It is acking that message from user was received by kernel and now
processing. The message from kernel to user are anyway unreliable [1],
so I don't understand on which handshake you are talking.
[1] "reliable transmissions from kernel to user are impossible in any
case"
https://linux.die.net/man/7/netlink
The SA messages are kernel initiated messages that target a user space daemon (e.g. ibacm) for cached PR data. If that fails because the daemon isn't running or the message is lost, the normal retry mechanism in the kernel should kick in and try sending the SA message to the remote (i.e. real) SA.
- Sean
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