Which RFC defines how VAD is treated in SIP Signalling

1. Brekeke Product Name and Version:3.9

2. Java version:1.8

3. OS type and the version:CENTOS7

4. UA (phone), gateway or other hardware/software involved:NA

5. Your problem:NA

My question has nothing to do with BSS but I am posting here in case a SIP guru happens to know.

Does anyone know which RFC dictates how VAD is communicated in the SIP signalling.

Our customer is sending us silence suppression as dictated by RFC3108 (a=silenceSupp: on/off - - - -).

But our provider wants us to send the SDP as

a=rtpmap:0 PCMU/8000
a=fmtp:0 vad=no

I am not able to find any RFC dictating the silence suppression in the above format. Can any SIP Guru help me on this?

It seems there is no RFC which defines " vad=no".

Appreciate your response.