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We know that UDP checksum's pseudo header contains the most important parts of the IP header, that is, source and destination address, protocol number and data length.

I hope that the concept of IP addresses is present in the network layer and the transport layer is above it, so how does the transport layer know the IP address at the sending side because at the sender side, first the transport layer header is first encapsulated around the data coming from application layer and then the IP header is attached. I mean, how does the transport layer get to know the IP address when this facility is present at the bottom layer (Network layer) to make pseudo header ?

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These layers are just abstract concepts. They don't actively do anything by their own. Instead such models are a tool to deal with complexity, get a common understanding of the functionality and to structure the code in a way which can also be understood and managed by others.

This means there is no "transport layer know ...". There is instead a specific implementation (usually in the OS) dealing with all the different layers and which can also easily exchange information between these layers, like the IP address.

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  • I have this related one more question, could I edit the question?
    – Alok Maity
    Sep 12 at 6:02
  • @AlokMaity: No. Questions should not be made a moving target. One can clarify the question or add more details while editing, but one should not add what was asked or add more questions to it. Please ask a new question instead with full context. Sep 12 at 6:48
  • see this question
    – Alok Maity
    Sep 12 at 9:23

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