How to create an SCTP association

[Michael Tuexen]

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.
> 
>>> 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.
> 
> My use case is a distributed hash table with many servers.
> Clients connect to servers via local proxy and the local proxy tries to
> stay in contact with all storage nodes - basicly to keep RTT values up
> to date.
> I initially plan 100 nodes per cluster, 2 cluster per location and
> 2 location.
> So such a proxy has to stay in touch with 400 peers of which 200 are
> on a remote location.
> Of course the remote location is rarely used, but in case of network
> failure 200 peers are unreachable.
> With TCP sockets the sockets are unrelated of each other.
> In this case I may try to sctp_connect 200 offline peers and still
> want to be able to reach the 200 local peers without additional latency.
> Is there anything special to care about to not block the traffic
> to the remaining peers in case a whole bunch of the others is offline?
> Raising socket buffer on the proxy servers is not a problem, since
> those have beafy RAM - the nodes however are tiny quad core ARM-A9 with
> 2G RAM only, which they mostly need to keep hashtable indexes.
I don't see a problem right now. I suggest to do some testing. In case
of problems, please let me know.

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