GRP address type in MeGaCo/SDP for TDM

Hi Albrecht, Tom and all,

About the use of the GRP address type, [draft-taylor-mmusic-sdp-tdm-01] 
proposes:

The GRP scheme designates a logical or physical grouping of circuits
within which individual 64 kbit/s channels may be identified by
integer values.  The GRP connection-address consists of two parts.
The first part identifies the logical or physical group, while the
second identifies the individual circuit within the group.

         grp-connection-address = group-part "/" member-part

         group-part  = non-ws-string
         member-part = non-ws-string


If we want to describe an ISDN mutichannel service with 3 B-channel (B1, 
B4, B7) belonging to the ISDN interface of the rack "A", I image a 
possible example would be:

v=0
c=TDM GRP  1111/1
c=TDM GRP  1111/4
c=TDM GRP  1111/7
m=application 1 TDM octect-stream
b=AS:64  # each channel of 64 kbps


But, then: How does MGC and MG agree the group number of each ISDN 
interface (in the example, the "1111" number for the ISDN interface of 
the rack "A" (and similarly for member number "1,4,7" with the B1,B4,B7 
channels, although the correlative numbering is a possibility here)?

Regards,

Javier Muñoz