Perhaps this is a "newbee" question, answered in the history of this
discussion thread, but how does a lower layer "know" what kind of MIC may be
applied by a higher layer (i.e. in the payload) unless it is explicitly told
by the next higher layer? The suggested wording requires such cross layer
knowledge. I see that only the source of the packet can know this, but
likewise in that source node, how would the LoWPAN layer make such a
determination? The obvious way is when informed by the upper layer via the
appropriate SAP mechanisms. Would the wording not be more 'crisp' if it the
only case is when the suppression of the UDP CRC is indicated by the upper
layer?
Mention is made of this being a layer violation in the thread, but what I
don't understand is why this is necessary in the standard?
Thanks
-Ben
----- Original Message -----
From: "Pascal Thubert (pthubert)" <[EMAIL PROTECTED]>
To: "6lowpan" <[email protected]>
Sent: Thursday, November 20, 2008 2:46 AM
Subject: [6lowpan] Proposed new text for the UDP checksum elide issue
Hi:
Carsten asked for a crisper wording for which node can elide the UDP
checksum and when that happens. There seems to be a consensus that
we are not ready to open the door for heuristics based operations.
So at this point, only the source of a packet MAY elide the checksum,
And the receiver of a packet MAY ignore the checksum if it is present.
This in only allowed in the case of tunneling or stronger integrity
check above UDP.
Here is a proposed text:
"
With this specification, the source of a packet MAY elide the UDP
checksum in the following cases:
Tunneling: The source of the packet is tunneling another packet that
possesses its own integrity mechanism.
Upper Layer MIC: The Upper Layer Protocol over UDP uses an end-to-
end Message Integrity Check (MIC) that has stronger properties
than what is provided by the UDP checksum. Such an integrity
check MUST be end-to-end and cover the IPv6 pseudo-header, UDP
header, and UDP payload.
Only the source of a packet can know what Upper Layer operation takes
place. A router on the way that is not aware of such operation
SHOULD NOT elide the UDP checksum when performing 6LoWPAN
compression.
The 6LoWPAN termination point has to recompute the elided checksum
based on the received packet. If that point is a router, then the
router reforms a proper IPv6/UDP packet that can be forwarded on any
interface. If the 6LoWPAN termination point is the destination of
the packet and is aware of the presence of the Upper Layer MIC for
the destination UDP port, it MAY omit the UDP checksum operation
completely.
"
What do you think?
Pascal
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