Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]

Brian E Carpenter <> Mon, 16 January 2017 19:57 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id F22871299F6 for <>; Mon, 16 Jan 2017 11:57:56 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2
X-Spam-Status: No, score=-2 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id lrYAqkmRuZbp for <>; Mon, 16 Jan 2017 11:57:55 -0800 (PST)
Received: from ( [IPv6:2607:f8b0:400e:c05::235]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 8F07012962C for <>; Mon, 16 Jan 2017 11:57:55 -0800 (PST)
Received: by with SMTP id 194so17858936pgd.2 for <>; Mon, 16 Jan 2017 11:57:55 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=subject:to:references:cc:from:organization:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding; bh=Xg9aF18//bHp2tZYoipF8xFHtm92bAsqyWp2CNtr598=; b=WAwjwuJsJVF8+QZgQiA4n7eZlulsHR3Ppg8JxGYxtd2y5jVOeATzaBtsgqHmZPVluc x6FSTVJTvMSawySpcmcwyocdS6LLNXBRPTS56Vuy87JwgeJNxLWXWnO3BGY1oxKaV6AW H404mmCUV1EHVTvXxo0af0Ad/rxSEoHoPpoCe268nSTYF+UtGCny9SIiC4JQAKi7xezp 9AM0TecAnSFMGuXjmsDAcWYEWB3xFqpVFkx0llmhEgX3hkPVx8c7szb2AgOqAAAKbLUj Neof1WYKD5cEGmoke9Cd02wlapyUdTlnHluxFmL0o+Vl/jfYY0bXFqCFJAW/EpritPFU 9y1w==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=x-gm-message-state:subject:to:references:cc:from:organization :message-id:date:user-agent:mime-version:in-reply-to :content-transfer-encoding; bh=Xg9aF18//bHp2tZYoipF8xFHtm92bAsqyWp2CNtr598=; b=ZB10JbekGtL3ycp1LEor+jgsnhhPJ1XSxILVToPcHP+w81/dPdrA9uhsgpfruZxhF2 v6SyulIn+qpAW0PypB+i6jDixTsyqjtjLBzyEvKM98MeJ5JW8v0KhwJkGTInUnkNPmDD Zkvlz7MKusuNIvQAZ5nGXAuw4U5WJGxAZAvOMXAHuligYtcxThy5l4q7EU/mjQboa0hJ Mzc0duamIe4MAFLc1IItwN8kgWlJ/D/HirONWAuTF3J84GqfYbLVhhklB2dZihCz+II2 FsaRyQEyWe5fguvVonTBWrJbbbNNhQkQfwYFCFKt9DaMMkSdTWifdh3XQb9i9Xee/Zxq JaPg==
X-Gm-Message-State: AIkVDXJOnLQcOzxoywAKfM3HaADtxdolZRjeZLFzpuYn+jPfgTELGvf3yDRbbFRnReJYtA==
X-Received: by with SMTP id n11mr11091329pfi.65.1484596675069; Mon, 16 Jan 2017 11:57:55 -0800 (PST)
Received: from ?IPv6:2406:e007:4961:1:28cc:dc4c:9703:6781? ([2406:e007:4961:1:28cc:dc4c:9703:6781]) by with ESMTPSA id i82sm1046345pfk.52.2017. (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 Jan 2017 11:57:54 -0800 (PST)
Subject: Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]
To: Lorenzo Colitti <>
References: <> <> <> <> <> <> <> <> <> <> <> <> <2A5073777007277764473D78@PSB> <> <> <> <> <>
From: Brian E Carpenter <>
Organization: University of Auckland
Message-ID: <>
Date: Tue, 17 Jan 2017 08:57:51 +1300
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0
MIME-Version: 1.0
In-Reply-To: <>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Archived-At: <>
Cc: 6man <>
X-Mailman-Version: 2.1.17
Precedence: list
List-Id: "IPv6 Maintenance Working Group \(6man\)" <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 16 Jan 2017 19:57:57 -0000

On 16/01/2017 22:36, Lorenzo Colitti wrote:
> Brian,
> what's the specific rationale for this change? Is it a bug in 4291 which
> you're proposing that we resolve in 4291 bis? If so, what is the bug?

The bug is that in SLAAC, the IID length is a parameter, not a constant,
and that in routing protocols, the prefix length is a parameter, not
a constant. The addressing architecture needs to recognise that.

> The "interface identifiers are 64 bits" long text in RFC 4291 goes back all
> the way to 1998 and the text below would be a major change to text that has
> likely been baked into implementations for almost two decades years. I
> don't see why we would change that now.

Any SLAAC implementation that has 64 baked into it is already non-conformant.
But I very much doubt if anyone will need to change their code as a result
(except for any non-conformant routers such as you mentioned recently).

> BTW: if the reason for the text is a perceived contradiction between the
> fact that "IIDs are 64 bits" and "IPv6 addresses are aggregatable on all
> bit lengths" - I don't see a contradiction. 

I suggest discussing that with Randy Bush.

> In general, IPv6 addresses are
> aggregatable on all bit lengths. But global unicast addresses (other than
> ::/3) have a 64-bit IID, so links in that space are assigned 64-bit prefix
> lengths. These two properties can coexist even in global unicast space. For
> example, you could load-balance traffic to a given global unicast /64 by
> announcing it to the backbone as two /65s. The "addresses are aggregatable
> on all bit lengths" text means that the /65 are valid prefixes that can be
> routed by routers.

Sure. And this text doesn't aim to change anything in any current (and
non-broken) implememtations. It aims to respond to the objections that have
been discussed on the IETF list recently.


> Cheers,
> Lorenzo
> On Sun, Jan 15, 2017 at 4:49 AM, Brian E Carpenter <
>> wrote:
>> NEW
>>    IPv6 routing is based on prefixes of any valid length up to 128
>> [BCP198].
>>    For example, [RFC6164] standardises 127 bit  prefixes on point-to-point
>>    links. However, consistent use of Stateless Address Autoconfiguration
>>    (SLAAC)[RFC4862] requires that all interfaces on a link use the same
>> length
>>    of Interface ID. In practice, this means that to guarantee
>> interoperability
>>    of SLAAC, a fixed length of Interface ID is necessary. For all currently
>>    allocated unicast addresses, except those that start with the binary
>>    value 000, that length is 64 bits. Note that this value is an arbitrary
>>    choice and might be changed for some future allocation of unicast
>> address
>>    space. Background on the 64 bit boundary in IPv6 addresses can be found
>>    in [RFC7421].
>> Regards
>>    Brian
>> --------------------------------------------------------------------
>> IETF IPv6 working group mailing list
>> Administrative Requests:
>> --------------------------------------------------------------------