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