Re: [spring] Is srv6 PSP a good idea

Mark Smith <markzzzsmith@gmail.com> Sun, 15 December 2019 03:08 UTC

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From: Mark Smith <markzzzsmith@gmail.com>
Date: Sun, 15 Dec 2019 14:08:32 +1100
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To: Ron Bonica <rbonica=40juniper.net@dmarc.ietf.org>
Cc: "Pablo Camarillo (pcamaril)" <pcamaril@cisco.com>, "Joel M. Halpern" <jmh@joelhalpern.com>, "spring@ietf.org" <spring@ietf.org>
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Subject: Re: [spring] Is srv6 PSP a good idea
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On Fri, 13 Dec 2019, 07:50 Ron Bonica, <rbonica=40juniper.net@dmarc.ietf.org>
wrote:

> Pablo,
>
> I am not convinced the benefit derived by the ultimate segment justifies
> the price paid by the penultimate segment. Specifically,
>
> - the ultimate segment benefits because it doesn't have to skip over the
> SRH with SL == 0
> - in order for the ultimate segment to derive this benefit, the
> penultimate segment needs to remove bytes from the middle of the packet and
> update two fields in the IPv6 header
>
> As Joel said, we typically don't add options (i.e., complexity) to a
> specification unless there is substantial benefit.
>


It's also contrary to the SPRING charter:

"SPRING WG should avoid modification to existing data planes that would
make them incompatible with existing deployments. Where possible,
existing control and management plane protocols must be used within
existing architectures to implement the SPRING function."

The only time significant changes to IPv6 operation should be proposed are
if what is being tried to be done is impossible using existing mechanisms,
or there is a very significant benefit that far outweighs the cost and
consequences of changing a protocol that is decades old and already has
millions of deployments.

When significant changes to a protocol are needed or would provide
significant advantages, that's a sign that either (a) it's the wrong
protocol to try to use or (b) it's time for a new version of the protocol.

The 'man' in 6man stands for 'maintenance'. IPv6 is way past the
development stage (that was 1995 and earlier).

It seems SPRING is commonly taking a "new protocol development" approach
regarding IPv6, rather than "use an existing protocol" approach. This "new
protocol development" approach is costing a lot of time and effort due to
long debates that really shouldn't be needed if the SPRING charter was
being followed, and IPv6 was just being used rather than attempted to be
being redeveloped.

Regards,
Mark.



>                                                     Ron
>
>
>
>
> Juniper Business Use Only
>
> -----Original Message-----
> From: spring <spring-bounces@ietf.org> On Behalf Of Pablo Camarillo
> (pcamaril)
> Sent: Wednesday, December 11, 2019 3:12 PM
> To: Joel M. Halpern <jmh@joelhalpern.com>om>; spring@ietf.org
> Subject: Re: [spring] Is srv6 PSP a good idea
>
> Joel,
>
> 1.- The use-case for PSP has already been provided at the mailer. There
> are scenarios where it provides benefits to operators.
>
> 2.- The PSP behavior is optional. It is up to the operator in his
> deployment to decide whether to enable it or not at one particular router.
> Similarly, a vendor may decide not to implement it. The PSP behavior has
> been implemented by several vendors and deployed (see the srv6 deployment
> draft).
>
> 3.- A network may have PSP enabled at some nodes and not at others.
> Everything is still interoperable and works fine.
>
> 4.- PSP is not a complex operation in hardware (doable at linerate on
> existing merchant silicon).
> Example: It has been implemented and deployed on Broadcom J/J+. If I
> recall correctly Broadcom Jericho+ started shipping in March 2016! PSP is
> supported on this platform at linerate with no performance degradation
> (neither PPPS nor BW).
> Given that this is doable in a platform from more than 3 years ago, I fail
> to see how you need "very special provision" to do this.
>
> Is it really something that horrible to provide freedom of choice to the
> operators deploying?
>
> In summary, it can be implemented without any burden in hardware and
> deployment experience prove this is beneficial to operators.
>
> Thanks,
> Pablo.
>
> -----Original Message-----
> From: spring <spring-bounces@ietf.org> on behalf of "Joel M. Halpern" <
> jmh@joelhalpern.com>
> Date: Wednesday, 11 December 2019 at 03:55
> To: "spring@ietf.org" <spring@ietf.org>
> Subject: [spring] Is srv6 PSP a good idea
>
>     For purposes of this thread, even if you think PSP violates RFC 8200,
>     let us assume that it is legal.
>
>     As I understand it, the PSP situation is:
>     o the packet arrives at the place (let's not argue about whether SIDs
>     are locators) identified by the SID in the destination address field
>     o that SID is the next to last SID in the SID list
>     o that sid is marked as / known to be PSP
>     o at the intended place in the processing pseudocode, the last (first)
>     entry in the SRH is copied into the destination IPv6 address field of
>     the packet
>     -> The SRH being used is then removed from the packet.
>
>     In order to evaluate whether this is a good idea, we have to have some
>     idea of the benefit.  It may be that I am missing some of the benefit,
>     and I would appreciate clarification.
>     As far as I can tell, the benefit of this removal is that in exchange
>     for this node doing the work of removing the SRH, the final node in
> the
>     SRH does not have to process the SRH at all, as it has been removed.
>
>     I have trouble seeing how that work tradeoff can be beneficial.
>     Removing bytes from the middle of a packet is a complex operation.
>     Doing so in Silicon (we expect this to be done in the fast path of
>     significant forwarders as I understand it) requires very special
>     provision.  Even in software, removing bytes from the middle of a
> packet
>     requires somewhere between some and a lot of extra work.  It is
>     distinctly NOT free.
>
>     In contrast, we have assumed that the work of processing SRH itself is
>     tractable, since otherwise all of SRv6 would be problematic.  So why
> is
>     this necessary.
>
>     Yours,
>     Joel
>
>     PS: Note that both the MPLS case and the encapsulation case are very
>     different in that the material being removed is at the front of the IP
>     packet.  Pop or prepend are MUCH easier than middle-removal (or
>     middle-insertion).
>
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