[Ns-developers] emulation use cases

[Gustavo Carneiro]

2009/1/15 Tom Henderson <tomh at tomh.org>

> Craig is working on Tap device support for emulation, and some questions
> have arisen about use cases and requirements to support, so I thought I'd
> raise the issue on the list for any input people might have.
>
> Presently, we have an emulation mode documented here:
> http://www.nsnam.org/docs/manual.html#SEC59
> it runs the simulator in real-time and allows nodes in a simulation to emit
> packets that are bridged onto a physical device on the host machine.  ns-3
> packets are therefore emitted onto real links.  The ns-3 device must be
> hand-configured with mac address and does not presently support DHCP
> configuration of the address.
>
> I've suggested that the next priority would be to invert the picture, to
> allow real hosts to emit packets onto ns-3 links.  The main use case I have
> in mind is to allow users to interconnect virtual machines such as OpenVZ
> containers over ns-3 links (mainly, WiFi is of immediate interest).  It
> would also be possible to use the simulator as a "black box" such as a
> "simulator in the loop" testbed between real nodes or test equipment.
>
> To accomplish this, what is needed is to create tap devices on the ns-3
> host machine, and bridge these tap devices to some new kind of ns-3
> NetDevice that reads and writes from the tap device.  Essentially, this new
> ns-3 NetDevice would be receiving fully formed layer-2 frames and bridging
> them onto our existing ns-3 NetDevice types.
>
> In this model, the ns-3 node that does this bridging would be kind of like
> a ghost node, possibly with an Internet stack (maybe not).  It may be the
> case that a simulation/emulation would involve a mix of ns-3 nodes and these
> special ghost nodes that exist for the purpose only of hosting the virtual
> machine; in such a case, I would envision that IP-level and above
> configuration of the mixed environment would have to be done separately.
>
> I think that a framework to try to configure all of this from a bird's eye
> view of both simulation and emulation portions is possible, but would be a
> later phase of the effort (possibly a GSOC project).
>
> Are there other suggestions for the above type of use case, or other use
> cases for emulation that people would like to see prioritized?


I was looking at this again (the manual), and I realize that I am confused
about something.

As far as I can tell, "tap" devices can be used to allow a userspace
simulation interact with applications running on the same kernel that runs
the simulator.  In this scenario no virtualization needs to be employed at
all.

On the other hand, in a virtualized environment I would expect the
virtualization environment to already provide the tools to create and
interconnect virtual devices to allow communication between guests or
between host and guests.  In this case I think NS-3 should not need to
create any tap device, and instead should just send/receive packets to an
existing device.  Therefore, what we need to use is just EmuNetDevice again,
not Tap.

However, the manual explicitly mentions a virtualization scenario as
motivation for Tap devices, which I think may work but EmuNetDevice may also
work and simpler, more efficient.

Am I missing something?

-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert