IPv6 Addresses for Ad Hoc Networks

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Table of Contents

1. Introduction

When two or more IPv6 [RFC8200] nodes come together within an Ad Hoc network operating region (e.g., such as in a Mobile Ad-hoc NETwork (MANET)), they must be able to assign unique addresses, discover multihop routes and exchange IPv6 packets with peers even if there is no Internetworking infrastructure present.¶

Ad Hoc networks often include IPv6 nodes that configure interface connections to links with undetermined connectivity properties such that multihop traversal may be necessary to span the network. These same principles may apply for both wireless and wired-line communications. The transitive property of connectivity for conventional shared media links is therefore not assured, while IPv6 nodes must still be able to assign and use IPv6 addresses that are unique within the local Ad Hoc network. This is true even for nodes that configure multiple interface connections to the same Ad Hoc network as a localized multihop forwarding domain with multiple links.¶

By its nature, the term "Ad Hoc network" implies logical groupings whereas the historical term "site" suggested physical boundaries such as a building or a campus. In particular, Ad Hoc networks can self-organize amorphously even if they overlap with other (logical) networks, split apart to form multiple smaller networks or join together to form larger networks. Clustering has been suggested as a means to organize these logical groupings, but Ad Hoc network ecosystems are often in a constant state of flux and likely to change over time. An address type that can be used by nodes that float freely between logical Ad Hoc network boundaries is therefore necessary.¶

The term "Ad Hoc" used throughout this document extends to include isolated localized IPv6 networks where peer to peer communications may require multihop traversal of multiple links whether or not the peers are particularly mobile or ad hoc. For any isolated Ad Hoc network (i.e., one for which IPv6 Internetworking routers are either absent or only intermittently available), a localized IPv6 addressing scheme that allows Ad Hoc nodes to communicate internally is necessary. Therefore, all IPv6 nodes that connect to Ad Hoc networks should be prepared to operate according to this Ad Hoc network multilink addressing model when necessary. Ad Hoc network multihop forwarding services are then coordinated at an architectural sublayer termed the "adaptation layer" below the IPv6 Internetworking layer but above the true link layer.¶

Section 6 of the "IP Addressing Model in Ad Hoc Networks" [RFC5889] states that: "an IP address configured on this (Ad Hoc) interface should be unique, at least within the routing domain" and: "no on-link subnet prefix is configured on this (Ad Hoc) interface". The section then continues to explain why IPv6 Link-Local Addresses (LLAs) are of limited utility on links with undetermined connectivity, to the point that they cannot be used exclusively within Ad Hoc network domains. (Note that [RFC5498] provides IANA allocations for MANET protocols as a complimentary publication.)¶

[RFC5889] suggests that Global Unicast [RFC4291] (aka "GUA") and Unique-Local [RFC4193] (aka "ULA") addresses are Ad Hoc network addressing candidates. However, provisioning of unique GUAs and ULAs must be coordinated either through administrative actions or through an automated address delegation service coordinated by IPv6 Internetworking routers that connect the Ad Hoc network to other networks. Since such routers may not always be available, this document asserts that new forms of self-generated and unique Ad Hoc network local IPv6 addresses are needed.¶

The key feature of these Ad Hoc network adaptation layer IPv6 addresses is that they must have excellent statistical uniqueness properties such that there is little/no chance of conflicting with an address assigned by another node. There is no requirement that the addresses include topologically-oriented prefixes, since the (newly-formed) Ad Hoc networks may not (yet) connect to established Internetworking topologies.¶

Ad Hoc network nodes must be able to use adaptation layer IPv6 addresses for continuous local communications and/or to coordinate topologically-oriented addresses for assignment on other interfaces. A new "Multilink Local" scope for the IPv6 scoped addressing architecture [RFC4007] with scope greater than link-local but lesser than GUA/ULA is therefore necessary. This document therefore defines a new unique local unicast address variant known as "Multilink Local Addresses (MLAs)".¶

The term "multilink interface" refers to a node's IPv6 interface connection to an Ad Hoc network with undetermined connectivity properties where neighbor relationships appear as point-to-point "links" and multiple adaptation layer forwarding hops between peers may be necessary. However, the same principles apply for Ad Hoc network interfaces with full neighborhood connectivity including multiple access links such as Ethernet.¶

2. IPv6 Ad Hoc Network Local Addressing

The IPv6 addressing architecture specified in [RFC4007], [RFC4193] and [RFC4291] defines the supported IPv6 unicast/multicast/anycast address forms with various scopes including link- and site-local. ULAs and GUAs are typically obtained through Stateless Address AutoConfiguration (SLAAC) [RFC4862] and/or the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415], but these services require the presence of IPv6 network infrastructure which may not be immediately available in spontaneously-formed Ad Hoc networks.¶

Multilink interface connections to Ad Hoc networks have the interesting property that a multihop router R will often need to forward packets between nodes A and B even though R uses the same interface in the inbound and outbound directions. Since nodes A and B may not be able to communicate directly even though both can communicate directly with R, the link connectivity property is intransitive and the IPv6 Neighbor Discovery (ND) Redirect service cannot be used. Conversely, R may need to forward packets between nodes A and B via different multilink interfaces within a single Ad Hoc network that includes multiple distinct links/regions. Due to these indeterminant multilink properties, exclusive use of IPv6 Link Local Addresses (LLAs) is also out of scope.¶

This document therefore introduces an IPv6 MLA address type that uses a well-formed IPv6 prefix "TBD::/N" (see IANA Considerations). After a node creates an MLA, it can use the address within the context of spontaneously-organized Ad Hoc networks in which two or more nodes come together in the absence of stable supporting infrastructure and can still exchange IPv6 packets with little or no chance of address collisions. The use could be limited to bootstrapping the assignment of topologically-oriented IPv6 addresses through other means mentioned earlier, or it could extend to longer term usage patterns such as sustained communications with single-hop neighbors on a local link or even between multihop peers within an Ad Hoc network.¶

3. Assigning an MLA to an Interface

IPv6 MLAs have no topological orientation and can therefore be assigned to any of a node's IPv6 multilink interfaces with a /128 prefix length (i.e., as a singleton address). The node can then begin to use an MLA as the source/destination address of IPv6 packets that are forwarded over the interface within an Ad Hoc network multihop forwarding region. The node can assign the same MLA to multiple multilink interfaces all members of the same Ad Hoc network according to the scoped addressing architecture.¶

MLAs may then serve as a basis for multihop forwarding over an IPv6 interface and/or for local neighborhood discovery over other IPv6 interface types. Due to their uniqueness properties, the node can assign these address types to a multilink interface as optimistic addresses per [RFC4429], however it should deprecate an MLA if it detects in-service duplication.¶

A node can also assign an MLA to an Overlay Multilink Network (OMNI) Interface as discussed in [I-D.templin-6man-omni3]. In that case, the MLA can be used to support global-scoped communications over the OMNI link if it is based on a global unicast IPv6 prefix.¶

4. MLAs in the Scoped Addressing Architecture

Returning to a debate from more than 20 years ago, a case could be made for reclaiming the deprecated site-local address prefix "fec0::/10" for use as the top-level MLA prefix. However, some implementations still honor the deprecation and continue to regard the prefix as a non-functional historical artifact.¶

[RFC3879] documents the deprecation rationale including the assertion that "Site is an Ill-Defined Concept". However, the concept of an Ad Hoc network is a coherent logical one based on time-varying (multilink) connectivity and not necessarily one constrained by physical boundaries. Especially in Ad Hoc networks that employ a proactive local routing protocol the list of available adaptation layer addresses in each network is continuously updated for temporal consistency.¶

For example, an IPv6 node may connect to multiple distinct Ad Hoc networks with a first set of multilink interfaces connected to network "A", a second set of interfaces connected to network "B", etc. According to the scoped IPv6 addressing architecture, the node would assign a separate MLA for each multilink interface set A, B, etc. and maintain separate Ad Hoc network multihop routing protocol instances for each set. MLAs A, B, etc. then become the router IDs for the separate routing protocol instances, but the IPv6 node may elect to redistribute discovered adaptation layer routes between the instances. The uniqueness properties of MLAs therefore transcends logical Ad Hoc network boundaries but without "leaking" into external networks.¶

A means for entering Ad Hoc network local IPv6 Zone Identifiers in user interfaces is necessary according to [I-D.ietf-6man-zone-ui]. Examples of an Ad Hoc network local unicast address qualified by a zone identifier are as follows:¶

• TBD::884e:9d2a:73fc:2d94%netA¶

• TBD::c63d:9724:fca:1237%netB¶

Upon publication as a standards track RFC, the RFC Editor is instructed to update [RFC4007] and [RFC5889] to reflect this new address type for Ad Hoc networks in the IPv6 scoped addressing architecture.¶

5. Obtaining and Assigning IPv6 GUAs/ULAs

IPv6 nodes assign MLAs to their IPv6 multilink interfaces for use only within the scope of locally connected Ad Hoc networks. These MLAs can appear in Ad Hoc network multihop routing protocol control messages and can also appear as the source and destination addresses for IPv6 packets forwarded within the locally connected Ad Hoc networks.¶

In order to support communications beyond the Ad Hoc local scope, each IPv6 node is required to obtain an IPv6 GUA/ULA pair through an IPv6 Internetworking border router or proxy that connects the Ad Hoc network to other networks. Since the border router/proxy may be multiple adaptation layer hops away, however, the IPv6 node configures and engages OMNI Interface as specified in [I-D.templin-6man-omni3]. The IPv6 node assigns the GUA/ULA to the OMNI interface which forwards original packets by inserting an adaptation layer IPv6 encapsulation header that uses MLAs as source/destination addresses while the original packet uses GUAs/ULAs.¶

The IPv6 Internetworking border router/proxy may be configured as an IPv6-to-IPv6 Network Prefix Translation (NPTv6) gateway that maintains a 1:1 relationship between the ULA on the "inside" and a GUA on the "outside" as discussed in [I-D.bctb-6man-rfc6296-bis]. The NPTv6 gateway will then statelessly translate each ULA into its corresponding GUA (and vice versa) for IPv6 packets that transit between the inside and outside domains.¶

The gateway provides service per the "ULA-Only" or "ULA+PA" [I-D.ietf-v6ops-ula-usage-considerations] connected network models. The IPv6 node can then use the ULA for local-scoped communications with internal peers and the GUA for global-scoped communications with external peers via the gateway as either a "NPTv6 translator" or "NPTv6 pass-through". IPv6 nodes can then select address pair combinations according to IPv6 default address selection rules [I-D.ietf-6man-rfc6724-update].¶

After receiving a ULA+PA GUA delegation, IPv6 nodes that require Provider-Independent (PI) GUAs can use the OMNI interface in conjunction with the Automatic Extended Route Optimization (AERO) global distributed mobility management service [I-D.templin-6man-aero3] to request and maintain IPv6 and/or IPv4 PI prefixes from the mobility service. The IPv6 node can then sub-delegate GUAs from the PI prefixes to its attached downstream local networks which may in turn engage an arbitrarily large IPv6 and/or IPv4 "Internet of Things".¶

6. Address Selection

"Default Address Selection for Internet Protocol Version 6 (IPv6)" [RFC6724] provides a policy table that specifies precedence values and preferred source prefixes for destination prefixes. "Preference for IPv6 ULAs over IPv4 addresses in RFC6724" [I-D.ietf-6man-rfc6724-update] updates the policy table entries for ULAs, IPv4 addresses and the 6to4 prefix (2002::/16).¶

This document proposes a further update to the policy table for IPv6 MLAs. The proposed update appears in the table below:¶

 draft-ietf-6man-rfc6724-update                           Updated
Prefix        Precedence Label        Prefix        Precedence Label
::1/128               50     0        ::1/128               50     0
::/0                  40     1        ::/0                  40     1
::ffff:0:0/96         20     4        ::ffff:0:0/96         20     4
2002::/16              5     2        2002::/16              5     2
2001::/32              5     5        2001::/32              5     5
fc00::/7              30    13        fc00::/7              30    13
::/96                  1     3        ::/96                  1     3
fec0::/10              1    11        fec0::/10              1    11
3ffe::/16              1    12        3ffe::/16              1    12
                                      TBD::/N                4    14 (*)
(*) value(s) changed in update

With the proposed updates, this new MLA address type appears as a lesser precedence than IPv6 GUAs, IPv6 ULAs and IPv4 addresses but as a greater precedence than deprecated IPv6 prefixes.¶

7. Requirements

IPv6 nodes MAY assign MLAs to their multilink interface connections to Ad Hoc networks. If the node becomes aware that the address is already in use by another node, it instead generates and assigns a new MLA.¶

IPv6 routers MAY forward IPv6 packets with MLA source or destination addresses over multiple hops within the same Ad Hoc network as an adaptation layer function.¶

IPv6 routers MUST NOT forward packets with MLA source or destination addresses to a link outside the packet's Ad Hoc network of origin as an adaptation layer service.¶

IPv6 nodes MAY assign MLAs to the OMNI interface allowing routers to forward original packets with MLA addresses at the IPv6 layer. In that case, the MLAs appear as /128 routes in the OMNI link IPv6 routing service.¶

8. Implementation Status

In progress.¶

9. IANA Considerations

Upon publication, the IETF will work with IANA to designate one or more short IPv6 prefixes "TBD::/N" as the basis for MLAs. Each such prefix TBD must include a prefix length N that is short enough to ensure ample statistical address uniqueness. Therefore, multiple different types of MLAs may be defined but that all serve the same functional purpose from an addressing perspective.¶

10. Security Considerations

IPv6 MLAs include very large uniquely-assigned bit strings in both the prefix and interface identifier components which together provide strong uniqueness properties.¶

With the random generation procedures expected for the various MLA types, the only apparent opportunity for MLA duplication would be through either intentional or unintentional misconfiguration.¶

An IPv6 node that generates an MLA and assigns it to an interface should therefore be prepared to deprecate the MLA and generate/assign a new one if it detects a legitimate duplicate.¶

11. Acknowledgements

This work was inspired by continued investigations into 5G MANET operations in cooperation with the Virginia Tech National Security Institute (VTNSI).¶

Emerging discussions both in-person and on the IPv6 maintenance (6man) and MANET mailing lists continue to shape updated versions of this document. The author acknowledges all those whose useful comments have helped further the understanding of this proposal.¶

Honoring life, liberty and the pursuit of happiness.¶

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