AE6382Fundamentals of Computer NetworkingAE6382Fundamentals of Computer Networking• Computer networks are an integral part of the modern computing infrastructure• The local network (LAN) is usually Ethernet• LAN’s are inter-connected with other LAN’s in a hierarchical fashion eventually forming the InternetAE6382Fundamentals of Computer Networking• The operation of computer networks is governed by hardware and software protocols• There are two standard network models• The 7-layer ISO model http://en.wikipedia.org/wiki/OSI_model•The 5-layer TCP/IP model http://en.wikipedia.org/wiki/TCP/IP_model• These models are abstract representations of the division of laborAE6382Fundamentals of Computer NetworkingNetwork ModelsAE6382Fundamentals of Computer Networking• Layer 1 (Physical) is the electrical specification• Layer 2 (Data Link) defines the interface to Layer 1• Layer 3 (Network) is generally responsible for transferring data between networks• Layer 4 (Transport) is generally responsible for packaging data into packets for transfer between computers• Layer 5 (Application) is generally responsible for communication between applicationsNetwork ModelAE6382Fundamentals of Computer Networking• The networking protocol of the Local Area Network is Ethernet http://en.wikipedia.org/wiki/Ethernet• Ethernet is defined in layers 1 and 2EthernetAE6382Fundamentals of Computer Networking• The networking protocol of the Internet is TCP/IP http://en.wikipedia.org/wiki/TCP/IP• The functionality of TCP/IP resides in layers 3, 4, and 5TCP/IP SuiteAE6382Fundamentals of Computer Networking• Ethernet was invented by Xerox in 1970’s• Speeds are currently 10, 100, 1000 Mbps• Data is packaged in frames• The major contents of a frame are• source address (6 bytes)• destination address (6 bytes)• payload (up to 1500 bytes)Layer 2 - EthernetAE6382Fundamentals of Computer Networking• An ethernet is 48 bits in length and each device has a unique address• An ethernet address is usually referred to by the terms• MAC address• Physical address• The address is normally written as• xx:xx:xx:xx:xx:xx• xx-xx-xx-xx-xx-xx• “x” is a hexi-decimal digit, 0-fLayer 2 - EthernetAE6382Fundamentals of Computer Networking• The ethernet address space is flat• Addressing is not scalable• Ethernet transmissions are “unreliable”, the receiver does not acknowledge receiptLayer 2 - EthernetAE6382Fundamentals of Computer Networking• Three common layer 3 TCP/IP protocols are• IP the inter-networking protocol• ICMP the IP control message protocol• ARP the address resolution protocol Layer 3AE6382Fundamentals of Computer Networking• Data is packaged in packets• Contents of an IP packet are• source address (4 bytes)• destination address (4 bytes)• payload• IP addresses are hierarchical• Written as• ddd.ddd.ddd.ddd• where ddd ranges from 0-255Layer 3 - IPAE6382Fundamentals of Computer Networking• An IP address consists of a network part and a host part• Displayed as address and subnet mask• Can also be displayed as 172.16.11.200/16• IPv4 addresses are 64 bits• IPv6 addresses are 128 bitsLayer 3 - IPAE6382Fundamentals of Computer Networking• These messages convey information about an IP connection• Host unreachable• Route not found• This is also the carrier for the ping command• ping www.ae.gatech.eduLayer 3 - ICMPAE6382Fundamentals of Computer Networking• Address resolution protocol• Fits between layers 2 and 3• Creates and maintains the mappings between ethernetaddresses and IP addresesLayer 3 - ARPAE6382Fundamentals of Computer Networking• User Datagram Protocol• Connectionless, send and forget• Packet ordering not guaranteed• Does not provide a reliable connection between nodes• Less expensive to setup than TCP• Uses source and destination ports for addressing• Requires IP addressing to locate remote computerLayer 4 - UDPAE6382Fundamentals of Computer Networking• Transmission Control Protocol• Connection oriented, creates a virtual connection between computers• Packet ordering is guaranteed• Does provide a reliable connection between computers• will retransmit as necessary• Uses source and destination ports for addressing• Requires IP addressing to locate remote computerLayer 4 - TCPAE6382Fundamentals of Computer Networking• There are many application level protocols• http, smtp, pop, telnet, ssh, smb, nfs, …• These all rely on the capabilities of the lower levels• Most are TCP but some are UDPLayer 5 - ApplicationAE6382Fundamentals of Computer Networking• Each layer will add additional content to the data to be transmitted• For example, an HTTP request will consist of,• An HTTP request enclosed in a TCP packet, which in turn is enclosed in an IP packet, which is finally enclosed in an ethernet frame• Each level add its own functionality and addressing schemePackaging DataAE6382Fundamentals of Computer Networking• http://www.ae.gatech.edu/• HTTP request (5) – GET / HTTP/1.0• TCP packet (4) – specifies port 80• IP packet (3) – specifies address of web server (128.61.191.2)• Ethernet frame (2) – puts request on the wire to server (00:50:da:b9:4b:c5)ExampleAE6382Fundamentals of Computer Networking• To uniquely identify a connection requires• source IP address (3)• source port number (4)• destination IP address (3)• source port number (4)• Sometimes written as 128.61.191.2:80IP AddressesAE6382Fundamentals of Computer Networking• The addressing scheme of TCP and UDP uses a source and destination port number• Both TCP and UDP are point to point in nature – they see only the other computer – they identify a resource on the other end• Port numbers are assigned and well-known• 22 ssh• 25 smtp• 80 http• 3389 rdp• Source port is usually randomly selectedIP Addresses - PortsAE6382Fundamentals of Computer Networking• The Domain Name System (DNS), a layer 5 protocol, provides a mapping service between names and addresses• A name consists of name.domain• The naming system is hierarchical• Examples•www.gatech.edu - 130.207.165.120•www.ae.gatech.edu - 128.61.191.2•asdlserver.asdl.ae.gatech.edu - 172.16.0.100IP Addresses - NamesAE6382Fundamentals of Computer Networking• A computer will usually acquire its IP address from• manual configuration• from a DHCP (dynamic host configuration protocol) server (a UDP