The MPLS WG Archive

["Mayank Kumar" <mkumar@aplion.stpn.soft.net>]

hi david
u have been repeatedly talking about the resource exhaustion problem. I
think that is taken care off by the Down stream on demand mode. What does
the Ordered control do here.
Do u agree with the author of the Book when he says
" this action would require the downstream
 lsr to have layer 3 lookup capability (such as if the downstream lsr would
 have no further downstream label for the requested destination). Therefore
,
 the atm swicthes never respond to a label allocation request unless they
 already have a corresponding downstream label allocated"

the author says that in a certain scenario, the atm lsr might require layer
3 lookup capability (probably in the data plane) , therefore it never
responds to a label allocation request unless it already has a corresponding
downstream mapping, and therefore is able to avoid layer 3 lookup.


I think , that all these discussions conclude to the following:-


--all lsr 's whether atm lsr's or frame lsr's , have layer 3 lookup
capability , which means to be able to determine the reachability of a
particular destination prefix by looking into a routing table (to determine
the next hop and output interface) . This also means that all lsr's have a
routing table too.
--all lsr's are able to orginate ip packets in the control plane (routing
protocol packets and Signalling protocol packets)
--all lsr's are able to receive ip packets in the control plane (routing
protocol packets and Signalling protocol packets)
--some lsr's might be able to forward ip packets in the control plane (for
eg an atm lsr might forward ip packets using its control vc by reassembling
an ip packet doing a routing table lookup and then determining the outgoing
interface and then further segmenting the cells again back when sending out
the outgoing interface determined using the routing table.)
--only edge lsr's are able to receive ip packets in the data plane.
--only frame based lsr 's are able to forward ip packets in the data plane


is it correct ???


i think that the author of the cisco book when saying layer 3 lookup
capability means having a layer 3 lookup capability in the Data plane which
most swicthes dont have as they forward based on the cell header. If a atm
lsr responded to  a label request  without it having a corresponding
downstream label binding , then it might start receiving labelled packets
for the said fec and because by that time if it does not have a downstream
label assigned, it may need to do a ip lookup of the received labelled
packet (after popping its label) in order to forward that packet (layer 3
lookup and forwarding in the data plane required) or it may choose to drop
it.

please tell me if i am correct ??




thanks and regards
mayank










-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of David
Charlap
Sent: Tuesday, August 06, 2002 12:33 AM
To: IETF MPLS List
Subject: Re: Layer 3 Lookup in ATM LSR's


Mayank Kumar wrote:
>
> then why is it recommended that in atm backbones running mpls,we should
use
> Downstream on Demand  with Ordered LSP Control.
> What advantages does it provide ???

ATM switches don't have a concept of the "per platform" label space.
ATM forwarding hardware is designed around ATM capabilities, which only
allocated VCIDs on a per-interface basis.  multipoint-to-point
connections may not be possible either.

This can quickly lead to resource exhaustion on the line cards and
switching fabric, since each interface would need to allocate a label
for evey IGP prefix, and you'd need to create fabric connections for
every interface/prefix combination - which can be tremendous.

> My doubt arose because i have been reading a book from cisco press named
> "MPLS And VPN architectures" by  Ivan Pepelnjak and Jim Guichard.
> 	the book on pg 57 says that
> "the label allocation and distribution process accross the atm-lsr domain
> has the following characteristics:-
> --Label allocation in devices with layer 3 lookup capabilites (routers )
is
> done regardless of whether the router has already received a label for the
> same prefix from its next hop router . Label allocation in routers is thus
> called independent control.

This description is assuming connectionless hardware.  A connection-
oriented layer-3 router that doesn't support per-platform label spaces
and multipoint-to-point connections can hit the same resource exhaustion
problems that an ATM switch is likely to hit.

> --label allocation in devices with no layer 3 lookup capabilites  (atm
> switches) is performed only if a corresponding downstream label has
already
> been allocated . Label allocation in atm  swicthes is thus Ordered
control."

See above.  They're talking about layer-3 hardware-forwarding
capabilities, not a lack of ability to receive an IP packet.

On-demand/ordered operation is better for these kind of switches because
you only create connections that are explicitly requested, which will
usually be less than the product of all prefixes and all interfaces.
This behavior comes at the cost of longer convergence times.

> the book further says that "the downstream lsr could simply allocate a
label
> and respond to the request from the upstream lsr with a corresponding
reply
> message. Under some circumstances , this action would require the
downstream
> lsr to have layer 3 lookup capability (such as if the downstream lsr would
> have no further downstream label for the requested destination). Therefore
,
> the atm swicthes never respond to a label allocation request unless they
> already have a corresponding downstream label allocated"
>
> What does that mean ???
>
> And do u want to say that an LSR always has a layer 3 lookup capabilites
> irrespective of whether it is an atm lsr or frame based lsr.

No.  You're confusing what "lookup capable" means.

A lookup capable router (or interface) is one that can receive an
unlabeled packet, determine the next-hop (which may be into an LSP),
push an appropriate label, and forward the packet to that next-hop.
With modern equipment, this mean that this is done in hardware, since no
processor is fast enough to do this when there are a lot of high-speed
interfaces.

Lookup capability is also needed for a router (or interface) that must
pop the label stack.  It must do a label-lookup (which all switches can
do), but it must then pop the stack and then do an IP lookup on the
result before forwarding.  This is beyond the capability of most ATM
forwarding hardware.

> I think Eric is right in his reply ???
> I think that Layer 3 Lookup implies , being able to process the IP header
> while forwarding packets and thus being able to make a forwarding decision
> based on the destination ip address. Atm swicthes do form routing
> adjacencies on the control vc but they dont have layer 3 lookup
capability.
>
> Why does the Book say then , that label allocation in atm swicthes is
> Ordered. What advantage does that provide ???

See above.  It helps to prevent the resource exhaustion that will happen
if you have a lot of prefixes and a lot of interfaces, wihtout a
per-platform label space or mp2p connection.

-- David