Re: failure detection

[Erik Nordmark <erik.nordmark@sun.com>]

[I'm still catching up on some mail]

Paul Jakma wrote:

> My gut feelings though are:
>
> - Failures typically are near the edges
> - Failures are typically bi-directional for a given path
> - Uni-directional failures tend to be due to /congestion/, not
>   actual failures - again, typically at the edges. Congestion related
>   "failures" tend to be very transient/sporadic.

When talking about actual uni-directional failures due to links or 
routers going bad, I'd agree.

But one assumption we've been making in the multi6/shim6 work is that we 
want to cover the SOHO multihoming case. Using basically retain ISP 
service from more than one ISP, where the ISPs might apply ingress 
filtering, and the site can't get the ISP to relax their ingress filtering.

If we don't do something in this case, then we'll have the routing (and 
selecting of exit ISP) be completely independent of the (hosts') source 
address selection, and we will have a very high incidence of packets 
being dropped by the ingress filters.

Some options for what to do is in 
draft-huitema-multi6-ingress-filtering-00 (now expired).
Even if such a scheme is standardized and deployed, it might not be 
deployed everywhere.
Which is why some of see utility in having the capability to try all 
<source,dest> combinations in both directions of the communication. But 
I sure do hope that in normal deployment we don't have link/router 
failures that cause shim6 to have to try close to n^2 locator pairs 
before finding a working one.

I think one can come up with strategies for the order in which locator 
pairs are tried that will work efficiently for the normal case of some 
source address dependent exit router selection (in the cases this is 
necessary to avoid ingress filtering dropping the packets), and 
link/router failures. For instance, if A has locators A1, A2 and B has 
B1, B2, B3, and A is communicating with B using <A1, B1> then when A 
suspects that the locator pair has stopped working it can try to 
maximize the probability of using a different path by trying to 
combinations which change both the source and destination first, and 
then the pairs that only change one of them. For example, try in the 
order of
	<A2, B2>
	<A2, B3>
followed by
	<A1, B2>
	<A1, B3>
	<A2, B1>

   Erik