Higher Computing Network Protocols, the OSI model and IP addresses

What is a protocol? A set of rules agreed upon in order to communicate successfully. These might include: Data format Transfer speed Handshaking Flow Control

Application Layer Protocols Remote access – Telnet File Transfer – FTP Information Access – HTTP HTTPS – SMTP and POP3

Application Layer Protocols Telnet – used to make a remote connection to computer FTP connect (password required) upload and download files – remains connected HTTP – connect to web server, retrieve file, then disconnect POP3 download mail from mail server – password required SMTP upload mail to mail server – originally no password required

Telnet

FTP Open hostname Connected to hostname 220-Microsoft FTP Service User (hostname): anonymous 331 Annonymous access allowed, send identity ( name) as password Password: 230 Anonymous user logged in. cd remote folder 250 CWD command successful. Get filename.doc 200 PORT command successful. 150 Opening ASCII mode data connection for searchrep.doc(20480 bytes). 226 transfer complete bytes received in 0.00 Seconds Kbytes/sec. quit

FTP GUI front-end

HTTP GET index.htm HTTP/1.0 HTTP/ OK Server: Microsoft-IIS/4.0 Date: Wed, 27 Feb :07:15 GMT Content-Type: text/html Accept-Ranges: bytes Last-Modified: Fri, 28 Apr :34:33 GMT ETag: "982f13df1eb1bf1:2f4d" Content-Length: 558

HTTP Front-end

POP3 OK khs.mail POP service ready [1] MDaemon v2.7 SP5 R user username +OK username recipient ok pass password +OK username's mailbox has 1 total messages (813 octets). list +OK 1 813

SMTP 220 khs.mail ESMTP service ready [1] MDaemon v2.7 SP5 R helo username 250 khs.mail Hello ianking, pleased to meet you mail from 250, Sender accepted rcpt 250, Recipient ok data 354 Enter mail, end with.

POP3 SMTP front-end

Ports FTP – 21 and 20 Telnet – 23 SMTP – 25 POP3 –110 HTTP – 80 and 8080

Lower level protocols Transmission Control protocol –TCP Internet Protocol – IP User Datagram Protocol UDP

TCP Transmission Control Protocol: Sending Breaks data to be transmitted into packets Each packet is given a header which contains: a sequence number The sender and recipient IP address If an acknowledgement is not received then the packet is retransmitted

TCP Transmission Control Protocol: Receiving Reassembles packets received according to sequence number Sends an acknowledgement Waits for any missing packets

IP Internet Protocol Adds a header to each packet (or datagram) for routing using sender and recipient IP IP does not always know the final destination of a packet – only the IP address of the next router along the line Packets may take different routes and take different lengths of time to be delivered

UDP User Datagram Protocol Like TCP but does not ask for re-delivery of lost packets Used for: – network troubleshooting –audio or video streaming

OSI Network Layers Open Systems Interconnection

Transparency and Independence Each layer has a specific task to perform. Functions which are similar are grouped together within layers. Each layer only knows about its immediately adjacent layers. A layer uses the services of the layer below and provides services to the layer above The internal design of a layer is independent of the functions it provides

Provides a set of interfaces for applications to obtain access to networked services This layer can be replaced with: client, Internet Browser, Web server, Mail server etc. Protocols: HTTP, SMTP, POP3, FTP, NNTP etc 7: Application Layer

6: Presentation Layer Handles data format information for networked communications. This is done by converting data into a standardised format that can be understood by both sides This layer deals with: Encryption and data conversion

5: Session Layer The Session layer permits two parties to hold on-going communications called a Session across a network.. This layer is responsible for : Starting or stopping a communication session, managing logon and password authentication. The Domain Name Service (DNS)

4: Transport Layer Ensures delivery of entire file or message using packet sequencing and packet acknowledgement This layer deals with: : Error detection and correction Protocols: TCP, UDP

3: Network Layer Routes data to different LANs or WANs based on network address This layer deals with: Packet Routing, IP forwarding Protocols: IP, ICMP

2: Data Link Layer Transmits packets from node to node based on its station address This layer deals with: : synchronisation, putting data into frames along with error checking data Protocol: Ethernet (CSMACD)

1: Physical Layer Provides a Physical Connection Examples are: Cat5 Ethernet Cable, Fibre optic cable, Wireless

Mapping TCP/IP to OSI ISO/OSI model Internet Architecture Model Protocols / Standards Application Layer Telnet, FTP, POP3, SMTP, DNS Presentation Layer Session Layer Transport Layer TCP, UDP Network LayerInternet Layer IP, ICMP Data Link Network Layer SLIP, PPP, Ethernet, Token Ring (FDDI), ATM Physical Layer

Network Identities For any network protocol every computer (host) must have a unique identity. Computer Name Ethernet – MAC address TCP/IP – IP address

Application Network Data Link Physical Application Network Data Link Physical File Packet (IP Address) Frame (MAC Address) Data ( )

Application Network Data Link Physical Application Network Data Link Physical Router Identifies packets by IP address

Application Network Data Link Physical Application Network Data Link Physical Switch Identifies frames by MAC address

Application Network Data Link Physical Layer Application Network Data Link HUB Repeats frames Physical Layer

IP addresses and DNS Every computer on the Internet must have a unique IP address to identify it. An IP address consists of four 8-bit numbers called octets Since these numbers are not easy to remember, domain names are used as a more user friendly way of identifying machines The Domain Name Service (DNS) translates Domain names into IP addresses

IP Addresses four 8 bit numbers (octets) which can range from to ie. 0 to 255 eg

Classes of IP Address Class A: nnn.hhh.hhh.hhh 1 st octet = 1 to 126 Class B: nnn.nnn.hhh.hhh 1 st octet = 128 to 191 Class C: nnn.nnn.nnn.hhh 1 st octet = 192 to 223 Class D nnn.hhh.hhh.hhh 1 st octet = 224 to 239 Class E nnn.hhh.hhh.hhh 1 st octet = 240 to 255 n = network h = host

Classes of IP Address Class A:16 million addresses per block Class B: addresses per block Class C: 254 addresses per block Class D Multicast Class E Broadcast (Experimental)

Private IP addresses (inside a LAN) Class A to Class B to Class C to Used with a proxy server and Network Address Translation (NAT) Localhost:

Static and Dynamic IP addressing Static – fixed IP address Dynamic – IP address allocated by a server (DHCP server on a LAN, or provided by your ISP on a WAN)

2013 SQA paper Q 29 The hospital network uses TCP/IP to transfer files across the network. (i) State three operations of the TCP part of this protocol. (ii) State two operations of the IP part of this protocol. (iii) Name one other common protocol that could be used to transfer files across the hospital network.

2013 SQA paper Q 29 (i) TCP splits the file into packets, adding header information which includes sender and receiver IP addresses and sequence number, reassembling the packets at the receiving end and acknowledging their arrival (ii) IP adds its own address header and routes the packets around the network (iii) HTTP or FTP could be used

2013 SQA paper Q31 A local tax office has 300 computers connected together in a Local Area Network with access to the Internet. (a) The network conforms to the Open Systems interconnection (OSI) model. (i) State which layer of the OSI model carries out encryption. (ii) State which layer of the OSI model carries out routing. (b) Explain which class of IP address is most appropriate for the tax office to use to network their computers.

2013 SQA paper Q31 (i)Presentation layer (ii)Network layer (iii)Class B (Class C only has 254)

2012 SQA paper Q28 It is important that computer networks are designed to agreed standards, such as the Open Systems Interconnection (OSI). (i) State the name of the layer of the OSI model at which a router functions. (ii) State the name of the layer of the OSI model that carries out data encryption. (b) TCP/IP is a set of protocols used in network communication. State the actions carried out by the IP part when transmitting data over a network.

2012 SQA paper Q28 (i) a router functions at the Network layer (ii) Data encryption is done at the Presentation layer (iii) IP adds its own address header and routes the packets around the network

2012 SQA paper Q31 A sports centre has a local area network of 10 computers and 2 printers. (a) Explain why class A IP addressing is not suitable for this network. (b) A network interface card is required to provide a physical link to the local area network. The network interface card contains a MAC address. Describe the purpose of a MAC address.

2012 SQA paper Q31 (a)Class A allows 16,777,214 addresses ( ) A small network would use class C (with 254 addresses) Only 12 IP addresses are needed (b)A MAC address uniquely identifies the network card on the network.

2011 SQA paper Q23 The IP addresses for some of the devices on the network are as follows: Computer File Server Computer Router Computer Printer (i) State the class of IP address used within this network. Justify your answer. A new computer is added to the network. It is allocated the IP address (ii) State one reason why the second octet is invalid. (iii) State one reason why the fourth octet is invalid.

2011 SQA paper Q23 (i)The network is Class C The first octet is between 192 and 223. The first three octets are identical for each IP address (ii) The second octet is different to the original which indicates a different network (iii)The fourth octet is over 255

2010 SQA paper Q25 The network conforms to the Open Systems Interconnection (OSI) model. Two layers of the OSI model are the Session layer and the Network layer. (i) State one task carried out at the Session layer. (ii) Name a networking device that operates at the Network layer.

2010 SQA paper Q25 (i) The session layer manages starting and stopping the connection, manages logon and password authentication. This is the layer where the the Domain Name Service (DNS) operates. (ii) A router operates at the network layer.