>> Yes.  Also badly misconfigured networking on your ISP's end.
> Is it really that way?

Well, that's my understanding - and I say that as someone who works at
a (smallish) ISP.

> I mean, a client forms a request NTP packet, and puts it into a UDP
> packet with my IP as the destination and his IP address as where he
> wants to get the response back.  A metric buttload of routers between
> him and me pass this packet on to me. My ntpd sees the request and
> replies back to the address he put in the UDP packet.

Right.

> Is it really the responsibility of the network layer to check the
> "from" address and see it's nonsensical?

Not so much that it's nonsensical as forged: the other host is emitting
packets with from-addresses that aren't its own.  If I've been
allocated, say, 100.200.30.40/29, I should not be emitting packets with
ip_src values not in that /29, and (my understanding of) best practice
is that my upstream should enforce this.  RFC1918-private addresses are
just a special (and especially glaring) case of this.

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