OSI MODEL (first four layers)

Transport Layer

The transport layer is possibly the most important layer for exam study purposes. A lot is going on here, and it is heavily tested.

The transport layer's main jobs

·         It sets up and maintains a session connection between two devices.

·         It can provide for the reliable or unreliable delivery of data across this connection.

·         It multiplexes connections, allowing multiple applications to simultaneously send and receive data. When

·         Implementing a reliable connection, sequence numbers and acknowledgments (ACKs) are used.

·         Flow control (through the use of windowing or acknowledgements)

·         Reliable connections (through the use of sequence numbers and Acknowledgement )

Transport layer use two protocols for sending data TCP and UDP.

TCP
TCP is connection oriented protocols. Connection-oriented transmission is said to be reliable. Thinks TCP as registry AD facility available in Indian post office. For this level of service, you have to buy extra ticket and put a bunch of extra labels on it to track where it is going and where it has been. But, you get a receipt when it is delivered, you are guaranteed delivery, and you can keep track of whether your shipment got to its destination. All of this costs you more—but it is reliable!

UDP
UDP is connection less protocols. Connection-less transmission is said to be unreliable. Now, don't get too wrapped up in the term "unreliable" this doesn't mean that the data isn't going to get there; it only means that it isn't guaranteed to get there. Think of your options when you are sending a postcard, put it in the mailbox, and chances are good that it will get where it's supposed to go—but there is no guarantee, and stuff does go missing once in a while. On the other hand, it's cheap.

The transport layer can use two basic flow control methods:

·         Ready/not ready signals

·         Windowing

There are two problems with the use of ready/not ready signals to implement flow control. 
First, the destination may respond to the source with a not ready signal when its buffer fills up. While this message is on its way to the source, the source is still sending information to the destination, which the destination will probably have to drop because its buffer space is full.
The second problem with the use of these signals is that once the destination is ready to receive more information, it must first send a ready signal to the source, which must receive it before more information can be sent.In many implementations, the window size is dynamically negotiated up front and can be renegotiated during the lifetime of the connection.

In windowing a window size is defined between two host engaged in data transmission. And sender host will wait for an acknowledgement signal after sending the segments equal to window size. If any packet lost in way receiver will respond with acknowledgement for lost packet. And sender will send lost packet again.

Reliability

When reliability is necessary, it should cover these four items:

·         recognizing lost packets and having them re-sent

·         recognizing packets that arrive out of order and reordering them

·         detecting duplicate packets and dropping the extra ones

·         Avoiding congestion

Connection Multiplexing/Application Mapping

Transport layer assigns a unique set of numbers for each connection. These numbers are called port or socket numbers. TCP, and UDP, provide a multiplexing function for a device: This allows multiple applications to simultaneously send and receive data.
Imagine a server that performs a number of functions—for example email, web pages, FTP, and DNS. The server has a single IP address, but can perform all these different functions for all the hosts that want to connect to it. The transport layer (layer 4) uses port numbers to distinguish between different types of traffic that might be headed for the same IP address.

Network Layer

The network layer provides a logical topology and layer-3 addresses. Routers function at the network layer. This layer is responsible for three main functions:

·         Defines logical addresses used at layer-3

·         Finds paths, based on the network numbers of logical addresses, to reach destination devices

·         Connects different data link types together, such as Ethernet, FDDI, Serial, and Token Ring

IP packet
Where the transport layer uses segments to transfer information between machines, the Internet layer uses datagram's. Datagram is just another word for packet.

The IP protocol is mainly responsible for these functions:

·         Connectionless data delivery: best effort delivery with no data recovery capabilities

·         Hierarchical logical addressing to provide for highly scalable internetworks

IP addresses are broken into two components:

·         Network component Defines on what segment, in the network, a device is located

·         Host component defines the specific device on a particular network segment

Two types of packets are used at the Network layer: data and route updates.

Data packets
Used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols; examples of routed protocols are IP and IPv6.

Route update packets
Used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called routing protocols; examples of some common ones are RIP, RIPv2, EIGRP, and OSPF. Route update packets are used to help build and maintain routing tables on each router.

IP Classes

·         Class A addresses range from 1-126: 00000001-01111111.

·         Class B addresses range from 128-191: 10000000-10111111.

·         Class C addresses range from 192-223: 11000000-11011111.

·         Class D addresses range from 224-239: 11100000-11101111.

·         Class E addresses range from 240-254:

1.  0 is reserved and represents all IP addresses;

2.  127 is a reserved address and is used for testing, like a loop back on an interface:

3.  255 is a reserved address and is used for broadcasting purposes.

Public addresses are Class A, B, and C addresses that can be used to access devices in other public networks, such as the Internet. Public IP address assign authority The Internet Assigned Numbers Authority (IANA) is ultimately responsible for handing out and managing public addresses. Normally you get public addresses directly from your ISP, which, in turn, requests them from one of five upstream address registries:

·         American Registry for Internet Numbers (ARIN)

·         Reseaux IP Europeans Network Coordination Center (RIPE NCC)

·         Asia Pacific Registry for Internet Numbers (APNIC)

·         Latin American and Caribbean Internet Address Registry (LACNIC)

·         African Network Information Centre (AfriNIC)

Private IP and ISP

Private ip address can be used to configure private network. You can use private ip to build your network without paying a single rupees. But one biggest problem with private ip is that with private you can not access the internet. This is the point where ISP comes from. ISP purchase a bulk of public ip address and provide them on rent. Whatever you pay to ISP for accessing internet is actually the charge of using public ip address.

Data link layer

Main functions of data link layer is

·         Defining the Media Access Control (MAC) or hardware addresses

·         Defining the physical or hardware topology for connections

·         Defining how the network layer protocol is encapsulated in the data link layer frame

·         Providing both connectionless and connection-oriented services

·         Defines hardware (MAC) addresses as well as the communication process that occurs within a media.

·         The first six hexadecimal digits of a MAC address form the OUI.

·         MAC addresses only need to be unique in a broadcast domain,

·         You can have the same MAC address in different broadcast domains (virtual LANs).

There are two specifications of Ethernet frame Ethernet II and 802

802.2 use a SAP or SNAP field to differentiate between encapsulated layer-3 payloads.

With a SNAP frame, the SAP fields are set to 0xAA and the type field is used to indicate the layer-3 protocol. One of the issues of the original SAP field in the 802.2 SAP frame is that even though it is eight bits (one byte) in length, only the first six bits are used for identifying upper-layer protocols, which allows up to 64 protocols.

802.2 SNAP frame support of up to 65,536 protocols

Ethernet II's Version of Ethernet

·         Ethernet II does not have any sub layers, while IEEE 802.2/3 has two: LLC and MAC.

·         Ethernet II has a type field instead of a length field (used in 802.3). IEEE 802.2 defines the type for IEEE Ethernet

Physical Layer

The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others utilize state transitions—changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to explain the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media’s attachment interface.