SCTP sendmsgx

[Doug Hardie]

> 
>> On 9. Mar 2020, at 11:01, Doug Hardie <bc979 at lafn.org> wrote:
>> 
>>> I am trying to get sctp_sendmsgx to work and not having a lot of success.  I have not been able to find any examples on the web of using it.  I have a client using sctp_sendmsg working fine.  I need to make use of the multihoming feature which requires sctp_sendmsgx.    I changed the call to sctp_sendmsgx, and changed the address count to 2.  However, all I get is an EINVAL response.  Looking through the code there are at least 2 different possible causes, but I can't distinguish between them. I do have two address structures in the address field.  Are there any examples of how to build a client with sctp_sendmsgx?
>> 
>> I am now making some progress.  If you are using the sctp_sendmsg function the sa_len or sin_len field is not used.  However, sctp_sendx does use it.  Leaving it at zero causes
> Yepp, filling out the sa_len field is important.
>> the problem.  sendx now sends a connection init to the remote host.  There is no server running there yet.  It hangs for quite some time and doesn't try the multihome address.  I seem to recall reading something about that so will investigate that tomorrow. 
> What do you see on the wire? I would expect INIT chunks to be sent to the two addresses you provided.

If I have both network connections working, there is an INIT chunk on the primary network followed by a ABORT chunk response.  Then the client sits waiting for a response which never comes.  If I disconnect the primary network cable, then after about 5 seconds of starting the client, I get the INIT and ABORT chunks on the secondary network.  I haven't figured out how to control the time, or how to make it switch to the secondary when the network connection is there, but the server is not responding.

> 
> Please note that sctp_sendx() is deprecated (like sctp_sendmsg()). Please consider using sctp_sendv().
> See https://tools.ietf.org/html/rfc6458#section-9.12

Thats quite interesting.  This must have occurred after the Steven's book came out.  There is no man entry for sctp_sendv in FBSD 12.1.  However, I do see it in sctp_sys_calls.c so I will have to figure it out from there.  Likewise there is no indication those calls are depreciated in the man pages.

-- Doug