Re: [Softwires] Problem in draft-ietf-softwire-multicast-prefix-option

Ted Lemon <mellon@fugue.com> Thu, 09 June 2016 12:34 UTC

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From: Ted Lemon <mellon@fugue.com>
Date: Thu, 9 Jun 2016 08:34:12 -0400
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To: Ian Farrer <ianfarrer@gmx.com>
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Cc: Softwires WG <softwires@ietf.org>, "dhcwg-chairs@ietf.org" <dhcwg-chairs@ietf.org>
Subject: Re: [Softwires] Problem in draft-ietf-softwire-multicast-prefix-option
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Actually, this is a classic example of what we try to avoid: special server
behavior for options.   Options are just payload, not protocol, unless they
are intended to affect the flow of the protocol.   It should not be
necessary for the server to have special code to support a different
payload, any more than the TCP stack should have to be modified in order to
support new extensions to HTTP.

So you could make an operational note about how servers ought to be
configured, but you should not require that the server check to make sure
it is configured that way.   You should not require that the server
understand the semantics of the option--what it means for there to be two
of them, or what order they should occur in, or what meaning that ordering
has.

If you have two different uses for an option, with different semantics, you
should define two different options, each of which has only one semantics.

On Thu, Jun 9, 2016 at 3:44 AM, Ian Farrer <ianfarrer@gmx.com> wrote:

> Hi Med,
>
> To be clear, both of the suggestions include the modification to the
> Section 5 client text? I think this is necessary. But is the error
> condition that all of the enclosed options are the same scope, or that two
> or more are?
>
> Between the two, I would prefer (1) - a MUST NOT with an exception is a
> SHOULD NOT. An alternative wording that tries to get the best of both:
>
> A server MUST NOT send more than on one instance of OPTION_V6_PREFIX64 per
> scope. Servers MAY send one instance of OPTION_V6_PREFIX64 for each
> distinct IPv6 multicast prefix with a distinct scope.
>
> Thanks,
> Ian
>
> On 9 Jun 2016, at 08:49, mohamed.boucadair@orange.com wrote:
>
> Hi Ian,
>
> There are two options:
>
> (1)
>
> Maintain the current text in the draft but add the following text to
> Section 4:
>
>    Servers SHOULD NOT send more than one instance of OPTION_V6_PREFIX64,
>    except if distinct IPv6 multicast prefixes with distinct scopes are
>    configured.
>
> This is the behavior defined in
> https://tools.ietf.org/html/rfc6334#section-5/.
>
> (2)
>
> Add this text in Section 4:
>
>    Servers MUST NOT send more than one instance of OPTION_V6_PREFIX64,
>    except if distinct IPv6 multicast prefixes with distinct scopes are
>    configured.
>
> And modify the text in Section 5 to indicate that the client must discard
> the options if all enclosed addresses are of the same scope.
>
> I agree with you that (2) will help to identify a server error, but with a
> risk to induce service disruptions.
>
> Which one do you prefer to be implemented in the draft?
>
> Thank you.
>
> Cheers,
> Med
>
> *De :* Ian Farrer [mailto:ianfarrer@gmx.com <ianfarrer@gmx.com>]
> *Envoyé :* mercredi 8 juin 2016 19:10
> *À :* BOUCADAIR Mohamed IMT/OLN; Softwires WG
> *Cc :* dhcwg-chairs@ietf.org
> *Objet :* Fwd: Problem in draft-ietf-softwire-multicast-prefix-option
>
> Hi Med,
>
> Forwarding this to the list (the left group mailing addresses were not
> working earlier).
>
> Please see inline below.
>
> Thanks,
> Ian
>
>
> Begin forwarded message:
>
> *From: *<mohamed.boucadair@orange.com>
> *Subject: RE: Problem in draft-ietf-softwire-multicast-prefix-option*
> *Date: *8 June 2016 16:50:51 CEST
> *To: *ian Farrer <ianfarrer@gmx.com>om>, "softwires@ietf.org" <
> softwires@ietf.org>gt;, Jacni Qin <jacni@jacni.com>om>, "dxhbupt@gmail.com" <
> dxhbupt@gmail.com>
> *Cc: *"dhcwg-chairs@ietf.org" <dhcwg-chairs@ietf.org>
>
> Hi Ian,
>
> Please see inline.
>
> Cheers,
> Med
>
> *De :* ian Farrer [mailto:ianfarrer@gmx.com <ianfarrer@gmx.com>]
> *Envoyé :* mercredi 8 juin 2016 16:11
> *À :* softwires@ietf.org; BOUCADAIR Mohamed IMT/OLN; Jacni Qin;
> dxhbupt@gmail.com
> *Cc :* dhcwg-chairs@ietf.org
> *Objet :* Problem in draft-ietf-softwire-multicast-prefix-option
>
> Hi,
>
> On reviewing this draft I would like to raise a problem with section 5 of
> the draft. The text is:
>
> "If all the enclosed IPv4-embedded IPv6 multicast prefixes have the same
> scope, the first instance of the option MUST be used."
>
> The problem is that this contravenes section 17 of RFC7227:
>
> Option order, either the order among many DHCPv6 options or the order
>
>    of multiple instances of the same option, SHOULD NOT be significant.
>
>    New documents MUST NOT assume any specific option processing order.
>
>
>
> [Med] That sentence does not assume any (preference) order. It does only provide one way to select one instance among that list. As a reminder, the server is supposed to return one instance (per scope).
>
>
> [if - If the client is taking the first occurrence within the option and
> attempting to configure it, then it is preferring this option over other
> instances based on the order in which it occurs in the message.
>
> The current text which describes the server behaviour (section 4) does not
> mention scope at all and does not specify any limitations on the number of
> instances or criteria for which they may be included - see RFC7227 sec 16.
>
> If the server is meant to only return a single instance of the option per
> scope, but it is sending more than one, then this is a server configuration
> error. A quasi-random mechanism for the client to try and work around this
> just means that the configuration error may get masked.
>
> As a suggestion, wouldn’t it be better to specify what the valid cases for
> the server including multiple option instances are and are not (with
> normative language). The client’s behaviour can then be defined to discard
> the options if they do not meet this criteria?]
>
>
>
>
>
>
> I raised this with the DHC WG chairs, and they have a couple of
> suggestions:
>
> 1. Define an encapsulating option - as the data inside an option can be
> order dependent.
> 2. Add a “preference” (octet?) and then a client can sort them based
> on this preference.
>
> Thanks,
> Ian
>
>
>
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