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I-D Internet-Draft Mobile nodes (e.g., aircraft of various configurations) act as mobile routers for their on-board networks, and may have multiple data links for communicating with networked correspondents. Mobile nodes configure a virtual interface (termed the "AERO interface") as a thin layer over their underlying data link interfaces. This document specifies the transmission of IPv6 packets over AERO interfaces.

Mobile Nodes (MNs) such as aircraft of various configurations may have multiple data links for communicating with networked correspondents. These data links often have differing performance, cost and availability characteristics that can change dynamically according to mobility patterns, flight phases, proximity to infrastructure, etc. Each MN receives an IPv6 Mobile Network Prefix (MNP) that can be used by on-board networks regardless of the actual link or links selected for data transport. The MN acts as a mobile router on behalf of its on-board networks, but appears as a multi-addressed host from the perspective of off-board correspondents. This implies the need for a virtual interface (termed the "AERO interface") configured as a thin layer over the underlying data link interfaces. The AERO interface is therefore the only interface abstraction exposed to the IPv6 layer, and behaves according to the Non-Broadcast, Multiple Access (NBMA) interface principle. This document specifies the transmission of IPv6 packets over AERO interfaces.

The terminology in the normative references applies; especially, the terms "link" and "interface" are the same as defined in the IPv6 and IPv6 Neighbor Discovery (ND) specifications. The following terms are defined within the scope of this document: a connected network region that has a coherent IP addressing plan and is either physically isolated or separated from other networks by packet filtering border routers. Examples include private enterprise networks, aviation networks and the global public Internet itself. a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured over an underlying Internetwork. Nodes on the AERO link appear as single-hop neighbors from the perspective of the virtual overlay even though they may be separated by many underlying Internetwork hops. An AERO link may comprise multiple segments joined by bridges the same as for any link; the underlying Internetwork addressing plans in each segment may be mutually exclusive and managed by different administrative entities. a node's attachment to an AERO link, and configured over one or more underlying interfaces a node with an AERO interface attached to an AERO link. an IPv6 link-local address constructed as specified in . a link that connects an AERO node to the underlying Internetwork. an AERO node's interface point of attachment to an underlying link.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Lower case uses of these words are not to be interpreted as carrying RFC2119 significance.

An AERO interface is a MN's virtual interface configured over one or more underlying links, which may be physical (e.g., an Ethernet) or virtual (e.g., an Internet or higher-layer "tunnel"). The MN discovers routers on the AERO link through Router Solicitation (RS) / Router Advertisement (RA) message exchanges. The AERO interface architectural layering model is the same as in , and reproduced here (in an augmented form) as shown in . The AERO interface is therefore a single network-layer interface with multiple link-layer addresses.

| | Address Binding | +----------------------------+ +---->| IPv6 | IP Address +---->| | Binding | +----------------------------+ +---->| AERO Interface | Logical-to- +---->| (AERO Address) | Physical | +----------------------------+ Interface +---->| L2 | L2 | | L2 | Binding |(IF#1)|(IF#2)| ..... |(IF#n)| +------+------+ +------+ | L1 | L1 | | L1 | | | | | | +------+------+ +------+ ]]>

The AERO interface Maximum Transmission Unit (MTU) is derived from the underlying interface MTUs and set to a value that ensures that the MTU for each underlying interface is respected. The AERO interface MTU may be common to all data flows or differ between data flows. Regardless of the strategy by which the MTU is determined, the AERO link administrative authority should configure routers to advertise a conservative MTU for all nodes noting that fragmentation should be avoided if possible. In common practice, there may be additional encapsulation headers inserted by various forms of Layer 2 tunnels on the path to an on-link neighbor. Such tunnels SHOULD be instrumented to accommodate the native MTU of the underlying interface, but in some cases it may be prudent to reduce the size of the underlying interface MTU to allow room for L2 encapsulation. Especially for underlying links with low-end performance characteristics, it is imperative that packets that successfully traverse the underlying link are not dropped in the network due to a size restriction. In a preferred approach, the AERO interface MTU should be set to a value no smaller than the largest MTU among all underlying interfaces. The AERO interface itself then MUST return locally-generated ICMPv6 "Packet Too Big" messages for packets that are too large to traverse the selected underlying interface in one piece. This ensures that the MTU is adaptive and reflects the underlying interface used for a given data flow. Alternatively, the AERO interface MTU may be determined as the minimum MTU among all underlying interfaces. However, this may result in under-utilization of robust underlying interfaces after a low-end underlying interface has degraded the common minimum MTU. For example, if the underlying interfaces have MTUs 1500, 1472 and 1400, then the minimum AERO interface MTU is 1400. If any underlying interface has an MTU smaller than 1280, the AERO interface MUST either perform IPv6 fragmentation when using this interface or disable the underlying interface. The MTU for an underlying interface is normally determined from information provided either statically or dynamically when the interface becomes active. If an underlying interface MTU dynamically reports an MTU smaller than any minimum MTU already determined then the AERO interface MUST either perform IPv6 fragmentation when using this interface, or disable the underlying interface. The AERO interface MAY also receive an RA with an MTU option. If the advertised MTU is no larger than 1500, the AERO interface MTU is set to the new value and the AERO interface MUST either perform IPv6 fragmentation over any underlying interface having a smaller MTU or disable the underlying interface. If the advertised MTU is larger than 1500, the AERO interface sets the new value and disables any underlying interface having a smaller MTU instead of fragmenting, since IPv6 destinations are not required to reassemble more than 1500 bytes.

AERO interfaces transmit IPv6 packets according to the frame format of the underlying interface. For example, for an Ethernet interface the frame format is exactly as specified in , for an IPv6 tunnel over IPv4 the frame format is exactly as specified in , etc.

A MN's AERO address is an IPv6 link-local address with an interface identifier based on its assigned MNP. AERO addresses begin with the prefix fe80::/64 followed by a 64-bit prefix taken from the MNP. For example, for the MNP: 2001:db8:1000:2000::/56 the corresponding AERO addresses are: fe80::2001:db8:1000:2000 fe80::2001:db8:1000:2001 fe80::2001:db8:1000:2002 ... etc. ... fe80::2001:db8:1000:20ff When the MN configures AERO addresses from its MNP, the lowest-numbered AERO address serves as the "base" address (for example, for the MNP 2001:db8:1000:2000::/56 the base AERO address is fe80::2001:db8:1000:2000). MNs and routers use the base address for the purpose of maintaining neighbor cache entries, but the MN accepts packets destined to all AERO addresses as equivalent. A router's AERO address is allocated from the range fe80::/96, and MUST be managed for uniqueness by the AERO link administrative authority. The lower 32 bits of the AERO address includes a unique integer value, e.g., fe80::1, fe80::2, fe80::3, etc. The address fe80:: is reserved as the IPv6 link-local Subnet Router Anycast address , and the address fe80::ffff:ffff is reserved as the unspecified AERO address; hence, these values are not available for general assignment. For multi-segment AERO links, the routers of each segment MUST assign AERO addresses that are unique among all routers on the (collective) link. Although the address assignment policy is completely at the discretion of the AERO link administrative authority, a useful technique may be to assign a different aggregated portion of the fe80::/96 prefix to each segment, e.g., fe80::/120, fe80::0100/120, fe80::0200/120, etc.

AERO interfaces maintain a neighbor cache for tracking per-neighbor state the same as for any interface. AERO interfaces use standard IPv6 Neighbor Discovery (ND) messages including Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA) and Redirect. AERO interface ND messages may include zero or more Source/Target Link-Layer Address Options (S/TLLAOs) formatted as shown in :

In this format: Type is set to '1' for SLLAO or '2' for TLLAO. Length is set to the constant value '5' (i.e., 5 units of 8 octets). Prefix Length is set to the MNP prefix length for the AERO address found in the source (RS), destination (RA) or target (NA) address of an ND message used for the purpose of asserting an MNP; otherwise set to 0. If the message contains multiple SLLAOs, only the Prefix Length value in the first SLLAO is consulted and the values in other SLLAOs are ignored. For RS messages, the router creates a neighbor cache entry and announces the MNP in the routing system, then returns an RA with Router Lifetime set to the MNP assertion lifetime. R (the "Release" bit) is set to '1' in the SLLAO of an RS message sent for the purpose of withdrawing an MNP; otherwise, set to '0'. If the message contains multiple SLLAOs, only the R value in the first SLLAO is consulted and the values in other SLLAOs are ignored. The router withdraws the MNP, then returns an RA with Router Lifetime set to '0'. D (the "Disable" bit) is set to '1' in the S/TLLAOs of an RS/NA message for each Interface ID that is to be disabled in the recipient's neighbor cache entry; otherwise, set to '0'. If the message contains an S/TLLAO with D=1 and Interface ID 255, the node disables the entire neighbor cache entry. If the message contains multiple S/TLLAOs the D value in each S/TLLAO is consulted. X (the "proXy" bit) is set to '1' in the SLLAO of an RS/RA message when there is a proxy in the path; otherwise, set to '0'. If the message contains multiple SLLAOs, only the X value in the first SLLAO is consulted and the values in other SLLAOs are ignored. T (the "Translator" bit) is set to '1' in the SLLAO of an RA message if there is a link-layer address translator in the path; otherwise, set to '0'. If the message contains multiple SLLAOs, only the T value in the first SLLAO is consulted and the values in other SLLAOs are ignored. Resvd is set to the value '0' on transmission and ignored on receipt. Interface ID is set to a 16-bit integer value corresponding to a specific underlying interface. Once the MN has assigned an Interface ID to an underlying interface, the assignment MUST remain unchanged until the MN disables the AERO interface. The value '255' is reserved as the router Interface ID, i.e., routers MUST use Interface ID '255', and MNs MUST number their Interface IDs with values in the range of 0-254. Port Number and Link-Layer Address are set to the addresses assigned to the underlying interface, or to '0' when the addresses are left unspecified. For transmission over physical interfaces such as Ethernet, the Link-Layer Address is set to the same format as in the appropriate interface specification (e.g., IPv6 over Ethernet ) beginning with the lowest-numbered byte of the field and ending in trailing null padding to a total of 16 bytes. For transmission over tunnel interfaces, the Link-Layer address is set to an IPv6 address for IPv6 encapsulation or an IPv4-mapped IPv6 address for IPv4 encapsulation. When TCP or UDP are used as part of the encapsulation, Port Number is set to the encapsulation protocol port number; otherwise, set to '0'. P(i) is a set of Preferences that correspond to the 64 Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0' ("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to indicate a QoS preference level for underlying interface selection purposes. 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. From the perspective of ND, the AERO interface would therefore appear to have multiple link-layer addresses. In that case, ND messages MAY include multiple S/TLLAOs -- each with an Interface ID that corresponds to a specific underlying interface. When the MN includes S/TLLAOs solely for the purpose of announcing new QoS preferences, it sets both Port Number and Link-Layer Address to 0 to indicate that the addresses are not to be updated in the router's neighbor cache. When an ND message includes multiple S/TLLAOs, the first S/TLLAO MUST correspond to the underlying interface used to transmit the message. Note that this S/TLLAO format includes network-layer information (e.g., Prefix Length) in a link-layer option. This is due to the fact that it is difficult to standardize new IPv6 ND options in a timely fashion. An experimental proposal defines a "Universal RA Option" intended for carrying generic network-layer information in RS/RA messages . However, there is no way at this time to predict how long the experiment would take nor whether it will be successful.

When the underlying network does not support multicast, aircraft map link-scoped multicast addresses to the link-layer address of a router, which acts as a multicast forwarding agent. The mobile router on board the aircraft also serves as an IGMP/MLD Proxy for its EUNs and/or hosted applications per while using the link-layer address of the router as the link-layer address for all multicast packets. When the underlying network supports multicast, AERO interfaces use the multicast address mapping specification found in for IPv4 underlying networks and use a TBD site-scoped multicast mapping for IPv6 underlying networks. In that case, border routers must ensure that the encapsulated site-scoped multicast packets do not leak outside of the AERO link.

MNs and routers coordinate their MNP assertions and per-link parameters through RS/RA exchanges, and use ND messages to maintain neighbor cache entries. Routers configure their AERO interfaces as advertising interfaces, and therefore send unicast RA messages with configuration information in response to a MN's RS message. Thereafter, the MN sends additional RS messages to the router's unicast address to refresh MNP and/or router lifetimes. To assert an MNP, the MN sends an RS message over any underlying interface with its base AERO address as the source address, all-routers multicast as the destination address and with an SLLAO with a valid Prefix Length for the MNP. The SLLAO also contains valid Interface ID and P(i) values appropriate for the underlying interface. When the router receives the RS message it injects the MNP into the routing system if the prefix assertion was acceptable, then registers the new Interface ID, Port Number, Link-Layer Address and P(i) values in a neighbor cache entry. The router then returns an RA with its AERO address as the source address, the AERO address of the MN as the destination address and with Router Lifetime set to a non-zero value if the MNP assertion was accepted; otherwise set to zero. The message also includes an SLLAO with Prefix Length set to the length of the MNP assertion. After the MN receives the RA confirming the MNP assertion, it registers each additional underlying interface with the router by sending an RS over the underlying interface with its base AERO address as the source address, the router's AERO address as the destination address, and with an SLLAO with Prefix Length set to 0. The SLLAO also contains valid Interface ID and P(i) values appropriate for the underlying interface. When the router receives the RS message it registers the new Interface ID, Port Number, Link-Layer Address and P(i) values in the neighbor cache already established during MNP assertion. The router then returns an RA message with its AERO address as the source address, the AERO address of the MN as the destination address and with Router Lifetime set to a non-zero value. The message also includes an SLLAO with Prefix Length set to 0. The MN is responsible for retrying each RS/RA exchange up to MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL seconds until an RA is received. If no RA is received, the MN declares the underlying interface unreachable, but MAY try again later (e.g., if underlying link conditions become more favorable). MNs that do not need to assert MNP, Port Number, Link-Layer Address, Interface ID or P(i) values MAY omit SLLAOs in RS messages. Responding routers MAY also omit SLLAOs in the corresponding RAs.

No IANA actions are required.

Security considerations are the same as defined for the underlying interface types, and readers are referred to the appropriate underlying interface specifications. IPv6 and IPv6 ND security considerations also apply, and are specified in the normative references.

This document was prepared per the consensus decision at the 8th Conference of the International Civil Aviation Organization (ICAO) Working Group-I Mobility Subgroup on March 22, 2019. Attendees and contributors included: Guray Acar, Danny Bharj, Francois D´Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg Saccone, Robert Segers, Stephane Tamalet, Fred Templin, Bela Varkonyi, Tony Whyman, and Dongsong Zeng. .

The S/TLLAO format specified in includes a Length value of 5 (i.e., 5 units of 8 octets). However, special-purpose AERO links may extend the basic format to include additional fields and a Length value larger than 5. For example, adaptation of AERO to the Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS) includes link selection preferences based on transport port numbers in addition to the existing DSCP-based preferences. ATN/IPS nodes maintain a map of transport port numbers to 64 possible preference fields, e.g., TCP port 22 maps to preference field 8, TCP port 443 maps to preference field 20, UDP port 8060 maps to preference field 34, etc. The extended S/TLLAO format for ATN/IPS is shown in , where the Length value is 7 and the 'Q(i)' fields provide link preferences for the corresponding transport port number.

<< RFC Editor - remove prior to publication >> First draft version (draft-templin-atn-aero-interface-00): Draft based on consensus decision of ICAO Working Group I Mobility Subgroup March 22, 2019.