Administrivia Homework 6 due today CMSC 411 Computer Systems Architecture Lecture 25 Networks cont Practice P ti final fi l exam posted t d Course evaluations open at http www CourseEvalUM umd edu p Final exam next Tuesday May 19 1 30 3 30PM Alan Sussman als cs umd edu 2 CMSC 411 25 some from Patterson Sussman others Connecting Multiple Computers Shared Media vs Switched pairs communicate at same time point topoint connections point Networks A Brief Overview Aggregate BW in switched network is many times shared point to point faster since no arbitration simpler interface Arbitration in Shared network Central arbiter for LAN Node Node Node Switched Media CM 5 ATM Node Node Switch Listen to check if being used Carrier Sensing g Listen to check if collision Collision Detection Shared Media Ethernet Random resend to avoid repeated collisions not fair arbitration OK if low utilization CMSC 411 25 some from Patterson Sussman others Node Node A K A data switching interchanges multistage interconnection net networks orks interface message processors 4 Connection Based vs Connectionless Connection Based vs Connectionless Telephone operator sets up connection between the caller and the receiver Connectionless every package of information must have an address packets 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 Each package is routed to its destination by looking at its address Analogy the postal system sending a letter also called Statistical multiplexing Note Split phase buses are sending packets Problem lines busy y based on number of conversations not amount of information sent Advantage reserved bandwidth CMSC 411 25 some from Patterson Sussman others 5 Routing Messages 6 CMSC 411 25 some from Patterson Sussman others Deterministic Routing g Examples p mesh dimension order routing Shared Media x1 y1 x2 y2 Broadcast to everyone first x x2 x1 Switched Media needs real routing Options then y y2 y1 Source based routing message specifies path to the destination changes of direction hypercube edge cube routing Virtual Circuit circuit established from source to destination message picks the circuit to follow destination X xox1x2 xn Y yoy1y2 yn Destination based routing message specifies destination switch must pick the path R X xor Y 111 Traverse dimensions of differing address in order deterministic always follow same path adaptive pick different paths to avoid congestion failures tree common ancestor g p pick between several good g Randomized routing paths to balance network load Deadlock free CMSC 411 25 some from Patterson Sussman others 7 110 010 011 100 000 001 CMSC 411 25 some from Patterson Sussman others 101 8 Cut Through vs Store and Forward Store and Forward vs Cut Through g Advantage Store and forward policy each switch waits for the full packet to arrive in switch before sending to the next switch good for WAN Latency reduces from function of number of intermediate switches X by the size of the packet Cut through routing or worm hole routing switch examines the header decides where to send the message and then starts forwarding it immediately message to ti time for f 1st 1 t partt off th the packet k t to t negotiate ti t the th switches it h the packet size interconnect BW In worm hole routing when head of message is blocked message stays strung out over the network potentially blocking other messages needs d only l buffer b ff the th piece i off the th packet k t that th t is i sentt between switches Cut through routing lets the tail continue when head is blocked accordioning di i the th whole h l message into i t a single i l switch it h Requires R i a buffer large enough to hold the largest packet CMSC 411 25 some from Patterson Sussman others 9 Congestion Control CMSC 411 25 some from Patterson Sussman others 10 Protocols HW SW Interface 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 Internetworking allows computers on independent and incompatible networks to communicate reliably and efficiently Enabling technologies SW standards that allow reliable communications without reliable networks 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 11 Hierarchy of SW layers giving each layer responsibility for portion of overall communications task called protocol 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 LAN CMSC 411 25 some from Patterson Sussman others 12 Protocol Family Concept Protocol Family Concept Key to protocol families is that communication occurs logically at the same level of the protocol called peer topeer Message but is implemented via services at the next lower level H Message T H H Message T T Encapsulation carry higher level information within lower level envelope TCP Fragmentation break packet into multiple smaller packets and reassemble IP Danger is each level increases latency if implemented as hierarchy e g multiple check sums Ethernet CMSC 411 25 some from Patterson Sussman others 13 TCP IP packet Ethernet packet protocols Application sends message TCP breaks into 64KB segments segments adds 20B header IP adds 20B header sends to network If Ethernet broken into 1500B packets with headers trailers 24B Ethernet Hdr IP Header TCP Header EHIP Data TCP data Message Ethernet Hdr 14 Example Networks Ethernet