The MPLS WG Archive

[neil.2.harrison@bt.com]

Ajay,  Let me take you back to the point of Shahram's orginal
question.....which in essence, if I have understood it correctly, was 'what
problem/application is LDP solving/addressing that cannot be done either (i)
using IP directly or (ii) ER LSPs?'.....and which must also take a wider
pro/con analysis of the implications of using LDP-LSPs vs ER-LSPs (or IP
directly).

Please also see below.   Regards, Neil

Ajay Simha wrote 01 June 2002 04:15
 
> The point is MPLS effectively decouples the forwarding 
> portion from the
> routing.
NH=> Well I agree, this is vital and it's what is needed.  And it's
something we in Telco-land have known/used for years, ie decoupling of the
traffic carrying data-plane and its control-plane (both its own data-plane,
where appropriate, and its routing and signalling protocols).  But LDP does
nothing more than instantiate labels per hop that are locked to the IGP.
That is (i) they take the same SPF routes as the IGP would use for IP
fowarding and (ii) the LSPs change as the IGP changes.  I would argue this
is not a required behaviour for applications where LSPs are long-holding
and/or must not be affected by ad hoc routing changes or failures of routing
protocols (eg VPNs).  Further, because LDP is so tightly coupled to the IGP
then one has to introduce 'layer violations' to make it work a bit better,
eg Load-balancing using IP-level hashing......its hard to take seriously
anyone claiming this is MPLS label forwarding when one has to look at the
client/IP level (which it may not always be, ie XoverMPLS) to make
switching/forwarding decisions.

> This makes MPLS a service enabler.
NH=> I don't agree that *LDP* makes MPLS a 'service-enabler'.  Considered
against a VPN service, the behaviour noted above is not what is needed.  Now
take the fact that QoS (a pkt forwarding attribute) and survivability (an
LSP attribute) need decoupling so that per VPN QoS/availability SLAs can be
defined/measured (independent of other VPNs) and look what LDP does to
that.....it mangles then both so they can't be decoupled, and now VPN
QoS/survivability behaviour is no longer independent on a per VPN basis.
Again this is not the required behaviour for VPNs.

> For instance I 
> did not see anyone
> mention things like QoS transparency that MPLS can achieve.
NH=> How can you claim this in view of the above observations?  Further, on
what basis are you discussing QoS?  That is, unless you can automatically
fault-manage MPLS I fail to see how anyone has any basis on which to discuss
QoS with any conviction......you have to define availability 1st, since QoS
metrics *only* have relevance to the up-state, and availability is dependent
of having defects defined.  So where are the defects defined and where is
availability defined?....once you can point at these then we can
meaningfully discuss QoS, but not before.
 
> Sure you can
> monkey around with IP to make IP have everything that MPLS 
> has.
NH=> No, you have to monkey around with LDP (eg load-balancing) because it
does not provide sufficient decoupling from IP behaviour as I noted above.

> May be even a
> fixed length tag and call it something else :).
> 
> Why do it when this extension already exists as an IETF based 
> standards where
> folks like us can debate issues and come up with solutions 
> that are acceptable
> to most?
NH=> I have no problems with people defining solution's for a restricted
perspective of the problem-space if that is all they need/want from MPLS,
just so long as they don't try to stop others defining solutions which have
a more general application.  I have a strong sense that some people want to
do this....and therein lies the real problem IMO. 
> 
> -ajay
> 
> 
> >:Robert Raszuk writes:
> >:
> >: > Currently deployed hardware & software.
> >:
> >:your current router hardware can't do many other things either, like
> >:learning mac addresses.  so when you re-spin it, you could 
> also consider
> >:adding real support for ip tunneling for those who don't like the
> >:complexity of mpls control plane.
> >:
> >:-- juha
> >:
> 
> -- 
>