Transport Layer

Question

In Transport layer either in TCP header or UDP header in checksum field we calculate checksum for Pseudo header of IP + header of TCP/UDP + data of TCP/UDP.

Transport layer should fill checksum field in header before the Segment pass to Network layer.

Here my doubt is how TL knows the IP header which is going to create in next step NL ?

Comments

https://stackoverflow.com/questions/359045/what-is-the-significance-of-pseudo-header-used-in-udp-tcp

Answer 1

Upper layer can communicate to lower layer so transport layer can communicate to network layer.

in Pseudo IP Header included important field which did not change during transmission like source ip,destination ip,protocol,TCP/UDP segment bit

Comments

But how ?

Can u explain it more clearly?

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Instead of computing the checksum over only the actual data fields of the TCP segment, a 12-byte TCP pseudo header is created prior to checksum calculation. This header contains important information taken from fields in both the TCP header and the IP datagram into which the TCP segment will be encapsulated.

But My Quetion is how TCP know IP pseudo header ???????

Answer 2

Yes, the layer concept is violated in this case but to provide reliable transmission it uses the IP header which is available in next layer