CCNA Rev Series Part 2- OSI

Here's a summary of the OSI model which you should always remember as a Networking/Security Engineer.

The definition and working method of the seven layer's, viz;

7] Application Layer: This layer comes into action when it's apparent that access to the network is going to be needed soon. The application layer acts as an interface between the actual application program. Internet Explorer does not reside in this layer. It interfaces with the application layer and also Microsoft word the same.
Examples: file transfer, email, enabling remote access network management activities, client/server process and information location.

6] Presentation Layer: It represents data to the application layer and is responsible for data translating and code formatting. It ensures that data transferred from the application layer of one system can be read by the application layer of another one. The OSI protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption and decryption and some presentation layer standard even involve multimedia operations.

5] Session Layer: The session layer is responsible for setting up, managing and then tearing down sessions between presentation layer entities. It also provides dialogue control between devices or nodes. It co-ordinates communication between systems and server to organize their communication by offering three different modes: Simplex, Half Duplex and Full duplex. To sum up, session layer basically keeps different applications data separate from different application data.

4] Transport Layer: The transport segments a data and reassembles data into a data stream. They provide end-to-end data transport service. It can be connection oriented(TCP) or connectionless(UDP). It is responsible for providing multiplexing upper-layer applications, establishing sessions and tearing down virtual circuits. Reliable networking means acknowledgment, flow control, windowing and sequencing will be used.
*If you want to know more about flow control, three way handshake and windowing CLICK HERE*

3] Network Layer: It manages device addressing, tracks the location of devices on the network and determines the best way to move data. It provides routing services within an internetwork. Logical addressing comes at this stage. Router act at layer 3.

2] Data Link Layer: It provides the physical transmission of the data and handles error notification, network topology. It uses hardware address, data frame comes under layer 2. It has two sub layers:
(a)Logical Link Control(LLC) [IEEE 802.2]: Responsible for identifying network layer protocols and encapsulating them. An LLC header tells the data link layer what to do with a packet once a frame is received.
(b) Media Access Control(MAC) [IEEE 802.3]: Defines how packets are placed on media. Physical addressing and logical topologies comes under this layer. Layer 2 switching is considered hardware based bridging because it uses specialized hardware hardware called an application specific integrated circuit.

1] Physical Layer: It does two main things, send bits and receives bits. this layer is also where you identify the interface between the Data Terminal Equipment(DTE) and Data Communication Equipment(DCE). The DCE is usually located at the service provider, while DTE is attached device. Hubs are at physical layers. A hub is a multiport repeater. Single collision and broadcast domain.

(Please go to Part 3 for more on TCP/IP Introduction)

Flow Control, Windowing and Three Way Handshake

Flow control, Acknowledgement, Sequencing, Windowing and Three way handshake are the reliable means of data transfer. TCP is a connection-oriented network.

Flow Control:

                       TCP uses an end-to-end flow control protocol, which prevents the sending host from overflowing the buffers in the receiving end. The purpose of the flow control is to provide a means for the receiver to govern the amount of data sent by the sender. For example, if a PC sends data to a Smart Phone or PDA, that is slowly processing the received data, the Smartphone/PDA must regulate the data flow so as not to overflow the buffer.

Three Way Handshake:

                        The three way handshake is also called as the virtual circuit. It describes the connection process of a sending host to a receiving host. The below figure will explain you in much detail about this process.

Consider Host A sending data to Host B.

When Host A tries to send data to Host B, 

  1] It tries to send a request of Synchronization (SYN) i.e. to connect. At the same time Host B is passive open for receiving connection and readying itself for the receipt of a connection request (SYN) from Host A.

  2] When Host B receives the Synchronization(SYN), it sends an Acknowledgement+Synchronization(SYN+ACK) to Host A. What it means is, Host B sends a single SYN+ACK message back to the Host A that contains an ACK for the Host A's SYN, and the Host B's own SYN. At the same time, Host A waits to receive an ACK to the SYN it has sent, as well as Host B's SYN.

  3]When the Host A receives the SYN+ACK containing the ACK to the Host A's SYN, and the SYN from the Host B, it sends the Host B an ACK for the Host A's SYN. The Host A is now done with the connection establishment. At the same time, Host B waits for an ACK to the SYN it had sent previously. When host B receives the final ACK from the Host A, it establishes the connection and finally ready for data Transfer.

This is the three way handshake.

Windowing:

                       The quantity of data segments(measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgement for them is called a window. The TCP window size field controls the amount of outstanding unacknowledged data segments(flow of data) and its value is limited to between 2 and 65,535 bytes. The window scale option is used only during the TCP 3-way handshake with sequence number acknowledgement. The initial window size is determined during the session startup via the three-way handshake. Consider the following figure:

In this example, the window size was decided in the Three Way Handshake itself which is 3000bytes. So, if the sender tries to sends more than three thousand bytes, the packet will be lost, as the receiver won't be able to handle it.

 When the sender has transmitted 3000 bytes, it waits for an acknowledgement of these bytes before transmitting more segments in this session. Once the sender has received this acknowledgement from the receiver, the sender can transmit an additional 3000 bytes. During the delay in receiving the acknowledgement, the sender will not be sending any additional segments for this session. It will send only after receiving the acknowledgement.

This is known as TCP windowing. 

CCNA Rev Series Part 1 – Internetwork

Internetwork Basics

• PC communicates in LAN by MAC address and not by IP address.

• Breaking up big network into number of smaller ones called as network segmentation. This is done with the help of devices like Routers, Switches and Bridges

• List of things that can cause traffic congestion's are:

                          a]Too many host's in a broadcast domain.
                          b] Broadcast Storms
                          c] Multicasting
                          d] Low bandwidth
                          e] Adding hubs to connectivity of network
                          f] A bunch of ARP or IPX traffic.

• Hubs never segments a network, they connect segments.

• Routers are used to connect networks together and route packets of data from one network to other.

• Routers break up broadcast domain by default and also break up collision domain.

• Router uses something called a serial interface for WAN connections, specifically, a v.35 physical interface on a cisco router.

• Router also called Layer 3 Switches and Switch is in Layer2.

• Functions of Router:

                          a] Packet Switching.
                          b] Packet Filtering.
                          c] Internetwork Communication.
                          d] Path Selection.

•  Router uses a routing table (Map Of Internet) to make up path selections and to foreward packets to remote networks.

• Switched don't breap up broadcast domains; They break up Collision domain. Even Bridges are the same.

• Although bridges and switches are used to segment network, they wi;; not isolate multicast or broadcast packets.

 OSI Reference Model

•  OSI stands for Open System Interconnection.

• It has seven layers:

                          Layer7- Application Layer
                          Layer6- Presentation Layer
                          Layer5- Session Layer
                          Layer4- Transport Layer
                          Layer3- Network Layer
                          Layer2- Datalink Layer
                                       LCC-Logical Link Control 802.2
                                       MAC-Media Access Control 802.3
                          Layer1- Physical Layer

• The top three layers viz., Application,Presentation and Session are called as UPPER LAYERS and the rest four are called as LOWER LAYERS.

• The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted from end to end.

(End of Part 1. OSI continuation in Part 2)

A quick 5min CCNA Information

This is a quick information about CCNA certification of Cisco. I have posted some links where you can find out and very easy to understand. No need of searching the entire web now, to learn all about CCNA!

These are the links from where you can know what is CCNA all about and how you can prepare by yourself:
https://learningnetwork.cisco.com/docs/DOC-1297
https://learningnetwork.cisco.com/community/certifications/ccna/ccna_exam
http://prajitnair.blogspot.com/2009/10/how-to-get-started-on-ccna.html
http://compnetworking.about.com/od/cisconetworking/g/ccna-certification-cisco.htm

Links where you can download any material of any course or books on any cert you want:
http://www.certbible.org/
http://www.ciscoguides.com/

Torrents:
www.demonoid.com
http://thepiratebay.org/