[mpls] Re: Potential MNA technical issue - late follow up

[Loa Andersson <[e-mail]>]

Tony.

Sorry for the very late response. Some comments and a question. Inline plz.

Den 21/02/2025 kl. 15:19, skrev Tony Li:
> 
> Hi Jie Dong,
> 
> 
>> Another open issue is where should the PSD indication mechanism be specified. To my understanding PSD indication is part of the general MNA solution and is orthogonal to the encoding of Post-stack data, thus it is more suitable to be covered in the MNA header draft.
> 
> 
> We still have not resolved how PSD will be indicated.  We have four proposals on the table.  When we come to consensus, then it will be obvious where to put it.
> 
> a) P-bit in the ISD header. This lacks any offset, which makes it difficult to find the PSD on mid-path nodes where other post-stack information might be above it.

I think that the P-flag can only be used if PSD immediately follows the 
LSE that has the BoS set.

You say that using the P-flag is a problem for transit nodes "where 
other post-stack information might be above it". Why does not the egress 
node have the same problem?


> b) An offset as an opcode.
> c) Offsets per PSD action.
> d) Some combination of the above.
> 
> I favor option b as it’s simple and sufficient.

You say you favor option b, I might agree with you. I option b that we 
have an OpCode (indicating "PSD present") followed by a 13 or 16 bit 
field (depending on type of LSE) containing the offset?

Like this:


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Opcode=PSD  |      Offset             |R|IHS|S|U|  NASL | NAL |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: LSE Format B: The initial opcode


    LSE Format C is used to encode the subsequent opcodes in the NAS.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Opcode=PSD  |            Offset             |S|U|  Data | NAL |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: LSE Format C: Subsequent opcodes

Then setting the P-flag would be semantically equal to having an offset 
of "0", and I see no harm in including it.

Did I understand you correctly?

/Loa
> 
> 
>> During the interim this week we didn't get chance to discuss the possible PSD indication mechanisms. This may be considered as a remaining open technical issue for further discussion.
> 
> 
> That was not on the agenda as we were hoping to resolve the SPL and ISD status issues first.
> 
> T
> 
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-- 
Loa Andersson
Senior MPLS Expert
Bronze Dragon Consulting
[e-mail]
[e-mail]