Thank you Jingrong for providing some of the other motivations. Two
furhter comments.
As far as I know, the only savings on the end box is the processing for
noticing the SRH, noticing that SL is 0 and there are no relevant TLVs,
and then moving on.
If the actual end device is not part of the SR domain, I assume that
encapsulation would have been used, so I think it is reasoanble to
assume that in the PSP case the end device is SR capable.
Do you have any comments on what appears to be the significant increase
in complexity on the device performing PSP? The question I am trying to
get at is about the tradeoff, which needs one to evaluate both sides.
Yours,
Joel
On 12/10/2019 11:13 PM, Xiejingrong (Jingrong) 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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