Introduction to Networking
* introduction: what is a network
* elements of network architecture
* layered network architecture
* examples
* history
* internetworking
* Reading: sections 1.1-1.5, 1.7-1.9
What is a Computer Network?
* a set of computer and/or switches connected by communication links
* many ``topologies'' possible:
* local area networks (LAN) versus wide-area networks (WAN)
* many different media: fiber optic, coaxial cable, twisted pair, radio, satellite
* for us: topology and media unimportant
What is a Computer Network?
a software/hardware infrastructure:
* original justification: allows shared access to computing resources (e.g., computers, files, data)
* a medium through which geographically dispersed users communicate (e.g., email, teleconferencing)
* a medium through distributed services/applications are implemented
* an electronic village
* an information highway, national information infrastructure
* cyberspace - "a consensual [environment] experienced daily by billions of operators, in every nation, ...." Hotlink: Wiliam Gibson on Cyberspace
Packet-Switching
* data entering network divided into chunks called "packets''
* packets traversing network share network resources (e.g., link bandwidth, buffers) with other packets
o on demand resource use: statistical resource sharing
* resources demands may exceed resources available:
o e.g., A and B packets arrive at R1, destined for C
o resource contention: queueing (waiting), delay
Circuit Switched Networks
* all resources (e.g. communication links) needed by call dedicated to that call for duration
* example: telephone network
* resource demands may exceed resources available
* A and B want to call C
* resource contention: blocking (busy signal)
* drawbacks: ??
* advantages: ?? Hotlink: Keith Ross on Circuit-switching versus packet-switching
Why statistically share resources?
* save/make money!
* example: 1 Mbit/sec link; each user requires 100 Kbits/sec when transmitting; each user has data to send only 10% of time.
o circuit-switching: give each caller 100 Kbits/sec capacity. Can support 10 callers.
o packet-switching: with 35 ongoing calls, probability that 10 or more callers simultaneously active < 0.0004!
+ Can support many more callers, with small probability of` "contention.''
* if users are ``bursty'' (on/off), then packet-switching is advantageous (Baran, 1965)
Elements of a Network
* communication links:
o point-to-point (e.g., A-to-B)
o broadcast (e.g.,: Ethernet LAN)
* host: computer running applications which use network (e.g.: H1)
* router: computer (often w/o applications-level programs) routing packets from input line to output line. (e.g., C)
* gateway: a router directly connected to two+ networks (e.g. A)
* network: set of nodes (hosts/routers/gateways) within single administrative domain
* internet: collection of interconnected networks
Protocols
* protocol: rules by which active network elements (applications, hosts, routers) communicate with each other
* protocols define :
o format/order of messages exchanged
o actions taken on receipt of message
* rules by which two or more people communicate to provide a service, or to get something done
* protocols in every day life:
Layered Architecture
* complex system architecture simplified by layering.
* layer N relies on services of layer N-1 to provide a service to layer N+1
* service from lower layer independent of how that service implemented
o information/complexity hiding
o layer N change doesn't affect other layers
* interfaces define how services requested
Layered network architecture
* the network consists of geographically distributed hardware/software components
* a distributed layered view:
Layering and protocols
* peer entities (e.g., processes) in layer N provide service by communicating (sending "packets") with each other, using communication service provided by layer N-1.
* logical versus physical communication:
The Internet and ISO/OSI reference models
Hot Link: an IETF view of standards
Layers of a protocol architecture
* application layer
* process-to-process communication
* examples: WWW, email, teleconferencing, info. retrieval
* socket layer (Internet only)
o buffering and delivery of data at end systems
* presentation layer (OSI only)
o conversion of data to a common format (e.g., little endian versus big-endian byte orders, integer and floating point numbers).
o Internet stack: data conversion a user-level concern
Layers of a protocol architecture (cont)
* session layer (OSI only)
o session set up (e.g., authentication), recovery from failure (broken session)
o a "thin" layer
* transport layer
o transport service: end-to-end delivery of data
o may multiplex several streams from higher layers
o sender/receiver speed matching
o Internet: TCP and UDP
Layers of a protocol architecture (cont)
* network layer
o at end hosts: start packets on their way
o at routers: control packet routing
o bottleneck avoidance, congestion control
o Internet: IP packets, BGP, RIP
Layers of a protocol architecture (cont)
* data link layer
o point-to-point error free communication over a single link
o multiaccess LAN protocols
o speed matching between sender/receiver
o Ethernet, HDLC, PPP
* physical layer: stuff of EE's
o transmitting raw bits (0/1) over wire
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