[Tinyos-devel] Packet.clear()

[Miklos Maroti]

On 10/17/07, Philip Levis <pal at cs.stanford.edu> wrote:
> On Oct 17, 2007, at 11:04 AM, David Gay wrote:
>
> > On 10/17/07, Miklos Maroti <mmaroti at math.u-szeged.hu> wrote:
> >> If the message to be sent is statically allocated in a component and
> >> its ownership does not change (other than passing via Send.send),
> >> then
> >> Packet.clear need not be called. In this case ALL metadata used for
> >> transmission MUST NOT be changed by the Send implementation.
> >
> > Presumablt this exception should be extended to only returning it via
> > Receive.receive.
> >
> > And in both cases it should be clear that this only applies to "things
> > going to the same comm stack", i.e., if you have 2 Send's, then you
> > must use clear if you don't know that they go the same stack.
> > Hopefully you should be able to switch between Receive and Send for
> > the same comm stack...
>
> This came up on the call briefly. I don't think that you can ever
> safely say that you can switch between Receive and Send, because the
> wiring of those two is outside your control. For example, it's
> entirely possible to take CtpForwardingEngineP and hook its receive
> up to the serial stack while its transmit is up to the radio stack.
> Furthermore, this puts the onus on the receive side to clear out
> these fields. Otherwise, they will be as they were the last time send
> was called with that buffer, which might not have been this same
> component. In that way, Receive by itself is not sufficient, because
> it's possible you returned it to a queue, which then another
> component uses for Receive.
>
> I'm not sure what the exact correct usage should be.
>
> Phil

You cannot reuse a received message buffer as a message to be sent
before you actually call Packet.clear EVEN IF the Send and Receive are
on the same interfece. Imagine, that ComponentA is receiving messages
into a buffer and it does nto care about metadata. These message
buffers go into the communication stack and resurface into ComponentB
at Receive.receive(). At this point the metadata of the message buffer
could be either set incorrectly (the buffer might never have been used
to send a message), or set according to somthing else (for example not
requiring acknowledgements, etc). In this situation you MUST set up
the send-metadata (e.g. ack requested, retries, etc), and if you do
you might as well clear the packet with Packet.clear for metadata that
ComponentB does not know or care about.

There is a BIG assumption here. Packet.clear clears only the
send-metadata, that is, stuff that is used to send the message. Maybe
we should make it explicit that it should not touch receive-metadata,
such as lqi and timestamps.

Miklos