Network Working Group F. Templin, Ed.
Internet-Draft The Boeing Company
Intended status: Standards Track A. Whyman
Expires: August 21, 2020 MWA Ltd c/o Inmarsat Global Ltd
February 18, 2020

Transmission of IPv6 Packets over Overlay Multilink Network (OMNI) Interfaces


Mobile nodes (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, mobile enterprise devices, etc.) communicate with networked correspondents over multiple access network data links and configure mobile routers to connect end user networks. A multilink interface specification is therefore needed for coordination with the network-based mobility service. This document specifies the transmission of IPv6 packets over Overlay Multilink Network (OMNI) Interfaces.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on August 21, 2020.

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Table of Contents

1. Introduction

Mobile Nodes (MNs) (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, mobile enterprise devices, etc.) often have multiple data links for communicating with networked correspondents. 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. MNs coordinate their data links in a discipline known as "multilink", in which a single virtual interface is configured over the underlying data link interfaces.

The MN configures a virtual interface (termed the "Overlay Multilink Network (OMNI) interface") as a thin layer over the underlying access network interfaces. The OMNI interface is therefore the only interface abstraction exposed to the IPv6 layer and behaves according to the Non-Broadcast, Multiple Access (NBMA) interface principle, while underlying access network interfaces appear as link layer communication channels in the architecture. The OMNI interface connects to a virtual overlay service known as the "OMNI link". The OMNI link spans a worldwide Internetwork that may include private-use infrastructures and/or the global public Internet itself.

Each MN receives a Mobile Network Prefix (MNP) for numbering downstream-attached End User Networks (EUNs) independently of the access network data links selected for data transport. The MN performs router discovery over the OMNI interface (i.e., similar to IPv6 customer edge routers [RFC7084]) and acts as a mobile router on behalf of its EUNs. The router discovery process is iterated over each of the OMNI interface's underlying access network data links in order to register per-link parameters (see Section 12).

The OMNI interface provides a multilink nexus for exchanging inbound and outbound traffic via the correct underlying Access Network (ANET) interface(s). The IPv6 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) from which OMNI link MNPs are derived. If there are multiple OMNI links, the IPv6 layer will see multiple OMNI interfaces.

The OMNI interface interacts with a network-based Mobility Service (MS) through IPv6 Neighbor Discovery (ND) control message exchanges [RFC4861]. The MS provides Mobility Service Endpoints (MSEs) that track MN movements and represent their MNPs in a global routing or mapping system.

This document specifies the transmission of IPv6 packets [RFC8200] and MN/MS control messaging over OMNI interfaces.

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. Also, 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 defined in [RFC4291] (with Link-Local scope assumed).

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

Mobile Node (MN)

an end system with multiple distinct upstream data link connections that are managed together as a single logical unit. The MN's data link connection parameters can change over time due to, e.g., node mobility, link quality, etc. The MN further connects a downstream-attached End User Network (EUN). The term MN used here is distinct from uses in other documents, and does not imply a particular mobility protocol.
End User Network (EUN)

a simple or complex downstream-attached mobile network that travels with the MN as a single logical unit. The IPv6 addresses assigned to EUN devices remain stable even if the MN's upstream data link connections change.
Mobility Service (MS)

a mobile routing service that tracks MN movements and ensures that MNs remain continuously reachable even across mobility events. Specific MS details are out of scope for this document.
Mobility Service Endpoint (MSE)

an entity in the MS (either singluar or aggregate) that coordinates the mobility events of one or more MN.
Mobility Service Prefix (MSP)

an aggregated IPv6 prefix (e.g., 2001:db8::/32) advertised to the rest of the Internetwork by the MS, and from which more-specific Mobile Network Prefixes (MNPs) are derived.
Mobile Network Prefix (MNP)

a longer IPv6 prefix taken from the MSP (e.g., 2001:db8:1000:2000::/56) and assigned to a MN. MNs sub-delegate the MNP to devices located in EUNs.
Access Network (ANET)

a data link service network (e.g., an aviation radio access network, satellite service provider network, cellular operator network, wifi network, etc.) that connects MNs. Physical and/or data link level security between the MN and ANET are assumed.
Access Router (AR)

a first-hop router in the ANET for connecting MNs to correspondents in outside Internetworks.
ANET interface

a MN's attachment to a link in an ANET.
Internetwork (INET)

a connected network region with a coherent IP addressing plan that provides transit forwarding services for ANET MNs and INET correspondents. Examples include private enterprise networks, ground domain aviation service networks and the global public Internet itself.
INET interface

a node's attachment to a link in an INET.
OMNI link

a virtual overlay configured over one or more INETs and their connected ANETs. An OMNI link can comprise multiple INET segments joined by bridges the same as for any link; the addressing plans in each segment may be mutually exclusive and managed by different administrative entities.
OMNI interface

a node's attachment to an OMNI link, and configured over one or more underlying ANET/INET interfaces.
OMNI link local address (LLA)

an IPv6 link-local address constructed as specified in Section 7, and assigned to an OMNI interface.
OMNI Option

an IPv6 Neighbor Discovery option providing multilink parameters for the OMNI interface as specified in Section 8.

an OMNI interface's manner of managing diverse underlying data link interfaces as a single logical unit. The OMNI interface provides a single unified interface to upper layers, while underlying data link selections are performed on a per-packet basis considering factors such as DSCP, flow label, application policy, signal quality, cost, etc. Multilinking decisions are coordinated in both the outbound (i.e. MN to correspondent) and inbound (i.e., correspondent to MN) directions.

The second layer in the OSI network model. Also known as "layer-2", "link-layer", "sub-IP layer", "data link layer", etc.

The third layer in the OSI network model. Also known as "layer-3", "network-layer", "IPv6 layer", etc.

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 MN virtual interface configured over one or more ANET interfaces, which may be physical (e.g., an aeronautical radio link) or virtual (e.g., an Internet or higher-layer "tunnel"). The MN receives a MNP from the MS, and coordinates with the MS through IPv6 ND message exchanges. The MN uses the MNP to construct a unique OMNI LLA through the algorithmic derivation specified in Section 7 and assigns the LLA to the OMNI interface.

The OMNI interface architectural layering model is the same as in [RFC7847], and augmented as shown in Figure 1. The IP layer (L3) therefore sees the OMNI interface as a single network layer interface with multiple underlying ANET 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 LLA)         |
           Physical         |     +----------------------------+
           Interface        +---->|  L2  |  L2  |       |  L2  |
           Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                  +------+------+       +------+
                                  |  L1  |  L1  |       |  L1  |
                                  |      |      |       |      |
                                  +------+------+       +------+

Figure 1: OMNI Interface Architectural Layering Model

The OMNI virtual interface model gives rise to a number of opportunities:


Other opportunities are discussed in

Figure 2 depicts the architectural model for a MN connecting to the MS via multiple independent ANETs. When an ANET interface becomes active, the MN's OMNI interface sends native (i.e., unencapsulated) IPv6 ND messages via the underlying ANET interface. IPv6 ND messages traverse the ground domain ANETs until they reach an Access Router (AR#1, AR#2, .., AR#n). The AR then coordinates with a Mobility Service Endpoint (MSE#1, MSE#2, ..., MSE#m) in the INET and returns an IPv6 ND message response to the MN. IPv6 ND messages traverse the ANET at layer 2; hence, the Hop Limit is not decremented.

                        |      MN      |
                        |OMNI interface|
               +--------|IF#1|IF#2|IF#n|------ +
              /         +----+----+----+        \
             /                 |                 \
            /    <---- Native  |  IP ---->        \
           v                   v                   v
        (:::)-.              (:::)-.              (:::)-.
   .-(::ANET:::)        .-(::ANET:::)        .-(::ANET:::)
     `-(::::)-'           `-(::::)-'           `-(::::)-'
       +----+               +----+               +----+
 ...   |AR#1|  ..........   |AR#2|   .........   |AR#n|  ...
.      +-|--+               +-|--+               +-|--+     .
.        |                    |                    |
.        v                    v                    v        .
.               <-----  Encapsulation ----->                .
.                                                           .
.      +-----+               (:::)-.                        .
.      |MSE#2|           .-(::::::::)          +-----+      .
.      +-----+       .-(:::   INET  :::)-.     |MSE#m|      .
.                  (:::::    Routing  ::::)    +-----+      .
.                     `-(::: System :::)-'                  .
.  +-----+                `-(:::::::-'                      .
.  |MSE#1|          +-----+               +-----+           .
.  +-----+          |MSE#3|               |MSE#4|           .
.                   +-----+               +-----+           .
.                                                           .
.                                                           .
.       <----- Worldwide Connected Internetwork ---->       .

Figure 2: MN/MS Coordination via Multiple ANETs

After the initial IPv6 ND message exchange, the MN can send and receive unencapsulated IPv6 data packets over the OMNI interface. OMNI interface multilink services will forward the packets via ARs in the correct underlying ANETs. The AR encapsulates the packets according to the capabilities provided by the MS and forwards them to the next hop within the worldwide connected Internetwork via optimal routes.

5. Maximum Transmission Unit (MTU) and Fragmentation

All IPv6 interfaces are REQUIRED to configure a minimum Maximum Transmission Unit (MTU) of 1280 bytes [RFC8200]. The network therefore MUST forward packets of at least 1280 bytes without generating an IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message [RFC8201].

The OMNI interface configures an MTU of 9180 bytes [RFC2492]; the size is therefore not a reflection of the underlying ANET interface MTUs, but rather determines the largest packet the OMNI interface can forward or reassemble.

The OMNI interface can employ link-layer IPv6 encapsulation and fragmentation/reassembly per [RFC2473], but its use is OPTIONAL since correct operation will result in either case. Implementations that omit link-layer IPv6 fragmentation/reassembly may be more prone to dropping large packets and returning a PTB, while those that include it may see improved performance at the expense of including additional code. In both cases, OMNI interface neighbors are responsible for advertising their willingness to reassemble.

The OMNI interface returns internally-generated PTB messages for packets admitted into the interface that it deems too large for the outbound underlying ANET interface (e.g., according to ANET performance characteristics, MTU, etc). For all other packets, the OMNI interface performs PMTUD even if the destination appears to be on the same link since a proxy on the path could return a PTB message. This ensures that the path MTU is adaptive and reflects the current path used for a given data flow.

The MN's OMNI interface forwards packets that are no larger than the MTU of the selected underlying ANET interface according to the ANET L2 frame format. When the OMNI interface forwards a packet that is larger than the ANET interface MTU, it drops the packet and returns a PTB if the AR is not willing to reassemble.

Otherwise, the OMNI interface encapsulates the packet in an IPv6 header with source address set to the MN's link-local address and destination address set to the link-local address of the MSE (see: Section 7). The OMNI interface then uses IPv6 fragmentation to break the encapsulated packet into fragments that are no larger than the ANET interface MTU and sends the fragments over the ANET where they will be intercepted by the AR. The AR then reassembles and conveys the packet toward the final destination.

When an AR receives a fragmented or whole packet from the INET destined to an ANET MN, it first determines whether to forward or drop and return a PTB. If the AR deems the packet to be of acceptable size, it first reassembles locally (if necessary) then forwards the packet to the MN. If the (reassembled) packet is no larger than the ANET MTU, the AR forwards according to the ANET L2 frame format. If the packet is larger than the ANET MTU, the AR instead uses link-layer IPv6 encapsulation and fragmentation as above if the MN accepts fragments or drops and returns a PTB otherwise. The MN then reassembles and discards the encapsulation header, then forwards the whole packet to the final destination.

Applications that cannot tolerate loss due to MTU restrictions SHOULD avoid sending packets larger than 1280 bytes, since dynamic path changes can reduce the path MTU at any time. Applications that may benefit from sending larger packets even though the path MTU may change dynamically MAY use larger sizes (i.e., up to the OMNI interface MTU).

Note that when the AR forwards a fragmented packet received from the INET, it is imperative that the AR reassembles locally first instead of blindly forwarding fragments directly to the MN to avoid attacks such as tiny fragments, overlapping fragments, etc.

6. Frame Format

The OMNI interface transmits IPv6 packets according to the native frame format of each underlying ANET 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 tunnels over IPv6 the frame format is specified in [RFC2473], etc.

7. Link-Local Addresses

OMNI interfaces assign IPv6 Link-Local Addresses (i.e., "OMNI LLAs") using the following constructs:

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 above OMNI LLA constructs.

Since MN OMNI LLAs are based on the distribution of administratively assured unique MNPs, and since MSE OMNI LLAs are guaranteed unique through administrative assignment, OMNI interfaces set the autoconfiguration variable DupAddrDetectTransmits to 0 [RFC4862].

8. Address Mapping - Unicast

OMNI interfaces maintain a neighbor cache for tracking per-neighbor state and use the link-local address format specified in Section 7. IPv6 Neighbor Discovery (ND) [RFC4861] messages on MN OMNI interfaces observe the native Source/Target Link-Layer Address Option (S/TLLAO) formats of the underlying ANET interfaces (e.g., for Ethernet the S/TLLAO is specified in [RFC2464]).

MNs such as aircraft typically have many 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 ANET interfaces in a single IPv6 ND message exchange.

OMNI interfaces use an IPv6 ND option called the "OMNI option" formatted as shown in Figure 3:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |      Type     |     Length    | Prefix Length |R|P|A| Reserved|
     |                                                               |
     ~                          Sub-Options                          ~
     |                                                               |

Figure 3: OMNI Option Format

8.1. Sub-Options

      0                   1                   2  
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5  
     |   Sub-Type    |  Sub-length   | Sub-Option Data ...  

Figure 4: Sub-Option Format

     Option Name            Sub-Type
     Pad1                        0
     PadN                        1
     ifIndex-tuple (Type 1)      2
     ifIndex-tuple (Type 2)      3
     Notification ID             4

Figure 5

The OMNI option includes zero or more Sub-Options, some of which may appear multiple times in the same message. Each consecutive Sub-Option is concatenated immediately after its predecessor. All Sub-Options except Pad1 (see below) are type-length-value (TLV) encoded in the following format:

During processing, unrecognized Sub-Options are ignored and the next Sub-Option processed until the end of the OMNI option.

The following Sub-Option types and formats are defined in this document:

8.1.1. Pad1

      0 1 2 3 4 5 6 7 
     |   Sub-Type=0  |

Figure 6: Pad1

8.1.2. PadN

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     |   Sub-Type=1  |Sub-length=N-2 | N-2 padding bytes ...

Figure 7: PadN

8.1.3. ifIndex-tuple (Type 1)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
     |  Provider ID  | Link  |S|I|RSV| Bitmap(0)=0xff|P00|P01|P02|P03|
     |P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31| Bitmap(1)=0xff|
     |P32|P33|P34|P35|P36|P37|P38|P39| ...

Figure 8: ifIndex-tuple (Type 1)

     |   Index=i     |   Bitmap(i)   |P[*] values ...

Figure 9

8.1.4. ifIndex-tuple (Type 2)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=3  | Sub-length=4+N|    ifIndex    |    ifType     |
     |  Provider ID  | Link  |S|Resvd|                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
     ~                                                               ~
     ~                RFC 6088 Format Traffic Selector               ~
     ~                                                               ~

Figure 10: ifIndex-tuple (Type 2)

8.1.5. Notification ID

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=4  | Sub-length=4  | Notification ID (bits 0 - 15) |
     | Notification ID (bits 16 - 32)|

Figure 11: Notification ID

9. Address Mapping - Multicast

The multicast address mapping of the native underlying ANET interface applies. The mobile router on board the aircraft also serves as an IGMP/MLD Proxy for its EUNs and/or hosted applications per [RFC4605] while using the L2 address of the router as the L2 address for all multicast packets.

10. Address Mapping for IPv6 Neighbor Discovery Messages

Per [RFC4861], IPv6 ND messages may be sent to either a multicast or unicast link-scoped IPv6 destination address. However, IPv6 ND messaging is coordinated between the MN and MS only without invoking other nodes on the ANET.

For this reason, ANET links maintain unicast L2 addresses ("MSADDR") for the purpose of supporting MN/MS IPv6 ND messaging. For Ethernet-compatible ANETs, this specification reserves one Ethernet unicast address TBD2. For non-Ethernet statically-addressed ANETs, MSADDR is reserved per the assigned numbers authority for the ANET addressing space. For still other ANETs, MSADDR may be dynamically discovered through other means, e.g., L2 beacons.

MNs map the L3 addresses of all IPv6 ND messages they send (i.e., both multicast and unicast) to an MSADDR instead of to an ordinary unicast or multicast L2 address. In this way, all of the MN's IPv6 ND messages will be received by MS devices that are configured to accept packets destined to MSADDR. Note that multiple MS devices on the link could be configured to accept packets destined to MSADDR, e.g., as a basis for supporting redundancy.

Therefore, ARs MUST accept and process packets destined to MSADDR, while all other devices MUST NOT process packets destined to MSADDR. This model has well-established operational experience in Proxy Mobile IPv6 (PMIP) [RFC5213][RFC6543].

11. Conceptual Sending Algorithm

The MN's IPv6 layer selects the outbound OMNI interface according to standard IPv6 requirements when forwarding data packets from local or EUN applications to external correspondents. The OMNI interface maintains default routes and neighbor cache entries for MSEs, and may also include additional neighbor cache entries created through other means (e.g., Address Resolution, static configuration, etc.).

After a packet enters the OMNI interface, an outbound ANET interface is selected based on multilink parameters such as DSCP, application port number, cost, performance, message size, etc. OMNI interface multilink selections could also be configured to perform replication across multiple ANET interfaces for increased reliability at the expense of packet duplication.

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

11.1. Multiple OMNI Interfaces

MNs may associate with multiple MS instances concurrently. Each MS instance represents a distinct OMNI link distinguished by its associated MSPs. The MN configures a separate OMNI interface for each link so that multiple interfaces (e.g., omni0, omni1, omni2, etc.) are exposed to the IPv6 layer.

Depending on local policy and configuration, an MN may choose between alternative active OMNI interfaces using a packet's DSCP, routing information or static configuration. Interface selection based on per-packet source addresses is also enabled when the MSPs for each OMNI interface are known (e.g., discovered through Prefix Information Options (PIOs) and/or Route Information Options (RIOs)).

Each OMNI interface can be configured over the same or different sets of ANET interfaces. Each ANET distinguishes between the different OMNI links based on the MSPs represented in per-packet IPv6 addresses.

Multiple distinct OMNI links can therefore be used to support fault tolerance, load balancing, reliability, etc. The architectural model parallels Layer 2 Virtual Local Area Networks (VLANs), where the MSPs serve as (virtual) VLAN tags.

12. Router Discovery and Prefix Registration

ARs process IPv6 ND messages destined to All-Routers multicast (ff02::2), Subnet-Router anycast (fe80::) and unicast IPv6 LLAs [RFC4291]. ARs configure the L2 address MSADDR (see: Section 10) and act as a proxy for MSE OMNI LLAs.

MNs interface with the MS by sending RS messages with OMNI options. For each ANET interface, the MN sends an RS message with an OMNI option, with L2 destination address set to MSADDR and with L3 destination address set to either a specific MSE OMNI LLA, link-local Subnet-Router anycast, or All-Routers multicast. The MN discovers MSE OMNI LLAs either through an RA message response to an initial anycast/multicast RS or before sending an initial RS message. [RFC5214] provides example MSE address discovery methods, including information conveyed during data link login, name service lookups, static configuration, etc.

The AR receives the RS messages and coordinates with the corresponding MSE in a manner outside the scope of this document. The AR returns an RA message with source address set to the MSE OMNI LLA, with an OMNI option and with any information for the link that would normally be delivered in a solicited RA message. (Note that if all MSEs share common state, the AR can instead return an RA with source address set to link-local Subnet-Router anycast.)

MNs configure OMNI interfaces that observe the properties discussed in the previous section. The OMNI interface and its underlying 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 underlying interfaces. When a first underlying interface transitions to UP, the OMNI interface also transitions to UP. When all underlying interfaces transition to DOWN, the OMNI interface also transitions to DOWN.

When an OMNI interface transitions to UP, the MN sends initial RS messages to register its MNP and an initial set of underlying ANET interfaces that are also UP. The MN sends additional RS messages to refresh lifetimes and to register/deregister underlying ANET interfaces as they transition to UP or DOWN.

ARs return RA messages with configuration information in response to a MN's RS messages. The AR sets the RA Cur Hop Limit, M and O flags, Router Lifetime, Reachable Time and Retrans Timer values as directed by the MSE, and includes any necessary options such as:

The AR coordinates with the MSE and sends immediate unicast RA responses without delay; therefore, the IPv6 ND MAX_RA_DELAY_TIME and MIN_DELAY_BETWEEN_RAS constants for multicast RAs do not apply. The AR MAY send periodic and/or event-driven unsolicited RA messages, but is not required to do so for unicast advertisements [RFC4861].

The MN sends RS messages from within the OMNI interface while using an UP underlying ANET interface as the outbound interface. Each RS message is formatted as though it originated from the IPv6 layer, but the process is coordinated wholly from within the OMNI interface and is therefore opaque to the IPv6 layer. The MN sends initial RS messages over an UP underlying interface with its OMNI LLA as the source and with destination set as discussed above. The RS messages include an OMNI option per Section 8 with a valid Prefix Length, (R,P,A) flags, and with ifIndex-tuples appropriate for underlying ANET interfaces. The AR processes RS message and conveys the OMNI option information to the MSE.

When the MSE processes the OMNI information, it first validates the prefix registration information. If the prefix registration was valid, the MSE injects the MNP into the routing/mapping system then caches the new Prefix Length, MNP and ifIndex-tuples. If the MN's OMNI option included one or more Notification IDs, the new MSE also notifies the former MSE(s). The MSE then directs the AR to return an RA message to the MN with an OMNI option per Section 8 and with a non-zero Router Lifetime if the prefix registration was successful; otherwise, with a zero Router Lifetime.

When the MN receives the RA message, it creates a default route with L3 next hop address set to the address found in the RA source address and with L2 address set to MSADDR. The AR will then forward packets between the MN and the MS.

The MN then manages its underlying ANET interfaces according to their states as follows:

The MN 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 multiple UP ANET interfaces, the MN declares this MSE unreachable and tries a different MSE.

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 that is consistent with the information received from the RAs generated by the MS.

Whether the OMNI interface IPv6 ND messaging process is initiated from the receipt of an RS message from the IPv6 layer is an implementation matter. Some implementations may elect to defer the IPv6 ND messaging process until an RS is received from the IPv6 layer, while others may elect to initiate the process proactively.

Note: The Router Lifetime value in RA messages indicates the time before which the MN must send another RS message over this underlying 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). For this reason, the MN should select a primary AR, which is responsible for keeping the MS prefix registration alive on the MN's behalf. If the MN does not select a primary, then it must perform more frequent RS/RA exchanges on its own behalf to refresh the MS prefix registration lifetime.

13. AR and MSE Resilience

ANETs SHOULD deploy ARs in Virtual Router Redundancy Protocol (VRRP) [RFC5798] configurations so that service continuity is maintained even if one or more ARs fail. Using VRRP, the MN is unaware which of the (redundant) ARs 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.

MSEs 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.

14. Detecting and Responding to MSE Failures

In environments where fast recovery from MSE failure is required, ARs SHOULD use proactive Neighbor Unreachability Detection (NUD) in a manner that parallels Bidirectional Forwarding Detection (BFD) [RFC5880] to track MSE reachability. ARs 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 aeronautical radio links) and can therefore be tuned for rapid response.

ARs perform proactive NUD for MSEs for which there are currently active ANET MNs. If an MSE fails, ARs can quickly inform MNs of the outage by sending multicast RA messages on the ANET interface. The AR sends RA messages to the MN via the ANET interface with source address set to the MSEs OMNI LLA, destination address set to All-Nodes multicast (ff02::1) [RFC4291], and Router Lifetime set to 0.

The AR SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays [RFC4861]. Any MNs on the ANET interface that have been using the (now defunct) MSE will receive the RA messages and associate with a new MSE.

15. IANA Considerations

The IANA is instructed to allocate an official Type number TBD from the registry "IPv6 Neighbor Discovery Option Formats" for the OMNI option. Implementations set Type to 253 as an interim value [RFC4727].

The OMNI option also defines an 8-bit Sub-Type field, for which IANA is instructed to create and maintain a new registry entitled "OMNI option Sub-Type values". Initial values for the OMNI option Sub-Type values registry are given below; future assignments are to be made through Expert Review [RFC8126].

   Value    Sub-Type name              Reference  
   -----    -------------              ----------  
   0        Pad1                       [RFCXXXX]  
   1        PadN                       [RFCXXXX]  
   2        ifIndex-tuple (Type 1)     [RFCXXXX]
   3        ifIndex-tuple (Type 2)     [RFCXXXX]  
   4        Notification ID            [RFCXXXX]  
   5-252    Unassigned  
   253-254  Experimental               [RFCXXXX]  
   255      Reserved                   [RFCXXXX]

Figure 12: OMNI Option Sub-Type Values

The IANA is instructed to allocate one Ethernet unicast address TBD2 (suggest 00-00-5E-00-52-14 [RFC5214]) in the registry "IANA Ethernet Address Block - Unicast Use".

16. Security Considerations

Security considerations for IPv6 [RFC8200] and IPv6 Neighbor Discovery [RFC4861] apply. OMNI interface IPv6 ND messages SHOULD include Nonce and Timestamp options [RFC3971] when synchronized transaction confirmation is needed.

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

17. 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: Michael Matyas, Madhu Niraula, Greg Saccone, Stephane Tamalet, Eric Vyncke. Pavel Drasil, Zdenek Jaron and Michal Skorepa are recognized for their many helpful ideas and suggestions.

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.

18. References

18.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998.
[RFC3971] Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971, March 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, DOI 10.17487/RFC4727, November 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007.
[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007.
[RFC6088] Tsirtsis, G., Giarreta, G., Soliman, H. and N. Montavont, "Traffic Selectors for Flow Bindings", RFC 6088, DOI 10.17487/RFC6088, January 2011.
[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by Hosts in a Multi-Prefix Network", RFC 8028, DOI 10.17487/RFC8028, November 2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.
[RFC8201] McCann, J., Deering, S., Mogul, J. and R. Hinden, "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017.

18.2. Informative References

[RFC2225] Laubach, M. and J. Halpern, "Classical IP and ARP over ATM", RFC 2225, DOI 10.17487/RFC2225, April 1998.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998.
[RFC2492] Armitage, G., Schulter, P. and M. Jork, "IPv6 over ATM Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999.
[RFC2863] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, DOI 10.17487/RFC3692, January 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004.
[RFC4605] Fenner, B., He, H., Haberman, B. and H. Sandick, "Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, August 2006.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K. and B. Patil, "Proxy Mobile IPv6", RFC 5213, DOI 10.17487/RFC5213, August 2008.
[RFC5214] Templin, F., Gleeson, T. and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, DOI 10.17487/RFC5214, March 2008.
[RFC5798] Nadas, S., "Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 5798, DOI 10.17487/RFC5798, March 2010.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010.
[RFC6543] Gundavelli, S., "Reserved IPv6 Interface Identifier for Proxy Mobile IPv6", RFC 6543, DOI 10.17487/RFC6543, May 2012.
[RFC7084] Singh, H., Beebee, W., Donley, C. and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, DOI 10.17487/RFC7084, November 2013.
[RFC7421] Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu, A. and A. Yourtchenko, "Analysis of the 64-bit Boundary in IPv6 Addressing", RFC 7421, DOI 10.17487/RFC7421, January 2015.
[RFC7847] Melia, T. and S. Gundavelli, "Logical-Interface Support for IP Hosts with Multi-Access Support", RFC 7847, DOI 10.17487/RFC7847, May 2016.
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017.

Appendix A. Type 1 ifIndex-tuple Traffic Classifier Preference Encoding

Adaptation of the OMNI option Type 1 ifIndex-tuple's traffic classifier bitmap to specific Internetworks such as the Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS) may include link selection preferences based on other traffic classifiers (e.g., transport port numbers, etc.) in addition to the existing DSCP-based preferences. Nodes on specific Internetworks maintain a map of traffic classifiers to additional P[*] preference fields beyond the first 64. For example, TCP port 22 maps to P[67], TCP port 443 maps to P[70], UDP port 8060 maps to P[76], etc.

Implementations use Simplex or Indexed encoding formats for P[*] encoding in order to encode a given set of traffic classifiers in the most efficient way. Some use cases may be more efficiently coded using Simplex form, while others may be more efficient using Indexed. Once a format is selected for preparation of a single ifIndex-tuple the same format must be used for the entire Sub-Option. Different Sub-Options may use different formats.

The following figures show coding examples for various Simplex and Indexed formats:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
     |  Provider ID  | Link  |S|0|RSV| Bitmap(0)=0xff|P00|P01|P02|P03|
     |P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31| Bitmap(1)=0xff|
     | Bitmap(2)=0xff|P64|P65|P67|P68| ...

Figure 13: Example 1: Dense Simplex Encoding

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
     |  Provider ID  | Link  |S|0|RSV| Bitmap(0)=0x00| Bitmap(1)=0x0f|
     | Bitmap(2)=0x00| Bitmap(3)=0x00| Bitmap(4)=0x00| Bitmap(5)=0x00|
     | Bitmap(6)=0xf0|192|193|194|195|196|197|198|199|200|201|202|203|
     |204|205|206|207| Bitmap(7)=0x00| Bitmap(8)=0x0f|272|273|274|275|
     |276|277|278|279|280|281|282|283|284|285|286|287| Bitmap(9)=0x00|
     |Bitmap(10)=0x00| ...

Figure 14: Example 2: Sparse Simplex Encoding

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
     |  Provider ID  | Link  |S|1|RSV|  Index = 0x00 | Bitmap = 0x80 |
     |P00|P01|P02|P03|  Index = 0x01 | Bitmap = 0x01 |P60|P61|P62|P63|
     |  Index = 0x10 | Bitmap = 0x80 |512|513|514|515|  Index = 0x18 |
     | Bitmap = 0x01 |796|797|798|799| ...

Figure 15: Example 3: Indexed Encoding

Appendix B. Prefix Length Considerations

The 64-bit boundary in IPv6 addresses [RFC7421] determines the MN OMNI LLA format for encoding the most-significant 64 MNP bits into the least-significant 64 bits of the prefix fe80::/64 as discussed in Section 7.

[RFC4291] defines the link-local address format as the most significant 10 bits of the prefix fe80::/10, followed by 54 unused bits, followed by the least-significant 64 bits of the address. If the 64-bit boundary is relaxed through future standards activity, then the 54 unused bits can be employed for extended coding of MNPs of length /65 up to /118.

The extended coding format would continue to encode MNP bits 0-63 in bits 64-127 of the OMNI LLA, while including MNP bits 64-117 in bits 10-63. For example, the OMNI LLA corresponding to the MNP 2001:db8:1111:2222:3333:4444:5555::/112 would be fe8c:ccd1:1115:5540:2001:db8:1111:2222, and would still be a valid IPv6 LLA per [RFC4291].

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: [].

Appendix D. Change Log

<< RFC Editor - remove prior to publication >>

Differences from draft-templin-atn-aero-interface-21 to draft-templin-6man-omni-interface-00:

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First draft version (draft-templin-atn-aero-interface-00):

Authors' Addresses

Fred L. Templin (editor) The Boeing Company P.O. Box 3707 Seattle, WA 98124 USA EMail:
Tony Whyman MWA Ltd c/o Inmarsat Global Ltd 99 City Road London, EC1Y 1AX England EMail: