Because the RS message is not complex, we will breeze through it to
save time for the RA, which is much more interesting.
The RS asks a simple question.
Where are the routers?
It's originated from a client's link‑local address and destined
for ff02::2, the all routers multicast group.
The type is 133 and code is 0.
Like the other ND messages we've seen so far, the RS is a
separate type of ICMP message entirely.
The message doesn't have any special flags, but does carry an ICMPv6
option of the source MAC address of the sender.
Why?
Because this allows any routers to reply directly to the host if needed.
It was designed for those routers that haven't gone through
the NS/NA process with a given host yet.
How does this question get answered?
The RA, of course. In my opinion, this is the most powerful ICMP message ever.
It's very gracious and supplies a wealth of
information about the network to clients.
The source of an RA is the link‑local address of the router sending it, and
the destination is fe80::4, the originator of the RS.
Like the NA, the RA can also be sent unsolicited, in which case the
destination would be the all hosts IPv6 multicast address of ff02::1. The type
is 134 and code is 0, so just like the other ND messages, it's a completely
different ICMP message type. The hop limit specifies the IPv4 TTL equivalent
that hosts should use when originating.
IPv6 traffic.
IPv6 gives us a knob to control this at the network layer.
There are many minor options available.
We won't be exploring each one here as they
typically tie in with other features.
I'll show the RFCs for reference in case you want to dig deeper. The managed
address flag, or M flag, is sent when the router offers stateful Dynamic Host
Configuration Protocol, or DHCP, for addressing. The other config flag, or O
flag, means DHCP is available, but not for addressing, perhaps just for the
domain name and network time servers instead. The home agent, or H flag, is
specific to IPv6 mobility and specifies whether the router lies at the home site
or the foreign site.
PRF stands for preference and can be used to set high, medium, and low
priorities for routers when multiple choices exist on a LAN.
The proxy flag allows IPv6 ND to have functionality
similar to IPv4 proxy ARP, allowing for bridging a subnet
even across Layer 3 boundaries.
Let's move on to the ICMPv6 options.
I won't detail all of them, but I'll hit the high points.
We've discussed the source MAC address option before, so I'll skip that one.
The MTU option is another polite piece of information that
tells the host what the MTU of the link is.
This could proactively prevent fragmentation.
The most important option is the IPv6 prefix.
IPv6 can signal to clients a set of prefixes available for
addressing. Clients can statelessly choose IPv6 addresses
using SLAAC from these ranges.
Each prefix will have detailed information about its prefix length and values.
We saw this earlier when talking about NS and NA messages.
Let's quickly cover what we learned in this module.
I introduced four new messages. The neighbor solicitation and neighbor
advertisement messages are similar to IPv4 ARP requests and replies.
The router solicitation and router advertisement messages are
enhancements that simplify host to router signaling.
Some describe IPv6 as a polite process whereby duplicate addresses
are averted and valuable information such as hop limit, MTU, and
routable prefixes are freely shared.
We've demonstrated several of these characteristics in this module.
These fancy features of IPv6 just work without much manual effort.
Like IPv4 ARP, it's still important to understand how it works
under the hood, and we covered that here.
Next, we'll tackle other common ICMPv6 messages as they relate to network operations.