[Freeswitch-dev] Can't negotiate L16 at 16000h with Asterisk

[Antonio Eugenio Burriel]

Yesterday I posted the subject "LIN16 at 16000h Freeswitch <->
Asterisk" in the user mailing list, but after having taken a peek at
the source, I thought it was better to share my analysis here:

Problem: Freeswitch can't negotiate L16 at 16000h codec with some peers
(like Aterisk) which assigns a dynamic PT (over 95) to L16.

Internally Freeswitch assigns a PT of 70 to L16; in
switch_core_media_negotiate_sdp() we have the following match check:
match = (!strcasecmp(rm_encoding, imp->iananame) &&
    ((map->rm_pt < 96 && imp->ianacode < 96) || (map->rm_pt > 95 &&
imp->ianacode > 95)) &&
    (remote_codec_rate == codec_rate || fmtp_remote_codec_rate ==
imp->actual_samples_per_second)) ? 1 : 0;
Which means that when our peer's codec is dynamic, our match must be
also dynamic and, if not, out match mustn't.

Except for L16 at 44100h, which has a PT of 10, its PT should be dynamic
(>95) according to RFC3551. But Freeswitch is internally assigning 70,
so it will never match.
Incidentally, Asterisk erroneously assigns to L16 at 8000h a PT of 10,
which passes the check.

I'm not quite sure which is the proper way to fix this.
If we change PT from 70 to something over 95, we'll break
interoperability with older Freeswitches and other peers, such
LN16 at 800h with Asterisk.

So maybe its cleaner to add a exception to the match check like:
match = (!strcasecmp(rm_encoding, imp->iananame) &&
    ((map->rm_pt < 96 && imp->ianacode < 96) || (map->rm_pt > 95 &&
(imp->ianacode > 95 || imp->ianacode == 70))) &&
    (remote_codec_rate == codec_rate || fmtp_remote_codec_rate ==
imp->actual_samples_per_second)) ? 1 : 0;

Any thoughts?