Lesson One: OSI anD TCP/IP Modle
OSI Model Layers
7- Application 6- Presentation 5- Session 4- Transport 3- Network 2- Data Link 1- Physical Note: This is not the standard it is a guideline. Focus on understanding how data flows between each layer.
Institute of Electrical and Electronic Engineers (IEEE) 802 Standards
A collection of IEEE network standards describing physical network hardware, transmission media, transmission methods, and protocols. Split the Data Layer into two sub-layers: Media Access Control (MAC) Logic Link Control (LLC)
WAN (Wide Access Network)
A network that spans multiple geographic locations
Node
Devices that communicate on the network
Determining how multiple nodes can share access to the network media
Bus-Based Topology: Uses contention as a media access method Ring-Based Topology: Uses a token passing access method
Protocol Data Unit (PDU)
Protocol: A set of rules enabling systems to communicate by exchanging data in a structured format The PDU name is given to data at different layers of the OSI model. Specifically, the PDU for Layer 4 is segment. The Layer 3 PDU is packet, the Layer 2 PDU is frame, and the Layer 1 PDU is bit. Two major functions: -Addressing: Where it should go -Encapsulation: How it should be packaged Header: Unit of information that precedes data objects but is considered to be part of the data packet and contains information about the file or the transmission Payload: The portion of a data packet that contains the actual message to be transferred, without headers for transport and without metadata
Transmission Control Protocol/Internet Protocol (TCP/IP) Model
Protocol: Rules used by network computers to communicate; must have protocol in common for two computers to communicate Protocol Suite: A collection of suites (or protocol stacks, collection of protocols and then logical order in which they work together) 4-LAYERS: 4- Application 3- Transport 2- Internet 1- Link/ Network Interface
Internet Layer
Provides addressing and routing functions; also provides the ability to fragment large frames from the Network Interface into smaller packets Uses several protocols, notably the IP and Address Resolution Protocol (ARP), to facilitate the delivery of packets ARP thought of as layer 2 in the OSI Model as it needs to reference the hardware address and isn't routable; While layer 2 protocol such as Ethernet doesn't require ARP to function; while a layer 3 protocol such as IP does ARP = layer 2.5
De-encapsulation
The reverse of encapsulation
Links
A pathway of communication between network nodes; created by using some type of transmission or physical media Example: -wired (copper or fiber optic cable) bound -wireless (Microwave radio) unbound
LAN (Local Area Network)
A type of network covering various sizes but generally considered to be restricted to a single geographic location and owned/managed by a single organization.
Node
Any physical device that can communicate on a network via one or more network interfaces Interface: A means of connection that enables nodes to communicate On a wireless network usually called a workstation (includes endpoints and forwarding devices) In TCP/IP called Host (doesn't include switch or routers)
Basic Encapsulation Process
At each Layer (excluding the Physical Layer) the protocol adds fields in a header to whatever the data (payload) it receives from an application or other protocol
Layer 5 Sessions
Communication management between devices Start, Stop, Reset Half-duplex, Full-duplex Control Protocols, Tunneling protocols Most application protocols require the exchange of multiple messages between the client and the server; this exchange of such a sequence of messages is called a session or dialogue Represents the dialogue control function that admins the process of establishing the dialog, managing data transfer, and then ending (or tearing down) the session Works in 3 modes: One-Way/Simplex: Only one system is allowed to send messages; the other one only receives Two-Way Alternative (TWA): The host establishes some system for taking turns to send messages such as exchanging a token Two-Way Simultaneous (TWS)/Duplex: Either host can send messages at any time
Backbone
Describes a fast link between other segments of a network Carries all communications occurring between nodes in separate segments High bandwidth backbone segments are used between the router and the switches
Open Systems Intercommunication (OSI) Model
Designed to aid in the understatement of how a network system works in terms of hardware and software components by separating them into layers
Link/Network Layer
Equal to OSI Layers 1 and 2 Defines the host connection to the network media Includes hardware and software involvement in the interchange of frames between hosts Technologies can be LAN-Based (Ethernet or Wi-Fi) or WAN-based (T-Carrier, ISDN, or DSL)
Transportation Layer
Establishes connections between the application that the source and destination host are communicating with Brakes the application layer information into segments Two methods for delivery: Connection: oriented delivery using the TCP TCP: Protocol in the TCP/IP suite operating at the Transport layer to provide connection-oriented, guaranteed delivery of packets. Hosts establish a session to exchange data and confirm the delivery of packets using acknowledgments. This overhead means the system is relatively slow. Connectionless: using the user datagram protocol (UDP) UDP: A protocol in the TCP/IP suite operating at the Transport layer to provide connectionless, non-guaranteed communication with no sequencing or flow control. Faster than TCP, but does not provide reliability.
Layer 1 - Physical
Is responsible for the transmission and receipt of bits from one node to another node Signaling, cabling, and connectors Problems with Layer 1: Fix your cabling, punch-downs, etc Run loopback tests, test/replace cables, swap adapter cards
Frame
Layer 2, Organizes the stream of 1s and 0s (bits) arriving from one physical layer into structured units each frame contains a Network Layer packet as its payload Layer 2 adds control information to the payload in the form of header fields; these fields include a source and destination hardware address Last part of the frame is usually an error check; Layer 2 only capable of basic error checking such as identifying truncated or corrupted frames (no function to retransmit damaged frames that is done at higher levels of the OSI)
Layer 4 - Transport Devices
Multi layer Switches: Usually working as load balances and many types of security applications such as more advanced firewalls and Intrusion Detection Systems (IDSs)
Layer 2 Data Link Devices
Network Adapters or Network Interface Cards (NICs): Joins a host to network media (wired or wireless)and enables it to communicate over the network by assembling and disassembling frames Bridges: A bridge joins two networks segments while minimizing the performance reduction of having more nodes on the same network; has multi ports, each of which function as a network interface Switches: An advanced type of bridge with many ports; can create links between a large number of nodes more efficiently Wireless Access Point/Access Point (WAP/AP): Allows nodes with wireless network cards to communicate and create a bridge between wireless networks and wired ones
Segment
Networks are typically divided into segments to cope with the physical restrictions of the network Used to improve performance or to improve security At Layer 1, a segment is one where all nodes share access to the same media Specifies: Physical Topology: Layout of nodes and links as established by the transmission media Physical Interface: Mechanical specifications for the network medium: The cable specifications, the media connector and pin-out details(the number and functions of various pins in a network connector), and radio transceivers specifications The process of transmitting and receiving signals over the network medium; includes modulation schemes and timing/sync
Packet Switched vs Circuit Switched Networks
Packet Switching: Method used to make more efficient use of available bandwidth by splitting data into small packets and routing them via any available path Recovers from link fails means it reroutes data packets around an interwork or available path; if transmission lines are damaged Circuit Switching form of switching that establishes a temporary dedicated path between nodes; if circuits it ends the communication Sending large chunks of data is a problem as it uses up a lot of bandwidth and causes delays Fragmentation capabilities of the IP to divide up large messages into numerous small packages means that a single packet can be resent at little cost if it is lost or damaged during transmission means that protocols can work with different types of underlying data link protocol frame formats and sizes
Layer 3 - Networking Devices
Router: A network device that links dissimilar networks and can support multiple alternate paths between location-based parameters such as speed, traffic loads, and cost. l3 Switches, these combine the functions of switches and routers and firewalls
TCP/IP and Internet Standards
Several Organizations are responsible for the Internet and TCP/IP development Internet Society (ISOC): Purpose is to encourage the development of the internet; Provides organizational resources to the Internet Architecture Board (IAB) Internet Architecture Board (IAB): Is the technical committee of ISOC and is responsible for setting Internet standards and publishing these standards as Requests For Comments (RFCs); Governs the following groups: Internet Engineering Task Force (IETF): Focuses on solutions to internet problems and the adoption of new standards Internet Assigned Numbers Authority (IANA): Manages allocations of the IP address and maintenance of the top-level domain space; currently run by Internet Corporation for Assigned Names and Numbers (ICANN); IANA allocated address to regional registries who can allocate them to local registries or Internet Service Providers (ISPs) Regional Registries: Asia/Pacific: APNIC North America/ South Africa: ARIN Latian America: LACNIC Europe/Northern Africa/ Central Asia/ Middle East: RIPENCC Requests for Comments (RFCs): Process for adopting Internet standards revolve around the production of documents called RFCs Old RFC arent updated; if changed a new one is made Always verify if you have the most recent one Not all RFCs describe standards; some are designated information while others are experimental; some describe networking services and policies and implementation while others summarize policies
Layer 4- Transport
The "Post Office" Layer TCP (Transmission Control Protocol) UDP (User Data Protocol) First 3 layers are mostly concerned with moving frames and datagrams between nodes and networks At the transport layer (aka end-to-end or host-to-host layer), the content of the packets start to become more significant critical functions of layer 4 is to ID each type of network app by assigning it a port number At layer 4 on the sending host data from the upper layer is packaged as a series of layer 4 PDUs referred to as segments; each segment is tagged with the apps port number which is then passed to the network layer for delivery; host could transmit multiple application packets at the same time as these are multiplexed using the port numbers onto the same network link Each host is assigned two port numbers; On the client, the destination port number is mapped to the service that the client is requesting; the client also assigned a random port number The service uses the client assigned port number as the destination port number for its replies and its application port number as its the source port number; this allows the host to track multiple conversations for the same application protocol Port number isn't significant at layer 2 and 3 becomes part of the payload and is invisible to the routers and switches that implement the addressing and forwarding functions of these layers At the receiving host; each packet is extracted from its frame and then identified by its port number and passed up to its relevant handler at the upper sessions and application layers; traffic is demultiplexed The transport layer is responsible for ensuring reliable data delivery, should the app require it; Reliable data delivery means that damaged or lost packets are resent
Layer 2 - Data Link
The Data Link Layer is responsible for transferring data between nodes on the same logical segments At Layer 2, a segment is one where all nodes can send traffic to one another using hardware addresses, regardless if they share access to the same media Each host is connected to a central node (hub, switch, or Wireless Access Point (WAP)); these provide forwarding functions receiving the communications from one node and sending it to another The addresses of interfaces in the same network are described as local/hardware addresses (MAC) May include multiple physical segments A.K.A Logical Topologies Basic network language Foundation of communication at data link layer Data Link Control (DLC) protocols Media Access Controls (MAC) address on the ethernet The switching layer
IEEE 802.3 Ethernet and The Mac Sub-Layer
The MAC sub-Layer defines, how multiple networking interfaces share a single transmission medium Covers: Logical Topology: Bus or RIng Media Access Method: Contention or Token Passing Addressing: format for the hardware address of each network interface Frame Format Error Checking Mechanisms Specifies, protocols that implement functions of the MAC sublayer, plus signaling and media specifications at the physical layers Based on Ethernet Networking Products called Ethernet: 802.3 Wired (LAN) 802.11 Wireless (WLAN)
Layer 6 - Presentation
The Presentation Layer, transforms data between the format required for the network and the data required for the application Can be conceived as supporting data compression and encryption; however in practical terms, these functions are often implemented by encryption devices and protocols running at lower levels of the stack or simply within a homogeneous Application Layer Character encoding Application encryption Often combined with the Application Layer
Layer 7 - Application
The layer we see HTTP, FTP, DNS, POP3 The application layer is at the top of the stack Doesn't encapsulate any other protocol or provide services to any other protocol Provides an interface for software programs on network hosts that have established a communications channel through the lower-level protocols to exchange data Upper layer protocols provide most of the services that make a network useful rather than just functional including network printing, e-mail, communications, directory lookup, and database services Important to establish between Application protocols and the software application (programs and shared programming libraries) that run on computers Software programs and Operating System (OS) make use of Application Programming Interface (APIs) to call functions of the relevant part of the network stack Allows programs on a network node (computer) access network services, such as file transfers, electronic mail, and database access
Layer 3 - Networking
The routing layer Internet Protocol (IP) Fragments frames to traverse different networks IP Fragmentation are always in multiples of 8 because of the number of fragment offset bits in the IP Header The Network Layer is responsible for moving data around the networks of networks (Internet) While Layer 2 forwards data by hardware addresses within a single segment; Layer 3 moves information around an internet by using logical networks and host IDs Networks are typically heterogeneous, meaning they use a variety of physical layer media and data link protocols The network layer transfers information between networks by examining the destination network layer or the logical network address and routing the packets through the internet by using intermediate systems (routers) Packets move router by router (or hop by hop)through the internet to the target network Once the destination is reached the hardware address can be used to deliver the packet to the target node The general connection is to describer PDUs packaged at the network layer as packets or datagram and the messages packaged at the data link layer as frames; keep in mind packet is ofter used to refer to PDUs at any layer
Switching vs Routing Concepts
The whole network is connected to the internet via a WAN router/firewall This router type can communicate using different layer 2 formats of the link to the internet and the Ethernet frame format used on the LAN Important security function, using a firewall to prevent unwanted traffic entering or leaving the LAN Another router is used to divide the network into three logical sub-networks; these sub-networks are mapped to layer 2 segments; two segments are served by switches and one by a legacy hub device Nodes within each subnet can address one another directly but can only communicate with nodes on another segment via the router Nodes within switched segments can use full 1 Gps link speed; whereas nodes in the legacy segment share access to 100 Mbps media bandwidth
Layer 1 - Physical Devices
Transceivers: The part of a network interface that that sends and receives signals over the network media; These signals will be converted into electromagnetic energy or optical light Repeaters: A device that amplifies the electrical signal to extend the maximum allowable distance for the media type Hubs: Multi-port repeater, deployed as the central point of connection for nodes Media Converter: Device that converts one media signaling type to another Modem: a device that converts between digital and analog signal transmission
Network
Two or more computer systems linked by transmission media to share information
IEEE 802.2 Logic Link Control
Used with other 802.2 protocols such as 802.3 Ethernet and 802.11 Wi-Fi Provides a standard network layer service interface, regardless of which MAC sub-layer protocol ios used
Application Layer
Where many TCP/IP services (High-Level protocols) can be run SUCH AS FTP, HTTP, and SMTP
