Lecture 17: Final Review CSE 123: Computer Networks Chris Kanich - PDF document

Lecture 17: Final Review CSE 123: Computer Networks Chris Kanich Last class!!! Overview  Signaling  Framing  Error detection  Reliable transmission  Flow control  Bridging/Switching  Congestion control  Routing  QoS  Wireless CSE 123: Lecture 17: Final Review 2 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 1

TCP/IP Protocol Stack host host HTTP Application Layer HTTP TCP Transport Layer TCP router router I I Network Layer I I P P P P Ethernet SONET Ethernet Ethernet Ethernet SONET Link Layer interface interface interface interface interface interface CSE 123: Lecture 17: Final Review 4 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 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 2

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 (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 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 CSE 123: Lecture 17: Final Review 9 3

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 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 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 4

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 Partitioning Visualization FDMA power TDMA power CDMA power Courtesy Takashi Inoue CSE 123: Lecture 17: Final Review 14 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 5

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 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 Congestion Control  How fast should a sending host transmit data?  Not to fast, not to slow, just right…  Should not be faster than the sender ’ s 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 6

TCP’s 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 Hubs/Repeaters  Physical layer device  One “port” for each LAN  Repeat received bits on one port out all other ports Hub LAN1 LAN2 LAN3 CSE 123: Lecture 17: Final Review 20 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 7

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 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 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 128.54.70.238  128.54 refers to the UCSD campus network  70.238 refers to the host ieng6.ucsd.edu  Subnetting/CIDR aggregation  Network mask/prefix CSE 123: Lecture 17: Final Review 24 8

Layers of Identifiers  Host name (e.g., www.ucsd.edu)  Used by humans to specify host of interest  Unique, selected by host administrator  Hierarchical, variable-length string of alphanumeric carachters  IP address (e.g., 128.54.70.238)  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 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 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 9