TCP checksum

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==> TCP checksum is combination of Data + TCP Header + Pseudo Header

We all aware about DATA and TCP header but what is this Pseudo Header?

Pseudo header is combination of [SIP + DIP + Protocol ID (TCP or UDP) + TCP length (including both header and data) + Reserved(all 0) ]

Pseudo Header:=>




Sometimes, question may arise whether router changes TCP checksum as it changes (decrement) TTL value
 
==>Pseudo header doesn't include TTL value 


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