Re: [discuss] How to enable "flow label" in IPv6?

[Toomas Soome via illumos-discuss]

> On 3. Feb 2024, at 07:50, Bill Sommerfeld via illumos-discuss 
> <discuss@lists.illumos.org> wrote:
> 
> On 2/2/24 21:41, Dan McDonald wrote:
>>> On Feb 2, 2024, at 11:07 PM, Jesus Cea <j...@jcea.es> wrote:
>>> 
>>> I am using IPv6 and I noticed that by default SmartOS uses a flow label of 
>>> 0 (no flow label) in its IPv6 datagrams. I wonder how could I enable the 
>>> use of flow labels in order to be more compatible with for example, load 
>>> balancers (that hash (origin ip, destination ip, flowlabel) to route to a 
>>> backend server). For example, as described in 
>>> https://datatracker.ietf.org/doc/rfc7098/
>> If you are using pre-compiled software, that could be a useful RFE to have 0 
>> == random if a IP netstack parameter is set.  (I'm curious how BSD and Linux 
>> do this?)
> 
> linux randomizes by default; this is explicitly encouraged by RFC6437 
> (https://www.rfc-editor.org/rfc/rfc6437) and its successors.
> 
> It's IMHO not really reasonable to expect applications to do this, and the 
> TCP accept/listen API doesn't give servers a good place to inject a flow 
> label for the server->client direction.
> 
> So we should do this by default; for TCP it's easy (set at connection 
> establishment time to a uniformly distributed value between 1 and 2^20-1).  
> Appendix A of RFC6437 has a recommended hash function for this but anything 
> sufficiently random should work.
> 
> For IPsec, you want to use 20 bits of the SPI.  You might think that in 
> tunnel mode you could use the inner flow label of the encapsulated packet, 
> but that would spread your SA traffic across multiple paths and blow up your 
> receiver's replay detection windows.
> 
> Another trick TCP can play with flow labels that I'm aware of: in networks 
> with significant multipathing it's been found useful to reroll the flow label 
> dice after loss or congestion - this juggles long-lived flows across 
> different paths and reduces overall congestion.  A paper on this worth 
> reading: https://dl.acm.org/doi/10.1145/3544216.3544226
> 
>                                       - Bill
> 
> 

is the flowadm(8) related there?

rgds,
toomas
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