mgcp_codec: be sensitive about IuFP when checking codecs

When two codecs are checked for convertibility we insist that
subtype_name and rate are equal since normally when those are different,
we assume a completely different codec that will require a transcoder,
which we do not have yet.

However when IuFP is used, the call agent will always negotiate IuFP as
VND.3GPP.IUFP with a rate of 16000, even though the IuFP payloads
contain regular AMR at a rate of 8000.

This means that if we detect IuFP on one side of the call leg and AMR on
the other side of the call leg, we must not insist on equal subtype_name
and rate.

This fixes the following TTCN3 testcases:
MGCP_Test.TC_two_crcx_mdcx_and_iuup_rtp
MGCP_Test.TC_two_crcx_mdcx_and_iuup_rtp_rfci_unordered

Related: OS#5461
Change-Id: I6bc1e6022efe21cb893ef213f3da35017960357d
---
M src/libosmo-mgcp/mgcp_codec.c
1 file changed, 37 insertions(+), 0 deletions(-)

diff --git a/src/libosmo-mgcp/mgcp_codec.c b/src/libosmo-mgcp/mgcp_codec.c
index 905795c..d340e11 100644
--- a/src/libosmo-mgcp/mgcp_codec.c
+++ b/src/libosmo-mgcp/mgcp_codec.c
@@ -330,10 +330,20 @@
 	/* OsmoMGW currently has no ability to transcode from one codec to another. However OsmoMGW is still able to
 	 * translate between different payload formats as long as the encoded voice data itself does not change.
 	 * Therefore we must insist on equal codecs but still allow different payload formatting. */
+
+	/* In 3G IuUP, AMR may be encapsulated in IuFP, this means even though the codec name and negotiated rate is
+	 * different, the formatting can still be converted by OsmoMGW. Therefore we won't insist on equal
+	 * subtype_name and rate if we detect IuFP and AMR is used on the same tandem. */
+	if (strcmp(codec_a->subtype_name, "AMR") == 0 && strcmp(codec_b->subtype_name, "VND.3GPP.IUFP") == 0)
+		goto iufp;
+	if (strcmp(codec_a->subtype_name, "VND.3GPP.IUFP") == 0 && strcmp(codec_b->subtype_name, "AMR") == 0)
+		goto iufp;
+
 	if (strcmp(codec_a->subtype_name, codec_b->subtype_name))
 		return false;
 	if (codec_a->rate != codec_b->rate)
 		return false;
+iufp:
 	if (codec_a->channels != codec_b->channels)
 		return false;
 	if (codec_a->frame_duration_num != codec_b->frame_duration_num)

