Re: [spring] Is srv6 PSP a good idea

I think it's a good idea.
Nothing new, but benefits that people have already said seems notable to me.

(1) reduce the load of final destination. This benefit can be notable for the following sub reasons.
(1.1) final destination tends to have heavy load. It need to handle all the EHs and do the delivery/demultiplex the packet to the right overlay service.
(1.2) example 1, the final destination may need to handle the DOH after the RH.
(1.3) example 2, the final destination may need to do the assembly of fragmented packets.
(1.4) example 3, the final destination may need to do AH/ESP after the Fragmentation Header.
(1.5) example 4, the final destination may need to deliver the packet to the right overlay service.

(2) support the incremental deployment when final destination(s) do not process/recognize SRH. This benefit can be notable for the following sub reasons.
(2.1) A core router may (fan-out) connected with a big number of low-end routers that do not support SRH but support tunnel-end/service-demultiplex function of SRv6.

Thanks
Jingrong

-----Original Message-----
From: spring [mailto:spring-bounces@ietf.org] On Behalf Of Joel M. Halpern
Sent: Wednesday, December 11, 2019 10:55 AM
To: 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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