Date:	Thu, 24 Jul 2014 16:45:11 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Andy Zhou <azhou@...ira.com>, David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [net-next 10/10] openvswitch: Add support for Geneve tunneling.

On Thu, Jul 24, 2014 at 3:59 PM, Jesse Gross <jesse@...ira.com> wrote:
> On Thu, Jul 24, 2014 at 12:43 PM, Tom Herbert <therbert@...gle.com> wrote:
>> On Wed, Jul 23, 2014 at 9:10 PM, Jesse Gross <jesse@...ira.com> wrote:
>>> On Wed, Jul 23, 2014 at 4:29 PM, Tom Herbert <therbert@...gle.com> wrote:
>>> > This check also applies in the OAM case where there is no data packet
>>> > but we still enforce the protocol field to be Ethernert (meaning of
>>> > prot_type when OAM bit is set is ambiguous in the draft). As I
>>> > mentioned on the nvo3 list, this OAM bit is really a 1-bit packet
>>> > type. If this bit is donated to version field (make it a type version
>>> > field) then we can switch on ver_type above and create another
>>> > processing path for OAM so that the prot_type is at least not
>>> > unnecessarily verified in that case and the bits could even be reused
>>> > for some OAM specific purpose.
>>>
>>> I think the draft is clear :) The value of the OAM bit does not change
>>> the interpretation of the protocol field. This is true in the other
>>> drafts as well.
>>
>>
>>>
>>> OAM packets are essentially just high priority packets (presumably
>>> with some kind of control semantics but that depends on the control
>>> plane). That means that they might be BFD or some other heartbeat,
>>> header fragments for tracing, or really anything else that should be
>>> treated as control. In all of these cases, the protocol type still
>>> needs to indicate the format of the data.
>>>
>> I see, I think you're saying that control messages still contain some sort
>> of message after Geneve header whose type is indicated by protocol field.
>> So, if OAM doesn't change the interpretation of the protocol field then this
>> must be an Ether_type protocol, hence control messages must be formatted as
>> Ether_types. So then I would extrapolate that control messages could be
>> directled to the control plane based on demux of protocol field (ie. type is
>> something like NSH, or a new control message Ethertype). Consequently, the
>> OAM bit is really just a hint to request expedited processing but does not
>> influence demux so could in theory be ignored without loss of functionality.
>> Is this interpretation correct?
>
> Yes, this is largely correct. There is one other possible significance
> to the OAM flag, which is to say "do not forward the resulting packet
> after decapsulation". I think this is slightly less likely to be an
> issue for Geneve (although I can imagine situations where it might be)
> but it was a problem with TRILL OAM, which used data-like packets and
> has no OAM bit. The result is that they had to use all kinds of
> special addresses and other things to separate out data traffic from
> OAM.
>
Thanks for the clarification.

>> A concrete example of a control message with headers would be useful in
>> understanding OAM bit. For instance, how would would a mechanism to
>> implement NAT keepalive of UDP flows be implemented (see RFC3948 for how
>> this was done in ESP/UDP)?
>
> A real example that comes to mind is if you want to have a heartbeat
> protocol such as CFM or BFD. You would set the appropriate EtherType
> for this protocol and then run it as the payload. As you said, it
> would already be obvious that this is a control protocol based on the
> EtherType and the OAM bit is not strictly required. However, it would
> allow easy (and generic) separation of control traffic from data
> traffic so in the event that you have a flood of data packets, the
> heartbeats can prioritized to avoid detecting a failure.
>

I don't see a whole lot of value in using a OAM bit as a priority bit,
we don't have any support in the network which is where we really need
to apply priority. There are already existing mechanisms in the
network to deal with prioritization which are more generic (IP TOS,
Ether priority) and could work today. Sender of these control message
should be setting priority to reflect it.


> If I was trying to implement just a totally simple keep alive, then
> there's really no need to put anything in the payload. I would
> probably just set the EtherType to zero (guaranteed to be unused) and
> leave it at that.

Interesting idea. I don't think 0 has be reserved for this purpose,
but it does seem like a NULL ether_type might be useful. Has any other
encapsulation protocol carrying ether_type specific special meaning of
zero?
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