RE: link capacity and reservable

["Francois Le Faucheur (flefauch)" <flefauch@cisco.com>]

Hello Dimitry,

>> -----Original Message-----
>> From: Dimitry Haskin [mailto:dhaskin@axiowave.com] 
>> Sent: 27 May 2003 17:37
>> To: Francois Le Faucheur (flefauch); Ash, Gerald R (Jerry), ALABS
>> Cc: te-wg@ops.ietf.org
>> Subject: RE: link capacity and reservable
>> 
>> 
>> Francois,
>> 
>> > My point below was about enforcing overbooking ratios on a
>> > per-CT-and-per-link basis (ie CT0 is overbooked by factor 
>> 2 on link 0
>> > and overbooked by factor 3 on link 1, while CT1 is not 
>> overbooked on
>> > link0 nor on link1). This could NOT be achieved just by 
>> combination of
>> > "LSP Size overbooking" and "Link Size Overbooking"; it 
>> could only be
>> > achieved via the use of the LOM method. But, as being 
>> discussed, it is
>> > not obvious that the nbeed for this justifies the extra complexity.
>> >
>> 
>> For my sake.. Since b/w constrains are advertised and 
>> handled per CT per TE
>> link (i.e. there are separate BCn values on each visible link in a TE
>> domain), why is LOM necessary to achieve overbooking per  CT 
>> per link?  

LOM is necessary because the BC Models we are considering enforce one
(or more) shared/aggregate constraint(s).

If we had a trivial BC model of pure independent bandwidth constraints,
then yes, each CT could operate independently and do its own computation
of how much is taken from its own BC. And we would not need LOM.

But since we have shared aggregate constraints (aka Max Reservable Bw)
you need to somehow configure what is the individual overbooking ratio
that is to be considered on that link for that CT, for the purpose of
deducting bandwidth from the shared/aggregate constraint. This is the
vectorisation issue that Waisum raised. The "euro vs dollar" question as
Wai put it.

In other words, considering MAM (with new definition), imagine you
configure BC0=1000 (probably reflecting some high overbooking) and
BC1=100 (probably reflecting some low overbooking), and
Max-Reservable-Bw=300 (probably reflecting an average overbooking on an
aggregate basis). One would not know exactly how to apply the Max
Reservable Bandwidth differntly to CT0 LSPs and CT1 LSPs in order to
compute the. For example if you have established one CT0 LSP of 500 and
one CT1 LSP of 50, how much exactly is left available to CT0 and CT1?

With the LOM solution, you configure BC0=100, BC1=50 , Max Reservable
Bw=130, LOM(0)=10 and LOM(1)=2. Then you know exactly how to apply the
Max Reservable Bandw constraints (see formulas in dste drafts) and how
to compute the available bandwidth for each CT dependeing on how much
LSPs are established acros ALL other CTs.

I suggest looking at section 6.2 of diff-te-russian where the formulas
make clear the role of LOM in various computation.

One could probably come up with solutions which are slightly different
to LOM, but the LOM solution is very attractive because, again, it is a
pure increment to the other overbooking methods which exist in TE today.

Cheers

Francois

>>Or
>> is it that link0 and link1 in your example form a some kind 
>> link bundle, i.e
>> form a single TE link as far as OSPF/ISIS TE extensions are 
>> concerned?
>> 
>> Thanks,
>>  Dimitry
>> 
>>