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cs7-config: Describe special meaning of M3UA routing context '0' 

Osmocom has a special interpretation of the routing-context '0'.

Let's document that.

Change-Id: Iac0bc1e754d04b2e798263bfa029d26376e4e92f
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Harald Welte 2021-01-28 23:36:45 +01:00
parent ae95711860
commit ee37483661
1 changed files with 26 additions and 0 deletions
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common/chapters/cs7-config.adoc
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@ -307,3 +307,29 @@ routing-key RCONTEXT DPC si (aal2|bicc|b-isup|h248|isup|sat-isup|sccp|tup)
routing-key RCONTEXT DPC ssn SSN
routing-key RCONTEXT DPC si (aal2|bicc|b-isup|h248|isup|sat-isup|sccp|tup) ssn SSN
----
==== M3UA without Routing Context IE / Routing Context '0'
As per the M3UA specification, the use of the routing context IE is
optional as long as there is only one AS within an ASP. As soon as
there are multiple different AS within one ASP, the routing context IE
is mandatory, as it is the only clue to differentiate which of the ASs a
given message belongs to.
In the Osmocom M3UA implementation, it is generally assumed that a
routing context IE is always used, for the sake of clarity.
However, the routing context ID of '0' has the special meaning of _do
not encode a routing context IE on transmit_.
So if you configure an application like OsmoBSC to use routing context
0, then no routing context IE will be included in outbound M3UA
messages.
This special interpretation of '0' within the Osmocom M3UA
implementation however means that we can not represent M3UA with a
routing context IE that actually contains '0' as a numeric identifier.
So you only have the following options:
* Using M3UA with routing context (1..N)
* Using M3UA without routing context (0)