Brian E Carpenter
brian.e.carpenter at gmail.com
Fri Oct 1 02:11:31 CEST 2010
On 2010-10-01 09:51, Gert Doering wrote:
> On Thu, Sep 30, 2010 at 09:37:23AM -0700, Tony Li wrote:
>> So if you have N entries then your network admins won't bother to make them work?
>> Seems to me like you need new network admins.
> What exactly can the network admin do here?
> Let me try to illustrate this with a specific example. No complex policies
> for now, just focusing on "shortest path to destination".
> There's a source prefix S1 that is routed via ISP 1, and a source prefix
> S2 that is routed via ISP 2 (both prefixes coming from the respective
> ISP's PA space, so BCP 38 filters will require that source address
> selection automatically selects egress ISP).
> Destination prefix D1 is connected to peer of ISP 1, while destination
> prefix D2 is connected to something 20 hops behind ISP 2 (assuming that
> "20 hops" means "more latency, less bandwidth", for the simplicity of
> the model).
> +--- S1 === ISP 1 === ISP X ======== D1 ---+
> source (*) destination
> +--- S2 === ISP 2 --- ISP A, B, C -- D2 ---+
> How exactly is the end host supposed to know that "S1->D1" is good, while
> "S2->D2" is bad and "S1->D2" is very bad? (Assume that "use the longest
> sequence of common bits in the addresses" won't help, which is the normal
> case when crossing RIR regions).
The only reasonable answer to this question is (IMHO) trial and error.
For some indication of what that means in practice and at large scale,
A Performance Study on REAchability Protocol in Large Scale IPv6 Networks, Habib Naderi and Brian Carpenter, Second
International Conference on Computer and Network Technology (ICCNT 2010), Bangkok, (April 2010) 28-32.
This is large scale simulation, but recently Habib has run small scale
lab tests with real code, giving comparable results.
> If I have a single S and D prefix, the end host does not *need* to know,
> and the network *will know* that the path via ISP 1 to its peer to D is
> shorter than via ISP 2.
> I hope I have succeeded this time in explaining where "we the operators"
> see a gap in the N*M model - and that has nothing whatsoever to do with
> "network admins not being up to the job".
> Gert Doering
> -- NetMaster
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