I disagree that layer2 collisions are unavoidable. If you follow RFC3307 then collisions between dynamically assigned and permanent assigned addresses are totally avoidable (like they are intended). The issue here is that this draft does not follow the intent of 3307.
As far as benign goes, everyone considers the effect of sending "Name Queries" and because of a collision a host that joined "another" group gets those queries. But the opposite case is the major one. Take for instance that a multicast group is being used to send large amounts of packets per second. Another host is running a real-time mission critical application that it not designed to receive a large number of packets per second. The second host just happens to join the same (layer 2) group because of the MD5 hash on its name. Then the host will be flooded with multicast traffic and may cause it to fail to perform its real-time task. The solution is to insure that certain ranges of address can be used to guarantee that collision will not happen. -----Original Message----- From: Elwyn Davies [mailto:[EMAIL PROTECTED] Sent: Wednesday, July 20, 2005 11:29 To: [EMAIL PROTECTED] Cc: Elwyn Davies; ipv6@ietf.org; Pashby, Ronald W CTR NSWCDD-B35 Subject: Re: Comments on draft-ietf-ipngwg-icmp-name-lookups-12.txt JINMEI Tatuya / 神明達哉 wrote: >>>>>>On Wed, 20 Jul 2005 11:43:45 +0100, >>>>>>Elwyn Davies <[EMAIL PROTECTED]> said: >>>>>> >>>>>> > > > >>As I said in my previous posting, I don't think you ought to think >>of either solicited node multicast groups or these groups as >>dynamically allocated. The groups exist statically whether any >>nodes are using them or not. The dynamically allocated ones are >>(IMO) those that an application needs to create with some guarantee >>of uniqueness in order to send on. No dynamic allocation is needed >>for the name lookup addresses. >> >> > >Whether it's really a 'dynamically allocated' address or not is just a >matter of definition. I won't surprise if different people have >different views on this. I just represented my own interpretation of >the sense of RFC3307. Of course, I could be wrong, in which case the >author of the RFC will hopefully correct me. > > Indeed RFC3307 has to some extent been overtaken by events and the exact meaning is now unclear. As you say it doesn't really matter for the purposes of this discussion. >In any case, I don't think the definition of 'dynamic' is a real >issue. The issue is whether it makes sense to try avoiding layer-2 >and layer-3 collisions for the multicast groups regarding the >icmp-name-lookup protocol. > > >In my understanding, a very rough summary of your argument against >this point is "the probability of collisions should be very rare due >to the use of MD5 hashing, so don't bother" (correct?). I see the >point, and I can live with that. > >In my undersatnding, however, the trend of the IETF is to not rely on >that type of arguments and is to try avoiding possible collisions more >and more explicitly. In fact, we rejected the idea of "duplicate >interface-ID detection' instead of 'duplicate address detection', >based on the argument of "the probability of collisions should be rare >since the IDs should be highly unique, so why not optimize?". Also, >RFC3307 seems to try avoiding collisions even though it recommends the >use of good random group IDs (according to the second paragraph of >Section 4.3.2). So, it seems more consistent to me to avoid >collisions in this case despite the use of reasonably distributed IDs. > > > The real point I was trying to get across is that Layer 2 multicast address collisions are essentially unavoidable, but that they are, at least in the name lookup case, benign, especially as far as performance is concerned. So I wouldn't change the algorithm. I agreed with the DAD decision, and making sure the layer 3 addresses are unique for a given purpose is clearly the right answer, because a collision could have disastrous effects unless nodes/applications are expecting it. The name lookup and ND solicited node Layer 3 addresses are taken from different spaces from any of the RFC3307 addresses so there can never be a collision between these different types of addresses at Layer 3. The name lookup (and ND) solution reduces the probability of multiple nodes using the same Layer 3 and Layer 2 addresses for this purpose but ensure that a collision doesn't matter, although there will be a small amount of wasted effort on nodes with Layer 3 collisions. As far as I can see, there is bound to be a small residual probability of a Layer 2 multicast address collision on account of there being many less Layer 2 multicast addresses than Layer 3 multicast addresses. In the vast majority of cases this will not matter because nodes will receive and process just the packets they would have received anyway. There will be a small number of (relatively) pathological cases where a packet goes to a number of listeners that aren't interested. However, especially in the case of name lookup, the number of packets involved at any one node is likely to be very small. Regards, Elwyn >It's not a strong opinion though. If a majority prefers keeping the >current range, I won't fight against that. > > JINMEI, Tatuya > Communication Platform Lab. > Corporate R&D Center, Toshiba Corp. > [EMAIL PROTECTED] > >
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