Issue 47: Handle unknown LLDP PDUs gracefully

When an unknown TLV is received, whose type does not match any of the ones that are currently handled, even though the PDU is good, pkt_lldp crashes as such a packet is not handled. Wireshark shows that packet as unknown TLV. Doing the same here.

include/pkt_lldp.hrl

@@ -140,6 +140,9 @@
 -record(organizationally_specific, { value = <<>> :: binary() }).
 -type organizationally_specific() :: #organizationally_specific{}.
 
+-record(unknown_lldp_tlv,{type = 0:: integer(), value = <<>> :: binary()}).
+-type unknown_lldp_tlv() :: #unknown_lldp_tlv{}.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 % LLDP Frame Format
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
@@ -153,7 +156,8 @@
              | system_desc()
              | system_capability()
              | management_address()
-             | organizationally_specific().
+             | organizationally_specific()
+             | unknown_lldp_tlv().
 
 -record(lldp, { pdus = [] :: [pdu()] }).
 -type lldp() :: #lldp{}.

src/pkt_lldp.erl

@@ -84,6 +84,10 @@ decode(<<?MANAGEMENT_ADDRESS:7, Length:9,
 decode(<<?ORGANIZATIONALLY_SPECIFIC:7, Length:9,
          Value:Length/bytes, Rest/bytes>>, Acc) ->
     Pdu = #organizationally_specific{ value = Value },
+    decode(Rest, [Pdu | Acc]);
+decode(<<Type:7, Length:9,
+         Value:Length/bytes, Rest/bytes>>, Acc) ->
+    Pdu = #unknown_lldp_tlv{ type = Type, value = Value },
     decode(Rest, [Pdu | Acc]).
 
 encode([], Binary) -> Binary;