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

[Stewart Bryant <[e-mail]>]

Talking of corner residing elephants how to make sequence number 
processing atomic at the egress detnet-T-PE?

This is not a problem at the S-PEs because it does not matter if two 
copies get through but it is critical at the egress T-PE.

Also what do you plan to do if a later packet overtakes an earlier one? 
Presumably declare all the late delivery packets as "lost" rather than 
attempt to re-order.

- Stewart


On 13/02/2017 10:55, Stewart Bryant wrote:
>
> Hi,
>
> I was given some background information on your thoughts on detnet-PW, 
> and pass on my thoughts in response.
>
> I think NSP issue  is a red herring. NSP can be NULL.
>
> An S-PE does no NSP, although in any case I suspect that you may need 
> some processing function at the detnet-S-PE - see below.
>
> The underlying DETNET PW is an SS-PW in the diagram I was shown, in 
> that the PW label is the same end to end, although of course it need 
> not be, you could have an equivalence label set and run pure MS-PW. 
> Indeed when you have multiple administrations you would like them to 
> be different for administrative purposes (that is why we designed 
> MS-PW like that).
>
> So if you create an equivalence relationship in the egress-PE, i.e. 
> two entries in the global label table pointing to the same duplicate 
> suppressor (sequencer), then you could use regular MS-PW for this.
>
> If the S-PEs are in the same administrative domain in both ingress and 
> egress, you can also use a single label value on egress and on egress 
> since you can give them the same label mappings, i.e. they have 
> identical swap instructions programmed into the L-FIB.
>
> We don't have NSP at the S-PE's in the current PW architecture, in the 
> data-plane it is essentially a simple MPLS LSR, swapping PW labels and 
> forwarding the packet on a new LSP. What you will almost certainly 
> want to do is to have the ability to replicate at nodes at the S-PEs, 
> and that is new functionality.
>
> An approach I would look at is as follows:
>
> Create a new detnet-T-PE. On ingress this adds the sequence number, 
> replicates and adds the PW label, which as I said above MAY be next 
> hop detnet-S-PE dependent. Then it delivers the copies to the 
> detnet-S-PEs over the LSPs. Now if you have an ECMP path between the 
> detnet-T-PE and a detnet-S-PE, or you have SR or RSVP-TE available you 
> can also deliver multiple copies to the detnet-S-PE and take advantage 
> of the variability of transit time in the MPLS underlay.
>
> Now you create a new detnet-S-PE that operates as follows. On it's 
> ingress side it looks for the first packet at a given sequence number 
> on this PW (or PW set) and suppresses all future packets on that 
> sequence number on that PW or PW-set. It then replicates the packet if 
> required, swaps the PW label (note that it may also use multiple 
> outgoing labels) and send the packet over the egress LSP set.
>
> At the egress T-PE it looks at the sequence number on this PW (or 
> PW-set), trims all duplicates, applies any required egress processing 
> and send the packet on it's way.
>
> In summary on ingress a detnet-T-PE replicates to multiple S-PEs using 
> the PW label the detnet-S-PE expects and potentially sends the packet 
> over multiple paths to the detnet-S-PEs. At egress a detnet-T-PE looks 
> at the sequence numbers across the detnet-PW set and selects the first 
> of the sequence number suppressing all others, and sends the 
> underlying packet on its way. A detnet-S-PE is a back to back 
> detnet-egress-T-PE and a detnet-ingress-T-PE with a PW label swap in 
> the middle and no other PW processing.
>
> 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.
>
> - Stewart
>
>
>
> On 13/02/2017 04:11, Andrew G. Malis wrote:
>> Loa et al,
>>
>> To be clear, there’s currently no definition of PWs encapsulated in 
>> PWs, and while it might be conceptually possible, such as an Ethernet 
>> PW carried within a SONET/SDH PW, I couldn’t imagine a use case for 
>> doing it as it’s very inefficient, and I asked Loa if he had one. And 
>> if you were to do so, each PW in the hierarchy would need NSP 
>> functionality and real or emulated CE access circuits at the 
>> endpoints. Also, thinking about it some more, you couldn’t have both 
>> PWs in the same label stack, since a PW emulates a physical circuit. 
>> So there would need to be a separate label stack (and MPLS LSP) 
>> inside the emulated circuit for the outer PW. By definition, PW 
>> labels terminate a label stack.
>>
>> As I haven’t been following Detnet at all, I don’t have the context 
>> for what you’re trying to accomplish. That said, I’ll take a look at 
>> the slides and let you know if I have any comments.
>>
>> Cheers,
>> Andy
>>
>>
>> On Sun, Feb 12, 2017 at 8:31 PM, Loa Andersson <[e-mail] 
>> <mailto:[e-mail]>> wrote:
>>
>>     Folks,
>>
>>     The mail from Yuanlong mmade me go back and check the PW
>>     architecture and consult with Andy Malis and Stewart Bryant. So I
>>     have one more thing
>>     that we should discuss on "Tuesday".
>>
>>     What Yaunlong said was: "IMHO, multiple layers of PW is a break from
>>     the PWE3 architecture, and all DP/CP/MP things will become more
>>     complicated."
>>
>>     It is correct that multi-layer pw's is problematic, though Andy said
>>     that "if you have a good use case, you can do it".
>>
>>     The problem is that there is a native service processing (NSP) at
>>     the end of the PW. Multi-layer PWs will only do NSP at one level.
>>     I think
>>     we should replace the MS-PW with an LSP. I've added one slide (9) and
>>     change slide 8,9 and 11 in the earlier package. The other slides
>>     arere
>>     for reference
>>
>>     I want Andy and Stewart to have a chance to review this prior to that
>>     we commit too hard to it. Copied them.
>>
>>
>>     /Loa
>>     -- 
>>
>>
>>     Loa Andersson                        email: [e-mail]
>>     <mailto:[e-mail]>
>>     Senior MPLS Expert [e-mail] <mailto:[e-mail]>
>>     Huawei Technologies (consultant)     phone: [phone]
>>     <tel:%2B46%20739%2081%2021%2064>
>>
>>
>