Re: [RFC v2] RoCE v2.0 Entropy - IPv6 Flow Label and UDP Source Port

From: Tom Talpey <tom@talpey.com>
To: Mark Zhang <markz@mellanox.com>, Jason Gunthorpe <jgg@ziepe.ca>
Cc: Alex Rosenbaum <rosenbaumalex@gmail.com>,
	RDMA mailing list <linux-rdma@vger.kernel.org>,
	Eran Ben Elisha <eranbe@mellanox.com>,
	Yishai Hadas <yishaih@mellanox.com>,
	Alex Rosenbaum <alexr@mellanox.com>,
	Maor Gottlieb <maorg@mellanox.com>,
	Leon Romanovsky <leonro@mellanox.com>
Subject: Re: [RFC v2] RoCE v2.0 Entropy - IPv6 Flow Label and UDP Source Port
Date: Fri, 21 Feb 2020 09:47:12 -0500
Message-ID: <33f075e2-b5c0-53cd-6954-7ac57eeb008f@talpey.com> (raw)
In-Reply-To: <c4fc4449-94ed-805e-76d1-6ce856a4fc05@mellanox.com>

On 2/19/2020 8:04 PM, Mark Zhang wrote:
> On 2/20/2020 1:41 AM, Tom Talpey wrote:
>> On 2/19/2020 8:06 AM, Jason Gunthorpe wrote:
>>> On Wed, Feb 19, 2020 at 02:06:28AM +0000, Mark Zhang wrote:
>>>> The symmetry is important when calculate flow_label with DstQPn/SrcQPn
>>>> for non-RDMA CM Service ID (check the first mail), so that the server
>>>> and client will have same flow_label and udp_sport. But looks like it is
>>>> not important in this case.
>>>
>>> If the application needs a certain flow label it should not rely on
>>> auto-generation, IMHO.
>>>
>>> I expect most networks will not be reversible anyhow, even with the
>>> same flow label?
>>
>> These are network flow labels, not under application control. If they
>> are under application control, that's a security issue.
>>
> 
> As Jason said application is able to control it in ipv6. Besides
> application is also able to control it for non-RDMA CM Service ID in ipv4.

Ok, well I guess that's a separate issue, let's not rathole on
it here then.

> Hi Jason, same flow label get same UDP source port, with same UDP source
> port (along with same sIP/dIP/sPort), are networks reversible?
> 
>> But I agree, if the symmetric behavior is not needed, it should be
>> ignored and a better (more uniformly distributed) hash should be chosen.
>>
>> I definitely like the simplicity and perfect flatness of the newly
>> proposed (src * 31) + dst. But that "31" causes overflow into bit 21,
>> doesn't it? (31 * 0xffff == 0x1f0000) >
> 
> I think overflow doesn't matter? We have overflow anyway if
> multiplicative is used.

Hmm, it does seem to matter because dropping bits tilts the
distribution curve. Plugging ((src * 31) + dst) & 0xFFFFF into
my little test shows some odd behaviors. It starts out flat,
then the collisions start to rise around 49000, leveling out
at 65000 to a value roughly double the initial one (528 -> 1056).
It sits there until 525700, where it falls back to the start
value (528). I don't think this is optimal :-)

Tom.