Subject: replying to anycast address

From: Richard Draves <[email-masked]>
Date: Fri, 4 Jun 1999 17:55:18 -0700

I recently came across this question while doing some interop testing. Our
MSR IPv6 implementation was failing to reply to ICMP Echo Requests. It turns
out the Echo Requests were sent with a source address that was a reserved
subnet anycast address, eg of the form a:b:c:d::.

Obviously the sending implementation was in error, because you shouldn't
send packets with an anycast source address. (RFC 2373, section 2.6)

However was our implementation behaving correctly by refusing to send a
reply to the anycast address?

RFC2463 section 2.4 calls out this case for ICMP errors, saying that errors
must not be sent to addresses known not to be unique. But this was an ICMP
Echo Reply, not an error.

Any opinions?

Thanks,
Rich
--------------------------------------------------------------------
IETF IPng Working Group Mailing List
IPng Home Page:                      http://playground.sun.com/ipng
FTP archive:                      ftp://playground.sun.com/pub/ipng
Direct all administrative requests to [email-masked]
--------------------------------------------------------------------

---

From: Steve Deering <[email-masked]>
Date: Fri, 4 Jun 1999 18:22:27 -0700

>reply to the anycast address?

In my opinion, yes, it was correct behavior to refuse to reply to an
illegal source address (same would be true if the address were multicast
or all-zeros, and for any upper-layer protocol, whether TCP, UDP, or
ICMP Echo).  Note, however, that a node cannot always tell that the
source address of a received packet is an anycast address, so we cannot
insist that a node never reply to such a packet.  Therefore, I would not
consider a node that *did* reply to a packet from an anycast source to
be faulty.  As you said, the fault was on the part of the original
sender of the packet with the anycast source address.

Steve