Re: [MEGACO] Mid call codec negotiation with unspecified Local descriptor

Hello Manpreet,

Please see my response below.

Regards, Christian

On 28/08/2014 2:17 AM, Manpreet Singh wrote:
>
> Hi All,
>
> Need you input in a MEGACO mid call codec negotiation scenario. In =

> this scenario Local descriptor was unspecified in mid call codec =

> change and we want to understand what will be correct behavior.
>
> Call flow in summary
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
> 1. Firstly there is negotiation of audio codec and it is successful =

> (first modify request and its reply from MG is shown in message 1 & =

> message 2).
>
> 2. Second Modify Request send by MGC with unspecified Local (Message 3)
>
> 3. MG reply Modify with =93515 Unsupported_Media_Type=94. (Message 4)
>
> As per the RFC
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
> An unspecified Local or Remote Descriptor is considered to be a =

> missing mandatory parameter. It requires the MG to use whatever was =

> last specified for that descriptor. It is
>
> possible that there was no previously specified value, in which case =

> the descriptor concerned is ignored in further processing of the command.
>
> Behavior
>
> =3D=3D=3D=3D=3D=3D=3D=3D
>
> In this case local Descriptor was unspecified in modify (message 3) =

> due to this at media gateway previously specified value was PCMA.
>
[CNG] Yes this is acceptable to leave a previously set descriptor =

unspecified.
>
> As there is no matching codec in local and new remote Descriptor so MG =

> return error.
>
[CNG] H.248/Megaco doesn't specify that the codec in the local and =

remote descriptors must be the same. So from a protocol perspective it =

is OK to only include one descriptor.

The issue relates to what the MG can support. If the MG only supports =

the same codec in local and remote then it is legitimate to return an =

error. If the issue that the setting of the codec is also required in =

both the local and remote descriptors by the MG then it could return =

"441 Missing Remote or Local Descriptor". This would indicate that the =

local descriptor should also have been updated.

> Question
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D
>
> Is MG behavior correct ?
>
[CNG] I didn't see where in the first message the MGC requested the =

local descriptor information? Would that have been requested in the =

initial ADD.req?
>
> Can anyone please confirm the right behavior of MG with second modify =

> request (Message 3) ?
>
[CNG] See above.
>
> Call follow
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
> [Message 1:: First modify from MGC]
>
> !/2 <nord2.interconnect.mgc.voip.telefonica.de =

> <http://nord2.interconnect.mgc.voip.telefonica.de>>:2945 =

> T=3D111526820{C=3D2913{mf=3Dds/stm1_2/e1_50/29{m{o{mo=3Dsr,TDMC/EC=3DOFF}=
},e,sg{}},mf=3Dtgwrtp/8456{m{o{mo=3Dsr},r{v=3D0
>
> o=3Danonymous 140490962784 140490962786 IN IP4 213.20.251.120
>
> s=3DSIP Call
>
> c=3DIN IP4 213.20.251.120
>
> t=3D0 0
>
> m=3Daudio 13076 RTP/AVP 8 0 125 101
>
> b=3DAS:82
>
> a=3Drtpmap:8 PCMA/8000/1
>
> a=3Drtpmap:0 PCMU/8000/1
>
> a=3Drtpmap:125 CLEARMODE/8000/1
>
> a=3Drtpmap:101 telephone-event/8000
>
> a=3Dfmtp:101 0-15
>
> a=3Dptime:20
>
> a=3Dsendrecv
>
> }},e,sg{}}}}
>
> [message 2 :: REPLY message for selected remote and local codecs form MG]:
>
> !/2 <tmgx-brln-de103-2>:2945 P =3D 111526820{ C =3D 2913 =

> {mf=3Dds/stm1_2/e1_50/29 ,mf=3DTGWRTP/8456 { m { l {
>
> v=3D0
>
> o=3D- 392 1404909627 IN IP4 62.53.160.73
>
> s=3D-
>
> c=3DIN IP4 62.53.160.73
>
> t=3D0 0
>
> m=3Daudio 30356 RTP/AVP 8 101
>
> a=3Dptime:20
>
> a=3Drtpmap:8 PCMA/8000/1
>
> a=3Drtpmap:101 telephone-event/8000/1
>
> a=3Dfmtp:101 0-15
>
> } ,r {
>
> v=3D0
>
> o=3Danonymous 140490962784 140490962786 IN IP4 213.20.251.120
>
> s=3D-
>
> c=3DIN IP4 213.20.251.120
>
> t=3D0 0
>
> m=3Daudio 13076 RTP/AVP 8 101
>
> a=3Drtpmap:8 PCMA/8000/1
>
> a=3Dptime:20
>
> a=3Drtpmap:101 telephone-event/8000
>
> a=3Dfmtp:101 0-15
>
> } } } }}
>
> [message 3 :: second Modify Request with unspecified Local with change =

> in codec]
>
> !/2 <nord2.interconnect.mgc.voip.telefonica.de =

> <http://nord2.interconnect.mgc.voip.telefonica.de>>:2945 =

> T=3D111528469{C=3D2913{mf=3Dds/stm1_2/e1_50/29{m{ts{CTYP/CALLTYP=3D[FAX,T=
EXT,DATA]},o{mo=3Dsr,TDMC/EC=3DOFF}},e=3D1{CTYP/DTONE},sg{}},mf=3Dtgwrtp/84=
56{m{o{mo=3Dsr},r{v=3D0
>
> o=3Danonymous 140490962784 140490962787 IN IP4 213.20.251.120
>
> s=3DSIP Call
>
> c=3DIN IP4 213.20.251.120
>
> t=3D0 0
>
> m=3Daudio 13076 RTP/AVP 0 101
>
> b=3DAS:82
>
> a=3Drtpmap:0 PCMU/8000/1
>
> a=3Drtpmap:101 telephone-event/8000
>
> a=3Dfmtp:101 0-15
>
> a=3Dptime:20
>
> a=3Dsendrecv
>
> }},e=3D1{IPFAX/FAXCONNCHANGE},sg{}}}}
>
> [message 4 :: Modify Reply]
>
> !/2 <tmgx-brln-de103-2>:2945 P =3D 111528469{ C =3D 2913 =

> {mf=3Dds/stm1_2/e1_50/29 ,mf=3DTGWRTP/8456 { er =3D 515 { =

> "Unsupported_Media_Type" } } }}
>
> Any information will be appreciated!
>
> Thanks &Regards,
>
> Manpreet
>
>
>
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