From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Mon Nov 17 2003 - 11:41:09 GMT-3
At 9:13 PM -0500 11/16/03, Jonathan V Hays wrote:
>
>Howard,
>
>Sorry about that but I couldn't resist; your "real-world taint" phrase
>triggered something there.
>
>If nobody else will bite, then let me be the one to ask. Exactly why
>will 30% of the response return via ISP2?
>
>Puzzled,
>
>Jonathan
>
Fair question, and I keep my caveats about rule of thumb.
Let's say you want to reach host A, which is in a customer AS.
Host A is served by ISP3 and ISP4, which do not have direct
connections to ISP1 and ISP2. There are some number of intermediate
AS between them, which have no business relationships beyond
bilateral peering. ISP3 will eventually deliver traffic to you via
ISP1 and ISP4 will eventually deliver traffic to you via ISP 2.
You prefer ISP 1 and prepend your AS once to ISP2. Clearly, you can
specify your outgoing policy.
But at the AS of host A, the route back to you comes in from ISP's 1
and 2. Due to intermediate ISP's between ISP1 and ISP2, the AS path
from AS "A" to your ISP is six AS through ISP3, and four AS through
ISP4. Even though you have prepended your AS once, the ISP 4 route
still is shorter, FROM THE PERSPECTIVE OF AS "A". AS "A" will send
the response via its connection to ISP 4, which will arrive at your
site via ISP 2.
Remember that unless you have explicit business arrangements with
every ISP in the path, such that they will carry out a specific
routing policy (e.g., signaled by an ISP 1 community recognized by
all), you have only advisory control over the paths that other AS
take, and you cannot bind any ISP with which you have no business
relationship. The message here is that if you want a specific,
QoS-predictable path between you and some distant AS, order a
QoS-enabled multiprovider VPN from one of your providers, and make
them responsible for the intermediate business arrangements to ensure
your policy is met.
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