The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

Hi,

> I am greatly concerned by the numerous layer violations in the current
> base MPLS drafts (MPLS Architecture and MPLS Label Stack Encoding, in
> particular).

So am I.  If the protocol requires that an LSR in the middle of a tunnel
look beyond the label stack (into the MPLS payload), something is
seriously broken.  Groping inside a packet that has no protocol
discriminator hoping to find evidence of IP-ness ("oh, look, the first
nibble is 0x4!") is a gross hack; and while gross hacks are hallmarks
of great implementations and proudly exchanged at beer parties, I would
rather not see them in the protocol specifications themselves.

The main reasons seem to be handling TTL expiry and the need for
fragmentation.  In both cases, in my opinion, the guilty packet MUST be
dropped with no further processing.

If TTL expiry is needed for traceroute, an important debugging tool,
then the answer is "do traceroute right", and indeed such an effort
is under way.

As for fragmentation, we've argued this several times.  In my opinion,
the only place that this should be handled is at the head end of the
LSP; and protocols/implementations that don't allow/do this are broken
and need to be fixed.

Finally, the L3PID (or equivalent) should only be used by the tail end
of an LSP (or the penultimate hop, if PHP is used) when the last label
is popped, and the packet forwarded as an unlabeled layer 3 packet.
Again, my opinion only.

Kireeti.