On Feb 25, 2009, at 1:18 AM, Akshitij Malik wrote:
> There is no Protocol ID field in the L2TPv3 Header that identifies
> the encapsulated protocol.
>
> Somehow, using preferences one specifies how to interpret the L2TPv3
> payload.
>
> So it is not possible to register a protocol based on some header-
> field value of L2TPv3 Header.
There's no requirement that the field used for a dissector table be a
field in the packet, so that, by itself, doesn't prevent you from
having a dissector table for the L2TPv3 pseudowire type.
This does bring up another issue. Currently, the only mechanism we
have for specifying the pseudowire type for an L2TPv3 session is a
global preference, but a capture might have multiple L2TPv3 sessions
which might be using different pseudowire types, and if we capture the
session setup we might see a Pseudowire Type AVP that specifies the
pseudowire type.
This might be done best with:
1) a mechanism that identifies L2TP flows by the endpoint addresses
and the session ID;
2) a mechanism to attach a pseudowire type to a flow;
3) a mechanism by which you can specify the pseudowire type for a
flow in the GUI (and perhaps via a command-line option for TShark);
4) the L2TPv3 dissector setting the pseudowire type for a flow if it
sees a Pseudowire Type AVP.
Ideally, we'd want:
1) a mechanism, subsuming the current conversation and circuit
mechanism, for identifying flows at multiple protocol layers;
2) a mechanism for attaching arbitrary attributes to a flow and
fetching the attributes for a flow;
3) a mechanism for a dissector to register user-settable per-flow
attributes, which could be edited through the GUI (and possibly set
via a command-line option);
which could be used by other dissectors.
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