Subject: RFC3697 [Re: [RRG] FLOWv6: IPv6 Flow Label to control DFZ forwarding]

From: Brian E Carpenter <[email-masked]>
Date: Fri, 01 Aug 2008 19:28:34 +1200

Robin,

> 1 - We would replace the current semantics of the Flow Label:
> 
>       http://tools.ietf.org/html/rfc2460#appendix-A

That appendix is non-normative and effectively obsolete. Please see
RFC 3697. (Unfortunately, this doesn't show in the RFC index as an
update to 2460, since the appendix is non-normative.)

You will find that the bounding semantics of the flow label
are quite well defined in 3697. The history is that there was
a very clear consensus in the IPv6 WG that the semantics
should be defined strictly end to end between hosts; use as
a routing handle was explicitly set aside.

I can't assert that there is no use of the flow label, since
it has consenting-host semantics. On the other hand, no use cases
have been documented to RFC level since RFC 3697 came out,
and I've never seen any statistics or traces indicating
non-zero flow labels. The flow-aware router people planned
to use it at one stage, but that is now caught up in ITU-T
process following on ITU-T Y.2121 (see
draft-adams-tsvwg-flow-signalling-codepoint-00). I don't
know for sure if they still plan to use the flow label.

So, if the mechanism you propose (I haven't read further
as I type this) is incompatible with the bounding semantics
in RFC 3697, quite some work in the IETF would be needed
to resolve this.

    Brian

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---

From: Robin Whittle <[email-masked]>
Date: Fri, 01 Aug 2008 18:40:47 +1000

Hi Brian,

Thanks very much for researching this.

Were you or any other RRG people involved in the creation of the
Flow Label?  I see you were a co-author of:

  http://tools.ietf.org/html/rfc3697

> a routing handle was explicitly set aside.

OK.

> know for sure if they still plan to use the flow label.

http://tools.ietf.org/html/draft-adams-tsvwg-flow-signalling-codepoint-00
(2008-06-24)

It is high-level discussion of a flow-related control packet,
implicitly one for IPv6. There is no specific mention of the Flow
Label, but I assume they are planning to use it.

Although IPv6 is not widely used, I think it has been extremely well
researched for all manner of networking enhancements.

If the Flow Label has been there from the start, and in 14 years has
yet to find a substantial use, maybe these bits could be renamed and
used for a different purpose with something like FLOWv6, if this
proves to be a markedly better approach to scalable routing than the
alternatives.

With something like FLOWv6, there would be nothing to stop the Flow
Label being used for any purpose in private networks.  However it
would be ugly to do this and not have it work if the packet went
through, or anywhere near, the DFZ.

  - Robin

---

From: "Tony Li" <[email-masked]>
Date: Fri, 1 Aug 2008 12:20:51 +0100

|Were you or any other RRG people involved in the creation of the
|Flow Label?  

As an onlooker, my perception was that the IPv6 header ended up having too
many free bits in it and that compressing it would have cause unfortunate
consequences for header alignment on 64 bit processors, so the thought was
to leave it effectively as reserved space.  However, there were so many
poorly justified uses for the space, that the IPv6 folks decided to allocate
it to the then-popular notion of a flow label.  This kept others from
requesting header bits and did not force them into a precise semantic
definition for the field.

Effectively, today, it's reserved space.

My $.02,
Tony

---

From: Brian E Carpenter <[email-masked]>
Date: Sat, 02 Aug 2008 18:40:48 +1200

> Effectively, today, it's reserved space.

One of the reasons that I helped write RFC3697 was to open the way
for efforts to change that, but not much has happened. There are lots
of things you could use it for today and stay conformant with
RFC2460 and RFC3697, but as I said, nobody does it.

However, I don't think 20 bits is enough for what Robin has in mind.
Effectively it only allows for 2^20 RLOCs and that is only 4 times
today's DFZ if I have the arithmetic right.

    Brian