MANET Internetworking: Problem Statement and Gap Analysis

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Table of Contents

1. Introduction

Mobile Ad-hoc Networks (MANETs) [RFC2501] often include mobile nodes with limited range wireless transmission media interfaces that establish links via a dynamically changing set of neighbors within operational range. Each mobile node engages a MANET routing protocol to discover links to first hop neighbors as well as multihop paths to reach other nodes beyond. MANET routers represent multihop paths as "host routes" established through either proactive or on-demand discovery. These host routes name singleton destinations based on either L2 addressing and forwarding in the case of L2-MANET or IP addressing and forwarding [RFC0791] [RFC8200] in the case of L3-MANET.¶

Individual MANETs typically include modest numbers of mobile nodes (e.g., O(10**1), O(10**2), etc.) which naturally limits the number of host routes needed in the local routing system. MANETs can merge to form larger MANETs and/or partition into smaller MANETs according to dynamic network conditions such as mobility. MANETs may also have internal clusters with IP addressed cluster heads that limit the extent over which host routes propagate to reduce control message overhead. Finally, MANETs often operate autonomously until they encounter Internetwork access points of opportunity.¶

A MANET should be regarded as a mobile network that is either disconnected from the global Internetwork or only intermittently connected to the global Internetwork with the ability to move rapidly between different Internetwork attachment points. Due to the dynamic multilink nature of MANETs, MANET routers must configure a unique Multilink Local Address (MLA) and often also use prefix delegation to obtain a unique Mobile Network Prefix (MNP) to connect nodes on their downstream-attached interfaces.¶

Data communications between two nodes within the same MANET local routing region follow host routes using MANET-internal links. When a MANET border router establishes an Internetwork link, it can provide "Internet connection-sharing" access to the rest of the MANET as a connected "stub" network. Per [RFC2501], "stub networks carry traffic originating at and/or destined for internal nodes, but do not permit exogenous traffic to "transit" through the stub network".¶

Practical applications however suggest that MANETs can act as either true stub networks (e.g., a cellphone providing an Internet connection sharing peer for a local multihop Wi-Fi region) or as "not-so-stubby" networks (e.g., Intelligent Transportation Systems where the 5G/6G "SideLink" service supports vehicle-to-vehicle (V2V) multihopping). In the former case, the cellphone acts as an IP router for a stub Wi-Fi MANET behind it and the individual Wi-Fi nodes act as dependent nodes. In the latter case, individual 5G/6G SideLink nodes can connect the stub MANETs they aggregate across not-so-stubby V2V multihop forwarding paths. MANET Internetworking must therefore be capable of accommodating all such scenarios.¶

A widely-accepted axiom at the time of this writing suggests that there are more cellphones than people on the planet [STATISTA]. According to Wikipedia, the world population reached 8 billion in 2022 and is expected to reach 10 billion by 2056 [WIKI]. Each mobile node that connects to the global public Internet can in some sense be regarded as a singleton "MANET" with the potential to connect still larger MANETs.¶

A common concern cited for regarding a cellphone as a MANET router is battery consumption, but this not an issue when the cellphone is connected to a power source such as when used inside of a vehicle. Even in dismounted scenarios, however, having a (limited-duration) multi-hop communications capability may be deemed worthwhile and could provide a safety-of-life capability such as for emergency response, disaster relief, search and rescue, etc. Mapping services, streaming video and many other applications drain cellphone batteries, but people use them all the time. An endless variety of cellphone apps already exists, with many more released every day. For these reasons, it is expected that MANET apps for cellphones will eventually emerge and proliferate.¶

With such a large population of candidate nodes, MANET Internetworking therefore regards the global Internet as a "network of (mobile ad-hoc) networks" with unrestricted dynamic relationships between distinct MANET local routing regions joined by a Non-Broadcast Multiple Access (NBMA) virtual overlay link manifested through encapsulation. Figure 1 illustrates an example of 2 distinct MANET local routing regions connected via the NBMA overlay using the Internet as transit:¶

                             .-(::::::::)
                          .-(::: Global ::)-.
               X==+======(===================)======+==X
                  |        `-(: Internet :)-'       |
                  |           `-(::::::)-'          |
                  |                                 |
           .-(::::::::)                       .-(::::::::)
        .-(::::::::::::)-.                 .-(::::::::::::)-.
       (::::: MANET 1 :::::)              (::::: MANET 2 :::::)
         `-(::::::::::::)-'                 `-(::::::::::::)-'
            `-(::::::)-'                       `-(::::::)-'

While the figure depicts just 2 MANET local routing regions, many others worldwide will also want to connect to the virtual link. Since a sustained increase in both the world population and number of mobile wireless devices is certain, MANET Internetworking must therefore accommodate populations on the order of O(10**10) or more. This includes address duplication avoidance through operational assurance since statistical properties alone may be insufficient to avoid duplication in such large populations.¶

Since each MANET itself is mobile, its border routers may continuously encounter different Internetwork access points as they roam while potentially configuring a different IP address at each access point. MANET border routers therefore must not continuously inject and withdraw mobile network prefixes into the Internetwork BGP routing system as they move since this would result in unacceptable churn. MANET routers instead maintain stable addresses in a Mobility Service Provider (MSP) overlay over the Internetwork such that the overlay addresses remain stable even if the border router's underlay address changes frequently.¶

The overlay network BGP routing system must similarly be protected from mobility churn by maintaining address to mobility anchor point mappings in a scalable directory service such as the domain name system (DNS). Efficient path traversal across the overlay must be supported through traffic flow diversity per [RFC6437][RFC6438] with differentiated services applied within each flow. This is especially important for the common case of multilink MANET nodes (such as cellphones with both 5G and Wi-Fi interfaces) where overlay flow bindings are necessary for multilink coordination.¶

BGP scaling in the modern Internet backbone accommodates O(10**6) prefixes that rarely change. Frequent advertisement and withdrawal of MLAs/MNPs can cause BGP churn resulting in instability in the global routing system such that BGP alone is not well suited to supporting mobile Internetworking by itself. With an overlay, the BGP routing service only needs to cover the addresses of Mobility Anchor Points (MAPs) which are stable nodes not subject to mobility. Within the overlay, the MAPs keep track of MLA/MNP to MAP associations which may change dynamically. This is accommodated through a combination of a hierarchical overlay BGP arrangement of the MAPs in combination with dynamic updates to the (reverse) DNS resource records that associate MLAs/MNPs with MAPs. Finally, the architecture is not limited to only one gigantic global overlay; there could be many smaller overlays operated by MSPs.¶

"IPv6 Wireless Access in Vehicular Environments (IPWAVE): Problem Statement and Use Cases" [RFC9365] provides a complimentary analysis but does not explore MANET-specific idiosyncrasies. This document presents a MANET Internetworking problem statement and gap analysis.¶

2. Terminology

The following terms are defined within the scope of this document:¶

Client

a MANET router that connects a mobile network to a multilink Internetworking service via Proxy/Servers in a Non-Broadcast, Multiple Access (NBMA) overlay.¶

Gateway

an overlay multilink network service node that runs an interdomain routing protocol (e.g., BGP) and directory service (e.g., DNS) to track Client-to-MAP associations. Gateways furthermore join multiple Internetworking segments in an overlay multilink virtual bridging service to form larger Internetworks.¶

Internetwork

a more stable and wide-area terrestrial, non-terrestrial or hybrid network that can serve as transit to interconnect disjoint (or partitioned) MANET local routing regions. The global public Internet is an example, as are private operator service networks either individually or in concatenations with other service networks.¶

Media Access Control (MAC) Address

an IEEE EUI-48 or EUI-64 address, or an L2 address format specific to a custom data link specification.¶

Mobile Ad-hoc Network (MANET)

the same as defined in [RFC2501]; often includes mobile nodes with limited range wireless transmission media interfaces that establish links via a dynamically changing set of neighbors within operational range and engage in a MANET local routing protocol. The MANET local routing protocol can be configured to operate as either an L3 (IP-based) or L2 (MAC address-based) service according to operational requirements. In both cases, each MANET can dynamically partition into multiple smaller MANETs or merge with others to form larger MANETs. This may present a challenge to the classical IP "on-link" subnet model.¶

MANET Border Router

a MANET router that also has a continuous or intermittent interface connection to a transit Internetwork.¶

MANET Cluster Head

a MANET router that joins multiple smaller MANET local routing regions to form a single larger local routing region. Each smaller region is seen as a cluster within the larger region.¶

MANET Interface

a node's (typically wireless) limited range transmission media interface with indeterminant connectivity properties.¶

MANET Router

a node that runs a routing protocol over one or more MANET interfaces to establish multihop forwarding paths within a local routing region. Forwarding nodes in both L2- and L3-MANETs are considered "routers" even when forwarding is based on MAC addresses. All MANET routers are also considered MANET nodes, but not all MANET nodes necessarily act as MANET routers.¶

Mobility Anchor Point (MAP)

a Proxy/Server that also provides mobility, address/prefix autoconfiguration and address resolution services to Clients. The MAP also runs an interdomain routing protocol (e.g., BGP) to announce its Client associations to Gateways. All (reasonably) stable Proxy/Servers are eligible to serve as MAPs as part of a Distributed Mobility Management (DMM) service.¶

Mobile Network Prefix (MNP)

a longer IP prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56, 2002:192.0.2.8::/46, etc.) and assigned to a Client. Clients receive MNPs from MAP Proxy/Servers and sub-delegate them to routers in downstream networks.¶

Mobility Service Prefix (MSP)

an aggregated IP GUA prefix (e.g., 2001:db8::/32, 2002:192.0.2.0::/40, etc.) assigned to a mobility service provider's overlay and from which more-specific MNPs are delegated.¶

Multilink Local Address (MLA)

an IPv6 address based on a cryptographic hash of the node's public key regarded as its full identify. The MLA is regarded as a unique identity tag that, when assigned to an interface, becomes a valid IPv6 address with multilink-local scope. The MLA must be assured globally unique and routable at least within the overlay limited domain to support MANET Internetworking. A candidate MLA type appears in [RFC9374].¶

Proxy/Server

an overlay multilink network service node in an Internetwork that provides proxy forwarding services to MANET border routers and other MANET router Clients.¶

3. MANET Use Cases

MANETs have an important role in emergency response communications, disaster relief situations, communications in remote and rural areas, military operations, vehicular and swarm communications, and low-powered Internet of things (IoT) applications. MANETs provide the ability to establish and maintain communications when infrastructure- based networks, such as 5G cellular communication systems, are not accessible. As described above, MANETs may also provide Internet connectivity to internal nodes, for example, as a "stub" network via MANET routers which possess an Internetworking capability and an external connection to a radio access network.¶

Example use cases of such MANETs include the following:¶

• Disaster Relief: Disaster situations may compromise network infrastructure, such as through the loss of base stations in a cellular radio access network (RAN). In this scenario, MANET networks can play a role in closing coverage gaps through multi-hop routing to nodes within the coverage area of uncompromised base stations. This use case is broadly applicable to any situation in which nodes are operating outside or at the periphery of RAN coverage.¶

• Tracking and Monitoring: Another example use case is the tracking and monitoring of data from low-cost low-power IoT devices ("tags") which may be placed on packages during shipment or storage. Such devices may transition in and out of coverage of infrastructure-based networks, often being located in environments that are not conducive to RF propagation (e.g., shipping container, warehouse, etc.). The ability to discover and connect to neighboring MANET-enabled devices and to establish Internet connectivity through such MANETs, enables real-time logistics and inventory data to be collected opportunistically.¶

• UAV Swarms: local communications within swarms for coordination and cooperation is a good use case for MANET networks due to the highly mobile dynamic nature of such networks. Yet swarms may also benefit from connectivity to the Internet, or other external networks. And in large swarm-based MANETs, routing of traffic through infrastructure networks to MANET endpoints, rather than traversing the entire MANET can improve communications throughput and reliability.
  
  Under mobility conditions, distinct UAV swarms defined by MANET local routing regions will encounter situations where the local regions enter into communications range of each other. In this case, it is desirable to establish cluster heads between these regions and to propagation host routes over them as new interconnections are available and discovered. Moreover, UAV swarms for which internetworking will be persistent should be able to perform local region merger. In this case, internetworking protocols must support seamless merger of the MANET local routing regions into a larger region. Conversely, nodes or collections of nodes which leave coverage of the local region should be capable of establishing and operating an independent local region at a future time.¶

4. MANET Architectural Layering

MANET routing for nodes operating in a common local routing region can be configured to operate as either a layer-3 (L3-MANET) or layer-2 (L2-MANET) service.¶

In L3-MANET, routing and addressing occurs at the IP layer and each MANET forwarding hop decrements the IP TTL/Hop-Limit. IP address uniqueness assurance is essential to avoid conflicts, while MAC address collisions may not pose significant operational issues. The MANET interface observes a partially-connected multi-access link with some nodes as single-hop IP neighbors and others reachable only over multi-hop paths. IP multicast transmissions only reach single-hop neighbors which must engage an IP layer service such as Simplified Multicast Forwarding (SMF) [RFC6621] to flood multicast messages to all MANET nodes. IP header compression state must be maintained at each MANET forwarding hop and may become invalidated due to path changes.¶

In L2-MANET, routing and addressing occurs at the MAC layer and the IP TTL/Hop-Limit is not decremented. Both IP address and MAC address uniqueness assurance are essential to avoid conflicts. The MANET interface observes a fully-connected multi-access link with all nodes appearing as single-hop IP neighbors even though some may be separated by multiple L2 hops. IP multicast transmissions are propagated to all MANET nodes even though the L2 service may be required to forward at the layer below IP. IP header compression state is only maintained at the MANET ingress and egress nodes and is therefore not invalidated due to interior MANET path changes. Instead, the L2 header must carry additional overhead (including a TTL, packet identification and ancillary addresses) to support forwarding at a layer below IP.¶

Since L3-MANETs propagate IP host routes, multicast-based address resolution services are unnecessary thereby reducing dependence on MANET multicast. By default, L2-MANETs require multicast-based address resolution services which can be minimized through proactive distribution of address resolution information.¶

In both the L2- and L3-MANET models, MANETs can partition into smaller separate MANETs or merge to form larger MANETs. This presents a challenge to the classical IP subnet model where all nodes within the subnet are presumed to be reachable as single-hop neighbors connected to the same (shared) link. All MANET routers should therefore only configure the MLA and MSP prefixes as on-link prefixes on an overlay interface. This causes the node to invoke address resolution for both MANET-local nodes and target peers in other networks reached via the overlay. MANET Internetworking concepts apply equally to both the L3-MANET and L2-MANET models as discussed throughout the document.¶

5. MANET Internetworking Problem Statement

MANETs present a unique set of Internetworking challenges not addressed by earlier works [RFC5889][RFC9365]. The following sub-sections provide MANET Internetworking problem statements for aspects not (fully) satisfied by existing services that must be addressed by any solution proposals.¶

6. MANET Internetworking Gap Analysis

Many current and past solution proposals address most aspects of the problems articulated in the previous section, but often have functional gaps that do not fully satisfy the requirements. The following gaps must be addressed by candidate solutions:¶

7. MANET Internetworking Solution Proposal Analysis

The following sections provide an analysis of solution proposals with respect to the problem statements and gap analysis. Any solution proposal must explain how it addresses the problems while also showing how it either does or does not satisfy the technological gaps.¶

8. IANA Considerations

This document is an informational problem statement and does not in itself request any IANA actions. IANA considerations can be found in solution space documents.¶

9. Security Considerations

MANET Internetworking assumes a multi-layer security architecture. Physical and data link layer security is assumed within each MANET local routing region and with proper network layer authentication for admitting nodes into the MANET Internetworking overlay service. Transport and higher layer security should then be applied for end-to-end confidentiality, integrity and authorization.¶

10. Acknowledgements

Discussions on the MANET working group mailing list helped shape concepts exposed in this document. The following are acknowledged for their helpful comments: Abdussalam Baryun, Lou Berger, Stuart Card, Juliusz Chroboczek, Christopher Dearlove, Donald Eastlake, Joel Halpern and others who provided valuable input on the list or through private discussions.¶

Authors' Addresses