Simon Matthews writes: > OK, this may be a reasonable approach, but how do I force it initiate > connections from the "fast" interface, yet allow it to fail over to the > slow interface if the sytem removes the route to the fast gateway because > it has detected that it is not responding? Off hand I don't know anything built in for this (I look forward to hearing an answer from someone who does), but I don't think this is really what you want anyway. It's not as if your link is the only one that could fail! If ISP1's upstream link fails then you want to use ISP2 for all traffic other than that intended for ISP1 itself. And of course, problems further upstream prevent you from reaching certain addresses but not others, and you don't really know which without a global view of the routing. I think the "right" solution involves monitoring the traffic. There's a wide range of things you could do, the simplest being simply detecting that the link is not responding. You could also try to detect tcp retransmits, measure RTT, aggregate data to measure how well individual connections are working, further aggregate data to determine which addresses blocks are working well and which poorly, etc. Then use that data to decide which of your links to use for a given destination. I actually sent a proposal to this list that I think provides a good solution to the general problem: an extension to TCP (possibly even IP) that supports multiple addresses/ports. This would even allow you to switch addresses in the middle of a connection. I think what I described before applies more to the machine on the other side of your connection, which now would know both of your addresses. Whenever it does a tcp retransmit it switches the address. It therefore tends to stay on the one that works most reliably. (Perhaps this algorithm could be improved to take speed into account too.) This discussion points out that something similar should be done on your end: you should switch the output interface you use when you retransmit. Of course this is not yet implemented. It's on my queue, but not close to the beginning. I'd be glad if someone out there could beat me to it. _______________________________________________ LARTC mailing list / LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
