Support Neighbor Address Resolution over PCUIF IPA multiplex
While NACC was initially developed, it became clear there was need for
a way to interact PCU<->BSC in order resolve ARFCN+BSIC into CGI-PS
for later RIM usage.
Hence, this resolution was first (until today) implemented using an out
of bands RPC system using the CTRL interface, which required specific
config to be written and matches in osmo-pcu and osmo-bsc VTY (ip+port
of the CTRL interface to use).
However, this has several shortcomings:
* As explained above, specific configuration is required
* Since recently, we do support BSC redundancy in osmo-bts. Hence the BTS
may switch to a BSC other than first one. If that happened, that'd mean
the CTRL interface would still point to the initially configured one,
which may not be the same currently serving the PCU.
During recent development of ANR related features, a similar need for
PCU<->BSC was required, but this time it was decided to extend the IPA
multiplex of the Abis OML connection to pass PCUIF messages,
transparently forwarded to each side by the BTS.
This has the advantage that connection PCU<->BTS is handled by BTS and
both sides send messages transparently.
Let's switch by default to using this new interface, while still
maintaing the old way for a while (announcing them as deprecated) to
avoid breaking existing deployments until they are upgraded to new
versions of osmo-pcu and osmo-bsc.
Related: SYS#4971
Change-Id: I6ad33c7ab10202840cf804dea9ba595978d0e920
diff --git a/doc/manuals/chapters/configuration.adoc b/doc/manuals/chapters/configuration.adoc
index d031ce6..d778929 100644
--- a/doc/manuals/chapters/configuration.adoc
+++ b/doc/manuals/chapters/configuration.adoc
@@ -252,8 +252,40 @@
CGI-PS, since it actually equals to the Circuit Switch CGI + RAC.
In order to apply this target cell identity translation, OsmoPCU uses the
-OsmoBSC Neighbor Resolution CTRL interface (see OsmoBSC User Manual), since the
-BSC is the node holding all the neighbor related information.
+OsmoBSC Neighbor Resolution Service. This service is nowadays provided by means
+of PCUIF container messages, which are transparently forwarded in both directions
+by the BTS using the IPA multiplex of the OML connection against the BSC. No
+specific configuration is required in any of the involved nodes, they should
+behave properly out of the box.
+
+These neighbor address resolutions (<ARFCN + BSIC> => <RAI + CI>) are by default
+cached for a while, in order to avoid querying the BSC frequently. As a result,
+the resolution time is also optimized.
+
+.Example: Configure Neighbor Resolution cache and timeouts
+----
+pcu
+ timer X1 500 <1>
+ timer X0 60 <2>
+----
+<1> Time out if the BSC doesn't answer our resolution request after 500 ms
+<2> Keep resolved neighbor addresses cached for 60 seconds
+
+===== OsmoBSC CTRL interface (deprecated)
+
+CAUTION: This interface is nowadays considered deprecated and should not be used
+anymore. Any related VTY options should be dropped from configuration files, to
+let OsmoPCU use the new interface instead. This section is kept here for a while
+as a reference for old deployments using old versions of the programs.
+
+This Neighbor Address Resolution Service was initially implemented by means of a
+separate CTRL interface (see OsmoBSC User Manual), where OsmoPCU would create a
+CTRL connection to the BSC each time an address resolution was required.
+
+Older versions of OsmoBSC may not support the current Neighbor Address
+Resolution Service over the IPA multiplex (see above). For those cases, OsmoPCU
+can be configured to use the old deprecated CTRL interface.
+
By default, the use of this interface is not configured and hence disabled in
OsmoPCU. As a result, until configured, the network won't be able to provide the
System Information to the MS prior to allowing the change during NACC against
@@ -261,20 +293,12 @@
to configure the interface, the OsmoBSC IP address and port to connect to must
be configured in OsmoPCU VTY.
-These neighbor address resolutions (<ARFCN + BSIC> => <RAI + CI>) are by default
-cached for a while in order to avoid querying the BSC frequently and, as a
-result, optimizing the resolution time too.
-
.Example: Configure Neighbor Resolution CTRL interface against OsmoBSC
----
pcu
neighbor resolution 172.18.13.10 4248 <1>
- timer X1 500 <2>
- timer X0 60 <3>
----
<1> Port 4248 is the default and hence could be omitted in this case
-<2> Time out if the BSC doesn't answer our CTRL resolution request after 500 ms
-<3> Keep resolved neighbor addresses cached for 60 seconds
==== System Information Resolution