Section 3: Host to Host Communications

  ICND1 CCENT

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11. Introduction

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Quick discussion of the OSI conceptual model and TCP/IP Suite

12. Basic Introduction to Networking

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  • LAN – Local Area Network
    • Printers
    • Laptops
    • Wire Access Points
    • All above connect directly to a switch.
  • Router
    • Makes advanced routing decisions to access the Internet and other networks.
    • Generally has different types of ports to support different types of networks.
  • Firewalls
    • Secures different parts of a network from each other.
  • WAN – Wide Area Network
    • Ability to connect different locations together

Network Characteristics

  • Topology
    • How the devices are connected to each other
    • Often has a drawing (Network Topology Diagram)
  • Speed
    • Faster the network, the more it costs
  • Costs
    • Speed
    • types of devices
    • Size of the network
  • Security
    • Firewall
    • features on routers and switches
    • IDS./ IPS
  • Availability
    • High Availability
  • Scalability
    • Easy to grow without redesign
  • Reliability
    • make sure it keeps working.

13. The OSI Reference Model Overview

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03-03-Open-Systems-Interconnection-OSI-Model-Overview

Open Systems Interconnect Model

  • The OSi reference model is a standard of the International Organization for Standardization (ISO)
  • General purpose framework that describes and standardizes how devices communicate over a network.
    • Not an actual technology
  • 7 layer approach divides the operations into specific related groups of actions per layer
  • A layer serves the layer above it and is served by the layer below it.

Sending

(7) Application >
(6) Presentation >
(5) Session >
(4) Transport (TCP/UDP, Port [Where Networking starts to get involved]) >
(3) Network (IP address – [Routers]) >
(2) Data-Link (Mac Address -[Switches]) >
(1) Physical (Hubs, Wires, etc.)

Receiving (Reverse the order)

(7) Physical >
(2) Data-Link (Check Layer 2 headerr for mac addr to see if is for this device or discards) >
(3) Network (Checks Layer 3 header for IP…) >
(4) Transport (Port, data control…)>
(5) Session >
(6) Presentation >
(7) Application

Benefits to OSI model

This is important.  Live it, know it, love it.

  • Engineers only need to design to work on the layer their devices is supposed to work at.  They do not need to worry about other layers other than the layer above and below.
  • This leads to open standards and multi-vendor interoperability
  • Troubleshooting is easier because you can analyze a problem in a logical fashion, layer by layer.

OSI Acronyms

  • Please – Physical
  • Do – Data Link
  • Not – Network
  • Throw – Transport
  • Sausage – Session
  • Pizza – Presentation
  • Away – Application

 

14. The TCP/IP Stack

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03-04-The-TCPIP-Stack-Transmission-Control-Protocol-Internet-Protocol

  • Developed by DOD in 1960s
  • Protocol stack consisting of multiple protocols including
    • TCP – Transmission Control Protocol
    • IP – Internet Protocol
  • While OSI is conceptual, TCP/IP stack is used to transfer data.
  • TCP/IP is also layered but does not use all of the OSI layers, although they are equivalent in operation and function.

TCP/IP Stack

  • Application
    • Application
    • Presentation
    • Session
  • Transport
    • Transport
  • Internet
    • Network
  • Network Access
    • Data Link
    • Physical

PDUs – Protocol Data Units

(This is commonly called a ‘Packet’ but is really a PDU)

  • When two hosts talk to each other, they exchange PDUs
    • This is the entire data unit, from Application to the Network Access layer
  • Application – Data
  • Transport – Segments
  • Internet – Packets
  • Network Access – Frames

 

15. The Upper OSI Layers

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  • Network engineers do not typically work directly with the upper 3 layers (Application layer in TCP/IP)
    • These are generally dealt with by application developers
  • Need to know the definitions because we will be tested on them.
  • Information in the upper layers would include Message body and subject line in an email message.

Layer 7 – Application Layer

  • The application layer provides network services to the applications of the user.
  • It differs from the other layers such that it does not provide services to any other OSI layer.
  • The application layer establishes the availability of intended communication partners
    • aka The host that the server is communicating with.
  • It then synchronizes and establishes agreement on procedures for error recovery and control of data integrity.
    • Data integrity ensures the data has not been corrupted or altered in transit.

Layer 6 – Presentation Layer

  • Ensures the information that is sent at the application layer of one system is readable by the application layer of another system
  • Can translate among multiple data formats using a common format
    • Eg Computers with different encoding schemes

Layer 5 – Session Layer

  • The session layer establishes, manages and terminates sessions between two communicating hosts.
  • This layer also synchronizes dialog between the presentation layer of the two hosts and manages their data exchange.
    • For example: web servers have many users, so there are many communication processes open at any given time to track.
  • It also offers efficient data transfer, CoS (Class of Service) and exception reporting of upper layer problems.

16. The Lower OSI Layers

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Layer 4 – Transport Layer

  • Main characteristics are whether TCP or UDP is used and the Port number
  • Definition
    • The transport layer defines services to segment, transfer and reassemble the data for individual communications between the end devices
    • It breaks down large files into smaller segments that are less likely to incur transmission problems.

Layer 3 – Network Layer

  • Most important information is the source and destination IP address.
  • Routers operate at Layer 3
  • Definition
    • The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.
    • The network layer is the layer that manages the connectivity of hosts by providing logical addressing (IP Addressing).

Layer 2 – Data Link Layer

  • Important information is the source and destination layer 2 address
    • Example: sorce and destination MAC address if Ethernet is the Layer 2 technology
  • Switches operate at Layer 2
  • Definition
    • The data link layer defines how data is formatted for transmission and how access to physical media is controlled.
    • It also typically includes error detection and correction to ensure a reliable delivery of the data.

Layer 1 – Physical Layer

  • This layer literally concerns the physical components of the network
    • Example, the cable being used.
  • Definition
    • The physical link enables bit transmission between end devices.
    • It defines specifications needed for activating, maintaining and deactivating the physical link between end devices.
    • For example, voltage levels, physical data rates, maximum transmission distances, physical connectors, etc.

 

 

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