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

[Stewart Bryant <[e-mail]>]

On 15/02/2017 04:45, Loa Andersson wrote:
> Folks,
>
> Actually I think (except for the nomenclature, which I think we should
> adopt) of what Stewart says is there in the new slides.
>
> There are a few "if" that I don't agree to a design (maybe it may be
> possible to collapse the design), e.g. assuming one single domain.
>
> I think there are some design decision that need to be there.
>
> - replication and elimination of packets at places/nodes that the
>   operator can control, i.e. not every node should do it

Sure, a regular S-PE would work like a regular S-PE and simply
forward all packets on the PW to the x-PE it was configured
to forward to.

> - with an "S-detnet-PE" in the path (for replication/elimination) we
>   need three levels of labels. The implication is that if we don't do
>   replication/elimination other than at the T-detnet-PWs then we only
>   need two layers.

I am not convinced that you need three labels. As I explained you can
do it with a PW over an LSP if you create a new PW or PW set if needed.

Apart from Ethernet, what types of packet do you anticipate carrying?


> - what we called "d-pw" is not a new pw, but a pw from the PWE3/PALS
>   inventory

I am not sure that makes sense.

>
> Now that last bullet is a problem, the sequence number is 16 bits in
> PWE3/PALS and we know that is to small. Maybe we have to define a detnet
> PW after all. A PW that carries sequence number + another PW.

So, you really don't want to use the existing Ethernet s/n in this 
anyway. It
has a horrible skip zero count. What I would think about is to follow
the CW with a 32bit counter. There are PWs that have an 8 octet CW.
You need to retain the first nibble design and the length design (to allow
for short packets), so I would use something like:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0| Flags |FRG|  Length   | Reserved                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Sequence Number                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Then do OAM with the normal 0001 GACH


>
> I've been  thinking hard about the equivalence relationship, but is 
> still not sure it will work in the generic case, equivalence
> relationship tells node D that it should treat L1 from B and L2 from C
> the same, which is fine, but it does still not tell us that the packet
> came from A originally.

You know from the PW label which PW it is. PWs are connection oriented.
In a MS-PW you may not know that A is A since you only really know about
the last hop S-PE.

However I am not sure I understand the underlying problem.

>
> BTW - I think we need some type of sign-off from Stewart/Andy before we
> commit to hard.
>
> Also the problem of comparing sequence numbers arriving over
> different physical interface seems to be a eal problem.

Yes, that is what worries me most.

- Stewart

>
> /Loa
>
> On 2017-02-13 18:55, 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>
>>>
>>>
>>
>>
>>
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