Hi Ben >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?
[Pascal] That's right. But for tunnels which are similar yet a bit different. >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? [Pascal] Saving bits is critical in this space, mostly sensors->backbone because that's most of the traffic and backbone routers have a power source anyway. Bits translate into days and weeks of battery operation. Bits can make the difference between a packet that can be sent on scavenged energy and a packet that can not be. 2 bytes are wealth in LoWPANs :) Pascal >----- 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 >> >> >> _______________________________________________ >> 6lowpan mailing list >> [email protected] >> https://www.ietf.org/mailman/listinfo/6lowpan >> > >_______________________________________________ >6lowpan mailing list >[email protected] >https://www.ietf.org/mailman/listinfo/6lowpan _______________________________________________ 6lowpan mailing list [email protected] https://www.ietf.org/mailman/listinfo/6lowpan
