gbproxy: Implement scaling of BVC flow control in SGSN pool
When there are multiple SGSNs inside a pool, we need to decide
how much of the per-BVC capacity advertised by the BSS in its
BVC-FLOW-CONTROL we should announce to each of the pool members.
A conservative approach would be to advertise 1/num_sgsn, but
there may also be use case where over-provisioning (announcing more
than an equal share of the capacity) is useful.
Hence, let's introduce "pool bvc-flow-control-ratio <1-100>" in order
to allow the administrator to decide.
Related: OS#4891
Change-Id: Ibe5addf657e7237499ca0205bacfe999ecd1e771
diff --git a/doc/manuals/chapters/gbproxy-configuration.adoc b/doc/manuals/chapters/gbproxy-configuration.adoc
index 599b3f7..e61af48 100644
--- a/doc/manuals/chapters/gbproxy-configuration.adoc
+++ b/doc/manuals/chapters/gbproxy-configuration.adoc
@@ -4,3 +4,29 @@
Osmocom has very limited funding and support resources; Feel free to help
us completing this documentation by contributing with code, documentation
or by supporting the developers financially.
+
+
+=== SGSN pool support
+
+In a SGSN pool, osmo-gbproxy is facing the problem of dividing the downlink
+capacity of a cell towards the SGSN. The BSS advertises the per-BVC capacity
+by means of the BSSGP FLOW-CONTROL-BVC messages, but as there are multiple
+SGSN in a pool, they all have to share / divide that total capacity.
+
+By default, osmo-gbproxy advertises the full capacity to _each_ of the SGSN
+pool members, which results in significant over-provisioning and can lead to
+overload situations.
+
+The administrator can configure the _percentage_ of the overall BSS-advertised
+capacity that shall be reported to each pool member SGSN using the
+`pool bvc-flow-control-ratio <1-100>` configuration command.
+
+A setting of 100 means that each pool member is informed of 100% of the
+BSS side capacity.
+
+A setting of 25 means that each pool member is informed of 25% of the
+BSS side capacity. This would make most sense in a set-up with four
+SGSN of equal share.
+
+More complex capacity division schemes are so far not supported by
+osmo-gbproxy.