The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

"Faisal S. Naik" wrote:
> 
> I need to know what are the similarities or differnces between the
> Flow label field of Ipv6 and the label field in the MPLS shim header.
> Incase of an IPv6 network working with MPLS, what role will each field
> of their respective technologies play?

Acording to RFC 2460, the flow label field is used to identify flows
that require special handling.  The actual use of the field is
documented as experimental (in section 6).  The details of what the
"special handling" means is something that must either be configured or
signalled and is beyond the scope of RFC 2460.

While there does appear to be some overlap between flow labels and MPLS
labels, there are some important differences:

1: IPv6 does not require all packets to have flow labels.  In fact,
packets that don't require special handling are not supposed to have
them at all.  (ie. the flow label is set to 0).

2: Flow labels are not swapped as the packet traverses the network.

3: The meaning of these flow labels must still be signalled or
configured.  In order to prevent network-wide conflicts, some kind of
signalling for label selection must also exist.  Which implies a need
for something like LDP or RSVP-TE.

I'm no expert here, but it seems to me that IPv6 flow labels would not
be used if another mechanism (like MPLS) is already in place.  Leaving
the flow label out (setting it to zero) is acceptible, and MPLS provides
its own, more robust, mechanism for labeling packets.

-- David