Re: Ethernet II Trailer identification

Many thanks for your acclarations, it has been very useful.

2006/6/14, Guy Harris <[email protected]>:
>
> Eduardo Escudero Sánchez wrote:
> > How can ethereal determine the size of the trailer based on the size of
> > rest of the frame? In the data part of the frame there could be a lot of
> > different protocol-dependent data.
>
> The Ethernet dissector relies on the dissector that it calls to set the
> actual length of the tvbuff it handed to that dissector to the actual
> length.  The IPv4 dissector, for example, sets it based on the total
> length in the IPv4 header.
>
> > What im doing now is analize the data
> > inside the ethernet II frame and seeing if there is more data than the
> > upper protocol expects.
>
> That is, in effect, what Ethereal does.
>
> > But
> > this is not a very efficient approach because if i want to stay
> > independent of upper protocols it means that if i want to analize a MAC
> > level frame i have to step onto uppper levels to achieve it. That means
> > too that i must have implememnted frame analizers for all the upper
> > protocols in order to ask any of them what is the length of their data
> > to obtain the real length of the ethernet II data.
>
> There is no alternative to that.  The payload length of a protocol
> encapsulated with an Ethernet type field is defined by the protocol, so
> the only way to find out what that length is would be to have code that
> understands enough of that protocol to determine the payload length.
> _______________________________________________
> Ethereal-dev mailing list
> [email protected]
> http://www.ethereal.com/mailman/listinfo/ethereal-dev
>

_______________________________________________
Ethereal-dev mailing list
[email protected]
http://www.ethereal.com/mailman/listinfo/ethereal-dev