HI Pablo:
Replies in-line prefaced with DA>
-----Original Message-----
From: spring <spring-boun...@ietf.org> On Behalf Of Pablo Camarillo (pcamaril)
Sent: Saturday, December 14, 2019 1:57 AM
To: Xiejingrong (Jingrong) <xiejingr...@huawei.com>; Joel M. Halpern
<j...@joelhalpern.com>; spring@ietf.org
Subject: Re: [spring] Is srv6 PSP a good idea
Hi Dave,
Thank you for participating in the discussion.
I fail to see how it "fragmentation becomes particularly ugly". What is the
negative impact to the final destination? I believe there is none.
DA> The work that needs to be performed doing PSP is substantially more than
the final destination to handle the SRH. That fragmentation multiplies this
was my point.
Intuitively I tend to agree with Jingrong that PSP is beneficial since the
egress PE will have easier access to the fragmentation header.
DA> I've seen the point raised that implementations can do this today. I would
still be concerned about the additional load we are imposing as the next
generation of interfaces comes along.
Regardless of whether it has any benefit: note that the source is the one
choosing which segments that go into the SRH. As such, it can encode into the
packet a penultimate segment with no PSP support. PSP is optional and it is
advertised in the control plane so that the source can decide whether to use it.
DA> I have to admit I find that to be a strange statement, effectively the
source can choose whether or not to do something that is of questionable
benefit?
Rgds
Dave
-----Original Message-----
From: David Allan I <david.i.al...@ericsson.com>
Date: Thursday, 12 December 2019 at 22:40
To: "Pablo Camarillo (pcamaril)" <pcama...@cisco.com>, "Xiejingrong (Jingrong)"
<xiejingr...@huawei.com>, "Joel M. Halpern" <j...@joelhalpern.com>,
"spring@ietf.org" <spring@ietf.org>
Subject: RE: [spring] Is srv6 PSP a good idea
Hi Pablo
Looking over the benefits list, Irrespective of any other merits, I'd
actually suspect that 1.3/example 2 is kind of a specious benefit.
My reasoning is if fragmentation has occurred, and the NSH has been
replicated in all fragments and needs to be removed from each fragment by the
penultimate SR. This offers minimal benefit to the reassembly function at the
ultimate Segment Router.
IMO SRH removal at an intermediate device is a more complex operation. In
a simplistic description of operation it adds at a minimum, parsing and a
memcpy of the payload. In effect it replaces Start_of_payload_ptr += length
(SRH) at the ultimate SR with memcpy (start_of_SRH_ptr,
start_of_SRH_ptr+length(SRH), length(SRH)) at the penultimate SR.
So skipping over the exhausted SRH would appear to be trivial compared to
the work at the penultimate segment router to remove it entirely. IMO it does
not offer benefits equivalent to PHP in MPLS
The same argument could be considered without fragmentation, but
fragmentation becomes a particularly ugly example.
Hope this helps
Dave
-----Original Message-----
From: spring <spring-boun...@ietf.org> On Behalf Of Pablo Camarillo
(pcamaril)
Sent: Wednesday, December 11, 2019 12:12 PM
To: Xiejingrong (Jingrong) <xiejingr...@huawei.com>; Joel M. Halpern
<j...@joelhalpern.com>; spring@ietf.org
Subject: Re: [spring] Is srv6 PSP a good idea
Jingrong,
> Nothing new, but benefits that people have already said seems notable to
me.
Agreed.
Cheers,
Pablo.
-----Original Message-----
From: spring <spring-boun...@ietf.org> on behalf of "Xiejingrong
(Jingrong)" <xiejingr...@huawei.com>
Date: Wednesday, 11 December 2019 at 05:15
To: "Joel M. Halpern" <j...@joelhalpern.com>, "spring@ietf.org"
<spring@ietf.org>
Subject: 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-boun...@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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