The Speex RTP payload is defined as a header, followed by any number of
requests to the remote encoder and all encoded speech frames.

+--------+----------+----------------+
| Header | Requests | Speech data... |
+--------+----------+----------------+

The header contains only the number of frames sent 
encoded in 6 bits

 0 1 2 3 4 5 
+-+-+-+-+-+-+
| NB frames |
+-+-+-+-+-+-+

There can be any number of requests of the form

 0 1 2 3 4 5 6 7 0 1
+-+-+-+-+-+-+-+-+-+-+
|R| ReqID | ReqVal  |
+-+-+-+-+-+-+-+-+-+-+

where R is 1 when a request is following and 0 when there is no more
request. Each request (if R=1) is composed of a 4-bit request ID (ReqID) and
a 5-bit value (ReqVal)

Possible values for ReqID are:
 0: REQ_PERSIST   ReqVal=1 for persistent requests/mode selection, 
                  0 otherwise
 1: PERSIST_ACK   Acknowledge a REQ_PERSIST from the other end, 
                  ReqVal equals the value received
 2: MODE          Choose the encoder mode directly
 3: QUALITY       Choose the encoder quality
 4: VBR           Set VBR on (ReqVal=1) or off (ReqVal=2)
 5: VBR_QUALITY   Set the encoder quality for VBR mode
 6: LOW_MODE      Set the encoder mode for low-band (wideband only)
 7: HIGH_MODE     Set the encoder mode for high-band (wideband only)

All requests should be considered at the receiver as a suggestion and
compliance is not mandatory. The PERSIST_ACK should be sent upon receiving a
REQ_PERSIST request to indicate that the request has been received.

The speech data part contains speech frames one after the other. The size of
the encoded frames can be found since the mode is directly encoded into each
frame.

For example, a frame where we request VBR to be on with quality 8 and we
transmit two frames encoded at 8.35 kbps (167 bits/frame) will be:

 0               1               2               3
 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   NB=2    |1|ReqID=2| ReqVal=0|1|ReqID=3|ReqVal=8 |0| frame 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 1                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 1                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 1                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 1                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 1                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|end|                     frame 2                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 2                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 2                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 2                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         frame 2                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    end frame 2    |P|P|P|P|P|P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
