Chapter 2

TCP

A connection-oriented transport protocol. Connection-oriented transport protocols provide reliable transport, in that if a segment is dropped, the sender can detect that drop and retransmit that dropped segment. Specifically, a receiver acknowledges segments that it receives. Based on those acknowledgements, a sender can determine which segments were successfully received and which segments need to be transmitted again.

UDP

A connectionless transport protocol. Connectionless transport protocols provide unreliable transport, in that if a segment is dropped, the sender is unaware of the drop, and no retransmission occurs.

Application Layer (TCP/IP stack)

Addresses concepts described by Layers 5, 6, and 7 (that is, the session, presentation, and application layers) of the OSI model.

Transport Layer (TCP/IP stack)

Also known as the DoD model, this four-layer model (as opposed to the seven-layer OSI model) targets the suite of TCP/IP protocols.

Data Link Layer

As Layer 2 of the OSI model, this layer is concerned with the packaging of data into frames and transmitting those frames on a network, performing error detection/correction, uniquely identifying network devices with an address, and handling flow control.

Transport Layer

As Layer 4 of the OSI model, it acts as a dividing line between the upper layers and the lower layers. Specifically, messages are taken from the upper layers (Layers 5-7) and encapsulated into segments for transmission to the lower layers (Layers 1-3). Similarly, data streams coming from lower layers are decapsulated and sent to Layer 5 (the session layer) or some other upper layer, depending on the protocol.

Session Layer

As Layer 5 of the OSI model, it's responsible for setting up, maintaining, and tearing down sessions.

OSI Reference Model

Commonly referred to as the OSI model or the OSI stack. This 7 layer model categorizes various network technologies.

Physical Layer

Layer 1 of the OSI model, it is concerned with the transmission of bits on a network.

Network Layer

Layer 3 of the OSI model, is it primarily concerned with forwarding data based on logical addresses.

Presentation Layer

Layer 6 of the OSI model, it is responsible for the formatting of data being exchanged and securing the data with encryption.

Application Layer

Layer 7 of the OSI model, it provides application services to a network. An important, and an often-misunderstood concept, is that end-user applications do not reside at the application layer. Instead, the application layer supports services used by end-user applications. Another function of the application layer is advertising available services.

Protocol Data Unit (PDU)

The name given to data at different layers of the OSI model. Specifically, the PDU for Layer 4 is segment. The Layer 3 PDU is packet, the Layer 2 PDU is frame, and the Layer 1 PDU is bit.

Network Interface Layer

The network interface layer of the TCP/IP stack (also known as the network access layer) encompasses the technologies addressed by Layers 1 and 2 (that is, the physical and data link layers) of the OSI model.

Internet Layer

This layer of the TCP/IP stack maps to Layer 3 (network layer) of the OSI model. Although multiple routed protocols (for example, IPv4 and IPv6) may reside at the OSI model's network layer, the Internet layer of the TCP/IP stack focuses on IP as the protocol to be routed through a network.