How to create an SCTP association

[Michael Tuexen]

On 02 May 2014, at 00:04, Bernd Walter <ticso at cicely7.cicely.de> wrote:

> On Thu, May 01, 2014 at 11:06:09PM +0200, Michael Tuexen wrote:
>> On 01 May 2014, at 22:42, Bernd Walter <ticso at cicely7.cicely.de> wrote:
>> 
>>> On Thu, May 01, 2014 at 08:07:19PM +0200, Michael Tuexen wrote:
>>>> On 01 May 2014, at 14:49, Bernd Walter <ticso at cicely7.cicely.de> wrote:
>>>> 
>>>>> I have an SOCK_SEQPACKET socket and want to setup an association
>>>>> without sending a message.
>>>> Just call connect() or sctp_connectx().
>>> 
>>> Cool - I wasn't sure about using *connect*() because tutorials only
>>> mention it for 1to1 sockets.
>> You might want to take a look at
>> http://tools.ietf.org/html/rfc6458#section-3.1.6
>> http://tools.ietf.org/html/rfc6458#section-9.9
>> sctp_connectx() also provides you a way to get the association id,
>> which is needed for 1-to-many style sockets.
> 
> Ah - there it is.
> So many sources to get things running, SCTP still is too new to
> easily find good sample code beyound basics.
> About the association id - so far I'm always using socket_addr.
> I may be able to track association IDs, but are they really unique?
> E.g. association shuts down and a new one gets the ID?
http://tools.ietf.org/html/rfc6458
guarantees them to be unique between all existing associations on
a given socket. So two associations on different sockets can have
the same id. The spec also allows them to be reused, as indicated
at the end of:
http://tools.ietf.org/html/rfc6458#section-8.1.8

In FreeBSD the assoc ID is a 32-bit counter which gets incremented
and wrapped around. So it is reused after a long time...
> I also use it to get the RTT, which is untested code so far.
> 
>>>>> The background is that I want to keep the round-trip times of peer
>>>>> servers.
>>>>> I've enabled regular heartbeat and can use SCTP_GET_PEER_ADDR_INFO
>>>>> to get the RTT.
>>>>> But in case a host is down or services are restarted I don't have
>>>>> an association to ask, so I will need some way to (re)establish
>>>>> associations.
>>>>> This is done in a different thread as the normal socket operation,
>>>>> which uses EOR to write.
>>>>> If I send a (dummy)message to the socket I would have to add a mutex
>>>>> to not disturb the sending thread.
>>>> Does the above solve your issue?
>>> 
>>> Yes.
>>> 
>>> But I have another question.
>>> Which resources will connecting an unreachable peer use on my one to
>>> many socket?
>> Basically the resources for an association and it will run a timer for
>> retransmitting the INITs.
> 
> Great - that's what I was hoping for.
> I may have enough troubles already with data for down hosts stuck in
> socket buffer, but this is a one time thing until the data times out.
If you enable the SCTP_SEND_FAILED_EVENT you can get that data back
if you want...

Best regards
Michael
> 
> Thank you
> 
> -- 
> B.Walter <bernd at bwct.de> http://www.bwct.de
> Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
>