computer science 457 networking and the internetgersch/cs457/01intro.pdf · computer science 457...
TRANSCRIPT
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Computer Science 457 Networking and the Internet
Fall 2014 Dr. Joseph Gersch
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Welcome to:
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Administrivia � Website: http://www.cs.colostate.edu/~cs457
� For both local and remote students � Syllabus, Outline, Grading Policies � Homework and Projects � Still deciding on RAMCT vs class mailing list. Stay tuned!!
� Instructor: Joseph Gersch � Office hours: 1-2 T,TH at CSB 360 or by appointment � Email: [email protected]
� Teaching Assistant: Awad Younis Mussa � Office hours: TBA � Email: [email protected]
� Recitation Sections (WF@ CSB 315) � Unix/C/Makefile tutorials � Homework/Project discussion/solutions
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Grading and Policies � Grading
� 10% Homework � 5% Project 1 � 10% Project 2 � 25% Project 3 � 20% Midterm � 30% Final
� Grading Policy � No credit for late homework. No exceptions! � See grading policy for late projects on the website � No make-up exams. It is your responsibility to check for
conflicts for Final Exam and make necessary arrangements.
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Workload • Frequent Homework Assignments
– Some from the book – Pen & paper problems, small programming exercises – Submissions MUST be typed! Be nice to the TA
• Exams – Midterm + Final (comprehensive)
• Course Projects – Language: C/C++ ONLY! (no Java) – Projects can be developed on your laptop, but MUST run on Linux
Lab machines
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What About the Book?
• All the material you need for the exams will be on the slides or in the lectures (hint: attend the lectures)
• However, the book is very valuable to clarify concepts and help you with the homework
• Don’t care what edition of the book you have, but make sure you solve the correct homework!
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How to Do Well in this Class
� Review your C/C++ now! Projects 2 and 3 are not trivial
� Do all the homework, but don’t lose sleep if you miss one or two assignments
� Understand the high-level picture during the lecture. You can review details later on your own, or ask me or the TA
� After each lecture, ask what you have learned and how it fits with the big picture
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Cheating Policy
• Simple cheating policy – Anyone caught cheating will FAIL the CLASS! – Regardless of what you cheated on, so don’t cheat. – We know that solutions to the book exercises are on the
web – don’t use those solutions!
• Plagiarism – see definition – http://writing.colostate.edu/guides/teaching/plagiarism/ – Rule of thumb: if you didn’t write it, it’s not yours – Cut and paste from the Web is plagiarism 99% of the
time
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Project 1: Warm-up (5%) • Simple socket programming assignment for
you to warm up – due next week Sunday!!
• Texting program: connect two processes using sockets and exchange messages up to 140 characters long
• Works by taking turns: first I type, then you type, etc.
• See description on the web, TA will answer questions during recitation
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Questions?
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Our Focus: the Internet
• There ARE other networks! – Cable, satellite, POTS, sensor nets, etc.
• Interested in Protocols and mechanisms – Protocol rules and algorithms – Investigate mechanism tradeoffs – Why this way and not another?
• First high-level picture, then the details
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By the End of the Class..
• You will have a good understanding of networking concepts
• You will know UDP/TCP/IP • You will understand how key applications
of the Internet work • You will start your own company and make
millions (well, maybe..)
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Chapter 1: Foundations
• Networking is a very broad topic – How does a wireless card talk to a base station? – How does a message get from CSU to Amazon.com? – How does an application (web browser, IM, P2P) work? – How to do deal with loss, delay, congestion, security?
• First Goal: Learn Big Picture and Terminology – What are network layers? – What is a protocol? – What are the basic network components?
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What is a Network?
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…
Point-to-Point
Multiple Access
… Wireless
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Definition of a Network
Two or more nodes connected by a link, or
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two or more networks connected via two or more nodes
Internet: network of networks
• A network can be defined recursively as...
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Some Basic Terms
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My Laptop - Running web
browser Web Server
www.cnn.com
My laptop and the web server are both End Systems = Hosts End systems can also include PDAs, sensors, cell phones, and generally any device using the network to communicate End systems are located at the network edge and connected to the network using communication links (wired or wireless)
Simple Example: Use web browser to lookup www.cnn.com
Internet
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Client/Server Communication Model
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My Laptop - Running web
browser Web Server
www.cnn.com
End systems may be classified as client, a server, both, or neither. Client - runs some program that requests services: web browser requests a page, email reader requests messages, ftp program requests files, etc. Server - runs some program that listens for requests and provides services. web server, email server, ftp server, etc. Client vs. server depends on what programs the end system is running.
Internet
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What is a Packet?
• Self-contained set of bits • Includes a header and (in most cases)
user data or payload • Header: needed by the network - contains
control information needed to deliver the packet to the destination
• Payload: can be anything – network does not care
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Example: IP Packet
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4-bit Version
4-bit Header Length
8-bit Type of Service (TOS) 16-bit Total Length (Bytes)
16-bit Identification 3-bit Flags 13-bit Fragment Offset
8-bit Time to Live (TTL)
8-bit Protocol 16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload
20-byte header
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Circuit vs. Packet Switching � Circuit switching:
� Reserve the circuit ahead of time, exclusive use by one connection � Like reserving your own lane in the highway, so your lane cannot be
used by others. But with networks, the highway has hundreds or thousands of lanes!!
� Example: Telephone network (POTS)
� Packet switching: � Nothing gets reserved, share the lanes with everyone else � Leads to Statistical Multiplexing!! (remember this term) � Example: Internet
� What are the advantages/disadvantages of each?
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Packet Switching Illustration
Interleave packets from different sources • Efficient: resources used on demand
– Accommodates multiple applications, bursty traffic
– Statistical multiplexing
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Statistical Multiplexing Gain Problem: Link is 1 Mbps; users require 0.1 mbps
when transmitting; users active only 10% of the time. How many users can we support?
Answer: • Circuit switching: can support 10 users. • Packet switching: with 35 users, probability that
>=10 are transmitting at the same time = 0.0004.
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Characteristics of Packet Switching
• Store and forward – Packets are self contained units – Can use alternate paths - reordering
• Contention – Congestion – Delay
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Network Components
• The network is composed of: • Hosts or endpoints (PCs, PDAs, cell-
phones, laptops, etc.) • Routers (specialized machines that route
packets) • The distinction is often blurred (caches,
content servers, etc.)
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A Network
• Network needs: – wires (from the phone or cable company) and
maybe some wireless equipment – Router(s) – a firewall? – an ISP to connect to the Internet – a network address block (e.g., 192.168.1.xxx) – servers
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An ISP
• ISP needs: – a (big?) block of addresses – connections to one or more other ISPs, peerings – multiple routers, some at exchange points – servers for your users: mail, web, etc. – servers for you: monitoring, billing, etc. – competent network admins (recommended) – an AUP (Acceptable Use Policy) – a lawyer
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Idealized Network Structure
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Backbones, Tier 1 (national, global)
Regional, Tier2
Campus LANs, Business
The Internet today looks nothing like this!
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Some Datacenter Pictures
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More Pictures
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Worst Datacenters!
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Host Configuration
• Host configuration needs: 1. a physical network cable or wireless card 2. an IP address 3. a network mask 4. a gateway 5. a DNS server (and other servers)
2-5 often obtained using DHCP (dynamic host configuration protocol)
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How Do Computers Find Each Other?
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Internet
Computer1 Computer 2
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What Are the Different Kinds of Names and Addresses?
• Have domain name (e.g., www.colostate.edu) – Global, human readable name
• DNS translates name to IP address (e.g. 128.82.103.106) – Global, understood by all IP networks
• Finally, we need local net address – e.g., Ethernet (08-00-2c-19-dc-45) – Local, works only on a particular type of link
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DHCP Simplified
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Local DHCP server
Broadcast: I need an IP address/mask
Computer 1
Here you go: 129.82.138.196/24 Here’s your gateway: 129.82.138.254/24 Here’s your DNS server: 129.82.103.106
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Finding Ether Address: Address Resolution (ARP)
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Ethernet
Broadcast: who knows the Ethernet address for 129.82.138.254?
(gateway address)
Ethernet
Broadcast: I do, it is 08-00-2c-19-dc-45
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Domain Naming System (DNS)
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Local DNS server
What’s the IP address for www.colostate.edu?
Computer 1
It is 129.82.103.106
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Sending a Packet Through the Internet
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R
R
R
R R H H
H
H
H
R
R H
R
Routers send packet to next closest point
H: Hosts
R: Routers
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How Do the Routers Know Where to Send Data?
• Each router has a forwarding table, which is populated by routing protocols
• Original Internet: forwarding table was manually updated! Network was small enough
• Routing protocols update tables based on “cost” (typically hop count) – Exchange tables with neighbors or everyone to build
a shortest path tree, then – Use neighbor leading to shortest path
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Programming with Layers � Sub-divide the problem
� Each layer relies on services from layer below � Each layer exports services to layer above
� Interface between layers defines interaction � Hides implementation details � Layers can change without disturbing other layers
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Host-to-host connectivity
Application-to-application channels
Application
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Network Layering
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Host Host
Application
Transport
Network
Link
User A User B Teleconferencing
Layering: technique to simplify complex systems
Peers
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IP Suite: End Hosts vs. Routers
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HTTP
TCP
IP
Ethernet interface
HTTP
TCP
IP
Ethernet interface
IP IP
Ethernet interface
Ethernet interface
SONET interface
SONET interface
host host
router router
HTTP message
TCP segment
IP packet IP packet IP packet
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Layer Encapsulation
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Get index.html
Connection ID
Source/Destination
Link Address
User A User B
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The Internet Hourglass Design
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[Deering98]
Remember this one! Why the narrow waist?
Applications
Application protocols
Transport protocols
Network protocol
Link layer protocols
Physical layer (hardware)
(framing, access ctl..)
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Designing Protocols
• What factors should be taken into account when designing protocols?
• Application requirements – can vary widely • Network characteristics – bandwidth,
delay, loss, etc. • Link characteristics – wired, wireless,
copper, optical, etc.
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Application Requirements vs. Network Service
• Application may provide input to network – Traffic data rate – Traffic pattern (bursty or constant bit rate) – Traffic target (multipoint or single destination, mobile
or fixed) – Internet does not accept any of this!
• Network service delivered to application – Delay characteristics – Loss characteristics – In the Internet applications measure and adapt
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Email, Reliable File Transfer
• Loss sensitive • Not delay sensitive relative to round trip
times • Point-to-point • Bursty
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Remote Login
• Loss sensitive • Delay sensitive
– Subject to interactive constraints – Can tolerate up to several hundreds of
milliseconds • Bursty • Point to point
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Network Audio
• Relatively low bandwidth – Digitized samples, packetized
• Delay variance sensitive • Loss tolerant • Possibly multipoint, long duration sessions
– Natural limit to number of simultaneous senders
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Network Video
• High bandwidth • Compressed video, bursty • Loss tolerance function of compression • Delay tolerance a function of interactivity • Possibly multipoint • Larger number of simultaneous sources
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Web
• Transactional traffic – Short requests, possibly large responses
• Loss (bug?) tolerant (hit refresh) • Delay sensitive
– Human interactivity • Point-to-point (multipoint is asynchronous)
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Bandwidth-Delay Product
• Very important consideration in network design
• Determines how much data is in transit (and thus cannot be stopped)
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Examples
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Link Type Bandwidth Distance RTT BW x Delay
Dial-up 56Kb/s 10Km 87 µs 5 bits
Wireless Lan 54Mb/s 50m 0.33µs 18 bits
Satellite 45 Mb/s 33,000 Km 230 ms 10 Mb
Cross-country fiber
10Gb/s 4,000 Km 40 ms 400 Mb
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Assignments
• Project 1 is posted on the web – the TA will cover it in recitation along with intro to socket programming and makefiles
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