In moving the HC draft from -00 to -01, we added support for multiple
compression contexts so that more than one prefix (other than link-
local) can be compressed.
The obvious question now is how many contexts are enough? It is clear
that two contexts are useful (one for the 6LoWPAN and another for the
common destination that all nodes may be sending to). But should we
add more?
Some thoughts:
- Fewer contexts allow the HC encoding to use fewer bits in
identifying the context in use.
- Specifying support for X contexts requires nodes to allocate enough
memory to maintain X contexts (ip addresses, timers, etc.).
- Additionally, we cannot simply say that we will change X to some
larger Y at some future date since the nodes that only support X will
not be able to support Y > X contexts.
- One argument for supporting more contexts is that it allows network
renumbering while allowing all nodes to communicate with compressed
addresses. My thought, however, is that renumbering of 6LoWPAN
networks should be rare and when they do occur it is okay to incur
some extra cost and communicate with full addresses during that time.
So my thought is to keep the number of supported contexts small (2).
What do other people think?
--
Jonathan Hui
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