[e2e] traffic dispersion and blocking probality

Curtis Villamizar curtis at workhorse.fictitious.org
Mon Apr 1 12:52:18 PST 2002

In message <39469E08BD83D411A3D900204840EC557630AE at vie-msgusr-01.dc.fore.com>, 
"Naidu, Venkata" writes:
>   I got to disagree with Bob - *Blocking probability*
>   indirectly signifies/derived from either loss probability or
>   impending congestion probability (regardless of
>   Layer 3 connection oriented or not). Almost all of
>   the datalink/physical layers are connection oriented
>   in that sense and there do exists some loss probability
>   as time varying function. The intelligent decision 
>   (*end-to-end path*) should depend upon such links/hops 
>   properties also.
>   You may want to look at the latest OSPF-OMP draft
>   and complete description of OMP applicability to
>   different areas (OSPF/ISIS/MPLS) in the official
>   OMP site (below).
>   http://www.fictitious.org/ospf-omp/ospf-omp.txt
>   http://www.fictitious.org/omp/
> Venkata.
> -> you might try to locate an old, expired, internet-draft by 
> -> Curtis Villamizar:
> -> 
> -> 	draft-ietf-ospf-omp-02.txt
> -> 
> -> at the time, Curtis had versions for various routing 
> -> protocols and, if i 
> -> remember correctly, mpls.  unfortunately, his attention was directed 
> -> elsewhere, and no one else has taken up this work.
> -> 
> -> cheers, Greg Minshall

Venkata, Greg,

Grumpy response follows.  Don't take it personally.

Blocking probability as an issue is a thing of the past for any well
designed network.  Unless we are discussing poorly designed networks,
or protocols there is no reason to talk about it.

The reason for this is twofold.  First IP was designed to work in the
presense of congestion and temporary congestion is not a problem.
Second, running out of bandwidth due to a transient (for example, a
link failure) *used to* result in blocking, but now it results in
temporary congestion.  It is better to oversubscribe and use MPLS
adaptivity to remove the congestion in the layout if possible after
the initial layout.  If it is not possible to remove the congestion it
is absolutely unacceptable in a the core of an ISP to not bring up an
LSP between city pairs.  A really good implenentation of adaptivity
would react faster when utilizations are high and/or there are
significant gains to be made moving LSPs and slow when there is not
much to gain, thereby reacting quickly yet stabilizing.  Using CAC
with MPLS is particulary a bad idea if QoS capability is available
(preferred traffic doesn't see the transient congestion at all).

Call blocking was relevant when we talking about calls over TDM.
Blocking was relevant when talking about SVCs or SPVCs over ATM.
That's one reason ATM is all but gone.  Blocking probability is no
longer a relevant topic in computer networks.  (IMHO, of course).

btw - There hasn't been commercial interest in OMP so it has been
idle.  For some ISPs dynamic is scary since getting an algorithm wrong
could have dire consequences and determinism in terms of where traffic
should be going is valued for operational simplicity (where to look
when you can't get from here to there is more straightforward).  That
could change, but so far it hasn't.  The next step for OMP would be to
implement and if there is commercial interest, I'll be happy to do so.


ps - feel free to discuss blocking probability anyway if you're in
love with the idea just don't bring me into it.  :-)

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