Thomas Narten wrote:
When NodeA issues a NS for NodeB, that usually means that NodeA has
traffic to send to NodeB, and NodeB will shortly thereafter send a
response packet back to/through NodeA. In IPv4, that requires both
sides issue ARPs independently. I.e., NodeA has to ARP for NodeB, and
NodeB then later has to ARP for NodeA. A total of 4 packets, 2 of them
broadcast.
>
With ND, we can short circuit the need for the second packet
exchange. When NodeA issues an NS for NodeB, NodeB (per the rule we
have been discussing) goes ahead and creates a Neighbor Cache Entry
for NodeA (i.e., from the source address of the NS). When NodeB then
generates a response packet, it doesn't need to do an additional NS/NA
exchange, because it already has an entry for the desired address.
FWIW RFC 826 specifies exactly the same thing - learn from the source
protocol address and hardware address so that only 2 ARP packets are needed.
The case where bullet three and four would potentially help is when
there is a NMBA network and as a result no on-link prefixes, or when
different nodes are configured with different on-link prefixes.
Let's look at those in turn.
NBMA:
-----
Host A and Host B have an empty on-link prefix list hence all packets
are initially sent to a default router. The default routers are assumed
to know (through magic) all the IP and L2 addresses of the nodes on the
link.
Host A sends packet destined to B to the router. Router responds with a
redirect.
When host B responds to A that packet also goes to the router and
results in a redirect.
That results in both host A and host B knowing that they are on-link
with no use of NA or NS, and no use of bullet 3 and bullet 4.
Differently configured hosts:
-----------------------------
Host A and host B are on the same link.
Host A has been configured with IP address P1:A and with on-link
prefixes P1 and P2.
Host B has been configured with IP address P2:B and on-link prefix P2.
(Note that this requires manual configuration since RFC 4861/4862 can't
create such a configuration.)
When A sends a packet to P2:B it will multicast a NS since it views P2
as on-link.
Taking bullet 4 into account B will treat P1:A as on-link, thus a single
NS/NA exchange is sufficient.
If we remove bullet 4 then B would respond by sending the packet to a
default router. Presumably that router knows P1 is on-link (so it can
forward the packet back out the link) hence it would send a redirect
back to B telling B that P1:A is on-link.
Thus in the case of statically inconsistently configured hosts we do
same 2 packets sent on the wire as a result of bullet 4.
I can't come up with a case where bullet 3 would make a difference.
Erik
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