Chi-Cheng Lin, Winona State University CS 313 Introduction to Computer Networking & Telecommunication Introduction

2 Topics l Introduction l Metric Units l Network Hardware l Network Software l Reference Models l Example Networks l Standards and Standards Organizations

3 Network Software l Old computer networks:  HW main concern  SW afterthought  Not working now! l Network SW is now highly structured  Approach: Protocol Hierarchies

4 Protocol Hierarchies l What is protocol?  Agreement between communication parties on HOW communication is processed l Layered architecture  Reduce design complexity: Lower layer offers service to higher layer  Hiding implementation details  Layer n on one machine talks to layer n on another  Rules and conventions used in layer n’s talk: Layer n protocol

5 Protocol Hierarchies l Peers  Entities comprising corresponding layers on different machines  Virtual communication using protocol  Peer process abstraction make network design becomes that of individual layers l Physical communication  Sender: Data and control passed to layer below  Data transmitted via physical media  Receiver: Data and control passed to layer above

6 Layers, Protocols, and Interfaces Virtual Communication Physical Communication

7 Protocol Hierarchies l Interface between two adjacent layers  Defines primitive operations and services a lower layer offers to the upper one  Minimizes amount of information passed between two layers  Simplifies replacement of implementation  E.g., telephone lines  satellite channels

8 Protocol Hierarchies l Network architecture  Set of layers and protocols  Implementation and interface specification not included l Protocol stack  A list of protocols used by a certain system, one protocol per layer

9 Multilayer Communication - Example l Philosopher-translator-secretary architecture  It is ok if  Dutch is replaced by Finnish  fax is replaced by

10 Information Flow - Example l Virtual communication for layer 5  Header: control information Layer 1 protocol …

11 Key Design Issues for the Layers l Reliability  Error control  Error-detecting  Error-correcting  Routing  Selecting the best path for sending a packet from one point to another

12 Key Design Issues for the Layers l Sender/receiver identification mechanism  Addressing/naming l Sequencing l Message disassembling, transmitting, reassembling

13 Key Design Issues for the Layers l Resource allocation  Multiplexing  The process of combining signals from multiple sources for transmission across a single data link  Multiple connections can share the link l Flow control  Needed for fast sender, slow receiver l Congestion control l Quality of service l Security

14 Connection-Oriented and Connectionless Services l Two basic types of services  Connection-oriented  Connectionless l Consider reliability …  ReliableConnection-oriented UnreliableConnectionless l Note that: Connection  Reliability

15 Connection-Oriented Service l A connection is established first, then used, and then released when done. l Works like a pipe:  Sender pushes data in at one end  Receiver takes them out, often in the same order, at the other end l Analogy  Telephone system

16 Connectionless Service l No need to set up a connection first l Each message carrying full destination address is routed independently of others  No guarantees on the order l Analogy  Postal system

17 Service Primitives l Service is formally specified by a set of primitives (e.g., OS’s system calls) available to users or entities l Five service primitives for implementing a simple connection-oriented service.

18 Service Primitives l Packets sent in a simple client-server interaction on a connection-oriented network.

19 Relationship of Services to Protocols l Service  Set of primitives a layer provides to the layer above it  Define WHAT operations not HOW implemented l Protocol  Set of rules governing format and meaning of message exchanged by peer entities within a layer  Used by entities to implement service definitions

20 Services to Protocols Relationship l The relationship between a service and a protocol.

21 Relationship of Services to Protocols l Analogy: object-oriented languages  Service :: ADT or Object  Users do not know the implementation of a service  Protocol :: Implementation  The protocol of the service is invisible to users  Do you have to understand http (hypertext transport protocol) before you can surf the Internet?

22 Reference Models l Two reference models will be discussed  OSI reference model  TCP/IP model

23 OSI Reference Model l ISO/OSI (Open Systems Interconnection) Reference Model l NOT a network architecture itself  Exact services and protocols are not specified  Just "what should be done" in each layer  However, standards are produced for all layers

24 OSI Reference Model l Seven layers  Layer 7: application layer  Layer 6: presentation layer  Layer 5: session layer  Layer 4: transport layer  Layer 3: network layer  Layer 2: data link layer  Layer 1: physical layer (lowest) l Diagram of OSI reference model  Note: this is one of the most important figures in the whole book!!

25 Physical medium

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27 Host A Host B Subnet Physical medium End-to-End Point-to-Point

28 Physical Layer l Transmitting raw bits (0s and 1s) over communication channel l Design issues  Representation of bits  How is 0/1 represented?  Data rate: number of bits sent per second  How long does a bit last?  Transmission mode (bi-directional?)  Mechanical, electrical, timing interfaces  Underlying physical transmission medium

29 Data Link Layer l Takes a raw transmission facility and transforms it into a line (link) that appears free of undetected transmission errors to network layer l Basic function  Breaks up input data to data frames  Transmits data frames sequentially  Processes acknowledgement frames sent back from receiver for reliable transmission

30 Data Link Layer l Responsibilities  Physical addressing (e.g. MAC address)  Framing  Creating and recognizing frame boundaries  Error control (adjacent nodes, node-to-node)  Errors: damaged, lost, duplicate  Flow control (adjacent nodes, node-to-node)  Traffic regulation between fast sender and slow receiver  Medium access control  Shared channel access control in broadcast networks

31 Network Layer l Subnet operation control l Responsibilities  Logical addressing (e.g., IP address)  Routing  Static tables  Determined at the start of conversation  Dynamic  Congestion control  Quality of service  Accounting  Heterogeneous network interconnection

32 Transport Layer l End-to-end layer  Talk to destination machine directly (virtually)  Layers 4 through 7 are end-to-end  Layers 1 through 3 are node-to-node (chained) l Basic function  Split data from session layer into smaller units  Pass units to network layer  Ensure units arrive correctly at the other end

33 Transport Layer l Determine services provided to session layer (and ultimately to users)  Error-free point-to-point channel that delivers messages in the order in which they were sent  Transport of isolated messages w/o guarantee about order  Broadcasting

34 Transport Layer l Responsibilities include  Service-point addressing (e.g., port number)  Which message belong to which connection (application):  (End-to-end) Flow control Compare to the  (End-to-end) Error control Data Link layer

35 Session layer l Session establishment between users on different machines l Responsibilities  Dialogue control  Deciding who sends, and when  Token management  Control of same critical operation not to be performed at the same time  Synchronization  Inserting checkpoints (checkpointing)

36 Presentation Layer l Syntax/semantics of information transmitted l Responsibilities  Make communication between computers with different internal data representations possible  Approach: standard encoding  Convert from data representation used in one host to the standard abstract data structure and back

37 Application Layer l Provides interface and support for services to users (human, software, robots) l Examples  File transfer   Network news  Hypertext transfer

38 TCP/IP Reference Model l Goals  Internetworking  Fault tolerance  Flexible architecture l Four layers of TCP/IP Reference Model  Host-to-network layer  Internet layer  Transport layer  Application layer

39 Internet Layer l Packet-switching, connectionless l Packets injected to network  Independent travel  Out-of-order arrival l Analogy  Mail system l IP (Internet Protocol)  Packet routing  Congestion control

40 Transport Layer l Two end-to-end protocols  UDP (User Datagram Protocol)  TCP (Transmission Control Protocol) l UDP (User Datagram Protocol)  Unreliable, connectionless  Widely used for  client-server type request-reply queries  speech, video

41 Transport Layer l TCP  Reliable connection-oriented  Incoming byte stream (form application layer) is fragmented into discrete messages and passed onto internet layer  Message is reassembled at destination  Flow control  Analogy A B Pipe

42 Applications and Host-to-Network Layers l Application layer  No session and presentation layers  TELNET, FTP, SMTP, DNS, NNTP, HTTP l Link layer  Defines what links must do to meet the need of connectionless internet layer

43 TCP/IP Protocols

44 OSI and TCP/IP Models l Correspondence

45 OSI and TCP/IP Models l Similarities  Stack of independent protocols  Layer functionality  Transport layer  Application layer

46 Differences between OSI and TCP/IP Models l OSI  Distinction between services, interfaces, and protocols (perhaps the biggest contribution)  Better Protocol-Hidden  Model first, then protocols  Pro: No bias, more general  Con: Designers did not have  much experience with the subject  a good idea of which functionality to put in which layer  No thought given to internetworking  7 layers  Communication  Connection-Oriented and connectionless in network layer  Only connection-oriented in transport layer

47 Differences between OSI and TCP/IP Models l TCP/IP:  No clear distinction between services, interfaces, and protocols  Worse protocol-hidden  Protocol first, then model  Pro: Protocols fit model perfectly  Con: Model does not fit any other protocol stacks (not general)  4 layers  Communication  Connectionless in network layer  Both in transport layer (good for request-response protocols)

48 Summary of Reference Models l OSI  OSI model exceptionally useful for discussing computer networks  OSI protocols not popular l TCP/IP  TCP/IP model practically nonexistent  TCP/IP protocols widely used l Modified framework is used in the text

49 Summary of Reference Models l Modified framework is used in the text

50 Example Networks l The Internet Overview of the Internet architecture

51 Example Networks l 3G mobile networks l Wireless LANs: l RFID and sensor networks

52 Standards and Standards Organizations l Why standards? l Categories  de facto  de jure l Organizations  ITU-T (formerly CCITT)  ISO  ANSI  IEEE  IETF  ATM Forum