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

<> Thu, 09 June 2016 06:49 UTC

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From: <>
To: Ian Farrer <>, Softwires WG <>
Thread-Topic: Problem in draft-ietf-softwire-multicast-prefix-option
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Date: Thu, 9 Jun 2016 06:49:09 +0000
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Subject: Re: [Softwires] Problem in draft-ietf-softwire-multicast-prefix-option
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Hi Ian,

There are two options:


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

This is the behavior defined in


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

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.


De : Ian Farrer []
Envoyé : mercredi 8 juin 2016 19:10
À : BOUCADAIR Mohamed IMT/OLN; Softwires WG
Cc :
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.


Begin forwarded message:

From: <<>>
Subject: RE: Problem in draft-ietf-softwire-multicast-prefix-option
Date: 8 June 2016 16:50:51 CEST
To: ian Farrer <<>>, "<>" <<>>, Jacni Qin <<>>, "<>" <<>>
Cc: "<>" <<>>

Hi Ian,

Please see inline.


De : ian Farrer []
Envoyé : mercredi 8 juin 2016 16:11
À :<>; BOUCADAIR Mohamed IMT/OLN; Jacni Qin;<>
Cc :<>
Objet : Problem in draft-ietf-softwire-multicast-prefix-option


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.