Internet TopologyReturning the Midterm ExamGoals of Today’s LectureIP RoutersInside a High-End RouterRouter Physical LayoutLine Cards (Interface Cards, Adaptors)Switching FabricPacket SwitchingRouter ProcessorError ReportingInternet Control Message ProtocolExample: Time ExceededPing: Echo and ReplyRouter-level topology of a networkIntra-AS TopologyHub-and-Spoke TopologyPrinceton ExampleSimple Alternatives to Hub-and-SpokeBackbone NetworksAbilene Internet2 BackbonePoints-of-Presence (PoPs)Where to Locate Nodes and LinksAS-level topology of the InternetInternet Routing ArchitectureAutonomous System NumbersAS TopologyWhat is an Edge, Really?Interdomain PathsBusiness RelationshipsCustomer-Provider RelationshipCustomer Connecting to a ProviderMulti-Homing: Two or More ProvidersSlide 34Peer-Peer RelationshipAS Structure: Tier-1 ProvidersEfficient Early-Exit RoutingAS Structure: Other ASesCharacteristics of the AS GraphCharacteristics of AS PathsShared RisksConclusions1Internet TopologyCOS 461: Computer NetworksSpring 2007 (MW 1:30-2:50 in Friend 004)Jennifer RexfordTeaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/2Returning the Midterm Exam•Exam scoring break down–Average: 90–High: 99–Median: low 90s•See the course Web site–Exam–Answer key3Goals of Today’s Lecture•IP routers–Interface cards–Switching fabric–Route processor•Router-level topology with a single network–Points of Presence (PoPs)–Backbone and enterprise network topologies•AS-level topology of the Internet–Autonomous System (AS) numbers–Tier-1 ISPs, regional providers, and stub ASes–Business relationships between ASes4IP Routers5Inside a High-End RouterSwitchingFabricProcessorLine cardLine cardLine cardLine cardLine cardLine card6Router Physical LayoutJuniper T seriesCisco 12000CrossbarLinecards7Line Cards (Interface Cards, Adaptors)•Interfacing –Physical link–Switching fabric•Packet handling–Packet forwarding–Decrement time-to-live–Buffer management–Link scheduling–Packet filtering–Rate limiting–Packet marking–Measurementto/from linkto/from switchlookupReceiveTransmit8Switching Fabric•Deliver packet inside the router–From incoming interface to outgoing interface–A small network in and of itself•Must operate very quickly–Multiple packets going to same outgoing interface–Switch scheduling to match inputs to outputs•Implementation techniques–Bus, crossbar, interconnection network, …–Running at a faster speed (e.g., 2X) than links–Dividing variable-length packets into cells9Packet SwitchingR1Link 1Link 2Link 3Link 4Link 1, ingress Link 1, egressLink 2, ingress Link 2, egressLink 3, ingress Link 3, egressLink 4, ingress Link 4, egressChooseEgressChooseEgressChooseEgressChooseEgress“4”“4”10Router Processor•So-called “Loopback” interface–IP address of the CPU on the router•Control-plane software–Implementation of the routing protocols–Creation of forwarding table for the line cards•Interface to network administrators–Command-line interface for configuration–Transmission of measurement statistics •Handling of special data packets–Packets with IP options enabled–Packets with expired Time-To-Live field11Error Reporting•Examples of errors a router may see–Router doesn’t know where to forward a packet–Packet’s time-to-live field expires•Router doesn’t really need to respond–Best effort means never having to say you’re sorry–So, IP could conceivably just silently drop packets•But, silent failures are really hard to diagnose–IP includes basic feedback about network problems–Internet Control Message Protocol (ICMP)12Internet Control Message Protocol•ICMP runs on top of IP–In parallel to TCP and UDP–Though still viewed as an integral part of IP•Diagnostics–Triggered when an IP packet encounters a problemE.g., time exceeded or destination unreachable–ICMP packet sent back to the source IP addressIncludes the error information (e.g., type and code)… and an excerpt of the original data packet for identification–Source host receives the ICMP packetAnd inspects the except of the packet (e.g., protocol and ports)… to identify which socket should receive the error13Example: Time Exceededhost DNS...hosthost DNS...router routerrouterhost1.2.3.78.9.10.115.6.7.156•Host sends an IP packet–Each router decrements the time-to-live field•If time-to-live field reaches 0–Router generates an ICMP message–Sends a “time exceeded” message back to the sourceTime exceeded14Ping: Echo and Reply•ICMP includes a simple “echo” function–Sending node sends an ICMP “echo” message–Receiving node sends an ICMP “echo reply”•Ping tool–Tests the connectivity with a remote host–… by sending regularly spaced echo commands–… and measuring the delay until receiving the reply•Pinging a host–“ping www.cs.princeton.edu” or “ping 12.157.34.212”–Used to test if a machine is reachable and alive–(However, some nodes have ICMP disabled… )15Router-level topology of a network16Intra-AS Topology•Node: router•Edge: link17Hub-and-Spoke Topology•Single hub node–Common in enterprise networks–Main location and satellite sites–Simple design and trivial routing•Problems–Single point of failure–Bandwidth limitations–High delay between sites–Costs to backhaul to hub18Princeton Example•Hub-and-spoke–Four hub routers and many spokes•Hub routers–Outside world (e.g., AT&T, USLEC, …)–Dorms–Academic and administrative buildings–Servers19Simple Alternatives to Hub-and-Spoke•Dual hub-and-spoke–Higher reliability–Higher cost–Good building block•Levels of hierarchy–Reduce backhaul cost–Aggregate the bandwidth–Shorter site-to-site delay…20Backbone Networks•Backbone networks–Multiple Points-of-Presence (PoPs)–Lots of communication between PoPs–Accommodate traffic demands and limit delay21Abilene Internet2 Backbone22Points-of-Presence (PoPs)•Inter-PoP links–Long distances–High bandwidth•Intra-PoP links–Short cables between racks or floors–Aggregated bandwidth•Links to other networks–Wide range of media and bandwidthIntra-PoPOther networksInter-PoP23Where to Locate Nodes and Links•Placing Points-of-Presence (PoPs)–Large population of potential customers–Other providers or exchange points–Cost and availability of real-estate–Mostly
View Full Document
Unlocking...