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Re: Slashdot: Providers Ignoring DNS TTL?
On Sun, 24 Apr 2005, Robert M. Enger wrote:
Steinar:I'm trying to sort out the various claims here, since I think right now this is a case of people talking past each other, and arguing completely different points.
First of all, let's ditch the term "PPLB." The usual alternative to per packet load balancing (what's been being talked about here) is per prefix load balancing, which would also be "PPLB." The abbreviation is therefore more confusing than anything else.
Now, onto the argument that's going on here.
Dean says "per packet load balancing is coming," and then goes on to assume it's going to be used in such a way that it will cause packets to route through widely divergent paths.
Several others have responded that that would cause packet reordering and break TCP.
Robert says that even used correctly (on identical circuits between the same set of routers), per packet load balancing can cause packet reordering.
Steiner says that when used correctly, per packet load balancing causes packet reordering only rarely, and speeds things up enough when it doesn't that the slowdowns caused by occasional packet reordering may be worth putting up with.
Robert says well known researchers say that packet reordering is bad.
So, as far as I can tell, everybody except perhaps Dean agrees that:
- Used incorrectly (on divergent paths), per packet load balancing can
cause packet reordering.
- Used correctly (on non-diverging paths), packet reordering doesn't
- Packet reordering is bad, and should be avoided.
I'm less clear on Dean's position, but I think it's something along the lines of:
"Per packet load balancing over divergent paths is coming, by fiat from marketing departments even if engineers don't like it, and anything that doesn't play well with it needs fixing." While Dean focuses on anycast, that would presumably extend to TCP and to anything jitter sensitive, such as streaming audio or video.
Anything that's being missed here, or does this sum it all up?