IP Packet SwitchingGoals of Today’s LectureSimple Network: Nodes and a LinkNetwork ComponentsLinks: Delay and BandwidthConnecting More Than Two HostsBeyond Directly-Connected NetworksCircuit Switching (e.g., Phone Network)Circuit Switching With Human OperatorCircuit Switching: Multiplexing a LinkAdvantages of Circuit SwitchingDisadvantages of Circuit SwitchingPacket Switching (e.g., Internet)Packet Switching: Statistical MultiplexingIP Service: Best-Effort Packet DeliveryIP Service Model: Why Packets?IP Service Model: Why Best-Effort?IP Service: Best-Effort is EnoughLayering in the IP ProtocolsHistory: Why IP Packets?Other Main Driving Goals (In Order)Other Driving Goals, Somewhat MetIP Packet StructureIP Packet Header FieldsIP Packet Header Fields (Continued)Time-to-Live (TTL) FieldApplication of TTL in TracerouteExample Traceroute: Berkeley to CNNTry Running Traceroute YourselfSlide 30Slide 31IP Packet Header (Continued)What if the Source Lies?Summary: Packet Switching ReviewNext Lecture1IP Packet SwitchingCOS 461: Computer NetworksSpring 2006 (MW 1:30-2:50 in Friend 109)Jennifer RexfordTeaching Assistant: Mike Wawrzoniak http://www.cs.princeton.edu/courses/archive/spring06/cos461/2Goals of Today’s Lecture•Connectivity–Links and nodes–Circuit switching–Packet switching•IP service model–Best-effort packet delivery–IP as the Internet’s “narrow waist”–Design philosophy of IP•IP packet structure–Fields in the IP header–Traceroute using TTL field–Source-address spoofing3Simple Network: Nodes and a Link•Node: computer–End host: general-purpose computer, cell phone, PDA–Network node: switch or router•Link: physical medium connecting nodes–Twisted pair: the wire that connects to telephones–Coaxial cable: the wire that connects to TV sets–Optical fiber: high-bandwidth long-distance links–Space: propagation of radio waves, microwaves, …Node Link Node4FibersCoaxial CableLinks Interfaces Switches/routersEthernet cardWireless cardLarge routerTelephoneswitchNetwork Components5Links: Delay and Bandwidth•Delay–Latency for propagating data along the link–Corresponds to the “length” of the link–Typically measured in seconds•Bandwidth–Amount of data sent (or received) per unit time–Corresponds to the “width” of the link–Typically measured in bits per secondbandwidthdelaydelay x bandwidth6Connecting More Than Two Hosts•Multi-access link: Ethernet, wireless –Single physical link, shared by multiple nodes–Limitations on distance and number of nodes•Point-to-point links: fiber-optic cable–Only two nodes (separate link per pair of nodes)–Limitations on the number of adapters per nodemulti-access linkpoint-to-point links7Beyond Directly-Connected Networks•Switched network–End hosts at the edge–Network nodes that switch traffic–Links between the nodes•Multiplexing–Many end hosts communicate over the network–Traffic shares access to the same links8Circuit Switching (e.g., Phone Network)•Source establishes connection to destination–Node along the path store connection info–Nodes may reserve resources for the connection•Source sends data over the connection–No destination address, since nodes know path•Source tears down connection when done9Circuit Switching With Human Operator10Circuit Switching: Multiplexing a Link•Time-division–Each circuit allocated certain time slots•Frequency-division–Each circuit allocated certain frequenciestimefrequencytime11Advantages of Circuit Switching•Guaranteed bandwidth –Predictable communication performance–Not “best-effort” delivery with no real guarantees•Simple abstraction–Reliable communication channel between hosts–No worries about lost or out-of-order packets•Simple forwarding –Forwarding based on time slot or frequency–No need to inspect a packet header•Low per-packet overhead–Forwarding based on time slot or frequency–No IP (and TCP/UDP) header on each packet12Disadvantages of Circuit Switching•Wasted bandwidth–Bursty traffic leads to idle connection during silent period–Unable to achieve gains from statistical multiplexing•Blocked connections–Connection refused when resources are not sufficient–Unable to offer “okay” service to everybody•Connection set-up delay –No communication until the connection is set up–Unable to avoid extra latency for small data transfers•Network state–Network nodes must store per-connection information–Unable to avoid per-connection storage and state13Packet Switching (e.g., Internet)•Data traffic divided into packets–Each packet contains a header (with address)•Packets travel separately through network–Packet forwarding based on the header–Network nodes may store packets temporarily•Destination reconstructs the message14Packet Switching: Statistical MultiplexingPackets15IP Service: Best-Effort Packet Delivery•Packet switching–Divide messages into a sequence of packets–Headers with source and destination address•Best-effort delivery–Packets may be lost–Packets may be corrupted–Packets may be delivered out of ordersourcedestinationIP network16IP Service Model: Why Packets?•Data traffic is bursty–Logging in to remote machines–Exchanging e-mail messages•Don’t want to waste reserved bandwidth–No traffic exchanged during idle periods•Better to allow multiplexing–Different transfers share access to same links•Packets can be delivered by most anything–RFC 2549: IP over Avian Carriers (aka birds) •… still, packet switching can be inefficient–Extra header bits on every packet17IP Service Model: Why Best-Effort?•IP means never having to say you’re sorry…–Don’t need to reserve bandwidth and memory–Don’t need to do error detection & correction–Don’t need to remember from one packet to next•Easier to survive failures–Transient disruptions are okay during failover•… but, applications do want efficient, accurate transfer of data in order, in a timely fashion18IP Service: Best-Effort is Enough•No error detection or correction–Higher-level protocol can provide error checking•Successive packets may not follow the same path–Not a problem as long as packets reach the destination•Packets can be delivered out-of-order–Receiver can put packets back in order (if necessary)•Packets may be lost or arbitrarily delayed–Sender can send the packets again (if desired)•No network congestion control
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