On Fri, 17 Dec 2004, Iljitsch van Beijnum wrote:
> Nope. Consider:
>
> +-------+ +-------+
> |ISPrtr1+---+ACinstA|
> +------+---+---+---+ +-------+
> |source| |
> +------+---+---+---+ +-------+
> |ISPrtr2+---+ACinstB|
> +-------+ +-------+
The setup above is not that uncommon. Further, the outbound links between
source and both ISPrtr1 and ISPrtr2 going out are probably OSPF or RIP or
static default routes. BGP is not really crucial to the problem statement.
Real life examples:
========================
Many customers that have redundant connections to their ISP will have for
example, 2 t1's, which go into different POP sites for their ISP. These
links have default routes and PPLB going out.
Many ISPs themselves have internal default routes going out to different
peering routers at potentially different physical locations, and some of
those may want PPLB exits.
Basically, anywhere you find an opportunity for an assymetric path, you
have a problem with anycast.
Let me restate and re-emphasize the rule for determining if an application
is suitable for anycast:
If you can't guarentee a single path to the anycast server,
the application must make use of some other solution, as
described in RFC 1546.
>From RFC 1546 Page 6
=====================================================================
In general, applications use anycast addresses like any other IP
address. The only worrisome application use of anycasting is applications
which try to maintain stateful connections over UDP and applications which
try to maintain state across multiple TCP connections. Because anycasting
is stateless and does not guarantee delivery of multiple anycast datagrams
to the same system, an application cannot be sure that it is communicating
with the same peer in two successive UDP transmissions or in two
successive TCP connections to the same anycast address.
The obvious solutions to these issues are to require applications
which wish to maintain state to learn the unicast address of their peer on
the first exchange of UDP datagrams or during the first TCP connection and
use the unicast address in future conversations.
=====================================================================
Because per-packet assymetric paths can occur for a variety of reasons,
Anycast can't be used for DNS TCP connections.
Work should begin on the possibilty of altering DNS protocol so that the
unicast address can be passed for a tcp connection.
--Dean
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