Depending the Layer 2 technology that you use , the header can change :
- Ethernet Frame => 18 bytes
- Frame Relay/FRF.12 => 6 bytes
- MLP => 6 bytes 13 bytes
- ATM => 5 bytes
- MLP over Frame Relay => 14 bytes
For Layer 3 , you can use the following values
- IP Header = 20 bytes
- UDP Header = 8 bytes
- RTP Header = 12 bytes
So it gives you the following amount
- IP+TCP+UDP Header => 40 bytes
With CRTP enabled, you can then benefit of a huge saving of compression of this payload.
Indeed , from 40 bytes , you can decrease this payload from 2 up 4 bytes.