Re: [spring] Is srv6 PSP a good idea

[Ron Bonica <[e-mail]>]

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.

                                                    Ron




Juniper Business Use Only

-----Original Message-----
From: spring <[e-mail]> On Behalf Of Pablo Camarillo (pcamaril)
Sent: Wednesday, December 11, 2019 3:12 PM
To: Joel M. Halpern <[e-mail]>; [e-mail]
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 <[e-mail]> on behalf of "Joel M. Halpern" <[e-mail]>
Date: Wednesday, 11 December 2019 at 03:55
To: "[e-mail]" <[e-mail]>
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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