== MGCP Extensions

The MGCP protocol is extendable. The following non-standard extensions are
understood by OsmoMGW.

=== `X-Osmo-IGN`

`X-Osmo-IGN` indicates to OsmoMGW that specific items of an endpoint should be
ignored, so that it is lenient on mismatching values that would normally
indicate collisions or configuration errors.

==== `X-Osmo-IGN` Format

The value part of X-Osmo-IGN must be one or more items separated by one or more
spaces. Each item consists of one or more non-whitespace characters.

.Example: `X-Osmo-IGN` format with three ficticious items "X", "abc" and "123".
----
X-Osmo-IGN: X abc 123
----

`X-Osmo-IGN` must be issued in the MGCP section (typically as its last item),
before the SDP section starts.

==== Supported `X-Osmo-IGN` Items

Currently, the following `X-Osmo-IGN` items are supported:

* `C`: ignore CallID mismatches, i.e. differing "C" values between connections
  on the same endpoint.

.Note:
`X-Osmo-IGN` does not support ignoring mismatches on the domain part of
an endpoint name, e.g. ignoring a mismatch on "example.com" in
`rtpbridge/123abc@example.com`. Instead, you may globally configure OsmoMGW
with `mgcp` / `domain *` to permit all domain parts.

===== `X-Osmo-IGN: C`

By default, OsmoMGW verifies that all CallIDs ("C" values) match for all
connections on any one given endpoint. To ignore CallID mismatches, pass a `C`
in the `X-Osmo-IGN` header, for the first or the second `CRCX` on an endpoint.
When the `X-Osmo-IGN: C` is sent for any one `CRCX` on an endpoint, CallID
mismatches will be ignored for that and all subsequent messages for that
endpoint. Hence this header only needs to be included once per endpoint, in any
`CRCX` message that precedes or coincides with a CallID mismatch.

This is particularly useful for a BSC that is connected to an A/SCCPlite MSC,
where the BSC and MSC each are expected to configure their respective own
connection on a shared endpoint. For A/SCCPlite, it is impossible for the BSC
to know the CallID that the MSC will use, so CallID mismatches are inevitable.
See also OsmoBSC, which will by default pass the `X-Osmo-IGN: C` header for
endpoints associated with an A/SCCPlite MSC.

.Example: `CRCX` message that instructs OsmoMGW to ignore CallID mismatches
----
CRCX 2 rtpbridge/123abc@mgw MGCP 1.0
M: recvonly
C: 2
L: p:20
X-Osmo-IGN: C

v=0
c=IN IP4 123.12.12.123
m=audio 5904 RTP/AVP 97
a=rtpmap:97 GSM-EFR/8000
a=ptime:40
----