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

Hi Les,

Thanks for the summary and citations. 

To my understanding, although DSCP based steering could be used in multi-topology scenarios, such usage is not defined in IETF specifications. Actually there can be many ways of choosing which topology is used for the forwarding of a particular packet. Thus the relationship between DSCP and MT is not that tightly coupled. 

Best regards,
Jie

> -----Original Message-----
> From: Lsr [mailto:lsr-bounces@ietf.org] On Behalf Of Les Ginsberg (ginsberg)
> Sent: Thursday, November 15, 2018 12:41 PM
> To: Toerless Eckert <tte@cs.fau.de>; lsr@ietf.org
> Subject: Re: [Lsr] LSR: Using DSCP for path/topology selection Q
> 
> 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 <lsr-bounces@ietf.org> On Behalf Of Toerless Eckert
> > Sent: Wednesday, November 14, 2018 6:29 PM
> > To: lsr@ietf.org
> > 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
> >
> > _______________________________________________
> > Lsr mailing list
> > Lsr@ietf.org
> > https://www.ietf.org/mailman/listinfo/lsr
> 
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