Layering of information can be compared with the regular postal service where there are several distinct functions:
Creating the letter
Placing the letter in an envelope, and writing the sender’s and recipient’s address
Choosing the type of delivery for the letter (same day service, same week and so on)
Placing the appropriate stamp on the letter to pay for the service
Physically sending the letter via carriers; for example, by truck or airplane
After the sender writes the letter, all of the functions listed above are relevant to transporting the letter to the appropriate destination. At the destination, the letter is received by the recipient, and depending upon the transport service, an acknowledgement may be sent to the sender confirming the receipt of the letter. The letter can then be removed from the envelope and its contents read.
The layering of information on the Internet occurs in a similar fashion. The objective of this data transfer is to inter-network with different computer systems. The applications need to send data to and receive data from other applications on different hosts/systems. In doing so, the application composes the data and requests a layering stack to transport the information.
Each layer of the protocol stack adds the pertinent information for that layer to the existing data. As the data is sent from the sender to the receiver, the data passes through several other systems. These systems only check the information that is relevant to the layers in which they have an interest. The systems use this information to assist in transmitting the data to the appropriate destination.
The Evolution of the Internet Section 3 - How the Internet Works – TCP/IP Layering
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How the Internet Works - TCP/IP Layering
TCP/IP Layers - Overview
TCP/IP Layers - Characteristics
Encapsulation
End-to-end Frame transfer
OSI Model
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TCP/IP Layers - Overview
The network protocol suite defines the protocols and technologies that support the interconnection of a diverse array of hardware and systems to support the operation of a wide range of applications over the network. Anyone who has used an Internet application, such as a web browser or e-mail can appreciate the complexity of the systems that are required to support these applications.
The layering of protocols simplifies this complex problem by dividing the protocol into a number of simpler functions. Each layer performs a specific function that contributes to the overall functioning of the network. The TCP/IP suite, also known as the Internet protocol suite, contains four layers of technology.
The application services layer provides all of the services that are available to users of the Internet.
The two intermediate layers (transport and Internet protocol) provide a common set of services that are available to all of the Internet applications and operate on the Internet hardware infrastructure.
The network interfaces layer includes all of the hardware that comprises the physical infrastructure of the Internet.
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TCP/IP Layers - Characteristics
User interface to the network
User applications
E-mail, Telnet, FTP, WWW
Application interface to IP Layer
Reliable/unreliable transfers
Unique network addressing scheme to
identify hosts
Routing protocols for path determination
End-to-end forwarding of datagrams
Physical transfer of data
ATM, Ethernet, frame relay
The application services layer is where the user interfaces with the network. This layer applies only to network applications, such as e-mail, Telnet, FTP, and WWW. Without network connectivity, these
applications would be useless. Applications such as word processors and database programs are not considered network applications because they do not require network connectivity.
The transport layer is the application’s interface to the network. The transport protocol provides a
mechanism for an application to communicate with another application that resides on another device in the network. In the TCP/IP suite, there are two transport protocols: TCP and user datagram protocol (UDP). TCP is a connection-oriented protocol that provides an ordered and reliable transfer of data over the network. UDP is a connectionless protocol that supports the transfer of a single datagram across the network with no delivery guarantee. UDP is simpler than TCP and operates with less overhead than TCP. Most Internet applications, such as HTTP (web-browsing), e-mail, Telnet, and file transfer protocol (FTP), use TCP for data transfer because it provides a reliable transfer service. Some applications, such as domain name system (DNS) and simple network management protocol (SNMP), use UDP because they only require a simple datagram transfer. Other
applications, such as reliable transfer protocol (RTP), use UDP to avoid the overhead of TCP and because there is no benefit in the retransmission of lost packets for the applications that use RTP.
The Internet protocol layer provides a common addressing plan for all of the hosts on the Internet as well as a simple, unreliable datagram transfer service between these hosts. IP is the common glue that defines the Internet. IP also defines the way a datagram (or packet) is routed to its final destination. In an IP network, packet forwarding across the network is handled by routers. IP routers examine the destination address of a datagram and determine which router is the next hop that will provide the best route to the destination (known as hop-by-hop routing). Routers communicate with each other using dynamic routing protocols to exchange information about the networks to which they are connected. The protocols allow routers to make forwarding decisions for the datagrams that they receive.
The network interface layer comprises the hardware that supports the physical interconnection of all of the network devices. The technologies of the network interface layer are often defined as multiple layers. The common trait of all technologies of this layer is that they can forward IP datagrams. There are many different technologies that operate at this layer, some of which are very complex. Some of the protocols commonly used at this layer include ATM, frame relay, point-to-point protocol (PPP), and Ethernet. However, many other protocols are used; some of the protocols are open standards and some are proprietary. The diversity of the network interfaces layer demonstrates one of the benefits of protocol layering. As new transmission
technologies are developed, it is not necessary to change the upper layers in order to incorporate these technologies in the network. The only requirement is that the new technology be able to support the forwarding of IP datagrams.
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