[mpls] Concerns about ISD

[Tianran Zhou <[e-mail]>]

I followed the discussion in the mailing list for a while,  and read all the proposed solutions.
As I see some proposals mentioned about ISD, I do not think we really need it.
The only reason I see is the lower parsing depth.
However, the complexity we add to the implementation and the backward compatibility issue is a disaster.

Let's see some concerns:
1. ISD breaks the MPLS stack. It's very different from existing implementations. Different label processions are required for the whole stack.
2. ISD has constrained space and format. For example, the s bit is reserved to not use. This will fragment the data space. Hence introduce a lot of complexities on the parsing, processing, encoding, hence damage the flexibility and extensibility.
3. Pop the entire ISD within a lsp/sr-path is a very different procession to existing implementation.
4. ISD itself will increase the depth of the label stack. Several ISDs encoded in the label stack will severely impact the payload efficiency.
5.  If ISD carries any field that can change en route, it will impact the ECMP hash.
6. ISD cannot replace PSD. If we can achieve with only PSD with one piece of code, I can find no reason to implement both ISD and PSD.

In all, my design philosophy is simplicity. However, ISD breaks the elegant MPLS label stack.
I would strongly object to ISD.

Best,
Tianran