Protocol Layering

• each layer provides its service by (1) performing certain actions within that layer and by (2) using the services of the layer directly below it.

• The application layer is where network applications and their protocols reside, such as HTTP for web document transfer, SMTP for email transfer, and FTP for file transfer. It also includes the domain name system (DNS) for translating human-friendly names to network addresses.
• The transport layer provides connection-oriented (TCP) or connectionless (UDP) services for transporting application-layer messages.
• The network layer is responsible for moving network-layer packets (datagrams) from one host to another, using the IP protocol and routing protocols to determine the routes between sources and destinations.
• The network layer is often referred to as the IP layer, as IP is the core protocol that holds the Internet together.
• The link layer is responsible for moving packets between nodes in a network. It works in conjunction with the network layer to route datagrams through routers.
• The link layer protocols, such as Ethernet, WiFi, and DOCSIS, provide services like reliable delivery and handle different types of links along the route.
• The physical layer, on the other hand, focuses on moving individual bits within a frame from one node to the next. It deals with the transmission medium, such as copper wire or fiber optics.
• The OSI model, proposed by the ISO, consists of seven layers: application, presentation, session, transport, network, data link, and physical. While the Internet protocol stack doesn't include the presentation and session layers, the functionality of the other layers is similar to their counterparts in the OSI model.
• The presentation layer handles data interpretation, compression, encryption, and description, while the session layer manages data exchange synchronization and recovery.
• The Internet leaves it to the application developer to decide if these services are necessary and to incorporate them into the application.

 

1.5.2 Encapsulation

• Routers and link-layer switches are packet switches that organize their networking hardware and software into layers, similar to end systems.
• However, they typically implement only the lower layers of the protocol stack.
• Link-layer switches implement layers 1 and 2, while routers implement layers 1 through 3.
• This means that routers can implement the IP protocol (layer 3), but link-layer switches cannot. Hosts, on the other hand, implement all five layers.
• Encapsulation is an important concept where each layer adds header information to the packet received from the layer above, creating a new packet at that layer.
• For example, at the sending host, the transport layer appends transport-layer header information to the application-layer message, creating a transport-layer segment. The network layer then adds network-layer header information, creating a network-layer datagram.
• Finally, the link layer adds its own link-layer header information, creating a link-layer frame. Each packet consists of header fields and a payload field, with the payload typically being the packet from the layer above.
• An analogy for encapsulation is sending an interoffice memo through the postal service.
• The memo represents the application-layer message, the interoffice envelope represents the transport-layer segment with header information, and the postal envelope represents the network-layer datagram with additional header information.
• The process of encapsulation is reversed at the receiving end, where each layer extracts the relevant information from the packet.
• Encapsulation can become more complex with large messages, which may be divided into multiple transport-layer segments and network-layer datagrams.
• At the receiving end, these segments and datagrams must be reconstructed to retrieve the original message.