Re: [spring] I-D Action: draft-ietf-spring-srv6-network-programming-10.txt

["Joel M. Halpern" <[e-mail]>]

While it is true that some traffic only needs steering in one direction, 
I have real trouble figuring out how an operator would dare deploy an SR 
edge device that could not steer incoming traffic.  Either they do not 
need SR, or they can expect that some traffic will need it in both 
directions.  So I am left supporting John's contention that there seems 
to be a gap in how one would used the hypothesized device that needs 
another device performing PSP.

also, and edge device that requires PSP would place constraints on the 
SRv6 traffic paths that could be used be used to it.  One would have to 
ensure that all SRv6 traffic ending there had a penultimate explicit hop 
to a device that could perform PSP.  Yes, this can be done.  It does 
seem to complicate life.  Thus, yet another way that PSP adds complications.

Yours,
Joel

On 2/28/2020 5:55 AM, Robert Raszuk wrote:
> Hi John,
> 
>  > I have an additional observation, or question, about Dan’s scenario. 
> Almost all communication is bidirectional.
>  > Presumably this means a router that’s the tail end of an SRv6 path in 
> one direction is the head end in the other.
> 
> While your observation is correct that most TCP connections are bidir SR 
> in a lot of cases can operate only in one direction. Needless to say it 
> can also be used with UDP streaming.
> 
> To extend Ketan's OTT video example let me observe that in a lot of 
> transactions queries from clients are tiny and do not TE 
> capabilities while responses are huge and bursty and may indeed benefit 
> from special handling.
> 
> Sure if you think of applications like VPNs than you are right .. 
> regardless of the size of the packets proper tagging must occur in 
> either direction - but this is just one use of SRv6 perhaps not even the 
> major one.
> 
> - - -
> 
> Now as one friend just asked me offline - putting all IPv6 dogmas aside 
> - what is the technical issue with removing previously applied extension 
> header from the packet within a given operator's network ? What breaks 
> when you do that ?
> 
> Thx,
> R.
> 
> 
> On Thu, Feb 27, 2020 at 10:11 PM John Scudder 
> <jgs=[e-mail] <mailto:[e-mail]>> 
> wrote:
> 
>     I have an additional observation, or question, about Dan’s
>     scenario.. Almost all communication is bidirectional. Presumably
>     this means a router that’s the tail end of an SRv6 path in one
>     direction is the head end in the other. Doesn’t a head end need to
>     add an SRH? If I’ve gotten that right, then we can extend Ron’s list
>     with one more item. That is, apparently the ultimate segment endpoint:
> 
>     • Can process a SID, received as an IPv6 DA, on the fast path
>     • Cannot process an SRH on receipt, even if Segments Left equal 0,
>     on the fast path.
>     • Can add an SRH on transmission, on the fast path
> 
>     Even though strictly speaking the second and third bullet points
>     aren’t mutually exclusive, it’s a little difficult to imagine a real
>     router that would have both these properties simultaneously. Perhaps
>     I’m not being creative enough in imagining deployment scenarios?
>     Since this scenario is claimed as an important reason this
>     problematic feature is needed, it would be great if someone who
>     understands it would elucidate, thanks.
> 
>     One further point, Ron says “I wonder whether it is a good idea to
>     stretch the IPv6 standard to accommodate IPv6-challenged devices.” I
>     also wonder this, especially because these devices will have a
>     relatively limited lifetime in the network.[*] I don’t find the
>     cost/benefit attractive of making a permanent detrimental change to
>     the IPv6 architecture to accommodate a temporary deployment issue.
> 
>     Regards,
> 
>     —John
> 
> 
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