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

[Norman Finn <[e-mail]>]

There is certainly a problem processing sequence numbers at very high speed.  Unless/until the hardware is able to do that, this technique probably won't be used at really high speed.  You can't do what you can't do.  In the meantime, it can be very useful for lower speeds.

Discarding out-of-order packets doesn't makes the whole exercise pointless, but does decrease its utility.  There are two major classes of use cases known:

 1. "Intermittent flows": Where the difference in latency between paths is smaller than the interval between transmissions.  This is a typical case in machine control applications.  In this case, the long-path repeated packet is received before the next short-path packet.  Discarding out-of-order only has the usual issues with reset transmitters.

 2. "Bulk flows":  Where there are several packets more in flight along the slow path than the fast path.  This is a typical case in super high-definition video applications.  Tossing out-of-order packets makes the packet replication/elimination pointless; The object is to receive every packet, even if out-of-order.

-- Norm

________________________________
From: Detnet-dp-dt [[e-mail]] on behalf of Stewart Bryant [[e-mail]]
Sent: Monday, February 13, 2017 4:56 AM
To: Andrew G. Malis; Loa Andersson
Cc: [e-mail]
Subject: Re: [Detnet-dp-dt] detnet LSPs and PWs


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>