Re: Mirja Kühlewind's Discuss on draft-ietf-bfd-multipoint-18: (with DISCUSS)

[Jeffrey Haas <[e-mail]>]

Mirja,

You keep waving around RFC 8085 as a panacea.  Consider:

: 4.1.  Multicast Congestion Control Guidelines
: 
:    Applications using multicast SHOULD provide appropriate congestion
:    control.

Firstly, it's a SHOULD, and not a MUST.  Also, I believe we've made the
point more than once that BFD is used by an application set and it's free as
part of using BFD in multipoint mode to tune BFD.  BFD is intentionally not
self-tuning.

By analogy to the single hop unicast scenario, one doesn't expect BFD in the
face of congestion protecting the forwarding path for fast re-route
behaviors to slow down.  The point is that it *starts dropping packets* and
the BFD session drops.  Similarly, if a multipoint consumer of BFD sessions
starts having issues with forwarding, or something along the same path
(typically a multicast S,G), the expectation is that *it does something*.
This may include switching to a different path.

This is also covered in section 4.1 of 8085:

:    o  Receiver-driven congestion control, which does not require a
:       receiver to send explicit UDP control messages for congestion
:       control (e.g., [RFC3738], [RFC5775]).  Instead, the sender
:       distributes the data across multiple IP multicast groups (e.g.,
:       using a set of {S,G} channels).  Each receiver determines its own
:       level of congestion and controls its reception rate using only
:       multicast join/leave messages sent in the network control plane.
:       This method scales to arbitrary large groups of receivers.

Multicast networks will set the BFD configuration based on what their
expectations are for protection.  IETF can't blanketly tell users "you're
not allowed ot use BFD protection greater than X" nor can we arbitrarily
tell them we won't let them use a protocol intended to run at a high rate
that they can't do that.  In most cases, vendors are pushed to go for faster
detection times, not slower.

You can readily argue that BFD Multipoint shouldn't be used for generic
Internet multicast (e.g. mbone).  Fine - don't use it there.  But mbone
hasn't been the useful scenario for the deployment of multicast for the
majority of my career, nor is it the one that is the one funding the
vendors.

I strongly request that if the above text from 8085 doesn't convince you to
clear your discuss that you help arrange for a voice call to iterate over
lingering concerns sooner than later.  The chairs won't be attending IETF
103, sadly.  We'll find an agreeable time.

-- Jeff



On Mon, Oct 15, 2018 at 01:59:30PM +0200, Mirja Kuehlewind (IETF) wrote:
> Hi Martin,
> 
> please see below.
> 
> > Am 05.10.2018 um 10:57 schrieb Martin Vigoureux <[e-mail]>:
> > 
> > Hello Mirja,
> > 
> > time has passed since the last exchanges on that. To reinitiate the discussion, I come back to your original points.
> > 
> > 
> > Le 2018-07-03 à 20:31, Mirja Kühlewind a écrit :
> >> Mirja Kühlewind has entered the following ballot position for
> >> draft-ietf-bfd-multipoint-18: Discuss
> >> When responding, please keep the subject line intact and reply to all
> >> email addresses included in the To and CC lines. (Feel free to cut this
> >> introductory paragraph, however.)
> >> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> >> for more information about IESG DISCUSS and COMMENT positions.
> >> The document, along with other ballot positions, can be found here:
> >> https://datatracker.ietf.org/doc/draft-ietf-bfd-multipoint/
> >> ----------------------------------------------------------------------
> >> DISCUSS:
> >> ----------------------------------------------------------------------
> >> This mechanism has the potentially to easily overload the network as there is
> >> no handshake and therefore also no feedback mechanism (as already noted by the
> >> TSV-ART review of Bob - Thanks!). Regarding the base spec in RFC5880, this
> >> mechanism can only be used under certain constrains which should be clearly
> >> stated in this doc, which are:
> >> 1) See sec 6.8.1 of RFC5880:
> >> "bfd.DesiredMinTxInterval
> >>       [...] The actual
> >>       interval is negotiated between the two systems.  This MUST be
> >>       initialized to a value of at least one second (1,000,000
> >>       microseconds) according to the rules described in section 6.8.3."
> >> As there no negotiation in this spec, bfd.DesiredMinTxInterval MUST always be
> >> at least one second. Actually RFC8085 even recommend 3 sec (see sec 3.1.3).
> > 
> > There are two aspects to this.
> > First, draft-ietf-bfd-multipoint is consistent with 5880 on the initialization. So I think we are on the safe side.
> > Second, limiting a variable to only take certain values seems to me as being outside the scope of a protocol spec. We are touching there operational considerations.
> 
> I completely disagree. That’s what a spec is for. 
> 
> The point here is, that if you would want a smaller value, the network and system low gets higher and RFC8085 require congestion control. If no congested control is used or cannot be used, a lower value is not safe.
> 
> > If a user needs and wants to set a variable to a given value in a specific environment we can't forbid that. We can however raise his awareness on the potential consequences of a given choice.
> 
> You can enforce people implementing the spec correctly but you can specify the protocol correctly in order to make it safe to deploy it. If there are actual environment where a lower is safe to use that can be explicitly stated in the spec, however, if the talk about a part of the Internet (but not a data center or another separated, fully controlled environment) I don’t an exception is safe.
> 
> > 
> >> 2) See sec 7 of RFC 8085
> >> "When BFD is used across multiple hops, a congestion control mechanism
> >>    MUST be implemented, and when congestion is detected, the BFD
> >>    implementation MUST reduce the amount of traffic it generates. "
> >> As there is no feedback and therefore no congestion control, this spec can only
> >> be used for one-hop scenarios and the TTL or Hop Count MUST be set to one.
> > 
> > Rather than limiting the use cases of bfd-multipoint, I think we should set the same constraints than in base BFD spec.
> 
> One-hop is the constraint given in the base spec.
> 
> > 
> >> 3) Also given the traffic load multipoint BFD generates depends on the number
> >> of active session, and there is no feedback mechanism, I recommend to also
> >> limit the number of active session of MultipointHead type to a small number
> >> (per link).
> > 
> > x.  Operational Considerations
> > 
> >   Use of BFD in multipoint networks, as specified in this document,
> >   over multiple hops requires consideration of the mechanisms to react
> >   to network congestion.  Requirements stated in Section 7 of the BFD
> >   base specification [RFC5880] equally apply to BFD in multipoint
> >   networks.
> > 
> >   Furthermore, because a tail does not transmit any BFD Control packets
> >   to the head of the BFD session, Min RX Interval cannot be used to
> >   control the BFD packet transmission rate at the MultipointHead.  The
> >   mechanism to control the load of BFD traffic MAY use BFD's
> >   configuration interface to control BFD state variable
> >   bfd.DesiredMinTxInterval. Details of the interface and the mechanism
> >   itself are outside the scope of this document.
> > 
> >   Also, enabling BFD in such environments should be done considering
> >   the recommendations laid out in RFC 8085 [RFC8085].
> 
> In this case it is really not enough to say „please read RFC8085“. RFC8085 are consideration for the design of UDP based protocol and must be applied by the spec/protocol designer not the user.
> 
> Mirja
> 
> 
> 
> > 
> > 
> > I really hope that to be ok for the document to move forward.
> > 
> > Thank you
> > -m
> > 
> >