Wireshark · Ethereal-dev: Re: [Ethereal-dev] in need of an idea to support l2 tunneling over mpls

[Guy Harris <gharris@xxxxxxxxx>]

On Sun, Mar 17, 2002 at 08:38:07PM -0500, Aamer Akhter wrote:
> We can get it to mostly work for Ethernet Over Mpls (except for the poor
> packets whose destination starts of 4 or 6), but there is no guarentee that
> it's that kind of packet even the above is finger crossing at best, and is no
> answer for frame-relay or ATM over mpls.
> 
> I am thinking that leaving the above change in is better than nothing

The above change is better than nothing but worse than declaring

	static dissector_handle_t eth_dissector;

at the top of "packet-mpls.c", doing

	eth_dissector = find_dissector("eth");

in "proto_reg_handoff_mpls()", and doing

    if (ipvers == 6) {
      call_dissector(ipv6_handle, next_tvb, pinfo, tree);
    } else if (ipvers == 4 ){
      call_dissector(ipv4_handle, next_tvb, pinfo, tree);
    } else {
      call_dissector(eth_handle, next_tvb, pinfo, tree);
    }

(why search a table on *every call* when you can do it only once?).

> and modifying the 'Decode as' user selectable option to include 'MPLS encaped'
> along with 'link', 'network' and 'transport'.

If you want to decode something as something, you need to have a way to
specify what the first "something" is.

For link-layer "Decode as", the "something" is specified by the Ethernet
type field in the packet.

For network-layer "Decode as", the "something" is specified by the IP
protocol type field in the packet.

For transport-layer "Decode as", the "something" can be specified by the
source TCP/UDP port number, the destination TCP/UDP port number, or both
in the packet.

What would specify that "something" for MPLS?

If the intent is to have *all* MPLS traffic be decoded as a particular
protocol, that'd be a preference item for the MPLS dissector.

If the intent is to have *some* MPLS traffic be decoded as a particular
protocol, that'd be the "something" in question.