The MPLS WG Archive

["Anu Divya" <anudivka@rediffmail.com>]

Below mail thread discusses regarding the Traffic Engineering 
Extension of OSPF and CSPF.

This discussion concludes that the Route selection through the 
network is performed using only the Unreserved Bandwidths for 
Class-Types.

The Maximum Reservable Aggregate Bandwidth (which can be thought 
of as the Maximum Reservable Bandwidth for Class-Type 0) is 
advertized for historical reasons, i.e. original IGP extensions 
for TE included it and we didn't want to simply remove it as our 
extensions are built on the original extensions.

If we are going to use only Unreservered bandwith (for 
class-types) during Route calculation, what is the use of Maximum 
bandwidth in CSPF. Is this TLV also retained for historical 
reasons like Maximum Reservable Bandwidth?


Question on BW advertisements for Diffserv aware TE based on Class 
Type
 From: "Sudhakar Ganti" <sganti@tropicnetworks.com>
Date: Wed, 16 May 2001 10:01:26 -0400
Cc: "mpls" <mpls@UU.NET>
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Hi Darek,

Thanks for your reply. If the Maximum Reservable Bandwidth for 
Class Type-N
is implicitly inferred, then I think the text in both these drafts 
should not refer to this
as it is implementation / configuration specific.

Second thing is that, it may be necessary to advertise this value. 
It gives
an option for Traffic Engineering not to take paths that are 
overbooked for a
given class if necessary. We may not be able to infer this from 
Unreserved
bandwidth. If not, is there any other way to know about it? 
Thanks

-Sudhakar
-----Original Message-----
 From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of 
Darek Skalecki
Sent: Wednesday, May 16, 2001 8:53 AM
To: Sudhakar Ganti
Cc: mpls
Subject: Re: Question on BW advertisements for Diffserv aware TE 
based on Class Type


Hi Sudhakar,
See inlined answers.

Thanks,

Darek


Sudhakar Ganti wrote:

Hello,
A couple of questions regarding the BW advertisements
on these drafts:
     draft-ietf-mpls-diff-te-ext-01.txt
     draft-ietf-ospf-diff-te-00.txt

a) The text in both the drafts refers to "the Class-Type N
    bandwidth currently unreserved (i.e. the difference
    between the Maximum Reservable Bandwidth for Class-Type N
    and the bandwidth reserved by existing Class-Type N LSPs)"

However I do not see any separate sub-TLV for Max reservable
bandwidth for each class type (class type N). There is only one
that advertises the Max Reservable Aggregate Bandwidth.

The Maximum Reservable Bandwidth for Class-Type N is reflected in 
the Unreserved Bandwidth for Class-Type N. That is, there is no 
need to advertize through IGP the Maximum Reservable Bandwidth for 
Class-Type N as it is implicitely advertized  in the Unreserved 
Bandwidth for Class-Type N. The intention is that the Maximum 
Reservable Bandwidth for Class-Type N is configured on links that 
support Class-Type N. However, when it is time to advertize 
bandwidth availability for that link, only the Unreserved 
Bandwidth for Class-Type N is computed, as you stated by quoting 
the draft, and then advertized through IGP. Route selection 
through the network is then performed using only the Unreserved 
Bandwidths for Class-Types.

The Maximum Reservable Aggregate Bandwidth (which can be thought 
of as the Maximum Reservable Bandwidth for Class-Type 0) is 
advertized for historical reasons, i.e. original IGP extensions 
for TE included it and we didn't want to simply remove it as our 
extensions are built on the original extensions.

b) Different classes can be oversubscribed differently
    depending upon the link (or network) configuration.
    If so, I think it may be necessary to advertise the
    Maximum Reservable bandwidth for each Class (Class-Type N).

    Is there any reason why this is omitted??

See my answer to a). Since Maximum Reservable Bandwidth for 
Class-Type N is implicitely "included" in the Unreserved Bandwidth 
for Class-Type N and it is the Unreserved Bandwidth for Class-Type 
N that is advertized through IGPs and used for route computation, 
there is no need to separately advertize Maximum Reservable 
Bandwidth for Class-Type N. To oversubscribe a link, the sum of 
Maximum Reservable Bandwidths for all Class-Types supported by the 
link needs to be greater than the capacity of the link. As I 
already mentioned, each Maximum Reservable Bandwidth for 
Class-Type N is configured on the link and used in the computation 
of the Unreserved Bandwidth for Class-Type N for advertizing 
purposes.

-Sudhakar

--
Darek Skalecki
Nortel
(613) 765-2252




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