IPv6 Neighbor Discovery
Extensions for Prefix DelegationBoeing 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. This document presents IPv6ND extensions for providing a
single unified autoconfiguration 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 two possible methods for
providing a single unified autoconfiguration 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 two methods for combining these separate
operations into a single, unified exchange. The first method is through
definition of 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 second method leverages the PIOX proposal
[I-D.pioxfolks-6man-pio-exclusive-bit] where the router sets the "X
(eXclusive)" bit in an RA Prefix Information Option (PIO) to inform the
node that the prefix is provided for the node's own exclusive use. This
document permits nodes to include PIOs in their RS messages for the
purpose of requesting prefix delegations from routers. The PIOX proposal
sugggests an unsolicited method of providing the node with the prefix,
whereby this document suggested a solicited method.The following sections present considerations for nodes that employ
these approaches.The first method discussed in this document is the inclusion of
DHCPv6 message options in IPv6ND RS and RA messages, as discussed in the
following sections.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 second method discussed in this document is the inclusion of PIO
options in IPv6ND RS messages, as discussed in the following
sections.The PIO option for solicited prefix delegation is formatted exactly
as specified in except including the "X" bit
as defined in [I-D.pioxfolks-6man-pio-exclusive-bit]. We refer to PIO
options with the "X" bit set as "PIOX" options. The format of the
option is as follows:In this format, all feilds are exactly as defined in
Section 4.6 of . The "X" bit is set to 1 if
the prefix is to be provided for the node's own exclusive use. If "X"
is set to 0, no statement is made about the prefix's exclusivity.When a node that wishes to request an eXclusive prefix first comes
onto the link, it creates a Router Solicitation (RS) message
containing a PIOX. It sets the Prefix Length to either the length of
the prefix it wishes to receive or '0' (unspecified) if it will defer
to the router's preference. The node then sets the Valid and Preferred
Lifetimes to either its preferred values or '0' (unspecified) if it
will defer to the router's preference. The node then sets the Prefix
to either the prefix it wishes to receive, or '0' (unspecified) if it
will defer to the router's preference. 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 PIOX option, if it is
not configured to accept PIOXs in RS messages it returns a Router
Advertisement (RA) message as normal and without including a PIOX. By
receiving the RA message with no PIOX, the node can determine that the
router does not recognize the option and/or does not support a PIOX
prefix delegation service. In this way, no harm will have come from
the node including the PIOX in the RS, and the function is fully
backwards compatible.When a router receives an RS message with a PIOX, if it recognizes
the option and can provide prefix delegation services it examines the
fields in the message and selects a prefix to delegate to the node. If
the PIOX included a non-zero Prefix, the router delegates the node's
preferred prefix if possible. Otherwise, the router selects a prefix
to delegate to the node with length based on the node's Prefix Length.
The router sets lifetimes matching the lifetimes requested by the node
if possible, or shorter lifetimes if the node's requested lifetimes
are too long. The router finally prepares a PIOX containing this
information and inserts it into an RA message to send back to the
source of the RS.Each router can implement a prefix delegation database management
service of their own choosing, but an attractive alternative would be
to use the already-existing DHCPv6 service as the back-end prefix
management service. In this way, all communications between the
router's link to the requesting node are via PIOX RS/RA messaging.
But, when the router receives an RS message with a PIOX it can create
a syntehsized DHCPv6 Solicit message to send to the DHCPv6 server.
This can be done exactly the same as for the approach discussed in
. In this way, the node on the link over which
the PIOX is advertised only ever sees RS/RA messages on the front end,
and the router gets to use the mature and widely deployed DHCPv6
service for prefix management on the back end.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.
Those individuals who provided encouragement and critical review are
acknowledged.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.