Re: [mpls] Concerns about ISD

[Tony Li <[e-mail]>]

Hi Haoyu,


> Switch ASICs all use TCAM/CAM to do parsing for performance and NPs use microcode. You are right that in NP microcode you just write “if” branches to parse the headers. But each “if” costs extra clock cycles to process which are very expensive in NPs.


Sorry, I disagree.  Again, compute cycles aren’t nearly as expensive as memory operations.


> Have you evaluated how much more complex (e.g., how many more “if”) it is to parse the FAI than a simple post-stack header chain? The result might surprise you. The inefficiency of the header encoding applies to any target platforms which will hurt the forwarding performance (or incur high hardware cost) and it will hit the old devices in use even harder.  



Yes, I have.  The memory reads for parsing a header chain are extremely high.  No thank you.  The bandwidth requirements for the header chain are extremely high.  No thank you.



> Ok, that saves you 4 octets, but costs you a bit in EHI. There’s only a finite number of bits available.  What do you do when they’re consumed?
>  
> [HS3] Resource is always limited everywhere and for any proposal, therefore we need to careful evaluate and admit use cases. 


Well, I prefer proposals where there is more flexibility.


> Yes, determining the length requires reading the full ISD.
>  
> [HS3]This then would further complicate the parsing.  


Considering that the parsing is probably trivial compared to performing the network action, I don’t understand the objection.


> Absolutely true. Allocate popular functions first. :) If there are too many actions, then we could also consider a second SPL.
>  
> [HS3] We only know what’s popular for known use cases. We don’t know if a use case emerging in future is actually more important. 


Agreed. Knowing the unknown is rather difficult.


> So EH is twice as expensive as FAI.
>  
> [HS3] In terms of header overhead, this is true but only to this specific case. The number depends on how many functions and what functions are supported in a packet. For EH, HEH is a fixed 4 byte overhead, and all the others are equivalent.   


This case seems like it might be pretty common.  EL is already common today and is likely to become more common as the use of ECMP proliferates.  I think we can anticipate that slicing will be common as the 5G folks seem to be strongly in favor of it. Mobile clearly isn’t going to go away.

Tony