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What is the significance of “received” tag in the Via header?

April 26, 2017header received significance

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What is the significance of “received” tag in the Via header?

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A. Whenever a SIP transport receives an incoming request, and if the address in the topmost via header is different from the source IP address from which the request is received then the SIP transport layer MUST add a received=tag in the request. This is so that when the response is to be sent back to the prev-hop then it could be sent on the same address from which the request was received. One of the reason why the source IP address can be different from the address in the Via header is because of a presence of a NAT in between.