Re: [spring] Different MSDs for different traffic types on the same headend.

[Nat Kao <[e-mail]>]

Hi, Jeff.

Consider a headend that can perform 1 of the following 2 modes(but not
both):
1) Plain IPv4: 6 transport labels + 0 service label => traffic can be
steered into a 6-label SR-TE policy.
2) Any type of VPN: 3 transport labels + 1~3 service labels => traffic
cannot be steered into a 6-label SR-TE policy.

a) As defined in RFC8491, the BMI-MSD is 6 for this headend. Do we have a
standardized way to signal the transport label depth in mode 2?
   Maybe in a different MSD type?
b) Since plain IPv4 and VPN routes can be steered into the same SR-TE
policy, do we have a standardized headend behavior in this situation?
   (Should I open a new thread to discuss this? It seems not quite
MSD-relative.)

Thanks.
Nat


On Tue, Dec 17, 2019 at 11:14 AM Nat Kao <[e-mail]> wrote:

> Hi, Jeff.
>
> Consider a headend that can perform 1 of the following 2 modes(but not
> both):
> 1) Plain IPv4: 6 transport labels + 0 service label => traffic can be
> steered into a 6-label SR-TE policy.
> 2) Any type of VPN: 3 transport labels + 1~3 service labels => traffic
> cannot be steered into a 6-label SR-TE policy.
>
> a) As defined in RFC8491, the BMI-MSD is 6 for this headend. Do we have a
> standardized way to signal the transport label depth in mode 2?
>    Maybe in a different MSD type?
> b) Since plain IPv4 and VPN routes can be steered into the same SR-TE
> policy, do we have a standardized headend behavior in this situation?
>    (Should I open a new thread to discuss this? It seems not quite
> MSD-relative.)
>
> Thanks.
> Nat.
>
>
> On Tue, Dec 17, 2019 at 7:19 AM Jeff Tantsura <[e-mail]>
> wrote:
>
>> Gyan,
>>
>> MSD is only relevant for a device that either imposes the label stack
>> (head-end) or manipulates it (BSID anchor). There are some other constrains
>> when it comes to entropy labels and ERLD, please read the respective drafts.
>> In general, SID stack would grow when TE is in use (any time you need to
>> use additional SID to deviate from SPT), another case is when additional
>> SID’s are used for services on the nodes, other than the tail-end.
>> That’s why we've designed MSD to be very flexible to accommodate all the
>> different use cases, it is upto computational logic to decide how to deal
>> with different constrains (MSD types)
>>
>> Cheers,
>> Jeff
>>
>> P.S. you might want to see the NANOG MSD presentation I did some time ago.
>>
>> https://pc.nanog.org/static/published/meetings/NANOG71/1424/20171004_Tantsura_The_Critical_Role_v1.pdf
>> On Dec 14, 2019, 11:59 PM -0800, Gyan Mishra <[e-mail]>,
>> wrote:
>>
>> Jeff
>>
>> With SR-MPLS with SR-TR let’s say if you use cSPF snd don’t have an ERO
>> strict explicit path defined or is a loose path, then the for the cSPF
>> would the transport labels be 1.  For loose would also be 1 also.  If the
>> path were explicit defined to egress PE and was 7 hops from ingress to
>> egress then transport would be 6.  And if L3 vpn service sid was signaled
>> that would be 1 vpn label.
>>
>> Let me know if I have that right.
>>
>> In Nats scenario for IPv6 he has 3 vpnv6 labels.
>>
>> Why is that?
>>
>> With both SR-MPLS and SRv6 the L3 vpn AFI/SAFI MBGP services overlay
>> single label sits on top off SR as if does today with MPLS so why 3 vpn
>> labels.
>>
>> So with this draft with BGP-LS and BMI-MSD you can flood into the IGP the
>> SID depth so all the nodes along the SR-TE path don’t go over the maximum
>> which would result in an error.
>>
>> If you set your MTU high enough in the core like 9216, does that overcome
>> the SID depth issues with SR-TE?
>>
>> Warm regards,
>>
>> Gyan
>>
>> On Sat, Dec 14, 2019 at 2:43 AM Nat Kao <[e-mail]> wrote:
>>
>>> Hi, Jeff.
>>>
>>> Thanks for the BMI-MSD reference. If I understand correctly:
>>>
>>> BMI-MSD = Transport Label Depth + Service Label Depth
>>> Only former can be utilized by SR-TE policies.
>>>
>>> Currently do we have any method to determine the composition of BMI?
>>> We need to know the transport label depth when doing service route
>>> per-destination steering.
>>>
>>> This problem arises when trying to steer a plain IPv4 route and a VPN
>>> service route into the same SR-TE policy that exceeds the transport label
>>> depth of the service route. I'm trying to figure out the standard behavior
>>> in this case since the headend we use currently produces some interesting
>>> results.
>>>
>>> Regards,
>>> Nat.
>>>
>>> On Sat, Dec 14, 2019 at 2:42 AM Jeff Tantsura <[e-mail]
>>> <[e-mail]>> wrote:
>>>
>>>> Hi Nat,
>>>>
>>>> Please read https://tools.ietf.org/html/rfc8491#section-5
>>>> Currently defined MSD types are:
>>>> 1: BMI
>>>> 2: ERLD
>>>>
>>>> Specifically to BMI:
>>>> Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS labels
>>>> that can be imposed, including all service/transport/special labels.
>>>> The answer to your question is 6
>>>>
>>>> Cheers,
>>>> Jeff
>>>> On Dec 13, 2019, 3:42 AM -0800, Nat Kao <[e-mail]>, wrote:
>>>>
>>>> Hello, SPRING WG.
>>>>
>>>> How do we deal with an SR-TE policy headend with different MSDs for
>>>> different types of traffic?
>>>> For example, a headend H can impose:
>>>> 6 transport labels for plain IPv4 packets;
>>>> 5 transport labels + 1 IPv6 ExpNull label for plain IPv6 packets;
>>>> 3 transport labels + 3 VPN  labels for VPN packets.
>>>>
>>>> a) For a plain IPv4 route R4 and a VPN route Rv both steered into the
>>>> SR-TE policy P1 with SID list <S1, S2, S3, S4, S5>, what will H perform in
>>>> this situation?
>>>> b) What is the MSD of H? 6, 5 or 3?
>>>>
>>>> Thanks,
>>>> Nat.
>>>> _______________________________________________
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>>>>
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>>>
>> --
>>
>> Gyan S. Mishra
>>
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