Lecture 17: Final Review"

Transcription

1 Lecture 17: Final Review" CSE 123: Computer Networks Alex C. Snoeren Last class!!!!

2 Overview" Signaling Framing Error detection Reliable transmission Flow control Bridging/Switching Congestion control Routing QoS Wireless CSE 123: Lecture 17: Final Review 2

3 Final Mechanics" Bulk of the final covers material after midterm Routing, QoS, Wireless Some material on signaing, framing, transport, etc. MAC, ARQ, TCP, IP Based upon lecture material, homeworks, and project May be a question regarding the projects Closed book, one page of notes Expect similar style to midterm, just longer CSE 123: Lecture 17: Final Review 3

4 TCP/IP Protocol Stack" host host HTTP Application Layer HTTP TCP Transport Layer TCP router router IP IP Network Layer IP IP Ethernet interface Ethernet interface SONET SONET interface Link Layer interface Ethernet interface Ethernet interface CSE 123: Lecture 17: Final Review 4

5 Signals and Channels"" A signal is some form of energy (light, voltage, etc) Varies with time (on/off, high/low, etc.) Can be continuous or discrete We assume it is periodic with a fixed frequency A channel is a physical medium that conveys energy Any real channel will distort the input signal as it does so How it distorts the signal depends on the signal CSE 123: Lecture 17: Final Review 5

6 Channel Properties" Bandwidth-limited Range of frequencies the channel will transmit Means the channel is slow to react to change in signal Power attenuates over distance Signal gets softer (harder to hear ) the further it travels Different frequencies have different response (distortion) Background noise or interference May add or subtract from original signal Different physical characteristics Point-to-point vs. shared media Very different price points to deploy CSE 123: Lecture 17: Final Review 6

7 Signaling" Digital modulation FSK, ASK, PSK Dealing with noise Shannon s law Sampling at the receiver Intersymbol Inteference: Nyquist Limit Synchronous vs. Asynchronous coding Clock recovery NRZ, Manchester, 4B/5B CSE 123: Lecture 17: Final Review 7

8 (Data) Link Layer" Framing Break stream of bits up into discrete chunks Error handling Detect and/or correct errors in received frames Media access Arbitrate which nodes can send frames at any point in time Not always necessary; e.g. point-to-point duplex links Multiplexing Determine appropriate destination for a given frame Also not always required; again, point-to-point CSE 123: Lecture 17: Final Review 8

9 Framing" Header Payload Trailer Framing determines when payload starts/stops Lots of different ways to do it, various efficiencies Sentinel-based framing requires stuffing Increases the size of the packet Alternatives include fixed size frames SONET uses timing-based framing Gets complicated when trying to switch links CSE 123: Lecture 17: Final Review 9

10 Error Detection" Error handling through redundancy Adding extra bits to the frame Hamming Distance When we can detect When we can correct Checksum Cyclic Remainder Check (CRC) CSE 123: Lecture 17: Final Review 10

11 Reliable Transmission" Automatic Repeat Request (ARQ) Acknowledgements (ACKs) and timeouts Stop-and-Wait Sliding Window Forward Error Correction CSE 123: Lecture 17: Final Review 11

12 Sliding Window" Single mechanism that supports: Multiple outstanding packets Reliable delivery In-order delivery Flow control At the core of all modern ARQ protocols Go-Back-N is a special case Receive window size of one CSE 123: Lecture 17: Final Review 12

13 Media Access Control" Methods to share physical media: multiple access Fixed partitioning Random access Channelizing mechanisms Contention-based mechanisms Aloha Ethernet CSE 123: Lecture 17: Final Review 13

14 Partitioning Visualization" FDMA TDMA power power CDMA power CSE 123: Lecture 17: Final Review Courtesy Takashi Inoue 14

15 Carrier Sense (CSMA) + CD" Aloha transmits even if another host is transmitting Thus guaranteeing a collision Instead, listen first to make sure channel is idle Useful only if channel is frequently idle Why? If nodes can detect collisions, abort! Requires a minimum frame size ( acquiring the medium ) Requires a full duplex channel Binary exponential back-off balances delay w/load CSE 123: Lecture 17: Final Review 15

16 Transport Layer" Provides process naming/demultiplexing Port numbers Two main protocols in use on the Internet User Datagram Protocol (UDP)» Unreliable, datagram service Transport Control Protocol (TCP)» Reliable byte-stream» Requires connection establishment/three-way handshake CSE 123: Lecture 17: Final Review 16

17 Transmission Control Protocol" Reliable bi-directional bytestream between processes Uses a sliding window protocol for efficient transfer Connection-oriented Conversation between two endpoints with beginning and end Flow control Prevents sender from over-running receiver buffers Congestion control Prevents sender from over-running network capacity CSE 123: Lecture 17: Final Review 17

18 Congestion Control" How fast should a sending host transmit data? Not to fast, not to slow, just right Should not be faster than the senders share Bandwidth allocation Should not be faster than the network can process Congestion control Congestion control & bandwidth allocation are separate ideas, but frequently combined CSE 123: Lecture 17: Final Review 18

19 TCPʼs Algorithm" Window-based congestion control Unified congestion control and flow control mechanism rwin: advertised flow control window from receiver cwnd: congestion control window» Estimate of how much outstanding data network can deliver in a round-trip time Sender can only send MIN(rwin,cwnd) at any time Complicated with slow start and fast recovery Ramp up quickly Avoid backing all the way down on isolated loss events CSE 123: Lecture 17: Final Review 19

20 Hubs/Repeaters" Physical layer device One port for each LAN Repeat received bits on one port out all other ports LAN1 Hub LAN2 LAN3 CSE 123: Lecture 17: Final Review 20

21 Bridges" Store and forward device Data-link layer device Buffers entire packet and then rebroadcasts it on other ports Creates separate collision domains Uses CSMA/CD for access to each LAN (acts like a host) Improves throughput Some bridges can learn topology Spanning Tree Algorithm CSE 123: Lecture 17: Final Review 21

22 internetworking " Switching still only moves frames on common link layer MAC addresses are unique but flat Routers forward packets from source to destination May cross many separate networks along the way All packets use a common Internet Protocol Any underlying data link protocol Any higher layer transport protocol CSE 123: Lecture 17: Final Review 22

23 Routers" A router is a store-and-forward device Routers are connected to multiple networks On each network, looks just like another host A lot like a switch, except at the network layer Must be explicitly addressed by incoming frames Not at all like a switch, which is transparent Removes link-layer header, parses IP header Looks up next hop, forwards on appropriate network Each router need only get one step closer to destination CSE 123: Lecture 17: Final Review 23

24 IP Addresses" 32-bits in an IPv4 address Dotted decimal format a.b.c.d Each represent 8 bits of address Hierarchical: Network part and host part E.g. IP address refers to the UCSD campus network refers to the host ieng6.ucsd.edu Subnetting/CIDR aggregation Network mask/prefix CSE 123: Lecture 17: Final Review 24

25 Layers of Identifiers" Host name (e.g., Used by humans to specify host of interest Unique, selected by host administrator Hierarchical, variable-length string of alphanumeric carachters IP address (e.g., ) Used by routers to forward packets Unique, topologically meaningful locator Hierarchical namespace of 32 bits MAC address (e.g., 58:B0:35:F2:3C:D9) Used by network adaptors to identify interesting frames Unique, hard-coded identifier burned into network adaptor Flat name space (of 48 bits in Ethernet) CSE 123: Lecture 17: Final Review 25

26 Naming Protocols" Domain Name System Distributed, hierarchical database Distributed collection of servers Caching to improve performance IP to MAC Address mapping Dynamic Host Configuration Protocol (DHCP) Address Resolution Protocol (ARP) CSE 123: Lecture 17: Final Review 26

27 Routing" How to choose best path? Defining best can be slippery How to scale to millions of users? Minimize control messages and routing table size How to adapt to failures or changes? Node and link failures, plus message loss CSE 123: Lecture 17: Final Review 27

28 Forwarding Options" Source routing Complete path listed in packet Virtual circuits Set up path out-of-band and store path identifier in routers Local path identifier in packet Destination-based forwarding Router looks up address in forwarding table Forwarding table contains (address, next-hop) tuples CSE 123: Lecture 17: Final Review 28

29 Intra-domain Routing" Routing within a network/organization A single administrative domain The administrator can set edge costs Overall goals Provide intra-network connectivity Adapt quickly to failures or topology changes Optimize use of network resources Non-goals Extreme scalability Lying, and/or disagreements about edge costs CSE 123: Lecture 17: Final Review 29

30 Basic Routing Approaches" Static Type in the right answers and hope they are always true So far Link state Tell everyone what you know about your neighbors Dijkstra s Algorithm Distance vector Tell your neighbors when you know about everyone Bellman-Ford Algorithm CSE 123: Lecture 17: Final Review 30

31 Dijkstraʼs Shortest Path" Graph algorithm for single-source shortest path tree S ß {} Q ß <remaining nodes keyed by distance> While Q!= {} u ß extract-min(q) ß u is done S ß S plus {u} for each node v adjacent to u relax the cost of v CSE 123: Lecture 17: Final Review 31

32 Bellman-Ford Algorithm" Define distances at each node X d x (y) = cost of least-cost path from X to Y Update distances based on neighbors d x (y) = min {c(x,v) + d v (y)} over all neighbors V u! 3 2 v! w! x! 5 s! 3 4 y! 1 z! t! d u (z) = min{c(u,v) + d v (z), c(u,w) + d w (z)} CSE 123: Lecture 17: Final Review 32

33 Link-state vs. Distance-vector" Message complexity LS: with n nodes, E links, O(nE) messages sent DV: exchange between neighbors only Speed of Convergence LS: relatively fast DV: convergence time varies May be routing loops Count-to-infinity problem Robustness: what happens if router malfunctions? LS: DV: Node can advertise incorrect link cost Each node computes only its own table Node can advertise incorrect path cost Each nodes table used by others (error propagates) CSE 123: Lecture 17: Final Review 33

34 Inter-domain Routing" Border routers summarize and advertise internal routes to external neighbors and viceversa Border routers apply policy R1 AS1 R3 Internal routers can use notion of default routes R2 Border router Autonomous system 1 Core is default-free; routers must have a route to all networks in the world R5 R4 Autonomous Border system router 2 R6 AS2 CSE 123: Lecture 17: Final Review 34

35 Business Relationships" Neighboring ASes have business contracts How much traffic to carry Which destinations to reach How much money to pay Common business relationships Customer-provider» E.g., Princeton is a customer of USLEC» E.g., MIT is a customer of Level3 Peer-peer» E.g., UUNET is a peer of Sprint» E.g., Harvard is a peer of Harvard Business School CSE 123: Lecture 17: Final Review 35

36 Path-vector Routing" Extension of distance-vector routing Support flexible routing policies Avoid count-to-infinity problem Key idea: advertise the entire path Distance vector: send distance metric per destination Path vector: send the entire path for each destination d: path (2,1) 3 data traffic d: path (1) 2 1 data traffic CSE 123: Lecture 17: Final Review 36 d

37 Border Gateway Protocol" Implement inter-domain routing on the Internet Used and abused to model business practices Default decision process for route selection Highest local pref, shortest AS path, lowest MED, prefer ebgp over ibgp, lowest IGP cost, router id Many policies built on default decision process, but Possible to create arbitrary policies in principal» Any criteria: BGP attributes, source address, prime number of bytes in message, CSE 123: Lecture 17: Final Review 37

38 QoS" Routers manage their own resources Buffer management may entail marking/dropping Scheduling discipline determines outgoing packet order Token bucket and RED Mechanisms to control traffic flowing through routers Networks can provide quality of service Combines per-router traffic policing with network signalling IntServ and DiffServ are contrasting approaches CSE 123: Lecture 17: Final Review 38

39 QoS Signaling" Integrated services Motivated by need for end-to-end guarantees On-line negotiation of per-flow requirements End-to-end per-router negotiation of resources Complex Differentiated services Motivated by economics (multi-tier pricing) No per-flow state Not end-to-end and not guaranteed services Simple CSE 123: Lecture 17: Final Review 39

40 Wireless" New challenges Hidden terminal problem Asymmetric ranges Half-duplex radios (can t do CD) /WiFi Common technology for local-area wireless Uses CSMA/CA RTS/CTS and NAV for virtual carrier sense CSE 123: Lecture 17: Final Review 40

41 For next time " There is no next time J Good luck on the final! If you haven t already, please complete your CAPE CSE 123: Lecture 17: Final Review 41