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Computer Networking. Eliezer Dor ( eliezer dor@ gmail.com ). Teaching Assistant: Allon Wagner. Course Information. Lectures: Thursday 5 – 8 Dach 005 Recitation: Tuesday 16 – 17, 17 – 18 Dan David 001. Web site: http://www.cs.tau.ac.il/~allonwag/comnet2011B/index.html.

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computer networking

Computer Networking

Eliezer Dor (eliezer dor@gmail.com)

Teaching Assistant: Allon Wagner

course information
Course Information

Lectures: Thursday 5 – 8Dach 005

Recitation: Tuesday 16 – 17, 17 – 18 Dan David 001

Web site:

http://www.cs.tau.ac.il/~allonwag/comnet2011B/index.html

Main Book:

  • Kurose-Ross: A Top-down Approach to Computer Networking

Additional Books:

  • Keshav : An Engineering Approach to Computer Networking
  • Tanenbaum : Computer Networks
  • Bertsekas and Gallager : Data Networks
practical information
Practical Information

Homework assignment:

Mandatory

Both theoretical and programming

Grades:

Final Exam: 60%

theory exercises: 20%

Programming exercises: 20%

chapter 1

Chapter 1

Introduction

motivation
Motivation
  • 1st stage society: Agriculture, handicraft
  • 2nd stage society: Industrial, labor intensive
  • Today’s society:
    • automated industry with sophisticated logistics
    • information intensive:
      • business, knowledge, advertising, news, social interaction, recreation
  • Future society is likely to be even more information-dominated
the purpose of the network
The Purpose of the Network
  • serves network applications residing in hosts
  • applications at distinct hosts need to co-ordinate actions / co-operate
  • thus they need to communicate information to each other
  • network must deliver that information
    • to the right host
    • to the right application process / thread
  • network serves applications which serve users
information
Information
  • A representation of knowledge
  • Examples:
    • text, music, video, technical specifications
    • software, instructions, reports, alarms
  • Can be represented in two ways
    • analog (pictures / ideograms)
    • digital (bits)
  • the Digital Revolution
    • convert information as pictures to information as bits
    • networks move around bits instead of pictures
challenges
Challenges
  • make order in the jungle of applications
  • organize information into manageable units
  • keep track of info units sent/ moving/ received
    • take account of errors / misunderstandings etc.
  • move the bits through the network
    • find the destination host in the network jungle
      • using an efficient path
      • learn automatically the current network topology
    • make efficient use of link / router capacities
    • resolve competition for use of same resource
    • Cheaply, Securely, with Quality of Service,
this course s challenge
This course’s Challenge
  • To discuss this complexity in an organized way, so that we
    • understand the issues / alternatives
    • can follow/design/troubleshoot processes
  • Need to divide the job into functional layers
  • Understand the interrelation between them
  • These problems are beyond a specific technology
early communications systems
Early communications systems
  • telegraph, telephone
  • first used direct point to point links
  • when number of users grew:
  • introduced switching points/ configurable circuits
  • each call had a dedicated circuit for its duration

trunk group

phone line

Switched connection

data networks
Data Networks
  • set of interconnected nodes exchanging information
  • links are common usage
  • switching node must:
    • choose for each data unit a link bringing it closer to dest.
    • schedule their transmission on the common usage links (resolve the competition for the usage of the link)
qwest backbone
Qwest backbone

http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf

slide14

Networking Tasks – phone net. sol’n

  • Addressing- identify the end user
    • phone number 1-201-222-2673 = country code + region code + exchange + number
  • Routing – Find route from source to destination.
    • determined from phone number by static routing tables
  • Forwarding – how information is moved
    • circuit switching::a fixed circuit along path to destination
  • Information Units- How information is sent
    • voice samples; no addressing attached
    • samples sent continuously , 8000/sec
    • network must prepare source-dest. circuit in advance
networking tasks internet solution
Networking Tasks – Internet Solution

Addressing - identify the end user

  • IP addresses 132.66.48.37, =network number || host #

Routing- How to get from source to destination

  • routers learn automatically network topology
  • build routing tables / updated frequently

Forwarding – how information is moved

  • packet switching: move packets 1 by 1 through routers.

Information Units - How information is sent.

  • self-descriptive packet = data + header
  • header contains destination address
slide16
Telephone networks support a high end-to-end quality of service, but is expensive

Internet supports no quality of service but is flexible and cheap

Future networks will have to support a wide range of service qualities at a reasonable cost

history
1961: Kleinrock shows effectiveness of packet-switching

1964: Baran - packet-switching concept in military networks

1967: ARPAnet – by Advanced Research Projects Agency

1972: ARPAnet demonstrated publicly (15 nodes total)

first transport. email, protocols

1970: ALOHAnet satellite network in Hawaii

1973: Metcalfe’s PhD thesis proposes Ethernet

1974: Cerf and Kahn - architecture for internetworking

late70’s: proprietary architectures: DECnet, SNA, XNA

1982-5: TCP/IP, SMTP mail, DNS, FTP

1988: TCP congestion control

1991:ARPAnet commercialized: NSFnet , Internet

1989-93: WWW, browser, http, html, URL

History

LAN idea

cerf and kahn s internetworking principles
Cerf and Kahn’s internetworking principles:
  • autonomy - no internal changes required to interconnect networks
  • best effort service model
  • stateless routers
  • decentralized control

Defines today’s Internet architecture

why do we need protocols
Why do we need Protocols
  • Communication is between applications or other S/W entities
  • Its objective: enable cooperation on a common task
  • Need protocols to understand each other
    • Semantics: what I report/ want of you to do
    • Syntax/ format: how write/ read this info
open proprietary protocols
Open/ Proprietary Protocols
  • Open protocol can be used by anyone
    • it is published by a standards organization or a public consortium
    • e.g. draft standard. standard
  • Proprietary protocol is owned by a company
    • may be used subject to company’s agreement
why do we need standards
Why do we need Standards
  • Communication happens between entities
    • Hosts (personal computers, servers)
    • Routers
  • H/W entities produced by different vendors
  • S/W applications/ OS entities also
  • Need agreement to ensure correct, efficient and meaningful communication
    • this is called Interworking
organizations that issue standards
Organizations that Issue Standards
  • IETF (Internet Engineering Task Force)
  • IEEE (Institute for Electrical and Electronic Engineers)
  • ITU (International Telecommunications Union)
  • ISO (International Organization for Standardization)
  • W3C (World Wide Web Consortium)
why layering
Why Layering
  • Communication is a very complex task
  • What we need is: communication btw applications at distant hosts
  • What is reasonably feasible in one piece is: the ability to transfer a series of bits over a link
  • We need to bridge between very sophisticated applications and very primitive physical layer
  • What is needed is to divide the task’s functionality into well chosen parts
    • each part should be reasonably ‘easy’ to do
    • they should work well together
how to do layering
How to do Layering
  • Define a conceptual Layering Model
    • means: what is the function of each layer
    • how they cooperate / use each other’s services
  • Set principles for proper usage of the model
  • Build protocols for each layer
    • protocol is between same layer entities @ distinct nodes
    • there may be several protocols in each layer providing different type service for the layer’s function
  • Define interfaces between layers
    • interface (here) is between distinct layer entities at same node (computing device)
layering principles
Layering Principles

Modularity

  • each layer works independently of the others
    • information exchange only according to Interfaces defined in the Model
    • analogous to the Object Oriented principle in S/W eng.
  • this means:
    • don’t change/peek into internal variables of other layers
  • modularity is bypassed very seldom
    • only when there is no other solution to a problem

Transparency

  • layering should be invisible to user
layering benefits
Layering Benefits

Layering enables:

  • discussion/understanding of the issues
    • enables clear visualizing of relationships btw. functions
    • it’s impossible to think about all layers @ once
  • efficient development of protocols
    • each layer has a different functional focus
    • no need to think other layers when designing it
  • easy replacement/maintenance of protocols
    • as long as modularity & interfaces are adhered to

Layering is a good reference model for discussion

how do we communicate
How do we Communicate?

Hollywood, California

Bob

  • Send a mail from Alice to Bob
    • Alice in Champaign, Bob in Hollywood
  • Example:
    • US Postal Service

Champaign, Illinois

Alice

what does alice do
What does Alice do?
  • Bob’s address (his mailbox)
  • Bob’s name – in case people share mailbox
  • Postage – have to pay!
  • Alice’s own name and address
    • in case Bob wants to return a message
    • In case the mail has to be returned.

Alice

200 Cornfield Rd.

Champaign, IL 61820

Bob

100 Santa Monica Blvd.

Hollywood, CA 90028

what does bob do
What does Bob do?
  • Install a mailbox
  • Receive the mail
  • Get rid of envelope
  • Read the message

Alice

200 Cornfield Rd.

Champaign, IL 61820

Bob

100 Santa Monica Blvd.

Hollywood, CA 90028

slide31

LayersPeer entities

Champaign

Hollywood

Usergive parcel to P.O pick up parcel at P.O

Post office (P.O)counter handlingput parcel in mailbox

Ground transfer: on truck to airportfrom airport to dest. P.O

Airport transfer: loading on airplane take off the airplane

Airplane routing from source to destination

each layer implements a service

  • via its own internal-layer actions
  • relying on services provided by layer below
  • Qn: Find scenarios justifying adding extra layers to the mail model.
  • Name the layers and specify their place in model
what layers are necessary
What Layers are Necessary?

msg

Application

Application:

I received your msg …

I want you to do …

NETWORK

how to make sense?

Host B

Router

Host A

how to get it to B ?

1011001…

Receiver

Transmitter:

Physical Layer

what layers are necessary1
What Layers are Necessary?

msg

Application

Application:

I received your msg …

I want you to do …

NETWORK

how to make sense?

Host B

Router

Host A

Frame

Frame

Link Layer

Link L.

how to get it to B ?

1011001…

1011001…

Receiver

Transmitter:

Physical Layer

what layers are necessary2
What Layers are Necessary?

msg

Application

Application:

I received your msg …

I want you to do …

too many pieces!!

too many details !!

NETWORK

Net L.

Net Layer

Host B

Router

Host A

packet

packet

Frame

Frame

Link Layer

Link L.

1011001…

1011001…

1011001…

Receiver

Transmitter:

Physical Layer

what layers are necessary3
What Layers are Necessary?

msg

Application

Application:

I received your msg …

I want you to do …

Transport

Transport

NETWORK

Net L.

Net Layer

Net Layer

Host B

Router

Host A

Frame

Frame

packet

packet

Link Layer

Link L.

1011001…

1011001…

1011001…

Receiver

Transmitter:

Physical Layer

THE FIVE LAYER MODEL

application layer l5
Application Layer L5
  • Tasks:
    • write messages serving needs of application
      • proper type of semantics (meaning, information)
      • appropriate syntax/format, so that semantics is understood
    • keep track of the interaction process / state machine
  • Focus: on needs of a specific application type
  • Data unit: Message
  • Peer: the Application Layer at destination host
  • Uses: the Transport Layer
  • Used by: the application itself
  • Run by: the application
transport layer l4
Transport Layer L4
  • Main Tasks:
    • prepare data for transfer
      • fragment data into proper size segments / reassemble at dest.
      • add header which enables delivery to the correct appl. process
    • optional: error- /flow- /congestion-control
  • Data Unit: Segment
  • Focus: on control of End-to-End data transfer
  • Peer: the Transport Layer at destination host
  • Uses: the Network Layer
  • Used by: the Application Layer
  • Run by: the OS of the host
network layer l3
Network Layer L3
  • Main Tasks:
    • learn network topology in real time
    • prepare routing tables for fast usage in forwarding data
    • network layer (WAN) addressing
    • forward data from source to destination
  • Data Unit: Datagram / “packet”
  • Focus: on network and data fowarding
  • Peers: the Network Layer along the whole path
  • Uses: the Link Layer
  • Used by: the Transport Layer
  • Run by: the OS of the host, the router S/W
link layer l2
Link Layer L2
  • Main Tasks:
    • insert delimiters so start/end of frame can be known
      • physical layer may transfer an endless stream of bits
      • this is part of the task of the Link header and Link trailer
    • in LAN, access control/ link layer addressing
  • Data Unit: Frame
  • Focus: data transfer over a link
  • Peer: Link Layer at the other end of the link
  • Uses: the Physical Layer
  • Used by: the Network Layer
  • Run by: the NIC (Network interface card, כרטיס רשת)
physical layer l1
Physical Layer L1
  • Main Tasks:
    • transmit signals that encode bits 1 and 0
    • receive such signals and decode bits from them
    • synchronize the bit rate clocks of the peer nodes
  • Data Unit: Bit
  • Focus: bit transfer over a link
  • Peer: the Physical Layer @ other end of the link
  • Uses: the raw media: cable/ space
  • Used by: the Link Layer
  • Run by: transmitter/ receiver /wave propagation
protocols
Protocols
  • A protocol is a set of rules and formats that govern the communication between communicating peers
    • set of valid message formats - syntax
    • meaning of each message - semantics
  • Necessary for any function that requires cooperation between peers
protocols1
Protocols
  • A protocol provides a service
    • For example: the post office “registered” protocol for reliable parcel transfer service
  • Peer entities use a protocol to provide a service to a higher-level peer entity
    • for example, truck drivers use a protocol to present post offices with the abstraction of an unreliable parcel transfer service
  • In the layering model:
    • each layer gives service to next higher layer
iso osi reference model
ISO OSI reference model
  • Reference model
    • formally defines what is meant by a layer, a service etc.
  • Service architecture
    • describes the services provided by each layer and the service access point
  • Protocol architecture
    • set of protocols that implement the service architecture
    • compliant service architectures may still use non-compliant protocol architectures
the seven five layers
The seven/five Layers

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

Network

Data Link

Data Link

Data Link

Physical

Physical

Physical

There are only 5 (!!) in most architectures

Application

Transport

Intermediate

system

End system

End system

the seven layers protocol stack
The seven Layers - protocol stack

data

Application

Application

AH

data

Presentation

Presentation

PH

data

Session

Session

SH

data

Transport

Transport

Network

Network

NH

data

Data Link

Data Link

Physical

Physical

TH

data

Network

Data Link

DH+data+DT

Physical

bits

  • Session and presentation layers are not so important, and are often ignored
postal network
Postal network
  • Application: people using the postal system
  • Session and presentation: chief clerk sends some priority mail, and some by regular mail ; translator translates letters going abroad.
  • Transport layer: mail clerk sends a message, retransmits if not acked
  • Network layer: postal system computes a route and forwards the letters
  • Datalink layer: letters loaded on planes, trains, trucks
  • Physical layer: the driver/pilot carrying letters in sack
slide47

Internet protocol stack

application

transport

network

link

physical

  • application: supporting network applications
    • ftp, smtp, http
  • transport: host-host data transfer
    • tcp, udp
  • network: routing of datagrams from source to destination
    • ip, routing protocols
  • link: data transfer between neighboring network elements
    • ppp, ethernet, WiFi, token ring
  • physical: bits “on the wire”

Network access

encapsulation
Encapsulation

network

link

physical

message

segment

packet

frame

Tl

1011………

Tl’

Tl

Tl’

M

M

Ht

Ht

M

M

Hn

Hn

Hn

Hn

Ht

Ht

Ht

Ht

M

M

M

M

Hl’

Hl’

Hl

Hl

Hn

Hn

Hn

Hn

Ht

Ht

Ht

Ht

M

M

M

M

source host

application

transport

network

link

physical

datagram/

M – message

Ht – transport header

Hn – network header

Hl – link header

Tl – link trailer

destination host

application

transport

network

link

physical

router

service protocol at layer k
Service & protocol at layer k

to layer k+1

Service received by layer k from layer k-1

packet structure sending host view
L5:applicationlayergenerates a messageand passes it to transport layer

L4: transport layer adds its header (H4=Ht)

this generates a segmentwhich is passed to netwk layer(one message may be fragmented into several segments)

L3:networklayeradds its header (H3=Hn)

this generates a datagram, which is passed to link layer

L2:linklayer adds header (H2=Hl), trailer (T2=Tl)

this generates a frame which is passed to physical layer

L1:physical layer sends the frame as a sequence of bytes is on link

Packet structure: sending host view

H2

H3

H4

T2

frame

datagram

segment

Message

packet structure router view
Packet structure: router view

Receiving stage:

L1:physicallayerreceives frame,passes it to L2

L2:linklayer checks H2+T2 and removes them

this makes a datagramwhich is passed to network layer

H2

H3

T2

frame

datagram

H2*

H3

L3 payload

T2*

datagram

L3 payload

Sending stage:

  • L3: network layerdecides on which link to send
    • transfers datagram to L2
  • L2:linklayer adds newH2+T2and makes a frame
    • frame is passed to L1 which sends its bits on link

frame

packet structure destination view
L1:physical layer receives frame from link

L2:linklayer recognizes frame boundaries

checks H2+T2 and removes them

this makes a datagram, passed to network layer

L3:networklayerchecksH3 and removes it

this generates a segment, passed to transport layer

L4:transportlayer checks H4 and removes it

this leaves the messagewhich is saved in receive buffer

L5:applicationlayertakes message from buffer

Packet structure: destination view

H2

H3

H4

T2

frame

datagram

segment

Message

physical layer l11
Physical layer L1
  • Link Types:
    • Point to Point (usually continuous transmission)
    • LAN/multiple access (intermittent transmission)
  • What is contained in a standard:
    • shapes/sizes/material of connectors and cables/media
    • coding scheme to represent a bit
    • bit-level synchronization
  • Nodes:
    • Repeater
    • Hub (on LAN only)
  • Located: in transmitter/receiver of NIC (כרטיס רשת)
datalink layer l2
Datalink layer L2
  • Protocol Types
    • PTP protocol (HDLC, PPP, LAPD)
    • LAN protocol (Ethernet, Token Ring, WiFI)
      • contains a MAC sublayer)
  • What is contained in a protocol
    • header & trailer format
    • indication of start & end frame (delimitation)
    • in cont. transmission links: filler frame/marker format
    • in LAN: media access (MAC) rules, addressing rules
  • Nodes (in LAN only)
    • Bridge, (L2-) Switch
  • Located: in NIC of hosts, routers, swithces
network layer l31
Network layer L3
  • Network Types
    • Circuit switching/Packet switching (datagram or VC)
  • Protocol Types: Routing/ Forwarding
  • What is contained in a forwarding protocol
    • header format, address formats
    • forwarding rules (how to use routing tables)
  • What is contained in a routing protocol
    • rules/ messages for learning topology info
    • rules for building routing tables
  • Nodes: Router
  • Metaphor: Welds links into Host into Host (ETE) “channel”
  • Location: Host OS, Router S/W
network layer more l3
Network layer (more) L3
  • In datagram networks
    • provides both routing and data forwarding
  • In connection-oriented network
    • separate data plane and control plane
    • data plane only forwards and schedules data
    • control plane prepares (virtual) circuits before data is sent
  • Internet
    • forwarding by IP protocol (a datagram protocol)
      • best effort service (no reliability tools)
    • several routing protocols (RIP, OSFP, BGP)
network layer contd
Network layer (contd.)
  • At intermediate systems
    • participates in routing protocol to create routing tables
    • responsible for forwarding packets
    • schedules the transmission order of packets
    • chooses which packets to drop
  • At end-systems
    • primarily hides details of datalink layer
    • segments and reassemble
    • detects errors
transport layer l41
Transport layer L4
  • Protocol Types:
    • Reliable stream protocols (TCP, SCTP, SSL)
    • Unreliable datagram protocols (UDP)
  • What is contained a protocol
    • header format
    • user-process multiplexing rules (using port)

in Reliable protocols, also:

    • error control (ack, seq. #s, retransmission)
    • flow control (don’t overwhelm destination)
    • congestion control (don’t overload network)
  • Metaphor: Gives a Process to Process ETE channel
  • Location: Hosts only, part of OS
application layer l51
Application layer L5
  • Application Types:
    • User-oriented applications (Web, Mail, File xfer..)
      • Protocols: HTTP, SMTP+POP, FTP
    • Infrastructure applications (DNS, NTP)
  • Protocol Types:
    • Each application type has a separate protocol
  • What is contained a protocol
    • header format
    • rules for mutual interaction of peer processes
  • Metaphor: Talks to peer application about common job
  • Location: Hosts only, run by the application S/W
layer model summary
Layer Model Summary
  • Studied (basically) the Internet 5 Layer Model
  • OSI model (defined earlier, by ISO)
    • Contains 2 more layers:
      • Layer 5 (Sessiion)
      • Layer 6 (Presentation)
    • Application Layer is pushed to Layer 7
    • Not used in the Internet
  • Session layer
    • Duplex ctrl, Data priority, Special session controls
  • Presentation layer
    • Data structure standardization, encoding, encryption
  • see Extra slides for more details
history 1961 1972 early packet switching principles
1961: Kleinrock - queuing theory shows effectiveness of packet-switching

1964: Baran - packet-switching in military networks

1967: ARPAnet – conceived by Advanced Research Projects Agency

1969: first ARPAnet node operational

1972:ARPAnet demonstrated publicly

NCP (Netwk Control Protocol) 1st host-host protocol

first e-mail program

ARPAnet has 15 nodes

History1961-1972: Early packet-switching principles
history 1972 1980 internetworking new and proprietary nets
1970: ALOHAnet satellite network in Hawaii

1973: Metcalfe’s PhD thesis proposes Ethernet

1974: Cerf and Kahn - architecture for interconnecting networks

late70’s: proprietary architectures: DECnet, SNA, XNA

late 70’s: switching fixed length packets (ATM precursor)

1979: ARPAnet has 200 nodes

History 1972-1980: Internetworking, new and proprietary nets
cerf and kahn s internetworking principles1
Cerf and Kahn’s internetworking principles:
  • minimalism, autonomy - no internal changes required to interconnect networks
  • best effort service model
  • stateless routers
  • decentralized control

Defines today’s Internet architecture

history 1980 1990 new protocols proliferation of networks
1983: deployment of TCP/IP

1982: SMTP e-mail protocol defined

1983: DNS defined for name-to-IP-address translation

1985: FTP protocol defined

1988: TCP congestion control

new national networks: CSnet, BITnet, NSFnet, Minitel

100,000 hosts connected to confederation of networks

History 1980-1990: new protocols, proliferation of networks
history 1990 commercialization and www
early 1990’s:ARPAnet decommissioned

1991:NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

early 1990s: WWW

hypertext [Bush 1945, Nelson 1960’s]

HTML, http: Berners-Lee

1994: Mosaic, later Netscape

late 1990’s: commercialization of WWW

History1990 - : commercialization and WWW
demand and supply
Demand and Supply
  • Huge growth in users
    • The introduction of the web
  • Faster home access
    • Better user experience.
  • Infrastructure
    • Significant portion of telecommunication.
  • New evolving industries
    • Although, sometimes temporary setbacks
penetration around the globe 2009
Penetration around the Globe (2009)

http://www.internetworldstats.com/stats.htm

today s options
Today’s options
  • Modem: 56 K
  • ISDN: 64K – 128K
  • Frame Relay: 56K ++
  • Today High Speed Connections
    • Cable, ADSL, Satellite.
    • All are available at
      • 5Mb (2005)
      • 30 Mb (2009)

OBSOLETE

session layer
Session layer
  • Not common
  • Provides full-duplex service, expedited data delivery, and session synchronization
  • Internet
    • doesn’t have a standard session layer
session layer cont
Session layer (cont.)
  • Duplex
    • if transport layer is simplex, concatenates two transport endpoints together
  • Expedited data delivery
    • allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint
  • Synchronization
    • allows users to place marks in data stream and to roll back to a prespecified mark
presentation layer
Presentation layer
  • Usually ad hoc
  • Touches the application data

(Unlike other layers which deal with headers)

  • Hides data representation differences between applications
    • characters (ASCII, unicode, EBCDIC.)
  • Can also encrypt data
  • Internet
    • no standard presentation layer
    • only defines network byte order for 2- and 4-byte integers
slide78
עיקרון השכבות

Destination

Source

VoIP

Email(smtp)

ftp

Application

UDP

TCP

Transport

Network (IPv4)

Network

Modem

Ethernet

WiFi

Data-Link

Network

slide79

app1

app2

app3

UDP

TCP

Network (IPv4)

Modem

Ethernet

WiFi

עיקרון השכבות

Destination

Source

app3

app1

app2

UDP

TCP

Network (IPv4)

Modem

Ethernet

WiFi

Network

discussion
Discussion
  • Layers break a complex problem into smaller, simpler pieces.
  • Why seven layers?
    • Need a top and a bottom  2
    • Need to hide physical link; so need datalink  3
    • Need both end-to-end and hop-by-hop actions; so need at least the network and transport layers  5