Erik - assume, for discussion, the MTU within the RPL network is
uniform (1500 bytes), and the RPL network includes only non-storing
routers, so all the forwarding headers are inserted by the RPL edge
router. Could that edge router manage the ICMP messages to the source
correctly; e.g., if the edge router needs 64 bytes for routing
headers, it sends the ICMP message with "please limit packets to 1436
bytes"?
- Ralph
On Jun 1, 2010, at 5:02 AM 6/1/10, Erik Nordmark wrote:
On 05/31/10 09:06 AM, culler wrote:
Since we route WITHIN a RPL network, rather than THROUGH a RPL
network,
it would seem that the case mentioned does not arise. Right?
It can still happen.
If the sender is outside of the RPL network and sends a packet, and
then at the edge of the RPL network the routing header is added and
the packet is forwarded. If then a downstream RPL router sees that
the packet is to big, it will generate an ICMP "packet too big" and
send it to the source of the packet.
Erik
On May 30, 2010, at 7:56 AM, Erik Nordmark wrote:
The draft says:
A RPL Router MAY insert a Type 4 Routing header if one does not
already exist. The conditions for inserting a Type 4 Routing header
are out of scope of this document.
Having routers insert headers in packets they forward is know to be
problematic in general, since it breaks path MTU discovery.
Taking an example of a host which sends a packet which is 1500
bytes,
and then receives an ICMP packet too big error saying "please limit
packets to 1500 bytes", will not change the host's behavior. But
that
can happen if some router grows the packet to be more than 1500
bytes.
Thus in general, the only safe way to insert headers in a packet
is to
have the "inserter" put an extra IP header, with itself as a source,
then the inserted header, and then the original packet.
That extra IP header ensures that any ICMP error go back to the
"inserter", which can then compensate for its insertion before
sending
the ICMP error towards the sender of the original packet.
Thus I think there is a question for 6man whether we think the
constrained environment of ROLL is such that we can relax this.
But it
does require that all the routers at the edge of the ROLL network
adjust the ICMP errors they forward to compensate for any inserted
RH4
headers.
Erik
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