| RFC 9762 | PIO P Flag | June 2025 |
| Colitti, et al. | Standards Track | [Page] |
This document defines the P flag in the Prefix Information Option (PIO) ofIPv6 Router Advertisements (RAs). The flag is used to indicate that thenetwork prefers that clients use the deployment model in RFC 9663instead of using individual addresses in the on-link prefix assigned usingStateless Address Autoconfiguration (SLAAC) or DHCPv6 address assignment.¶
This document updates RFC 4862 to indicate that the Autonomous flag in a PIOneeds to be ignored if the PIO has the P flag set. It also updates RFC 4861to specify that the P flag indicates DHCPv6 prefix delegation support forclients.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc9762.¶
Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
[RFC9663] documents an IPv6 address assignment model where IPv6 devices obtain dedicated prefixes from the network via DHCPv6 prefix delegation (DHCPv6-PD)[RFC8415]. This model provides devices with a large IPv6 address space they can use to create addresses for communication, individually number virtual machines (VMs) or containers, or extend the network to downstream devices. It also provides scalability benefits on large networks because network infrastructure devices do not need to maintain per-address state, such as IPv6 neighbor cache, Source Address Validation Improvement (SAVI)[RFC7039] mappings, Virtual eXtensible Local Area Network (VXLAN)[RFC7348] routes, etc.¶
On networks with fewer devices, however, this model may not be appropriate, because scaling to support multiple individual IPv6 addresses per device is less of a concern. Also, many home networks currently offer prefix delegation but assume that a limited number of specialized devices and/or applications will require delegated prefixes and thus do not allocate enough address space to offer prefixes to every device that connects to the network. For example, if clients assume the[RFC9663] deployment model on a home network that only receives a /60 from the ISP and each client obtains a /64 prefix, then the network will run out of prefixes after 15 devices have been connected.¶
Therefore, to safely roll out the support of the deployment model defined in[RFC9663] on the client side, it is necessary to have a mechanism for the network to signal to the client which address assignment method is preferred.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14[RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The network administrator might want to indicate to clients that requesting a prefix via DHCPv6-PD and using that prefix for address assignment (see[RFC9663]) should be preferred over using individual addresses from the on-link prefix.The information is passed to the client via a P flag in the PIO. The reasons for it being a PIO flag are as follows:¶
Note that setting the P flag in a PIO expresses the network operator's preference that clients should attempt using DHCPv6-PD instead of performing individual address configuration on the prefix.For clients that honor this preference by requesting prefix delegation, the actual delegated prefix will necessarily be a prefix different from the one from the PIO.¶
The P flag (also called the DHCPv6-PD Preferred Flag) is a 1-bit PIO flag, located after the R flag[RFC6275].The presence of a PIO with the P flag set indicates that the network prefers that clients use prefix delegation instead of acquiring individual addresses via SLAAC or DHCPv6 address assignment. This implies that the network has a DHCPv6 server capable of making DHCPv6 prefix delegations to every device on the network, as described in[RFC9663].¶
Figure 1 shows the resulting format of the PIO.¶
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|P| Rsvd1 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Valid Lifetime |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Preferred Lifetime |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Reserved2 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ +| |+ Prefix +| |+ +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The P flag is independent of the value of the M and O flags in the RA.If the network desires to delegate prefixes to devices that support DHCPv6 prefix delegation but do not support the P flag, itSHOULD also set the M or O bits in the RA to 1, because some devices, such as Customer Edge (CE) routers[RFC7084], might not initiate DHCPv6 prefix delegation if both the M and O bits are set to zero.¶
RoutersSHOULD set the P flag to zero by default, unless explicitly configured by the administrator, andSHOULD allow the operator to set the P flag value for any given prefix advertised in a PIO.RoutersMUST allow the P flag to be configured separately from the A flag.In particular, enabling or disabling the P flagMUST NOT trigger automatic changes in the A flag value set by the router.¶
This specification only applies to clients that support DHCPv6 prefix delegation.Clients that do not support DHCPv6 prefix delegationMUST ignore the P flag.The P flag is meaningless for link-local prefixes, and any PIO containingthe link-local prefixMUST be ignored as specified inSection 5.5.3 of [RFC4862].In the following text, all prefixes are assumed not to be link-local.¶
For each interface, the clientMUST keep a list of every prefix that was receivedfrom a PIO with the P flag set and currently has a non-zero preferred lifetime.The list affects the behavior of the DHCPv6 client as follows:¶
When a client requests a prefix via DHCPv6-PD, itMUST use the prefix length hint (Section 18.2.4 of [RFC8415]) to request a prefix that is short enough to form addresses via SLAAC.¶
In order to achieve the scalability benefits of using DHCPv6-PD, the clientSHOULD prefer to form addresses from the delegated prefixinstead of using individual addresses in the on-link prefix(es). Therefore,when the client requests a prefix using DHCPv6-PD, the clientSHOULD NOT use SLAAC to obtain IPv6 addresses from PIOs with the P and A bitsset. Similarly, if all PIOs processed by the client have the P bit set, theclientSHOULD NOT request individual IPv6 addresses fromDHCPv6, i.e., itSHOULD NOT include any IA_NA options inSolicit messages[RFC8415]. The clientMAYcontinue to use addresses that are already configured.¶
If the client does not obtain any suitable prefixes via DHCPv6-PD that are suitable for SLAAC, itMAY choose to disable further processing of the P flag on that interface, allowing the client to fall back to other address assignment mechanisms, such as forming addresses via SLAAC (if the PIO has the A flag set to 1) and/or requesting individual addresses via DHCPv6.¶
If the delegated prefix is too long to be used for SLAAC, the clientMUST ignore it, asSection 7 of [RFC9663] requires the network to provide a SLAAC-suitable prefix to clients. If the prefix is shorter than required for SLAAC, the clientSHOULD accept it, allocate one or more longer prefixes suitable for SLAAC, and use the prefixes as described below.¶
For every accepted prefix:¶
The clientMUST NOT send or forward packets with destination addresses within a delegated prefix to the interface that it obtained the prefix on, as this can cause a routing loop. This problem will not occur if the client has assigned the prefix to another interface. Another way the client can prevent this problem is to add to its routing table a high-metric discard route for the delegated prefix.¶
The P bit is purely a positive indicator, telling nodes that DHCPv6prefix delegation is available and the network prefers that nodes use it,even if they do not have any other reason to run a prefix delegation client.The absence of any PIOs with the P bit does not carry any kind of signal tothe opposite andMUST NOT be processed to mean that DHCPv6-PD is absent. Inparticular, nodes that run DHCPv6-PD due to explicit configuration or by default(e.g., to extend the network)MUST NOT disable DHCPv6-PD on the absence of PIOswith the P bit set.A very common example of this are CE routers as describedby[RFC7084].¶
When the network delegates unique prefixes to clients, each client will consider other clients' destination addresses to be off-link, because those addresses are from the delegated prefixes and are not within any on-link prefix. When a client sends traffic to another client, packets will initially be sent to the default router. The router may respond with an ICMPv6 redirect message (Section 4.5 of [RFC4861]). If the client receives and accepts the redirect, then traffic can flow directly from device to device. Therefore, hosts supporting the P flagSHOULD process redirects unless configured otherwise.Hosts that do not process ICMPv6 redirects, and routers that do not act on ICMPv6 redirects, may experience higher latency while communicating to prefixes delegated to other clients on the same link.¶
For the purpose of source address selection[RFC6724], if the host creates any addresses from a delegated prefix, itSHOULD treat those addresses as if they were assigned to the interface on which the prefix was received. This includes placing them in the candidate set and associating them with the outgoing interface when implementing Rule 5 of the source address selection algorithm[RFC6724].¶
In multi-prefix multihoming, the host generally needs to associate the prefix with the router that advertised it (for example, see Rule 5.5 in[RFC6724]). If the host supports Rule 5.5, then itSHOULD associate each prefix with the link-local address of the DHCPv6 server or relay from which it received the REPLY packet.When receiving multiple REPLYs carrying the same prefix from distinctlink-local addresses, the hostSHOULD associate that prefix with all of theseaddresses. This can commonly happen in networks with redundant routers andDHCPv6 servers or relays.¶
This document makes the following changes toSection 4.2 of [RFC4861]:¶
OLD TEXT:¶
Note: If neither M nor O flags are set, this indicates that no information is available via DHCPv6.¶
NEW TEXT:¶
Note: If the M, O, or P (RFC 9762) flags are not set, this indicates that no information is available via DHCPv6.¶
This document makes the following changes toSection 5.5.3 of [RFC4862]:¶
OLD TEXT:¶
For each Prefix-Information option in the Router Advertisement:¶
- a)
- If the Autonomous flag is not set, silently ignore the Prefix Information option.¶
- b)
- If the prefix is the link-local prefix, silently ignore the Prefix Information option.¶
- c)
- If the preferred lifetime is greater than the valid lifetime, silently ignore the Prefix Information option. A nodeMAY wish to log a system management error in this case.¶
- d)
- If the prefix advertised is not equal to the prefix of an address configured by stateless autoconfiguration already in the list of addresses associated with the interface (where "equal" means the two prefix lengths are the same and the first prefix-length bits of the prefixes are identical), and if the Valid Lifetime is not 0, form an address (and add it to the list) by combining the advertised prefix with an interface identifier of the link as follows:¶
NEW TEXT:¶
For each Prefix Information Option in the Router Advertisement:¶
- a)
- If the prefix is the link-local prefix, silently ignore the Prefix Information Option.¶
- b)
- If the P flag is set and the node implements RFC 9762, itSHOULD treat the Autonomous flag as if it was unset and use prefix delegation to obtain addresses as described in RFC 9762.¶
- c)
- If the Autonomous flag is not set, silently ignore the Prefix Information Option.¶
- d)
- If the preferred lifetime is greater than the valid lifetime, silently ignore the Prefix Information Option. A nodeMAY wish to log a system management error in this case.¶
- e)
- If the prefix advertised is not equal to the prefix of an address configured by stateless autoconfiguration already in the list of addresses associated with the interface (where "equal" means the two prefix lengths are the same and the first prefix-length bits of the prefixes are identical) and if the Valid Lifetime is not 0, form an address (and add it to the list) by combining the advertised prefix with an interface identifier of the link as follows:¶
The mechanism described in this document relies on the information provided in the RA and therefore shares the same security model as SLAAC. If the network does not implement RA-Guard[RFC6105], an attacker might send RAs containing the PIO used by the network, set the P flag to 1, and force hosts to ignore the A flag. In the absence of DHCPv6-PD infrastructure, hosts would either obtain no IPv6 addresses or, if they fall back to other IPv6 address assignment mechanisms such as SLAAC and IA_NA, would experience delays in obtaining IPv6 addresses. If the network does not support DHCPv6-Shield[RFC7610], the attacker could also run a rogue DHCPv6 server, providing the host with invalid prefixes or other invalid configuration information.¶
The attacker might force hosts to oscillate between DHCPv6-PD and PIO-based SLAAC by sending the same set of PIOs with and then without the P flag set. That would cause the clients to issue REBIND requests, increasing the load on the DHCP infrastructure. However,Section 14.1 of [RFC8415] requires that DHCPv6-PD clients rate-limit transmitted DHCPv6 messages.¶
It should be noted that if the network allows rogue RAs to be sent, the attacker would be able to disrupt hosts' connectivity anyway, so this document doesn't introduce any fundamentally new security considerations.¶
Security considerations inherent to the PD-per-device model are documented inSection 15 of [RFC9663].¶
The privacy implications of implementing the P flag and using DHCPv6-PD to assign prefixes to hosts are similar to the privacy implications of using DHCPv6 to assign individual addresses. If the DHCPv6 infrastructure assigns the same prefix to the same client, then an observer might be able to identify clients based on the highest 64 bits of the client's address. Those implications and recommended countermeasures are discussed inSection 13 of [RFC9663].¶
Implementing the P flag support on a host and receiving will enable DHCPv6 on that host if the host receives an RA containing a PIO with the P bit set.Sending DHCPv6 packets may reveal some minor additional information about the host, most prominently the hostname. This is not a new concern and would apply for any network that uses DHCPv6 and sets the M flag in RAs.¶
No privacy considerations result from supporting the P flag on the sender side.¶
IANA has made the following allocation in the "IPv6 Neighbor Discovery Prefix Information Option Flags" registry[RFC8425]:¶
| PIO Option Bit | Description | Reference |
|---|---|---|
| 3 | P - DHCPv6-PD Preferred Flag | RFC 9762 |
Thanks toNick Buraglio,Brian Carpenter,Tim Chown,David Farmer,Fernando Gont,Susan Hares,Mahesh Jethanandani,Suresh Krishnan,Ted Lemon,Andrew McGregor,Tomek Mrugalski,Erik Nordmark,Michael Richardson,Patrick Rohr,John Scudder,Ole Trøan,Dirk Von Hugo,Éric Vyncke andTimothy Winters for the discussions, reviews, input, and contributions.¶