SCCP connection identifiers vs. local references - OpenBSC

20 Mar 2019

Hi Neels,

...
  21:14 < neeels> LaF0rge, I need a solution for
picking a conn_id as an SCCP SAP user. The way osmo-bsc
                 does it isn't much good either. There has to be some way to obtain an
unused local
                 reference without running into potential collisions
 21:16 < neeels> (I submitted the patch expecting your -1, but just to make sure
that I understood
                 correctly I wanted to submit as code that has no room for
misunderstandings)
 21:18 < neeels> if I can't use the internal "next_id" thing, then
what I would have to do is mimick the
                 exact same in osmo-msc by looking up what conn_ids are already in use and
pick an unused
                 one. That's quite brain damaged IMHO, it's the same thing as that
layer violation patch,
                 only with much more effort and more chances for id collisions. 
Nobody is suggesting that.

...
  21:21 < neeels> ideas: add some primitive to
hand out a local reference -- but that would be non-standard
                 IIUC 
I had mentioned and discarded that option in Message-ID:
<20190311204817.GA725@nataraja>
here on this list.  The reason is not that it's non-standard, but that it again
wouldn't
work over any type of asynchronous / message based SAP [and that's how a SAP is
conceptualized].

...
  21:21 < neeels> give each application some
unique number space for local references, like the highest
                 byte is configured in .cfg file, and then each SCCP user cycles through
its own number
                 space of local references
 21:23 < neeels> ...and libosmo-sccp would have to be constrain-able to its own
number space for incoming
                 connections 
I was quite sure we've had that discussion before.  Unfortuantely it doesn't
seem to be on this mailing list or in gerrit.  It may have been on IRC or I am
starting to be delusional :/

The correct solution from the spec / SCCP architecutre point of view is rather clear, I
think:

* don't recycle the SCCP local reference (protocol) from the SCCP connection ID
(SAP),
  but use distinct number spaces for that.  It was an oversimplification to do that
  in the original implementation.  I simply missed the different scoping of those
  two identifiers.  The scoping is the root of our problems here.

* only once we have distinct identifiers, we can fix the scope: One of
  the two identifiers has a per-SCCP-user scope, while the other has a
  per-SCCP-instance scope

* this way, every SCCP user knows exactly which SCCP connection identifiers are
  currently in use between this specific user and the SCCP provider, and it hence
  can choose any unused ID for a new connection

* right now, the primitive exchange across the SCCP user SAP is synchronous,
  so there can be no races where the SCCP provider and the SCCP user could
  chose the same connection ID at the same time.   If you want to prepare for
  some kind of possible future asynchronous, message-queue based SAP, then you
  could use the highest bit as "allocation flag", e.g. highest bit = 0,
allocated
  by the SCCP provider and highest bit = 1, allocated by SCCP user.

Let me know if you see any problem with above proposal.

Regards,
	Harald
-- 
- Harald Welte &lt;laforge(a)gnumonks.org&gt;           http://laforge.gnumonks.org/
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