Re: draft-bonica-tunneltrace-02

[Giles Heron <giles@packetexchange.net>]

David Allan wrote:

> Eric:
> 
> I would phrase it differently. MPLS has a dual nature which tends to be 
> a function of the control plane used.
> 
> Most of what you describe as non-CO stuff permits LSPs to mimic the 
> topology generated by IP routing with as ruthlessly simple an 
> implementation as possible, and that's OK, that was a design goal.
> 
> However if that were the only application of MPLS, I would delegate it 
> to being a useless appendage on the IP forwarding plane. It only adds 
> baggage and complexity to no net value. I require things like liberal 
> label retention, supporting additional protocols (LDP), and lots of 
> extra logic on the node to approach the reliability that IP routing by 
> itself would be capable of without MPLS. This is because it adds steps 
> after routing convergence so that MPLS can configure the forwarding 
> table to mimic what the routing protocols have told it and that may 
> include having to solicit new labels from neighbors, or remebering a lot 
> of extra labels it would not otherwise need "just in case".


Actually there are other things you can do with "just" LDP that you 
can't do with IP.

1)  draft-martini etc.  Sure, IP equivalents of this are now being 
defined (UTI^H^H^H L2TPv3 et al) but they aren't as widely available, 
and are much less efficient encapsulation-wise (as they require an IP 
header plus a tunnel header instead of a couple of labels).

2)  MPLS VPN (RFC2547).  I won't go into the pros and cons of this right 
now ;-)

3)  BGP free network core.  This may improve stability (though there's 
certainly a strong argument that BGP is less destabilising than LDP 
right now) and security (the core doesn't need to know how to get to the 
Internet.)  Also this allows you to layer multiple distinct IP backbones 
over one MPLS core.

> It is when I get into topology manipulation independent of dynamic IP 
> routing that MPLS starts to add value and to claim this is not 
> connection oriented use of MPLS forwarding IMHO smacks of the same 
> "tunnel"/"layer" semantic debate that can also be said to waste much time.


this is your view.  And it is certainly one view.  But it isn't 
necessarily the only valid view ;-)

 
> I would also similarly invert the "saddled with CO mechanisms" statement 
> as it would appear that bringing some CO properties to IP is what MPLS 
> is all about. Otherwise IP without MPLS would be just fine....


again, some may differ on this.

Giles


> cheers
> Dave
> 
>  > -----Original Message-----
>  > From: Eric Rosen [mailto:erosen@cisco.com]
>  > Sent: Tuesday, March 05, 2002 9:52 AM
>  > To: Shahram Davari
>  > Cc: 'Thomas D. Nadeau'; ccamp@ops.ietf.org
>  > Subject: Re: draft-bonica-tunneltrace-02
>  >
>  >
>  >
>  > Shahram> What is  the difference between ATM,  FR, or any  CO
>  > technology and
>  > Shahram> MPLS
>  >
>  > Well, for  one thing, MPLS  isn't a CO  technology.  It's got
>  > lots  of stuff
>  > that doesn't really  make sense from a CO perspective.   Of
>  > course, when the
>  > CO  guys  run  across  these  features  (multipoint-to-point,
>  >  php,  liberal
>  > retention, independent  mode, equal  cost load balancing, 
>  > dynamic rerouting
>  > with no signaling,  ttl, lack of packet sequencing,  IP
>  > control plane, etc.,
>  > etc.) they assume that those features are there by mistake!
>  >
>  > This misconception  that MPLS  is a CO  technology, shared 
>  > as it is  by the
>  > "pure  IP" crowd and  by the  ITU/ATM crowd,  is the  source
>  > of  much wasted
>  > time. 
>  >
>  > MPLS is  actually an IP-derived technology that  facilitates
>  > the application
>  > of  certain   circuit-like  characteristics  to  IP 
>  > networks.    As  an  IP
>  > technology,  the usual  IP diagnostic  tools, such  as ping 
>  > and traceroute,
>  > should be applicable.   That's what this whole useless 
>  > discussion is about.
>  > One  the one  side are  people  who think  that IP  networks
>  > are  inherently
>  > unmanageable (after all, they don't  follow ITU standards),
>  > and on the other
>  > side are people who think that the IP-based paradigms are the best.
>  >
>  > It is true that  MPLS can be used to provide something  which
>  > is very like a
>  > CO network,  but it  doesn't have to  be used  this way, and 
>  > usually isn't.
>  > Those of use  who are not particularly interested in  CO
>  > networks just don't
>  > want to be saddled with the  legacy CO mechanisms. 
>  > Presumably, if all these
>  > legacy CO mechanisms were  so great there would not be such 
>  > a great rush to
>  > IP.
>  >
>  >
>  >
>  >
>  >
>  >
> 


-- 
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Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
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