[net2-wg] header format changes (?)

[Stephen Dawson-Haggerty]

Hmm, good point.  This actually could be an issue no matter how we do the
packet layout.  Will do some more thinking...

Steve

On Sun, Jan 18, 2009 at 2:27 PM, Philip Levis <pal at cs.stanford.edu> wrote:

> The problem with this is it means you can't pass packets between link
> layers. E.g., you have to do a payload copy between the CC2420 and serial
> stack. This is why the am type is in the CC2420 header.
>
> Phil
>
>
> On Jan 18, 2009, at 2:00 PM, Stephen Dawson-Haggerty wrote:
>
>  The cc2420 radio stack will need a few small patches to accommodate both
>> tinyos active messages and non-tinyos 6lowpan messages.
>>
>> The goal is to provide standard tinyos AM dispatch when the network byte
>> in a packet is TINYOS_6LOWPAN_NETWORK_ID, and a more low level 802.14.5-ish
>> interface when it is not.  That way IP stacks can wire to the 802.15.4
>> interface and get the packets they need, while other tinyos code using the
>> AM interface will still work.  My thought is that we should provide both of
>> these interfaces from ActiveMessageC, although we could potentially also
>> split them apart.
>>
>> My proposal: change cc2420_header_t: remove the 'network' and 'type'
>> fields, and create a new
>> typedef struct {
>>  nxle_uint8_t network;
>>  nxle_uint8_t type;
>> } am_header_t;
>>
>> The active message layer is now a dispatch layer on top of the lower-level
>> 802.15.4-ish interface.
>>  pros: this is clean, and layering-wise the right thing to do
>>  cons: (1) this means that the 2-byte AM header is part of the payload
>> region of the message_t therefore, getMaximumPayload() will return
>> TOSH_DATA_LENGTH -2 , not TOSH_DATA_LENGTH
>>       also, (2) components attempting to access these two fields from the
>> cc2420 header struct will no longer compile.
>>
>> grepping for code doing (2) didn't turn up anything in core; component's
>> __shouldn't__ be doing this...  (2) shouldn't matter so long as applications
>> are using getMaximumPayload and not just assuming it's TOSH_DATA_LENGTH,
>> although if they've optimized memory use to set TOSH_DATA_LENGTH to be
>> __exactly__ right it will break.
>>
>> We could clearly also do this without changing the header; what I'm trying
>> to feel out here is how much this will really break, and how much we care
>> since code-wise this does feel like the right thing to do.
>>
>> Steve
>> _______________________________________________
>> net2-wg mailing list
>> net2-wg at millennium.berkeley.edu
>> https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/net2-wg
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.millennium.berkeley.edu/pipermail/net2-wg/attachments/20090118/8a811164/attachment.htm