Re: [mpls] MPLS-RT review of draft-bryant-mpls-sfl-framework

Stewart Bryant <stewart.bryant@gmail.com> Thu, 04 May 2017 13:19 UTC

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To: Curtis Villamizar <curtis@ipv6.occnc.com>, mpls@ietf.org
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From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Thu, 4 May 2017 14:19:07 +0100
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Subject: Re: [mpls] MPLS-RT review of draft-bryant-mpls-sfl-framework
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On 28/04/2017 20:26, Curtis Villamizar wrote:
> Loa,
>
> The document is coherent and possibly useful.  The question of IPR
> should be resolved before accepting this as a WG document.
I will leave the IPR issue to the chairs.

>
> Briefly:
>
>    1.  The document proposes a change to forwarding.  Whenever a change
>        in forwarding is considered, there should be a very substantial
>        benefit and interoperability with older hardware should be
>        considered in the document.
The important thing is that the change to forwarding is confined solely 
to the
PEs that are using this function. There is no change to any of the P 
routers needed.

Furthermore, in the application that triggered this work, pro-active packet
loss measurement, the action required is to count the number of packets
received on a label, and that is something that a lot of chips already 
do. In such
a case the change is a technical change to the forwarding behaviour 
specifying
an action that is already deployed.

In other cases, where a net-processor is deployed, it is a case of 
vectoring from
the LFIB to a different  routine. Given that the additional 
functionality is needed
by the PEs such a change is not unreasonable.

>
>    2.  The document leaves open the question of dealing with ECMP in
>        section 4.  The first option in section 4 is not viable.
I do not understand why this is not viable, other than perhaps due to 
stack size limits.
The EL solution requires support for EL along the LSP which may not be 
available.
Indeed I am not sure how widely available it is. Thus I see option 1 as 
a method to be
deployed in the absence of EL support.

We can discuss whether this is listed first or second.

>        The
>        document should drop this and expand on the second "worthy of
>        consideration" solution, explaining how it will work in the same
>        level of detail as provided in 3.1, 3.2, and 3.3.

Sure, that can be added. Adding it is fairly simple, although I do not 
see why
that cannot wait until after adoption, since it is the duty of the editor to
reflect the consensus of the WG.

>
>    3.  In the VPN example a perceived advantage of MP2P LSP is lost.
>        With MP2P LSP, the number of application labels needed is the
>        number of VPN IDs.  The number of labels needed becomes number
>        of VPN IDs times the number in ingress.  If Section 3.3 is
>        supposed to address this, then this is not clear and therefore
>        could benefit by referring back to the example and stating that
>        the SFL could indicate source only and the application label
>        indicate VPN ID only.
Sure we can text to address the scaling concern, and expand the 
processing text.
Note that whether scaling is a concern or not will depend on how many VPNs
need SFL actions, and on the chosen ECMP approach.

>
> More detail:
>
> Forwarding of labeled packets currently involves some special
> processing of labels 0-15 and some form of lookup in a table or TCAM
> or other structure with the lookup resolving to a set of operations.
> At mimimum these operation are SWAP and forward to an egress
> optionally after considering ECMP (EL or rest of stack), POP and look
> at next label (or assume IP payload if no next label).  Forwarding
> hardware currently need not support a POP and look for IP payload.
> That operation is only required of the IPv4 Explicit NULL Label and
> IPv6 Explicit NULL Label.

I do not see what in the text causes you to raise this point. An SL has 
the same
semantics as the label it replaces plus a side-effect. If the original 
label implied
IP, so will the SL. As I said earlier, if the side-effect is other than 
increment a
counter (which is common anyway) then the side effect is new functionality
which needs implementing anyway.

> Second, both Explicit NULL labels in
> RFC3032 are required to be at the bottom of stack in RFC3032 with this
> restriction removed in RFC4182 for proper operation of the pipe model
> when not using an ordinary label POP at egress.
>
> The changes to forwarding are:
>
>    1.  Processing of an ordinary label (label not in the 0-15 range)
>        must be able to POP and look at the IPv4 or IPv6 payload to
>        conform to section 3.2 of this draft.  Currently this processing
>        can be fixed to label values 0 and 2 respectively.
>
>    2.  Processing of an ordinary label taking on the function of an
>        explicit NULL and processing of an explicit NULL label must be
>        capable of handling ELI and EL below the SFL or Explicit NULL
>        and then take the same action as would be taken for Explicit
>        NULL after the ELI and EL POP.  This would be needed to support
>        pipe model as described in RFC4182 and not change the
>        penultimate LSR to SWAP a lable for SFL or Explicit NULL and
>        remove ELI and EL from underneath.
>
> I'm assuming that SFL would not map to special labels other than
> Explicit NULL but such a statement should be made explicit.  Although
> if applied to RFC 6374 then SFL might also map to GAL.
I had been assuming that if you wanted a GAL you would put it in there.
I wonder if the EN equivalence is confusing and we should look at other
equivalences.

There are a number of alternative equivalence models we can look at that
have the same properties:

If you assume that the LSP label has already been popped (due to PHP) it
the SFL could be considered as equivalent to a non-popped LSP label.
Alternatively I suppose you could model the SFL as a VRF label with
the lookup performed in the base topology.
> If SFL can map
> to other special labels, then those others from the set of 0-15
> including mapping to ESPL should be mentioned.

We will put text in on the subject.

> Each time a new
> special label is defined there are complaints about changing
> forwarding.  If now any ordinary label can map to a specific set of
> special labels, or worse to any special label, then this is a
> substantial change in the definition of MPLS forwarding.
That is not where I think this should go either. I think that it needs to
behave exactly as MPLS normally behaves except for the agreed side
effect in PEs configured to have the required behaviour.

>
> btw- In practice the IPv6 Explicit NULL is rarely if ever used but
> instead POP of an ordinary label at BOS, label zero (IPv4 Explicit
> NULL) at BOS, or lack of a label stack after PHP looks for the IP
> version in the payload.  Does any RFC say to do this (combine the
> meaning of IPv4 Explicit NULL and IPv6 Explicit NULL and look at IP
> version in the payload) or is this just common practice for almost 20
> years that escaped getting documented?

I am not sure this impacts this draft.

>
> The second issue relates to ECMP.  Section 4.1 item 1 suggests that
> "The operator can elect to always run with the SFL in place".  If the
> value of SFL is different for data traffic and PM traffic, then the PM
> would not take the same path.  This would put each "flow" in a
> multipoint to point LSP (ie: LDP) on a separate path which for ECMP is
> not an issue.
I would expect that the instrumentation packet go with the same SFL it is
collecting results for.

>
> If the only purpose of the SFL was maintain separately counters for
> specific multipoint to point LSP ingress then lack of SFL for other
> ingress has no effect.  In this case the "solution" described in
> Section 4.1 item 1 is a NOOP.
>
> If the purpose is to also provide separate PM for specific (or all)
> multipoint to point LSP ingress or provide separate counters for PM
> for a point to point LSP, then the "solution" described in Section 4.1
> item 1 fails.
>
> Section 4.1 item 1 therefore needs to be removed.

I don't think so.

If you instrument one flow on a long term basis and leave the SFL in place
the behaviour is always the same.  How does that fail?

>
> The implications of Section 4.1 item 2 need to be better explored.
> The section as-is constitutes a "punt" on the problem of ECMP with SFL
> used for PM.  A diagram of each case in Section 3 with ELI and EL can
> be added in Section 4 or the optional placement of ELI and EL.  The
> behaviour when ELI and EL are present can then be described in Section
> 4 with penultimate LSR behaviour in addition to egress LSR behaviour
> described for both PHP and non-PHP case.
That text can be expanded in a future version.

> Nits:
>
> In "Abstract" the phrase "on the on the" should be "on the".
Ack
>
> In Section 2 "defined to be a label" is awkward.
s/to be/as/

>
> An alternate:
>
> An alternate to the proposal here would be to make something like what
> is in Section 3.3 the only case, but where a ESPL range is used.  This
> would add egress processing without stepping on ECMP since SPL and
> ESPL are excluded from ECMP according to RFC 7274.

An interesting idea, but I am not sure how widely EL is supported, and 
it has the
disadvantage that it needs new dataplane changes to process the EL, whereas
the advantage of the proposal in the text is that for the common VPN and PW
case the existing h/w works out of the box in a lot of cases.

> An ESPL with a few
> top bits always set and less than 20 bits of range could be considered
> (for example XL + 1111xxxxxxxxxxxxxxxx for 16 bits of range and 1/16
> of the ESPL space in RFC 7274).  This range would be defined such that
> it is application specific, is added at ingress, completely ingnored
> along the way and provides the some egress specific functionality.

Not only is that new functionality in the core (ignoring a label 
following and EL)
but it then remains unclear how ECMP then works.

On the other hand we could (which is what I thought you meant) assign a 
meaning
to the EL value agreed between PEs and have the load balancing use the 
EL value
and have the SL behaviour be based on the SL value. However this is a 
forwarding
change and an ECMP path change, and can only be applied once whereas the
proposed design could have multiple SFLs in the stack, say to monitor
an LSP and a PW.

> Most implementations that support RFC 6790 are likely to also support
> the prohibition on using ESPL for ECMP since the documents were in the
> MPLS WG at about the same time, though publication of the latter
> lagged about a year.  This alternative also solves any scalability
> problem associated with combining source and application label
> functionality into one label.  It does add two labels to the stack
> depth but newer hardware should no longer be constrained to very
> shallow stack depth.  This alternate also might be an even more
> attractive option if IPR becomes an issue with the current proposal.

I have no idea how to respond to this given the IETF rules that prohibit 
discussion
of IPR terms.

>
> Summary:
>
> well written document.  not a great design.  multiple problems as
> defined.  possible IPR issue.
I agree with the first :)

I disagree with the second :(

Working through problems is part of the WG phase of development, and as 
far as
I can see the alternative you propose has problems. What I do know is 
that the
underlying problem we seek to solve is important and as far as I can 
tell this is
the best proposal on the table.

You keep saying IPR issues, but it is a deficiency of the IETF rules of 
conduct that
we cannot have a detailed discussion. The chairs and or ADs need to provide
guidance on how to respond to this.

- Stewart

>
> consider alternative.  any alternative but I provided one.  feel free
> to use the idea.  :-)
>
> Curtis
>
>
> In message <5774d63c-6980-103e-0b3f-a277db822462@pi.nu>
> Loa Andersson writes:
>> Huub, Andy, Curtis and Kamran,
>>   
>> You have been selected as MPLS-RT reviewers for
>> draft-bryant-mpls-sfl-framework-04
>> .
>>   
> [ ...]
>
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