On 2/2/24 21:41, Dan McDonald wrote:
On Feb 2, 2024, at 11:07 PM, Jesus Cea <[email protected]> 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
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