Overview

1
Overview

• COS 461 Computer Networks
• Spring 2006 (MW 130-250 in Friend 109)
• Jennifer Rexford
• Teaching Assistant Mike Wawrzoniak
• http//www.cs.princeton.edu/courses/archive/spring
  06/cos461/

2
Goals of Todays Class

• Course overview (45 minutes)
• Review of the material in the course
• Preparation for Wednesdays exam
• Course evaluations (15 minutes)
• Scan-tron form
• Written responses
• Assignment 3 (20 minutes)
• Overview of assignment from Mike W
• Opportunity to ask questions

3
Important Dates

• Second midterm exam Wednesday May 3
• Exam during class time (130-250pm)
• Room 104 in the CS building, just as last time
• Open notes, open book, and open slides
• Covering material from lecture 11 onward
• Assignment 3 Tuesday May 16 at 9pm
• Assignment 3 is due on Deans Date at 9pm
• Office hours and mailing list during reading
  period
• Research projects Tuesday May 16 at 9pm
• Write-up of research projects due on Deans Date
• Send via e-mail to jrex_at_cs.princeton.edu

4
Goals of This Course

• Skill network programming
• Socket programming
• Designing and implementing protocols
• Knowledge how the Internet works
• IP protocol suite
• Internet architecture
• Applications (Web, e-mail, P2P, VoIP, )
• Insight key concepts in networking
• Protocols
• Layering
• Resource allocation
• Naming

5
IP Suite End Hosts vs. Routers
host
host
HTTP message
HTTP
HTTP
TCP segment
TCP
TCP
router
router
IP packet
IP packet
IP packet
IP
Ethernet interface
Ethernet interface
SONET interface
Ethernet interface
SONET interface
6
Shuttling Data at Different Layers

• Different devices switch different things
• Physical layer electrical signals (repeaters and
  hubs)
• Link layer frames (bridges and switches)
• Network layer packets (routers)

Application gateway
Transport gateway
Frameheader
Packetheader
TCPheader
User data
Router
Bridge, switch
Repeater, hub
7
Physical Layer Repeaters and Hubs

• Analog electronic devices
• Continuously monitors electrical signals on each
  LAN
• Transmits an amplified copy

hub
hub
hub
hub
8
Link Layer Bridges and Switches

• Connects two or more LANs at the link layer
• Extracts destination address from the frame
• Looks up the destination in a table
• Forwards the frame to the appropriate LAN segment

B
host
host
host
host
A
C
Bridge
switch
host
host
host
host
D
9
Self Learning Building the Table

• When a frame arrives
• Inspect the source MAC address
• Associate the address with the incoming interface
• Store the mapping in the switch table
• Use a time-to-live field to eventually forget the
  mapping
• When frame arrives with an unfamiliar
  destination
• Forward out all interfaces
• except where frame arrived
• Hopefully wont happen often

B
C
A
D
10
Network Layer Routers
Processor
Switching Fabric
Line card
Line card
Line card
Line card
Line card
Line card
11
Hubs, Switches, and Routers

• Repeaters and hubs
• Analog devices
• Relay electrical signals
• Bridges and switches
• Forwards frames based on the MAC address
• Self-learning to construct the switch table
• Constructing a spanning tree to broadcast frames
• Routers
• Forwards packets based on the IP address
• Routing protocols to construct the forwarding
  tables
• Require more configuration than switches do

12
Two-Tiered Routing Architecture

• Goal distributed management of resources
• Internetworking of multiple networks
• Networks under separate administrative control
• Solution two-tiered routing architecture
• Intradomain inside a region of control
• Okay for routers to share topology information
• Routers configured to achieve a common goal
• Interdomain between regions of control
• Not okay to share complete information
• Networks may have different/conflicting goals
• Led to the use of different protocols

13
Autonomous Systems (ASes)
Path 6, 5, 4, 3, 2, 1
4
3
5
2
6
7
1
Web server
Client
14
Internet Routing Architecture

• Divided into Autonomous Systems
• Distinct regions of administrative control
• Routers/links managed by a single institution
• Service provider, company, university,
• Hierarchy of Autonomous Systems
• Large, tier-1 provider with a nationwide backbone
• Medium-sized regional provider with smaller
  backbone
• Small network run by a single company or
  university
• Interaction between Autonomous Systems
• Internal topology is not shared between ASes
• but, neighboring ASes interact to coordinate
  routing

15
Interdomain Routing (Between ASes)

• Support local routing policies
• Advertise the AS-level paths for each prefix
• Allow each AS to decide which path to use
• and whether to announce path to neighbors
• Common business relationships
• Customer-provider
• Customer can reach all destinations through
  provider
• Provider ensures rest of Internet can reach
  customer
• Peer-peer
• Peers transit traffic between respective
  customers
• but not to other peers and providers

16
Two Kinds of Routing Protocols
Link State
Vectoring

• Each router knows little about network topology
• Only best next-hops are chosen by each router for
  each destination.
• Best end-to-end paths result from composition of
  all next-hop choices
• Does not require any notion of distance
• Does not require uniform policies at all routers
• Examples RIP, BGP

• Topology information is flooded within the
  routing domain
• Best end-to-end paths are computed locally at
  each router.
• Best end-to-end paths determine next-hops.
• Based on minimizing some notion of distance
• Works only if policy is shared and uniform
• Examples OSPF, IS-IS

17
Overlay Routing

• Overlay on the network
• Hosts serve as nodes and make forwarding
  decisions
• Tunnels serve as links that carry the packets

Princeton
Yale
Two-hop (application-level) Berkeley-to-Princeton
route
application-layer router
Berkeley
18
Peer-to-Peer Protocols

• End hosts communicating directly with each other
• File sharing (music, movies)
• Voice over IP (telephone calls)
• Challenge
• Determining who has the data you want
• Handling churn as users come and go
• Three approaches
• Central server client sends query to the server
• Flooding client floods query throughout the
  network
• Hybrid client sends query to super-node, and
  super-nodes flood amongst themselves

19
E-Mail

• End hosts sending e-mail messages
• Asynchronous communication
• Determining how to relay the message to recipient
• Mail agents and mail servers
• Special DNS query to identify the mail server
• Forwarding of messages from one server to the
  next
• Protocols for recipients to retrieve the e-mail

20
Web

• Simpler client-server paradigm
• Clients (e.g., browsers) send requests
• Servers send responses
• Optional proxies in between
• Ingredients of the Web
• Uniform Resource Locator (URL)
• HyperText Markup Language (HTML)
• HyperText Transfer Protocol (HTTP)
• Stateless protocol
• Each request-response pair treated independently
• Improves the scalability of the server
• Separate mechanisms (e.g., cookies) for state

21
Resource Meta-Data

• Meta-data
• Information relating to a resource
• but not part of the resource itself
• Example meta-data
• Size of a resource
• Last modification time
• Type of the content
• Transfer encoding format
• Concept borrowed from e-mail protocols
• Multipurpose Internet Mail Extensions (MIME)
• Data format classification (e.g., Content-Type
  text/html)
• Enables clients to automatically launch a viewer

22
Internet of Today

• Internet has evolved substantially
• From a small research curiousity
• To a world-wide communications infrastructure
• Yet, some early design decisions remain
• Packet switching
• The narrow waist of IP
• Best-effort service model
• Limited state inside the network
• Protocols based on trust
• And these are meeting real challenges
• Demands for quality of service guarantees
• Serious security threats
• Challenges of network management

23
Internet of the Future

• A need for a change?
• Circuit switching?
• Guaranteed service?
• Strict notions of identity?
• Central authorities?
• Open question
• Is it possible to have an inter-network that
  supports such rapid evolution of new services
• while providing performance guarantees
  security?
• New initiatives
• Clean-slate rethinking of the Internet design
• See www.geni.net