Re: Overview of MPLS PW bugs

[Guy Harris <guy () alum mit edu>]

On Jan 7, 2017, at 1:15 PM, Francesco Fondelli <francesco.fondelli () gmail com> wrote:

> the pw_eth_heuristic is too strong, it does not take into
> consideration locally-assigned MAC addresses and multicast (as noted
> in some bugs by Guy Harris and Michael Mann). Patches are welcome :-)

The heuristic used in the pw_eth_heuristic dissector is both too strong *and* too weak:

        it's too strong because it only recognizes globally-assigned MAC addresses;

        it's too weak because it only checks the MAC address - it doesn't check whether, if the type/length field is a 
type field, the type is one we know or, if it's a length field, whether the headers following the MAC header are 
something we'll dissect.

Bugs for *both* of those problems have been filed.

> That said, I think the current situation is a good trade-off.

The *current heuristics* are clearly *not* a good trade-off, given the bugs that have been filed.  They need to be 
improved, in both directions.

> The not edulcorated version reads "Ethernet PW without control word is
> a pain in the ass, do not use it".

That may be the case, but apparently people *do* use it, and if we can make life less painful for them without making 
life more painful for the people "doing the right thing", we should do so.

> an other improvement could be to add logic to signalling dissectors
> (e.g. LDP, BGP) in order to add explicit label-to-dissector bindings.
> This would be useful only in case signalling and data plane are
> captured together. Therefore, I guess this is not common and it isn't
> worth it.

We *already* do that for some other control and data plane protocols; for example, RTSP and SDP dissectors can set up 
UDP traffic to be dissected as RTP (there are other examples as well), so I don't consider that a sufficiently good 
reason not to do it.
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