Re: [Detnet-dp-dt] detnet LSPs and PWs

[Stewart Bryant <[e-mail]>]

On 13/02/2017 11:58, Andrew G. Malis wrote:
>
> On Mon, Feb 13, 2017 at 5:55 AM, Stewart Bryant 
> <[e-mail] <mailto:[e-mail]>> wrote:
>
>     Now for the elephant in the corner of all of the schemes I have
>     seen. If you have multiple paths to an X-PE, packets will likely
>     arrive on different line cards. Sequence number co-ordination
>     amongst different line cards, and at high speed even amongst
>     different ports on the same line card is a hard problem. Indeed
>     depending on the pipeline design on the line card, ANY sequence
>     number processing can be hard. You could mitigate this (at the
>     cost of availability) by requiring a common ingress port at any
>     detnet X-PE. This would normally require an RSVP-TE or SR underlay.
>
> I would like to second Stewart’s comment above. Back in the early days 
> of PWs, some vendors implemented PWs without a sequence number because 
> they couldn’t do sequence number processing across multiple parallel 
> forwarding engines even on the same physical port, as the forwarding 
> engines didn’t have common shared memory. Of course, their customers 
> had to not use ECMP, at least for those PWs. I think most vendors have 
> gotten past that these days, but you can never tell. But keep these 
> possible limitations in mind as you design Detnet.
>
> Cheers,
> Andy
>

Andy it is not common shared memory but the pipelining and the intercard 
communication that stops it working.

As you know we prohibit ECMP on most PWs via the control word, with the 
exception of the Ethernet PW where it is optional and in place for 
historic reasons. However an Ethernet PW can re-order and as such does 
not fully comply with the Ethernet spec.

The problem is still live - which is why we need the packet marking 
scheme for packet loss measurement.

- Stewart