LKSCTP SCTP_PRIMARY_ADDR query

Question

I've got a question about primary SCTP addresses, what it exactly means to be primary.

Consider an association with two addresses (ie. two sockaddr:s were supplied to sctp_connectx()). SCTP starts using the primary path, then after a while the connection goes down. SCTP tries the secondary path, which is functional. After a while the primary path becomes operational again.

Now, as far as we've seen the SCTP stack delivers a SCTP_ADDR_AVAILABLE event, but doesn't revert back to the primary path, which would be desirable to us. We've discovered the SCTP_PRIMARY_ADDR socket option, but looking at the kernel sources the zeroth address specified with sctp_connectx()
is always made the primary one, so there would be no benefit in trying to reinforce the primaryness with SCTP_PRIMARY_ADDR on SCTP_ADDR_AVAILABLE.

In summary, I'd like to know how to tell the SCTP stack to prefer the zeroth address of sctp_connectx() to transmit data whenever its path is available?

Answer 1

All primary addr means is that for as long as that address exists as an active association, it will be the source address used when routing packets to a destination. And yes, the primary address might change over the life of a connection.

I don't understand your concern though. you've found the SCTP_PRIMARY_ADDR socket option, how come you can't issue a setsockopt call in response to the reception of a ADDR_AVAILABLE event (or any other address changing event)?