The DHCPv6 Option for IPv6
Neighbor DiscoveryBoeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftIPv6 Neighbor Discovery (IPv6ND) specifies a control message set for
nodes to discover neighbors, routers, prefixes and other services on the
link. It also supports a manner of StateLess Address AutoConfiguration
(SLAAC). The Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
specifies a service for the stateful delegation of addresses and
prefixes.Currently, at least two round-trip message exchanges are necessary in
order to perform the IPv6ND router discovery and DHCPv6 address/prefix
delegation functions. This document presents a protocol for combining
these two exchanges into a single exchange by joining the two services
into a single unified service.IPv6 Neighbor Discovery (IPv6ND) specifies a
control message set for nodes to discover neighbors, routers, prefixes
and other services on the link. It also supports a manner of StateLess
Address AutoConfiguration (SLAAC). The Dynamic Host Configuration
Protocol for IPv6 (DHCPv6) specifies a service for the stateful
delegation of addresses and prefixes .Currently, at least two round-trip message exchanges are necessary in
order to perform the IPv6ND router discovery and DHCPv6 address/prefix
delegation functions. This document presents a protocol for combining
these two exchanges into a single exchange by joining the two services
into a single unified service.When a node first comes onto the link, it sends a Router Solicitation
(RS) message to elicit a Router Advertisement (RA) message from one or
more routers for the link. If the node also needs to acquire managed
addresses and prefixes (and, if the 'M' bit is set in the RA message) it
then sends a DHCPv6 Solicit message with Rapid Commit to elicit a Reply
message from a DHCPv6 server that is authoritative for the link. This
two round-trip message exchange can add delay as well as waste critical
link bandwidth on low-end links (e.g., aeronautical wireless links).
While it is possible to conceive of starting both round trip exchanges
at the same time (i.e., under the leap-of-faith assumption that the link
supports DHCPv6 before examining the 'M' bit) this would result in twice
as many channel access transactions as necessary.This document proposes a new IPv6 ND option called the "DHCPv6
Option" that combines the IPv6 ND router discovery and DHCPv6 managed
address/prefix acquisition processes into a single message exchange.
Nodes include the DHCPv6 option in RS messages to solicit an RA message
with a DHCPv6 option in return. This allows the IPv6 ND and DHCPv6
functions to work together to supply the client with all needed
configuration information in a single message exchange instead of
multiple.The following sections present considerations for nodes that employ
the IPv6 ND DHCPv6 option.The DHCPv6 option is a new IPv6 ND option that simply embeds a
standard DHCPv6 message per section 6 of ,
beginning with the 'msg-type' followed by the 'transaction-id' and all
DHCPv6 'options'. The format of the option is as follows:In this format, 'Type' and 'Length' are exactly as defined in
Section 4.6 of , 'Pad' encodes the number of
trailing zero octets (between 0 - 7) that appear at the end of the
option to pad to an integral number of 8-octets, 'Reserved' is included
for alignment and potential future use, and the rest of the option is
exactly as defined in Section 6 of . The length
of the full DHCPv6 message is determined by ((('Length' * 8) - 4) -
'Pad'), for a maximum message length of 2044 octets.The 'Reserved' field MUST be set to 0 on transmission and ignored on
reception. Future specifications MAY define new uses for these bits.When a node first comes onto the link, it creates a Router
Solicitation (RS) message containing a DHCPv6 option that embeds a
DHCPv6 Solicit message with Rapid Commit. The node then sends the RS
message either to the unicast address of a specific router on the link,
or to the All-Routers multicast address.When a router receives an RS message with a DHCPv6 option, if it does
not recognize the option and/or does not employ a DHCPv6 relay agent or
server, it returns a Router Advertisement (RA) message as normal and
without including a DHCPv6 option. By receiving the RA message with no
DHCPv6 option, the node can determine that the router does not recognize
the option and/or does not support a DHCPv6 relay/server function. In
this way, no harm will have come from the node including the DHCPv6
option in the RS, and the function is fully backwards compatible.When a router receives an RS message with a DHCPv6 option, if it
recognizes the option and employs a DHCPv6 relay agent or server, it
extracts the DHCPv6 message from the RS message and forwards the message
to the DHCPv6 relay agent or server. When the DHCPv6 message reaches a
DHCPv6 server, the server processes the DHCPv6 Solicit message and
prepares a DHCPv6 Reply message containing any delegated addresses,
prefixes and/or any other information the server is configured to send.
The server then returns the Reply message to the router.When the router receives the DHCPv6 Reply message, it creates a
Router Advertisement (RA) message that includes any autoconfiguration
information necessary for the link and also embeds the Reply message in
a DHCPv6 option within the body of the RA. The router then returns the
RA as a unicast message reply to the node that sent the RS.At any time after the initial RS/RA exchange, the node may need to
issue a DHCPv6 Renew, Release or Rebind message, e.g., to extend
address/prefix lifetimes. In that case, the node prepares a DHCPv6
message option and inserts it in an RS message which it then sends via
unicast to the router. The router in turn processes the message the same
as for DHCPv6 Solicit/Reply.At any time after the initial RS/RA exchange, the DHCPv6 server may
need to issue a DHCPv6 Reconfigure message. In that case, when the
router receives the DHCPv6 Reconfigure message it prepares a unicast RA
message with a DHCPv6 option that encodes the Reconfigure and sends the
RA as an unsolicited unicast message to the node.The IPv6ND function and DHCPv6 function are typically implemented in
separate router modules. In that case, the IPv6 ND function extracts the
DHCPv6 message from the option included in the RS message and wraps it
in IP/UDP headers. The source address in the IP header is set to the
node's link-local address, and the source port in the UDP header is set
to the port number associated with the IPv6 ND function. The IPv6 ND
function then acts as a Lightweight DHCPv6 Relay Agent (LDRA) to forward the message to the DHCPv6 relay or server
function on-board the router.The forwarded DHCPv6 message then traverses any additional relays on
the reverse path until it reaches the DHCPv6 server. When the DHCPv6
server processes the message, it delegates any necessary resources and
sends a Reply via the same relay agent path as had occurred on the
reverse path so that the Reply will eventually arrive back at the IPv6
ND function. The IPv6 ND function then prepares an RA message with any
autoconfiguration information associated with the link, embeds the
DHCPv6 message body in an IPv6 ND DHCPv6 option, and returns the message
via unicast to the node that sent the RS.In a preferred implementation, however, the IPv6ND and DHCPv6
functions could be co-located in the same module on the router. In that
way the two functions would be coupled as though they were in fact a
single unified function without the need for any LDRA processing.The IANA is instructed to assign an IPv6 ND option Type value TBD for
the DHCPv6 option.The IANA is instructed to create a registry for the DHCPv6 option
"Reserved" field so that future uses of bits in the field can be
coordinated.Security considerations for IPv6 Neighbor Discovery and DHCPv6 apply to this document.SEcure Neighbor Discovery (SEND) can provide
authentication for the combined DHCPv6/IPv6ND messages with no need for
additional securing mechanisms..This work was motivated by discussions on the 6man and v6ops
list.The following individuals provided useful comments that improved the
document: Bernie Volz.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program and the Boeing Research & Technology (BR&T)
enterprise autonomy program.