Re: [Stackevo-discuss] some comments on draft-iab-protocol-transitions-05

Tom Herbert <tom@herbertland.com> Tue, 10 January 2017 23:52 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Tue, 10 Jan 2017 15:52:37 -0800
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To: Joe Touch <touch@isi.edu>
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Cc: "stackevo-discuss@iab.org" <stackevo-discuss@iab.org>, Dave Thaler <dthaler@microsoft.com>
Subject: Re: [Stackevo-discuss] some comments on draft-iab-protocol-transitions-05
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On Tue, Jan 10, 2017 at 2:50 PM, Joe Touch <touch@isi.edu> wrote:
> Hi, Dave (et al.),
>
> I had some thoughts on this doc, summarized below. I hope they're useful.
>
> Joe
>
> -------
>
> The discussion of IPv4-6 transition might include a discussion of paths
> not taken in IPv4, e.g., earlier versions included variable length
> addressing.
>
> Regarding new features, it might be useful to discuss how options are
> handled. We include them to future-proof a protocol, to ease the
> transition to new features. However, we too often succumb to commercial
> pressures to ignore this flexibility in favor of performance or economy.
> That's why IPv4 fragments are being dropped, why IPv6 options are now
> limited in total length, and why options are generally deprecated for
> traffic that expects to successfully traverse the Internet. It's also
> how we're boxing ourselves into needing IPv-next rather than augmenting
> what we have in our hands.
>
Agreed. The topic of extensibility to facilitate protocol transition
needs more discussion. The principle to "Design for extensibility
[RFC6709] so that things can be fixed up later." really doesn't seem
to have worked out very well for either IPv4 (IP options) or IPv6
(ext. hdrs.) in practice.

Tom

> There are a few design lessons here that might be useful to point out. A
> discussion of TLV vs. fixed tag encodings would be useful. Reasons to
> put version IDs, or demuxing tags in most protocols would be useful too
> (as we learned for the TCP Experimental option codepoints). It might be
> useful to have a more detailed discussion of handling "TBD" fields,
> e.g., when they MUST be set to X by legacy sources, MUST be ignored vs.
> discarded if not zero by legacy receivers, and when to use each strategy.
>
> I.e., we're constantly oscillating between considering a "thin waist"
> either ossification or stability; the former when we actually need new
> capabilities (like more addresses), the latter when we actually want
> things to work (like running DNS over HTTP). We need to understand that
> to know whether and how we want to support transitions like these.
>
> ------
>
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