CMSC 411CMSC 411Computer Systems ArchitectureLecture 25Networks, cont.Networks, cont.Alan [email protected]• Homework 6 due todayPtifil td•Practice final exam posted• Course evaluations open, at http://www.CourseEvalUM.umd.edup• Final exam next Tuesday, May 19, 1:30-3:30PMCMSC 411 - 25 (some from Patterson, Sussman, others)2Networks–A Brief OverviewNetworks A Brief OverviewConnecting Multiple Computers• Shared Media vs. Switched: pairs communicate at same time: “point-to-point”connectionsShared Media (Ethernet)Node Node Nodepoint connections• Aggregate BW in switched network is many times sharedSwitched Media (CM-5,ATM)Node Node Node;;;– point-to-point faster since no arbitration, simpler interface• Arbitration in Shared network?Switched Media (CM-5,ATM)NodeNode;;– Central arbiter for LAN?– Listen to check if being used (“Carrier Sensing”)NodeNodeSwitch;;g)– Listen to check if collision (“Collision Detection”)–Random resend to avoid repeated collisions; NodeNode;;(A. K. A. data switching interchanges, multistageinterconnection net orksCMSC 411 - 25 (some from Patterson, Sussman, others)4not fair arbitration; – OK if low utilizationinterconnection networks,interface message processors)Connection-Based vs. Connectionless• Telephone: operator sets up connection between the caller and the receiver– Once the connection is established, conversation can continue for hours• Share transmission lines over long distances by using switches to multiplex several conversations on the same lines– “Time division multiplexing” divide B/W transmission line into a fixed number of slots, with each slot assigned to a conversation• Problem: lines busy based on number of yconversations, not amount of information sent• Advantage: reserved bandwidthCMSC 411 - 25 (some from Patterson, Sussman, others)5Connection-Based vs. Connectionless•Connectionless: every package of informationConnectionless: every package of information must have an address => packets – Each package is routed to its destination by looking at its addressaddress– Analogy, the postal system (sending a letter)– also called “Statistical multiplexing”– Note: “Split phase buses” are sending packetsCMSC 411 - 25 (some from Patterson, Sussman, others)6Routing Messages•Shared MediaShared Media– Broadcast to everyone• Switched Media needs real routing. Options:– Source-based routing: message specifies path to the destination (changes of direction)– Virtual Circuit: circuit established from source to destination message picks the circuit to followdestination, message picks the circuit to follow– Destination-based routing: message specifies destination, switch must pick the path»deterministic: always follow same path»deterministic: always follow same path» adaptive: pick different paths to avoid congestion, failures» Randomized routing: pick between several good CMSC 411 - 25 (some from Patterson, Sussman, others)7gpgpaths to balance network loadDeterministic Routing Examples• mesh: dimension-order routing(xy)>(xy)gp–(x1, y1) -> (x2, y2)– first Δx = x2 -x1,– then Δy = y2-y1,2 1• hypercube: edge-cube routing– X = xox1x2 . . .xn -> Y = yoy1y2 . . .yn010110– R = X xor Y– Traverse dimensions of differing address in order100111011address in order• tree: common ancestorDeadlock free?000100CMSC 411 - 25 (some from Patterson, Sussman, others)8•Deadlock free?001101Store and Forward vs. Cut-Throughg•Store-and-forward policy: each switch waits for the•Store-and-forward policy: each switch waits for the full packet to arrive in switch before sending to the next switch (good for WAN)• Cut-through routing or worm hole routing: switch examines the header, decides where to send the message and then starts forwarding it immediatelymessage, and then starts forwarding it immediately – In worm hole routing, when head of message is blocked, message stays strung out over the network, potentially blocking other ( d l b ff th i f th k t th t i tmessages (needs only buffer the piece of the packet that is sent between switches). – Cut through routing lets the tail continue when head is blocked, di i th h l i t i l it h (R iCMSC 411 - 25 (some from Patterson, Sussman, others)9accordioning the whole message into a single switch. (Requires a buffer large enough to hold the largest packet).Cut-Through vs. Store and Forward•AdvantageAdvantage– Latency reduces from function of:number of intermediate switches X by the size of the packetnumber of intermediate switches X by the size of the packet toti f 1 t t f th k t t ti t th it htime for 1st part of the packet to negotiate the switches + the packet size ÷ interconnect BWCMSC 411 - 25 (some from Patterson, Sussman, others)10Congestion Control• Packet switched networks do not reserve bandwidth; this leads to contention• Solution: prevent packets from entering until contention is reduced (e.g., freeway on-ramp metering lights)• Options:–Packet discarding: If packet arrives at switch and no room in buffer, packet is discarded (e.g., UDP)– Flow control: between pairs of receivers and senders; use feedback to tell sender when allowed to send next packet» Back-pressure: separate wires to tell to stop» Window: give original sender right to send N packets before getting permission to send more; overlaps latency of interconnection with overhead to send & receive packet (e.g., TCP), adjustable window– Choke packets: aka “rate-based”; Each packet received by busy switch in warning state sent back to the source via choke packet. Source reduces traffic to that destination by a fixed % (e.g., ATM)CMSC 411 - 25 (some from Patterson, Sussman, others)11Protocols: HW/SW Interface• Internetworking: allows computers on independent and incompatible networks to communicate reliablyand incompatible networks to communicate reliably and efficiently;– Enabling technologies: SW standards that allow reliable communications without reliable networks– Hierarchy of SW layers, giving each layer responsibility for portion of overall communications task, calledprotocol families or protocol suites• Transmission Control Protocol/Internet Protocol (TCP/IP)– This protocol family is the basis of the Internet– IP makes best effort to deliver; TCP guarantees delivery– TCP/IP used even when communicating locally: NFS uses IP even though communicating across homogeneous LANCMSC 411 - 25 (some