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Computer Networks (Graduate level) Lecture 1: Introduction University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Computer Network 1 Objectives of course Understand the state-of-the-art in network protocols, architectures and applications Understand how networking research is done and prepare student for research in networking. How is class different from undergraduate networking course Training network programmers vs. training network researchers They are really different courses! Univ. of Tehran Computer Network 2 Course Materials Course Web page Research papers visit regularly Printed papers or pdf/ps version on the Web page ~40 papers, Combination of classic and recent work. Some Optional reading! Recommended textbook “Computer Networks, A system approach. Peterson & Davie 3rd edition. Univ. of Tehran Computer Network 3 Grading Homework assignments, around %20 Paper review Problems and hands-on assignments Severe late penalties! Class discussion, participation around %5 point. Project, ~ %30 Exam(s), Midterm and final, ~%45. Univ. of Tehran Computer Network 4 Covered Topics (we try!) Traditional Layering Interconnecting (LAN) Internet architecture Routing (IP) Switching Transport (TCP) Queue management (FQ, RED) Naming (DNS) Univ. of Tehran Recent Topics Multicast Wireless/Mobility Active networks QOS Security Network measurement Ad-hoc Networks Sensor Networks Computer Network 5 A tour of networking Goal and objective needs design requirement Whirlwind tour of networking Univ. of Tehran Computer Network 6 Information, Computers and Networks Information: anything that is represented in bits Form (can be represented) vs substance (cannot) Properties: Infinitely replicable Computers can “manipulate” information Networks create “access” to information Potential of networking: move bits everywhere, cheaply, and with desired performance characteristics Break the space barrier for information Univ. of Tehran Computer Network 7 Objective of Networking ? Direct or indirect access to every other node in the network Connectivity is the magic needed to communicate if you do not have a link. Must understand many connection factors Traffic data rate Traffic pattern (bursty or constant bit rate) Traffic target (multipoint or single destination, mobile or fixed) Application requirements, Delay sensitivity Loss sensitivity. Univ. of Tehran Computer Network 8 Another view Building a network to support diverse ranges of applications Distributed computing. Multimedia. Telecommunication. E-commerce, etc. What kind of technology do we need? Hardware. Software. Univ. of Tehran Computer Network 9 What is a Network? What is computer Network? Different views. Differences from other networks, Its generality. What is requirements? Different perspective: Network provider Network designer Application programmer Univ. of Tehran Computer Network 10 Design goals Connectivity Scalability Simplicity Efficiency For designers. Most importantly for users. cost performance Support for common user services. Univ. of Tehran Computer Network 11 Levels of Networking Communicating across a link, LANs. Connecting together multiple links (Bridges) Finding and routing data to nodes on Internet. Communicating on the application level, matching application requirements Univ. of Tehran Computer Network 12 A First Step Creating a link between nodes Link: path followed by bits Wired or wireless Broadcast or point-to-point (or both) Node: any device connected to a link Univ. of Tehran Computer Network 13 Types of Links Point-to-Point Multiple Access … Univ. of Tehran Computer Network 14 Packet Transmission Modes Unicast Broadcast Transmission to all network nodes Multicast Transmission to single specific receiver Transmission to specific subset of nodes Anycast Transmission to one of a specific subset of nodes Univ. of Tehran Computer Network 15 What are Switched Networks? Switch: moves bits between links Switched Network Packet switching Circuit switching Univ. of Tehran Computer Network 16 Back in the Old Days… Univ. of Tehran Computer Network 17 Then Came TDM… • Synchronous time division multiplexing Multiplex (mux) Univ. of Tehran Demultiplex (demux) Computer Network 18 TDM Logical Network View Univ. of Tehran Computer Network 19 Packet Switching (Internet) Packets Univ. of Tehran Computer Network 20 Packet Switching Interleave packets from different sources Efficient: resources used on demand General Statistical multiplexing Multiple types of applications Accommodates bursty traffic Addition of queues Univ. of Tehran Computer Network 21 Statistical Multiplexing Gain 1 Mbps link; users require 0.1 Mbps when transmitting; users active only 10% of the time Circuit switching: can support 10 users Packet switching: with 35 users, probability that >=10 are transmitting at the same time < 0.0017 Univ. of Tehran Computer Network 22 Characteristics of Packet Switching Store and forward Packets are self contained units Can use alternate paths - reordering Contention Congestion Delay Univ. of Tehran Computer Network 23 Second Step: Internet[work] A collection of interconnected networks Host: network endpoints (computer, PDA, light switch, …) Router: node that connects networks Internet vs. internet Univ. of Tehran Computer Network Internet[work] 24 Challenge Many differences between networks Address formats Performance – bandwidth/latency Packet size Loss rate/pattern handling Routing How to translate between various network technologies Univ. of Tehran Computer Network 25 Third Step: How To Find Nodes? internet Computer 1 Univ. of Tehran Computer 2 Computer Network 26 Naming Humans use readable host names E.g. www.cmu.edu Globally unique (can correspond to multiple hosts) Naming system translates to physical address E.g. DNS translates name to IP Address (e.g. 128.2.11.43) Address reflects location in network Univ. of Tehran Computer Network 27 Domain Name System What’s the IP address for www.cmu.edu? It is 128.2.11.43 Computer 1 Local DNS Server DNS server address manually configured into OS Univ. of Tehran Computer Network 28 Packet Routing/Delivery Each network technology has different local delivery methods Address resolution provides delivery information within network E.g., ARP maps IP addresses to Ethernet addresses Local, works only on a particular network Routing protocol provides path through an internetwork Univ. of Tehran Computer Network 29 Network:Address Resolution Protocol Broadcast: who knows the Ethernet address for 128.2.11.43? Ethernet Broadcast: Yes, it is 08-00-2c-19-dc-45 Ethernet Univ. of Tehran Computer Network 30 Internetwork: Datagram Routing Routers send packet to next closest point H H R R R H R R R H R R R: Routers H Univ. of Tehran H: Hosts Computer Network 31 Routing Forwarding tables at each router populated by routing protocols. Original Internet: manually updated Routing protocols update tables based on “cost” Exchange tables with neighbors or everyone Use neighbor leading to shortest path Univ. of Tehran Computer Network 32 Fourth Step: Application Demands Reliability Corruption Lost packets Flow and congestion control Fragmentation In-order delivery Etc… Univ. of Tehran Computer Network 33 What if the Data gets Corrupted? Problem: Data Corruption GET index.html Internet GET windex.html Solution: Add a checksum 0,9 9 Univ. of Tehran 6,7,8 21 X 4,5 7 Computer Network 1,2,3 6 34 What if the Data gets Lost? Problem: Lost Data GET index.html Internet Solution: Timeout and Retransmit GET index.html Internet GET index.html GET index.html Univ. of Tehran Computer Network 35 What if Network is Overloaded? Problem: Network Overload Solution: Buffering and Congestion Control Short bursts: buffer What if buffer overflows? Packets dropped and retransmitted Sender adjusts rate until load = resources Called “Congestion control” Univ. of Tehran Computer Network 36 What if the Data Doesn’t Fit? Problem: Packet size On Ethernet, max IP packet is 1.5kbytes Typical web page is 10kbytes Solution: Fragment data across packets ml x.ht inde GET GET index.html Univ. of Tehran Computer Network 37 What if the Data is Out of Order? Problem: Out of Order ml inde x.th GET GET x.thindeml Solution: Add Sequence Numbers ml 4 inde 2 x.th 3 GET 1 GET index.html Univ. of Tehran Computer Network 38 Network Functionality Summary Link Multiplexing Routing Addressing/naming (locating peers) Reliability Flow control Fragmentation Etc…. Univ. of Tehran Computer Network 39 What is Layering? Modular approach to network functionality The idea of divide and conquer Use abstraction to hide complexity. Example: Application Application-to-application channels Host-to-host connectivity Link hardware Univ. of Tehran Computer Network 40 Protocols Module in layered structure Set of rules governing communication between network elements (applications, hosts, routers) Protocols define: Interface to higher layers (API) Interface to peer Format and order of messages Actions taken on receipt of a message Univ. of Tehran Computer Network 41 Protocols Building blocks of a network architecture Each protocol object has two different interfaces service interface: operations on this protocol peer-to-peer interface: messages exchanged with peer Term “protocol” is overloaded specification of peer-to-peer interface module that implements this interface Univ. of Tehran Computer Network 42 Layering Characteristics Each layer relies on services from layer below and exports services to layer above Interface defines interaction Hides implementation - layers can change without disturbing other layers (black box) Univ. of Tehran Computer Network 43 Layering User A User B Application Transport Network Link Host Host Layering: technique to simplify complex systems Univ. of Tehran Computer Network 44 Layer Encapsulation User A User B Get index.html Connection ID Source/Destination Link Address Univ. of Tehran Computer Network 45 Protocol Demultiplexing Multiple choices at each layer FTP HTTP NV TCP UDP IPX NET1 TFTP Network IP Type Field Protocol Field TCP/UDP IP NET2 Univ. of Tehran … NETn Computer Network Port Number 46 E.g.: OSI Model: 7 Protocol Layers Physical: how to transmit bits Data link: how to transmit frames Network: how to route packets Transport: how to send packets end2end Session: how to tie flows together Presentation: byte ordering, security Application: everything else Univ. of Tehran Computer Network 47 OSI Layers and Locations Application Presentation Session Transport Network Data Link Physical Host Univ. of Tehran Switch Computer Network Router Host 48 Example: Transport Layer First end-to-end layer End-to-end state May provide reliability, flow and congestion control Univ. of Tehran Computer Network 49 Example: Network Layer Point-to-point communication Network and host addressing Routing Univ. of Tehran Computer Network 50 Is Layering Harmful? Sometimes.. Layer N may duplicate lower level functionality (e.g., error recovery) Layers may need same info (timestamp, MTU) Strict adherence to layering may hurt performance Univ. of Tehran Computer Network 51 Class Coverage Brief coverage of physical and data link layer Focus on network to application layer We will deal with: Protocol rules and algorithms Investigate protocol trade-offs Why this way and not another? Univ. of Tehran Computer Network 52 Next Lecture: Links How to make computers talk across a wire Assigned reading Chap. 2 of book (Recommended!) Univ. of Tehran Computer Network 53