Data & Computer Communication

Transcription

1 Basic Networking Concepts A network is a system of computers and other devices (such as printers and modems) that are connected in such a way that they can exchange data. A bridge is a device that connects networking cables without examining the addresses of messages or making decisions as to the best route for a message to take. A router contains addressing and routing information that lets it determine from a message's address the most efficient route for the message. A message can be passed from router to router several times before being delivered to its destination. In order for nodes to exchange data, they must use a common set of rules defining the format of the data and the manner in which it is to be transmitted. A protocol is a formalized set of procedural rules for the exchange of information and for the interactions among the network's interconnected nodes. A network software developer implements these rules in software modules that carry out the functions specified by the protocol. Data & Computer Communication William Stallings 1 2 A gateway converts addresses and protocols to connect dissimilar networks. A set of networks connected by routers or gateways is called an internet. The term Internet (note the capitalization) is often used to refer to the largest worldwide system of networks, also called the Worldwide Internet. The basic protocol used to implement the WorldWide Internet is called the Internet Protocol, or IP. Because the word internet is used in several different ways, it is important to note capitalization and context whenever you see this word. Types of Protocols Networking protocols can be characterized as connectionless or connection-oriented, and as transactionless or transaction-based. A connectionless protocol is one in which a node that wants to communicate with another simply sends a message without first establishing that the receiving node is prepared to receive it. Each message sent must include addressing information so that it can be delivered to its destination. A connection-oriented protocol is one in which two nodes on the network that want to communicate must go through a connection-establishment process called a handshake. This involves the exchange of predetermined signals between the nodes in which each end identifies itself to the other. Once a connection is established, the communicating applications or processes on the nodes at either end can send and receive data without having to add addresses to the messages or repeat the handshake process. Connection-oriented protocols provide support for sessions. 3 4 A Communications Model A session is a logical (as opposed to physical) connection between two entities on a network or internet. A session must be set up at the beginning, maintained by the periodic exchange of information, and broken down at the end. All of these services entail overhead compared to a connectionless protocol, for which no connection setup or breakdown is required and for which no session must be maintained. A connection-oriented session is analogous to a telephone call. The party who initiates the call knows whether the connection is made because someone at the other end of the line either answers or not. As long as the connection is maintained, neither party needs to dial the other telephone number again. A connectionless protocol is analogous to mail. A person sends a letter expecting it will be delivered to its destination. Although the mail usually arrives safely, the sender doesn't know this unless the recipient initiates a response affirming it. Each letter sent by either party requires a complete address. 5 6

2 Data Communications Model Networking Communication Categories Local area network (LAN) Wide area network (WAN) 7 8 Local Area Networks Metropolitan Area Networks Smaller scope Building or small campus Usually owned by same organization as attached devices Data rates much higher Switched lans,, e.g. ethernet Wireless lans Middle ground between LAN and WAN Private or public network High speed Large area 9 10 Internet Elements Internet Architecture 11 12

3 Example Configuration Circuit Switching Dedicated communications path between two nodes for the duration of conversation. Comprising a sequence of physical links. e.g. Telephone network Packet Switching Frame Relay Different Technique Data can be sent out of sequence Small chunks called packets of data each time Packets passed through various number of nodes to reach destination ion Used for terminal to computer and computer to computer communications Packet switching systems have large overheads to compensate for errors Modern systems are more reliable Errors can be caught in end system Frame relay provides higher speeds With most error control overhead removed Asynchronous Transfer Mode (ATM) Simplified Network Architecture Evolved from frame relay Fixed packet (called cell) length With little overhead for error control Anything from 10mbps to gbps Constant data rate using packet switching technique 17 18

4 TCP/IP Layers Physical Layer No official model but a working one: Concerned with physical interface between computer and network, deals with: 1. Application layer 2. Host-to to-host, or transport layer 3. Internet layer 4. Network access layer 5. Physical layer Characteristics of transmission medium Signal levels Data rates Network Access Layer Internet Layer (IP) Exchange of data between an end system and attached network, deals with: Destination address provision Invoking specific services like priority Access to & routing data across a network link between two attached systems Routing functions across multiple networks For systems attached to different networks Using IP protocol Implemented in end systems and routers Routers connect two networks and relays data between them Transport Layer (TCP) Application Layer Common layer shared by all applications Provides reliable delivery of data In same order as sent The most commonly used protocol is TCP Provide support for user applications Need a separate module for each type of application 23 24

5 Addressing Requirements Operation of TCP and IP Two levels of addressing required 1. Each host on a subnet needs a unique global network address Its IP address 2. Each application on a (multi-tasking) tasking) host needs a unique address within the host Known as a port Operation of TCP/IP Transmission Control Protocol (TCP) Usual transport layer is (TCP) Provides a reliable connection for transfer of data between applications A TCP segment is the basic protocol unit TCP tracks segments between entities for duration of each connection TCP Header User Datagram Protocol (UDP) An alternative to TCP No guaranteed delivery No preservation of sequence No protection against duplication Minimum overhead Adds port addressing to IP 29 30

6 UDP Header IP Header Header Fields Header Fields Version Currently 4 IP v6 - see later Internet header length In 32 bit words Identification Sequence number Identify datagram uniquely with addresses / protocol Flags More bit Don t fragment DS/ECN, type of service Differentiated services (provides build-in aggregation mechanism, e.g., multiple voice connections are not handled individually but in aggregate. Explicit congestion notification (process of standardization. Fragmentation offset Time to live Total length Of datagram, in octets Protocol Next higher layer to receive data field at destination Header Fields IPv6 Header Header checksum Re-verified and recomputed at each router 16 bit ones complement sum of all 16 bit words in header Set to zero during calculation Source address Destination address Options Padding To fill to multiple of 32 bits long 35 36

7 TCP/IP Applications Some TCP/IP Protocols Simple mail transfer protocol (SMTP) File transfer protocol (FTP) Telnet Open System Interconnection (OSI) Developed by the international organization for standardization (ISO) Consists of seven layers Is a theoretical system TCP/IP is the de facto standard OSI Layers Standardized Protocol Architectures 41 42

8 Service Primitives and Parameters Primitive Types Define services between adjacent layers using: Primitives to specify function performed Parameters to pass data and control info REQUEST INDICATION RESPONSE CONFIRM A primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service A primitive issued by a service provider either to: indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or notify the service user of a provider-initiated action A primitive issued by a service user to acknowledge or complete some procedure previously invoked by an indication to that user A primitive issued by a service provider to acknowledge or complete some procedure previously invoked by a request by the service user Traditional vs Multimedia Applications Elastic and Inelastic Traffic Traditionally internet dominated by info retrieval applications Typically using text and image transfer e.g. , file transfer, web There is increasing growth in multimedia applications Involving massive amounts of data Such as streaming audio and video Elastic traffic Can adjust to delay & throughput changes over a wide range e.g. Traditional data style TCP/IP traffic Some applications more sensitive though Inelastic traffic Does not adapt to such changes e.g. Real-time voice & video traffic Need minimum requirements on net arch Multimedia Technologies 47