IPv6 Router Advertisement Prefix Information Option Exclusive Bit

This document defines a new control bit in the Internet Protocol version 6 (IPv6) Router Advertisement (RA) Prefix Information Option (PIO) flags octet that indicates that the node receiving this RA is the exclusive receiver of all traffic destined to any address with that prefix. Subject to the lifetime constraints within the PIO, the receiving node effectively has exclusive use of the prefix, and will be the next hop destination for the sending router, and possibly other routers, for all traffic destined toward the prefix. Termed the eXclusive bit (or "X bit"), nodes that recognize this can perform some traffic-saving optimizations (e.g. disable Neighbor Discovery (ND) and Duplicate Address Detection (DAD) for addresses within this prefix) and more immediately pursue the benefits of being provided multiple addresses (vis. section 3). Additionally, network infrastructure nodes (routers, switches) can benefit by minimizing the number of {link layer, IP} address pairs required to offer network connectivity (vis. section 9.3). A router, for example, need not create any {link layer, IP} address pair entries for IP address within a proffered exclusive-use prefix--it can reliably forward all traffic to the network node to which it advertised the prefix. This solves a potential link layer state exhaustion problem, i.e excessive number of {link layer, IP address pairs}, using IP layer forwarding. Use of the X bit is backward compatible with existing IPv6 standards compliant implementations. -compliant nodes that do not understand the X bit are not negatively impacted. They must ignore it, and can process the PIO under existing standards, making use of the information exactly as if the X bit were not set.

There are several initiatives that propose network side practices that provide customer isolation, enhanced operational scalability, power efficiency, security and other benefits in IPv6 network deployments. Some of these involve isolating a host (or RA accepting client node) so that the host is the only node to receive a specific prefix, including DHCPv6 Prefix Delegation to hosts (), and advertising a unique prefix per host via unique RAs. (). Some architectures further isolate the host layers below IPv6, for improved client node security. Regardless of the specific level of isolation, the host can best make optimizations and choices about its use of a prefix exclusively forwarded to itself if the host can be informed of the exclusivity. In the case of a DHCPv6 Prefix Delegation the prefix can be assumed to be of exclusive use by the requesting node (in accordance with the model in ). This memo documents an additional bit in the IPv6 RA PIO that makes this information explicit to receiving node.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Throughout this document the following terminology is used purely for the sake of brevity.

The term "PIO-X" is used to refer to a Prefix Information Option (PIO) that has the X bit set.

The phrase "PIO-X RA" is used to refer to an IPv6 Router Advertisement (RA) that contains one or more PIO-X entries (the same RA may also contain one or more PIOs without the X bit set).

The term "host" may be used interchangeably throughout this document to mean a network node receiving and processing an RA. The receiving node may itself be a router, or may temporarily become one by routing all or a portion of an exclusive use prefix.

This document updates the Prefix Information Option specification in RFC 4861, section 4.6.2 with the definition of a bit from the former Reserved1 field as follows.

Fields: X The eXclusive use indicator flag, defined by this document. When set, the receiving node can be assured that all traffic destined to any address within the specified Prefix will be forwarded to itself by, at a minimum, the router from which the encapsulating RA was received, but possibly other routers as well. When not set, the receiving node MUST NOT make any assumptions of exclusive use of the specified Prefix, i.e. processing is unchanged from previous standards behaviour. Rsrvd1 Retains the same meaning as Reserved1 from section 4.6.2. All other fields Retain their same meaning from section 4.6.2.

Nodes compliant with this specification perform the following additional processing of RAs and PIO-X options when a PIO-X option is present.

A node receiving a PIO-X option MUST verify that it is the (likely) sole intended recipient of the PIO-X RA. This done by verifying that the RA is unicast to the node at the IPv6 layer and, if applicable, at the link layer. On links that provide the node with a guarantee that it is the only possible PIO-X RA recipient (e.g. PPP, 3GPP links) this validation step SHOULD NOT be performed. If an address other than :: (the unspecified address) was used as the source address for one or more Router Solicitations (RS) on this link, the node MUST verify that the IPv6 destination address of the PIO-X RA is one of the RS source addresses in use. If the link over which this communication takes place is known to be point-to-point, i.e. the nature of the link ensures that the node is the only possible recipient of an RA this check SHOULD NOT be performed. If the node receives a PIO-X RA over a link-layer medium that supports link-layer addresses, it MUST verify that the link-layer destination address of the PIO-X RA is its own link-layer address. If the node received a PIO-X RA over a point-to-point medium (such as PPP) this step is unnecessary. If any part of this "sole unicast recipient" verification fails, the node MUST ignore the PIO-X bit and continue processing as if it were not set (X=0).

Nodes compliant with this specification, i.e. those that understand the X-bit, MUST, when the X-bit is set, ignore the actual values of the L and A flags and instead interpret them as follows: interpret the L bit as if it were 0 (L=0) interpret the A bit as it it were 1 (A=1) The rationale for this is as follows.

Because a PIO-X aware node will know that it has exclusive use of a prefix with non-zero valid lifetime, the prefix itself cannot be considered to be on-link with respect to the link on which the PIO-X RA was received. Note that a given address from within the prefix may be considered on-link according to the definition in section 4, item 1, should the receiving node chose to configure that address on said link, but this is in no way synonymous with the entire prefix being considered on-link.

Because a PIO-X aware node will know that it has exclusive use of a prefix with non-zero valid lifetime, autoconfiguration of addresses according to any desired scheme, e.g. , , et cetera, is implicit in the setting of the X bit. Accordingly, the A bit can be interpreted as having been set, should the host choose to apply standard address generation schemes that require the bit to be set. It is free to assign any address formed from an exclusive prefix to any available interface; it is not required to configure the address on the link over which the PIO-X RA was received (i.e. it is under no obligation to form addresses such that they would be classified as on-link (according to the definition in section 4, item 1).

When a router transmits an RA containing one or more PIO-X options it MUST unicast the PIO-X RA to its intended recipient at the IPv6 layer and, if applicable, at the link-layer. It is RECOMMENDED that a PIO with the X-bit set also have the PIO flags L=0 and A=1 explicitly configured, for backward compatibility (i.e. use by non X-bit aware nodes).

TODO: This section needs some work.

A receiving node compliant with this document processes an RA with a PIO entry with the X flag set according the requirements in previous standards documents (chiefly section 6.3.4) subject to the additional requirements documented in .

Whatever use the host makes of the exclusive prefix during its valid lifetime, it SHOULD NOT perform Duplicate Address Detection ("DAD", section 5.4) on any address it configures from within the prefix if that address is configured on either the interface over which the PIO-X RA was received or on a loopback interface. Note that this does not absolve the host from performing DAD in all scenarios; if, for example, the host uses the prefix for 64sharing it MUST at a minimum defend via DAD any addresses it has configured for itself as documented in Requirement 2 of section 3.

Routers announcing PIO-X RAs do so via IPv6 unicast to the intended receiving node and may note the IPv6 unicast destination address of an RS as the next hop for the exclusive prefix. As such, hosts compliant with this SHOULD NOT use the unspecified address (::) when sending RSes; they SHOULD prefer issuing Router Solicitations from a link-local address. It is possible for a node to receive multiple RAs with a mix of exclusive and non-exclusive PIOs and even non-zero and zero default router lifetimes. While it is not possible for a host (receiving node) to be sure it has received all the RA information available to it, hosts compliant with this specification SHOULD implement Packet-Loss Resiliency for Router Solicitations so that the host continues to transmit Router Solicitations at least until an RA with a non-zero default router lifetime has been seen.

Routers announcing PIO-X RAs may record the source (link-local) address of an RS as the next hop for the exclusive prefix. A node compliant with this specification MUST continue to respond to Neighbor Solicitations for the source address used to send RSes (alternatively: the destination address of unicast PIO-X RAs received). Hosts that deprecate or even remove this address may experience a loss of connectivity.

No change to existing source address selection behaviour is required or specified by this document.

No change to existing next hop router selection behaviour is required or specified by this document.

The intent of networks that use PIO-X RAs is not to enable sophisticated routing architectures that could be far better handled by an actual routing protocol but rather to propagate a prefix's exclusive use information to enable the receiving node to make better use of the available addresses. As such: A PIO-X receiving node SHOULD NOT issue ICMPv6 Redirects ( section 4.5) for any address within an exclusive use prefix via the link over which the PIO-X RA was received. Redirecting portions of exclusive prefixes to other "upstream" on-link nodes is not a supported configuration. A PIO-X receiving node SHOULD NOT transmit RAs with any subset of its exclusive prefixes via the same interface through which the exclusive prefix was learned.

TODO: This section needs some work.

Since the host will not perform DAD for addresses within prefix announced via PIO-X, it's very important that only a single host receives the PIO-X RA. Therefore, the router MUST only include PIO-X in RAs that are sent using unicast RAs to destination unicast link-layer address and IPv6 link-local unicast address for a specific host. For point-to-point media without link-layer addresses or where there is guaranteed to only be single host that will receive the PIO-X RA (e.g. as enforced by link layer mechanisms), the router MAY send PIO-X RA with multicast destination IPv6 address. Under all circumstances the router MUST maintain a binding table of state information as discussed in .

When the host starts using a network connection it normally sends out an RS (Router Solicitation) packet. This is one way for the router to detect that a new host is connected to the network and detects its link-local address. If the router is configured to use PIO-X, it can now perform necessary processing/configuration and then send the PIO-X RA. For some networks, the host information regarding link-layer and link-local address might be available through other mechanism(s). Examples of this are PPP, 802.1x and 3GPP mobile networks. In that case this information MAY be used instead of relying on the host to send RS. It is however RECOMMENDED that these networks also provide indication whether the host is no longer connected to the network so that the router can invalidate the prefix binding prior to binding expiration (timeout).

The router SHOULD keep track of which PIO-X prefix has been issued to each node. The router SHOULD keep the binding between prefix and link-local address for the advertised valid lifetime, plus some operationally determined delay prior to reissuing a prefix ("grace period"), of the prefix. The router MUST monitor the reachability of each node in the binding table via Neighbor Unreachability Detection ("NUD", section 7.3) or an equivalent link-layer mechanism. The binding SHOULD be considered refreshed every time a periodic PIO-X RA is sent to a node. If the router is informed by some other mechanism (link-layer indication for instance) that a node is no longer connected to the link, it MAY immediately invalidate the prefix binding. (DISCUSS: Is this the correct approach? Do we want to point to some definition somewhere else?)

When the router intends to send a PIO-X RA, it SHOULD before sending the PIO-X RA, complete all necessary processing needed for the host to start using the PIO-X prefix to communicate through the router to other networks. This is so that the host can start using PIO-X based addresses immediately after reception of the PIO-X RA.

TODO: Out of scope things that are worth careful consideration include... Routers SHOULD NOT announce the same prefix to two different nodes within the valid lifetime of the earlier of the two PIO-X announcements. A link may operate in a mode where routers announce RAs to all nodes, possibly with non-exclusive PIO data, and non-zero default router lifetimes. Separately, one or more other nodes on the link may announce exclusive PIO information to nodes along with zero default router lifetimes. Except in the presence of a non-expired more specific route, e.g. learning from an Route Information Option (RIO), the receiving node should send exclusive use prefix originated or forwarded traffic destined off-link through routers with non-zero default router lifetimes.

This memo contains no requests of IANA.

This document fundamentally introduces no new protocol or behaviour substantively different from existing behaviour on a link which guarantees a unique /64 prefix to every attached host. It only describes a mechanism to convey that topological reality, allowing the host to make certain optimizations as well as share the exclusive prefix as it sees fit with other nodes according to its capabilities and policies.