Re: [Lsr] LSR: Using DSCP for path/topology selection Q

[Toerless Eckert <[e-mail]>]

On Fri, Nov 16, 2018 at 12:28:27AM +0100, Robert Raszuk wrote:
> Hey Toerless,
> 
> Please observe that DSCP based steering may be very useful vehicle for end
> hosts/applications influencing how their packets are transported.
> 
> So instead of closing that option I recommend we at least take a good look
> at what alternative mechanism exists.

I think back when DSCP was selected for MTR in implementations, it was
done so due to the absence of better options. But as i mentioned, the
overloading of DSCP between QoS and routing made it (in my memory)
undesirable to operators. And when designing new solutions with new QoS
like in DetNet its yet a similar possible problem.

If we hopefully do not need to find solutions for IPv4, i would rather
like to see something like what SR did, e.g.: IPv6 header extension. These
are AFAIK now also easily settable from linux programs (ok., maybe not
yet from javascript). Or much easier of course the existing IPv6
multi-addressing solutions where hosts can also lern semntics of these
addresses. And map to diffeent flex-topologies instead of just
multi-homing exit points (which i think was the main use-case so far).

> And btw I read Peter's note as possibility (or invitation) to define
> algorithm which takes into account DSCP rather then a announcement that
> this is not there and it should never be.

Sure, i am only talking about the solutions that tried to use DSCP for
routing so far. I think those failed. And when other agree and we codify
that, then that would not exclude the option for new work (like what
Peter may have in mind) to superceed that recommendation. 

Cheers
    Toerless

> Cheers,
> R.
> 
> 
> 
> On Fri, Nov 16, 2018 at 12:22 AM Toerless Eckert <[e-mail]> wrote:
> 
> >
> > Thanks, Les, Peter
> >
> > So... is there any opinion about creating a normative or BCP
> > recommendation to
> > not use DSCP to distinguish topologies/flex-algos ? Mabybe this would
> > not be appropriate for LSR, but TSVWG, but i think it would be
> > participants in LSR that would know if there is actually still any
> > customer demand for this option.
> >
> > Cheers
> >     Toerless
> >
> > P.S.: Context of the email is DetNet trying to define what a DetNet flow
> > is and currently this includes the thinking that DetNet flows could be
> > 6-tuple flows including DSCP because different DSCP could be routed
> > differently in the network via MTR, and that concept IMHO would just
> > result in making a foal (DetNet) a lot more complex because of a dead
> > horse (DSCP MTR).
> >
> > On Thu, Nov 15, 2018 at 04:41:06AM +0000, Les Ginsberg (ginsberg) wrote:
> > > Toerless -
> > >
> > > It's pretty hard to understand the context for your email.
> > >
> > > What leads you to believe that any of the MT specifications you mention
> > say anything normative about DSCP and topologies??
> > >
> > > RFC4915 does not mention DSCP at all - but does make the statement:
> > >
> > > https://tools.ietf.org/html/rfc4915#section-3.8
> > > "It is outside of the scope of this document to specify how the
> > >    information in various topology specific forwarding structures are
> > >    used during packet forwarding or how incoming packets are associated
> > >    with the corresponding topology."
> > >
> > > RFC 5120 does mention DSCP, but only as an example of something that
> > "could" be used to determine on what topology a packet should be forwarded.
> > >
> > > RFC 7722 also mentions DSCP as an example, but there is a statement in
> > Section 3:
> > >
> > > "It is assumed, but
> > >    outside the scope of this specification, that the network layer is
> > >    able to choose which topology to use for each packet"
> > >
> > > IGP WGs have never attempted to recommend (let alone normatively define)
> > any relationship between DSCP and MT.
> > >
> > > ???
> > >
> > >    Les
> > >
> > > > -----Original Message-----
> > > > From: Lsr <[e-mail]> On Behalf Of Toerless Eckert
> > > > Sent: Wednesday, November 14, 2018 6:29 PM
> > > > To: [e-mail]
> > > > Subject: [Lsr] LSR: Using DSCP for path/topology selection Q
> > > >
> > > > Whats the current best guidance on using DSCP for selection of path,
> > > > specifically for selection of topology with MTR (RFCs 4915, 5120,
> > 7722) ?
> > > >
> > > > My understanding from history is that this looked like a good idea
> > > > to customers first, but when implementations became available,
> > > > customers really did not want to implement it because of the
> > overloading
> > > > of DSCP between QoS and routing and the resulting management
> > > > complexity.
> > > >
> > > > Has the idea of using DSCP for path selection been re-introduced in any
> > > > later work like flex-Algos ?
> > > >
> > > > If there could be rough consensus that this is in general a bad idea,
> > i wonder
> > > > if it would be appropriate to have a short normative document from LSR
> > > > defining that the use of DSCP for topology selection is historic and
> > > > not recommended anymore and make this an update to above three RFCs,
> > > > maybe also pointing out that there are other methods to select a
> > > > topology and those remain viable:
> > > >
> > > > I specifically would not like to see the actual MTR RFCs to be changed
> > > > in status, because MTR actually does work quite well and is supported
> > > > in products and deployed with IP multicast, even with multiple
> > > > topologies for IP multicast in live-live scenarios.
> > > >
> > > > Thanks!
> > > >     Toerless
> > > >
> > > > _______________________________________________
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> > > > [e-mail]
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> > >
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