James Kempf wrote:
Correct. However, the v6 addressing spec prohibits the use of an anycast address from being used as the source address in a datagram, or being bound to an interface on a host. These two restrictions effectively prohibit the use of anycast as a service distribution mechanism.
Why was this prohibition was put into the specification?
My recollection was that it was a combination of issues. 1. ICMP and pmtu discovery not working, means you can't use it as a source 2. We don't have a protocol that allow hosts to tell the routers which anycast addresses they want to receive 3. TCP and higher level protocols might be confused when things change due to routing changes
The "rebuttals" to those could be 1. Add text to say "SHOULD limit packets with an anycast source to 1280 bytes". Add note that ICMP-based tools don't work with an anycast source address. 2. Any IGP (RIPng, OSPF, etc) can be used for this. (There was also some work to extend MLD to do this). But this really isn't a need to architecturally prohibit hosts from being anycast receivers; if/when there is a protocol by which they can advertise things, they should be able to use anycasts. 3. From the IPv6 WG we can point out that this is an issue, but leave it to some other WG to solve (whether it is solved by a protocol mechanisms, or operational constraints such as not using TCP with anycast, or ensuring that routing stays stable enough)
Erik
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