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

[Stewart Bryant <[e-mail]>]

Andy

I think we need to ask around about this, because I have never run into 
a mainstream box that supported s/n, much less re-order. The normal 
process was to strip off the PW label and CW only checking for the ACH, 
and then forward the packet straight out of the interface.

I just did a quick look at a couple of config manuals and there seemed 
to be no CW related options other than include it or not.

As I said, implementing other than that is actually very hard in a high 
speed interface.

Clearly we need to check.

Stewart


On 13/02/2017 14:35, Andrew G. Malis wrote:
> Stewart,
>
> In at least one implementation I’m familiar with, lack of common 
> shared memory between parallel microprogrammed forwarding engines for 
> the same physical port was the issue. I can see lack of intercard or 
> interport communications also being a problem.
>
> We added the control word and sequence number to Ethernet PWs later 
> on, and I believe most implementations today do use it to reorder and 
> comply with the Ethernet spec.
>
> Cheers,
> Andy
>
>
>
> On Mon, Feb 13, 2017 at 8:02 AM, Stewart Bryant 
> <[e-mail] <mailto:[e-mail]>> wrote:
>
>
>
>     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
>
>