COMP750. Distributed Systems. Network Overview

Transcription

1 COMP750 Distributed Systems Network Overview

2 Network Standards The purpose of a network is to allow two computers to communicate. Ex: The electrical power network in North America follows a standard to ensure that any electrical device can be used. Any standard electrical device 110 volts of 60 Hertz providing up to 15 amps

3 International Standards Organization International Standards Organization Open System Interface A standard and network architecture model. Not very often followed as a standard. OSI networks are more popular in Europe. Popular and important model for network architectures. Similar to the Internet Protocol model but not the same.

4 ISO OSI Model International Standards Organization Open System Interface Model OSI Stack Divides the many networking functions into seven different layers Called a stack because each layer provides functions or services to the layer above it.

5 OSI Flow Chart OSI Stack sending computer receiving computer When a layer wants to send something to its peer layer in another computer, it calls a function in the layer below it to actually send the data. Layer 7 Layer 6 Layer 5 Only the lowest layer actually sends bits to another computer Layer 4 Layer 3 Layer 2 Information flow indicator Layer 1

6 OSI (an Analogy) Ex: U.S. Mail You Your friend Your post office Their post office Airline Airline You do not have to worry about how to find your friends house in the distant city. The post office does not need to know how to fly the airplane. Each layer assumes that the layer below it will provide certain functions. Each layer provides additional functionality

7 Information chart Seven Layer Specs. (OSI) layer purpose example 1. Application Provides network services. X.400 , HTTP, FTP, telnet 2. Presentation Converts the data to the representation used by the local computer. 3. Session Establishes sessions. 4. Transport 5. Network 6. Data link - logical data link Multiplexes data streams from different applications. Directs packets to the correct user on a computer. This is the first end-toend layer. May also provide error correction. Finds a route for packets to take through the network. Directs packets to the correct computer Detects and corrects any errors on the link. Provides flow control. Transport Control Protocol (TCP) Internet Protocol (IP) - media access control 7. Physical Provides network services. Defines the characteristics of the physical connections. This is the only layer that actually sends bits to another computer. Ethernet, Token Ring SONET, RS-232C

8 Internet Protocol Stack The Internet Protocol uses a similar, but slightly different model than OSI. The Internet Protocol does not define the lower levels. layer purpose OSI equivalent example 1. Application Provides network services. Application, Session and Presentation HTTP, FTP, Telnet 2. Transport 3. Internet Multiplexes data streams from different applications. May also provide error correction. Routing. Transport Network TCP, UDP IP 5. Network Interface Provides access to the Data Link and lower protocols. The IP stack does not define the lower levels. Data Link Ethernet

9 Transport Layer The first, or lowest, end to end layer. The transport layer delivers packets to the appropriate application on the host. Some transport layer protocols correct transmission errors. There are several popular transport layer protocols including TCP and UDP.

10 Internet Port Numbers Applications are identified by a 16 bit integer number known as a port number. Internet ports do NOT refer to plugs in the back of the machine. The full address of an application is InternetName:port Applications bind to a port number to receive data sent to that port.

11 Well Known Ports Port numbers under 2K are reserved for specific well known application servers 21 ftp 23 telnet 79 finger 80 HTTP web servers 443 HTTPS secure web servers 17 Quote of the Day

12 Lesser Known Ports Well Known Ports are only used by servers. Servers for non-standard applications use higher numbered ports. Applications accessing a server use a higher numbered port. When a program connects to a remote system, it is automatically assigned a port.

13 TCP and UDP TCP Connection Oriented Complete reliability corrects lost, corrupted and out-of-order packets Full Duplex communication Point to Point communication Stream Interface Reliable connection startup Connectionless best effort delivery Full Duplex communication Point to Point communication or broadcast Packet Interface no connection UDP

14 Popular TCP Applications HTTP - web protocol telnet - terminal protocol ftp - file transfer protocol any program that has lengthy transfers that require reliability.

15 Popular UDP Applications DNS requests WINS requests Streaming Audio Any application that needs to send a short amount of data that can be resent if necessary (idempotent or at-least-once). Time critical applications

16 Connection Oriented A program using TCP must connect to the remote host before sending any data. Connection verifies ability to communicate with the destination. To establish a connection, TCP sends: message to the server requesting a connection. Response from server accepting connection. Message to the server specifying parameters.

17 Error Correction TCP corrects lost, corrupted, delayed and out-of-order packets. Applications do not have to worry about the transmission reliability. Data is delivered exactly as it was sent. UDP does not correct any errors. Higher level software must recover from problems. UDP is far more efficient than TCP.

18 Point to Point Communications TCP connects one sender to one receiver. All data sent over a TCP connection goes to the same receiving application. TCP connections do not support broadcasts. Connections are full duplex allowing communication in both directions.

19 Packet Interface UDP sent 50 bytes 50 bytes 50 bytes UDP receive 50 bytes 50 bytes UDP receive 25 bytes truncated 25 UDP receive 75 bytes Received less than request 50 bytes

20 Stream Interface TCP sent 50 bytes 50 bytes 50 bytes TCP receive 50 bytes 50 bytes TCP receive 25 bytes 25 TCP receive 75 bytes bytes

21 TCP Header

22 UDP Header Source Port Destination Port Message Length Checksum Data

23 Nested Protocol Headers The nested protocol headers that appear in a frame as the frame travels across a network. Each layer of protocol software adds a header to an outgoing frame.

24 Nested Protocol Headers The data link layer often adds a trailer to the packet that contains a cyclic redundancy check (CRC) to detect errors. The physical layer might, or might not, append a header or trailer to the packet. It is the bottom frame, with all of the headers, that is actually sent across the network. When it is received at the other end, the headers are stripped off as the packet is passed up the stack to the user application.

25 Packet Switching Data is sent in packets or frames. Intermediate nodes must receive an entire packet before sending on towards the destination, but they do not have to receive the entire message. Each packet needs a header to identify its destination.

26 Packet Switching The packets can be variable sized or (more often) fixed sized. The size of a packet is usually much smaller than the total data size. Packet sizes range from 48 bytes for ATM to 1500 bytes for Ethernet to 8K bytes for frame relay.

27 Packet Switching

28 Comparison of Methods Circuit switching works well if the time to transfer the data is long compared to the circuit setup time. Packet switching easily allows multiple independent data streams to be combined one a channel. The Internet uses packet switching.

29 Transmission Time When calculating the transmission time of data sent over a packet switched system, you have to account for the header and trailer overhead. Each packet has a header and can only hold some maximum amount of data. time databits = PktSize * PktSize + headersize transmissionrate

30 Delay with Intermediates X = number of bits to be sent (bits). B = transmission line speed (bits/second) D = propagation delay (seconds) = distance/c P = packet size (bits). H = header size (bits). S = switching time (seconds). K = number of hops or data lines used.