IPv6 Neighbor Discovery Extensions for Prefix Delegation
draft-templin-6man-dhcpv6-ndopt-02.txt

Abstract

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. This document presents IPv6ND extensions for providing a single unified autoconfiguration service.

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Table of Contents

• 1. Introduction
• 2. DHCPv6 Options in IPv6 ND Messages
• 2.1. The DHCPv6 Option
• 2.2. DHCPv6 Option Usage
• 2.3. Implementation Considerations
• 3. PIO Options in RS Messages
• 3.1. The PIOX Option
• 3.2. PIOX Option Usage
• 3.3. Implementation Considerations
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

IPv6 Neighbor Discovery (IPv6ND) [RFC4861] 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 [RFC3315][RFC3633].

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.

2. DHCPv6 Options in IPv6 ND Messages

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.

2.1. The DHCPv6 Option

The DHCPv6 option is a new IPv6 ND option that simply embeds a standard DHCPv6 message per section 6 of [RFC3315], beginning with the 'msg-type' followed by the 'transaction-id' and all DHCPv6 'options'. The format of the option is as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Type = TBD   |    Length     | Pad |        Reserved         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    msg-type   |               transaction-id                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                            options                            .
       .                           (variable)                          .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: IPv6 ND DHCPv6 Option Format

[RFC4861], '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 [RFC3315]. 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.

2.2. DHCPv6 Option Usage

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.

2.3. Implementation Considerations

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) [RFC6221] 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.

3. PIO Options in RS Messages

The second method discussed in this document is the inclusion of PIO options in IPv6ND RS messages, as discussed in the following sections.

3.1. The PIOX Option

The PIO option for solicited prefix delegation is formatted exactly as specified in [RFC4861] 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:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     | Prefix Length |L|A|R|X| Rsrvd1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Valid Lifetime                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Preferred Lifetime                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved2                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                            Prefix                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: PIOX Option Format

[RFC4861]. 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.

3.2. PIOX Option Usage

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.

3.3. Implementation Considerations

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 Section 2.3. 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.

4. IANA Considerations

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.

5. Security Considerations

Security considerations for IPv6 Neighbor Discovery [RFC4861] and DHCPv6 [RFC3315][RFC3633] apply to this document.

SEcure Neighbor Discovery (SEND) [RFC3971] can provide authentication for the combined DHCPv6/IPv6ND messages with no need for additional securing mechanisms.

.

6. Acknowledgements

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.

7. References

7.1. Normative References

7.2. Informative References

Author's Address