MPEG4 Decoder Problem

Somewhere in the middle the MPEG4 Decoder is failed.
if we used the MPEG4 Decoder with the MPEG4 Demux , then there will be no problem.
But if we used it with the RTP Video Source Filter, then we got the problem.
Reason:----------- This is the problem with MPEG4 Decoder.if we used any DivX free decoder filter, then it is working well.if we are starting from the middle, then the RTP source Filter will not send the Keyframe first.This is the problem
Solution:
--------------
I modified the Decoder Filter code to accept it as follows:
For the First time, if we are receiving VOS header then only allow it to display . otherwise just drop the frame.By checking the VOS Start code (00 00 01 B0) But it is not working. It seems like the ffmpeg is not sending VOS header.
So I added the code to check for the VOL start code and frame type.
if (bFirstFrame)
{
if( VOS start code or ( VOL startcode and frametype == KeyFrame))
{
bFirstFrame = false;
}
else { return S_FALSE; //Drop the Frame. }
}