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

[Toerless Eckert <[e-mail]>]

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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> 
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