The MPLS WG Archive

[Eric Gray <EGray@zaffire.com>]

Tan,

	No information is added to the IP header to
aid in determining what LSP it is to be propagated
on except for the MPLS label (which is prepended as
part of the MPLS stack and is not part of the IP
header).  When the last useful label is removed from 
a data packet (either by popping it or by using an
explicit NULL label), there is no longer any way to 
positively identify the LSP that it was forwarded on.

	I had the impression - because of the way that
you asked your question - that you were trying to 
find out how LSP information might be inferred from
the label on a packet.  Again, (and as Kireeti already
pointed out) it cannot be easily inferred from the IP
packet once all useful label information is gone.  I
doubt that it can be determined absolutely at that 
point through much less than magic.

--
Eric Gray

> -----Original Message-----
> From: Tan Su Wei [mailto:swtan@mmu.edu.my]
> Sent: Monday, May 29, 2000 7:33 PM
> To: Kireeti Kompella; dwilder@baynetworks.com; EGray@zaffire.com;
> Shahram_Davari@pmc-sierra.com
> Cc: mpls@UU.NET
> Subject: Re: can egress know the ingress of a packet?
> 
> 
> Dear All,
>     Thanks for answering my question.
> As Kireeti wrote:
>     > Perhaps we've gone off on an tangent here.  The question
>     > as I understood it is "when a *data* packet arrives at
>     > the egress, can we know which ingress and which LSP it
>     > travelled on?" -- and the answer is, not without some
>     > extracurricular work.
>     >
>     > Tan, can you clarify your question?
> 
> and my question is exactly as above. I'm sorry if there is any
> misunderstanding.
> For the posted answer from Mr. Eric Gray
> 
>     It is possible to know the LSP traversed, only if
>     the LSP is "identified".  This is possible with either
>     CR-LDP (using LSP-ID) or RSVP-TE (using SESSION).
> 
> Lets if the LSP is "identified" using either lspid or session 
> object (during
> the LSP setup time, right?) , to know the lsp and ingress the 
> *data* packet
> from, is that mean we need adding a field in the data packet on this
> information? (Maybe ip header option field?).
> Since information get from the incoming label and interface might be
> incorrect when there is LSP merge.
> Is my interpretation correct?
> 
>     Thank you for your reply.
> 
> Regards
> Tan Su Wei
> 
>