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joel jaeggli
If you're working up at the linux sockets layer or above the selection of a (random) flow-label is taken care of for you. all you're speficying is (c example) sockfd6 = socket (r->ai_family, r->ai_socktype, r->ai_protocol); can take a look at: https://www.lugod.org/presentations/ipv6programming/PortMeth.pdf RFC's tend not to speficy API's at least historically. but as with source port numbers this is soupposed to be handled automatically unless you have a really good reason to mess with it. I do agree that it's a bit of a mess from my vantage point, hence the motivation for writing the above missive.