[patch] BLIP fragment handling leaves last fragment buffer unused

What steps will reproduce the problem?
1. Note the configured value of N_FRAGMENTS in IPDispatch.h
2. Send the largest packet you can from TinyOS
3. Observe that the number of fragments on the air is really N_FRAGMENTS-1, 
even though memory has been set aside for N_FRAGMENTS.

What is the expected output? What do you see instead?

All the memory reserved by the N_FRAGMENTS sized array of message_t should be 
available for use. Currently, IPDispatchP uses a round-about way of working out 
whether there are more fragments to send, and allocates a fragment buffer even 
if there are no fragments left to send, effectively wasting an entire message_t 
worth of RAM. The attached patch introduces a very small helper function in 
lib6lowpan_frag.c, and IPDispatchP is tweaked to check whether there are more 
fragments _before_ allocating a fragment buffer. This allows BLIP to send 
packets using N_FRAGMENTS fragments, up from N_FRAGMENTS-1.

What version of the product are you using? On what operating system?

TinyOS 2.x, trunk/r5884

Please provide any additional information below.

Some fuzz possible, as we're using BLIP with an RF212 radio here and as such 
our tree has diverged slightly from trunk. Hand-merge should be trivial if it 
doesn't apply automatically.

Original issue reported on code.google.com by jmatts...@dius.com.au on 9 Feb 2012 at 10:36

Attachments:

tinyos-blip-unused-frag.patch

AhnSeung / tinyos-main

[patch] BLIP fragment handling leaves last fragment buffer unused #117