title: Scapy - Referencing multiple fields from a FieldLenField
body: <p>I have reached the next situation, I need to define a new protocol layer in Scapy and one of the fields has a variable size (1,2,3 or 4 bytes) that depends on the size of other specific layer. This is the situation:</p>
  
  <pre><code>class newLayer(Packet):
        name = "newLayer fixed header"
        fields_desc = [
            ByteField("msgtype",0),
                # Some other fields ...
            FieldLenField("length", XXX), # length of newLayer_specific layer
                          # This field can be 1,2,3 or 4 bytes long depending
                          # on the length of the variable header
          ]
  
  class newLayer_specific(Packet):
        name = "variable header"
        fields_desc = [
                FieldLenField("length2","field1"), # This field is always 2 bytes long
                StrLenField("field1","",length_from = lambda pkt:pkt.length2),
                StrLenField("field2","", length_from = lambda pkt:pkt.underlayer.length - pkt.length - 2)
                # 2 is because the length of the length2 field 
  
  bind_layers(newLayer, newLayer_specific, msgtype=1)
  </code></pre>
  
  <p>I don´t know how I can refer to the size of the <code>newLayer_specific</code> layer from the field <code>length</code> of the layer <code>newLayer</code>
  Any suggestion? Thanks in advance.</p>