Hi Phil: We do agree on the problem.
Per-hop reassembly, though, comes at the expense of that traffic fluidity that was so desirable to the ATM designers. In other words, because per-hop reassembly forces each intermediate node to store and forward a full packet, it is augmenting the latency of the flow and the memory requirements on the forwarding nodes on the way. This is why I had in mind to couple a few per-hop recovery with a more solid end-to-end recovery and flow control mechanism between the LoWPAN endpoints. Now I wonder what this discussion becomes in route over v.s. mesh under. In route over, it seems that the forwarding node terminates a link so it has to reassemble before it forwards on a different link that might or might not be a LoWPAN; so maybe we end up doing the same thing in that case. Jonathan, what do you think? Pascal >-----Original Message----- >From: Philip Levis [mailto:[EMAIL PROTECTED] >Sent: vendredi 23 mai 2008 19:46 >To: Pascal Thubert (pthubert) >Cc: Jean Philippe Vasseur (jvasseur); Mark Townsley (townsley); [email protected] >Subject: Re: [6lowpan] New charter for 6lowpan > > >On May 23, 2008, at 3:50 AM, Pascal Thubert (pthubert) wrote: > >> Hi JP: >> >> I see your concern. >> >> I'd argue that ECN is now pretty well defined now in RFC 3168, and >> that >> the basic operation is pretty simple: emulate a packet loss in RED but >> save the packet. So the real problem is not ECN itself but what RED >> becomes in a LowPAN forwarding node. And that is something that the >> node >> will have to define whether it does ECN or RED. >> >> If we do not have ECN then the LowPAN forwarding nodes will have to >> destroy frames which might be fragments. Considering the cost of >> energy >> and bandwidth to getting the frame up to that forwarding node, the >> latency introduced by transport layer time out, and the risk of >> congestion collapse introduced by resending the whole segment, I'd >> suggest that ECN is actually a good idea. > >This gets back to the point I brought up December 14, 2006. In lossy >networks, end-to-end fragmentation and assembly is a real problem. It >makes much more sense to do per-hop fragmentation and assembly. In the >former, the number of retransmitted fragments is exponential in the >number of hops; in the latter, it is linear. > >Phil _______________________________________________ 6lowpan mailing list [email protected] https://www.ietf.org/mailman/listinfo/6lowpan
