netdev - Re: [PATCH 3/3] vxlan: virtual extensible lan

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Date:	Thu, 4 Oct 2012 09:25:48 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH 3/3] vxlan: virtual extensible lan

On Thu, Oct 4, 2012 at 8:12 AM, Stephen Hemminger <shemminger@...tta.com> wrote:
> On Thu, 04 Oct 2012 08:34:50 +0200
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>> On Wed, 2012-10-03 at 21:02 -0700, Stephen Hemminger wrote:
>>
>> >
>> > Since port is always a pair, there is no need to restrict range, unless
>> > there was a broken firewall in the way. One bug there is that random32()
>> > can return 0 which is not a valid port number.  A better fallback
>> > would be a hash of the MAC header.
>>
>> But using up to 65536 values for the port means that if vxlan traffic
>> crosses a statefull firewall (eg netfilter with conntrack), this might
>> need 65536 contexts/sessions.
>
> This is UDP, there is no usable state to be found.
> But as a practical matter it only needs to be wide enough to hit the
> number of receive channels. Therefore even 64 is probably enough for
> most people.

I don't think it actually matters all that much either way because
there will be one vxlan flow for each encapsulated flow.  If you're
running iptables on a hypervisor, for example, you probably end up
seeing the same number of flows as you do today.

There is some benefit to using a slightly wider range than the number
of paths you can take because devices in the path will do their own
hash.  Having a larger range on the vxlan side will reduce the impact
of collisions in the second hash.

>> So random32() is not needed.
>>
>> What might be needed is additional parameters for a vxlan tunnel, to
>> give an optional range for the source port.
>
> It could be per vxlan without a lot of pain. or just derive it value
> from existing ephermeral port range.

In the STT draft we recommended using the ephemeral range.  It seemed
like a good balance.
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