The OSI reference modelis the primary model for network communications. The early development of LANs, MANs, and WANs was confused in many ways. The early 1980s saw great increases in the number and sizes of networks. As companies realized that they could save money and gain productivity by using networking technology, they added networks and expanded existing networks as rapidly as new network technologies and products were introduced.
Facilitates modular engineering:
Interoperability between Vendors
Ensures interoperable technology:
Simplifies teaching and learning:
The Session Layer
He transport layer is possibly the most important layer for exam study purposes. A lot is going on here, and it is heavily tested.
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 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.
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.
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.
Common TCP and UDP Port Numbers
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:
Where the transport layer uses segments to transfer information between machines, the Internet layer uses datagram's. Datagram is just another word for packet.
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.