Connections to a Network; Modelling Network Functions TCP/IP versus OSI the basic hardware how its use is organized (chapter 2 of Meyers)
The ISO/OSI Reference Model Open Systems Interconnection - 1977: ISO begins work on OSI» ISO International Organization for Standardization ("Iso" as in "equal") - 1984: OSI Reference Model published» Based on Honeywell s Distributed Systems Architecture (DSA), IBM s System Network Architecture (SNA) both 7- layer designs Seven layers describe distinct functions needed for a network transmission
The OSI Layers Top to Bottom Application set - What must an app do to talk to another app? Transport set - How does data get from one computer to another computer? Cisco divides the layers into the Application set and the Transport set.
The OSI Layers Bottom to Top From a network engineering perspective, the layers make more sense starting at the bottom and building up From a forensic perspective, the upper layers get more attention The Transport layer ties these groups together
and some Mnemonics for the layer names Top to bottom: - All People Seem To Need Data Processing - All People Studying This Need Drastic Psychotherapy Bottom to top: - Please Do Not Throw Sausage Pizza Away - People Design Networks To Send Packets Accurately - Purple Dragons Need To Smell Pleasant Aromas bob,mon!
All Phelines Seem To Need Dainty Paws
what's the point? Many different things may need to be done to successfully transmit some data from one computer to another. These things must interact. They must work even while one or both of the computers are also doing unrelated tasks. The OSI model is an attempt to categorize and organize the various necessary networking functions, so that programs from different vendors can interoperate.
Upper-Layer Functions Application layer - Programs that do things users care about, using a network to do so» viz., a web browser, an email client, or an FTP program Presentation layer - Convert data between system-readable formats and network-standard formats - Not so necessary, thanks to standardized document formats Session layer - Managing connections between applications on computers
Middle-Layer Functions Transport layer - Packaging of applications data (messages) into transportable chunks - Keeping data in order - Reliability Network layer - Transporting and routing data between networks
Lower-Layer Functions Datalink layer - Gives unique identities to senders and receivers - Organizes data packets into frames for transmission Physical layer - Convert logical data bits into physical signals - transport signals from sender to receiver(s)
What is Data Called In Each Layer? The content is called by different names in different layers Mnemonic:» "Messages" - Many Sick People Feel Bad» "Data" - Deathly Sick People Feel Bad Message or Data Segment Packet Frame Bits 7 - Application 6 Presentation 5 - Session 4 - Transport 3 - Network 2 - Datalink 1 - Physical
Encapsulation and the OSI Model OSI upper layers data Appl data User's data is formatted according to Application layer - Presentation layer, Session layer also involved here Forms a "message" Pres Appl data Sess Pres Appl data data or message Trans Sess Pres Appl data segment Result has Transport layer header added - May be split into smaller "segments"
Encapsulation and the OSI Model OSI lower layers Trans Network Datalink Physical Sess Pres Appl data segment Trans Sess Pres Appl data packet Network Trans Sess Pres Appl data frame Datalink Network Trans Sess Pres Appl data Network "packets" are packaged into "frames", then transmitted as "bits" over a medium bits
OSI and Reality No strictly OSI-based protocol stacks are in use ARPANET s TCP/IP protocol stack is widespread - Upper-level, message-oriented services are not differentiated - Lowest-level layers are not specified the whole point of TCP/IP is to work between different low-level networks» Ethernet is common in LANs, moving into the backbones DoD formalized this as a 4-layer model TCP/IP, "Arpanet Reference Model" show a 4- or 5-layer structure
The TCP/IP Model (and Protocol Stack) Centered on Transport, Internet layers - Transport: TCP, UDP - Internet: IP Link layer exists, but not explicitly specified - Ethernet is common at the edge - Core uses various protocols - Includes physical layer
Comparing Models Know these! OSI - Rigorously specified» moreso than the others - Real protocols not organized this way TCP/IP (Internet) - Transport layer includes some session-oriented functionality - Link layer not specified, includes physical layer - Based on actual implementations DoD - Basically an official description of TCP/IP OSI TCP/IP DoD
Use Wireshark to see layers Start Wireshark Use capture filter Use display filter - Look for frames with:» cups» stp» arp» igmp
Usage of TCP/IP A typical use of TCP/IP is for a webpage request: a) Web browser application uses HTTP to formulate request message» "GET / HTTP/1.1 \cr\lf " b) Request is put in one segment, for transport to port 80 on the web server c) Packet is addressed to the IP address of the web server Packet, w/ address, is supplied to the NIC for transmission to the Internet This only uses three layers of TCP/IP itself, plus whatever happens in the NIC and below.
Other Protocol Options Novell Netware - Oriented and marketed to LANs - Primarily file- and printer-sharing - Complete protocol stack AppleTalk - Apple-only protocol suite - Discontinued in Mac OS X v10.6, 2009 NetBEUI - Microsoft protocol suite - No routing capability, so no Internet
Netware
AppleTalk
NetBEUI NetBEUI - Microsoft implementation - NetBIOS (session layer) - NBF (Datalink layer) - different from IBM's NetBEUI source: en.wikipedia.org/wiki/netbios_frames _Protocol - Unsupported in Windows XP source: support.microsoft.com/kb/306059
and some relevant hardware
Building a Local Area Network NICs (Network Interface Controllers) - Computer components that convert data to physical signals Physical media - Carry signals between LAN nodes Switches, hubs, access points - Mediate transmission of signals These work with frames Routers - Connect one LAN to others Work with packets
Physical Media Typically cable of some sort Could be radio-frequency (RF) transmissions
NICs, Switches, Hubs This older NIC can work with either twisted-pair cabling or coaxial cable but not both at once Hubs and switches look very similar they differ in their internal electronics NICs are usually connected to a hub or switch, but can be directly connected for special circumstances
Routers Can look like switches One connection to LAN, one or more connections to Internet Often combined with a switch and an access point, into a single appliance
What Devices Use Which Layers? A message from one host node to another typically passes through intermediate nodes: Hosts: entire protocol stack is used NIC: handles Link, Physical layers for the host Switch: message is forwarded according to hardware (Link-layer) address Hub or bridge: physical translations only Router: message is forwarded according to the Internet (network) address