[net2-wg] header format changes (?)

[Philip Levis]

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
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