RE: [dhcwg] Sending SOLICIT/CONFIRM/REBIND messages to a relay-agentunicast address

["Templin, Fred L" <[e-mail]>]

Responding to my own post, I was able to gain some insights from
the RFC3315 text. Please review and comment on whether I may be
missing something, or if there is some fundamental flaw with
this logic: 

> If a DHCPv6 client is pre-configured with a list of IP addresses
> of DHCPv6 relay agents on the link, and if the client knows that
> there are no DHCPv6 servers on the link, can the client send
> SOLICIT/CONFIRM/REBIND messages to the unicast address of one
> of the relay agents?

Combing through RFC3315, I see only the following passages
relating to the IP destination address in client messages:

in Section 1.1:
  "DHCP servers receive messages from clients using a reserved,
   link-scoped multicast address.  A DHCP client transmits most messages
   to this reserved multicast address, so that the client need not be
   configured with the address or addresses of DHCP servers."

in Section 1.2:
  "In this case, the client sends a
   Solicit message to the All_DHCP_Relay_Agents_and_Servers requesting
   the assignment of addresses and other configuration information."

in Section 1.3:
  "The client sends a Solicit message to the
   All_DHCP_Relay_Agents_and_Servers address to find available DHCP
   servers."

in Section 3:
  "The availability of these features means that
   a client can use its link-local address and a well-known multicast
   address to discover and communicate with DHCP servers or relay agents
   on its link."

in Section 13:
  "Unless otherwise specified in this document, or in a document that
   describes how IPv6 is carried over a specific type of link (for link
   types that do not support multicast), a client sends DHCP messages to
   the All_DHCP_Relay_Agents_and_Servers.

   A client uses multicast to reach all servers or an individual server.
   An individual server is indicated by specifying that server's DUID in
   a Server Identifier option (see section 22.3) in the client's message
   (all servers will receive this message but only the indicated server
   will respond).  All servers are indicated by not supplying this
   option."

finally, in Section 15:
  "A server MUST discard any Solicit, Confirm, Rebind or
   Information-request messages it receives with a unicast destination
   address."

In these passages, I see only one "MUST" which occurs in the
Section 15 text wrt a server's validation for messages received
with a unicast destination address. But, if the client sends
messages to the unicast address of a *relay*, the relay would
forward them to the server with 'All_DHCP_Servers' as the
destination address, so the server message validation would pass.

> If not, is this something that can be specified "in a document
> that describes how IPv6 is carried over a specific type of link"
> as suggested by ([RFC3315], Section 13)?

None of the RFC3315 text speaks to whether clients could send
messages to the unicast address of a relay other than to say
in Section 1.1 that: "The operation of the relay agent is
transparent to the client". Furthermore, the spec is silent
on a relay's message validation which I take to mean that a
relay agent's behavior wrt receiving a unicast message from
a client is unspecified and presumed to behave the same as
for multicast messages.

Using this information, plus the first paragraph of the Section
13 text cited above, I assume that "a document that describes how
IPv6 is carried over a specific type of link (for link types that
do not support multicast)" could specify that a client's messages
are sent to the unicast address of a relay on the link (if the
client is configured with the IP address of the relay).

Comments?

Fred
[e-mail]

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