Subject: Taking away bits from the flow label - a blast from the past

From: Suresh Krishnan <[email-masked]>
Date: Mon, 19 Mar 2018 11:26:36 -0400

<AD hat off>

Hi all,
  This is the draft I talked about at the 6man meeting today. I realized it was further back than I stated (8 years rather than 5).

https://tools.ietf.org/html/draft-krishnan-6man-header-reserved-bits-00 <https://tools.ietf.org/html/draft-krishnan-6man-header-reserved-bits-00>

I would not mind if we have that discussion again in the WG to see if something has changed. To summarize the goal of the draft

“ The IPv6 header does not contain any reserved bits and all bits in
   the header are currently accounted for.  This means that it is not
   possible to develop any future extension mechanisms that require bits
   in the IPv6 header.  Mechanisms such as ECN were possible in IPv4
   because there were bits available in the IPv4 header."

Cheers
Suresh

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From: Erik Kline <[email-masked]>
Date: Mon, 19 Mar 2018 08:39:48 -0700

Interesting idea, but...is it not already too late?

OSes randomizing flow labels is already a thing now...

The future utility of the bits would have to take into account the
possibility that they might be set randomly by a sender who doesn't
understand whatever the newly defined meaning might be.

:-/

On 19 March 2018 at 08:26, Suresh Krishnan <[email-masked]> wrote:
> <AD hat off>
>
> Hi all,
>   This is the draft I talked about at the 6man meeting today. I realized it
> was further back than I stated (8 years rather than 5).
>
> https://tools.ietf.org/html/draft-krishnan-6man-header-reserved-bits-00
>
> I would not mind if we have that discussion again in the WG to see if
> something has changed. To summarize the goal of the draft
>
> --------------------------------------------------------------------
> IETF IPv6 working group mailing list
> [email-masked]
> Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6
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>

---

From: Tom Herbert <[email-masked]>
Date: Mon, 19 Mar 2018 09:12:41 -0700

> Interesting idea, but...is it not already too late?
+1

> OSes randomizing flow labels is already a thing now...
Sending them as well as processing them in receive for flow steering.

> understand whatever the newly defined meaning might be.
Or received by a reciever that doesn't understand the new meaning!

I'd also point out that ECMP is not the only use case of the flow
label. For instance, we are using this for RFS (Receive Flow Steering)
in which case the flow label+IP addresses is used to create a 32-bit
hash for a flow and the that result is used to lookup per flow state.
The full 20 bits of flow label is important in the case of tunneling
(many flows sharing same IP addresses), reducing the space of usable
flow label reduces reduces entropy and increases chances of collisions
and hence reduces the effectiveness of the algorithm. RFS has been
deployed for more than five years.

Tom

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From: Suresh Krishnan <[email-masked]>
Date: Mon, 19 Mar 2018 12:41:47 -0400

Hi Erik/Tom,

> On Mar 19, 2018, at 12:12 PM, Tom Herbert <[email-masked]> wrote:
> 
> On Mon, Mar 19, 2018 at 8:39 AM, Erik Kline <[email-masked] <mailto:[email-masked]>> wrote:

Yes. I fear so as well and I said so during the meeting (for the exact same reasons you two stated). My mail was in response to comments in the room which said things might have changed and we should try to reserve some bits now.

Thanks
Suresh

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From: Fernando Gont <[email-masked]>
Date: Mon, 19 Mar 2018 15:56:50 +0000

+1

-- 
Fernando Gont
SI6 Networks
e-mail: [email-masked]
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492

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From: Brian E Carpenter <[email-masked]>
Date: Tue, 20 Mar 2018 08:36:39 +1300

> Yes. I fear so as well and I said so during the meeting (for the exact same reasons you two stated). My mail was in response to comments in the room which said things might have changed and we shou…

We pretty much excluded that in the discussions that led to RFC 6437. For some history and rationale, see RFC 6294 and RFC 6436.

    Brian

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From: joel jaeggli <[email-masked]>
Date: Mon, 19 Mar 2018 21:47:59 -0700

On 3/19/18 09:41, Suresh Krishnan wrote:
> Hi Erik/Tom,
> 
>> On Mar 19, 2018, at 12:12 PM, Tom Herbert <[email-masked]
>> <mailto:[email-masked]>> wrote:
>>
> Yes. I fear so as well and I said so during the meeting (for the exact
> same reasons you two stated). My mail was in response to comments in the
> room which said things might have changed and we should try to reserve
> some bits now.

My observation that flow labels on the internet did not always remain
constant for the duration of a flow in some cases, caused me to pen the
following epistle...

https://blog.apnic.net/2018/01/11/ipv6-flow-label-misuse-hashing/

Proposals  like the one that resulted in this thread. As well as the
temptation to do similar things  on a private label basis threaten to
make the flow label even less suitable for use in ip forwarding than it
is at present.