Subject: do we need to have disjoint label space for cr-ldp and rsvp-te

From: "Mayank Kumar" <[email-masked]>
Date: Mon, 8 Jul 2002 16:13:30 +0530

hi
1: i wanted to know that do we need to support disjoint label spaces for
cr-ldp and rsvp-te in case of their simultaneous operations.
If yes , then what is the reason for this ???

2:is there a specific reason for having interface specific label space for
atm and frame relay ????
What is the most basic reason for going for interface specifc label space
???
can a interface like atm which supports interface specifc label space , be
not assigned a label from the platform wide label space.

thanks
and regards

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From: "Amarendra" <[email-masked]>
Date: Mon, 8 Jul 2002 19:18:14 +0530

Please read inline comments.

----- Original Message -----
From: "Mayank Kumar" <[email-masked]>
To: <[email-masked]>
Sent: Monday, July 08, 2002 4:13 PM
Subject: do we need to have disjoint label space for cr-ldp and rsvp-te

> If yes , then what is the reason for this ???
***: No you don't need to have disjoint label space for different label
distribution protocols, if you have a central entity responsible for
assigning different labels to different protocols. Otherwise you need to.
> 2:is there a specific reason for having interface specific label space for
***:In the cassse of ATM and Frame realy we encapsulate label in L2 header,
e.g in the case of ATM we use VPI/VCI field of ATM header for label.
>** No.

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From: "Mayank Kumar" <[email-masked]>
Date: Mon, 8 Jul 2002 19:42:06 +0530

Hi
got answer for my first question:-
but then the answer to my second ques, does not give me a good reason. Why
is it necessary to have interface specific labels for atm or frame relay???
how does it matter that , we have the label encapuslated in the L2 header in
case of atm or frame-relay.Am i missing something in saying that ,we can
always assign a label from the platform wide label space , as long as that
label confirms to be a valid dlci (for frame-relay) or a valid vpi/vci
air( for atm).

regds
Mayank

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From: David Charlap <[email-masked]>
Date: Mon, 08 Jul 2002 10:59:29 -0400

> label confirms to be a valid dlci (for frame-relay) or a valid vpi/vci

The nature of ATM is to assign VCIDs on a per-interface basis.  No 
existing ATM switch supports the concept of per-platform VCIDs, because 
doing so would break all of the ATM protocols.

Since ATM switches, and by extensions ATM interfaces, all use 
per-interface VCIDs, it makes no sense to specify per-platform labels 
for these interfaces.  You'll just end up specifying something that will 
never be implemented.

I assume a similar argument holds for FR.

And, as was already said, label spaces for ATM and FR are fundamentally 
different from shim-label label spaces.  Shim-header label spaces use a 
20-bit integer for a label.  FR uses two different size DLCIs, neither 
of which is 20 bits - and there are per-interface range limits.  ATM 
uses the two-dimensional VPI/VCI values, where VPI may be two different 
widths, and there are per-interface range limits on each dimension.

Trying to combine all three of these incompatible technologies into one 
huge pool of labels is asking for disaster.

-- David

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From: "Mayank Kumar" <[email-masked]>
Date: Mon, 8 Jul 2002 21:04:57 +0530

Hi
ur argument makes sense. Thank you . But i do not understand what do you
mean by VCID's. Is it some combination of VPI and VCI . The CISCO shows some
vcid kind of thing in its 'show atm vc' command. When u say that VCID's are
allocated on aper interface basis , do u mean that vcid's are unique for a
specific interface only and thus there can be two identical VCID's on two
different atm interfaces or for that matter ports. Then it makes sense to
not assign atm vpi/vci (labels) from a platform wide label space.
Also then i can infer one more thing that, the platform wide label space
contains only shim-labels (that means the generic encapsulation specified in
rfc 3032). Also  the pw wide label space contains just the 20 bit labels .
It does not make sense that the label space contain the 3 bit EXP and the S
and the TTl fields too.

regds
Mayank

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of David
Charlap
Sent: Monday, July 08, 2002 8:29 PM
To: IETF MPLS List
Subject: Re: do we need to have disjoint label space for cr-ldp and
rsvp-te

-- David

---

From: David Charlap <[email-masked]>
Date: Mon, 08 Jul 2002 12:17:42 -0400

Mayank Kumar wrote:
> mean by VCID's. Is it some combination of VPI and VCI.

Yes.  It's the combined VPI/VCI value that represents a single ATM 
connection.

> When u say that VCID's are
> different atm interfaces or for that matter ports.

Yes.  The same VCID can exist on two different ports.  When this 
happens, there is absolutely no relationship between the two connections.

> rfc 3032). Also  the pw wide label space contains just the 20 bit labels .

This is my understanding.  If platform-wide labels are implemented on a 
platform, then they are used only on interfaces where shim headers are 
used to store the label.  And shim headers only store 20-bit labels.

> It does not make sense that the label space contain the 3 bit EXP and the S

EXP and TTL fields are not part of the labels.  While your forwarding 
hardware may manipulate all three of these, they serve different purposes.

Labels serve to identify the route from ingress to egress (and may also 
identify QoS, in the case of an IntServ LSP or a DiffServ L-LSP.)

The EXP bits serve to identify the QoS in the case of a DiffServ E-LSP.

The TTL is decremented as packets are forwarded and only affects traffic 
flow if it decrements to zero.

-- David