20 Mar
2019
20 Mar
'19
8:22 a.m.
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 <laforge(a)gnumonks.org> http://laforge.gnumonks.org/
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