Transmission of IP Packets over Overlay Multilink Network (OMNI) Interfaces

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Table of Contents

1. Introduction

Mobile nodes (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, space systems, enterprise wireless devices, pedestrians with cellphones, etc.) configure mobile routers with multiple interface connections to wireless and/or wired-line data links. These data links may have diverse performance, cost and availability properties that can change dynamically according to mobility patterns, flight phases, proximity to infrastructure, etc. The mobile router acts as a Client of a network-based Mobility Service (MS) by configuring a virtual interface over its underlay interface data link connections.¶

Each Client configures a virtual interface (termed the "Overlay Multilink Network Interface (OMNI)") as a thin layer over its underlay network interfaces (which may themselves connect to virtual or physical links). The OMNI interface is therefore the only interface abstraction exposed to the IP layer and behaves according to the Non-Broadcast, Multiple Access (NBMA) interface principle, while underlay interfaces appear as link layer communication channels in the architecture. The OMNI interface internally employs the "OMNI Adaptation Layer (OAL)" to ensure that original IP packets or parcels [I-D.templin-intarea-parcels] are adapted to diverse underlay interfaces with heterogeneous properties.¶

The OMNI interface connects to a virtual overlay known as the "OMNI link". The OMNI link spans one or more Internetworks that may include private-use infrastructures (e.g., enterprise networks) and/or the global public Internet itself. Together, OMNI and the OAL provide the foundational elements required to support the "6 M's of Modern Internetworking", including:¶

1. Multilink - a Client's ability to coordinate multiple diverse underlay interfaces as a single logical unit (i.e., the OMNI interface) to achieve the required communications performance and reliability objectives.¶
2. Multinet - the ability to span the OMNI link over a segment routing topology with multiple diverse administrative domain network segments while maintaining seamless end-to-end communications between mobile Clients and correspondents such as air traffic controllers, fleet administrators, etc.¶
3. Mobility - a Client's ability to change network points of attachment (e.g., moving between wireless base stations) which may result in an underlay interface address change, but without disruptions to ongoing communication sessions with peers over the OMNI link.¶
4. Multicast - the ability to send a single network transmission that reaches multiple Clients belonging to the same interest group, but without disturbing other Clients not subscribed to the interest group.¶
5. Multihop - a mobile Client vehicle-to-vehicle relaying capability useful when multiple forwarding hops between vehicles may be necessary to "reach back" to an infrastructure access point connection to the OMNI link.¶
6. MTU assurance - the ability to deliver packets/parcels of various robust sizes between peers without loss due to a link size restriction, and to dynamically adjust packet/parcels sizes to achieve the optimal performance for each independent traffic flow.¶

Client OMNI interfaces interact with the MS and/or other OMNI nodes through IPv6 Neighbor Discovery (ND) control message exchanges [RFC4861]. The MS consists of a distributed set of service nodes (including Proxy/Servers and other infrastructure elements) that also configure OMNI interfaces. Automatic Extended Route Optimization (AERO) in particular provides a companion MS compatible with the OMNI architecture [I-D.templin-intarea-aero]. AERO discusses details of ND message based route optimization, mobility management, and multinet traversal while the fundamental aspects of OMNI link operation are discussed in this document.¶

Each OMNI interface provides a multilink nexus for exchanging inbound and outbound traffic via selected underlay interface(s). The IP layer sees the OMNI interface as a point of connection to the OMNI link. Each OMNI link has one or more associated Mobility Service Prefixes (MSPs), which are typically IP Global Unicast Address (GUA) prefixes assigned to the link and from which Mobile Network Prefixes (MNPs) are derived. If there are multiple OMNI links, the IP layer will see multiple OMNI interfaces.¶

Each Client receives an MNP through IPv6 ND control message exchanges with Proxy/Servers over Access Networks (ANETs) and/or open Internetworks (INETs). The Client sub-delegates the MNP to downstream-attached End-user Networks (ENETs) independently of the underlay interfaces selected for data transport. The Client acts as a fixed or mobile router on behalf of ENET peers, and uses OMNI interface control messaging to coordinate with Hosts, Proxy/Servers and/or other Clients. The Client iterates its control messaging over each of the OMNI interface's ANET/INET underlay interfaces in order to register each interface with the MS (see Section 15). The Client can also provide Proxy/Server-like services for a recursively nested chain of other Clients located in downstream-attached ENETs.¶

Clients may connect to multiple distinct OMNI links within the same OMNI domain by configuring multiple OMNI interfaces, e.g., omni0, omni1, omni2, etc. Each OMNI interface is configured over a distinct set of underlay interfaces and provides a nexus for Safety-Based Multilink (SBM) operation. The IP layer applies SBM routing to select a specific OMNI interface, then the selected OMNI interface applies Performance-Based Multilink (PBM) internally to select appropriate underlay interfaces. Applications select SBM topologies based on IP layer Segment Routing [RFC8402], while each OMNI interface orchestrates PBM internally based on OAL Multinet traversal.¶

OMNI provides a link model suitable for a wide range of use cases. For example, the International Civil Aviation Organization (ICAO) Working Group-I Mobility Subgroup is developing a future Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS) and has issued a liaison statement requesting IETF adoption [ATN] in support of ICAO Document 9896 [ATN-IPS]. The IETF IP Wireless Access in Vehicular Environments (ipwave) working group has further included problem statement and use case analysis for OMNI in a document now in AD evaluation for RFC publication [I-D.ietf-ipwave-vehicular-networking]. Still other communities of interest include AEEC, RTCA Special Committee 228 (SC-228) and NASA programs that examine commercial aviation, Urban Air Mobility (UAM) and Unmanned Air Systems (UAS). Pedestrians with handheld mobile devices represent another large class of potential OMNI users.¶

This document specifies the transmission of original IP packets/parcels and control messages over OMNI interfaces. The operation of both IP protocol versions (i.e., IPv4 [RFC0791] and IPv6 [RFC8200]) is specified as the network layer data plane, while OMNI interfaces use IPv6 ND messaging in the control plane independently of the data plane protocol(s). OMNI interfaces also provide an OAL based on encapsulation and fragmentation over heterogeneous underlay interfaces as an adaptation sublayer between L3 and L2. Both OMNI and the OAL are specified in detail throughout the remainder of this document.¶

2. Terminology

The terminology in the normative references applies; especially, the terms "link" and "interface" are the same as defined in the IPv6 [RFC8200] and IPv6 Neighbor Discovery (ND) [RFC4861] specifications. Additionally, this document assumes the following IPv6 ND message types: Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited NA (uNA) and Redirect. Hosts, Clients and Proxy/Servers that implement IPv6 ND maintain per-neighbor state in Neighbor Cache Entries (NCEs). Each NCE is indexed by the neighbor's network layer address(es) while the neighbor's OAL encapsulation address provides context for Identification verification.¶

The Protocol Constants defined in Section 10 of [RFC4861] are used in their same format and meaning in this document. The terms "All-Routers multicast", "All-Nodes multicast" and "Subnet-Router anycast" are the same as defined in [RFC4291] (with Link-Local scope assumed).¶

The term "IP" is used to refer collectively to either Internet Protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200]) when a specification at the layer in question applies equally to either version.¶

The terms Host, Client and Proxy/Server are intentionally capitalized to denote a node of that particular node type that also configures an OMNI interface and engages the OMNI Adaptation Layer.¶

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

L3

The Network layer in the OSI network model. Also known as "layer-3", "IP layer", etc.¶

L2

The Data Link layer in the OSI network model. Also known as "layer-2", "link-layer", "sub-IP layer", etc.¶

Adaptation layer

A mid-layer that adapts L3 to a diverse collection of L2 underlay interfaces and their encapsulations. (No layer number is assigned, since numbering was an artifact of the legacy reference model that need not carry forward in the modern architecture.) The adaptation layer sees the upper layer as "L3" and sees all lower layer encapsulations as "L2 encapsulations", which may include UDP, IP and true link-layer (e.g., Ethernet, etc.) headers.¶

Access Network (ANET)

a connected network region (e.g., an aviation radio access network, corporate enterprise network, satellite service provider network, cellular operator network, residential WiFi network, etc.) that connects Clients to the Mobility Service. Physical and/or data link level security is assumed (sometimes referred to as "protected spectrum" for wireless domains). ANETs such as private enterprise networks and ground domain aviation service networks often provide multiple secured IP hops between the Client's physical point of connection and the nearest Proxy/Server.¶

Internetwork (INET)

a connected network region with a coherent IP addressing plan that provides transit forwarding services between ANETs and/or OMNI nodes that coordinate with the Mobility Service over unprotected media. Since physical and/or data link level security cannot always be assumed, security must be applied by upper layers if necessary. The global public Internet itself is an example.¶

End-user Network (ENET)

a simple or complex "downstream" network that travels with the Client as a single logical unit. The ENET could be as simple as a single link connecting a single Host, or as complex as a large network with many links, routers, bridges and Hosts. The ENET could also provide an "upstream" link in a recursively-descending chain of additional Clients and ENETs. In this way, an ENET of an upstream Client is seen as the ANET of a downstream Client.¶

{A,I,E}NET interface

a Client's attachment to a link in an {A,I,E}NET.¶

*NET

a "wildcard" term used when a given specification applies equally to both ANET/INET cases. From the Client's perspective, *NET interfaces are "upstream" interfaces that connect the Client to the Mobility Service, while ENET interfaces are "downstream" interfaces that the Client uses to connect downstream ENETs, Hosts and/or other Clients.¶

underlay interface

an ANET/INET/ENET interface over which an OMNI interface is configured. The OMNI interface is seen as an L3 interface by the IP layer, and each underlay interface is seen as an L2 interface by the OMNI interface. The underlay interface either connects directly to the physical communications media or coordinates with another node where the physical media is hosted.¶

Mobile Ad-hoc NETwork (MANET)

a connected network region that shares the same properties as an ANET except that physical and/or data link layer security cannot always be assumed and multihop forwarding between Clients acting as MANET routers may be necessary. Proxy/Servers that connect the MANET to outside networks act as Clients on their MANET interfaces and act as ordinary Proxy/Servers on their ANET/INET interfaces, while Clients configure MANET interfaces and provide a multihop forwarding service for other Clients.¶

MANET Interface

a node's underlay interface connection to a local network with indeterminant neighborhood properties over which multihop relaying may be necessary.¶

OMNI link

a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured over one or more INETs and their connected ANETs/ENETs. An OMNI link may comprise multiple distinct "segments" joined by L2 forwarding devices the same as for any link; the addressing plans in each segment may be mutually exclusive and managed by different administrative entities. Proxy/Servers and other infrastructure elements extend the link to support communications between Clients as single-hop neighbors.¶

OMNI interface

a node's attachment to an OMNI link, and configured over one or more underlay interfaces. If there are multiple OMNI links in an OMNI domain, a separate OMNI interface is configured for each link. The OMNI interface configures a Maximum Transmission Unit (MTU) and a Maximum Reassembly Unit (MRU) the same as any interface.¶

OMNI Adaptation Layer (OAL)

an OMNI interface sublayer service that encapsulates original IP packets/parcels admitted into the interface in an IPv6 header and/or subjects them to fragmentation and reassembly. The OAL is also responsible for generating MTU-related control messages as necessary, and for providing addressing context for OMNI link SRT traversal. The OAL presents a new layer in the Internet architecture known simply as the "adaptation layer".¶

Host

an end user device that extends the OMNI link over an ENET interface serviced by a Client. (As an implementation matter, the Host either assigns the same IP address from the ENET (underlay) interface to an (overlay) OMNI interface, or configures an OMNI-like function as a virtual sublayer of the ENET interface itself.) The IP addresses assigned to each Host ENET interface remain stable even if the Client's upstream *NET interface connections change.¶

Client

a network platform/device mobile router that configures one or more OMNI interfaces over distinct sets of underlay interfaces grouped as logical OMNI link units. The Client coordinates with the Mobility Service via upstream networks over *NET interfaces, and provides Proxy/Server services for Hosts and other Clients on ENET interface downstream networks. The Client's *NET interface addresses and performance characteristics may change over time (e.g., due to node mobility, link quality, etc.) while downstream-attached Hosts and other Clients see the ENET as a stable ANET.¶

Proxy/Server

a segment routing topology edge node that configures an OMNI interface and connects Clients to the Mobility Service. As a server, the Proxy/Server responds directly to some Client IPv6 ND messages. As a proxy, the Proxy/Server forwards other Client IPv6 ND messages to other Proxy/Servers and Clients. As a router, the Proxy/Server provides a forwarding service for ordinary data messages that may be essential in some environments and a last resort in others. Proxy/Servers at ANET boundaries configure both an ANET downstream interface and *NET upstream interface, while INET-based Proxy/Servers configure only an INET interface.¶

First-Hop Segment (FHS) Proxy/Server

a Proxy/Server connected to the source Client's *NET that forwards OAL packets sent by the source into the segment routing topology. FHS Proxy/Servers also act as intermediate forwarding nodes to facilitate RS/RA exchanges between Clients and Hub Proxy/Servers.¶

Last-Hop Segment (LHS) Proxy/Server

a Proxy/Server connected to the target Client's *NET that forwards OAL packets received from the segment routing topology to the target.¶

Hub Proxy/Server

a single Proxy/Server selected by the Client that provides a designated router service for all of the Client's*NET underlay networks. Since all Proxy/Servers provide equivalent services, Clients normally select the first FHS Proxy/Server they coordinate with to serve as the Hub. However, the Hub can instead be any available Proxy/Server for the OMNI link, i.e., and not necessarily one of the Client's FHS Proxy/Servers.¶

Segment Routing Topology (SRT)

a multinet forwarding region configured over one or more INETs between the FHS Proxy/Server and LHS Proxy/Server. The SRT spans the OMNI link on behalf of source/target Client pairs using segment routing in a manner outside the scope of this document (see: [I-D.templin-intarea-aero]).¶

Mobility Service (MS)

a mobile routing service that tracks Client movements and ensures that Clients remain continuously reachable even across mobility events. The MS consists of the set of all Proxy/Servers plus any other OMNI link supporting infrastructure nodes. Specific MS details are out of scope for this document, with an example found in [I-D.templin-intarea-aero].¶

Mobility Service Prefix (MSP)

an aggregated IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and from which more-specific Mobile Network Prefixes (MNPs) are delegated. OMNI link administrators typically obtain MSPs from an Internet address registry, however private-use prefixes can also be used subject to certain limitations (see: Section 10). OMNI links that connect to the global Internet advertise their MSPs to their interdomain routing peers.¶

Mobile Network Prefix (MNP)

a longer IP prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and assigned to a Client. Clients receive MNPs from Proxy/Servers and sub-delegate them to routers, Hosts and other Clients located in ENETs.¶

original IP packet/parcel

a whole IP packet/parcel or fragment admitted into the OMNI interface by the network layer prior to OAL encapsulation and fragmentation, or an IP packet/parcel delivered to the network layer by the OMNI interface following OAL decapsulation and reassembly.¶

OAL packet

an original IP packet/parcel encapsulated in an OAL IPv6 header before OAL fragmentation, or following OAL reassembly.¶

OAL fragment

a portion of an OAL packet following fragmentation but prior to L2 encapsulation, or following L2 decapsulation but prior to OAL reassembly.¶

(OAL) atomic fragment

an OAL packet that does not require fragmentation is always encapsulated as an "atomic fragment" with a Fragment Header with Fragment Offset and More Fragments both set to 0, but with a valid Identification value.¶

(OAL) carrier packet

an encapsulated OAL fragment following L2 encapsulation or prior to L2 decapsulation. OAL sources and destinations exchange carrier packets over underlay interfaces, and may be separated by one or more OAL intermediate nodes. OAL intermediate nodes may perform re-encapsulation on carrier packets by removing the L2 headers of the first hop network and replacing them with new L2 headers for the next hop network. (The term "carrier" honors agents of the service postulated by [RFC1149] and [RFC6214].)¶

OAL source

an OMNI interface acts as an OAL source when it encapsulates original IP packets/parcels to form OAL packets, then performs OAL fragmentation and encapsulation to create carrier packets.¶

OAL destination

an OMNI interface acts as an OAL destination when it decapsulates carrier packets, then performs OAL reassembly and decapsulation to derive the original IP packet/parcel.¶

OAL intermediate node

an OMNI interface acts as an OAL intermediate node when it removes the L2 encapsulation headers of carrier packets received from a first segment to obtain the original OAL packet/fragment, then re-encapsulates in new L2 headers appropriate for the next segment and sends these new carrier packets into the next segment. OAL intermediate nodes decrement the Hop Limit in OAL packets/fragments during forwarding, and discard the OAL packet/fragment if the Hop Limit reaches 0. OAL intermediate nodes do not decrement the TTL/Hop Limit of the original IP packet/parcel, which can only be examined by upper layers.¶

OMNI Option

an IPv6 Neighbor Discovery option providing multilink parameters for the OMNI interface as specified in Section 12.¶

Interface Identifier (IID)

the least significant 64 bits of an IPv6 address, as specified in the IPv6 addressing architecture [RFC4291].¶

(OMNI) Link Local Address (LLA)

an IPv6 address beginning with fe80::/64 per the IPv6 addressing architecture [RFC4291] and with either a 64-bit MNP (LLA-MNP) or a 56-bit random value (LLA-RND) encoded in the IID as specified in Section 8.¶

(OMNI) Unique Local Address (ULA)

an IPv6 address beginning with fd00::/8 followed by a 40-bit Global ID followed by a 16-bit Subnet ID per [RFC4193] and with either a 64-bit MNP (ULA-MNP) or a 56-bit random value (ULA-RND) encoded in the IID as specified in Section 9. (Note that [RFC4193] specifies a second form of ULAs based on the prefix fc00::/8, which are referred to as "ULA-C" throughout this document to distinguish them from the ULAs defined here.)¶

(OMNI) Temporary Local Address (TLA)

a ULA beginning with fd00::/16 followed by a 48-bit randomly-initialized value followed by an MNP-based (TLA-MNP) or random (TLA-RND) IID as specified in Section 9. Clients use TLAs to bootstrap autoconfiguration in the presence of OMNI link infrastructure or for sustained communications in the absence of infrastructure. (Note that in some environments Clients can instead use a (Hierarchical) Host Identity Tag ((H)HIT) instead of a TLA - see: Section 22.)¶

(OMNI) eXtended Local Address (XLA)

a TLA beginning with fd00::/64 followed by an MNP-based (XLA-MNP) or random (XLA-RND) IID as specified in Section 9. An XLA is simply a TLA with an all-0 48-bit value following fd00::/16, and can be used to supply a "wildcard match" for IPv6 ND cache entries, a routing table entry for the OMNI link routing system, etc. (Note that XLAs can also be statelessly formed from LLAs (and vice-versa) simply by inverting prefix bits 7 and 8.)¶

Multilink

a Client OMNI interface's manner of managing multiple diverse *NET underlay interfaces as a single logical unit. The OMNI interface provides a single unified interface to upper layers, while underlay interface selections are performed on a per-flow basis considering traffic selectors such as DSCP, flow label, application policy, signal quality, cost, etc. Multilink selections are coordinated in both the outbound and inbound directions based on source/target underlay interface pairs.¶

Multinet

an intermediate node's manner of spanning multiple diverse IP Internetwork and/or private enterprise network "segments" at the OAL layer below IP. Through intermediate node concatenation of SRT network segments, multiple diverse Internetworks (such as the global public IPv4 and IPv6 Internets) can serve as transit segments in an end-to-end L2 forwarding path. This OAL concatenation capability provides benefits such as supporting IPv4/IPv6 transition and coexistence, joining multiple diverse operator networks into a cooperative single service network, etc. See: [I-D.templin-intarea-aero] for further information.¶

Multihop

an iterative relaying of carrier packets between Client's over an OMNI underlay interface technology (such as omnidirectional wireless) without support of fixed infrastructure. Multihop services entail Client-to-Client relaying within a Mobile/Vehicular Ad-hoc Network (MANET/VANET) for Vehicle-to-Vehicle (V2V) communications and/or for Vehicle-to-Infrastructure (V2I) "range extension" where Clients within range of communications infrastructure elements provide forwarding services for other Clients.¶

Mobility

any action that results in a change to a Client underlay interface address. The change could be due to, e.g., a handover to a new wireless base station, loss of link due to signal fading, an actual physical node movement, etc.¶

Safety-Based Multilink (SBM)

A means for ensuring fault tolerance through redundancy by connecting multiple OMNI interfaces within the same domain to independent routing topologies (i.e., multiple independent OMNI links).¶

Performance Based Multilink (PBM)

A means for selecting one or more underlay interface(s) for carrier packet transmission and reception within a single OMNI interface.¶

OMNI Domain

The set of all SBM/PBM OMNI links that collectively provides services for a common set of MSPs. All OMNI links within the same domain configure, advertise and respond to the same OMNI IPv6 Anycast address(es).¶

AERO Forwarding Information Base (AFIB)

A multilink forwarding table on each OAL source, destination and intermediate node that includes AERO Forwarding Vectors (AFV) with both next hop forwarding instructions and context for reconstructing compressed headers for specific underlay interface pairs used to communicate with peers. See: [I-D.templin-intarea-aero] for further discussion.¶

AERO Forwarding Vector (AFV)

An AFIB entry that includes soft state for each underlay interface pairwise communication session between peers. AFVs are identified by both a next-hop and previous-hop AFV Index (AFVI), with the next-hop established based on an IPv6 ND solicitation and the previous hop established based on the solicited IPv6 ND advertisement response. The AFV also caches underlay interface pairwise Identification sequence number parameters to support carrier packet filtering.See: [I-D.templin-intarea-aero] for further discussion.¶

AERO Forwarding Vector Index (AFVI)

A locally-unique 2-octet or 4-octet value automatically generated by an OAL node when it creates an AFV. OAL intermediate nodes assign two distinct 4-octet AFVIs (called "A" and "B") to each AFV, with "A" representing the forward path and "B" representing the reverse path. Meanwhile, the OAL source assigns a single "B" AFVI, and the OAL destination assigns a single "A" AFVI. Each OAL node advertises its "A" AFVI to previous hop nodes on the reverse path toward the source and advertises its "B" AFVI to next hop nodes on the forward path toward the destination. Clients in MANETs also assign distinct 2-octet AFVIs (called "C" and "D") to support local multihop forwarding. The same as for the A/B AFVIs, the "C" AFVI represents the forward path and the "D" AFVI represents the reverse path. For unidirectional MANET paths, only the forward path ("C") AFVI is used. See: [I-D.templin-intarea-aero] for further discussion.¶

IP Jumbogram

an IPv4 or IPv6 packet with a Jumbo Payload option that includes a 32-bit length field to be used instead of the 16-bit {Total, Payload} Length field (see: Section 5.1). For IPv4, the Total Length field must be set to the length of the IPv4 header only. For IPv6, the Payload Length must be set to 0. Original IP packets, OAL packets and carrier packets may all appear as IP Jumbograms.¶

IP Parcel

a special form of an IP Jumbogram with a segment length value included in the {Total, Payload} Length field and also with a Jumbo Payload option that encodes an 8-bit "Nsegs" field followed by a 24-bit length field (see: Section 5.2). Only original IP packets may appear as IP Parcels.¶

(OMNI) L2 encapsulation

the OMNI protocol encapsulation of OAL packets/fragments in an outer header or headers to form carrier packets that can be routed within the scope of the local {A,I,E}NET underlay network partition. Common L2 encapsulation combinations include UDP/IP/Ethernet, etc. using a port/protocol/type number for OMNI.¶

L2 address (L2ADDR)

an address that appears in the OMNI protocol L2 encapsulation for an underlay interface and also in IPv6 ND message OMNI options. L2ADDR can be either an IP address for IP encapsulations or an IEEE EUI address [EUI] for direct data link encapsulation. (When UDP/IP encapsulation is used, the UDP port number is considered an ancillary extension of the IP L2ADDR.)¶

3. Requirements

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.¶

An implementation is not required to internally use the architectural constructs described here so long as its external behavior is consistent with that described in this document.¶

4. Overlay Multilink Network (OMNI) Interface Model

An OMNI interface is a virtual interface configured over one or more underlay interfaces, which may be physical (e.g., an aeronautical radio link, a cellular wireless link, etc.) or virtual (e.g., an Internet or higher-layer "tunnel"). The OMNI interface architectural layering model is the same as in [RFC5558][RFC7847], and augmented as shown in Figure 1. The IP layer therefore sees the OMNI interface as a single L3 interface nexus for multiple underlay interfaces that appear as L2 communication channels in the architecture.¶

                                  +----------------------------+
                                  |    Upper Layer Protocol    |
           Session-to-IP    +---->|                            |
           Address Binding  |     +----------------------------+
                            +---->|           IP (L3)          |
           IP Address       +---->|                            |
           Binding          |     +----------------------------+
                            +---->|       OMNI Interface       |
           Logical-to-      +---->|   (OMNI Adaptation Layer)  |
           Physical         |     +----------------------------+
           Interface        +---->|  L2  |  L2  |       |  L2  |
           Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                  +------+------+       +------+
                                  |  L1  |  L1  |       |  L1  |
                                  |      |      |       |      |
                                  +------+------+       +------+

Each underlay interface provides an L2/L1 abstraction according to one of the following models:¶

• ANET interfaces connect to a protected and secured ANET that is separated from the open INET by Proxy/Servers. The ANET interface may be either on the same L2 link segment as a Proxy/Server, or separated from a Proxy/Server by multiple L2 hops. (Note that NATs may appear internally within an ANET or on the Proxy/Server itself and may require NAT traversal the same as for the INET case.)¶
• INET interfaces connect to an INET either natively or through one or several IPv4 Network Address Translators (NATs). Native INET interfaces have global IP addresses that are reachable from any INET correspondent. NATed INET interfaces typically configure private IP addresses and connect to a private network behind one or more NATs with the outermost NAT providing INET access.¶
• ENET interfaces connect a Client's downstream-attached networks, where the Client provides forwarding services for ENET Host and Client communications to remote peers. An ENET may be as simple as a small stub network that travels with a mobile Client (e.g., an Internet-of-Things) to as complex as a large private enterprise network that the Client connects to a larger ANET or INET. Downstream-attached Hosts and Clients see the ENET as an ANET and see the (upstream) Client as a Proxy/Server.¶
• VPNed interfaces use security encapsulation over an underlay network to a Client or Proxy/Server acting as a Virtual Private Network (VPN) gateway. Other than the link-layer encapsulation format, VPNed interfaces behave the same as for Direct interfaces.¶
• Direct (aka "point-to-point") interfaces connect directly to a Client or Proxy/Server without crossing any networked paths. An example is a line-of-sight link between a remote pilot and an unmanned aircraft.¶

The OMNI interface forwards original IP packets/parcels from the network layer (L3) using the OMNI Adaptation Layer (OAL) (see: Section 5) as an encapsulation and fragmentation sublayer service. This "OAL source" then further encapsulates the resulting OAL packets/fragments in underlay network headers (e.g., UDP/IP, IP-only, Ethernet-only, etc.) to create L2-encapsulated "carrier packets" for transmission over underlay interfaces. The target OMNI interface receives the carrier packets from underlay interfaces and discards the L2 encapsulation headers. If the resulting OAL packets/fragments are addressed to itself, the OMNI interface acts as an "OAL destination" and performs reassembly if necessary, discards the OAL encapsulation, and delivers the original IP packet/parcel to the network layer. If the OAL fragments are addressed to another node, the OMNI interface instead acts as an "OAL intermediate node" by re-encapsulating the carrier packets in new underlay network L2 headers and forwarding them over an underlay interface without reassembling or discarding the OAL encapsulation. The OAL source and OAL destination are seen as "neighbors" on the OMNI link, while OAL intermediate nodes provide a virtual bridging service that joins the segments of a (multinet) Segment Routing Topology (SRT).¶

The OMNI interface can forward original IP packets/parcels over underlay interfaces while including/omitting various lower layer encapsulations including OAL, UDP, IP and Ethernet (ETH) or other link-layer header. The network layer can also engage the underlay interfaces directly while bypassing the OMNI interface entirely when necessary. This architectural flexibility may be beneficial for underlay interfaces (e.g., some aviation data links) for which encapsulation overhead is a primary consideration. OMNI interfaces that send original IP packets/parcels directly over underlay interfaces without invoking the OAL can only reach peers located on the same OMNI link segment. Source Clients can instead use the OAL to coordinate with target Clients in the same or different OMNI link segments by sending initial carrier packets to a First-Hop Segment (FHS) Proxy/Server. The FHS Proxy/Sever then sends the carrier packets into the SRT spanning tree, which transports them to a Last-Hop Segment (LHS) Proxy/Server for the target Client.¶

Original IP packets/parcels sent directly over underlay interfaces are subject to the same path MTU related issues as for any Internetworking path, and do not include per-packet identifications that can be used for data origin verification and/or link-layer retransmissions. Original IP packets/parcels presented directly to an underlay interface that exceed the underlay network path MTU are dropped with an ordinary ICMPv6 Packet Too Big (PTB) message returned. These PTB messages are subject to loss [RFC2923] the same as for any non-OMNI IP interface.¶

The OMNI interface encapsulation/decapsulation layering possibilities are shown in Figure 2 below. Imaginary vertical lines drawn between the Network Layer and Underlay interfaces in the figure denote the encapsulation/decapsulation layering combination possibilities. Common combinations include IP-only (i.e., direct access to underlay interfaces with or without using the OMNI interface), IP/IP, IP/UDP/IP, IP/UDP/IP/ETH(ERNET), IP/OAL/UDP/IP, IP/OAL/UDP/ETH, etc.¶

 +------------------------------------------------------------+  ^
 |          Network Layer (Original IP packets/parcels)       |  |
 +--+---------------------------------------------------------+ L3
    |         OMNI Interface (virtual sublayer nexus)         |  |
    +--------------------------+------------------------------+  -
                               |      OAL Encaps/Decaps       |  |
                               +------------------------------+ OAL
                               |        OAL Frag/Reass        |  |
                  +------------+---------------+--------------+  -
                  | UDP Encaps/Decaps/Compress |                 |
             +----+---+------------+--------+--+  +--------+     |
             | IP E/D |            | IP E/D |     | IP E/D |    L2
        +----+-----+--+----+    +--+----+---+     +---+----+--+  |
        |ETH E/D|  |ETH E/D|    |ETH E/D|             |ETH E/D|  |
 +------+-------+--+-------+----+-------+-------------+-------+  v
 |                    Underlay Interfaces                     |
 +------------------------------------------------------------+

The OMNI/OAL model gives rise to a number of opportunities:¶

• Clients coordinate with the MS and receive MNP delegations through IPv6 ND message exchanges with Proxy/Servers. Clients use the MNP to construct Link-Local and Unique-Local Addresses (LLA-MNP / ULA-MNP) through the algorithmic derivation specified in Section 8 and assign the addresses to the OMNI interface. Since the LLA and ULA are derived from a unique MNP, no Duplicate Address Detection (DAD) or Multicast Listener Discovery (MLD) messaging is necessary.¶
• since Temporary ULAs with random IIDs (TLA-RNDs) are statistically unique, they can be used without DAD until an MNP is obtained.¶
• underlay interfaces on the same L2 link segment as a Proxy/Server do not require any L3 addresses (i.e., not even link-local) in environments where communications are coordinated entirely over the OMNI interface.¶
• as underlay interface properties change (e.g., link quality, cost, availability, etc.), any active interface can be used to update the profiles of multiple additional interfaces in a single message. This allows for timely adaptation and service continuity under dynamically changing conditions.¶
• coordinating underlay interfaces in this way allows them to be represented in a unified MS profile with provisions to support the "6 M's of Modern Internetworking".¶
• exposing a single virtual interface abstraction to the IPv6 layer allows for multilink operation (including QoS based link selection, carrier packet replication, load balancing, etc.) at L2 while still permitting L3 traffic shaping based on, e.g., DSCP, flow label, etc.¶
• the OMNI interface supports multinet traversal over the SRT when communications across different administrative domain network segments are necessary. This mode of operation would not be possible via direct communications over the underlay interfaces themselves.¶
• the OAL supports lossless and adaptive path MTU mitigations not available for communications directly over the underlay interfaces themselves. The OAL supports "packing" of multiple original IP payload packets/parcels within a single OAL "super-packet" and also supports transmission of IP packets/parcels of all sizes up to and including Jumbograms.¶
• the OAL assigns per-packet identification values that allow for link-layer reliability and data origin authentication.¶
• L3 sees the OMNI interface as a point of connection to the OMNI link; if there are multiple OMNI links, L3 will see multiple OMNI interfaces.¶
• Multiple independent OMNI interfaces can be used for increased fault tolerance through Safety-Based Multilink (SBM), with Performance-Based Multilink (PBM) applied within each interface.¶
• Multiple independent OMNI links can be joined together into a single link without requiring renumbering of infrastructure elements, since the ULAs assigned to the different links will be mutually exclusive.¶
• the OMNI/OAL model supports transmission of a new form of IP packets known as "IP Parcels" that improve performance and efficiency for both upper layer protocols and networked paths.¶

Note that even when the OMNI virtual interface is present, applications can still access underlay interfaces either through the network protocol stack using an Internet socket or directly using a raw socket. This allows for intra-network (or point-to-point) communications without invoking the OMNI interface and/or OAL. For example, when an OMNI interface is configured over an underlay IP interface, applications can still invoke intra-network IP communications directly over the underlay interface as long as the communicating endpoints are not subject to mobility dynamics.¶

Figure 3 depicts the architectural model for a source Client with an attached ENET connecting to the OMNI link via multiple independent ANETs/INETs (i.e., *NETs). The Client's OMNI interface forwards adaptation layer IPv6 ND solicitation messages over available *NET underlay interfaces using any necessary L2 encapsulations. The IPv6 ND messages traverse the *NETs until they reach an FHS Proxy/Server (FHS#1, FHS#2, ..., FHS#n), which returns an IPv6 ND advertisement message and/or forwards a proxyed version of the message over the SRT to an LHS Proxy/Server near the target Client (LHS#1, LHS#2, ..., LHS#m). The Hop Limit in IPv6 ND messages is not decremented due to encapsulation; hence, the source and target Client OMNI interfaces appear to be attached to a common link.¶

                        +--------------+
                        |Source Client |
                        +--------------+        (:::)-.
                        |OMNI interface|<-->.-(::ENET::)
                        +----+----+----+      `-(::::)-'
               +--------|IF#1|IF#2|IF#n|------ +
              /         +----+----+----+        \
             /                 |                 \
            /                  |                  \
           v                   v                   v
        (:::)-.              (:::)-.              (:::)-.
   .-(::*NET:::)        .-(::*NET:::)        .-(::*NET:::)
     `-(::::)-'           `-(::::)-'           `-(::::)-'
      +-----+              +-----+              +-----+
 ...  |FHS#1|  .........   |FHS#2|   .........  |FHS#n|  ...
.     +--|--+              +--|--+              +--|--+     .
.        |                    |                    |
.        \                    v                    /        .
.         \                                       /         .
.           v                 (:::)-.           v            .
.                        .-(::::::::)                       .
.                    .-(::: Segment :::)-.                  .
.                  (:::::   Routing   ::::)                 .
.                     `-(:: Topology ::)-'                  .
.                         `-(:::::::-'                      .
.                  /          |          \                  .
.                 /           |           \                 .
.                v            v            v
.     +-----+              +-----+              +-----+     .
 ...  |LHS#1|  .........   |LHS#2|   .........  |LHS#m|  ...
      +--|--+              +--|--+              +--|--+
          \                   |                    /
           v                  v                   v
                    <-- Target Clients -->

After the initial IPv6 ND message exchange, the source Client (as well as any nodes on its attached ENETs) can send carrier packets to the target Client via the OMNI interface. OMNI interface multilink services will send the carrier packets via FHS Proxy/Servers for the correct underlay *NETs. The FHS Proxy/Server then re-encapsulates the carrier packets and sends them over the SRT which delivers them to an LHS Proxy/Server, and the LHS Proxy/Server in turn re-encapsulates and sends them to the target Client. (Note that when the source and target Client are on the same SRT segment, the FHS and LHS Proxy/Servers may be one and the same.)¶

Clients select a Hub Proxy/Server (not shown in the figure), which will often be one of their FHS Proxy/Servers but could also be any Proxy/Server on the OMNI link. Clients then register all of their *NET underlay interfaces with the Hub Proxy/Server via the FHS Proxy/Server in a pure proxy role. The Hub Proxy/Server then provides a designated router service for the Client, and the Client can quickly migrate to a new Hub Proxy/Server if the first becomes unresponsive.¶

Clients therefore use Proxy/Servers as gateways into the SRT to reach OMNI link correspondents via a spanning tree established in a manner outside the scope of this document. Proxy/Servers forward critical MS control messages via the secured spanning tree and forward other messages via the unsecured spanning tree (see Security Considerations). When AERO route optimization is applied, Clients can instead forward directly to SRT intermediate nodes (or directly to correspondents in the same SRT segment) to reduce Proxy/Server load.¶

Note: while not shown in the figure, a Client's ENET may connect many additional Hosts and even other Clients in a recursive extension of the OMNI link. This OMNI virtual link extension will be discussed more fully throughout the document.¶

5. OMNI Interface Maximum Transmission Unit (MTU)

The OMNI interface observes the link nature of tunnels, including the Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and the role of fragmentation and reassembly [I-D.ietf-intarea-tunnels]. The OMNI interface is configured over one or more underlay interfaces as discussed in Section 4, where the interfaces (and their associated underlay network paths) may have diverse MTUs. OMNI interface considerations for accommodating original IP packets/parcels of various sizes are discussed in the following sections.¶

IPv6 underlay interfaces are REQUIRED to configure a minimum MTU of 1280 octets and a minimum MRU of 1500 octets [RFC8200]. Therefore, the minimum IPv6 path MTU is 1280 octets since routers on the path are not permitted to perform network fragmentation even though the destination is required to reassemble more. The network therefore MUST forward original IP packets/parcels of at least 1280 octets without generating an IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message [RFC8201]. (While the source can apply "source fragmentation" for locally-generated original IPv6 packets/parcels up to 1500 octets and larger still if it knows the destination configures a larger MRU, this does not affect the minimum IPv6 path MTU.)¶

IPv4 underlay interfaces are REQUIRED to configure a minimum MTU of 68 octets [RFC0791] and a minimum MRU of 576 octets [RFC0791][RFC1122]. Therefore, when the Don't Fragment (DF) bit in the IPv4 header is set to 0 the minimum IPv4 path MTU is 576 octets since routers on the path support network fragmentation and the destination is required to reassemble at least that much. The OMNI interface therefore SHOULD set DF to 0 in the IPv4 encapsulation headers of carrier packets that are no larger than 576 octets, and SHOULD set DF to 1 in larger carrier packets unless it has a way to determine the encapsulation destination MRU and has carefully considered the issues discussed in Section 6.14.¶

When the network layer admits an original IP packet/parcel into the OMNI interface the OAL prepends an IPv6 encapsulation header (see: Section 6) where the 16-bit Payload Length field limits the maximum-sized original IP packet/parcel to (2**16 -1) = 65535 octets; this is also the maximum size that the OAL can accommodate with IPv6 fragmentation. The OMNI interface therefore sets an MTU and MRU of 65535 octets to support assured delivery of original IP packets/parcels no larger than this size even if OAL fragmentation is required. (The OMNI interface MAY instead set a larger MTU to support best-effort delivery for IP Jumbograms and/or assured delivery of IP parcels; see below.) The OMNI interface then employs the OAL as an encapsulation sublayer service to transform original IP packets/parcels into OAL packets/fragments, and the OAL in turn uses underlay network L2 encapsulation to send carrier packets over underlay interfaces (see: Section 6).¶

6. The OMNI Adaptation Layer (OAL)

When an OMNI interface forwards an original IP packet/parcel from the network layer for transmission over one or more underlay interfaces, the OMNI Adaptation Layer (OAL) acting as the OAL source applies encapsulation to form OAL packets subject to fragmentation producing OAL fragments suitable for L2 encapsulation and transmission as carrier packets over underlay interfaces as described in Section 6.1. These carrier packets travel over one or more underlay networks spanned by OAL intermediate nodes in the SRT, which re-encapsulate by removing the L2 headers of the first underlay network and appending L2 headers appropriate for the next underlay network in succession. (This process supports the multinet concatenation capability needed for joining multiple diverse networks.) After re-encapsulation by zero or more OAL intermediate nodes, the carrier packets arrive at the OAL destination.¶

When the OAL destination receives the carrier packets, it discards the L2 headers and reassembles the resulting OAL fragments (if necessary) into an OAL packet as described in Section 6.3. The OAL destination next decapsulates the OAL packet to obtain the original IP packet/parcel which it then delivers to the network layer. The OAL source may be either the source Client or its FHS Proxy/Server, while the OAL destination may be either the LHS Proxy/Server or the target Client. Proxy/Servers (and SRT Gateways as discussed in [I-D.templin-intarea-aero]) may also serve as OAL intermediate nodes.¶

The OAL presents an OMNI sublayer abstraction similar to ATM Adaptation Layer 5 (AAL5). Unlike AAL5 which performs segmentation and reassembly with fixed-length 53-octet cells over ATM networks, however, the OAL uses IPv6 encapsulation, fragmentation and reassembly with larger variable-length cells over heterogeneous networks. Detailed operations of the OAL are specified in the following sections.¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Parcel ID  |A|      Fragment Offset    |P|S|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   a) First fragment


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |   Ordinal   |A|      Fragment Offset    |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   a) Non-first fragment

     +----------+----------------+
     |OAL Header|     Frag #0    |
     +----------+----------------+
         +----------+----------------+
         |OAL Header|     Frag #1    |
         +----------+----------------+
             +----------+----------------+
             |OAL Header|     Frag #2    |
             +----------+----------------+
                               ....
                 +----------+----------------+----+
                 |OAL Header|   Frag #(N-1)  |Csum|
                 +----------+----------------+----+
     a) OAL fragmentation (Csum in final fragment)


     +----------+-----------------------------+----+
     |OAL Header|  Original IP packet/parcel  |Csum|
     +----------+-----------------------------+----+
     b) An OAL atomic fragment


     +--------+----------+----------------+
     |L2 Hdrs |OAL Header|     Frag #i    |
     +--------+----------+----------------+
     c) OAL carrier packet after L2 encapsulation

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Hop Limit |ECN|  Parcel ID  |R|X|P|S|M|   Ident. (0)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Identification (1-3)             |    AFVI (0)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    AFVI (1)   |           AFVI (2-3)          |
   +-+-+-+-+-+-+-+-+-~~~-+-+-+-+-+-+-+-+-+-+-+-~~~-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Hop Limit |   Ordinal   |    Fragment Offset      |X|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           AFVI(0-1)           |           AFVI (2-3)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-~~~-+-+-+-+-+-+-+-+-+-+-+-~~~-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type |   Ordinal   |A|  Next Header  |  Parcel ID  |R|J|P|S|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Jumbo Payload Length (when J = 1)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   a) First fragment


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type |   Ordinal   |A| Resvd |    Fragment Offset      |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   a) Non-first fragment

    Send Sequence Variables (current, previous and pending)

      SND.NXT - send next
      SND.WND - send window
      ISS     - initial send sequence number

    Receive Sequence Variables (current and previous)

      RCV.NXT - receive next
      RCV.WND - receive window
      IRS     - initial receive sequence number

                <------- Original IP packets ------->
                +-----+-----+
                | iHa | iDa |
                +-----+-----+
                      |
                      |     +-----+-----+
                      |     | iHb | iDb |
                      |     +-----+-----+
                      |           |
                      |           |     +-----+-----+
                      |           |     | iHc | iDc |
                      |           |     +-----+-----+
                      |           |           |
                      v           v           v
     +----------+-----+-----+-----+-----+-----+-----+----+
     |  OAL Hdr | iHa | iDa | iHb | iDb | iHc | iDc |Csum|
     +----------+-----+-----+-----+-----+-----+-----+----+
     <--- OAL "Super-Packet" with single OAL Hdr/Csum --->

7. Frame Format

When the OMNI interface forwards original IP packets/parcels from the network layer it first invokes OAL encapsulation and fragmentation, then wraps each resulting OAL packet/fragment in any necessary L2 headers to produce carrier packets according to the native frame format of the underlay interface. For example, for Ethernet-compatible interfaces the frame format is specified in [RFC2464], for aeronautical radio interfaces the frame format is specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical Manual), for various forms of tunnels the frame format is found in the appropriate tunneling specification, etc.¶

When the OMNI interface encapsulates an OAL packet/fragment directly over an Ethernet-compatible link layer, the over-the-wire transmission format is shown in Figure 10:¶

   +--- ~~~ ---+--------+--------+-------- ~~~ --------+--- ~~~ ---+
   |  eth-hdr  |    OAL Length   | OAL Packet/Fragment | eth-trail |
   +--  ~~~ ---+--------+--------+-------- ~~~ --------+--- ~~~ ---+
               |<-------   Ethernet Payload   -------->|

The format includes a standard Ethernet Header ("eth-hdr") with EtherType TBD2 (see: Section 25.2) followed by an Ethernet Payload that includes a 2-octet OAL Length field followed by an OAL (or native IPv6/IPv4) Packet/Fragment. The Ethernet Payload is then followed by a standard Ethernet Trailer ("eth-trail").¶

The OAL Packet/Fragment begins with a 4-bit "Type/Version" as discussed in Section 6.2. When "Type/Version" encodes '1' or '2', the OAL Packet/Fragment includes a compressed OAL IPv6 header and OAL Length MUST encode the value that would appear in the uncompressed header Payload Length. When "Type/Version" encodes '3', the OAL header is compressed away entirely and only a Fragment/Jumbo header appears as the leading portion of the Packet/Fragment.¶

When "Type/Version" encodes '0', '4' or '6', the OAL Packet/Fragment instead includes an uncompressed OAL IPv6, native IPv4, or native IPv6 header (respectively). In that case, the IP header {Total, Payload} and/or Jumbo Payload Length fields determine the packet/fragment length and the OAL Length field in the Ethernet Payload is unused (noting that future documents MAY specify an alternate use).¶

See Figure 2 for a map of the various L2 layering combinations possible. For any layering combination, the final layer (e.g., UDP, IP, Ethernet, etc.) must have an assigned number and frame format representation that is compatible with the selected underlay interface.¶

Note: For IP parcels/jumbograms larger than 65535 octets submitted for direct link layer encapsulation, the Ethernet Payload following the OAL Length must begin with a native IPv4/IPv6 header, an OCH-0 or an OCH-3 with J flag set to 1.¶

8. Link-Local Addresses (LLAs)

[RFC4861] requires that nodes assign Link-Local Addresses (LLAs) to all interfaces, and that routers use their LLAs as the source address for RA and Redirect messages. OMNI interfaces honor the first requirement, but do not honor the second since the OMNI link could consist of the concatenation of multiple links with diverse ULA prefixes (see Section 9) but for which multiple nodes might configure identical interface identifiers (IIDs). OMNI interface LLAs are therefore considered only as context for IID formation as discussed below and have no other operational role.¶

OMNI interfaces assign IPv6 LLAs through pre-service administrative actions. Clients assign "LLA-MNPs" with IIDs that embed the Client's unique MNP, while Proxy/Servers assign "LLA-RNDs" that include a randomly-generated IIDs generated as specified in [RFC7217]. LLAs are configured as follows:¶

• IPv6 LLA-MNPs encode the most-significant 64 bits of an MNP within the least-significant 64 bits of the IPv6 link-local prefix fe80::/64, i.e., in the IID portion of the LLA. The LLA prefix length is determined by adding 64 to the MNP prefix length. e.g., for the MNP 2001:db8:1000:2000::/56 the corresponding LLA-MNP prefix is fe80::2001:db8:1000:2000/120. (The base LLA-MNP for each "/N" prefix sets the final 128-N bits to 0, but all LLA-MNPs that match the prefix are also accepted.) Non-MNP IPv6 prefix-based LLAs are also represented the same as for LLA-MNPs, but include a GUA prefix that is not properly covered by the MSP.¶
• IPv4-Compatible LLA-MNPs are constructed as fe80::{IPv4-Prefix}, i.e., the IID consists of 32 '0' bits followed by a 32 bit IPv4 address/prefix, which may be either public or private in correspondence with the network layer addressing plan. The IPv4-Compatible LLA-MNP prefix length is determined by adding 96 to the IPv4 prefix length. For example, the IPv4-Compatible LLA-MNP for 192.0.2.0/24 is fe80::192.0.2.0/120, also written as fe80::c000:0200/120. (The base LLA-MNP for each "/N" prefix sets the final 128-N bits to 0, but all LLA-MNPs that match the prefix are also accepted.) Non-MNP IPv4 prefix-based LLAs are also represented the same as for LLA-MNPs, but include a GUA prefix that is not properly covered by the MSP.¶
• LLA-RNDs are randomly-generated and assigned to Proxy/Servers and other SRT infrastructure elements. They may also be assigned by Clients to support the MNP delegation process. The upper 72 bits of the LLA-RND encode the prefix fe80::/72, and the lower 56 bits include a randomly-generated candidate pseudo-random value configured as specified in [RFC7217][RFC8981]; if the most significant 24 bits of the 56 bit candidate encodes the value '0', the node generates a new candidate to obtain one with a different most significant 24 bits to avoid overlap with IPv4-Compatible LLAs.¶
• The address fe80::/128 (i.e., the LLA /64 prefix followed by an all-zero IID) is considered the LLA Subnet Router Anycast address¶

Since the prefix 0000::/8 is "Reserved by the IETF" [RFC4291], no MNPs can be allocated from that block ensuring that there is no possibility for overlap between the different MNP and RND LLA constructs discussed above.¶

Since LLA-MNPs are based on the distribution of administratively assured unique MNPs, and since LLA-RNDs are assumed unique through pseudo-random assignment, OMNI interfaces set the autoconfiguration variable DupAddrDetectTransmits to 0 [RFC4862].¶

Note: If future protocol extensions relax the 64-bit boundary in IPv6 addressing, the additional prefix bits of an MNP could be encoded in bits 16 through 63 of the LLA-MNP. (The most-significant 64 bits would therefore still be in bits 64-127, and the remaining bits would appear in bits 16 through 48.) However, this would interfere with the relationship between OMNI LLAs and ULAs (see: Section 9) and render many OMNI functions inoperable. The analysis provided in [RFC7421] furthermore suggests that the 64-bit boundary will remain in the IPv6 architecture for the foreseeable future.¶

9. Unique-Local Addresses (ULAs)

OMNI links use IPv6 Unique-Local Addresses (ULAs) as the source and destination addresses in both IPv6 ND messages and OAL packet IPv6 encapsulation headers. ULAs are routable only within the scope of an OMNI link, and are derived from the IPv6 Unique Local Address prefix fd00::/8 (i.e., the prefix fc00::/7 followed by the L bit set to 1). When the first 16 bits of the ULA encode the value fd00::/16, the address is considered as either a Temporary ULA (TLA) or an eXtended ULA (XLA) - see below. For all other ULAs, the 56 bits following fd00::/8 encode a 40-bit Global ID followed by a 16-bit Subnet ID as specified in Section 3 of [RFC4193]. All OMNI link ULA types finally include a 64-bit value in the IID portion of the address ULA::/64 as specified below.¶

When a node configures a ULA for OMNI, it selects a 40-bit Global ID for the OMNI link initialized to a candidate pseudo-random value as specified in Section 3 of [RFC4193]; if the most significant 8 bits of the candidate encodes the value '0', the node selects a new candidate until it obtains one with a different most significant 8 bits. All nodes on the same OMNI link use the same Global ID, and statistical uniqueness of the pseudo-random Global ID provides a unique OMNI link identifier allowing different links to be joined together in the future without requiring renumbering.¶

Next, for each logical segment of the same OMNI link the node selects a 16-bit Subnet ID value between 0x0000 and 0xffff. Nodes on the same logical segment configure the same Subnet ID, but nodes on different segments of the same OMNI link can still exchange IPv6 ND messages as single-hop neighbors even if they configure different Subnet IDs. When a node moves to a different OMNI link segment, it resets the Global ID and Subnet ID value according to the new segment but need not change the IID.¶

ULAs and their associated prefix lengths are configured in correspondence with LLAs through stateless prefix translation where "ULA-MNPs" simply copy the IIDs of their corresponding LLA-MNPs and "ULA-RNDs" simply copy the IIDs of their corresponding LLA-RNDs. For example, for the OMNI link ULA prefix fd{Global}:{Subnet}::/64:¶

• the ULA-MNP corresponding to the LLA-MNP fe80::2001:db8:1:2 with a 56-bit MNP length is simply fd{Global}:{Subnet}:2001:db8:1:2/120 (where, the ULA prefix length becomes 64 plus the IPv6 MNP length).¶
• the ULA-MNP corresponding to fe80::192.0.2.0 with a 28-bit MNP length is simply fd{Global}:{Subnet}::192.0.2.0/124 (where, the ULA prefix length becomes 96 plus the IPv4 MNP length).¶
• the ULA-RND corresponding to fe80::0012:3456:789a:bcde is simply fd{Global}:{Subnet}::0012:3456:789a:bcde/128.¶
• the Subnet Router Anycast ULA corresponding to fe80::/128 is simply fd{Global}:{Subnet}::/128.¶

The ULA presents an IPv6 address format that is routable within the OMNI link routing system and can be used to convey link-scoped (i.e., single-hop) IPv6 ND messages across multiple hops through IPv6 encapsulation [RFC2473]. The OMNI link extends across one or more underlying Internetworks to include all Proxy/Servers and other service nodes. All Clients are also considered to be connected to the OMNI link, however unnecessary encapsulations are omitted whenever possible to conserve bandwidth (see: Section 14).¶

Clients can configure TLAs when they have no other ULA addresses by setting the ULA prefix to fd00::/16 followed by a 48-bit randomly-generated number followed by a random or MNP-based IID the same as specified in Section 8. XLAs are special-case TLAs that use the prefix fd00::/64; XLAs can also be formed from LLAs simply by inverting bits 7 and 8 of 'fe80' to form 'fd00'.¶

OMNI nodes use XLA-MNPs as "default" ULAs for representing MNPs in the OMNI link routing system. Clients use {TLA,XLA}-MNPs when they already know their MNP but need to express it outside the context of a specific ULA prefix, and Proxy/Servers advertise XLA-MNPs into the OMNI link routing system instead of advertising fully-qualified {TLA,ULA}-MNPs and/or non-routable LLA-MNPs.¶

{TLAs,XLAs} provide initial "bootstrapping" addresses while the Client is in the process of procuring an MNP and/or identifying the ULA prefix for the OMNI link segment; TLAs are not advertised into the OMNI link routing system but can be used for Client-to-Client communications within a single {A,I,E}NET when no OMNI link infrastructure is present. Within each individual {A,I,E}NET, TLAs employ optimistic DAD principles [RFC4429] since they are statistically unique.¶

Each OMNI link may be subdivided into SRT segments that often correspond to different administrative domains or physical partitions. Each SRT segment is identified by a different Subnet ID within the same ULA ::/48 prefix. Multiple distinct OMNI links with different ULA ::/48 prefixes can also be joined together into a single unified OMNI link through simple interconnection without requiring renumbering. In that case, the (larger) unified OMNI link routing system may carry multiple distinct ULA prefixes.¶

OMNI nodes can use Segment Routing [RFC8402] to support efficient forwarding to destinations located in other OMNI link segments. A full discussion of Segment Routing over the OMNI link appears in [I-D.templin-intarea-aero].¶

Note: IPv6 ULAs taken from the prefix fc00::/7 followed by the L bit set to 0 (i.e., as fc00::/8) are never used for OMNI OAL addressing, however the range could be used for MSP/MNP addressing under certain limiting conditions (see: Section 10). When used within the context of OMNI, ULAs based on the prefix fc00::/8 are referred to as "ULA-C's".¶

Note: When they appear in the OMNI link routing table, ULA-RNDs always use prefix lengths between /48 and /64 (or, /128) while XLA-MNPs always use prefix lengths between /65 and /128. {TLA,ULA}-MNPs and {TLA,XLA}-RNDs should never appear in the OMNI link routing table, but may appear in {A,I,E}NET routing tables.¶

10. Global Unicast Addresses (GUAs)

OMNI domains use IP Global Unicast Address (GUA) prefixes [RFC4291] as Mobility Service Prefixes (MSPs) from which Mobile Network Prefixes (MNP) are delegated to Clients. Fixed correspondent node networks reachable from the OMNI link are represented by non-MNP GUA prefixes that are not derived from the MSP, but are treated in all other ways the same as for MNPs.¶

For IPv6, GUA MSPs are assigned by IANA [IPV6-GUA] and/or an associated Regional Internet Registry (RIR) such that the OMNI link can be interconnected to the global IPv6 Internet without causing inconsistencies in the routing system. An OMNI link could instead use ULAs with the 'L' bit set to 0 (i.e., from the "ULA-C" prefix fc00::/8)[RFC4193], however this would require IPv6 NAT if the domain were ever connected to the global IPv6 Internet.¶

For IPv4, GUA MSPs are assigned by IANA [IPV4-GUA] and/or an associated RIR such that the OMNI link can be interconnected to the global IPv4 Internet without causing routing inconsistencies. An OMNI ANET/ENET could instead use private IPv4 prefixes (e.g., 10.0.0.0/8, etc.) [RFC3330], however this would require IPv4 NAT at the INET-to-ANET/ENET boundary. OMNI interfaces advertise IPv4 MSPs into IPv6 routing systems as IPv4-Compatible IPv6 prefixes [RFC4291] (e.g., the IPv6 prefix for the IPv4 MSP 192.0.2.0/24 is ::192.0.2.0/120).¶

OMNI interfaces assign the IPv4 anycast address TBD3 (see: IANA Considerations), and IPv4 routers that configure OMNI interfaces advertise the prefix TBD3/N into the routing system of other networks (see: IANA Considerations). OMNI interfaces also configure global IPv6 anycast addresses formed according to [RFC3056] as:¶

2002:TBD3{32}:MSP{64}:Link-ID{16}¶

where TBD3{32} is the 32 bit IPv4 anycast address and MSP{64} encodes an MSP zero-padded to 64 bits (if necessary). For example, the OMNI IPv6 anycast address for MSP 2001:db8::/32 is 2002:TBD3{32}:2001:db8:0:0:{Link-ID}, the OMNI IPv6 anycast address for MSP 192.0.2.0/24 is 2002:TBD3{32}::c000:0200:{Link-ID}, etc.).¶

The 16-bit Link-ID in the OMNI IPv6 anycast address identifies a specific OMNI link within the domain that services the MSP. The special Link-ID value '0' is a wildcard that matches all links, while all other values identify specific links. Mappings between Link-ID values and the ULA Global IDs assigned to OMNI links are outside the scope of this document.¶

OMNI interfaces assign OMNI IPv6 anycast addresses, and IPv6 routers that configure OMNI interfaces advertise the corresponding prefixes into the routing systems of other networks. An OMNI IPv6 anycast prefix is formed the same as for any IPv6 prefix; for example, the prefix 2002:TBD3{32}:2001:db8::/80 matches all OMNI IPv6 anycast addresses covered by the prefix. When IPv6 routers advertise OMNI IPv6 anycast prefixes in this way, Clients can locate and associate with either a specific OMNI link or any OMNI link within the domain that services the MSP of interest.¶

OMNI interfaces use OMNI IPv6 and IPv4 anycast addresses to support Service Discovery in the spirit of [RFC7094], i.e., the addresses are not intended for use in long-term transport protocol sessions. Specific applications for OMNI IPv6 and IPv4 anycast addresses are discussed throughout the document as well as in [I-D.templin-intarea-aero].¶

11. Node Identification

OMNI Clients and Proxy/Servers that connect over open Internetworks include a unique node identification value for themselves in the OMNI options of their IPv6 ND messages (see: Section 12.2.3). An example identification value alternative is the Host Identity Tag (HIT) as specified in [RFC7401], while Hierarchical HITs (HHITs) [I-D.ietf-drip-rid] may be more appropriate for certain domains such as the Unmanned (Air) Traffic Management (UTM) service for Unmanned Air Systems (UAS). Another example is the Universally Unique IDentifier (UUID) [RFC4122] which can be self-generated by a node without supporting infrastructure with very low probability of collision.¶

When a Client is truly outside the context of any infrastructure, it may have no MNP information at all. In that case, the Client can use a TLA or (H)HIT as an IPv6 source/destination address for sustained communications in Vehicle-to-Vehicle (V2V) and (multihop) Vehicle-to-Infrastructure (V2I) scenarios. The Client can also propagate the ULA/(H)HIT into the multihop routing tables of (collective) Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using only the vehicles themselves as communications relays.¶

When a Client connects via a protected-spectrum ANET, an alternate form of node identification (e.g., MAC address, serial number, airframe identification value, VIN, etc.) embedded in a ULA may be sufficient. The Client can then include OMNI "Node Identification" sub-options (see: Section 12.2.3) in IPv6 ND messages should the need to transmit identification information over the network arise.¶

HHITs provide an especially useful construct since they appear as properly-formed IPv6 GUAs and can therefore be assigned to interfaces. Clients may assign an HHIT to their OMNI interface to support peer-to-peer communications with other OMNI nodes that configure HHITs within the same OMNI link segment without the need for encapsulation. Clients may inject their HHIT into the local routing system of each OMNI link segment, but Proxy/Servers must not inject HHITs into the OMNI link global routing system.¶

12. Address Mapping - Unicast

OMNI interfaces maintain a network layer conceptual neighbor cache per [RFC1256] or [RFC4861] the same as for any IP interface, and (for IPv6) use the link-local address format specified in Section 8. The network layer maintains state through static and/or dynamic Neighbor Cache Entry (NCE) configurations.¶

Each OMNI interface also maintains a separate internal adaptation layer conceptual neighbor cache that includes a NCE for the unique-local address of each of its active OAL neighbors (see: Section 8). For each peer NCE, OAL neighbors also maintain AERO Forwarding Vectors (AFVs) which map per-interface-pair parameters. Throughout this document, the terms "neighbor cache", "NCE" and "AFV" refer to this OAL neighbor information unless otherwise specified.¶

IPv6 Neighbor Discovery (ND) [RFC4861] messages sent over OMNI interfaces without OAL encapsulation observe the native underlay interface Source/Target Link-Layer Address Option (S/TLLAO) format (e.g., for Ethernet the S/TLLAO is specified in [RFC2464]). IPv6 ND messages sent from within the OMNI interface using OAL encapsulation do not include S/TLLAOs, but instead include a new option type that encodes OMNI link-specific information. Hence, this document does not define a new S/TLLAO format but instead defines a new option type termed the "OMNI option" designed for these purposes. (Note that OMNI interface IPv6 ND messages sent without encapsulation may include both OMNI options and S/TLLAOs, but the information conveyed in each is mutually exclusive.)¶

For each IPv6 ND message, the OMNI interface includes one or more OMNI options (and any other ND message options) then completely populates all option information. OMNI options should be padded when necessary to ensure that they end on their natural 64-bit boundaries the same as for any IPv6 ND message option.¶

If the OMNI interface includes an OMNI option with an authentication signature, it first sets the signature field to 0 then calculates the authentication signature beginning after the IPv6 ND message header checksum field. The OMNI interface extends the calculation over the entire length of the ND message (as well as any concatenated extensions in the case of a super-packet) then writes the authentication signature value into the appropriate OMNI authentication sub-option field.¶

The OMNI interface then applies any non-OMNI authentication signatures, then calculates the IPv6 ND message checksum per [RFC4443] beginning with a pseudo-header of the IPv6 header and writes the value into the Checksum field. OMNI interfaces verify first integrity then authenticity of each IPv6 ND message or super-packet received, and process the message further only following successful verification.¶

OMNI interface Clients such as aircraft typically have multiple wireless data link types (e.g. satellite-based, cellular, terrestrial, air-to-air directional, etc.) with diverse performance, cost and availability properties. The OMNI interface would therefore appear to have multiple L2 connections, and may include information for multiple underlay interfaces in a single IPv6 ND message exchange. OMNI interfaces manage their dynamically-changing multilink profiles by including OMNI options in IPv6 ND messages as discussed in the following subsections.¶

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |     Length    |         Sub-Options           ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     | Sub-Type|      Sub-Length     | Sub-Option Data ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

     +-+-+-+-+-+-+-+-+
     | S-Type=0|x|x|x|
     +-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     | S-Type=1|    Sub-length=N     | N padding octets ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=2|    Sub-length=N     |    ID-Type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~            Node Identification Value (N-1 octets)             ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |         DUID-Type (2)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Enterprise Number (45282)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    ID-Type    |                                               |
     +-+-+-+-+-+-+-+-+                                               ~
     ~                   Node Identification Value                   ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=3|    Sub-length=N     |      Type     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~          Hashed Message Authentication Code (HMAC)            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=4|    Sub-length=12    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Acknowledgment Number                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R|R|O|A|R|R|S|R|                                               |
     |E|E|P|C|E|S|Y|E|                   Window                      |
     |S|S|T|K|S|T|N|S|                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         |                     |               |N|A|R|S|       |
     | S-Type=5|    Sub-length=1     |    Preflen    |U|R|P|N| Resv1 |
     |         |                     |               |D|R|T|R|       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |               |               |
     |   Reserved2   |   Reserved3   |   Reserved4   |
     |               |               |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=6|    Sub-length=N     |  Link |TS Form|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifIndex                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifType                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifProvider                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      SRT      |      FMT      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
     ~                  LHS Proxy/Server ULA/L2ADDR                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                RFC 6088 Format Traffic Selector               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=7|    Sub-length=N     |   TS Format   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifIndex                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                RFC 6088 Format Traffic Selector               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=8|    Sub-length=N     |  A  |  B  |Job|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~           AERO Forwarding Vector Index (AFVI) List            ~
     ~                (5 consecutive 4-octet AFVIs)                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       FHS Client ifIndex                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                FHS Proxy/Server FMT/ULA/L2ADDR                ~
     ~                  FHS Gateway FMT/ULA/L2ADDR                   ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       LHS Client ifIndex                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                LHS Proxy/Server FMT/ULA/L2ADDR                ~
     ~                   LHS Gateway FMT/ULA/L2ADDR                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=9|     Sub-length=N    |    Geo Type   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        Geo Coordinates                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=10|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |               transaction-id                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        DHCPv6 options                         ~
     ~                 (variable number and length)                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=11|    Sub-length=N     |PIM Ver| Type  |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         PIM-SM Message                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=12|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0| Packet Type |Version| RES.|1|           Controls            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Sender's Host Identity Tag (HIT)               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~               Receiver's Host Identity Tag (HIT)              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        HIP Parameters                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=13|     Sub-length=N    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         QUIC-TLS Message                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=14|    Sub-Length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Identification #1                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                            Bitmap #1                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Identification #2                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                            Bitmap #2                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +                              ...                              +

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=15|     Sub-length=N    |     Type      |     Code      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         Message Body                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=16|   Sub-length=32     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Old FHS Proxy/Server ULA (16 octets)           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Old Hub Proxy/Server ULA (16 octets)           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |S-Type=30|     Sub-length=N    | Extension-Type|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                       Extension-Type Body                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=30|      Sub-length=N   |   Ext-Type=0  |   Header Type |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                      Header Option Value                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=30|      Sub-length=N   |   Ext-Type=1  |  Trailer Type |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                     Trailer Option Value                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

13. Address Mapping - Multicast

The multicast address mapping of the native underlay interface applies. The Client mobile router also serves as an IGMP/MLD Proxy for its ENETs and/or hosted applications per [RFC4605].¶

The Client uses Multicast Listener Discovery (MLDv2) [RFC3810] to coordinate with Proxy/Servers, and underlay network elements use MLD snooping [RFC4541]. The Client can also employ multicast routing protocols to coordinate with network-based multicast sources as specified in [I-D.templin-intarea-aero].¶

Since the OMNI link model is NBMA, OMNI links support link-scoped multicast through iterative unicast transmissions to individual multicast group members (i.e., unicast/multicast emulation).¶

14. Multilink Conceptual Sending Algorithm

The Client's IPv6 layer selects the outbound OMNI interface according to SBM considerations when forwarding original IP packets/parcels from local or ENET applications to external correspondents. Each OMNI interface maintains an internal OAL neighbor cache maintained the same as discussed in [RFC4861], but also includes additional state for multilink coordination. Each Client OMNI interface maintains default routes via Proxy/Servers discovered as discussed in Section 15, and may configure more-specific routes discovered through means outside the scope of this specification.¶

For each original IP packet/parcel it forwards, the OMNI interface selects one or more source underlay interfaces based on PBM factors (e.g., traffic attributes, cost, performance, message size, etc.) and one or more target underlay interfaces for the neighbor based on Interface Attributes received in IPv6 ND messages (see: Section 12.2.7). Multilink forwarding may also direct carrier packet replication across multiple underlay interface pairs for increased reliability at the expense of duplication. The set of all Interface Attributes and Traffic Selectors received in IPv6 ND messages determines the multilink forwarding profile for selecting target underlay interfaces.¶

When the OMNI interface forwards an original IP packet/parcel over a selected source underlay interface, it first employs OAL encapsulation and fragmentation as discussed in Section 5, then performs L2 encapsulation as directed by the appropriate AFV. The OMNI interface also performs L2 encapsulation (following OAL encapsulation) when the nearest Proxy/Server is located multiple hops away as discussed in Section 15.2.¶

OMNI interface multilink service designers MUST observe the BCP guidance in Section 15 [RFC3819] in terms of implications for reordering when original IP packets/parcels from the same flow may be spread across multiple underlay interfaces having diverse properties.¶

15. Router Discovery and Prefix Registration

Clients engage the MS by sending RS messages with OMNI options under the assumption that one or more Proxy/Server will process the message and respond. The RS message is received by a FHS Proxy/Server, which may in turn forward a proxyed copy of the RS to a Hub Proxy/Server located on the same or different SRT segment. The Hub Proxy/Server then returns an RA message either directly to the Client or via an FHS Proxy/Server acting as a proxy.¶

To support Client to service coordination, OMNI defines three flag bits in the OMNI Neighbor Coordination sub-option discussed in Figure 20. Clients set or clear the NUD, ARR and/or RPT flags in RS messages as directives to the Mobility Service FHS and Hub Proxy/Servers. Proxy/Servers interpret the flags as follows:¶

• When an FHS Proxy/Server forwards or processes an RS with the NUD flag set, it responds directly to NS Neighbor Unreachability Detection (NUD) messages by returning NA(NUD) replies; otherwise, it forwards NS(NUD) messages to the Client.¶
• When the Hub Proxy/Server receives an RS with the ARR flag set, it responds directly to NS Address Resolution (AR) messages by returning NA(AR) replies; otherwise, it forwards NS(AR) messages to the Client.¶
• When the Hub Proxy/Server receives an RS with the RPT flag set, it maintains a Report List of recent NS(AR) message sources for the source or target Client and sends uNA messages to all list members if any aspects of the Client's underlay interfaces change.¶

Mobility Service Proxy/Servers function according to the NUD, ARR and RPT flag settings received in the most recent RS message to support dynamic Client updates.¶

Clients and FHS Proxy/Servers include an authentication signature in their RS/RA exchanges when necessary but always include a valid IPv6 ND message checksum. FHS and Hub Proxy/Server RS/RA message exchanges over the SRT secured spanning tree instead always include the checksum and omit the authentication signature. Clients and Proxy/Servers use the information included in RS/RA messages to establish NCE state and OMNI link autoconfiguration information as discussed in this section.¶

For each underlay interface, the Client sends RS messages with OMNI options to coordinate with a (potentially) different FHS Proxy/Server for each interface but with a single Hub Proxy/Server. All Proxy/Servers are identified by their ULA-RNDs and accept carrier packets addressed to their anycast/unicast L2ADDRs; the Hub Proxy/Server may be chosen among any of the Client's FHS Proxy/Servers or may be any other Proxy/Server for the OMNI link. Example ULA/L2ADDR discovery methods are given in [RFC5214] and include data link login parameters, name service lookups, static configuration, a static "hosts" file, etc. In the absence of other information, the Client can resolve the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the OMNI link (e.g., "example.com"). The name resolution will retain a set of DNS resource records with the addresses of Proxy/Servers for the domain.¶

Each FHS Proxy/Server configures a ULA-RND based on a /64 ULA prefix for the link/segment with randomly-generated Global ID to assure global uniqueness then administratively assigned to FHS Proxy/Servers for the link to assure global consistency. The Client can then configure ULA-MNPs derived from the 64-bit ULA prefix assigned to a FHS Proxy/Server for each underlay interface. The FHS Proxy/Servers discovered over multiple of the Client's underlay interfaces may configure the same or different ULA prefixes, and the Client's ULA-MNP for each underlay interface will fall within the ULA (multilink) subnet relative to each FHS Proxy/Server.¶

Clients configure OMNI interfaces that observe the properties discussed in previous sections. The OMNI interface and its underlay interfaces are said to be in either the "UP" or "DOWN" state according to administrative actions in conjunction with the interface connectivity status. An OMNI interface transitions to UP or DOWN through administrative action and/or through state transitions of the underlay interfaces. When a first underlay interface transitions to UP, the OMNI interface also transitions to UP. When all underlay interfaces transition to DOWN, the OMNI interface also transitions to DOWN.¶

When a Client OMNI interface transitions to UP, it sends RS messages to register its MNP and an initial set of underlay interfaces that are also UP. The Client sends additional RS messages to refresh lifetimes and to register/deregister underlay interfaces as they transition to UP or DOWN. The Client's OMNI interface sends initial RS messages over an UP underlay interface with its XLA-MNP as the source (or with a HHIT or TLA-RND as the source if it does not yet have an MNP) and with destination set to link-scoped All-Routers multicast or the ULA of a specific (Hub) Proxy/Server. The Client sets the RS NUD, ARR and RPT flags, then includes an OMNI option per Section 12 with an OMNI Window Coordination sub-option, a Neighbor Control or DHCPv6 Solicit sub-option if necessary, an Interface Attributes sub-option for the underlay interface, and with any other necessary OMNI sub-options such as authentication, Proxy/Server Departure, etc. The OMNI interface finally sets or clears the Interface Attributes FMT-Forward and FMT-Mode bits according to the behavior it would like to receive from the FHS Proxy/Server as described in Section 12.2.7.¶

The Client then calculates the authentication signature checksum and prepares to forward the RS over the underlay interface using OAL encapsulation and fragmentation if necessary. The OMNI interface selects an Identification value (see: Section 6.6), sets the OAL source address to the ULA-MNP corresponding to the RS source if known (otherwise to an HHIT/TLA), sets the OAL destination to an OMNI IPv6 anycast address or a known Proxy/Server ULA, optionally includes a Nonce and/or Timestamp, then performs fragmentation if necessary. When L2 encapsulation is used, the Client includes the discovered FHS Proxy/Server L2ADDR or an anycast address as the L2 destination then forwards the resulting carrier packet(s) into the underlay network. Note that the Client does not yet create a NCE, but instead caches the Identification, Nonce and/or Timestamp values included in its RS message transmissions to match against any received RA messages.¶

When an FHS Proxy/Server receives the carrier packets containing an RS it sets aside the L2 headers, verifies the Identifications and reassembles if necessary, sets aside the OAL header, then verifies the RS authentication signature/checksum. The FHS Proxy/Server then creates/updates a NCE indexed by the Client's RS source address and caches the OMNI Interface Attributes and any Traffic Selector sub-options while also caching the L2 (UDP/IP) and OAL source and destination address information. The FHS Proxy/Server next caches the RS NUD flag and Window Synchronization parameters (see: Section 12.1) then examines the RS destination address. If the destination matches its own ULA, the FHS Proxy/Server assumes the Hub role and acts as the sole entry point for injecting the Client's XLA-MNP into the OMNI link routing system (i.e., after performing any necessary prefix delegation operations) while setting the prefix to fd00::/64 and suffix to the 64-bit MNP, then including a prefix length set to the MNP prefix length plus 64. (For example, if the MNP prefix length is 48, the prefix length field encodes the value 112.) The FHS/Hub Proxy/Server then caches the RS ARR and RPT flags to determine its role in processing NS(AR) messages and generating uNA messages (see: Section 12.1).¶

The FHS/Hub Proxy/Server then prepares to return an RA message directly to the Client by first populating the Cur Hop Limit, Flags, Router Lifetime, Reachable Time and Retrans Timer fields with values appropriate for the OMNI link. The FHS/Hub Proxy/Server next includes as the first RA message option an OMNI option with a Window Synchronization sub-option, an authentication sub-option if necessary and a (proxyed) copy of the Client's original Interface Attributes sub-option with its INET-facing interface information written in the FMT, SRT and LHS Proxy/Server ULA/L2ADDR fields. The Proxy/Server also sets or clears the FMT-Forward and FMT-Mode flags if necessary to convey its capabilities to the Client, noting that it should honor the Client's stated preferences for those parameters if possible or override otherwise. The FMT-Forward/Mode flags thereafter remain fixed unless and until a new RS/RA exchange produces different values (see: Section 12.2.7 for further discussion). If the FHS/Hub Proxy/Server's Client-facing interface is different than its INET-facing interface, the Proxy/Server next includes a second Interface Attributes sub-option with ifIndex set to '0' and with a unicast L2 address for its Client-facing interface in the L2ADDR field.¶

The FHS/Hub Proxy/Server next includes an Origin Indication sub-option that includes the RS L2 source L2ADDR information (see: Section 12.2.18.1), then includes any other necessary OMNI sub-options (either within the same OMNI option or in additional OMNI options). Following the OMNI option(s), the FHS/Hub Proxy/Server next includes any other necessary RA options such as PIOs with (A; L=0) that include the OMNI link MSPs [RFC8028], RIOs [RFC4191] with more-specific routes, Nonce and Timestamp options, etc. The FHS/Hub Proxy/Server then sets the RA source address to its own ULA and destination address to the Client's ULA-MNP (i.e., relative to the ULA /64 prefix for its Client-facing underlay interface) while also recording the corresponding XLA-MNP as an (alternate) index to the Client NCE, then calculates the authentication signature/checksum. The FHS/Hub Proxy/Server finally performs OAL encapsulation with source set to its own ULA and destination set to the OAL source that appeared in the RS, then calculates the OAL checksum, selects an appropriate Identification, fragments if necessary, encapsulates each fragment in appropriate L2 headers with source and destination address information reversed from the RS L2 information and returns the resulting carrier packets to the Client over the same underlay interface the RS arrived on.¶

When an FHS Proxy/Server receives an RS with a valid authentication signature/checksum and with destination set to link-scoped All-Routers multicast, it can either assume the Hub role itself the same as above or act as a proxy and select the ULA of another Proxy/Server to serve as the Hub. When an FHS Proxy/Server assumes the proxy role or receives an RS with destination set to the ULA of another Proxy/Server, it forwards the message while acting as a proxy. The FHS Proxy/Server creates/updates a NCE for the Client (i.e., based on the RS source address) and caches the OAL source, Window Synchronization, NUD flag, Interface Attributes addressing information as above then writes its own INET-facing FMT, SRT and LHS Proxy/Server ULA/L2ADDR information into the appropriate Interface Attributes sub-option fields (while also setting/clearing FMT-Forward and FMT-Type as above). The FHS Proxy/Server then calculates and includes the checksum, performs OAL encapsulation with source set to its own ULA and destination set to the ULA of the Hub Proxy/Server, calculates the OAL checksum, selects an appropriate Identification, fragments if necessary, encapsulates each fragment in appropriate L2 headers and sends the resulting carrier packets into the SRT secured spanning tree.¶

When the Hub Proxy/Server receives the carrier packets, it discards the L2 headers, reassembles if necessary to obtain the proxyed RS, verifies checksums, then performs DHCPv6 Prefix Delegation (PD) to obtain the Client's MNP if the RS source is not already MNP-based. The Hub Proxy/Server then creates/updates a NCE for the Client's XLA-MNP and caches any state (including the ARR and RPT flags, OAL addresses, Interface Attributes information and Traffic Selectors), then finally performs routing protocol injection. The Hub Proxy/Server then returns an RA that echoes the Client's (proxyed) Interface Attributes sub-option and with any RA parameters the same as specified for the FHS/Hub Proxy/Server case above. The Hub Proxy/Server then sets the RA source address to its own ULA and destination address to the RS source address; if the RS source address is an HHIT/TLA, the Hub Proxy/Server also includes the MNP in a DHCPv6 PD Reply OMNI sub-option. The Hub Proxy/Server next calculates the checksum, then encapsulates the RA as an OAL packet with source set to its own ULA and destination set to the ULA of the FHS Proxy/Server that forwarded the RS. The Hub Proxy/Server finally calculates the OAL checksum, selects an appropriate Identification, fragments if necessary, encapsulates each fragment in appropriate L2 headers and sends the resulting carrier packets into the secured spanning tree.¶

When the FHS Proxy/Server receives the carrier packets it discards the L2 headers, reassembles if necessary to obtain the RA message, verifies checksums then updates the OMNI interface NCE for the Client and creates/updates a NCE for the Hub. The FHS Proxy/Server then sets the P flag in the RA flags field [RFC4389] and proxys the RA by changing the OAL source to its own ULA, changing the OAL destination to the OAL address found in the Client's NCE, and changing the RA destination address to the ULA-MNP of the Client relative to its own /64 ULA prefix while also recording the corresponding XLA-MNP as an alternate index into the Client NCE. (If the RA destination address was an HHIT/TLA, the FHS Proxy Server determines the MNP by consulting the DHCPv6 PD Reply message sub-option.) The FHS Proxy/Server next includes Window Synchronization parameters responsive to those in the Client's RS, an Interface Attributes sub-option with ifIndex '0' and with its Client-facing interface unicast L2 address if necessary (see above), an Origin Indication sub-option with the Client's cached L2ADDR and an authentication sub-option if necessary. The FHS Proxy/Server finally selects an Identification value per Section 6.6, calculates the authentication signature/checksum, fragments if necessary, encapsulates each fragment in L2 headers with addresses taken from the Client's NCE and sends the resulting carrier packets via the same underlay interface over which the RS was received.¶

When the Client receives the carrier packets, it discards the L2 headers, reassembles if necessary and removes the OAL header to obtain the RA message. The Client next verifies the authentication signature/checksum, then matches the RA message with its previously-sent RS by comparing the RS Sequence Number with the RA Acknowledgement Number and also comparing the Nonce and/or Timestamp values if present. If the values match, the Client then creates/updates OMNI interface NCEs for both the Hub and FHS Proxy/Server and caches the information in the RA message. In particular, the Client caches the RA source address as the Hub Proxy/Server ULA and uses the OAL source address to configure both an underlay interface-specific ULA for the Hub Proxy/Server and the ULA of this FHS Proxy/Server. The Client then uses the ULA-MNP in the RA destination address to configure its address within the ULA (multilink) subnet prefix of the FHS Proxy/Server. If the Client has multiple underlay interfaces, it creates additional FHS Proxy/Server NCEs and ULA-MNPs as necessary when it receives RAs over those interfaces (noting that multiple of the Client's underlay interfaces may be serviced by the same or different FHS Proxy/Servers). The Client finally adds the Hub Proxy/Server ULA to the default router list if necessary.¶

For each underlay interface, the Client next caches the (filled-out) Interface Attributes for its own ifIndex and Origin Indication information that it received in an RA message over that interface so that it can include them in future NS/NA messages to provide neighbors with accurate FMT/SRT/LHS information. (If the message includes an Interface Attributes sub-option with ifIndex '0', the Client also caches the L2ADDR as the underlay network-local unicast address of the FHS Proxy//Server via that underlay interface.) The Client then compares the Origin Indication L2ADDR information with its own underlay interface addresses to determine whether there may be NATs on the path to the FHS Proxy/Server; if the L2ADDR information differs, the Client is behind a NAT and must supply the Origin information in IPv6 ND message exchanges with prospective neighbors on the same SRT segment. The Client finally configures default routes and assigns the OMNI Subnet Router Anycast address corresponding to the MNP (e.g., 2001:db8:1:2::) to the OMNI interface.¶

Following the initial exchange, the FHS Proxy/Server MAY later send additional periodic and/or event-driven unsolicited RA messages per [RFC4861]. (The unsolicited RAs may be initiated either by the FHS Proxy/Server itself or by the Hub via the FHS as a proxy.) The Client then continuously manages its underlay interfaces according to their states as follows:¶

• When an underlay interface transitions to UP, the Client sends an RS over the underlay interface with an OMNI option with sub-options as specified above.¶
• When an underlay interface transitions to DOWN, the Client sends unsolicited NA messages over any UP underlay interface with an OMNI option containing Interface Attributes sub-options for the DOWN underlay interface with Link set to '0'. The Client sends isolated unsolicited NAs when reliability is not thought to be a concern (e.g., if redundant transmissions are sent on multiple underlay interfaces), or may instead set the SNR flag in an OMNI Neighbor Control sub-option to trigger an unsolicited NA reply (see: [I-D.templin-intarea-aero]).¶
• When the Router Lifetime for the Hub Proxy/Server nears expiration, the Client sends an RS over any underlay interface to receive a fresh RA from the Hub. If no RA messages are received over a first underlay interface (i.e., after retrying), the Client marks the underlay interface as DOWN and should attempt to contact the Hub Proxy/Server via a different underlay interface. If the Hub Proxy/Server is unresponsive over additional underlay interfaces, the Client sends an RS message with destination set to the ULA of another Proxy/Server which will then assume the Hub role.¶
• When all of a Client's underlay interfaces have transitioned to DOWN (or if the prefix registration lifetime expires), the Hub Proxy/Server withdraws the MNP the same as if it had received a message with a release indication.¶

The Client is responsible for retrying each RS exchange up to MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL seconds until an RA is received. If no RA is received over an UP underlay interface (i.e., even after attempting to contact alternate Proxy/Servers), the Client declares this underlay interface as DOWN. When changing to a new FHS or Hub Proxy/Server, the Client also includes a Proxy/Server Departure OMNI sub-option in new RS messages; the (new) FHS Proxy/Server will in turn send uNA messages to the old FHS and/or Hub Proxy/Server to announce the Client's departure as discussed in [I-D.templin-intarea-aero].¶

The IPv6 layer sees the OMNI interface as an ordinary IPv6 interface. Therefore, when the IPv6 layer sends an RS message the OMNI interface returns an internally-generated RA message as though the message originated from an IPv6 router. The internally-generated RA message contains configuration information consistent with the information received from the RAs generated by the Hub Proxy/Server. Whether the OMNI interface IPv6 ND messaging process is initiated from the receipt of an RS message from the IPv6 layer or independently of the IPv6 layer is an implementation matter. Some implementations may elect to defer the OMNI interface internal RS/RA messaging process until an RS is received from the IPv6 layer, while others may elect to initiate the process proactively. Still other deployments may elect to administratively disable IPv6 layer RS/RA messaging over the OMNI interface, since the messages are not required to drive the OMNI interface internal RS/RA process. (Note that this same logic applies to IPv4 implementations that employ "ICMP Router Discovery" [RFC1256].)¶

Note: The Router Lifetime value in RA messages indicates the time before which the Client must send another RS message over this underlay interface (e.g., 600 seconds), however that timescale may be significantly longer than the lifetime the MS has committed to retain the prefix registration (e.g., REACHABLETIME seconds). Proxy/Servers are therefore responsible for keeping MS state alive on a shorter timescale than the Client may be required to do on its own behalf.¶

Note: On certain multicast-capable underlay interfaces, Clients should send periodic unsolicited multicast NA messages and Proxy/Servers should send periodic unsolicited multicast RA messages as "beacons" that can be heard by other nodes on the link. If a node fails to receive a beacon after a timeout value specific to the link, it can initiate Neighbor Unreachability Detection (NUD) exchanges to test reachability.¶

Note: If a single FHS Proxy/Server services multiple of a Client's underlay interfaces, Window Synchronization will initially be repeated for the RS/RA exchange over each underlay interface, i.e., until the Client discovers the many-to-one relationship. This will naturally result in a single window synchronization that applies over the Client's multiple underlay interfaces for the same FHS Proxy/Server.¶

Note: Although the Client's FHS Proxy/Server is a first-hop segment node from its own perspective, the Client stores the Proxy/Server's FMT/SRT/ULA/L2ADDR as last-hop segment (LHS) information to supply to neighbors. This allows both the Client and Hub Proxy/Server to supply the information to neighbors that will perceive it as LHS information on the return path to the Client.¶

Note: The Hub Proxy/Server injects Client XLA-MNP into the OMNI link routing system by simply creating a route-to-interface forwarding table entry for fd00::{MNP}/N via the OMNI interface. The dynamic routing protocol will notice the new entry and propagate the route to its peers. If the Hub receives additional RS messages, it need not re-create the forwarding table entry (nor disturb the dynamic routing protocol) if an entry is already present. If the Hub ceases to receive RS messages from any of the Client's interfaces, it removes the Client XLA-MNP from the forwarding table (i.e., after a short delay) resulting in its removal also from the routing system.¶

Note: If the Client's initial RS message includes an anycast L2 destination address, the FHS Proxy/Server returns the solicited RA using the same anycast address as the L2 source while including an Interface Attributes sub-option with ifIndex '0' and its true unicast address in the L2ADDR. When the Client sends additional RS messages, it includes this FHS Proxy/Server unicast address as the L2 destination and the FHS Proxy/Server returns the solicited RA using the same unicast address as the L2 source. This will ensure that RS/RA exchanges are not impeded by any NATs on the path while avoiding long-term exposure of messages that use an anycast address as the source.¶

Note: The Origin Indication sub-option is included only by the FHS Proxy/Server and not by the Hub (unless the Hub is also serving as an FHS).¶

Note: Clients should set the NUD, ARR and RPT flags consistently in successive RS messages and only change those settings when an FHS/Hub Proxy/Server service profile update is necessary.¶

Note: Although the Client adds the Hub Proxy/Server ULA to the default router list, it also caches the ULAs of the FHS Proxy/Servers on the path to the Hub over each underlying interface. When the Client needs to send an original IP packet/parcel to a default router, it engages OAL encapsulation/fragmentation while using a destination ULA corresponding to the selected interface which directs the packet to an FHS Proxy/Server for that interface. The FHS Proxy/Server then performs L2 encapsulation and sends the resulting carrier packets without disturbing the Hub.¶

16. Secure Redirection

If the underlay network link model is multiple access, the FHS Proxy/Server is responsible for assuring that address duplication cannot corrupt the neighbor caches of other nodes on the link. When the Client sends an RS message on a multiple access underlay network, the Proxy/Server verifies that the Client is authorized to use the address and responds with an RA (or forwards the RS to the Hub) only if the Client is authorized.¶

After verifying Client authorization and returning an RA, the Proxy/Server MAY return IPv6 ND Redirect messages to direct Clients located on the same underlay network to exchange OAL packets directly without transiting the Proxy/Server. In that case, the Clients can exchange OAL packets according to their unicast L2 addresses discovered from the Redirect message instead of using the dogleg path through the Proxy/Server. In some underlay networks, however, such direct communications may be undesirable and continued use of the dogleg path through the Proxy/Server may provide better performance. In that case, the Proxy/Server can refrain from sending Redirects, and/or Clients can ignore them.¶

17. Proxy/Server Resilience

*NETs SHOULD deploy Proxy/Servers in Virtual Router Redundancy Protocol (VRRP) [RFC5798] configurations so that service continuity is maintained even if one or more Proxy/Servers fail. Using VRRP, the Client is unaware which of the (redundant) FHS Proxy/Servers is currently providing service, and any service discontinuity will be limited to the failover time supported by VRRP. Widely deployed public domain implementations of VRRP are available.¶

Proxy/Servers SHOULD use high availability clustering services so that multiple redundant systems can provide coordinated response to failures. As with VRRP, widely deployed public domain implementations of high availability clustering services are available. Note that special-purpose and expensive dedicated hardware is not necessary, and public domain implementations can be used even between lightweight virtual machines in cloud deployments.¶

18. Detecting and Responding to Proxy/Server Failures

In environments where fast recovery from Proxy/Server failure is required, FHS Proxy/Servers SHOULD use proactive Neighbor Unreachability Detection (NUD) in a manner that parallels Bidirectional Forwarding Detection (BFD) [RFC5880] to track Hub Proxy/Server reachability. FHS Proxy/Servers can then quickly detect and react to failures so that cached information is re-established through alternate paths. Proactive NUD control messaging is carried only over well-connected ground domain networks (i.e., and not low-end links such as aeronautical radios) and can therefore be tuned for rapid response.¶

FHS Proxy/Servers perform proactive NUD for Hub Proxy/Servers for which there are currently active Clients. If a Hub Proxy/Server fails, the FHS Proxy/Server can quickly inform Clients of the outage by sending multicast RA messages. The FHS Proxy/Server sends RA messages to Clients with source set to the ULA of the Hub, with destination address set to All-Nodes multicast (ff02::1) [RFC4291] and with Router Lifetime set to 0.¶

The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays [RFC4861]. Any Clients that have been using the (now defunct) Hub Proxy/Server will receive the RA messages.¶

19. Transition Considerations

When a Client connects to an *NET link for the first time, it sends an RS message with an OMNI option. If the first hop router recognizes the option, it responds according to the appropriate FHS/Hub Proxy/Server role resulting in an RA message with an OMNI option returned to the Client. The Client then engages this FHS Proxy/Sever according to the OMNI link model specified above. If the first hop router is a legacy IPv6 router, however, it instead returns an RA message with no OMNI option and with a non-OMNI unicast source LLA as specified in [RFC4861]. In that case, the Client engages the *NET according to the legacy IPv6 link model and without the OMNI extensions specified in this document.¶

If the *NET link model is multiple access, there must be assurance that address duplication cannot corrupt the neighbor caches of other nodes on the link. When the Client sends an RS message on a multiple access *NET link with an OMNI option, first hop routers that recognize the option ensure that the Client is authorized to use the address and return an RA with a non-zero Router Lifetime only if the Client is authorized. First hop routers that do not recognize the OMNI option instead return an RA that makes no statement about the Client's authorization to use the source address. In that case, the Client should perform Duplicate Address Detection to ensure that it does not interfere with other nodes on the link.¶

An alternative approach for multiple access *NET links to ensure isolation for Client-Proxy/Server communications is through link-layer address mappings as discussed in Appendix D. This arrangement imparts a (virtual) point-to-point link model over the (physical) multiple access link.¶

20. OMNI Interfaces on Open Internetworks

Client OMNI interfaces configured over IPv6-enabled underlay interfaces on an open Internetwork without an OMNI-aware first-hop router receive IPv6 RA messages with no OMNI options, while OMNI interfaces configured over IPv4-only underlay interfaces receive no IPv6 RA messages at all (but may receive IPv4 RA messages [RFC1256]). Client OMNI interfaces that receive RA messages with OMNI options configure addresses, on-link prefixes, etc. on the underlay interface that received the RA according to standard IPv6 ND and address resolution conventions [RFC4861] [RFC4862]. Client OMNI interfaces configured over IPv4-only underlay interfaces configure IPv4 address information on the underlay interfaces using mechanisms such as DHCPv4 [RFC2131].¶

Client OMNI interfaces configured over underlay interfaces connected to open Internetworks can apply security services such as VPNs to connect to a Proxy/Server, or can establish a direct link to the Proxy/Server through some other means (see Section 4). In environments where an explicit VPN or direct link may be impractical or undesirable, Client OMNI interfaces can instead send IPv6 ND messages with OMNI options that include authentication signatures.¶

OMNI interfaces use UDP/IP as L2 encapsulation headers for transmission over open Internetworks with UDP service port number 8060 (see: Section 25.13 and Section 3.6 of [I-D.templin-intarea-aero]) for both IPv4 and IPv6 underlay interfaces. The OMNI interface submits original IP packets/parcels for OAL encapsulation, then encapsulates the resulting OAL fragments in UDP/IP L2 headers to form carrier packets. (The first four bits following the UDP header determine whether the OAL headers are uncompressed/compressed as discussed in Section 6.4.) The OMNI interface sets the UDP length to the encapsulated OAL fragment length and sets the IP length to an appropriate value at least as large as the UDP datagram.¶

When necessary, sources include an OMNI option with an authentication sub-option in IPv6 ND messages. The source can employ a simple Hashed Message Authentication Code (HMAC) as specified in [RFC2104][RFC6234] or a message-based authentication service such as HIP [RFC7401], QUIC-TLS [RFC9000][RFC9001], etc. using the IPv6 ND message OMNI option as a "shipping container". Before calculating any authentication signature, the source fully populates any necessary OMNI sub-options as well as any ordinary IPv6 ND options as necessary.¶

The source then sets both the IPv6 ND message Checksum and authentication signature fields to 0 and calculates the authentication signature over the full length of the IPv6 ND message beginning after the IPv6 ND message checksum field and extending over the length of the message. (If the IPv6 ND message is part of an OAL super-packet, the source instead continues to calculate the authentication signature over the entire length of the super-packet.) The source next writes the authentication signature into the appropriate sub-option field and forwards the message.¶

After establishing a VPN or preparing for UDP/IP encapsulation, OMNI interfaces send RS/RA messages for Client-Proxy/Server coordination (see: Section 15) and NS/NA messages for route optimization, window synchronization and mobility management (see: [I-D.templin-intarea-aero]). These control plane messages must be authenticated while other control and data plane messages are delivered the same as for ordinary best-effort traffic with source address and/or Identification window-based data origin verification. Upper layer protocol sessions over OMNI interfaces that connect over open Internetworks without an explicit VPN should therefore employ transport- or higher-layer security to ensure authentication, integrity and/or confidentiality.¶

Clients should avoid using INET Proxy/Servers as general-purpose routers for steady streams of carrier packets that do not require authentication. Clients should therefore perform route optimization to coordinate with other INET nodes that can provide forwarding services (or preferably coordinate directly with peer Clients directly) instead of burdening the Proxy/Server. Procedures for coordinating with peer Clients and discovering INET nodes that can provide better forwarding services are discussed in [I-D.templin-intarea-aero].¶

Clients that attempt to contact peers over INET underlay interfaces often encounter NATs in the path. OMNI interfaces accommodate NAT traversal using UDP/IP encapsulation and the mechanisms discussed in [I-D.templin-intarea-aero]. FHS Proxy/Servers include Origin Indications in RA messages to allow Clients to detect the presence of NATs.¶

Note: Following the initial IPv6 ND message exchange, OMNI interfaces configured over INET underlay interfaces maintain neighbor relationships by transmitting periodic IPv6 ND messages with OMNI options that include authentication signatures. Other authentication services that use their own IPv6 ND option types such as [RFC3971] and [RFC8928] can also be used in addition to any OMNI authentication services.¶

Note: OMNI interfaces configured over INET underlay interfaces should employ the Identification window synchronization mechanisms specified in Section 6.6 in order to exclude spurious carrier packets that might otherwise clutter the reassembly cache. This is especially important in environments where carrier packet spoofing and/or corruption is a threat.¶

Note: NATs may be present on the path from a Client to its FHS Proxy/Server, but never on the path from the FHS Proxy/Server to the Hub where only INET and/or spanning tree hops occur. Therefore, the FHS Proxy/Server does not communicate Client origin information to the Hub where it would serve no purpose.¶

21. Time-Varying MNPs

In some use cases, it is desirable, beneficial and efficient for the Client to receive a constant MNP that travels with the Client wherever it moves. For example, this would allow air traffic controllers to easily track aircraft, etc. In other cases, however (e.g., intelligent transportation systems), the Client may be willing to sacrifice a modicum of efficiency in order to have time-varying MNPs that can be changed every so often to defeat adversarial tracking.¶

The prefix delegation services discussed in Section 15.3 allows Clients that desire time-varying MNPs to obtain short-lived prefixes to send RS messages with an HHIT/TLA source address and/or with an OMNI option with DHCPv6 Option sub-options. The Client would then be obligated to renumber its internal networks whenever its MNP (and therefore also its OMNI address) changes. This should not present a challenge for Clients with automated network renumbering services, but may disrupt persistent sessions that would prefer to use a constant address.¶

22. (H)HITs and Temporary ULA (TLA)s

Clients that generate (H)HITs but do not have pre-assigned MNPs can request MNP delegations by issuing IPv6 ND messages that use the (H)HIT instead of a TLA. For example, when a Client creates an RS message it can set the source to a (H)HIT and destination to link-scoped All-Routers multicast. The IPv6 ND message includes an OMNI option with a Node Identification sub-option, then encapsulates the message in an IPv6 header with the (H)HIT as the source address. The Client then sends the message as specified in Section 15.2.¶

When the Hub Proxy/Server receives the RS message, it notes that the source was a (H)HIT, then invokes the DHCPv6 protocol to request an MNP prefix delegation while using the (H)HIT (in the form of a DUID) as the Client Identifier. The Hub Proxy/Server then prepares an RA message with source address set to its own ULA and destination set to the source of the RS message. The Hub Proxy/Server next includes an OMNI option with a Node Identification sub-option and any DHCPv6 prefix delegation parameters. The Proxy/Server finally encapsulates the RA in an OAL header with source address set to its own ULA and destination set to the RS OAL source address, then returns the encapsulated RA to the Client either directly or by way of the FHS Proxy/Server as a proxy.¶

Clients can also use (H)HITs and/or TLAs for direct Client-to-Client communications outside the context of any OMNI link supporting infrastructure. When two Clients encounter one another they can use their (H)HITs and/or TLAs as original IPv6 packet/parcel source and destination addresses to support direct communications. Clients can also inject their (H)HITs and/or TLAs into an IPv6 multihop routing protocol to enable multihop communications as discussed in Section 15.2. Clients can further exchange other IPv6 ND messages using their (H)HITs and/or TLAs as source and destination addresses.¶

Lastly, when Clients are within the coverage range of OMNI link infrastructure a case could be made for injecting (H)HITs and/or TLAs into the global MS routing system. For example, when the Client sends an RS to an FHS Proxy/Server it could include a request to inject the (H)HIT / TLA into the routing system instead of requesting an MNP prefix delegation. This would potentially enable OMNI link-wide communications using only (H)HITs or TLAs, and not MNPs. This document notes the opportunity, but makes no recommendation.¶

23. Address Selection

Clients assign LLAs to the OMNI interface, but do not use LLAs as IPv6 ND message source/destination addresses nor for addressing ordinary original IP packets/parcels exchanged with OMNI link neighbors.¶

Clients use ULA-MNPs as source/destination IPv6 addresses in the encapsulation headers of OAL packets and use XLA-MNPs as the IPv6 source addresses of the IPv6 ND messages themselves. Clients use TLAs when an MNP is not available, or as source/destination IPv6 addresses for communications within a MANET/VANET local area. Clients can also use (H)HITs instead of TLAs for local communications when operation outside the context of a specific ULA domain and/or source address attestation is necessary.¶

Clients use MNP-based GUAs as original IP packet/parcel source and destination addresses for communications with Internet destinations when they are within range of OMNI link supporting infrastructure that can inject the MNP into the routing system. Clients can also use MNP-based GUAs within multihop routing regions that are currently disconnected from infrastructure as long as the corresponding ULA-MNPs have been injected into the routing system.¶

Clients use anycast GUAs as OAL and/or L2 encapsulation destination addresses for RS messages used to discover the nearest FHS Proxy/Server. When the Proxy/Server returns a solicited RA, it must also use the same anycast address as the RA OAL/L2 encapsulation source in order to successfully traverse any NATs in the path. The Client should then immediately transition to using the FHS Proxy/Server's discovered unicast OAL/L2 address as the destination in order to minimize dependence on the Proxy/Server's use of an anycast source address.¶

24. Error Messages

An OAL destination or intermediate node may need to return ICMPv6-like error messages (e.g., Destination Unreachable, Packet Too Big, Time Exceeded, etc.) [RFC4443] to an OAL source. Since ICMPv6 error messages do not themselves include authentication codes, OAL nodes can instead return error messages as an OMNI ICMPv6 Error sub-option in a secured IPv6 ND uNA message.¶

25. IANA Considerations

The following IANA actions are requested in accordance with [RFC8126] and [RFC8726]:¶

   Addresses      Usage                                         Reference
   ---------      -----                                         ---------
   00-52-14       Overlay Multilink Network (OMNI) Interface    [RFCXXXX]

   Code      Name                         Reference
   ---       ----                         ---------
   0         PTB Hard Error               [RFC4443]
   1         PTB Soft Error (loss)        [RFCXXXX]
   2         PTB Soft Error (no loss)     [RFCXXXX]
   3         Parcel Reply                 [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Pad1                           [RFCXXXX]
   1        PadN                           [RFCXXXX]
   2        Node Identification            [RFCXXXX]
   3        Authentication                 [RFCXXXX]
   4        Window Synchronization         [RFCXXXX]
   5        Neighbor Control               [RFCXXXX]
   6        Interface Attributes           [RFCXXXX]
   7        Traffic Selector               [RFCXXXX]
   8        AERO Forwarding Parameters     [RFCXXXX]
   9        Geo Coordinates                [RFCXXXX]
   10       DHCPv6 Message                 [RFCXXXX]
   11       PIM-SM Message                 [RFCXXXX]
   12       HIP Message                    [RFCXXXX]
   13       QUIC-TLS Message               [RFCXXXX]
   14       Fragmentation Report           [RFCXXXX]
   15       ICMPv6 Error                   [RFCXXXX]
   16       Proxy/Server Departure         [RFCXXXX]
   17-29    Unassigned
   30       Sub-Type Extension             [RFCXXXX]
   31       Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        UUID                           [RFCXXXX]
   1        HIT                            [RFCXXXX]
   2        HHIT                           [RFCXXXX]
   3        Network Access Identifier      [RFCXXXX]
   4        FQDN                           [RFCXXXX]
   5        IPv6 Address                   [RFCXXXX]
   6-252    Unassigned                     [RFCXXXX]
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        NULL                           [RFCXXXX]
   1-252    Unassigned                     [RFCXXXX]
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        RFC4380 UDP/IP Header Option   [RFCXXXX]
   1        RFC6081 UDP/IP Trailer Option  [RFCXXXX]
   2-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Origin Indication (IPv4)       [RFC4380]
   1        Authentication Encapsulation   [RFC4380]
   2        Origin Indication (IPv6)       [RFCXXXX]
   3-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Unassigned
   1        Nonce                          [RFC6081]
   2        Unassigned
   3        Alternate Address (IPv4)       [RFC6081]
   4        Neighbor Discovery Option      [RFC6081]
   5        Random Port                    [RFC6081]
   6        Alternate Address (IPv6)       [RFCXXXX]
   7-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

26. Security Considerations

Security considerations for IPv4 [RFC0791], IPv6 [RFC8200] and IPv6 Neighbor Discovery [RFC4861] apply. OMNI interface IPv6 ND messages SHOULD include Nonce and Timestamp options [RFC3971] when transaction confirmation and/or time synchronization is needed.¶

OMNI interfaces configured over secured ANET/ENET interfaces inherit the physical and/or link-layer security properties (i.e., "protected spectrum") of the connected networks. OMNI interfaces configured over open INET interfaces can use symmetric securing services such as VPNs or can by some other means establish a direct link. When a VPN or direct link may be impractical or undesirable, however, security services such as those specified in [RFC7401], [RFC4380], [RFC6234], [RFC9000], etc. can be employed.¶

OMNI link mobility services MUST provide for strong network layer security for control plane messages and forwarding path integrity for data plane messages. For example, the AERO service [I-D.templin-intarea-aero] constructs an SRT spanning tree with Proxy/Servers as leaf nodes and secures the spanning tree links with network layer security mechanisms such as IPsec [RFC4301] or WireGuard [WGD]. Secured control plane messages are then constrained to travel only over secured spanning tree paths and are therefore protected from attack or eavesdropping. Other control and data plane messages can travel over route optimized paths that do not strictly follow the secured spanning tree, therefore end-to-end sessions should employ transport- or higher-layer security services (e.g., TLS/SSL [RFC8446], DTLS [RFC6347], etc.). Additionally, the OAL Identification value can provide a first level of data origin authentication to mitigate off-path spoofing.¶

Identity-based key verification infrastructure services such as iPSK may be necessary for verifying the identities claimed by Clients. This requirement should be harmonized with the manner in which (H)HITs are attested in a given operational environment.¶

Security considerations for specific access network interface types are covered under the corresponding IP-over-(foo) specification (e.g., [RFC2464], [RFC2492], etc.).¶

Security considerations for IPv6 fragmentation and reassembly are discussed in Section 6.14. In environments where spoofing is considered a threat, OMNI nodes SHOULD employ Identification window synchronization and OAL destinations SHOULD configure an (end-system-based) firewall.¶

27. Implementation Status

AERO/OMNI Release-3.2 was tagged on March 30, 2021, and is undergoing internal testing. Additional internal releases expected within the coming months, with first public release expected end of 1H2021.¶

Many AERO/OMNI functions are implemented and undergoing final integration. OAL fragmentation/reassembly buffer management code has been cleared for public release.¶

28. Document Updates

This document does not itself update other RFCs, but suggests that the following could be updated through future IETF initiatives:¶

• [RFC1191]¶
• [RFC2675]¶
• [RFC4291]¶
• [RFC4443]¶
• [RFC8201]¶

Updates can be through, e.g., standards action, the errata process, etc. as appropriate.¶

29. Acknowledgements

The first version of this document was prepared per the consensus decision at the 7th Conference of the International Civil Aviation Organization (ICAO) Working Group-I Mobility Subgroup on March 22, 2019. Consensus to take the document forward to the IETF was reached at the 9th Conference of the Mobility Subgroup on November 22, 2019. Attendees and contributors included: Guray Acar, Danny Bharj, Francois D'Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo, Bernhard Haindl, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg Saccone, Robert Segers, Michal Skorepa, Michel Solery, Stephane Tamalet, Fred Templin, Jean-Marc Vacher, Bela Varkonyi, Tony Whyman, Fryderyk Wrobel and Dongsong Zeng.¶

The following individuals are acknowledged for their useful comments: Amanda Baber, Stuart Card, Donald Eastlake, Adrian Farrel, Michael Matyas, Robert Moskowitz, Madhu Niraula, Greg Saccone, Stephane Tamalet, Eliot Lear, Eduard Vasilenko, Eric Vyncke. Pavel Drasil, Zdenek Jaron and Michal Skorepa are especially recognized for their many helpful ideas and suggestions. Akash Agarwal, Madhuri Madhava Badgandi, Sean Dickson, Don Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron Sackman, Bhargava Raman Sai Prakash and Katherine Tran are acknowledged for their hard work on the implementation and technical insights that led to improvements for the spec.¶

Discussions on the IETF 6man and atn mailing lists during the fall of 2020 suggested additional points to consider. The authors gratefully acknowledge the list members who contributed valuable insights through those discussions. Eric Vyncke and Erik Kline were the intarea ADs, while Bob Hinden and Ole Troan were the 6man WG chairs at the time the document was developed; they are all gratefully acknowledged for their many helpful insights. Many of the ideas in this document have further built on IETF experiences beginning in the 1990s, with insights from colleagues including Ron Bonica, Brian Carpenter, Ralph Droms, Christian Huitema, Thomas Narten, Dave Thaler, Joe Touch, Pascal Thubert, and many others who deserve recognition.¶

Early observations on IP fragmentation performance implications were noted in the 1986 Digital Equipment Corporation (DEC) "qe reset" investigation, where fragment bursts from NFS UDP traffic triggered hardware resets resulting in communication failures. Jeff Chase, Fred Glover and Chet Juzsczak of the Ultrix Engineering Group led the investigation, and determined that setting a smaller NFS mount block size reduced the amount of fragmentation and suppressed the resets. Early observations on L2 media MTU issues were noted in the 1988 DEC FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde represented architectural considerations for FDDI networking in general including FDDI/Ethernet bridging. Jeff Mogul (who led the IETF Path MTU Discovery working group) and other DEC colleagues who supported these early investigations are also acknowledged.¶

Throughout the 1990's and into the 2000's, many colleagues supported and encouraged continuation of the work. Beginning with the DEC Project Sequoia effort at the University of California, Berkeley, then moving to the DEC research lab offices in Palo Alto CA, then to Sterling Software at the NASA Ames Research Center, then to SRI in Menlo Park, CA, then to Nokia in Mountain View, CA and finally to the Boeing Company in 2005 the work saw continuous advancement through the encouragement of many. Those who offered their support and encouragement are gratefully acknowledged.¶

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) Mobility Vision Lab (MVL) program.¶

Appendix A. OAL Checksum Algorithm

The OAL Checksum Algorithm adopts the 8-bit Fletcher algorithm specified in Appendix I of [RFC1146] as also analyzed in [CKSUM]. [RFC6247] declared [RFC1146] historic for the reason that the algorithms had never seen widespread use with TCP, however this document adopts the 8-bit Fletcher algorithm for a different purpose. Quoting from Appendix I of [RFC1146], the OAL Checksum Algorithm proceeds as follows:¶

• "The 8-bit Fletcher Checksum Algorithm is calculated over a sequence of data octets (call them D[1] through D[N]) by maintaining 2 unsigned 1's-complement 8-bit accumulators A and B whose contents are initially zero, and performing the following loop where i ranges from 1 to N:¶

  • A := A + D[i]¶
  • B := B + A¶

  It can be shown that at the end of the loop A will contain the 8-bit 1's complement sum of all octets in the datagram, and that B will contain (N)D[1] + (N-1)D[2] + ... + D[N]."¶

To calculate the OAL checksum, the above algorithm is applied over the N-octet concatenation of the OAL pseudo-header and the encapsulated original IP packet(s)/parcel(s). Specifically, the algorithm is first applied over the 40 octets of the OAL pseudo-header as data octets D[1] through D[40], then continues over the entire length of the original IP packet(s)/parcel(s) as data octets D[41] through D[N].¶

Appendix B. IPv6 ND Message Authentication and Integrity

OMNI interface IPv6 ND messages are subject to authentication and integrity checks at multiple levels. When an OMNI interface sends an IPv6 ND message over an INET interface, it includes an authentication sub-option with a valid signature if necessary and always includes an IPv6 ND message checksum. The OMNI interface that receives the message verifies the OAL checksum as a first-level integrity check, then verifies the IPv6 ND message checksum followed by the authentication signature (if present) to ensure IPv6 ND message integrity and authenticity.¶

When an OMNI interface sends an IPv6 ND message over an underlay interface connected to a secured network, it omits authentication (sub-)options but always calculates/includes an IPv6 ND message checksum beginning with a pseudo-header of the IPv6 header and extending to the end of the IPv6 ND message only with the Checksum field itself set to 0. When an OMNI interface sends an IPv6 ND message over an underlay interface connected to an unsecured network, it first includes an authentication (sub-)option and calculates the signature beginning with the first octet following the IPv6 ND message header Checksum field and extending to the end of the entire packet or super-packet with the authentication signature field set to 0. The OMNI interface next writes the signature into the signature field, then calculates the IPv6 ND message checksum as above.¶

The OMNI interface that receives the message applies any lower-layer authentication and integrity checks, then verifies both the OAL checksum and the IPv6 ND message checksum. If the checksums are correct, the OMNI interface next verifies the authentication signature. The OMNI interface then processes the packet further only if all checksums and authentication signatures were correct.¶

OAL destinations also discard carrier packets with unacceptable Identifications and submit the encapsulated fragments in all others for reassembly. The reassembly algorithm rejects any fragments with unacceptable sizes, offsets, etc. and reassembles all others. Following reassembly, the OAL checksum algorithm provides an integrity assurance layer that compliments any integrity checks already applied by lower layers as well as a first-pass filter for any checks that will be applied later by upper layers.¶

Appendix C. VDL Mode 2 Considerations

ICAO Doc 9776 is the "Technical Manual for VHF Data Link Mode 2" (VDLM2) that specifies an essential radio frequency data link service for aircraft and ground stations in worldwide civil aviation air traffic management. The VDLM2 link type is "multicast capable" [RFC4861], but with considerable differences from common multicast links such as Ethernet and IEEE 802.11.¶

First, the VDLM2 link data rate is only 31.5Kbps - multiple orders of magnitude less than most modern wireless networking gear. Second, due to the low available link bandwidth only VDLM2 ground stations (i.e., and not aircraft) are permitted to send broadcasts, and even so only as compact layer 2 "beacons". Third, aircraft employ the services of ground stations by performing unicast RS/RA exchanges upon receipt of beacons instead of listening for multicast RA messages and/or sending multicast RS messages.¶

This beacon-oriented unicast RS/RA approach is necessary to conserve the already-scarce available link bandwidth. Moreover, since the numbers of beaconing ground stations operating within a given spatial range must be kept as sparse as possible, it would not be feasible to have different classes of ground stations within the same region observing different protocols. It is therefore highly desirable that all ground stations observe a common language of RS/RA as specified in this document.¶

Note that links of this nature may benefit from compression techniques that reduce the bandwidth necessary for conveying the same amount of data. The IETF lpwan working group is considering possible alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].¶

Appendix D. Client-Proxy/Server Isolation Through Link-Layer Address Mapping

Per [RFC4861], IPv6 ND messages may be sent to either a multicast or unicast link-scoped IPv6 destination address. However, IPv6 ND messaging should be coordinated between the Client and Proxy/Server only without invoking other nodes on the underlay network. This implies that Client-Proxy/Server control messaging should be isolated and not overheard by other nodes on the link.¶

To support Client-Proxy/Server isolation on some links, Proxy/Servers can maintain an OMNI-specific unicast link-layer address ("MSADDR"). For Ethernet-compatible links, this specification reserves one Ethernet unicast address TBD5 (see: IANA Considerations). For non-Ethernet statically-addressed links MSADDR is reserved per the assigned numbers authority for the link-layer addressing space. For still other links, MSADDR may be dynamically discovered through other means, e.g., link-layer beacons.¶

Clients map the L3 addresses of all IPv6 ND messages they send (i.e., both multicast and unicast) to MSADDR instead of to an ordinary unicast or multicast link-layer address. In this way, all of the Client's IPv6 ND messages will be received by Proxy/Servers that are configured to accept carrier packets destined to MSADDR. Note that multiple Proxy/Servers on the link could be configured to accept carrier packets destined to MSADDR, e.g., as a basis for supporting redundancy.¶

Therefore, Proxy/Servers must accept and process carrier packets destined to MSADDR, while all other devices must not process carrier packets destined to MSADDR. This model has well-established operational experience in Proxy Mobile IPv6 (PMIP) [RFC5213][RFC6543].¶

Appendix E. Change Log

<< RFC Editor - remove prior to publication >>¶

Differences from earlier versions:¶

• Submit for Intarea Standards Track RFC Publication.¶

Author's Address