Re: [spring] Spirit and Letter of the Law (was: Question about SRv6 Insert function)

[Robert Raszuk <[e-mail]>]

Hi Tom,


> insertion. For instance, I'd invite you do the thought experiment
> about what happens when an intermediate node inserts a header that
> causes the packet to exceed the MTU of some downstream forwarding
> node.


Absolutely.

But SRH is meaningful only within a given domain.

At least I would worry much less about MTU being exceeded when you insert
EH as compared when you encapsulate with complete new header including EH,
DO etc ... which somehow everyone seems to be happy about. I am not seeing
much logic here.

Moreover if you look at telco transport today a lot of "circuits" are
emulated circuits which use encap in the middle of the transports which in
turn runs over their IP networks. How do you deal with such cases when one
day it works and when telco underly reroutes such that it goes via
bottleneck it does not ?

I am seeing this daily. We even wrote IETF drafts to detect this. Now the
BFD draft stuffing is a third attempt to have a tool to detect it. If IPv6
works end to end, hosts to hosts such MTU bottleneck should be part of the
IPv6 transport.

There is no robust protocol mechanism to deal with this.


That's a problem much bigger then SR. See when I am building transport over
third parties I use SLA probes which periodically validates not only SLA
data but also end to end max MTU. When I see an issue I choose
automatically different underlay path such that end applications do not
notice the issue.

Many thx,
R.


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