Re: [mpls] Concerns about ISD

[Tony Li <[e-mail]>]

Hi Haoyu,

> If you continue to read the indicator draft, you’ll find it may be possible to reach the EHs faster.


I’m sorry, I’m not sure which document you’re referring to.

Yes, I realize that you could store a length of the label stack in the EHI LSE.  That would allow you to index directly to PSD rather than linear parsing for the bottom of stack.  However, that’s not the established MPLS mechanism. :-)  Moreover, it causes a redundancy and inconsistency problem: both the length and the bottom of stack must been coordinated.  What happens if they are inconsistent? Is an implementation supposed to cross-check them? If so, we’re back to linear search for the S bit. And the redundancy seems wasteful of bits.


> Even we don’t use that mechanism, the parsing latency is indeed linear to the label stack depth.


Thank you.  


> But if you check the cost of parsing ISD, this can still be a better choice.


Please explain that.  As previously discussed, parsing the full stack to find the bottom of the stack requires more read/load operations. That would seem to mandate poorer performance.


> Moreover, the stack of stack can help to reduce the label stack depth to reach the EHs.


I’m not following that.  It seems to me that if there is a hierarchical stack, then for an implementation to support all of the network actions, it has to parse all of the hierarchy plus all of the extension headers.


> For the EH encoding efficiency, I appreciate your input. Currently it follows the IPv6 EH type (i.e., using NH + LENGTH to delineate every EH). There could certainly be room for improvement.


Ok.  If I understand your proposal correctly, there are 4 octets of overhead (HEH) at the start. Then, for each network action, there would seem to be at least 3 octets of overhead, plus any associated data, plus alignment.

Let’s suppose that we want to encode NFFRR, entropy, and GISS in one packet.  By my math, the cost is:

EHI: 4 octets
HEH: 4 octets
NFFRR: 4 octets
Entropy: 8 octets
GISS: 8 octets

Total: 28 octets

Do I have that right?

Tony