[barcaroller <barcaroller@xxxxxxxxx>]

I asked a similar question in comp.protocols.tcp-ip.  How does 
wireshark correctly identify the type of Layer 2 header?  For example, 
how does it know that the Layer 2 header is PPP and not Ethernet (as 
happened in my case)?  Also, in my case the PPP header was missing the 
0x7E flag field, but wireshark was still able to correctly parse the 
packet.