Subject: RE: [NDProxy#22] Unclear text about ICMP errors

From: "Dave Thaler" <[email-masked]>
Date: Wed, 16 Feb 2005 10:18:49 -0800

Erik Nordmark writes:
> > Whenever any IPv4 packet is to be forwarded out an interface
> > whose MTU is smaller than the size of the packet, and the Dont
> > Fragment bit is set, the ARP proxy drops the packet and sends
> > a Fragmentation Needed message back to the source.
> >
> > Similarly, whenever any IPv6 packet is to be forwarded out an
> > Interface whose MTU is smaller than the size of the packet,
> > the ND proxy drops the packet and sends a Packet Too Big message
> > back to the source, as described in [ICMPv6].
> 
> Given that the proxy is invisible at L3, I think it would make sense
to
> add explicit statements about any constraints on the IP source address
> to use when the proxy generates ICMP errors.
> 
> I believe the ICMPv* specifications say to use an IP address assigned
to
> either the interface where the packet in error arrived, or assigned to
> the interface on which the error is sent, but in the case of a proxy
> there might be a single IP address assigned to all the proxy
interfaces
> for all I know.

RFC 792 doesn't say how to select the source address, but RFC 2463 does.
>From section 2.2 (a-c only apply to responses, not PacketTooBig errors):
    (d) Otherwise, the node's routing table must be examined to
        determine which interface will be used to transmit the message
        to its destination, and a unicast address belonging to that
        interface must be used as the Source Address of the message.

Beyond the rule above, RFC3484 (address selection) would apply when
choosing from among addresses on the same interface.

However, none of this is specific to ICMP error generation, it's the
same as for locally originated packets (section 4.2).  Hence, I'm not
convinced that the rules need to be repeated here.

If you still disagree, please suggest text :)

-Dave

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