Hi Jonathan: You are assuming explicitly that I am assuming implicitly... mesh under... Hum.
Please have a look at http://www.ietf.org/internet-drafts/draft-thubert-lowpan-backbone-router -00.txt and it should be apparent that I'm not. In particular the backbone router is the device that will set and use the hop limit bit. So will all those interconnecting devices used in pseudowire, split bridges, IP Mobility, etc... if they are deployed in this space. What I'm really assuming is a gateway or an edge server on the link. It's there in most architectures that I know of at this point that's why I included it in the case we shoot for and saved 2 bits. Note that when the ULA and global addresses are used on top of link local, it makes sense that we define a mechanism to compress them as well. But this is not what HC3 is about. The goal of my initial proposal is illustrative, to foster this very discussion around the idea that we can shoot for a favorite case and make an HC3 out of HC1 and HC2 for that case. Now, if we as a group think that the GW assumption is not reasonable, then, fine, let us affect 2 bits in HC3 to indicate whether the addresses are compressed. What do the others think? Pascal >-----Original Message----- >From: Jonathan Hui [mailto:[EMAIL PROTECTED] >Sent: mardi 4 mars 2008 18:05 >To: Pascal Thubert (pthubert) >Cc: 6lowpan; [EMAIL PROTECTED]; Jay Werb >Subject: Re: [6lowpan] Compressing HC1 and HC2 into HC3 > > >Hi Pascal, > >Thanks for sharing your thoughts. > >By assuming the first 5 bits in HC1 are '1', you're staking a claim that >mesh-under link-local communication within a PAN is going to be most >common case and the one we've "shot for". If link-local is all we focus >on, then we are missing the point of IP. Frankly, I don't see much >benefit of IP if communication is limited to the PAN. > >W.r.t mesh-under, there are efforts within the IETF that assume 6LoWPAN >nodes will communicate via route-over using routable addresses (e.g. >ROLL). This brings us back to a broader topic of route-under vs. >mesh-under. While both will probably exist for some time, it raises the >question of whether 6LoWPAN implementations can support one or do they >have to support both. > >W.r.t compressing Hop Limit, I don't quite understand why you're using a >bit in HC3 to indicate where the message originated. If you're assuming >link-local communication, then messages shouldn't be forwarded across >links anyway. > >I do agree that there is work to do on compression and agree that there >are cases where HC1 and HC2 are not sufficient. This is why I've been >working, albeit slowly, on HC1g. > >-- >Jonathan Hui > > >Pascal Thubert (pthubert) wrote: >> Dear 6LoWPANers, >> >> Though HC1 and HC2 provide us with the capability to express any >> combination of compression, the most expected (or at least shot for) >> case does not lead to a better compression of the HC headers themselves. >> >> Seems that we could define an HC3 that would compress the pair of HC1 >> and HC2 for that best case. My proposal for doing this would be to: >> >> - define a new dispatch >> | 01 000011 | LOWPAN_HC3 - LOWPAN_HC3 compressed IPv6 and >> transport >> >> - define HC3 itself to: >> >> - implicitly say that the first 5 bits of HC1 are all ones >> - carry HC1 bits 5 and 6 (Next Header) >> - replace HC2 so bit 7 of HC1 is useless >> - provide transport specific bits such as those defined for >> HC-UDP, >> then again assuming a predefined best case if room is missing >> >> An example of what the HC3 octet could be: >> >> bits 0 1 2-3 >> 4-7 >> >> +------------------+-------------+-----------------+-------------------- >> ---------------------+ >> | reserved | reserved | Next Header | transport >> specific control bits | >> >> +------------------+-------------+-----------------+-------------------- >> ---------------------+ >> >> Next header: as defined in HC1, bits 5 and 6 >> >> transport specific control bits for UDP: >> bit 4 echoes HC-UDP bit 0 on UDP source port >> bit 5 echoes HC-UDP bit 1 on UDP destination port >> bit 6 echoes HC-UDP bit 2 on UDP length >> bit 7 is reserved >> >> 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
