Subject: RE: Layer 3 Lookup in ATM LSR's

From: "Gray, Eric" <[email-masked]>
Date: Mon, 5 Aug 2002 13:59:06 -0400

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)."

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.

Eric W. Gray
Systems Architect
Celox Networks, Inc.
[email-masked]
508 305 7214

-----Original Message-----
From: Mayank Kumar [mailto:[email-masked]] 
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

---

From: Eric Osborne <[email-masked]>
Date: Mon, 5 Aug 2002 14:19:23 -0400

>   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.

> 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

---

From: "Mayank Kumar" <[email-masked]>
Date: Tue, 6 Aug 2002 00:15:18 +0530

hi Eric,
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 ???
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.
--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."

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.

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 ???

thanks and reagards
Mayank

-----Original Message-----
From: Gray, Eric [mailto:[email-masked]]
Sent: Monday, August 05, 2002 11:29 PM
To: [email-masked]'
Cc: Mpls (E-mail)
Subject: RE: Layer 3 Lookup in ATM LSR's

Mayank,

Eric W. Gray
Systems Architect
Celox Networks, Inc.
[email-masked]
508 305 7214

-----Original Message-----
From: Mayank Kumar [mailto:[email-masked]]
Sent: Monday, August 05, 2002 1:32 PM
To: Mpls (E-mail)
Subject: Layer 3 Lookup in ATM LSR's

hi

thanks and regards
mayank

---

From: Eric Osborne <[email-masked]>
Date: Mon, 5 Aug 2002 14:57:47 -0400

> What advantages does it provide ???

DoD ordered means 
	- you only get a label (VPI/VCI) when you need one
	- you know that when you get one, things are all set to go
downstream of you

> "MPLS And VPN architectures" by  Ivan Pepelnjak and Jim Guichard.

...which isn't a book that pretends to be particularly authoritative
on ATM LSR stuff, IIRC..:)

> been allocated . Label allocation in atm  swicthes is thus Ordered control."

This is really explaining how things are done, not why they're done
that way.  I recommend you read the RFCs that are out there (rfc3031)
to get another perspective on the different modes.

> already have a corresponding downstream label allocated"

Not sure; I think it means "go away, this is a frame-mode book"...:)

> Ordered. What advantage does that provide ???

The advantage of Ordered is that if you receive a label binding
upstream, you are 100% certain that the downstream forwarding topology
is set up, and that if you forward packets down that LSP, they will
arrive at their destination (modulo congestion, etc), and that you
don't have to worry about needing to do a route lookup in an ATM
cloud.  But I think you might be reading too much into it.

eric

---

From: David Charlap <[email-masked]>
Date: Mon, 05 Aug 2002 15:02:43 -0400

> 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.

> 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.

> 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.

> 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.

> 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

---

From: "Mayank Kumar" <[email-masked]>
Date: Tue, 6 Aug 2002 01:40:38 +0530

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: [email-masked] [mailto:[email-masked]]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

-- David

---

From: David Charlap <[email-masked]>
Date: Mon, 05 Aug 2002 16:29:00 -0400

> the Ordered control do here.

I think Eric put it best.  You don't want to advertise a label mapping 
if you can't forward traffic arriving on that label.  If you don't have 
a downstream label yet, then you can't forward it as labeled traffic. 
If you're lookup capable, you can pop the label and forward based on the 
IP header.  If you're not, then you have no choice but to drop traffic 
until the downstream label arrives.

It isn't nice to your ingress nodes to give them labels that will result 
in dropped traffic.  Those nodes would probably prefer to forward the 
packet through some other means until the LSP is completely up.

>  have no further downstream label for the requested destination). Therefore,

I think that's a roundabout way of saying what I (and Eric) just said.

> protocol packets and Signalling protocol packets)

Absolutely.  You couldn't run a signaling protocol without these three 
capabilities (a routing table of some form, and the ability to transmit 
and receive unlabeled packets.)

> the outgoing interface determined using the routing table.)

That would be correct.  But it would be unreasonable to expect such 
forwarding capability to be robust enough to use for the data plane. 
Especially when people are selling switches with dozens of high-speed 
(like OC-12 and higher) interfaces.  No software-based forwarder could 
keep up with a load like that.

> --only edge lsr's are able to receive ip packets in the data plane.

Yes.  But "edge" can be a fuzzy term.

If a transit router should pop a label (perhaps an LDP-signaled LSP ends 
in the middle of the cloud), the next router will receive an unlabeld 
packet.  It may choose to encapsulate that packet in another label 
header and forward it through a second LSP.  From the standpoint of 
those two routers where this happened, they are edges.

> --only frame based lsr 's are able to forward ip packets in the data plane

I wouldn't go that far.  There are hop-by-hop routers with ATM 
interfaces.  And there are intelligent lookup-capable interfaces for 
some ATM switches.

Don't blur the lines between legacy implementations, current 
implementations, future implementations, and theoretical possibility.

It is true that legacy ATM switches can't do the kind of IP lookup 
necessary for data-plane forwarding.  But it is also true that there are 
modern switches with cell-based backplanes that can do IP lookup and 
forwarding.  Your use of the word "only" sounds like you're trying to 
draw a line in the sand somewhere, when in reality, that line is always 
in motion.

-- David

---

From: "Mayank Kumar" <[email-masked]>
Date: Tue, 6 Aug 2002 10:46:09 +0530

Hi
Thanks David for ur comments. They seem correct.  About my last line that
only frame based lsr's are capable of ip forwarding  in the data plane, when
u are talking  about atm interfaces capable of doing ip packet forwarding ,
then they are actually not operating in the cell mode but in frame mode.
Thus frame based lsr's include atm swicthes operating in frame mode (ie
receiving an ip packet as cells , reassembling it into a ip packet, doing a
routing table lookup and then forwarding it as cells ).

Also please see inline with prefix MAY for more :-

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of David
Charlap
Sent: Tuesday, August 06, 2002 1:59 AM
To: IETF MPLS List
Subject: Re: Layer 3 Lookup in ATM LSR's

MAY:does all the above amount to a good reason that why Atm lsr's shouldnt
respond to a label allocation request unless they have a corresponding
downstream mapping ????

MAY: can we differentiate between the following . I mean are these three
capabilities different:-
-- ability to orginate ip packets
-- ability to receive ip packets
-- ability to forward ip packets

can an implementation  only have the first two and not the second one.I
think that if its possible then we can say that atm swicthes do not have
layer 3 forwarding capability in either the control plane or the data plane.
Because in the data plane they do cell swicthing and in the control plane
they either receive routing protocols packets destined locally or signalling
protocol packets destined locally., or they orginate one but never forward
any. But then i doubt how do targetted hellos would get forwarded  ???

cisco routers allow u to configure an atm interface as either operating in
cell mode or frame mode. however cisco switches do not allow an atm
interface to operate in frame mode and cisco routers any way have layer 3
lookup and forwarding capability in the data plane.
What i want to say that atm swicthes do not have layer 3 forwarding
capability in the data plane because they alsways swicth cells and never
operate in frame mode

MAY:  u are write . Ok i agree that's true ;-)
thanks

-- David

---

From: Giles Heron <[email-masked]>
Date: 06 Aug 2002 14:49:55 +0000

just one nit...

On Mon, 2002-08-05 at 19:02, David Charlap wrote:
[snip]
> result before forwarding.  This is beyond the capability of most ATM 
[rest snipped]

this isn't strictly, true - is it?

in some cases you may pop the stack but then forward the IP payload
without actually performing an IP lookup (e.g. PHP).

Giles

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