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Transcription:

Module 2 Overview of Computer Networks

Networks and Communication Give me names of all employees Who earn more than $00,000 ISP intranet backbone satellite link desktop computer: server: network link: CS454/654 2-2

Issues How do the request and response get transmitted between the requestor and the server? Protocols to facilitate communication Moving the request and reply messages through the network What are the modes of communication between the requestor and the responder? Connectionless service Connection-oriented service CS454/654 2-3

What is a protocol? A protocol defines the format and the order of messages sent and received among network entities, and the actions taken on message transmission and receipt Human protocols: What s the time? I have a question introductions Network protocols: machines rather than humans all communication activity in Internet governed by protocols CS454/654 2-4

What s a protocol? Human protocol Computer protocol Hi Hi Got the time? 2:00 time TCP connection req. TCP connection reply. Get http://www.cs.uwaterloo.ca/ <index.html> CS454/654 2-5

Layered Protocols Divide functionality to different layers and let each layer provide one function. ISO OSI Layered network architecture Function layers Interfaces Protocols at each layer From Tanenbaum and van Steen, Distributed Systems: Principles and Paradigms CS454/654 Prentice-Hall, Inc. 2002 2-6

Message Format From Tanenbaum and van Steen, Distributed Systems: Principles and Paradigms CS454/654 Prentice-Hall, Inc. 2002 2-7

Message Transmission (Assume only 3 layers) CS454/654 2-8

OSI Protocol Summary Layer Description Examples Application Protocols that are designed to meet the communication requirements of HTTP, FTP, SMTP, specific applications, often defining the interface to a service. CORBA IIOP Presentation Protocols at this level transmit data in a network representation that is independent of the representations used in individual computers, which may differ. Encryption is also performed in this layer, if required. Secure Sockets (SSL),CORBA Data Rep. Session At this level reliability and adaptation are performed, such as detection of failures and automatic recovery. Transport This is the lowest level at which messages (rather than packets) are handled. Messages are addressed to communication ports attached to processes, Protocols in this layer may be connection-oriented or connectionless. TCP, UDP Network Data link Physical Transfers data packets between computers in a specific network. In a WAN or an internetwork this involves the generation of a route passing through routers. In a single LAN no routing is required. Responsible for transmission of packets between nodes that are directly connected by a physical link. In a WAN transmission is between pairs of routers or between routers and hosts. In a LAN it is between any pair of hosts. The circuits and hardware that drive the network. It transmits sequences of binary data by analogue signalling, using amplitude or frequency modulation of electrical signals (on cable circuits), light signals (on fibre optic circuits) or other electromagnetic signals (on radio and microwave circuits). IP, ATM virtual circuits Ethernet MAC, ATM cell transfer, PPP Ethernet base- band signalling, ISDN From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-9

Internet Protocols Application (7) Presentation (6) Session (5) HTML, CGI, Java, HTTP FTP Telnet NFS SMTP HTTP Transport (4) TCP UDP Network (3) IP Data Link (2) Physical () X.25 Ethernet RNIS ATM FDDI CS454/654 2-0

TCP/IP Layers Layers Message Application Transport Internet Network interface Messages (UDP) or Streams (TCP) UDP or TCP packets IP datagrams Network-specific frames Underlying network From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-

Internet Protocol Stack application: supporting network applications ftp, smtp, http transport: host-host data transfer tcp, udp network: routing of datagrams from source to destination ip, routing protocols link: data transfer between neighboring network elements ppp, ethernet physical: bits on the wire application transport network link physical message segment datagram frame CS454/654 2-2

Applications and Application- Layer Protocols Application: communicating, distributed processes running in network hosts in user space application transport network data link physical exchange messages to implement app e.g., email, file transfer, the Web Application-layer protocols one piece of an app define messages exchanged by apps and actions taken use services provided by lower layer protocols application transport network data link physical application transport network data link physical router workstation server mobile CS454/654 2-3

Application-Layer Protocols (cont). API: application programming interface Defines interface between application and transport layer socket: Internet API two processes communicate by sending data into socket, reading data out of socket What transport services does an application need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 00% reliable data transfer Bandwidth some apps (e.g., multimedia) require a minimum amount of bandwidth to be effective other apps ( elastic apps ) make use of whatever bandwidth they get Timing some apps (e.g., Internet telephony, interactive games) require low delay to be effective CS454/654 2-4

Transport Service Requirements of Common Applications Application Data loss Bandwidth Time Sensitive file transfer e-mail Web documents real-time audio/video stored audio/video interactive games financial apps no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5Kb-Mb video:0kb-5mb same as above few Kbps up elastic no no no yes, 00 s msec yes, few secs yes, 00 s msec yes and no CS454/654 2-5

Internet Transport Protocol Services TCP service: connection-oriented: setup required between client, server reliable transport between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee CS454/654 2-6

Internet Apps: Their Protocols and Transport Protocols Application e-mail remote terminal access Web file transfer streaming multimedia remote file server Internet telephony Application layer protocol smtp [RFC 82] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NFS proprietary (e.g., Vocaltec) Underlying transport protocol TCP TCP TCP TCP TCP or UDP TCP or UDP typically UDP CS454/654 2-7

TCP Message Encapsulation over an Ethernet Application message TCP header port IP header TCP Ethernet header IP Ethernet frame From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-8

The programmer's conceptual view of a TCP/IP Internet Application TCP Application UDP IP From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-9

Internet Address Structure 7 24 Class A: 0 Network ID Host ID 4 6 Class B: 0 Network ID Host ID 2 8 Class C: 0 Network ID Host ID Class D (multicast): 0 Multicast address Class E (reserved): 0 unused 28 27 From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-20

Decimal Representation of Internet Addresses octet octet 2 octet 3 Network ID Host ID Class A: to 27 0 to 255 0 to 255 0 to 255 Network ID Host ID Class B: 28 to 9 0 to 255 0 to 255 0 to 255 Network ID Host ID Class C: 92 to 223 0 to 255 0 to 255 to 254 Multicast address Class D (multicast): 224 to 239 0 to 255 0 to 255 to 254 Range of addresses.0.0.0 to 27.255.255.255 28.0.0.0 to 9.255.255.255 92.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 Class E (reserved): 240 to 255 0 to 255 0 to 255 to 254 240.0.0.0 to 255.255.255.255 From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-2

IP Packet Layout header IP address of source IP address of destination up to 64 kilobytes data From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-22

How to Transfer Data Through the Network Network is a mesh of interconnected routers Two ways of setting up a connection between two computers circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete chunks CS454/654 2-23

Circuit Switching End-end resources reserved for call link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required CS454/654 2-24

Circuit Switching Network resources (e.g., bandwidth) divided into pieces pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into pieces frequency division time division CS454/654 2-25

Packet Switching each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time transmit over link wait turn at next link CS454/654 2-26

Packet Switching A 0 Mbs Ethernet statistical multiplexing C B queue of packets waiting for output link.5 Mbs D 45 Mbs E CS454/654 2-27

Message Switching Message switching = Packet-switching without segmentation Packet switching: Store and Forward Message remains intact as it traverses the network Message 0 Message 5 Message 0 Message 5 Time (sec.) CS454/654 2-28

Packet switching vs Circuit Switching Packet switching allows more users to use network! Mbit link; each user: 00Kbps when active active 0% of time circuit-switching: 0 users packet switching: 35 users, probability > 0 active less than.004 N users Mbps link CS454/654 2-29

Packet Switching vs Circuit Switching (2) Packet switching is great for bursty data resource sharing no call setup It incurs excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem, but solutions such as ATM have been developed CS454/654 2-30

Delays in Packet Switching Packets experience delay on end-toend path Sources of delay at each hop: Nodal processing check bit errors determine output link Queuing time waiting at output link for transmission; depends on congestion level of router Transmission delay R=link bandwidth (bps), L=packet length (bits) time to send bits into link = L/R Propagation delay d = length of physical link, s = propagation speed in medium (~2x0 8 m/sec) propagation delay = d/s A B transmission propagation nodal processing queueing CS454/654 2-3

Routing Goal: move packets among routers from source to destination It is an issue in packet switched networks datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next hop fixed path determined at call setup time, remains fixed thru call routers maintain per-call state CS454/654 2-32

Routing in a Wide Area Network A B Hosts or local networks 3 D Links 6 E 4 5 2 C Routers From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000 CS454/654 2-33

CS454/654 2-34 Routing Tables Routings from D Routings from E To Link Cost To Link Cost A B C D E 3 3 6 local 6 2 2 0 A B C D E 4 4 5 6 local 2 0 Routings from A Routings from B Routings from C To Link Cost To Link Cost To Link Cost A B C D E local 3 0 2 2 A B C D E local 2 4 0 2 A B C D E 2 2 local 5 5 2 0 2 From Coulouris, Dollimore and Kindberg, Distributed Systems: Concepts and Design, 3rd ed. Addison-Wesley Publishers 2000

Network Taxonomy telecommunication networks Circuit-switched networks Packet-switched networks FDM TDM Networks with VCs connectionoriented Datagram Networks connection-oriented & connectionless CS454/654 2-35

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