Other use cases for header insertion (was Re: Header Insertion and TI-FA)

On Mon, May 11, 2020 at 5:58 AM Ron Bonica
<rbonica=40juniper.net@dmarc.ietf.org> wrote:
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> Folks,
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> Happy Monday!
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> As I was painting this weekend, I remembered that the use-case for IPv6 Header insertion is TI-LFA. This made the following questions come to mind:
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Ron,

There is at least one other use case for IPv6 Header insertion, that
being IOAM. For example, an operator may wish to collect per packet
statistics as packets traverse its network. An ingress router would
insert headers containing IOAM, routers would annotate the
information, and the egress router is expected to consume and remove
the headers. The big difference between this use case and SR is that
there's no source routing involved so the ingress router doesn't
necessarily know what the egress router is, hence the ingress router
wouldn't know the right destination address to use in encapsulation.

I believe that this is currently being developed and maybe even being
deployed, but not under the auspices of IETF. Frankly, figuring out
how to do header insertion in a sensible and standard way would be a
step up from some of the methods being proposed. The most egregious
proposal I've seen was one in which the ingress router would place the
IOAM information in the TCP payload and then use diff serv bits to
indicate the TCP payload is modified. Of course the day a network
fails to remove the inserted information this completely breaks TCP!

This use case makes me think the WG should consider how to make header
insertion safe and viable.

Tom
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> How does TI-LFA work when the original packet already contains a routing header? Will it insert a second, so that the packet has two routing headers?
> How does TI-LFA work when the node directly upstream of the link (i.e., the PLR) is not a segment endpoint? Will it insert an routing header? Is that consistent with 8200?
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>                                                                                                                         Ron
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