Course Overview and Introduction

1
Course Overview and Introduction

• CS 4251 Computer Networking IINick
  FeamsterSpring 2008

2
Goals

• You have presumably already learned the basics,
  so we will focus on
• Depth
• More in-depth treatment of various topics
• Hands-on experience and skills
• Testbeds Emulab, PlanetLab, VINI
• Tools Scriptroute, Click, XORP
• Analysis of real traces

3
Goals

• Design Experience and Insights
• Internet was based on design priorities
• Applications and requirements have changed
• You will gain experience re-evaluating design
  decisions and changing protocols
• Many recurring design tricks
• Tree forming
• Layering
• Resource allocation and sharing
• Naming

4
Logistics

• Course Web page
• http//www.gtnoise.net/classes/cs4251/fall_2008/
• Check this page regularly for updates to the
  syllabus, assignments, readings, etc.
• Course mailing list
• Sign up now/today if you are not already on it
• http//www.gtnoise.net/mailman/listinfo/cs4251

5
Who Am I?

• Nick Feamster
• Assistant Professor
• Networking Operations and Security
• Office Klaus 3348
• Email address on web page use subject CS
  4251
• Office Hours Wednesday, 2-3 p.m., by appt

6
Overview of Lectures

• Holistic approach
• Lectures organized by theme
• Tree forming/path finding
• Layering
• Resource allocation and sharing
• Naming
• Textbook reading, research papers, current
  events
• Read the readings before class!
• Historically, many things covered in class that
  are not in texts

7
Lecture Structure User-Generated

• One strongly positive review of last years
  course just in time topics
• This year Formalize this notion
• Every Friday Post a link to the course wiki
  (link soon) with a paper and one-line topic
  summary
• Voting over weekend
• Discuss paper in second half of Wednesday lecture
• I will do this, too

8
Networking in Current Events
Threats to the Internets naming system
Network Neutrality
9
Other Things Youll Learn

• How does BitTorrent find your file?
• How does the Georgia Tech wireless network allow
  you to roam across campus with the same IP
  address?
• How do ISPs connect to one another?
• Protocols, Economics,
• What could you do with two (or more) Internet
  connections at home?

10
Still More Things Youll Learn

• How many bits can you push over a physical
  channel?
• How can you use encoding to increase this?
• Whats inside a router?
• Function, power issues, trends (e.g.,
  programmability)
• Performance guarantees (e.g., telephony, video)?
• Can a networks resources be subdivided?

11
Still More Things Youll Learn

• Are we running out of IP addresses? Who cares,
  and how can we combat this?
• How do we reduce power utilization in data
  centers?
• What are the bad guys doing?
• Can we stop unwanted traffic?
• How do we make it easier to run the network?
• How do we make the network go faster?
• Why is it so hard to figure out whats wrong?
• Social networks?

12
Class Components and Grading

• Problem sets (20)
• Paper and pencil
• First assignment September 3
• Hands-on Assignments (30)
• Experience with tools and traces
• 2 Quizzes (25)
• Quiz March 3
• Final will set date soon (perhaps last week of
  class)
• 1 Project (25)
• TBD. Work in groups. Programming/analysis/etc.
• Most likely Pict from a list, or propose your
  own
• Late policy Maximum of 72 hours late throughout
  the term

13
Collaboration Policy

• See the Georgia Tech Honor Code
• Working together on assignments is fine, but you
  must turn in your own assignments, and ultimately
  write your own code, analysis, etc.

14
Who are you?

• Why are you taking this class?
• What do you hope to learn?
• (What have you learned already)
• What do you want out of a class project?
• Did you take 3251?

15
Overview of Course Content
16
Themes

• Routing Trees and Paths
• The Protocol Stack Protocols and Layering
• Resource Allocation
• Naming
• Trust
• Other themes
• Hierarchy
• Caching
• Randomization

17
The Internet A Network of Networks
Autonomous Systems (ASes)
Abilene
Comcast
ATT
Cogent

• Interconnected of the Internet Service Providers
  (ISPs) provide data communications services
• Networks are connected using routers that support
  communication in a hierarchical fashion
• Often need other special devices at the
  boundaries for security, accounting,
• Hosts and networks have to follow a common set of
  rules (protocols)

18
Challenges

• Scale 100,000,000s of hosts
• Heterogeneity
• 25,000 administrative domains (competing!)
• Thousands of applications
• Lots of users
• Diversity of network technologies and media
• Security Adversarial environment

19
Trends and Open Problems

• Reducing power consumption
• E.g., in data centers
• Making networks easier to manage
• Improving trust/identity in networks
• Spam, phishing attacks, etc.
• Policy-related issues (net neutrality)
• Programmability in routers/switches

20
Tree Forming and Route Finding
21
Computing Routes

• To deal with large scale, Internet routing
  employs hierarchy
• Internet Service Providers connect to one another
  with interdomain routing protocols (BGP)
• ISPs have business relationships with one another
• ISPs have PoPs that are connected with
  intradomain routing protocols

22
Gateways Routers and Switches

• Interconnect nodes to nodes
• And networks to networks
• No state about ongoing connections
• Stateless packet switches
• We can also think of your home router/NAT as
  performing the function of a gateway

68.211.6.12050878
Home Network
Internet
68.211.6.12050879
(more on NATs in lecture 17)
23
Challenge Scale
24
The Protocol Stack
25
Protocols Interconnection

• The syntax and semantics by which hosts and nodes
  agree on how to talk
• Must be standardized and agreed upon by all
  parties
• Standardization process
• IETF Requests for Comments (RFC)
• De-facto standards
• Format of messages
• Expectations for message delivery

26
Layering

• Helps manage complexity
• Each layer
• Relies on services from layer below
• Provides services to layer above
• For example IP (network) layer
• IP relies on connectivity to next hop, access to
  medium
• IP provides a datagram service
• Best effort delivery
• Packets may be lost, corrupted, reordered, etc.
• Layers on top of IP (e.g., TCP) may guarantee
  reliable, in-order delivery

27
Layering Mechanism Encapsulation
User A
User B
Application(message)Transport(segment)Networ
k(datagram)Link (frame)
Get index.html
Connection ID
Source/Destination
Link Address

• This can be more complex
• Example Network layers can be encapsulated
  within another network layer

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The Internet Protocol Stack

• Need to interconnect many existing networks
• Hide underlying technology from applications
• Decisions
• Network provides minimal functionality
• IP as the Narrow waist

Applications
Technology
29
The Narrow Waist

• Facilitates interconnection and interoperability
• IP over anything, anything over IP
• Has allowed for much innovation both above and
  below the IP layer of the stack
• Any device with an IP stack can get on the
  Internet
• Drawback very difficult to make changes to IP

30
Resource Sharing
31
Resource Sharing

• How? Multiplexing
• Switched network
• Party A gets resources sometimes
• Party B gets them sometimes
• Interior nodes (Routers or Switches)
  arbitrate access to resources

32
Circuit Switching

• Resources are reserved
• Source first establishes a connection (circuit)
  to the destination
• Source sends the data over the circuit
• Constant transmission rate
• Example telephone network
• Early early versions Human-mediated switches.
• Early versions End-to-end electrical connection
• Today Virtual circuits or lambda switching

33
Resource Sharing in Circuit-Switched Networks

• Frequency-Division Multiplexing (FDM)
• Link dedicates a frequency to each connection
• Width of this frequency band is called
  bandwidth
• We will discuss the capacity in Lecture 10
• Time-Division Multiplexing
• Each circuit gets all of the bandwidth on a link
  for brief periods of time

34
Circuit Switching

• Advantages
• Fast and simple data transfer, once the circuit
  has been established
• Predictable performance since the circuit
  provides isolation from other users
• Guaranteed bandwidth
• Disadvantages
• What about bursty traffic?
• Users with differing needs for bandwidth
• What if all resources are allocated?

35
Packet Switching

• Resources are not reserved
• Packets are self-contained
• Each has a destination address
• Source may have to break up single message
• Each packet travels independently to the
  destination host
• Routers and switches use the address in the
  packet to determine how to forward the packets

36
Resource Sharing Packet Switching

• Statistical multiplexing
• Switches arbitrate between inputs
• Can send from any input thats ready
• Links are never idle when traffic to send
• Efficiency!
• Requires buffering/queues
• Implies a service model/discipline (Lecture 21)

37
Delay in Packet Switched Networks

• Four contributors to hop-by-hop delay
• Processing Lookup, etc. (Lectures 6 and 7)
• Queueing Time the packet must wait before being
  transmitted (Lecture 21)
• Transmission time to push the packet onto the
  link
• Propagation time for the packet to propagate
  from A to B
• End-to-end performance metric throughput
• What (else) affects throughput

38
Forwarding Packet-Switched Networks

• Each packet contains a destination in the header
• Much like a postal address on an envelope
• Each hop (router or switch) inspects the
  destination address to determine the next hop
• Will a packet always take the same path?
• How do the hops know how to forward packets?