1 Network Models and Protocol Foundations
Learn how network models organize communication, how protocols cooperate across layers, and how data is encapsulated, forwarded, and troubleshot.
Models, layers, and protocols
A organizes communication functions into layers. It helps make complex systems easier to design, explain, and troubleshoot, but it is a guide to how communication functions fit together—not a itself.
A is an agreed set of rules for exchanging information. Protocols specify matters such as message format, the meaning of fields, and how devices respond. A groups related communication functions and provides services to the above it. Different protocols can perform similar functions within a .
The OSI and Internet models
The OSI model has seven layers, while the Internet architecture commonly groups functions into four layers. The correspondence is approximate: real protocols do not always fit neatly into one model category.
OSI application, presentation, and session layers: grouped as the application in the common Internet model. They cover network services used by applications, data representation and formatting (and sometimes encryption), and coordination of communication sessions.
OSI transport : corresponds to the transport , which handles communication between processes on end devices.
OSI network : corresponds to the internet , responsible for addressing and forwarding packets between networks.
OSI data link and physical layers: grouped as the link . The data link function delivers data across a directly connected network link; the physical function conveys signals over a physical medium or wireless channel.
The Internet model is also described using the names application, transport, internet, and link. Layering separates responsibilities: an application can use a transport service without managing Wi-Fi or Ethernet details, and a network link can change without requiring every application to be redesigned. These boundaries are useful, but implementations may combine functions or allow interaction across layers.
and data delivery
During transmission, each passes information to the below and typically adds control information, such as a header and sometimes a trailer. This process is called . At the receiving device, corresponding layers interpret and remove that information in reverse order; this is .
For a browser sending an HTTP request, the layers can work as follows:
Application: HTTP represents the request data.
Transport: TCP may add information used to deliver and manage the data between application processes.
Internet: IP adds source and destination network addresses so the can be forwarded across networks.
Link: Ethernet or Wi-Fi packages the IP for transmission over the current link.
Physical transmission: The link's bits are conveyed as electrical, optical, or radio signals.
At the destination, processing proceeds upward. The link passes the IP to IP; IP passes transport data to the appropriate transport ; and transport delivers the data to an application that can handle it. Common data-unit names are at the link , at the internet , and for TCP at the transport .
Forwarding and cooperation
A router typically examines the IP to choose where to forward it, then places it inside a new link- for the next link. Link- framing is therefore usually specific to each link along the path, while the IP is forwarded toward its destination.
Protocols cooperate by using services and information from other layers. For a website request, the application defines the request, a transport carries it between processes, IP moves packets between networks, and a link carries each across a local link. Each handles its own part; no single delivers the entire application service by itself.
Delivery guarantees and troubleshooting
IP provides a . It does not guarantee that packets arrive, arrive only once, or arrive in order. A transport such as TCP can provide reliable, ordered delivery for applications that need it; other applications may use a different transport service.
A useful troubleshooting sequence follows the layered communication path:
Check whether the local link is working.
Check whether IP addressing and forwarding are functioning.
Check whether transport communication is established or receiving data.
Check whether the application is behaving as expected.
A problem at one can prevent communication at higher layers even when those higher layers are correctly configured.