[mpls] Re: Working Group Adoption Poll on draft-mb-mpls-ioam-dex

[Fabian Ihle <[e-mail]>]

Stewart and Loa,

the "a bit trickier" part of PSD is:

 1. The presence of PSD needs to be indicated efficiently. With a single
    bit for indication (the "P-bit" from the jags draft), we still need
    8 bytes (Format A for in-stack NAS indication and Format B for a
    flag-based opcode with the P-bit set). If select-scoped PSD
    indication is to be a thing, even more overhead is added.
 2. In P4, the MPLS stack and the PSD must be within RLD. We cannot skip
    parts of the MPLS stack that are not relevant to the current node
    but have to parse the entire stack. While these irrelevant LSEs do
    not need to be processed, we still have to allocate resources to
    parse them. However, with an RLD of 51 LSEs, this constraint is not
    a showstopper and PSD is definitely doable. But it does emphasize
    the importance of the first point. Other hardware may work
    differently and not have this limitation, or may be even more
    constrained. In the latter case, PSD might be difficult.

Our intention with a P4 implementation is not to deploy commercial MNA 
solutions but rather to show that a protocol extension such as the MNA 
framework can be implemented on constrained hardware. If it works on 
constrained P4 hardware, hardware engineers will most likely also be 
able to implement it. However, the reverse assumption, i.e., "if it does 
not work on P4 hardware, it will also not work on other hardware" does 
not apply.

We have mainly focused on an ISD implementation for now, the next step 
will be an extensive evaluation of the PSD version.

Best,

Fabian


On 25.10.24 14:34, Loa Andersson wrote:
> Stewart and Fabian,
>
> I think all implementation experiences are useful, also if the they 
> are done using the commercially non-deployed P4, you can always draw 
> some conclusions.
>
> What I would like is why PSD is trickier, and if "a bit trickier" is 
> significant enough to not go with PSD. Especially since we agree that 
> draft-mb-mpls-ioam-dex is not compliant with RFC 9326.
>
> Is "a bit trickier" the prize we pay for compliance with RFC 9326?
>
> /Loa
>
> Den 25/10/2024 kl. 13:13, skrev Stewart Bryant:
>>
>>
>>> On 14 Oct 2024, at 10:34, Fabian Ihle <[e-mail]> 
>>> wrote:
>>>
>>> I think that ISD is the way to go for the DEX option as an PSD 
>>> implementation has proven to be a bit trickier in our P4 version.
>>
>> Is this an issue with the protocol or an indication of deficiencies 
>> in P4?
>>
>> Is P4 deployed or likely to be deployed in a production environment, 
>> i.e. is this a consideration for protocol standardisation?
>>
>> Best regards
>>
>> Stewart
>>
>>
>>
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-- 
Fabian Ihle
Universität Tübingen
Fachbereich Informatik Lehrstuhl Kommunikationsnetze
Wilhelm-Schickard-Institut für Informatik
Sand 13, 72076 Tübingen

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