| Network Working Group | F. Templin, Ed. |
| Internet-Draft | The Boeing Company |
| Updates: rfc4193, rfc4291, rfc4443, | A. Whyman |
| rfc8201 (if approved) | MWA Ltd c/o Inmarsat Global Ltd |
| Intended status: Standards Track | June 2, 2020 |
| Expires: December 4, 2020 |
Transmission of IPv6 Packets over Overlay Multilink Network (OMNI) Interfaces
draft-templin-6man-omni-interface-24
Mobile nodes (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, enterprise wireless 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.
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This Internet-Draft will expire on December 4, 2020.
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Mobile Nodes (MNs) (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, enterprise wireless 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 links.
The MN configures a virtual interface (termed the "Overlay Multilink Network (OMNI) interface") as a thin layer over the underlying Access Network (ANET) 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 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 one or more Internetworks 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 interfaces 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 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.
MNs may connect to multiple distinct OMNI links by configuring multiple OMNI interfaces, e.g., omni0, omni1, omni2, etc. Each OMNI interface is configured over a set of underlying interfaces and provides a nexus for Safety-Based Multilink (SBM) operation. The IP layer selects an OMNI interface based on SBM routing considerations, then the selected interface applies Performance-Based Multilink (PBM) to select the correct underlying interface. Applications can apply Segment Routing [RFC8402] to select independent SBM topologies for fault tolerance.
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.
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 the same as defined in [RFC4291] (with Link-Local scope assumed).
The following terms are defined within the scope of this document:
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.
An OMNI interface is a MN virtual interface configured over one or more underlying 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 [RFC5558][RFC7847], and augmented as shown in Figure 1. The IP layer therefore sees the OMNI interface as a single L3 interface with multiple underlying 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 underlying interface becomes active, the MN's OMNI interface sends native (i.e., unencapsulated) IPv6 ND messages via the underlying 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.
OMNI links span one or more underlying Internetwork via a mid-layer overlay encapsulation based on [RFC2473] and using [RFC4193] addressing. Each OMNI link corresponds to a different overlay (differentiated by an address codepoint) which may be carried over a completely separate underlying topology. Each MN can facilitate SBM by connecting to multiple OMNI links using a distinct OMNI interface for each link.
The OMNI interface observes the link nature of tunnels, including the Maximum Transmission Unit (MTU) and the role of fragmentation and reassembly[I-D.ietf-intarea-tunnels]. The OMNI interface is configured over one or more underlying interfaces that may have diverse MTUs.
IPv6 underlying interfaces are REQUIRED to configure a minimum 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 minimum MTU for IPv4 underlying interfaces is only 68 bytes [RFC1122], meaning that a packet smaller than the IPv6 minimum MTU may require fragmentation when IPv4 encapsulation is used. Therefore, the Don't Fragment (DF) bit in the IPv4 encapsulation header MUST be set to 0
The OMNI interface configures an MTU of 9180 bytes [RFC2492]; the size is therefore not a reflection of the underlying interface MTUs, but rather determines the largest packet the OMNI interface can forward or reassemble. The OMNI interface therefore accommodates packets as large as the OMNI interface MTU while generating IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) messages [RFC8201] as necessary (see below). For IPv4 packets with DF=0, the IP layer performs IPv4 fragmentation if necessary to admit the fragments into the OMNI interface. The interface may then internally apply further IPv4 fragmentation prior to encapsulation to ensure that the IPv4 fragments are delivered to the final destination.
OMNI interfaces internally employ OMNI link encapsulation and fragmentation/reassembly per [RFC2473]. The encapsulation inserts a mid-layer IPv6 header between the inner IP packet and any outer IP encapsulation headers. The OMNI interface returns internally-generated PTB messages for packets admitted into the interface that it deems too large (e.g., according to link performance characteristics, reassembly cost, etc.) while either dropping or forwarding the packet as necessary. The OMNI interface performs PMTUD even if the destination appears to be on the same link since an OMNI link node on the path may return a PTB. This ensures that the path MTU is adaptive and reflects the current path used for a given data flow.
OMNI interfaces perform encapsulation and fragmentation/reassembly as follows:
OMNI interfaces unconditionally drop all OMNI link fragments smaller than 640 bytes. In order to set the correct context for reassembly, the OMNI interface that inserts the IPv6 header MUST also be the one that inserts the IPv6 Fragment Header Identification value. While not strictly required, sending all fragments of the same fragmented mid-layer packet consecutively over the same underlying interface with minimal inter-fragment delay can in some cases increase the likelihood of successful reassembly.
Note that the OMNI interface can forward large packets via encapsulation and fragmentation while at the same time returning "advisory" PTB messages (subject to rate limiting). The receiving node that performs reassembly can also send advisory PTB messages if reassembly conditions become unfavorable. The OMNI interface can therefore continuously forward large packets without loss while returning advisory PTB messages recommending a smaller size.
OMNI interfaces that send advisory PTB messages set the ICMPv6 message header Code field to the value 1. Receiving nodes that recognize the code reduce their estimate of the path MTU the same as for ordinary "diagnistic" PTBs but do not regard the message as a loss indication. Nodes that do not recognize the code treat the message the same as a diagnostic PTB, but should heed the advice in [RFC8201] regarding retransmissions. This document therefore updates [RFC4443] and [RFC8201].
As discussed in Section 3.7 of [I-D.ietf-intarea-frag-fragile], there are four basic threats concerning IPv6 fragmentation; each of which is addressed by a suitable mitigation as follows:[RFC4963]. Since IPv6 provides a 32-bit Identification value, however, this is not a concern for IPv6 fragmentation.
Additionally, IPv4 fragmentation includes a 16-bit Identification (IP ID) field with only 65535 unique values, meaning that for even moderately high data rates the field could wrap and apply to new packets while the fragments of old packets using the same ID are still alive in the network
The OMNI interface transmits IPv6 packets according to the native frame format of each underlying 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.
OMNI interfaces construct IPv6 Link-Local Addresses (i.e., "OMNI LLAs") as follows:
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 MS OMNI LLAs are guaranteed unique through administrative assignment, OMNI interfaces set the autoconfiguration variable DupAddrDetectTransmits to 0 [RFC4862].
OMNI links use IPv6 Unique Local Addresses (i.e., "OMNI ULAs") [RFC4193] as the source and destination addresses in OMNI link IPv6 encapsulation headers. This document updates [RFC4193] by reserving the ULA prefix fc80::/10 for mapping OMNI LLAs to routable OMNI ULAs.
Each OMNI link instance is identified by bits 10-15 of the OMNI service prefix fc80::/10. For example, OMNI ULAs associated with instance 0 are configured from the prefix fc80::/16, instance 1 from fc81::/16, etc., up to instance 63 from fcbf::/16. OMNI ULAs are configured in one-to-one correspondence with OMNI LLAs through stateless prefix translation. For example, for OMNI link instance fc80::/16:
Each OMNI interface assigns the Anycast OMNI ULA specific to the OMNI link instance, e.g., the OMNI interface connected to instance 3 assigns the Anycast OMNI ULA fc83:. Routers that configure OMNI interfaces advertise the OMNI service prefix (e.g., fc83::/16) into the local routing system so that applications can direct traffic according to SBM requirements.
The OMNI ULA presents an IPv6 address format that is routable within the OMNI link routing system and can be used to convey link-scoped messages across multiple hops using IPv6 encapsulation [RFC2473]. The OMNI link extends across one or more underling Internetworks to include all ARs and MSEs. All MNs are also considered to be connected to the OMNI link, however OMNI link encapsulation is omitted over ANET links when possible to conserve bandwidth (see: Section 11).
The OMNI link can be subdivided into "segments" that often correspond to different administrative domains or physical partitions. OMNI nodes can use IPv6 Segment Routing [RFC8402] when necessary to support efficient packet 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-6706bis].
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 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 underlying 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| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Sub-Options ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: OMNI Option Format
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
MS-Register 4
MS-Release 5
Network Access Identifier 6
Geo Coordinates 7
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:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Sub-Type=0 |
+-+-+-+-+-+-+-+-+
Figure 6: Pad1
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
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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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
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)
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 | MSID (bits 0 - 15) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSID (bits 16 - 32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: MS-Register Sub-option
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=5 | Sub-length=4 | MSID (bits 0 - 15) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSID (bits 16 - 32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: MS-Release Sub-option
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=6 | Sub-length=N |Network Access Identifier (NAI)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 13: Network Access Identifier (NAI) Sub-option
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=7 | Sub-length=N | Geo Coordinates
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 14: Geo Coordinates Sub-option
The multicast address mapping of the native underlying interface applies. The mobile router on board the MN also serves as an IGMP/MLD Proxy for its EUNs and/or hosted applications per [RFC4605] while using the L2 address of the AR as the L2 address for all multicast packets.
The MN uses Multicast Listener Discovery (MLDv2) [RFC3810] to coordinate with the AR, and ANET L2 elements use MLD snooping [RFC4541].
The MN's IPv6 layer selects the outbound OMNI interface according to SBM considerations when forwarding data packets from local or EUN applications to external correspondents. Each OMNI interface maintains a neighbor cache the same as for any IPv6 interface, but with additional state for multilink coordination.
After a packet enters the OMNI interface, an outbound underlying interface is selected based on PBM traffic selectors such as DSCP, application port number, cost, performance, message size, etc. OMNI interface multilink selections could also be configured to perform replication across multiple underlying interfaces for increased reliability at the expense of packet duplication.
When an OMNI interface sends a packet over a selected outbound underlying interface, it omits OMNI link encapsulation if the packet does not require fragmentation and the neighbor can determine the OMNI ULAs through other means (e.g., the packet's destination, neighbor cache information, etc.). Otherwise, the OMNI interface inserts an IPv6 header with the OMNI ULAs and performs fragmentation if necessary. The OMNI interface also performs enacpsulation when the nearest AR is located multiple hops away as discussed in Section 12.1.
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 underlying interfaces having diverse properties.
MNs may connect to multiple independent OMNI links concurrently in support of SBM. Each OMNI interface is distinguished by its Anycast OMNI ULA (e.g., fc80::, fc81::, fc82::). 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. A different Anycast OMNI ULA is assigned to each interface, and the MN injects the service prefixes for the OMNI link instances into the EUN routing system.
Applications in EUNs can use Segment Routing to select the desired OMNI interface based on SBM considerations. The Anycast OMNI ULA is written into the IPv6 destination address, and the actual destination (along with any additional intermediate hops) is written into the Segment Routing Header. Standard IP routing directs the packets to the MN's mobile router entity, and the Anycast OMNI ULA identifies the OMNI interface to be used for transmission to the next hop. When the MN receives the message, it replaces the IPv6 destination address with the next hop found in the routing header and transmits the message over the OMNI interface identified by the Anycast OMNI ULA.
Multiple distinct OMNI links can therefore be used to support fault tolerance, load balancing, reliability, etc. The architectural model is similar to Layer 2 Virtual Local Area Networks (VLANs).
MNs interface with the MS by sending RS messages with OMNI options under the assumption that a single AR on the ANET will process the message and respond. This places a requirement on each ANET, which may be enforced by physical/logical partitioning, L2 AR beaconing, etc. The manner in which the ANET ensures single AR coordination is link-specific and outside the scope of this document.
For each underlying interface, the MN sends an RS message with an OMNI option with prefix registration information, ifIndex-tuples, MS-Register/Release suboptions containing MSIDs, and with destination address set to link-scoped All-Routers multicast (ff02::2) [RFC4291]. Example MSID discovery methods are given in [RFC5214], including data link login parameters, name service lookups, static configuration, etc. Alternatively, MNs can discover individual MSIDs by sending an initial RS with MS-Register MSID set to 0x00000000.
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 RS messages to register its MNP and an initial set of underlying interfaces that are also UP. The MN sends additional RS messages to refresh lifetimes and to register/deregister underlying interfaces as they transition to UP or DOWN. The MN sends initial RS messages over an UP underlying interface with its OMNI LLA as the source and with destination set to All-Routers multicast. The RS messages include an OMNI option per Section 9 with valid prefix registration information, ifIndex-tuples appropriate for underlying interfaces and MS-Register/Release sub-options.
ARs process IPv6 ND messages with OMNI options and act as a proxy for MSEs. ARs receive RS messages and create a neighbor cache entry for the MN, then coordinate with any named MSIDs in a manner outside the scope of this document. The AR returns an RA message with destination address set to the MN OMNI LLA (i.e., unicast), with source address set to its MS OMNI LLA, with the P(roxy) bit set in the RA flags [RFC4389][RFC5175], with an OMNI option with valid prefix registration information, ifIndex-tuples, MS-Register/Release sub-options, and with any information for the link that would normally be delivered in a solicited RA message. 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, and includes any necessary options such as:
The AR coordinates with each Register/Release MSID then sends an immediate unicast RA response 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 according to the standard [RFC4861].
When the MSE processes the OMNI information, it first validates the prefix registration information. The MSE then injects/withdraws the MNP in the routing/mapping system and caches/discards the new Prefix Length, MNP and ifIndex-tuples. The MSE then informs the AR of registration success/failure, and the AR adds the MSE to the list of Register/Release MSIDs to return in an RA message OMNI option per Section 9.
When the MN receives the RA message, it creates an OMNI interface neighbor cache entry with the AR's address as an L2 address and records the MSIDs that have confirmed MNP registration via this AR. If the MN connects to multiple ANETs, it establishes additional AR L2 addresses (i.e., as a Multilink neighbor). The MN then manages its underlying 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 a an UP underlying interface, the MN declares this underlying interface as DOWN.
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). ARs are therefore responsible for keeping MS state alive on a shorter timescale than the MN is required to do on its own behalf.
Note: On multicast-capable underlying interfaces, MNs should send periodic unsolicited multicast NA messages and ARs 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 a unicast exchange to test reachability.
On some ANET types (e.g., omni-directional ad-hoc wireless) a MN may be located multiple hops away from a node which has connectivity to the nearest AR. Forwarding through these multiple hops would be conducted through the application of a Mobile Ad-hoc Network (MANET) routing protocol operating across the ANET interfaces.
A MN located potentially multiple ANET hops away from the nearst AR prepares an RS message as normal then encapsulates the message in an IPv6 header with source address set to the ULA corresponding to the RS LLA source address, and with destination set to site-scoped All-Routers multicast (ff05::2)[RFC4291]. The MN then sends the encapsulated RS message via the ANET interface, where it will be received by zero or more nearby intermediate MNs.
When an intermediate MN that particpates in the MANET routing protocol receives the encapsulated RS, it forwards the message according to its (ULA-based) MANET routing tables. This process repeats iteratively until the RS message is received by an ultimate MN that is within communications range of an AR, which forwards the message to the AR.
When the AR receives the RS message, it coordinates with the MS the same as if the message were received as an ordinary link-local RS, since the inner Hop Limit will not have been decremented by the MANET multihop forwarding process. The AR then prepares an RA message with source address set to its own LLA and destination address set to the LLA of the original MN, then encapsulates the message in an IPv6 header with source and destination set to the ULAs corresponding to the inner header.
The AR then forwards the message to an MN within communications range, which forwards the message according to its MANET routing tables to an intermediate MN. The MANET forwarding process continues repetitively until the message is delivered to the original MN, which decapsulates the message and performs autoconfiguration the same as if it had received the RA directly from an AR.
Note: An alternate approach to encapsulation of IPv6 ND messages for multihop forwarding would be to statelessly translate the IPv6 LLAs into ULAs and forward the messages without encapsulation. This would violate the [RFC4861] requirement that certain IPv6 ND messages must use link-local addresses and must not be accepted if received with Hop Limit less than 255. This document therefore advocates encapsulation since the overhead is nominal considering the infrequent nature and small size of IPv6 ND messages. Future documents may consider encapsulation avoidance through translation while updating [RFC4861].
If the ANET link model is multiple access, the AR is responsible for assuring that address duplication cannot corrupt the neighbor caches of other nodes on the link. When the MN sends an RS message on a multiple access ANET link, the AR verifies that the MN is authorized to use the address and returns an RA with a non-zero Router Lifetime only if the MN is authorized.
After verifying MN authorization and returning an RA, the AR MAY return IPv6 ND Redirect messages to direct MNs located on the same ANET link to exchange packets directly without transiting the AR. In that case, the MNs can exchange packets according to their unicast L2 addresses discovered from the Redirect message instead of using the dogleg path through the AR. In some ANET links, however, such direct communications may be undesirable and continued use of the dogleg path through the AR may provide better performance. In that case, the AR can refrain from sending Redirects, and/or MNs can ignore them.
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.
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 ANET links such as aeronautical radios) and can therefore be tuned for rapid response.
ARs perform proactive NUD for MSEs for which there are currently active MNs on the ANET. 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 MNs via the ANET interface with an OMNI option with a Release ID for the failed MSE, and with destination address set to All-Nodes multicast (ff02::1) [RFC4291].
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.
When a MN connects to an ANET link for the first time, it sends an RS message with an OMNI option. If the first hop AR recognizes the option, it returns an RA with its MS OMNI LLA as the source, the MN OMNI LLA as the destination, the P(roxy) bit set in the RA flags and with an OMNI option included. The MN then engages the AR according to the OMNI link model specified above. If the first hop AR 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 MN engages the ANET according to the legacy IPv6 link model and without the OMNI extensions specified in this document.
If the ANET 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 MN sends an RS message on a multiple access ANET link with an OMNI LLA source address and an OMNI option, ARs that recognize the option ensure that the MN is authorized to use the address and return an RA with a non-zero Router Lifetime only if the MN is authorized. ARs that do not recognize the option instead return an RA that makes no statement about the MN's authorization to use the source address. In that case, the MN should perform Duplicate Address Detection to ensure that it does not interfere with other nodes on the link.
An alternative approach for multiple access ANET links to ensure isolation for MN / AR communications is through L2 address mappings as discussed in Appendix C. This arrangement imparts a (virtual) point-to-point link model over the (physical) multiple access link.
OMNI interfaces configured over INET interfaces that connect to the open Internet can apply symmetric security services such as VPNs or establish a direct link through some other means. In environments where an explicit VPN or direct link may be impractical, OMNI interfaces can instead use Teredo UDP/IP encapsulation [RFC6081][RFC4380]. (SEcure Neighbor Discovery (SEND) and Cryptographically Generated Addresses (CGA) [RFC3971][RFC3972] can also be used if additional authentication is necessary.)
The IPv6 ND control plane messages used to establish neighbor cache state must be authenticated while data plane messages are delivered the same as for ordinary best-effort Internet traffic with basic source address-based data origin verification. Data plane communications via OMNI interfaces that connect over the open Internet without an explicit VPN should therefore employ transport- or higher-layer security to ensure integrity and/or confidentiality.
OMNI interfaces in the open Internet are often located behind Network Address Translators (NATs). The OMNI interface accommodates NAT traversal using UDP/IP encapsulation and the mechanisms discussed in [RFC6081][RFC4380][I-D.templin-intarea-6706bis].
In some use cases, it is desirable, beneficial and efficient for the MN to receive a constant MNP that travels with the MN 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 MN 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.
Prefix delegation services such as those discussed in [I-D.templin-6man-dhcpv6-ndopt] and [I-D.templin-intarea-6706bis] allow OMNI MNs that desire time-varying MNPs to obtain short-lived prefixes. In that case, the identity of the MN can be used as a prefix delegation seed (e.g., a DHCPv6 Device Unique IDentifier (DUID) [RFC8415]). The MN 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 MNs with automated network renumbering services, however presents limits for the durations of ongoing sessions that would prefer to use a constant address.
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 IANA is instructed to assign a new Code value "1" in the "ICMPv6 Code Fields: Type 2 - Packet Too Big" registry. The registry should read as follows:
Code Name Reference --- ---- --------- 0 Diagnostic Packet Too Big [RFC4443] 1 Advisory Packet Too Big [RFCXXXX]
Figure 15: 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".
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 MS-Register [RFCXXXX] 5 MS-Release [RFCXXXX] 6 Network Acceess Identifier [RFCXXXX] 7 Geo Coordinates [RFCXXXX] 8-252 Unassigned 253-254 Experimental [RFCXXXX] 255 Reserved [RFCXXXX]
Figure 16: OMNI Option Sub-Type Values
Security considerations for 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 interfaces inherit the physical and/or link-layer security properties of the connected ANETs. 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, however, an asymmetric security service such as SEcure Neighbor Discovery (SEND) [RFC3971] with Cryptographically Generated Addresses (CGAs) [RFC3972] and/or the Teredo Authentication option [RFC4380] may be necessary.
While the OMNI link protects control plane messaging as discussed above, applications should still employ transport- or higher-layer security services to protect the data plane.
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 5.1.
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.
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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31| Bitmap(1)=0xff|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitmap(2)=0xff|P64|P65|P67|P68| ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 17: 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 18: 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 19: Example 3: Indexed Encoding
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].
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 MN and AR only without invoking other nodes on the ANET. This implies that MN / AR control messaging should be isolated and not overheard by other nodes on the link.
To support MN / AR isolation on some ANET links, ARs can maintain an OMNI-specific unicast L2 address ("MSADDR"). For Ethernet-compatible ANETs, this specification reserves one Ethernet unicast address TBD2 (see: Section 19). 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 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 ARs that are configured to accept packets destined to MSADDR. Note that multiple ARs 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].
<< RFC Editor - remove prior to publication >>
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