Wireshark · Wireshark-dev: Re: [Wireshark-dev] G.722 and G.726 decoders for Wireshark

[Jaap Keuter <jaap.keuter@xxxxxxxxx>]

On 01/25/2011 05:48 PM, Dietfrid Mali wrote:
> The problem with e.g. G.726 is that Wireshark gives those packets RTP
> type 102 which afaik is an error code ("unknown encoding").

No, that's your RTP endpoint configured to label these as such. RFC 3550 says:
"A profile MAY specify a default static mapping of payload type codes to payload 
formats. Additional payload type codes MAY be defined dynamically through 
non-RTP means (see Section 3)."
RFC 1890/RFC 3551 defines the "RTP Profile for Audio and Video Conferences with 
Minimal Control", which lists several static payload types. The old RFC lists 
G.721 (aka G.726-32), while the new one dropped that one and added references to 
G.726 at various bit rate, with a dynamic payload type.
RFC 3550 says in Section 3: "Non-RTP means: Protocols and mechanisms that may be 
needed in addition to RTP to provide a usable service.  In particular, ..., and 
define dynamic mappings between RTP payload type values and the payload formats 
they represent for formats that do not have a predefined payload type value." 
with reference to Session Description Protocol (SDP)

So, payload type 102 is a dynamic payload type which has to be given meaning 
(through SDP for instance) within the session. In your case Wireshark didn't 
pick that up from the trace, hence cannot give you the proper interpretation of 
that payload type within that session.

> I would need to know where and how Wireshark maps dynamic payload types
> (negotiated via SDP) to internal static ones. Above that RFC3551 notes that
> static G.726 payload types are obsolete, and afaik there aren't even
> (obsolete) static payload types for all G.726 variants, so Wireshark
> would need to
> take care of that by using some (more or less arbitrary) internal static
> type numbers.

Yep, that is done by the SDP dissector. It tries to interpret the SDP offer 
(should be the answer, but that a whole other story) and create conversations 
(see doc/README.developer, section 2.2) for the RTP dissector, feeding it 
dynamic payload type information it has learned from the media attributes.

The RTP dissector does then the heavy lifting on the RTP packets, based on the 
information feed in by the SDP dissector.

> I will do my best to provide a patch once I have fully integrated all
> codecs (currently only G.726-32 has been implemented as proof of
> concept, but since
> this is working adding more is no big deal).

Just one to get started is fine. Does it integrate into codecs/ directory 
besides G711a and G711u (and G729 and G723, if you have them)?

> Getting G.726 to work was a bit of a pain btw because of the weird frame
> sync calculation in rtp_player.c::play_channels() as this function seems to
> assume 1:1 relationships of decoder input and output stream sizes and
> thus simply halves the decoder output batch sizes to determine whether
> frames
> are properly sync'd. This doesn't work for compressed audio. To
> compensate, my decodeG726_32() function doubles the number of bytes returned
> (as it has a 1:2 relationship of input and output buffer sizes). Before
> it did that, lots of silence frames were inserted and half of the audio
> data was
> dropped by the player.

I'm no sure if I understand you correctly. Working with these decode functions 
there is an input buffer with its length as input, and two output parameters, 
being the output buffer and it a pointer to store its size. This size of the 
output buffer has to be set, by the decoder, to the number of samples in output 
buffer. That should be enough, see for instance rtp_player.c:decode_rtp_packet() 
the handling of G.279 and G.723.
Be aware that you have to store 16 bit linear samples in the output buffer, 
maybe that's your factor 2?

Thanks,
Jaap


> Dietfrid
>
>  > From: jaap.keuter@xxxxxxxxx
>  > Date: Tue, 25 Jan 2011 16:54:25 +0100
>  > To: wireshark-dev@xxxxxxxxxxxxx
>  > Subject: Re: [Wireshark-dev] G.722 and G.726 decoders for Wireshark
>  >
>  > Hi,
>  >
>  > That would be interesting. Can you put the code in a patch on bugzilla?
>  >
>  > Can't work on it right now, but would be nice to have.
>  >
>  > btw: their are already static RTP types assigned for both codecs. The
> dynamic types should come in through protocols like SDP, or a dissector
> preference.
>  >
>  > Thanks,
>  > Jaap
>  >
>  > Send from my iPhone
>  >
>  > On 25 jan. 2011, at 16:07, Dietfrid Mali <karx11erx@xxxxxxxxxxx> wrote:
>  >
>  > > Hi,
>  > >
>  > > using spandsp, I have added G.722 and G.726 decoders to Wireshark.
>  > >
>  > > Currently this is a bit of a hack job, particularly regarding
> inclusion of the spandsp lib, and I could need a bit help to properly
> integrate it into Wireshark's automake hell (configure.in).
>  > >
>  > > There also isn't a proper Wireshark signature for that RTP type (I
> am simply reacting to RTP type 102, which actually is an error code), so
> some help getting this straight and introducing proper codec types would
> be appreciated, too.
>  > >
>  > >
>  > >