On 4/13/11 12:53 AM, Pascal Thubert (pthubert) wrote:
Hi Erik:
The RPL (DAO) sequence number allows the node to increment rapidly in
case of rapid changes and then lazily when the situation is stable and
DAO are scarce. The increase is strictly monotonous which is critical to
us.
A time stamp imposes a synchronization between the routers. We do not
have such mechanism in RPL. A time unit (a granularity) must be agreed
upon. Within that unit, movements go undetected so the time unit must be
thin grained to cover rapid changes. Yet, depending on the medium, the
time unit, and the size of the network, it is not necessarily
easy/possible to guarantee a strictly monotonous value with a thin
grained time unit. And we have limited space (2 octets) and have to deal
with wrap around, which divides the space by at least 3.
So RPL went for a sequence number.
But the unstated assumption that RPL made is that all host-to-router
protocols have to now be RPL aware. That doesn't sound like good design.
A host isn't aware of whether the routers speak IS-IS or OSPF, so why do
the hosts need to be aware of RPL by passing some TID around?
I think ND has the same need as MIP for a TID == Sequence # . We know of
MIP; We know of RPL. We know of the backbone router operation. We know
we'll need the TID and we know exactly why. I think we should have it in
the 6LowPAN ND spec right away to avoid interop issues when we add RPL
and BR operations.
I don't see a need in 6lowpan-nd for a TID; the protocol works fine
without it.
I think RPL needs to be improved to deal with reality. Isn't there a
desire for RPL to handle 4861 hosts? Those would never know about a TID.
Erik
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