The two main parts of a protocol data unit (PDU) are protocol-specific control information and user data.
Let's break down what this means in more detail:
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Protocol-Specific Control Information (Header/Trailer): This section contains the necessary information for the protocol itself to function correctly. This information helps in routing, error checking, and sequencing of the data. It can be found in the header (at the beginning of the PDU) or trailer (at the end). Examples include:
- Source and destination addresses.
- Sequence numbers for reassembly.
- Error detection codes (checksums).
- Protocol version information.
- Control flags for managing the connection.
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User Data (Payload): This is the actual information that the application or user wants to transmit. It's the reason the PDU exists in the first place. This data is encapsulated within the control information. Examples include:
- Text messages.
- Image files.
- Audio or video streams.
- Database records.
Essentially, the protocol-specific control information acts like an envelope that tells the network how to handle and deliver the user data (the letter inside). The structure and content of the control information are determined by the specific protocol being used at that layer of the network stack.
