Network Fundamentals
Basics
Layers
Application Layer (Layer 7): This is where network applications operate. It provides services directly to user applications, like web browsers or email clients.
Presentation Layer (Layer 6): This layer translates data from the application layer into a format that can be sent over a network. It's responsible for data compression, encryption, and conversion.
Session Layer (Layer 5): This layer establishes, manages, and terminates connections (sessions) between applications. It's like a moderator of the conversation between two computers.
Transport Layer (Layer 4): This layer ensures that data are transferred from point A to point B reliably and without errors. It's responsible for breaking data into packets, checking for errors, and reassembling them at the destination.
Network Layer (Layer 3): Its primary function is routing and forwarding packets across the network. It decides the physical path that data will take based on network conditions, bandwidth, and availability.
Data Link Layer (Layer 2): This layer is responsible for transferring data over a single physical link in the network. It deals with the physical addressing and the handling of frames.
Physical Layer (Layer 1): The lowest layer, it's concerned with the transmission and reception of the unstructured raw data over a physical medium, like cables, radio frequencies, etc.
Encapsulation
A process in networking where each OSI layer adds its own header (or footer) to the data as it passes down the layers. This header contains control information for that layer. When data is received, each layer removes and processes its respective header.
Application Layer Header is added: The user's data is first processed by the application layer, which attaches a header containing application-level information like the type of service being requested.
Presentation Layer Header is added: As the data moves to the presentation layer, it gets another header that may include information for data translation or encryption.
Session Layer Header is added: Next, the session layer may add its header to manage the session control between applications.
Transport Layer Header is added: The transport layer then wraps the data with its header, which will include information necessary for data segmentation, reassembly, and error checking.
Network Layer Header is added: The network layer adds its header, which includes information needed for routing and addressing the data packet to the correct destination.
Data Link Header and Trailer are added: The data link layer adds a header and a trailer to frame the data for transmission. The header includes physical addressing, and the trailer typically contains error checking information.
Encapsulated data is sent across the network: Finally, the fully encapsulated data stream, now a series of bits, is sent across the physical medium to reach its destination.
Each header (and the trailer at the data link layer) provides the necessary control and addressing information required by each layer to fulfill its role in data transmission and delivery. When the data is received by the destination device, these headers (and the trailer) are stripped off in the reverse order to extract the original user data.
Handshake
SYN: The client begins the handshake by sending a SYN (synchronize) message to the server. This message is saying, "Hey, I want to start a conversation."
SYN/ACK: The server responds to the client's SYN message with a SYN/ACK (synchronize-acknowledge) message. In essence, the server is saying, "Okay, I got your request to start a conversation, and I'm ready to talk. Here's my own request to start a conversation (SYN), please acknowledge."
ACK: The client receives the server's SYN/ACK message and responds with an ACK (acknowledge) message. This final message from the client is saying, "I received your response, and I'm ready to start the conversation."
Packages with Wireshark
Layer-Modell - DD
Decodierung
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