The MPLS WG Archive

[Eric Osborne <eosborne@cisco.com>]

On Mon, Aug 05, 2002 at 02:35:53PM -0400, Gray, Eric wrote:
> Eric O.
> 
> 	You're interpreting confusion into existence.  :-)
> 
> 	L3 lookup should mean the ability to make a 
> determination on how to forward packets _without_
> necessarily implying capability to forward packets 
> with any particular capacity.  

But L3 lookup doesn't have to have anything to do with *forwarding*
packets.  It is entirely possible that a device may have a L3
forwarding table used _only_ for locally originated packets
(incl. RSVP), and no such thing even at the software level at the
receiving side of 0/32. 

However, this is semantics, and moreover, it's semantics I'm not
particularly opinionated about one way or the other, so it's not like
I'm looking to get into a big argument here. :)

> L3 forwarding, on 
> the other hand, might encompass what you refer to,
> but not necessarily.  Lookup and forwarding functions 
> are distinct, even if clearly not orthogonal.
> 
> 	The ability to look something up in no way 
> implies the ability to do anything in particular 
> with the results.
> 
> 	However, Mayank's question implies that - by 
> L3 lookup - Mayank meant the ability to determine how
> to forward packets.  Otherwise, the question of how
> to determine which downstream to request a label of
> would have been unnecessary since the ability to 
> determine how to forward packets would be used in
> determining how to forward requests.

An ATM LSR combines 2 control plane protocols (LDP/RSVP +
ISIS/OSPF/etc) to build both a routing topology (via IGP) and a
forwarding path (via LDP/RSVP).  Having either one is not good enough,
and I think that an ATM LSR is under no obligation to have to
forward *anything* as an IP packet, except control traffic.  And the
only control traffic I can think of that works like this is RSVP.




eric

> 
> Eric W. Gray
> Systems Architect
> Celox Networks, Inc.
> egray@celoxnetworks.com
> 508 305 7214
> 
> 
> -----Original Message-----
> From: Eric Osborne [mailto:eosborne@cisco.com] 
> Sent: Monday, August 05, 2002 2:19 PM
> To: Gray, Eric
> Cc: 'mkumar@aplion.stpn.soft.net'; Mpls (E-mail)
> Subject: Re: Layer 3 Lookup in ATM LSR's
> 
> On Mon, Aug 05, 2002 at 01:59:06PM -0400, Gray, Eric wrote:
> > Mayank,
> > 
> > 	The standards say that an ATM LSR does have L3
> > lookup capability.  Specifically, RFC 3035 says about
> > ATM LSRs: 
> > 
> > " ... ATM switches run network layer routing algorithms 
> >   (such as Open Shortest Path First (OSPF), Intermediate
> >   System to Intermediate System (IS-IS), etc.), and their 
> >   data forwarding is based on the results of these routing 
> >   algorithms.  No ATM-specific routing or addressing is 
> >   needed.  ATM switches used in this way are known as 
> >   ATM-LSRs (Label Switching Routers)."
> 
> To me, that doesn't say L3 lookup capability.  I could certainly see
> an ATM LSR implementation that used L3 for two things:
> - maintaining IGP adjacencies
> - deciding which interfaces to signal on-demand labels
> 
> ...but none of that implies that an ATM LSR should be prepared to
> forward received data packets.
> 
> > 
> > I am not sure what standards you were referring to.
> > 
> > If an ATM Switch is to participate in label switching, it
> > must be able to unambiguously determine how to forward 
> > labeled packets.  It is possible to implement ATM switches
> > that participates in label switching without themselves
> > participating in routing by ensuring that the determination
> > on how to forward labeled packets is made for them.  As an
> > example, this might be done by always using strict explicit 
> > routing relative to the ATM switch in question and immediate 
> > downstream neighbors.  Such ATM switches are not ATM LSRs.
> 
> I disagree; I think there's a difference between "L3 lookup" (which to
> me implies forwarding) and simply running a local IP stack for things
> which originate/terminate on that router.
> 
> 
> 
> eric
> 
> > 
> > Eric W. Gray
> > Systems Architect
> > Celox Networks, Inc.
> > egray@celoxnetworks.com
> > 508 305 7214
> > 
> > 
> > -----Original Message-----
> > From: Mayank Kumar [mailto:mkumar@aplion.stpn.soft.net] 
> > Sent: Monday, August 05, 2002 1:32 PM
> > To: Mpls (E-mail)
> > Subject: Layer 3 Lookup in ATM LSR's
> > 
> > hi
> > i have a query which , you all might be able to answer:-
> > 
> > it is said that atm switches in the core atm networks do not have layer 3
> > lookup capability.
> > Now the standards say that when atm lsr's do not have a layer 3 lookup
> > capability then they operate in Ordered control with down stream on demand
> > mode of label distribution.
> > 
> > lets say that i have the following mpls domain :-
> > 
> > a--------b---------c-------d
> >           |
> >           |-------k
> > 
> > 'b' and 'c' are atm switches with no layer 3 lookup capabilities and are
> > operating in Ordered Control with DownStream on Demand Label Distribution
> > method.
> > Suppose a lsp has to travel the path 'a'----'b'---'c'---'d'
> > 
> > 'a' requests a mapping for a particular fec from 'b'. 'b' upon receving a
> > label request for a particular fec , sees that it has no downstream
> mapping
> > for that fec. Since 'b' is operating in ordered control , it further sends
> a
> > label request to c (and not to k) since it cannot allocate a label for the
> > requested fec . 'c' would also further send the label request to 'd' and
> > then 'd' would reply with a label mapping message.
> > 
> > I hope that it's correct till this point.
> > 
> > My question is, how does 'b' know , that in order to send a label request
> > for a particular fec, what is the next hop for that fec when it does not
> > have a layer 3 capability.
> > why would 'b' send a label request to 'c' and not to 'k' ????
> > 
> > 
> > i hope i have made my point  clear
> > 
> > 
> > Please tell me where am i wrong
> > 
> > thanks and regards
> > mayank
> > 
> > 
> >