Re: [spring] Is srv6 PSP a good idea

Wed, 11 Dec 2019 21:48:50 -0800 

There are several aspects of your reply that leave me wondering.
First, optional behaviors in a protocol spec have signficiant cost. So if they are nbot needed, we generally prefer not to have options.
Second, I am confused by your comments about complexity. From my conversations with multiple ASIC folks at different companies, middle removal is generally not a trivial undertaking. My congratulations to the Broadcom folks for achieving it. Even if it is relatively simple, it is pretty clear that middle removal of an SRH is significantly more effort than skipping an exhausted SRH. particularly when such skipping is in a device inside the SRH domain. (Since it is outside the domain, one would have encapsulated, and removed the entire encapsulating header, which is indeed simpler.) 

Yours,
Joel

On 12/11/2019 3:11 PM, Pablo Camarillo (pcamaril) wrote:

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-boun...@ietf.org> on behalf of "Joel M. Halpern" 
<j...@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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