Layers in the Internet Protocol Suite Stack
The IP suite uses encapsulation to provide abstraction of protocols and services. Generally, a protocol at a higher level uses a protocol at a lower level to help accomplish its aims. The internet protocol stack can be roughly fitted into the four fixed layers shown before.
Application Layer
This layer is broadly equivalent to the application, presentation, and session layers of the OSI model. It gives an application access to the communication environment. Examples of protocols found at this layer are Telnet, FTP (File Transfer Protocol), SNMP (Simple Network Management Protocol), HTTP (Hyper Text Transfer Protocol), and SMTP (Simple Mail Transfer Protocol). An application is a user process co-operating with another process usually on a different host (there is also a benefit to application communication within a single host). The interface between the application and transport layers is defined by port numbers and sockets.
Transport Layer
The transport layer is similar to the OSI transport model but with elements of the OSI session layer functionality. This layer provides an application layer delivery service. The two protocols found at the transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Either of these two protocols is by the application layer process, the choice depends on the application’s transmission reliability requirements. The transport layer provides end-to-end data transfer by delivering data from an application to its remote peer. Multiple applications can be supported simultaneously.
The most-used transport layer protocol is the Transmission Control Protocol (TCP), which provides connection-oriented reliable data delivery, duplicate data suppression, congestion control, and flow control. TCP is a reliable, connection-oriented protocol that provides error checking and flow control through a virtual link that it establishes and finally terminates. This gives a reliable service, therefore TCP would be utilized by FTP and SNMP File transfer and email delivery have to be accurate and error-free.
UDP is an unreliable, connectionless protocol that provides data transport with lower network traffic overheads than TCP. UDP does not error check or offer any flow control, this is left to the application process. SNMP uses UDP. SNMP is used to monitor network performance, so its operation must not contribute to congestion.
Network Layer or Internet Layer
This layer is responsible for the routing and delivery of data across networks. It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes. The internetwork layer, also called the internet layer or the network layer, provides the “virtual network” image of the internet (this layer shields the higher levels from the physical network architecture below it). Internet Protocol (IP) is the most important protocol in this layer.
It is a connection-less protocol that doesn’t assume reliability from lower layers. IP does not provide reliability, flow control, or error recovery. These functions must be provided at a higher level. A message unit in an IP network is called an IP datagram. This is the basic unit of information transmitted across TCP/IP networks. Other internetwork layer protocols are IP, ICMP, IGMP, ARP, and RARP. With the advent of the concept of Internetworking, additional functionality was added to this layer, namely getting data from the source network to the destination network.
This generally involves routing the packet across a network of networks, known as the internet. In the Internet protocol suite, IP performs the basic task of getting packets of data from source to destination. IP can carry data for many different upper-layer protocols; these protocols are each identified by a unique protocol number. ICMP and IGMP are protocols 1 and 2, respectively.
Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage multicast data) are layered on top of IP but perform internetwork layer functions, illustrating an incompatibility between the internet and the IP stack and OSI model.
All routing protocols, such as BGP, OSPF, and RIP are also really part of the network layer, although they might seem to belong higher in the stack.
Layers 2 and 1 (Network Access Layers)
The combination of the data link and physical layers deals with pure hardware (wires, satellite links, network interface cards, etc.) and access methods such as 16 CSMA/CD (carrier sensed multiple access with collision detection).
Ethernet exists at the network access layer – its hardware operates at the physical layer and its medium access control method (CSMA/CD) operates at the data link layer. The network interface layer, also called the link layer or the data-link layer, is the interface to the actual network hardware. This interface may or may not provide reliable delivery, and may be packet or stream-oriented.
TCP/IP does not specify any protocol here but can use almost any network interface available, which illustrates the flexibility of the IP layer. The link layer is not part of the Internet protocol suite but is the method used to pass packets from the network layer on two different hosts. This process can be controlled both in the software device driver for the network card, as well as on firmware or specialist chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission and then actually transmitting the frame over a physical medium.
The link layer can also be the layer where packets are intercepted to be sent over a virtual private network. When this is done, the link layer data is considered the application data and proceeds back down the IP stack for actual transmission. On the receiving end, the data goes up the IP stack twice (once for the VPN and the second time for routing).
The physical layer is made up of the actual physical network components (hubs, repeaters, network cable, fiber optic cable, coaxial cable, network cards, Host Bus Adapter cards, and the associated network connectors: RJ-45, BNC, etc).