The MPLS WG Archive

["Kavi, Prabhu" <prabhu_kavi@tenornetworks.com>]

Eric,

I interpreted David's statements to mean that you can get 
completely bounded delay with variable size packets.  David,
correct me if I am wrong.

For example, a commonly used MTU of 1500 bytes requires only
5 microseconds of transmission time on an OC-48.  Therefore
it is possible to create a queueing mechanism where a high
priority packet is at most 5 microseconds away from the 
start of transmission. Queuing points in the rest of the 
system (if any) can be handled in a similar fashion.

Implementations of these systems might not be as far away
as you think.

Prabhu


> -----Original Message-----
> From: Eric Gray [mailto:eric.gray@sandburst.com]
> Sent: Friday, April 20, 2001 8:39 AM
> To: David Charlap
> Cc: mpls@UU.NET
> Subject: Re: ATM-LSR do they use OSPF/IS-IS or PNNI??
> 
> 
> David,
> 
>     I'm sure some smart person or another has written a paper
> on this, but I don't have the time to do a full-blown literature
> search right now.  While what you are saying is technically
> true, I believe it has very little applicability in practice.
> You can see that this works by imagining that an implementation
> might treat MTU propagation time exactly the same way that ATM
> treats cell propagation time.
> 
>     But this is very ugly.  It is so coarse that many would be
> hard pressed to consider it a useful guarantee.  It also seems
> to require deliberately inserting latency (N times an MTU
> propagation time) for all affected traffic (to allow for packet
> shuffling to accomodate the guarantee).  And it would be
> very difficult to allocate forwarding resources efficiently and
> still deliver guaranteed delay variation deterministically.
> 
>     Without making any moral judgement about why it is so,
> many people are more concerned about having a bounded
> delay than they are about having a bounded delay variation.
> And with a bounded delay, you get bounded delay variation
> for free.
> 
>     My opinion: there are two models here and trying to make
> technologies support both models may not be a good idea.
> 
> --
> Eric Gray
> 
> You wrote:
> 
> > Eric Gray wrote:
> > >
> > >     You can't exactly do ATM style QoS over non-ATM media.
> > >
> > >     The thing that allows an ATM switch to contemplate delay
> > > variation separately from delay is the fact that ATM switches
> > > switch data in fixed size chunks.  When the size of data chunks
> > > can vary, the only practical way to control delay variation is to
> > > try to control delay.  Therefore, with IP packets on non-ATM
> > > media, you can't even begin support two knobs for delay and
> > > delay variation.  So why would you undertake to add complexity
> > > by trying?
> >
> > Variable-size frames don't make it impossible.  It just 
> means you are
> > forced to work with a coarser granularity (defined by your 
> configured
> > MTU size.)
> >
> > -- david
> 
>