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UW-Madison CS 740 - CS740 - Review

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CS740 - ReviewNetwork Communication: Lots of Functions NeededWhat is Layering?Power of LayeringExample of LayeringLayering vs NotPacket SwitchingCircuit SwitchingSwitching in the Telephone NetworkCircuit Switching DiscussionVirtual Circuits SwitchingPackets vs. CircuitsOutlineEthernet MAC (CSMA/CD)Minimum Packet SizeEthernet Frame StructureEthernet Frame Structure (cont.)Transparent Bridges / SwitchesFrame ForwardingSpanning Tree BridgesSlide 21IP AddressesSubnet Addressing RFC917 (1984)Aside: Interaction with Link LayerClassless Inter-Domain Routing (CIDR) – RFC1338IP Service ModelIP Fragmentation ExampleImportant ConceptsDistance-Vector RoutingDistance-Vector UpdateLink State Protocol ConceptSending Link States by FloodingComparison of LS and DV AlgorithmsInter-domain Routing: HierarchyInternet’s HierarchyAn exampleBGP PreliminariesAS Numbers (ASNs)Distance Vector with PathHop-by-hop ModelPolicy with BGPExamples of BGP PoliciesBGP MessagesBGP UPDATE MessagePath Selection CriteriaAS_PATHDecision Process (First cut)A Logical View of the InternetInter-ISP Relationships: Transit vs. PeeringIllustrating BGP PoliciesPolicy I: Prefer Customer routingPolicy II: Import RoutesPolicy II: Export RoutesPolicy II: Valley-Free RoutesBGP Route Selection SummaryCS740 - ReviewAditya Akella01/25/08Network Communication:Lots of Functions Needed•Links•Multiplexing •Routing•Addressing/naming (locating peers)•Reliability•Flow control•FragmentationHow do you implement these functions?Key: Layering and protocolsWhat is Layering?•A way to deal with complexity–Add multiple levels of abstraction–Each level encapsulates some key functionality–And exports an interface to other components–Example?•Layering: Modular approach to implementing network functionality by introducing abstractions•Challenge: how to come up with the “right” abstractions?Power of Layering •Solution: Intermediate layer that provides a single abstraction for various network technologies–O(1) work to add app/media–variation on “add another level of indirection”SMTP SSH NFS802.11LANCoaxial cableFiberopticApplicationTransmissionMediaHTTPIntermediate layerExample of Layering•Software and hardware for communication between two hosts•Advantages:–Simplifies design and implementation–Easy to modify/evolveLink hardwareHost-to-host connectivityApplication-to-application channelsApplication semanticsLayering vs Not•Layer N may duplicate layer N-1 functionality –E.g., error recovery•Layers may need same info (timestamp, MTU)•Strict adherence to layering may hurt performance•Some layers are not always cleanly separated–Inter-layer dependencies in implementations for performance reasons–Many cross-layer assumptions, e.g. buffer management•Layer interfaces are not really standardized.–It would be hard to mix and match layers from independent implementations, e.g., windows network apps on unix (w/o compatibility library)Packet Switching•Packet-switching: Benefits–Ability to exploit statistical multiplexing–More efficient bandwidth usage•Packet switching: Concerns –Needs to buffer and deal with congestion:–More complex switches–Harder to provide good network services (e.g., delay and bandwidth guarantees)Circuit Switching•Source first establishes a circuit to destination–Switches along the way stores info about connection•Possibly allocate resources•Different srs-dst’s get different paths•Source sends the data over the circuit–No address required since path is established beforehand•The connection is explicitly set up and torn down•Switches use TDM (digital) or FDM (analog) to transmit data from various circuitsSwitching in the Telephone NetworkCircuit Switching Discussion•Positives–Fast and simple data transfer, once the circuit has been established–Predictable performance since the circuit provides isolation from other users•E.g. guaranteed max bandwidth•Negatives–How about bursty traffic•Circuit will be idle for significant periods of time•Also, can’t send more than max rate–Circuit set-up/tear down is expensive–Also, reconfiguration is slow•Fast becoming a non-issueVirtual Circuits Switching•Advantages–Efficient lookup (simple table lookup)–Can reserve bandwidth at connection setup–Easier for hardware implementations•Disadvantages–Still need to route connection setup request–More complex failure recovery – must recreate connection state•Typical use  fast router implementations–ATM – combined with fix sized cells–MPLS – tag switching for IP networksPackets vs. Circuits•Efficient –Can send from any input that is ready–No notion of wastage of resources that could be used otherwise•Contention (i.e. no isolation)–Congestion–Delay•Accommodates bursty traffic–But need packet buffers•Address look-up and forwarding–Need optimization•Packet switching pre-dominant–Circuit switching used on large time-scales, low granularitiesOutline•Switching and Multiplexing•Link-Layer–Ethernet and CSMA/CD–Bridges/Switches•Routing-Layer•Physical-LayerEthernet MAC (CSMA/CD)Packet?Sense CarrierDiscard PacketSendDetect CollisionJam channel b=CalcBackoff(); wait(b);attempts++;NoYesattempts < 16attempts == 16•Carrier Sense Multiple Access/Collision Detection15Minimum Packet Size•What if two people sent really small packets–How do you find collision?•Consider:–Worst case RTT–How fast bits can be sentEthernet Frame Structure•Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet Frame Structure (cont.)•Addresses: 6 bytes–Each adapter is given a globally unique address at manufacturing time•Address space is allocated to manufacturers–24 bits identify manufacturer–E.g., 0:0:15:*  3com adapter•Frame is received by all adapters on a LAN and dropped if address does not match–Special addresses•Broadcast – FF:FF:FF:FF:FF:FF is “everybody”•Range of addresses allocated to multicast–Adapter maintains list of multicast groups node is interested inTransparent Bridges / Switches•Design goals:–Self-configuring without hardware or software changes–Bridge do not impact the operation of the individual LANs•Three parts to making bridges transparent:1) Forwarding frames2) Learning addresses/host locations3) Spanning tree algorithmFrame Forwarding•A machine with MAC Address lies in the direction of number


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