[Tinyos-devel] Request for comments on TEP 111

[Ben Greenstein]

On 11/14/06, Matt Welsh <mdw at eecs.harvard.edu> wrote:
> Wei,
>
> Here are some comments on TEP 111.
>
> 1) Section 2 states that "This format keeps data at a fixed offset", but
> this does not seem to be the case: different platforms with different
> header sizes will have different offsets to the data field. This seems
> to be an ambiguity of wording; I think the correct wording is "This
> format keeps data at a fixed offset on a given platform".

Phil, correct me if I'm wrong, but one goal in the design of message_t
was to avoid data copies even when transititioning across link formats.
Thus message_t's header is 'right justified' so it always ends at the
same location in the buffer. Data immediately follows and thus always
begins at the same offset, irrespective of platform.

>
> 2) It is not clear why the metadata fields need to be nx_ types. These
> fields are not sent over the radio, so there is no need for
> endianness-matching on other processors; the extra overhead of using nx_
> types for metadata access is not warranted. Some justification should be
> given for this.
>
> 3) It would be very helpful to have some example interfaces for
> accessing header/footer/metadata fields externally from the link layer.
> The TEP mentions that implementations SHOULD provide such interfaces but
> no examples are given. A reference to TEP 116 is given, but looking at
> that TEP I don't see any concrete examples of interfaces to access the
> link-layer header, footer, or metadata fields.
>
> I think it would be useful to document a suggested approach to doing
> this so we can avoid having every implementation adopt its own
> conventions.
>
> 4) I am unclear on how the computation of the header offset will obey
> word-alignment issues on certain platforms (e.g., the MSP430). In the
> "getHeader(...)" example:
>
> serial_header_t* getHeader(message_t* msg) {
>   return (serial_header_t*)(msg->data - sizeof(serial_header_t));
> }
>
> There is no guarantee that the pointer returned here is word-aligned; it depends
> on the alignment of msg->data as well as the size of serial_header_t.
> This issue will also arise on 32-bit platforms. I am concerned
> about the use of pointer arithmetic here -- if this is a non-issue perhaps
> the TEP can point this out or provide guidelines to implementers to avoid
> common bugs.
>
> 5) Finally, what happens if this message_t format does not correspond to the
> format expected by the underlying hardware? As an example, an implementation
> might need to set up structures for the hardware to DMA packets to/from the
> interface directly, using a different layout than can be provided here. My
> guess is that this would require the implementation to copy to/from internal
> structures. The assumption built into this TEP, however, seems to be that this
> general format will be appropriate for interfacing to the hardware. The TEP might
> want to clarify the assumptions it is making in this regard.
>
> Matt
>
>
>
> On Mon, 2006-11-13 at 20:54 -0800, Wei Ye wrote:
> > Dear TinyOS developers and users,
> >
> > I was asked to shepherd TEP 111 for TinyOS 2.0. In case you are not
> > familiar with the TEPs (TinyOS Enhancement Proposals), they are the
> > major technical documentation of TinyOS. They describe major OS
> > abstractions and subsystems and source code documentation. For details,
> > please check TinyOS website.
> >
> > As a shepherd, my role is to collect comments and feedback from the wide
> > developers and users, and help the authors to address these comments in
> > revising the TEP. The TEP 111 describes the message buffer abstraction,
> > message_t. Probably every TinyOS user is familiar with TOS_Msg, and you
> > may have different experience on its limitations and how to work around
> > them. The purpose of TEP 111 is to define a new message buffer structure
> > to overcome these limitations. For details of TEP 111, please look at
> >
> > http://www.tinyos.net/tinyos-2.x/doc/html/tep111.html
> >
> > If you have any comments on TEP 111, you are welcome to email me and cc
> > the above lists (which may trigger more discussions). Your feedback will
> > help to shape the future directions of TinyOS.
> >
> > Thanks,
> > Wei Ye
> >
> >
> > _______________________________________________
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>
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