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Which software and hardware parts of a computer are responsible for adding or removing values to the initial payload of a data packet in every layer of OSI or TCP/IP model? In other words, which parts (software or hardware) of a computer are able to capsulate and de-capsulate the packets?
Also, how can I practically add an additional value to the packets in layer transport or perform an additional operation to the packet in the last layer of 1 (physical)?
The transport layer and the network layer are usually handled by the OS's stack (software), link layer and physical layer are usually handled by NIC hardware. (Ignoring offloading options for the sake of simplicity.)
Each layer encapsulates and decapsulates the payload from/to the layer above.
Since the physical layer is realized in hardware you can't perform any arbitrary, additional operations that aren't explicitly supported by the hardware and its driver.
However, the physical layer consists of bits (or symbols). IP packets are the data entity (PDU) in the network layer.
