Mark,
Both the SRv6 control plane and dataplane operate between PEs. The legacy
egress PE only executes End.DT/End.DX and is not capable of doing SRH
processing at linerate in the use-case described by Dan. And as a reminder this
is only one of the use-cases of PSP.
Cheers,
Pablo.
-----Original Message-----
From: spring <spring-boun...@ietf.org> on behalf of Mark Smith
<markzzzsm...@gmail.com>
Date: Thursday, 27 February 2020 at 11:43
To: "Voyer, Daniel" <daniel.vo...@bell.ca>
Cc: "Xiejingrong (Jingrong)" <xiejingr...@huawei.com>, "spring@ietf.org"
<spring@ietf.org>
Subject: [spring] A permanent change to/violation of RFC8200 for a temporary
situation. (Re: Is srv6 PSP a good idea)
On Sat, 14 Dec 2019 at 09:14, Voyer, Daniel <daniel.vo...@bell.ca> wrote:
>
> I agree 100% with Jingrong,
>
> PSP allows us to bring SRv6 to legacy PE devices that are not capable of
processing the SRH in the dataplane, but are capable of supporting SRv6 in the
control plane.
>
> See this example:
> I am streaming traffic from a server to a customer;
> The ingress PE (near the server) encapsulates the packet and adds an SRH
with a low-latency list of segments;
> The penultimate node in the SRH executes PSP;
> The egress PE (near the customer) decapsulates the IPv6 header and
forwards the inner packet to the customer.
>
I want to understand this example better, because it sounds very strange to
me.
So the SRv6 control plane is extended past the edge of the SRv6
forwarding plane?
In all other protocols and networks I'm aware of, the control plane
domain and devices, and the forwarding plane domain and devices
it/they controls are either congruent, or the control plane is
"smaller" than the forwarding plane e.g. a couple of BGP Route
Reflectors controlling a cluster of many more routers.
What value is an SRv6 control plane on a router/PE that doesn't
implement the SRv6 forwarding plane?
This value would only exist during a temporary period until the router
forwarding plane could be upgraded to an SRv6 forwarding plane,
returning to the common convention of congruent control and forwarding
planes.
So it seems in this case that RFC 8200 is being violated with the PSP
proposal to accommodate extending an SRv6 control plane past a
network's SRv6 forwarding plane for a relatively short temporary
period, for any particular network, perhaps no more than 2 to 3 years
maximum.
Why not have the SRv6 control and forwarding domains always match, as
is usual and conventional for other matched pairs of control and
forwarding plane protocols and deployments, including new
protocol/forwarding plane deployments, and entirely avoid the issue of
fundamental violations of or making fundamental changes to a full
Internet standard protocol?
> We can include SLA unidirectionally from the server to the customer even
though that the egress PE has a legacy ASIC. Legacy equipment are a reality and
are not easy to replace, hence interoperability with brownfield is key for any
innovative approach.
>
This is exactly the fundamental justification for not violating RFC
8200, and only minimally extending it where necessary and permitted,
fitting within the architecture rather than trying to change it. As
much SR magic as possible should be put into the much easier to
upgrade control plane, ideally avoiding or at least minimising
forwarding IPv6 plane changes.
Regards,
Mark.
> dan
>
> On 2019-12-10, 11:15 PM, "spring on behalf of Xiejingrong (Jingrong)"
<spring-boun...@ietf.org on behalf of xiejingr...@huawei.com> wrote:
>
> 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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