2012/3/12 Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>:
William Herrin wrote:
When I ran the numbers a few years ago, a route had a global cost impact in the neighborhood of $8000/year. It's tough to make a case that folks who need multihoming's reliability can't afford to put that much into the system.
The cost for bloated DFZ routing table is not so small and is paid by all the players, including those who use DFZ but do not multihome.
Hi, http://bill.herrin.us/network/bgpcost.html If you believe there's an error in my methodology, feel free to take issue with it.
Often overlooked is that multihoming through multi-addressing could solve IP mobility too.
Not.
What is often overlooked is the fact that they are orthogonal problems.
I respectfully disagree. Current mobility efforts have gone down a blind alley of relays from a home server and handoffs from one network to the next. And in all fairness, with TCP tightly bound to a particular IP address pair there aren't a whole lot of other options. Nevertheless, it's badly suboptimal. Latency and routing inefficiency rapidly increases with distance from the home node among other major problems. However, there's another way to imagine the problem: Networks become available. Networks cease to be available. No handoff. No home server. Just add and drop. Announce a route into the global system to each available network with priority set based on the node's best estimate of the network's bandwidth, likely future availablilty, etc. Cancel the announcement for any network that has left or is leaving range. Modify the announcement priority as the node's estimate of the network evolves. This is quite impossible with today's BGP core. The update rate would crush the core, as would the prefix count. And if those problems were magically solved, BGP still isn't capable of propagating a change fast enough to be useful for mobile applications. But suppose you had a TCP protocol that wasn't statically bound to the IP address by the application layer. Suppose each side of the connection referenced each other by name, TCP expected to spread packets across multiple local and remote addresses, and suppose TCP, down at layer 4, expected to generate calls to the DNS any time it wasn't sure what addresses it should be talking to. DNS servers can withstand the update rate. And the prefix count is moot. DNS is a distributed database. It *already* easily withstands hundreds of millions of entries in the in-addr.arpa zone alone. And if the node gets even moderately good at predicting when it will lose availability for each network it connects to and/or when to ask the DNS again instead of continuing to try the known IP addresses you can get to where network drops are ordinarily lossless and only occasionally result in a few packet losses over the course of a a single-digit number of seconds. Which would be just dandy for mobile IP applications.
The only end to end way to handle multiple addresses is to let applications handle them explicitly.
For connection-oriented protocols, that's nonsense. Pick an appropriate mapping function and you can handle multiple layer 3 addresses just fine at layer 4. Just like we successfully handle layer 2 addresses at layer 3. For connectionless protocols, maybe. Certainly layer 7 knowledge is needed to decide whether each path is operational. However, I'm not convinced that can't be reliably accomplished with a hinting process where the application tells layer 4 its best guess of which send()'s succeeded or failed and lets layer 4 figure out the resulting gory details of address selection. Regards, Bill Herrin -- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004