Re: [spring] “SRV6+” complexity in forwarding

[Robert Raszuk <[e-mail]>]

Hi Gyan,

> So now talking SRv6 with Ti LFA why is there an EH insertion as we are
> not using mpls LDP and not doing remote LFA and this is not the
traditional
> mpls TE FRR.

TI - stands for Topology Independent ... all other LFA modes rely on
topologies to be able to compute or not the backup path.

So why insertion/EH modification is needed ... well as I stated before
there are  few aspects of this and all must be taken into consideration.

A) In most cases of topologies it may not be required - only in some
special cases you may need to engineer your backup flows and then you
either insert new EH or add another IP encap with such SRH in current SRv6
architecture.

B) When you are doing node protection and PLR is adjucent to segment end
node you should fixup such SRH (so modify it on PLR which is not packet
destination) such that the packet will not try to return when LFA routed to
N+1 segment end.

Why A is option B clearly needs to be fixed or LFA style node protection of
any segment end declared as not supported.

Of course in bigger picture there any more protection methods to consider
and only specific operator of transit or enterprise is in position to
decide what works best for him.

Many thx,
R.















On Fri, Sep 20, 2019 at 3:06 AM Gyan Mishra <[e-mail]> wrote:

> Hi Robert
>
> I know we have gone through many many lengthy discussions adnosium and I
> know the question has come up a few times and I know you replied back a few
> times related to the service provider use case where we end up with the
> multiple violations of RFC 8200 related to intermediate nodes EH insertion
> as well as many EH insertions occurring and what was mentioned was Ti-LFA.
> So since LFA and Remote LFA are extensions of the IGP providing the 50ms
> failover similar to traditional mpls Fast Reroute capabilities NH & NNH
> link/node/path protection in the legacy TE FRR pce the head end PE LSR adds
> and additional mpls shim for FRR.
>
> With IP LFA and remote LFA used with LDP there are IGP extensions opaque
> LSA’s that provide the pre programmed backup path provided by LFA loop free
> backup path.  In that scenario with LDP there is not any additional label
> with LFA but with remote LFA is added for the Remote LFA backup path with
> LDP session protection enabled with targeted LDP session tunnled through
> the RLFA node.
>
> So now talking SRv6 with Ti LFA why is there an EH insertion as we are not
> using mpls LDP and not doing remote LFA and this is not the traditional
> mpls TE FRR.
>
> I am not getting it from a network engineering technical standpoint what
> the use case is and why EH insertion would occur on any intermediate node
> as that even in the legacy mpls TE world its on the ingress LSR PE and for
> that matter in this case the benefit of SRv6 is “native TE” source routing
> via SRH pssi instructions and not maintaining state on the intermediate
> nodes which just do the PSSI and copy the SRH destination rewrite of IPv6
> destination for the traffic engineered path and then do PSP or USP on the
> egress node of the service provider core.
>
> I designed and manage a fairly large mpls core for Verizon and if their is
> a asic processing penalty hit due to intermediate node EH insertion we
> don’t want it and will stay course with LDP and stick with our path
> targeting SR-MPLS and ditch any ideas of ever going to SRv6.
>
> Thanks in advance for help in clarification of this topic and I think
> understanding the use case would help 6man overall understand the
> justification of the RFC 8200 violations.
>
> Thank you
>
> Gyan Mishra
> Verizon Communications
> Cell 301 502-1347
>
> Sent from my iPhone
>
> On Sep 19, 2019, at 3:50 PM, Robert Raszuk <[e-mail]> wrote:
>
> Hi Reji,
>
> Notice what it says: *" ... explicitly listed intermediate nodes ... "*
>
> CRH which is used in SRv6+ does not explicitly list intermediate nodes so
> I do not think the procedures in IPv6 spec apply as the way you interpret
> them.
>
> But I am i no way authoritative ... still learning IPv6 and this thread
> become great education.
>
> An real example where those procedure apply is documented in RFC6554 which
> does put the addresses explicitly.
>
> Many thx,
> Robert.
>
>
>
>
>
>
>
>
> On Thu, Sep 19, 2019 at 9:14 PM Reji Thomas <[e-mail]>
> wrote:
>
>> Hi Robert,
>>
>> >>I do not know what is the difference between IPv6 Destination Address
>> in the fixed header and "final destination". Where do you carry "final
>> destination" address ?
>>
>> See Section 4.4 in RFC 8200.  Hope its clear what's the final destination
>> and the context in which it is used.
>>
>>       Segments Left       8-bit unsigned integer.  Number of route
>>                           segments remaining, i.e., number of explicitly
>>                           listed intermediate nodes still to be visited
>>                           before reaching the final destination.
>>
>>
>> Regards
>>
>> Reji
>>
>>
>> On Thu, Sep 19, 2019 at 10:26 PM Robert Raszuk <[e-mail]> wrote:
>>
>>> IPv6 fixed header has only one destination address. So TE midpoint is
>>> either a packet's destination or not. It can not be both.
>>>
>>> I do not know what is the difference between IPv6 Destination Address in
>>> the fixed header and "final destination". Where do you carry "final
>>> destination" address ?
>>>
>>> Many  thx,
>>> R.
>>>
>>> On Thu, Sep 19, 2019 at 6:17 PM Reji Thomas <[e-mail]>
>>> wrote:
>>>
>>>> Hi Robert,
>>>>
>>>>
>>>> >>Well the crux of the matter is that you still need to process all EHs at each IPv6 destination which here means each transit node per RFC8200
>>>>
>>>>
>>>>  From RFC 8200 that doesn't seem to be the case or at least as I understand. See  Section 4.1 note 1 and note 3. Am I missing something?
>>>>
>>>>
>>>>
>>>> IPv6 header
>>>>       Hop-by-Hop Options header
>>>>       Destination Options header (note 1)
>>>>       Routing header
>>>>       Fragment header
>>>>       Authentication header (note 2)
>>>>       Encapsulating Security Payload header (note 2)
>>>>       Destination Options header (note 3)
>>>>       Upper-Layer header
>>>>
>>>>       note 1: for options to be processed by the first destination that
>>>>               appears in the IPv6 Destination Address field plus
>>>>               subsequent destinations listed in the Routing header.
>>>>
>>>>       note 2: additional recommendations regarding the relative order of
>>>>               the Authentication and Encapsulating Security Payload
>>>>               headers are given in [RFC4303 <https://tools.ietf.org/html/rfc4303>].
>>>>
>>>>       note 3: for options to be processed only by the final destination
>>>>               of the packet.
>>>>
>>>>
>>>> Regards
>>>> Reji
>>>>
>>>> On Thu, Sep 19, 2019 at 9:00 PM Robert Raszuk <[e-mail]>
>>>> wrote:
>>>>
>>>>>
>>>>> I disagree. PPSI and PSSI leverages the DOHs in IPv6 architecture
>>>>>> better. The SRv6+ drafts explain the usecases better FYI.
>>>>>>
>>>>>
>>>>> Well the crux of the matter is that you still need to process all EHs
>>>>> at each IPv6 destination which here means each transit node per RFC8200.
>>>>> That is regardless what any other spec says .... unfortunately.
>>>>>
>>>>> Best,
>>>>> R.
>>>>>
>>>>>
>>>>>
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