What is SIP VIA header ?
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SIP Via Header
SIP via header is used to route the responses back to the originator, the entity which generates the header inserts its address in VIA filed, all the subsequent proxies that fall in the path also insert their own address as VIA
Any entity which replies to the request, keeps all the VIA addresses same and in same order as received, the subsequent proxies that fall in the reply path then remove their own via address and forward the response back. Hence, when the response reaches the originating UA it contains only the VIA address of the originator UA , the originator UA strips off this VIA and founds that there is no more VIA left, so the response it for its processing