distributed GSM / some thoughts

historical

Hi Oliver, Neels, community,

I had some comments on the D-GSM work that didn't really fit directly to
the gerrit code review, and I thought I'd post it here.

== DNS zone / .msisdn suffix ===

One question I had was regarding the use of the .{msisdn,imsi} TLD.  I would argue
it is probably besser to use something that fits within the existing DNS hierarchy
without contesting IANA's authority on gTLDs.

Historically, ETSI/3GPP made the mistake of using ".gprs" for resolving APNs
on the GRX.  This was later changed to something with 3gppnetwork.com or the like,
hwere that domain would actually be registered by 3GPP with normal domain registrars,
but without any publicly accessible zone records.  This way the name is reserved
in the public hierarchy and no risk of clashes.

I'm not sure how much of a concern this is to us, given that our use
case is much more niche than the GRX.  However, the "cost" is probably
rather small to change this to something like
.{msisdn,imsi}.dgsm.osmocom.org ?  Sure, the packets will get larger by
a few bytes, but given all the other overhead I think it's not really
going to have any impact?

What are your thoughts on that?

== MSISDN format ==

Another thought is whether or not there are any concerns regarding the
MSISDN format.  Historically, this is one of the weaknesses of OsmoMSC,
inherited from the OpenBSC days where we just thought in terms of PBX
extension numbers.  In reality, a MSISDN consist of a TON (type of
number), NPI (numbering plan indicator) and the related digits.  IIRC,
in the TON one can also specify if it's supposed to be national or
international, i.e. if it's prefixed with the country code or not.

It would be great to make sure that the format used in the mDNS queries
is somewhat standardized, if not at least only by the documentation
requiring that all queries should be done in fully qualfiied form with
country code present.   NPI is sort-of bogus as IMOH E.164 is the only
one applicable for MSISDN.

Any thoughts?

== The use of 'age' vs. absolute timestamp ==

In my original D-GSM idas I always thought we'd send an absolute UTC
timestamp when a given HLR/MSC has ever seen that subscriber.  The idea
here being that any rural GSM network will have some kind of GNSS
recevier for clock stability in the BTS anyway, and one can hence assume
that timestamps are synchronized.

The advantage of a relative 'age' is obvious: You don't care about the
absolute clock value being correct anymore.  The potential downside is
that propagation delay might matter.  If you have a rather slow / loaded
geostationary sattelite link from one village, but a faster terrestrial
link from another village, the 'age' will be ambiguous while an absolute
timesetamp wouldn't have that.

Given that the delays we're talking about are probably all sub-second or
maybe possible about 1s, it's probably not a problem.

== GSUP keepalives / connection loss detection ==

In the presence of unreliable back-haul mesh between villages, the GSUP
connection can also not be seen as reliable.  We would expect to see TCP
stalls due to packet loss, etc.

Have you considered this in your implementation and/or done any testing
based on simulated lossy networks to ensure we properly use either TCP
keepalives or IPA application-level PING/PONG to detect lost connections
and recover from such situations (by closing the old and
re-establishing)?

Unreliable networks can be easily simulated by Linux built-in 'tc netem'
for providing configurable packet loss / latency / jitter.

I also saw some comments / code related to "if a second connection using
the same IPA ID arrives, we're screwed" (paraphrasing here).   I would
expect this not to be uncommon even if every MSC/HLR out there is
configred correctly exactly because e.g .the remote MSC/HLR has already
decided that the TCP/GSUP is dead and starts to reconnect by performing
a local-end release, while the "local" MSC/HLR still thinks the old
connection is alive.  If the old connection "wins" (i.e. is preferred)
I see potential trouble here.

Situations like that probably warrant some carefully designed tests to
create exactly those situations.

Regards,
	Harald

-- 
- Harald Welte <laforge at osmocom.org>            http://laforge.gnumonks.org/
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