Re: [patch v1, kernel version 3.2.1] net/ipv4/ip_gre: Ethernetmultipoint GRE over IP

[Jesse Gross]

2012/1/16 Åtefan Gula <steweg@xxxxxxxxx>:
> DÅa 16. januÃra 2012 17:36, Stephen Hemminger <shemminger@xxxxxxxxxx> napÃsal/a:
>> On Mon, 16 Jan 2012 13:13:19 +0100
>> Åtefan Gula <steweg@xxxxxxxxx> wrote:
>>
>>> From: Stefan Gula <steweg@xxxxxxxxx
>>>
>>> This patch is an extension for current Ethernet over GRE
>>> implementation, which allows user to create virtual bridge (multipoint
>>> VPN) and forward traffic based on Ethernet MAC address informations in
>>> it. It simulates the Bridge bahaviour learing mechanism, but instead
>>> of learning port ID from which given MAC address comes, it learns IP
>>> address of peer which encapsulated given packet. Multicast, Broadcast
>>> and unknown-multicast traffic is send over network as multicast
>>> enacapsulated GRE packet, so one Ethernet multipoint GRE tunnel can be
>>> represented as one single virtual switch on logical level and be also
>>> represented as one multicast IPv4 address on network level.
>>>
>>> Signed-off-by: Stefan Gula <steweg@xxxxxxxxx>
>>
>> Thanks for the effort, but it is duplicating existing functionality.
>> It possible to do this already with existing gretap device and the
>> current bridge.
>>
>> The same thing is also supported by OpenVswitch.
>>
>
> gretap with bridge will not do the same as gretap allows you to only
> encapsulate L2 frames inside the GRE - this one part is actually
> utilized in my code. GRE multipoint implementation is also utilized in
> my code as well. But what is missing is forwarding logic here, which
> prevents the traffic going not optimal way. Scenario one - e.g. if you
> connect through 3 sites with using 1 gretap multipoint VPN, it always
> forwards frames between site 1 and site 2 even if they are unicast.
> That represents waste of bandwidth for site 3. Now assume that there
> will be more than 40 sites and I hope you see that single current
> multipoint gretap is not also good solution here
>
> The second scenario - e.g. using 3 sites using point-to-point gretap
> interfaces between each 2 sites (2 gretap VPN interfaces per site) and
> bridging those interfaces with real ones results in looped topology
> which needs to utilized STP inside to prevent loops. Once STP
> converges the topology will looks like this, traffic from site 1 to
> site 2 will go always directly by the way of unicast (on GRE level),
> from site 2 to site 3 always directly by the way of unicast (on GRE
> level) and from site 1 to site 3 will go indirectly through site 2 due
> STP limitations, which results in another not optimalized traffic
> flows. Now assume that the number of sites rises, so gretap+standard
> bridge code is also not a good solution here.
>
> My code utilizes it that way that I have extended the gretap
> multipoint interface with the forwarding logic e.g. using 3 sites,
> each site uses only one gretap VPN interface and if destination MAC
> address is known to bridge code inside the gretap interface forwarding
> logic, it forwards it towards only VPN endpoint that actually need
> that by the way of unicasting on GRE level. On the other hand if the
> destination MAC address is unknown or destination MAC address is L2
> multicast or L2 broadcast than the frame is spread out through
> multicasting on GRE level, providing delivery mechanism analogous to
> standard switches on top of the multipoint GRE tunnels.
>
> I also get through briefly over OpenVswitch documentation and found
> that it is more related to virtualization inside the box like VMware
> switches or so and not to such technologies interconnecting two or
> more separate segments over routed L3 infrastructure - there is a
> mention about the CAPWAP UDP transport but this is more related to
> WiFi implementations than generic ones. My patch also doesn't need any
> special userspace api to be configured. It utilizes the existing one.

I understand what you're trying to do and I think that the goal makes
sense but I agree with Stephen that this is not the right way to go
about it.  I see two issues:

 * It copies a lot of bridge code, making it unmaintainable and
inflexible to other use cases.
 * The implementation exists in the GRE protocol stack but it applies
equally to other tunneling protocols as well (VXLAN comes to mind).

Open vSwitch doesn't quite do this level of learning yet but it's the
direction that we want to move in (and there's nothing particularly
virtualization specific about it).  What I think makes the most sense
is to create some internal interfaces to the GRE stack that exposes
the information needed to do learning.  That way there is only one
instance of the protocol code for each tunneling mechanism and then
each way of managing those addresses (i.e. the current device-based
mechanism, Open vSwitch, potentially a direct bridge-based mechanism,
etc.) can be reused as well.
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