The MPLS WG Archive

[David Charlap <dcharlap@fore.com>]

Markus Jork wrote:
> 
> There was a minor change from draft-ietf-mpls-rsvp-lsp-tunnel-04
> to draft-ietf-mpls-rsvp-lsp-tunnel-05. The section about the
> CoS Tspec/Flowspec maximum packet size (M) parameter now says:
> "When the object is contained in Path
> messages, this parameter is updated at each hop with the lesser
> of the received value and the MTU of the outgoing interface."
> 
> This is clearly the wrong thing to do and unnecessary. It conflicts
> with the traditional rsvp model where the sender_tspec characterizes
> the sender's traffic and the adspec is used to gather information
> along the path. Now requiring that a tspec has to be updated along
> the path with link specific information is not a good idea, especially
> when we don't gain any advantage by doing so. The adspec already
> has the path mtu information and there is no point in collecting
> the same information in the tspec where it doesn't belong.
> And doing this because it is more convenient than parsing
> an intserv adspec is a rather weak argument, too.

Personally, I agree with you here.  I think the current draft breaks
with the spirit of RSVP.  I think it would be best to simply require the
ADSPEC (as straight RSVP does for router-to-router Path messages).  The
egress node, seeing the CoS TSPEC will then know to ignore all
parameters other than "M".

By making the TSPEC behave as an ADSPEC, you effectively eliminate the
TSPEC.  The egress node no longer knows what MTU the ingress node is
requesting.  It only knows the network's capabilities - which may be
greater than the amount required.

I do realize that MTU isn't bandwidth here, but I think the egress node
should get both the requested value as well as the ADSPEC value.  On
some routers, resources may be wasted if an MTU greater than required is
signalled.

I would also add as a side comment that there doesn't seem to be any
purpose to signalling the MTU in the CoS FLOWSPEC if the sender's
requirements are not propagated all the way the recipient.  If you only
want to signal "whatever the network's capable of", you can do that
without specifying anything.

-- David