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CSci4211: Introduction
1
Chapter 1: Introduction What is a Network? What is Internet?
Compared with postal service & telephone system Services provided “Nuts and Bolts” description
Packet Switching vs. Circuit Switching Fundamental Issues in Computer
Networking Protocol and Layered Architecture Internet Protocols, Architecture &
History Readings: Chapter 1, Lecture Notes
Goal and Motivating Questions
Our goal: • get “feel” and terminology
• more depth, detail later in course
• approach:– use Internet as example
Motivating Questions:
• What is internet? What’s so special about it?
• What’s a protocol?• How do I build a network?• How do I deal with the complexity?
• What does real Internet look like now?
• Why I download slowly?
CSci4211: Introduction 2
Internet is the network!
• It’s big!• It’s diverse!• It’s complex!• It’s everywhere (almost)!• … and it keeps growing and changing!
CSci4211: Introduction
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Inter-networking
– two or more nodes connected by a link, or
two or more networks connected by two or more nodes
A network can be defined recursively as...
Internet: networks of networks started as ARPAnet with only 4
nodesCSci4211:
Introduction
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Map of Internet
csci4211 Introduction
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csci4211 Introduction
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More gadgets are plugged in …
• servers, desktops, laptops, …
High-tier
Low-tier
High Mobility Low MobilityWide Area
Local Area
Wireless technologies revolutionizing Internet! WiFi, bluetooth, 3/4G cellular networks, …
mobile computinglocation services
• smart mobile phones, iPads, e-readers, … • now TVs, thermostats, smart meters, etc., soon toasters, fridges, …
CSci4211: Introduction
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Internet:a huge transformative & disruptive force!
What has become of the Internet: •Information Service and E-Commerce Platform
– deliver all kinds of information, news, music, video, shopping
– web, spotify, iTune, youtube, Netflix, Hulu, …
• Global Information Repository– store and search for all kinds of information
– google, flickr, dropbox, icloud, … •Cyberspace and Virtual Communities
– keep in touch with friends and strangers – email, facebook, twitter, …
• Enormous Super-Computer– mobile, cloud computing and servicesWe’re increasingly depending on it !
CSci4211: Introduction
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CSci4211: Introduction
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So what’s so special about the Internet?
But first, what is a Network?
CSci4211: Introduction
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What is a Network? There are many types of networks! Key Features of Networks
Providing certain services• transport goods, mail, information or data
Shared resources used by many users, often concurrently
Basic building blocks • nodes (active entities): process and transfer goods/data
• links (passive medium): passive “carrier” of goods/data
Typically distributed & “multi-hop”: two “end points” cannot directly reach each other
need other nodes/entities to relay
CSci4211: Introduction
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What is a Network …
Compare Internet with Postal Service and Telephone
System Services Provided Various Key Pieces and Their
Functions How the pieces work together to
provide services
CSci4211: Introduction
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Nuts and Bolts Description
Network is fundamentally distributed in nature: a collection of distinct entities: “nodes” and “links” Postal:
Mailboxes Local/Branch Postal Offices, Regional, Central Postal Offices Mail Sorting Machines Postmen, Delivery Trucks/Trains/Planes, Roads, …
Telephone: Phones Local Switching Office, Central Switching Offices, … Telephone Switches Wires
Internet ?
CSci4211: Introduction
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Internet: Building Blocks
• Nodes: PCs, special-purpose hardware, …– Hosts (or end systems): servers, PCs, laptops, mobile devices, smart meters, ……
– Switches: routers, switches, …
• Links: coax cable, optical fiber, wireless, …– point-to-point
– multiple access …
CSci4211: Introduction
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Inter-networking
– two or more nodes connected by a link, or
– two or more networks connected by two or more nodes
• A network can be defined recursively as...
• Internet: networks of networks
CSci4211: Introduction
16
Service PerspectiveBasic Services Provided Postal: deliver mail/package from people to people
First class, express mail, bulk rate, certified, registered, … Telephone: connect people for talking
You may get a busy dial tone Once connected, consistently good quality, unless using cell
phones Internet: transfer information between
people/machines Reliable connection-oriented or unreliably connectionless
services! You never get a busy dial tone, but things can be very slow! You can’t ask for express delivery (not at the moment at least!)
CSci4211: Introduction
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Fundamental Issues in NetworkingNetwork is a shared resource
– Provide services for many people at same time– Carry bits/information for many people at same
time •Switching and Multiplexing
– How to share resources among multiple users, and transfer data from one node to another node
•Naming and Addressing– How to find name/address of the party (or parties)
you would like to communicate with– Address: byte-string that identifies a node
• unicast, multicast and broadcast addresses
•Routing and (end-to-end) Forwarding: – Routing: process of determining how to send
packets towards the destination based on its address• find out neighbors, build “maps” (routing tables), …
– transfer data from source to destination “hop-by-hop”
CSci4211: Introduction
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What’s so special about the Internet?
• Internet is based on the notion of “packet switching”– enables statistical multiplexing– better utilization of network resources for transfer of “bursty” data traffic
CSci4211: Introduction
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Switching & Multiplexing• Network is a shared resource
– Provide services for many people at same time– Carry bits/information for many people at same time
• How do we do it? – Switching: how to deliver information from point A
to point B?– Multiplexing: how to share resources among many
users
Think about postal service and telephone system!
Switching and multiplexing are closely related!
CSci4211: Introduction
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Switching Strategies• Circuit switching
– set up a dedicated route (“circuit”) first – carry all bits of a “conversation” on one circuit
• original telephone network• Analogy: railroads and trains/subways
• Packet switching– divide information into small chunks (“packets”)– each packet delivered independently – “store-and-forward” packets
• Internet (also Postal Service, but they don’t tear your mail into
pieces first!)• Analogy: highways and cars
• Pros and Cons? - think taking subways vs. driving cars, during off-peak vs. rush
hours!
Analogy: railroad and train
CSci4211: Introduction
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Analogy: Highway and cars
CSci4211: Introduction
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Circuit Switchingnetwork resources (e.g., bandwidth) divided into “pieces”
• pieces allocated to calls
• resource piece idle if not used by owning call (no sharing)
dividing link bandwidth into “pieces” frequency division
time division code division Trivia Q:
You must have heard of the term “CDMA” (think the company Qualcom, for which it is most associated with), what does “CD” in CDMA stands for?
CSci4211: Introduction
23
Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
CSci4211: Introduction
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Numerical example• How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?– All links are 1.536 Mbps– Each link uses TDM with 24 slots/sec– 500 msec to establish end-to-end circuit
Let’s work it out!
10.5 seconds
CSci4211: Introduction
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Networks with Circuit Switching
e.g., conventional (fixed-line) telephone networks
End-end resources reserved for “call”
• link bandwidth, switch capacity
• dedicated resources: no sharing
• circuit-like (guaranteed) performance
• call setup requiredCSci4211:
Introduction26
CSci4211: Introduction
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Circuit Switched Networks
• All resources (e.g. communication links) needed by a call dedicated to that call for its duration– Example: telephone network– Call blocking when all resources are used
Packet SwitchingEach end-end “data stream” divided into packets
• users A, B packets share network resources
• each packet uses full link bandwidth
• resources used as needed
resource contention: aggregate resource demand can exceed amount available
congestion: packets queue, wait for link use
store and forward: packets move one hop at a time Node receives
complete packet before forwarding
Packets may suffer delay or losses!
Bandwidth division into “pieces”
Dedicated allocationResource reservation
28CSci4211: Introduction
CSci4211: Introduction
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Statistical Multiplexing
• Time division, but on demand rather than fixed• Reschedule link on a per-packet basis• Packets from different sources interleaved on the
link• Buffer packets that are contending for the link• Buffer buildup is called congestion• This is packet switching, used in computer networks
Packet Switching: Statistical Multiplexing
Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
A
B
C100 Mb/sEthernet
1.5 Mb/s
D E
statistical multiplexing
queue of packetswaiting for output
link
30CSci4211: Introduction
Packet-switching: store-and-forward
• Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps
• Entire packet must arrive at router before it can be transmitted on next link: store and forward
• delay = 3L/R (assuming zero propagation delay)
Example:• L = 7.5 Mbits• R = 1.5 Mbps• delay = ?
R R RL
more on delay later …
15 sec
CSci4211: Introduction
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Packet switching versus circuit switching
• 1 Mb/s link• each user:
– 100 kb/s when “active”
– active 10% of time
• circuit-switching: – 10 users
• packet switching: – with 35 users, probability > 10 active less than .0004
Packet switching allows more users to use network!
N users
1 Mbps link
Q: how did we get value 0.0004?
M
Nn
nMn ppn
M
1
1
CSci4211: Introduction
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CSci4211: Introduction
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Circuit Switching vs Packet Switching
Item Circuit-switched
Packet-switched
Dedicated “copper” path Yes No
Bandwidth available Fixed Dynamic
Potentially wasted bandwidth
Yes No (not really!)
Store-and-forward transmission
No Yes
Each packet/bit always follows the same route
Yes Not necessarily
Call setup Required Not Needed
When can congestion occur At setup time
On every packet
Effect of congestion Call blocking
Queuing delay
Packet switching vs. circuit switching
• Great for bursty data– resource sharing– simpler, no call setup
• Excessive congestion: packet delay and loss– protocols needed for reliable data transfer, congestion control
• Q: How to provide circuit-like behavior?– bandwidth guarantees needed for audio/video apps
– still an unsolved problem (chapter 7)
Is packet switching a “slam dunk winner?”
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
CSci4211: Introduction
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CSci4211: Introduction
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What’s so special about the Internet?
• Internet is based on the notion of “packet switching”– enables statistical multiplexing– better utilization of network resources for transfer of “bursty” data traffic
• Internet’s key organizational/architectural principle: “smart” end systems + “dumb” networks– architecture: functional division & function placement– hourglass Internet architecture: enables diverse applications and accommodates evolving technologies
– “dumb” network (core): simple packet-switched, store-forward, connectionless “datagram” service, with core functions: global addressing, routing & forwarding
– “smart” end systems/edges: servers, PCs, mobile devices, …; diverse and ever-emerging new applications!
CSci4211: Introduction 36
Internet Hourglass Architecture
WiFi, Bluetooth,Docsis, gMPLS, DWDM/fiber, …,3G/4G cellular, ….
p2p file sharing, skype, YouTube, Netflix, Cloud Computing
bitTorrent, DHT, SIP, DASH, ….
enabling diverse applications & new types of end devices
accommodating evolving & new technologies
netw
ork
core
netw
ork
edge
/end
hos
ts
37
“Dumb” Networks & “Smart” End Systems
• Five Layer Architecture:– Lower three layers are implemented everywhere– Top two layers are implemented only at hosts
Network
Datalink
Physical
Network
Datalink
Physical
Network
Datalink
Physical
Physical medium
Application
Transport
Host A
Application
Transport
Host B
Router
CSci4211: Introduction
An Overview of Network Structure:
a “horizontal view”• network edge: applications and hosts
• network core: – routers– network of networks
• access networks, physical media: communication links
CSci4211: Introduction
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What’s the Internet: “nuts and bolts” view
• millions of connected computing devices: hosts = end systems
• running network apps
• communication links– fiber, copper, radio, satellite
– transmission rate = bandwidth
• routers: forward packets (chunks of data)
local ISP
companynetwork
regional ISP
router workstation
servermobile
39CSci4211:
Introduction
The network edge:• end systems (hosts):
– run application programs– e.g. Web, email– at “edge of network”
• client/server model– client host requests, receives service from always-on server
– e.g. Web browser/server; email client/server
• peer-peer model:– minimal (or no) use of dedicated servers
– e.g. Skype, BitTorrent, KaZaA
CSci4211: Introduction
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The network edge:• end systems (hosts):
– run application programs– e.g. Web, email– at “edge of network”
• client/server model– client host requests, receives service from always-on server
– e.g. Web browser/server; email client/server
– Cloud & Mobile Computing
• peer-peer model:– minimal (or no) use of dedicated servers
– e.g. Skype, BitTorrent, KaZaA
cloud computing
CSci4211: Introduction
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Network edge: connection-oriented service
Goal: data transfer between end systems
• handshaking: setup (prepare for) data transfer ahead of time– Hello, hello back human protocol
– set up “state” in two communicating hosts
• TCP - Transmission Control Protocol – Internet’s connection-oriented service
TCP service [RFC 793]• reliable, in-order byte-stream data transfer– loss: acknowledgements and retransmissions
• flow control: – sender won’t overwhelm receiver
• congestion control: – senders “slow down sending rate” when network congested
CSci4211: Introduction
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Network edge: connectionless service
Goal: data transfer between end systems– same as before!
• UDP - User Datagram Protocol [RFC 768]: – connectionless – unreliable data transfer
– no flow control– no congestion control
App’s using TCP: • HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email), Flash videos, DASH stream videos
App’s using UDP:• streaming media, teleconferencing, DNS, Internet telephony
CSci4211: Introduction
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The Network Core
• mesh of interconnected routers shared by many users
• the fundamental questions: – how network is shared – how to find the other party (person, website, …) you want
– how is data transferred through net?
CSci4211: Introduction
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On the Internet Edge …
InternetInternet home users
banking &e-commercedumb &
smart phones POTS
VoIP
musicstreaming games
surveillance& security
video streaming & IPTV
web
• Large # of (mobile & stationary) users
• Large # of “dumb” or smart devices & appliances
• Some “always-on,” high-speed connection
• Others intermittent connectivity with varying bandwidth
• Diverse applications and services
• Heterogeneous technologies
smart pads &e-readers
social networks
sensors &smart home
others
CSci4211: Introduction
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Within the Internet “Cloud” Network Core:•big ISPs (& cellular providers) with large geographical span•As well as medium & smaller ISPs
And the “other end/edge”: •big content providers with huge data centers
High bandwidth, dense and rich topology
Enormous computing & storage capacities to support cloud, mobile computing/services
CSci4211: Introduction
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Well, Internet is too complex for me to learn.
How can they even build it?
And what’s a protocol & why do we need protocols?
Motivating Questions 3-5
CSci4211: Introduction
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Network Architecture(or organizational principles)
Networks are complex!
• many “pieces”:– hosts– routers– links of various media
– hardware, software
– applications– protocols– …..
Question: Is there any hope of organizing structure or principle of network?
Or at least our discussion of networks?
Network architecture: “blue prints” (or principles) regarding
functional division and function placement
CSci4211: Introduction
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Organization of air travel
• a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
CSci4211: Introduction
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ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departureairport
arrivalairport
intermediate air-trafficcontrol centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
Layering of airline functionality
Layers: each layer implements a service– via its own internal-layer actions– relying on services provided by layer below
CSci4211: Introduction
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Why Layering?
Dealing with complex systems:• explicit structure allows identification, relationship of complex system’s pieces– layered reference model for discussion
• modularization eases maintenance, updating of system– change of implementation of layer’s service transparent to rest of system
– e.g., change in gate procedure doesn’t affect rest of system
CSci4211: Introduction
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Internet Protocol Stack• application: supporting network applications– FTP, SMTP, HTTP, DASH, …
• transport: process-process data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
CSci4211: Introduction
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CSci4211: Introduction
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Layered Architecture
• Layering simplifies the architecture of complex system
• Layer N relies on services from layer N-1 to provide a service to layer N+1
• Interfaces define the services offered
• Service required from a lower layer is independent of its implementation– Layer N change doesn’t affect other layers
– Information/complexity hiding– Similar to object oriented methodology
CSci4211: Introduction
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Protocols and Services• Protocols are used to implement services
– Peering entities in layer N provide service by communicating with each other using the service provided by layer N-1
• Logical vs physical communication
What’s a protocol?human protocols:• “what’s the time?”
• “I have a question”
• introductions
network protocols:• machines rather than humans
• all communication activity in Internet governed by protocols (why this concept is so important!!!)
CSci4211: Introduction
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Make sure Bob is awake
Bob can speak English
Bob can understand English
Bob is willing to talk
1.
3
2
4
Human protocol
• protocols define:– Format.– Order of msgs sent and received among network entities (two or more)
– Actions taken on msg transmission, receipt
Hi
Hi
Got thetime?
AliceBob
Q: What are the purposes of first hi-hi exchange
2:00pm
CSci4211: Introduction
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What’s a protocol?a human protocol and a computer network protocol:
Q: Other human protocols? (e.g., in-class interaction)
Hi
Hi
Got thetime?
2:00
TCP connection request
TCP connectionresponse
Get http://www.cnn.com
<file>time
CSci4211: Introduction
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CSci4211: Introduction
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Protocols• Protocol: rules by which network elements communicate• Protocols define the agreement between peering entities
– The format and the meaning of messages exchanged
• Protocols in everyday life– Examples: traffic control, open round-table discussion etc
CSci4211: Introduction
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Protocol Packets• Protocol data units (PDUs):
– packets exchanged between peer entities• Service data units (SDUs):
– packets handed to a layer by an upper layer• Data at one layer is encapsulated in packet at a lower layer
– Envelope within envelope: PDU = SDU + (optional) header or trailer
sourceapplicat
iontranspor
tnetworklink
physical
HtHn M
segment Ht
datagram
destination
application
transport
networklink
physical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hnframe
CSci4211: Introduction
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CSci4211: Introduction
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Internet and ISO/OSI Reference Models
CSci4211: Introduction
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ISO/OSI Reference Model• Application layer
• Examples: smtp, http, ftp, dash, etc– process-to-process communication– all layers exist to support this layer
• Presentation layer (OSI only)– conversion of data to common format
• Example: “little endian” vs. “big endian” byte orders– multimedia streaming presentation (e.g., mpeg-dash)
• Session layer (OSI only)– session setup (and authentication)– recovery from failure (broken session)
• Internet applications perform presentation/session layer functions, e.g., “little” & “big” endian conversions
CSci4211: Introduction
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ISO/OSI Reference Model (cont’d)• Transport layer: end-to-end data delivery, e.g.,
– connection-oriented (TCP) or connection-less (UDP) services– error control, flow/congestion control, …
• Network layer: examples: IP, X.25– (global) naming and addressing, routing (build routing tables)– forwarding packets hop-by-hop across networks– avoidance of congested/failed links, traffic engineering, …
• Data link layer: data transfer between “neighboring” elements– Examples: Ethernet, 802.11 WiFi, PPP– framing and error/flow control– media access control
• Physical layer (EE stuff)– encoding/decoding information (bits) into physical media – modulating & transmitting raw bits (0/1) over wire
CSci4211: Introduction
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Comments on Layering• Layering simplifies the architecture of complex system
• Advantages– modularization eases maintenance and updating– hide lower layer complexity/implementation details from higher layers
• Layering considered harmful?– Q: which layer should implement what functionality?
• e.g., reliability, hop-by-hop basis or end-to-end basis?
• Possible Drawbacks?– possible duplication of functionality between layers
• error recovery at link layer and transport layer
– Other possible drawbacks?
CSci4211: Introduction
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Internet Protocol “Zoo”ap
plic
atio
n
SMTP telnet, ssh
NFS/RPC
FTP, SCP
DNSHTTP
RealAudio RealVideo
802.11 WiFi
Flash DASH
SOAP
…..…..
VoIP
IPTV
2.5G/3G/4G (GPRS,UMTS, WiMAX, LTE, …) Cellular Radio Networks
DWDM
MPLS/gMPLS
DSL or DOCSIS
PPP
ICMP, OSPF, RIP,BGP, …
P2P
What real Internet looks like now?
CSci4211: Introduction
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CSci4211: Introduction
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Internet Structure
LANs
International lines
Regional or local
ISPlocal ISPs
company university
National or tier-1 ISP
National or tier-1
ISP
IXPsor private peering
Regional ISPs
company
access via WiFi
hotspots
Internet: “networks of networks”!
Home users
Internet eXcangePoints
Home users
Internet structure: network of networks
• Roughly hierarchical• At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, L3, Cable and Wireless), national/international coverage– treat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
IXP
Tier-1 providers also interconnect at Internet Exchange Point
CSci4211: Introduction
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Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
….
………
POP: point-of-presence
CSci4211: Introduction
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Internet structure: network of networks
• “Tier-2” ISPs: smaller (often regional) ISPs– Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
IXP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider
Tier-2 ISPs also peer privately with each other, interconnect at IXP
CSci4211: Introduction
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Internet structure: network of networks
• “Tier-3” ISPs and local ISPs – last hop (“access”) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
IXP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet
CSci4211: Introduction
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Internet structure: network of networks
• a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
IXP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
traceroute www.cnn.com
CSci4211: Introduction
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Routing & forwarding:how do packets gofrom A to B?
B
A
Map of Internet
Why it takes so long to download my friends’ pictures from web?
Or why $#@! can’t I access the Internet now? Motivating Question 6
CSci4211: Introduction
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CSci4211: Introduction
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Fundamental Problems in Networking …
Or what can go wrong?• Bit-level errors: due to electrical interferences
• “Frame-level” errors: media access delay or frame collision due to contention/collision/interference
• Packet-level errors: packet delay or loss due to network congestion/buffer overflow
• Out of order delivery: packets may takes different paths
• Link/node failures: cable is cut or system crash
Four sources of packet delay
1. nodal processing: • check bit errors• determine output link
A
B
propagation
transmission
nodalprocessing queueing
2. queueing•time waiting at output link for transmission •depends on congestion level of router
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Delay in packet-switched networks
3. Transmission delay:• R=link bandwidth (bps)
• L=packet length (bits)
• time to send bits into link = L/R
4. Propagation delay:• d = length of physical link
• s = propagation speed in medium (~2x108 m/sec)
• propagation delay = d/sA
B
propagation
transmission
nodalprocessing queueing
Note: s and R are very different quantitites!
Nodal delay
• dproc = processing delay– typically a few microsecs or less
• dqueue = queuing delay– depends on congestion
• dtrans = transmission delay– = L/R, significant for low-speed links
• dprop = propagation delay– a few microsecs to hundreds of msecs
proptransqueueprocnodal ddddd
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Statistical Multiplexing and Queueing
A
B
C10 MbsEthernet
1.5 Mbs
45 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
CSci4211: Introduction
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Queueing delay (revisited)
• R=link bandwidth (bps)• L=packet length (bits)• a=average packet arrival rate
traffic intensity = La/R
• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large• La/R > 1: more “work” arriving than can be serviced, average delay infinite!
Queueing delay and Packet loss
• Queue (aka buffer) preceding link in buffer has finite capacity
• When packet arrives to full queue, packet is dropped (aka lost)
• lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all
CSci4211: Introduction
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“Real” Internet delays and routes
• What do “real” Internet delay & loss look like? • Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i:– sends three packets that will reach router i on path towards destination
– router i will return packets to sender– sender times interval between transmission and reply.
3 probes
3 probes
3 probes
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“Real” Internet delays and routes
Let’s Traceroute to www.bbc.com
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Throughput
• throughput: rate (bits/time unit) at which bits transferred between sender/receiver– instantaneous: rate at given point in time– average: rate over longer period of time
server, withfile of F bits
to send to client
link capacity
Rs bits/sec
link capacity
Rc bits/sec pipe that can
carryfluid at rate
Rs bits/sec)
pipe that can carry
fluid at rate
Rc bits/sec)
server sends bits
(fluid) into pipe
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Throughput (cont’d)
• Rs < Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
link on end-end path that constrains end-end throughput
bottleneck link
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Throughput: Internet scenario
10 connections (fairly) share backbone bottleneck
link R bits/sec
Rs
Rs
Rs
Rc
Rc
Rc
R
• per-connection end-end throughput: min(Rc,Rs,R/10)
• in practice: Rc or Rs is often bottleneck
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What’s the Internet: Recap
• protocols control sending, receiving of messages– e.g., TCP, IP, HTTP, FTP, PPP
• Internet: “network of networks”– loosely hierarchical– public Internet versus private intranet
• Internet standards– RFC: Request for comments– IETF: Internet Engineering Task Force
– IEEE
local ISP
companynetwork
regional ISP
router workstation
servermobile
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Fundamental Issues in NetworkingNetwork is a shared resource
– Provide services for many people at same time– Carry bits/information for many people at same
time
•Switching and Multiplexing – How to share resources among multiple users,
and transfer data from one node to another node
•Naming and Addressing– How to find name/address of the party (or
parties) you would like to communicate with– Address: byte-string that identifies a node
• unicast, multicast and broadcast addresses
•Routing and Switching/Forwarding: – process of determining how to send packets
towards the destination based on its address: finding out neighbors, building routing tables
– transferring data from source to destination
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Fundamental Problems in Networking …
Or what can go wrong?• Bit-level errors: due to electrical interferences
• “Frame-level” errors: media access delay or frame collision due to contention/collision/interference
• Packet-level errors: packet delay or loss due to network congestion/buffer overflow
• Out of order delivery: packets may takes different paths
• Link/node failures: cable is cut or system crash
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Fundamental Problems in Networking
What can be done?• Add redundancy to detect and correct erroneous packets
• Acknowledge received packets and retransmit lost packets
• Assign sequence numbers and reorder packets at the receiver
• Sense link/node failures and route around failed links/nodes
Goal: to fill the gap between what applications expect and what underlying technology provides
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The Internet Network layer
routingtable
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Data Link layer (Ethernet, WiFi, PPP, …)
Physical Layer (fiber optics, radio, …)
Networklayer
Introduction: SummaryAnswers to 6 motivating questions
• What is internet? What so special about it?
• What internet looks like now?
• How I deal with the complexity?
• What’s a protocol?• How I build a network?• Why do I suffer delays?
You now have: • context, overview, “feel” of networking
• more depth, detail to follow!
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Internet Summary• Computer networks/Internet use packet switching• Layered architecture for handling complexity & attaining maintainability– Key notions: protocols, services and interfaces
• Internet is based on TCP/IP protocol suite– Networks of networks!– Shared, distributed and complex system in global scale– No centralized authority
• Fundamental issues in networking– addressing/naming – routing/forwarding– error/flow/congestion control, media access control
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Readings for Next Week
• Read Chapter 1 • Review these lecture notes
– Read the supplementary notes that follow these one if you have time
• Read Chapter 2: sections 2.1 –2.6– Learn how web works – Learn how email works– Understand what Domain Name System does for us
– P2P File Sharing– Glance through Chapter 7: sections 7.1-7.2
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Supplementary Readings
• How big is Internet, Who pays for it?
• Access Network Technologies • NAPs, Private Peering and ISPs• Internet “Governing” Bodies• History of Internet
Access networks and physical media
Q: How to connect end systems to edge router?
• residential access nets
• institutional access networks (school, company)
• mobile access networks
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Physical Media
• physical link: transmitted data bit propagates across link
• guided media: – signals propagate in
solid media: copper, fiber
• unguided media: – signals propagate
freely e.g., radio
Twisted Pair (TP)• two insulated copper wires– Category 3:
traditional phone wires, 10 Mbps Ethernet
– Category 5 TP: 100Mbps Ethernet
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Physical Media: coax, fiber
Coaxial cable:• wire (signal carrier) within a wire (shield)– baseband: single
channel on cable– broadband: multiple
channel on cable
• bidirectional• common use in 10Mbs Ethernet
Fiber optic cable:
• glass fiber carrying light pulses
• high-speed operation:– 100Mbps Ethernet– high-speed point-to-
point transmission (e.g., 5 Gps)
• low error rate
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Physical media: radio
• signal carried in electromagnetic spectrum
• no physical “wire”• bidirectional• propagation environment effects:– reflection – obstruction by objects– interference
Radio link types:• microwave
– e.g. up to 45 Mbps channels
• LAN (e.g., waveLAN)– 2Mbps, 11Mbps
• wide-area (e.g., cellular)– e.g. CDPD, 10’s Kbps
• satellite– up to 50Mbps channel
(or multiple smaller channels)
– 270 Msec end-end delay– geosynchronous versus
LEOS
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Access networks
Q: How to connection end systems to edge router?
• residential access nets
• institutional access networks (school, company)
• mobile access networks
Keep in mind: • bandwidth (bits per second) of access network?
• shared or dedicated?
telephonenetwork Internet
homedial-upmodem
ISPmodem(e.g., AOL)
homePC
central office
Uses existing telephony infrastructure Home is connected to central office
up to 56Kbps direct access to router (often less)
Can’t surf and phone at same time: not “always on”
Residential access: Dial-up Modem
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telephonenetwork
DSLmodem
homePC
homephone
Internet
DSLAM
Existing phone line:0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data
splitter
centraloffice
Residential access: Digital Subscriber Line (DSL)
Also uses existing telephone infrastruture up to 1 Mbps upstream (today typically < 256 kbps)
up to 8 Mbps downstream (today typically < 1 Mbps)
dedicated physical line to telephone central office
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Residential access: cable modems
• HFC: hybrid fiber coax– asymmetric: up to 30Mbps downstream, 2 Mbps upstream
• Network of cable and fiber attaches homes to ISP router– homes share access to router
• Deployment: available via cable TV companies– Comcast triple play: Internet, vioce and TV
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Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html104CSci4211:
Introduction
Physical Media used in HFC
Coaxial cable:• two concentric copper conductors
• bidirectional• baseband:
– single channel on cable
– legacy Ethernet
• broadband:– multiple channels on cable
– HFC
Fiber optic cable: glass fiber carrying light pulses, each pulse a bit
high-speed operation: high-speed point-to-point transmission (e.g., 10’s-100’s Gps)
low error rate: repeaters spaced far apart ; immune to electromagnetic noiseCSci4211:
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5,000 homes
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork
Channels
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
DATA
DATA
CONTROL
1 2 3 4 5 6 7 8 9
FDM:
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100 Mbps
100 Mbps
100 Mbps1 Gbps
server
Ethernetswitch
Institutionalrouter
To Institution’sISP
Ethernet Internet access
• Typically used in companies, universities, etc
10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet Today, end systems typically connect into
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Wireless access networks• shared wireless access network connects end system to router– via base station aka “access point”
• wireless LANs:– 802.11b/g (WiFi): 11 or 54 Mbps
• wider-area wireless access– provided by telco operator– ~1Mbps over cellular system (3G)– next up (?): WiMAX (10’s Mbps) over
wide area and LTE (100’s Mbps)
• satellite– Kbps to 45Mbps channel (or multiple
smaller channels)– 270 msec end-end delay– geosynchronous versus low altitude
basestation
mobilehosts
router
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Case Study: Home networks
Typical home network components: • DSL or cable modem• router/firewall/NAT• Ethernet• wireless access point
wirelessaccess point
wirelesslaptops
router/firewall
cablemodem
to/fromcable
headend
Ethernet
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Origin of Internet? Started by U.S. research/military organizations:• Three Major Actors:
– DARPA: Defense Advanced Research Projects Agency
•funds technology with military goals– DoD: U.S. Department of Defense
•early adaptor of Internet technology for production use
– NSF: National Science Foundation
•funds university research
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Pre-Internet Modes of Human Telecommunications
The Dark Age before the Internet: before 1960Non-electrical (source: wikipedia)• Prehistoric: Fires, Beacons, Smoke signals, drums, Horns• 6th century BCE: (snail) mail (e.g., delivered by human couriers on
horse)• 5th century BCE: Pigeon post• 4th century BCE: Hydraulic semaphores, heliographs (shield signals)• 15th century CE: Maritime flag semaphores• 1672: First experimental acoustic (mechanical) telephone• 1790: Semaphore lines (optical telegraphs)• 1867: Signal lamps; 1877: Acoustic phonographElectrical:• 1830: telegraph• 1876: circuit-switching (telephone)• 1896: radio• TV (1940?) , and later cable TV (1970s)
Internet History
• 1961: Kleinrock - queueing theory shows effectiveness of packet-switching
• 1964: Baran - packet-switching in military nets
• 1967: ARPAnet conceived by Advanced Research Projects Agency
• 1969: first ARPAnet node operational
• 1972: – ARPAnet public demonstration
– NCP (Network Control Protocol) first host-host protocol
– first e-mail program– ARPAnet has 15 nodes
1961-1972: Early packet-switching principles
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Internet History
• 1970: ALOHAnet satellite network in Hawaii
• 1974: Cerf and Kahn - architecture for interconnecting networks
• 1976: Ethernet at Xerox PARC
• ate70’s: proprietary architectures: DECnet, SNA, XNA
• late 70’s: switching fixed length packets (ATM precursor)
• 1979: ARPAnet has 200 nodes
Cerf and Kahn’s internetworking principles:– minimalism, autonomy
- no internal changes required to interconnect networks
– best effort service model
– stateless routers– decentralized control
define today’s Internet architecture
1972-1980: Internetworking, new and proprietary nets
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Internet History
• 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
1980-1990: new protocols, a proliferation of networks
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Internet History
• Early 1990’s: ARPAnet decommissioned
• 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)
• early 1990s: Web– hypertext [Bush 1945, Nelson 1960’s]
– HTML, HTTP: Berners-Lee– 1994: Mosaic, later Netscape
– late 1990’s: commercialization of the Web
Late 1990’s – 2000’s:• more killer apps: instant
messaging, P2P file sharing• network security to
forefront• est. 50 million host, 100
million+ users• backbone links running at
Gbps• Napster, BitTorrent, …• Myspace, Facebook,
twitter,..• YouTube, Netflix, Hulu, …Now to the future:• … (your invention here!)
1990, 2000’s: commercialization, the Web, new apps
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Who Runs the Internet“nobody” really!• standards: Internet Engineering Task Force (IETF)
• names/numbers: The Internet Corporation for Assigned Names and Numbers (ICANN)
• DNS root server operators, domain name registrars
• networks: ISPs (Internet Service Providers), IXPs (Internet Exchange Points), ……
• fibers: telephone companies (mostly)• content: companies, universities, governments, individuals, …;
• content distribution networks, …
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Internet “Governing” Bodies
• Internet Society (ISOC): membership organization– raise funds for IAB, IETF& IESG, elect IAB
• Internet Engineering Task Force (IETF):– a body of several thousands or more volunteers– organized in working groups (WGs) – meet three times a year + email
• Internet Architecture Board– architectural oversight, elected by ISOC
• Steering Group (IESG): approves standards, – Internet standards, subset of RFC
• RFC: “Request For Comments”, since 1969– most are not standards, also
• experimental, informational and historic(al)
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Internet Names and Addresses
• Internet Corporation for Assigned Names and Numbers (ICAAN):– coordinate IPv4 & IPv6 address spaces, keep track of
numbers (e.g., protocol identifiers), delegates Internet address assignment to regional Internet registries
– manage top-level domain names & operations of root name servers
– designate authority for each top-level domain; create new TLDs • Regional Internet Registries: AfriNIC,
APNIC, ARIN, LACMIC, RIPE NCC:– manage the allocation and registration of Internet number resources
– e.g., hand out blocks of addresses to ISPs; assign AS numbers
– maintain WHOIS registries– ….