CN

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Weiqiang Sun

Internet standards


Internet Engineering Task Force (IETF)


Development on near term internet standards


Open body


Meets 3 times a year


Request For Comments (RFCs)


Official internet standards


Available from IETF web page: http://www.ietf.org/

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Weiqiang Sun

The Internet protocol (IP)


Routing packets across the network


Unreliable service


Best effort delivery


Recovery from lost packet must be done at higher layers


Connectionless


Packets are delivered independently


Can arrive out of order


Re
-
ordering must be done at higher layers


Current version V4, IPv4


Future IPv6

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Weiqiang Sun

IP header

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0 bit

Version

Header
length

Type of
Service

Total Length

32 bit

Identification

Flags

Fragment Offset

64 bit

Time
To
Live

Protocol

Header Checksum

96 bit

Source
IP Address

128 bit

Destination IP
Address

160 bit

Options

160or

192+


Data

Note that the minimum header size is 20 bytes, or 160 bits

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IP header

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Dynamic Host Configuration (DHCP)


Automated method for assigning network numbers


IP addresses, default routers


Computer contact DHCP server at Boot
-
up time


Server assigns IP address


Allows sharing of address space


More efficient use of address space


Adds scalability


Addresses are “leased” for some time


Not permanently assigned

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Weiqiang Sun

Address Resolution Protocol (ARP)


The role of the IP routing is to deliver the packet to its destination subnet


To the last hop router


Addressing inside a subnet, or a LAN, is based on local addresses, such as
Ethernet addresses


ARP provides a mapping between IP addresses and LAN addresses


RARP provides mapping from LAN addresses to IP addresses


Both accomplished by sending out a broadcast message


An ARP cache is maintained at each node with recent mappings to avoid
frequent address resolution (for better performance)

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Weiqiang Sun

ARP at source subnet

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R1

S

(4) I am here at

00
-
01
-
21
-
32
-
32
-
32

(3) Hi all~ Where is
my lovely router R1?

(1)
Computer S is configured to have a default router R1

(2)
S wants to send a message to D, and D is outside of the same LAN

(3)
S sends an ARP request for Ethernet Address of R1

(4)
R1 sends ARP responds to S

(5)
S sends the message to R1 with Ethernet addressing

(6)
R1 routes the packet to the next hop in the internet and the message
will be subsequently routed further toward D

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ARP at destination subnet

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R2

D

(3) Hi all~ I got a message for
202.120.39.134. Where is he?

(4) Hi R2~ I am here

at 00
-
01
-
01
-
11
-
AB
-
ED

(1)
An IP packet is delivered by the network from its source subnet to
router R2.

(2)
Router R2 realizes that the packet has reached its destination subnet
by comparing the destination address in the IP packet and its local
interface configurations (subnet address and mask)

(3)
Router R2 sends an ARP request on the interface to the subnet

(4)
Destination node D responses to the request

(5)
Packet is delivered to D with Ethernet addressing

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Routing in the multi
-
AS Internet


The Internet is divided into sub
-
networks, each under the control of a
single authority known as an Autonomous Systems (AS)


Routing algorithms are divided into two categories


Interior protocols (within an AS)


Exterior protocols (between ASs)


Interior protocols use shortest path algorithms


Distance vector proto. Based on Bellman
-
Ford


Link state proto. Based on Dijkstra’s algorithm


Exterior protocols route packets across ASs


Issue: no single cost metric, policy routing, etc


Hierarchical routing based on “peering” agreements


Example: Exterior Gateway Protocol (EGP) and Border Gateway Protocols (BGP)

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Weiqiang Sun

Border Gateway Protocol (BGP)


Routing between Autonomous systems


Find a path (no optimality) to destination (AS)


Path must satisfy policy criteria

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AS
corporation

AS

Large service
provider

AS

Large service
provider

AS

Small ISP

AS
corporation

AS
corporation

AS

Small ISP

AS

Small ISP

Transit AS

Multi
-
homed AS

(No transit traffic)

Stub AS

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BGP overview


BGP speaker


one per AS


Establishes (TCP) sessions with other “speakers” to exchange reachability
information


Border “gateways”


routers that interface between AS’s


BGP advertises complete paths to destination AS


Avoid loop problems


Enable policy decisions (e.g. avoid certain ASs)


AS numbers


centrally assigned 16 bit numbers for transit ASs

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128.64.3

128.61.2

192.12.2

Path to 128.64.2:
(AS
-
144, AS
-
367)

AS
-

12

AS
-

144

AS
-

367

AS
-

298

Weiqiang Sun

Relationship between ASs


ISP “tiers”


Tier
-
1 ISPs


provide global reachability


Tier
-
2 ISPs


regional/country


Tier
-
3 ISPs


local


Provider
-
customer relationship (transit)


Smaller ASs purchase internet access from
larger ones


Peering


ISPs of similar size are “peers” and
forward each other’s traffic at no charge


Paid peering: a small ISP may purchase
the right to peer with a larger provider


Policy issue


Which route would an ISP advertise?

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Tier
-
1 ISP

Tier
-
2 ISP

Tier
-
1 ISP

Tier
-
2 ISP

Tier
-
3 ISP

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IPv6


Effort started in 1991 as IPng


Motivation


Need to increase IP address space


Support for real
-
time applications


QoS


Security, mobility and auto
-
configuration


Major changes


Increased address space (128bit)


Support for QoS via Flow Label field


Simplified header


Transition to IPv6


Cannot be done at once; must support co
-
existance


Dual
-
stack: routers run both IPv4 and IPv6


Tunneling: IPv6 packets carried in payload of IPv4 packets, or vice versa

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Weiqiang Sun

QoS in the Internet


Quality of Service parameters


Dropped packets


Delay


Jitter


Out
-
of
-
order delivery


Error


Applications that require QoS


Multimedia streaming


IPTV


IP telephony, or VoIP


Video conferencing


Online game


Remote control




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Weiqiang Sun

QoS mechanisms


IntServ
: integrated services


best
-
effort service, real
-
time service, and
controlled link sharing


Resource reserved prior to data transfer


Resource released after transfer completes

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request

grant

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QoS mechanisms (cont.)


DiffServ
: differentiated services


Tagging on ingress edge node


Un
-
tagging on egress edge node


Routed/processed in network according to the tag/label


Realizes service differentiation through per
-
hop behavior (PHB)

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Weiqiang Sun

DiffServ

and MPLS

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MPLS
:
M
ulti
-
P
rotocol

L
abel
S
witching

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ATM
-

Asynchronous Transfer Mode


1980’s effort by the phone companies to develop an integrated network
standard (B
-
ISDN) that can support voice, data, video, etc.


ATM uses small (53 Bytes) fixed size packets called “cells”


Why cells? Cell switching has properties of both packet and circuit switching
Easier to implement high speed switches


Why 53 bytes?


Small cells are good for voice traffic (limit sampling delays) For 64Kbps voice it
takes 6 ms to fill a cell with data


ATM networks are connection oriented


Virtual circuits

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Weiqiang Sun

ATM Reference Architecture


Upper layers


Applications


TCP/IP


ATM adaptation layer


Similar to transport layer


Provides interface between upper layers
and ATM


Break messages into cells and
reassemble


ATM layer


Cell switching


Congestion control


Physical layer


ATM designed for SONET


Synchronous optical network


TDMA transmission scheme with 125
μs

frames

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Weiqiang Sun

ATM Cell format

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VPI/VCI

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ATM cell switches

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ATM summary


ATM is mostly used as a “core” network technology


ATM Advantages


Ability to provide QoS


Ability to do traffic management


Fast cell switching using relatively short VC numbers


ATM disadvantages


It not IP
-
most everything was design for TCP/IP


It’s not naturally an end
-
to
-
end protocol


Does not work well in heterogeneous environment


Was not design to inter
-
operate with other protocols


Not a good match for certain physical media (e.g., wireless)


Many of the benefits of ATM can be “borrowed” by IP


Cell switching core routers


Label switching mechanisms

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