Re: [Detnet] [mpls] [Pals] draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review the PALS/MPLS/DetNet Joint Session minutes)

[Gyan Mishra <[e-mail]>]

Hi Bruno

Responses in-line

On Fri, Apr 1, 2022 at 1:56 PM <[e-mail]> wrote:

> Hi Gyan,
>
>
>
> Thanks for the clarifications questions.
>
> I’ll take the liberty to top post to re-organize in two distinct points.
>
>
>
>    1. RFC 6790 specification of the use the EL’s TTL field.
>
>
>
> You have correctly highlighted the two relevant text:
>
> On the receiver side, §4.1 says “The EL's TTL MUST be ignored.”
>
> On the sender side, §4.2 says “The TTL for the EL MUST be zero”
>
>
>
> “a” So do we agree that RFC 6790 says “MUST be sent as zero and MUST
> ignored on receipt” ?
>
    Gyan Yes…but ..RFC 6790 defines the value of the TTL, however RFC 3032
defines the rules for processing all the fields of the label stack
encoding.  RFC 3032 Section 2.4.2 describes the processing when TTL is set
to 0 which means the TTL has expired and the packet must not be forwarded.
RFC 6790 section 4.2 bullet 4 states “The TTL for the EL MUST be zero to ensure
that it is not used inadvertently for forwarding.”  My interpretation of
that is even though the specification states the label stack is {TL,ELI,EL}
and that at the PHP node when the TL is popped in the case of implicit null
packets is forwarded to the egress LER which pops both ELI and EL.
However, the sentence is stating the TTL must be 0 for EL so the packet
cannot be inadvertently used for forwarding which I am interpreting as
there is a possibility that the EL could be exposed at the top of the
stack.  If hypothetically the case where the EL were not exposed were not
possible then it would have been sufficient to state that the TTL is
ignored on receipt and not worry about TTL processing which is a concern it
seems as stated in RFC 6790.  I think we have to understand why RFC 6790 is
requiring the TTL of EL MUST be set to 0.  So based on implementations of
MPLS their maybe possibilities that that even though the ELI must proceed
the EL, maybe they are corner cases where the ELI is missing label
imposition on the LER and thus the EL is exposed and required the EL to
have its TTL set to 0.  I think for this we would have to dig into
implementations of MPLS label stack and see what implementations have done
and if they are setting the TTL to 0 or not.  If we find that most all
implementations of RFC 6790 are not setting the EL TTL to 0 then it’s
possible we can reuse the TTL field as entropy label control field.  We
would have to really understand the corner case with missing ELI and how
the EL can possibly be exposed and that possibility as reasons the authors
of RFC 6790 stated the verbiage that has drawn my attention.

> “b” Do we agree that this how reserved field are defined at the IETF (e.g.
> https://datatracker.ietf.org/doc/html/rfc7176#section-2.1.1) ?
>
>    Gyan> So this I am guessing in RFC 7176 is an example of how Reserved
field is encoded with TRILL.  I don’t see the TTL field being marked as
reserved in RFC 3032.  I am missing your point here.

“c” Do we agree that the way this field is specified (cf “a”) has always
> allowed the IETF to further extend this field?
>
>  Gyan> RFC 3032 defines the label stack and RFC 6790 defines an entropy
> label for load balancing two new 4 byte label shims ELI and EL which use
> the same label stack encoding and processing rules defined in RFC 3032
> MPLS-SHIM.  I don’t see anywhere in RFC 3031 or 3032 that states that the
> TTL field can be further extended or repurposed for other uses.  I am
> afraid that if we use the field repurposed for something else it may have
> dire consequences with load balancing.
>
> 2)    Implicit null /PHP
>
> In case of PHP, the transport label is removed by the PHP and the ultimate
> node (egress LER) receives a label stack with the ELI, EL as top two labels.
>
> As per MPLS architecture, LER looks at the top label (ELI) and either:
>
> - supports RFC 6790 and then applies “Y then recognizes the ELI and pops two labels: the ELI and the EL.” In no way the EL is exposed and used for forwarding.
>
> - does not support RFC 6790 and hence will drop the packet as per RFC 3031
> (§3.18). In no way the EL is exposed and used for forwarding.
>
>
>
> So in summary, the EL is never exposed (as top label) and can never be
> used for forwarding.
>
>  Gyan>  Please see my first long comment related to RFC 6790 section 4.2
> bullet 4 states “The TTL for the EL MUST be zero to ensure that it is not
> used inadvertently for forwarding.”
>

    Gyan> Section 3.5 of NS packet draft mentions how a IETF network slices
map to a slice flow aggregate.
“ the routers correlate markers

present in the packets that belong to the Slice-Flow Aggregate.


So is the idea that you would split the 20 bit EL label carve out

bits to be used for slice flow aggregate and remaining bits to be

preserved for LB function.  The common size that has been used for flow

identifier has been a 20 bit field which is the size of MPLS label used

as well for IPv6 flow label where the 5 tuple header hash keys are used

to generate the 20 bit flow label which is an input key to hashing

function for stateless uniform load balancing.  VXLAN source port entropy uses

a 5 tuple header hash to generate a 16 bit source port input key to
hashing function.

I think the tradeoff here is how many bits to use for slice aggregate without

diminishing the load balancing functionality.


Gyan> RFC 8662 SR entropy refers to RFC 6790 on the ELI, EL semantics
and does not

get into the details described in RFC 6790. So as far as what I mentioned above

related to discussion that TTL Must be ignored and be set to 0 is not
mentioned in RFC 8662.  So there

maybe a gray areas as to RFC 8662 implementations as to what was actually done.



>
> Regards,
>
> --Bruno
>
>
>
>
>
> Orange Restricted
>
> *From:* Gyan Mishra <[e-mail]>
>
> *Sent:* Thursday, March 31, 2022 4:58 PM
> *To:* DECRAENE Bruno INNOV/NET <[e-mail]>
> *Cc:* Henderickx, Wim (Nokia - BE/Antwerp) <[e-mail]>;
> John E Drake <[e-mail]>; detnet WG <[e-mail]>; mpls <
> [e-mail]>; [e-mail]
> *Subject:* Re: [Detnet] [mpls] [Pals]
> draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review
> the PALS/MPLS/DetNet Joint Session minutes)
>
>
>
> Hi Bruno
>
>
>
> Please provide clarification on how existing implementations using Entropy
> Label ELI/EL RFC 6790 with your proposal to reuse the entropy label.
>
>
>
> In section 2 you talk about the new entropy label control field in your
> proposal to reuse the TTL field as the entropy label control field.
>
>
>
> I have some questions related below that are concerning with your proposal.
>
>
>
> RFC 6790 section 4 talks about the TTL processing below excerpt.
>
>
>
>    If an ingress LSR X chooses to impose an EL, then Y will receive a
>
>    tunnel termination packet with label stack <TL, ELI, EL> <remaining
>
>    packet header>.  Y recognizes TL as the label it distributed to its
>
>    upstreams for the tunnel and pops it.  (Note that TL may be the
>
>    implicit null label, in which case it doesn't appear in the label
>
>    stack.)  Y then recognizes the ELI and pops two labels: the ELI and
>
>    the EL.  Y then processes the remaining packet header as normal; this
>
>    may require further processing of tunnel termination, perhaps with
>
>    further ELI+EL pairs.  When processing the final tunnel termination,
>
>    Y MAY enqueue the packet based on that tunnel TL's or ELI's TC value
>
>    and MAY use the tunnel TL's or ELI's TTL to compute the TTL of the
>
>    remaining packet header.  The EL's TTL MUST be ignored.
>
>
>
>
>
> So the TL or ELI is used to compute the TTL of the remaining packet
> header.  States that EL’s TTL is ignored.
>
>
>
> Section 4.2 mentions that the TTL for the EL MUST be set to 0 so it’s not
> used for forwarding.  The issue here is related to implicit null PHP case
> where the TL is popped and ELI,EL are exposed and to ensure that the EL is
> not used for forwarding the EL MUST be set to 0.
>
>
>
>    4.  If, for the chosen tunnel, Y has not indicated that it can
>
>        process ELs, push <TL> onto the packet.  If Y has indicated that
>
>        it can process ELs for the tunnel, push <TL, ELI, EL> onto the
>
>        packet.  X SHOULD put the same TTL and TC fields for the ELI as
>
>        it does for TL.  X MAY choose different values for the TTL and TC
>
>        fields if it is known that the ELI will not be exposed as the top
>
>        label at any point along the LSP (as may happen in cases where
>
>        PHP is used and the ELI and EL are not stripped at the
>
>        penultimate hop (see Section 4.4 <https://datatracker.ietf.org/doc/html/rfc6790#section-4.4>).  The BoS bit for the ELI MUST
>
>        be zero (i.e., BoS is not set).  The TTL for the EL MUST be zero
>
>        to ensure that it is not used inadvertently for forwarding.  The
>
>        TC for the EL may be any value.  The BoS bit for the EL depends
>
>        on whether or not there are more labels in the label stack.
>
>
>
> The EL is not used for forwarding as long as the field is set to 0 which
> is a MUST.  However if you reuse the TTL field as the entropy label control
> field it will not be set to 0 and thus that could break implementations in
> the PHP case where the ELI/EL are exposed.
>
>
>
> Also the TTL being set to 0 is different then the field being actually a
> Reserved or not applicable field.
>
>
>
> I disagree with sentence below in section 2.
>
>
>
>    Hence essentially the TTL field of the EL behaves as a reserved field
>
>    which must be set to zero when sent and ignored when received.
>
>
>
> The TTL field as I stated MUST be set to 0 so it’s not used for
> forwarding.  So it’s not reserved and it’s read by the LSR looking for the
> field to be set to 0 so it’s not used for forwarding.  I can’t see how that
> won’t break existing implementations.
>
>
>
> Kind Regards
>
>
>
> Gyan
>
>
>
> On Thu, Mar 31, 2022 at 12:30 AM Gyan Mishra <[e-mail]>
> wrote:
>
>
>
> I like Bruno’s idea of reusing the entropy label as indicator of MEH in
> the label stack and is backwards compatibility for devices not supporting
> can continue to use for ECMP load balancing.
>
>
>
> I think this is a solid interim solution to get the ball rolling with
> minimal software updates and being able to support ancillary data in the
> label stack and as other solutions are progressed that may take longer or
> implement and deploy at least in the near term we have a quick solution
> that could be promising for operators.
>
>
>
> I think we do have to vett out the backwards compatibility and scenario I
> can think of is if you want to be able to use the entropy label for ECMP
> load balancing and simultaneously want to also use as ancillary data
> indicator I am guessing won’t work and that is something we would have to
> be cognizant of if deployed.
>
>
>
> Kind Regards
>
>
>
> Gyan
>
>
>
> On Wed, Mar 30, 2022 at 4:04 PM John E Drake <jdrake=
> [e-mail]> wrote:
>
> Wim,
>
>
>
> I think I would term it a thought experiment.  An RFC 6790 compliant node
> will take the value in the EL label field and use it to select an outgoing
> interface.  If the value in the EL field is a slice ID, such an node will
> select an outgoing interface which is not necessarily part of the slice in
> question and that outgoing interface will be to a node which is not
> necessarily part of the slice in question.
>
>
>
> Yours Irrespectively,
>
>
>
> John
>
>
>
>
>
> Juniper Business Use Only
>
> *From:* Henderickx, Wim (Nokia - BE/Antwerp) <[e-mail]>
> *Sent:* Wednesday, March 30, 2022 3:21 PM
> *To:* John E Drake <[e-mail]>; [e-mail]
> *Cc:* mpls <[e-mail]>; detnet WG <[e-mail]>; [e-mail]
> *Subject:* Re: [mpls] [Pals]
> draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review
> the PALS/MPLS/DetNet Joint Session minutes)
>
>
>
> *[External Email. Be cautious of content]*
>
>
>
> John, do you have evidence of this or is this a theoretical claim ?
>
>
>
> *From: *mpls <[e-mail]> on behalf of John E Drake <
> jdrake=[e-mail]>
> *Date: *Wednesday, 30 March 2022 at 19:13
> *To: *[e-mail] <[e-mail]>
> *Cc: *mpls <[e-mail]>, detnet WG <[e-mail]>, [e-mail] <
> [e-mail]>
> *Subject: *Re: [mpls] [Pals]
> draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review
> the PALS/MPLS/DetNet Joint Session minutes)
>
> Except that putting a slice ID in the Entropy Label field will break
> existing  ELI/EL Implementations because their hashing of the slice ID
> won’t necessarily place a packet on the correct outgoing I/F
>
> Sent from my iPhone
>
>
>
> On Mar 30, 2022, at 1:00 PM, [e-mail] wrote:
>
> 
>
> *[External Email. Be cautious of content]*
>
>
>
>
>
>
>
> *From:* Tony Li <[e-mail]> *On Behalf Of *Tony Li
> *Sent:* Wednesday, March 30, 2022 4:08 PM
>
> > [Kireeti]: suggest attending talk by Tony on danger of reusing ELI
> before making any decision.
>
> https://notes.ietf.org/notes-ietf-113-pals
> <https://urldefense.com/v3/__https:/notes.ietf.org/notes-ietf-113-pals__;!!NEt6yMaO-gk!Sw9ofU9AyD7Z-JKwyAqMlHk5xhNLxZNMSu31Yt6-K7yh-6JehvlSPLDcqrP3gOo$>
>
>
>
> Done. The talk raised no “danger of reusing ELI” for draft
> draft-decraene-mpls-slid-encoded-entropy-label-id; quite the contrary.
>
> I quote: “claims of backward compatibility apply to
> draft-decraene-mpls-slid-encoded-entropy-label-id-03”. With more details on
> slide 18
>
>
> https://datatracker.ietf.org/meeting/113/materials/slides-113-mpls-05-policy-on-mpls-special-purpose-labels-reuse-00
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/meeting/113/materials/slides-113-mpls-05-policy-on-mpls-special-purpose-labels-reuse-00__;!!NEt6yMaO-gk!Sw9ofU9AyD7Z-JKwyAqMlHk5xhNLxZNMSu31Yt6-K7yh-6JehvlSPLDcNEC7QKk$>
>
>
>
>
>
> Yes, the issue with this proposal is that it has no space for in-stack
> data and not enough space for possible expansion of additional actions.
>
>
>
> [Bruno] There are two steps:
>
> - This proposal allows for carrying 8 Indicators and a slice ID while been
> backward compatible with egress LER hance providing faster deployment with
> incremental benefit.
>
> - If more in-stack data is required the proposal is extensible (e.g.
> draft-jags-mpls-ext-hdr) but at the cost of losing the above benefits for
> the ASes & uses-cases requiring more than 8 Indicators per AS or In-Stack
> Data.
>
> So we can have both worlds: simple first step and extensibility for those
> who need it.
>
>
>
> Independently, we also/already have the post stack data option to carry
> ancillary data, which may limit the need for In-Stack data extension.
>
>
>
> --Bruno
>
>
>
> Tony
>
>
>
>
>
>
>
> Orange Restricted
>
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>
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> --
>
> <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions Architect *
>
> *Email [e-mail] <[e-mail]>*
>
> *M 301 502-1347*
>
>
>
> --
>
> <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions Architect *
>
> *Email [e-mail] <[e-mail]>*
>
> *M 301 502-1347*
>
>
>
> _________________________________________________________________________________________________________________________
>
> Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc
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*Gyan Mishra*

*Network Solutions A**rchitect *

*Email [e-mail] <[e-mail]>*



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