Course Overview and Introduction CS 4251: Computer Networking II Nick Feamster Spring 2008

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

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

Logistics Course Web page – –Check this page regularly for updates to the syllabus, assignments, readings, etc. Course mailing list –Sign up now/today –

Who Am I? Nick Feamster –Assistant Professor –Networking: Operations and Security Office: Klaus on web page, use CS 4251 Office Hours: Monday, 2-4 p.m.

Overview of Lectures Holistic approach Various themes recur throughout –Tree forming/path finding –Layering –Resource allocation and sharing –Naming Textbook reading, plus some research (and other) papers –Read the readings before class!

Things Youll Learn How does BitTorrent find your file? How does the GT wireless network allow you to roam across campus with the same IP address? How do ISPs connect to one another? –Interconnection: Protocols and business What could you do with two (or more) Internet connections at home?

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? Can you guarantee performance or service for certain types of applications (e.g., telephony, video)? Can a networks resources be subdivided?

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…?

Grading 3 Problem sets (20%) –Paper and pencil 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. Late policy: Maximum of 72 hours late throughout the term

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.

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?

Key Concepts in Networking Protocols Tree formation/Route Finding Layering Resource allocation and sharing Naming Lots of minor recurring themes –Hierarchy –Caching –Randomization

Georgia Tech The Internet: A Network of Networks Comcast Abilene AT&T Cogent Autonomous Systems (ASes) 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)

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

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

Layering Key technique for managing 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

Layering: Encapsulation This can be more complex Example: Network layers can be encapsulated within another network layer Get index.html Connection ID Source/Destination Link Address User AUser B Application (message) Transport (segment) Network (datagram) Link (frame)

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 Technology Applications WWW phone... SMTP HTTP RTP... TCP UDP… IP ethernet PPP… CSMA async sonet... copper fiber radio...

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

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

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

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

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?

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

Sharing in Packet-Switched Networks 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)

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

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?

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

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 Home Network Internet : :50879 (more on NATs in lecture 17)

Naming ClientLocal DNS resolver root,.edu troll-gw.gatech.edu NS troll-gw.gatech.edu NS burdell.cc.gatech.edu A burdell.cc.gatech.edu Recursive query Iterative queries Note the diversity of Georgia Techs authoritative nameservers Example: DNS –Maps names to IP addresses –Hierarchical

This set of goals might seem to be nothing more than a checklist of all the desirable network features. It is important to understand that these goals are in order of importance, and an entirely different network architecture would result if the order were changed. The Internets Design Goals Interconnection/Multiplexing Resilience/Survivability Heterogeneity –Different types of services –Different types of networks Distributed management Cost effectiveness Ease of attachment Accountability

Survivability Network should continue to work, even if some devices fail, are compromised, etc. How well does the current Internet support survivability?

Distributed Management Addressing (ARIN, RIPE, APNIC, etc.) –Though this was recently threatened. Naming (DNS) Routing (BGP) Many examples: No single entity in charge. Allows for organic growth, scalable management. Tradeoff: No one party has visibility/control

Heterogeneous Services TCP/IP designed as a monolithic transport –TCP for flow control, reliable delivery –IP for forwarding Became clear that not every type of application would need reliable, in-order delivery –Example: Voice and video over networks –Example: DNS –Why dont these applications require reliable, in-order delivery? –Narrow waist: allowed proliferation of transport protocols

Accountability Note: Accountability mentioned in early papers on TCP/IP, but not prioritized Datagram networks make accounting tricky –Circuit-switched networks are easier to bill –Payments/billing on the Internet is much less precise Tradeoff: Broken payment models and incentives.

So…what has changed? Security and Accountability Availability Mobility Scaling Management Support for disconnected/intermittent operation (e.g., in developing regions) … Would you make the same decisions about layering, resource sharing, protocol semantics and agreements, etc.?

Security February 2000March 2006 Lectures 23-25

Scaling Lecture 13 and others

Availability Is it possible to get to 5 nines of availability? If so, how? Lecture 5 and others

Mobility …and social networking. Lectures 13, 18, 28, …

Management Maintaining availability and correcting problems requires constant vigilance, intuition, etc. Failures at various layers can make things hard Lecture 25

Connectivity in Developing Regions Challenges –Power –Management –High cost and scarcity of upstream connectivity –Little-to-no wired infrastructure