Transmission of IP Packets over
Overlay Multilink Network (OMNI) InterfacesThe Boeing CompanyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgMWA Ltd c/o Inmarsat Global Ltd99 City RoadLondonEC1Y 1AXEnglandtony.whyman@mccallumwhyman.comI-DInternet-DraftMobile 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 presented that enables mobile nodes
to coordinate with a network-based mobility service and/or with other
mobile node peers. This document specifies the transmission of IP
packets over Overlay Multilink Network (OMNI) Interfaces.Mobile Nodes (MNs) (e.g., aircraft of various configurations,
terrestrial vehicles, seagoing vessels, enterprise wireless devices,
pedestrians with cellphones, etc.) often have multiple interface
connections to wireless and/or wired-line data links used 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 node's underlying interface connections to the data links.The MN configures a virtual interface (termed the "Overlay Multilink
Network Interface (OMNI)") as a thin layer over the underlying
interfaces. The OMNI interface is therefore the only interface
abstraction exposed to the IP layer and behaves according to the
Non-Broadcast, Multiple Access (NBMA) interface principle, while
underlying interfaces appear as link layer communication channels in the
architecture. The OMNI interface internally employs the "OMNI Adaptation
Layer (OAL)" to ensure that original IP packets are delivered without
loss due to size restrictions. 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 ) 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 ).The OMNI interface provides a multilink nexus for exchanging inbound
and outbound traffic via the correct underlying interface(s). The IP
layer sees the OMNI interface as a point of connection to the OMNI link.
Each OMNI link has one or more associated Mobility Service Prefixes
(MSPs), which are typically IP Global Unicast Address (GUA) prefixes
from which 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 within the same OMNI
domain 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. Each OMNI interface within the same OMNI domain configures a
common ULA prefix [ULA]::/48, and configures a unique 16-bit Subnet ID
'*' to construct the sub-prefix [ULA*]::/64 (see: ). The IP layer applies SBM routing to select an
OMNI interface, which then applies Performance-Based Multilink (PBM) to
select the correct underlying interface. Applications can apply Segment
Routing 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
. The MS provides Mobility Service Endpoints
(MSEs) that track MN movements and represent their MNPs in a global
routing or mapping system.Many OMNI use cases have been proposed. In particular, the
International Civil Aviation Organization (ICAO) Working Group-I
Mobility Subgroup is developing a future Aeronautical Telecommunications
Network with Internet Protocol Services (ATN/IPS) and has issued a
liaison statement requesting IETF adoption in
support of ICAO Document 9896 . The IETF IP
Wireless Access in Vehicular Environments (ipwave) working group has
further included problem statement and use case analysis for OMNI in a
document now in AD evaluation for RFC publication . Still other communities
of interest include AEEC, RTCA Special Committee 228 (SC-228) and NASA
programs that examine commercial aviation, Urban Air Mobility (UAM) and
Unmanned Air Systems (UAS). Pedestrians with handheld devices represent
another large class of potential OMNI users.In addition to many other aspects, OMNI supports the "6M's" of modern
Internetworking including:Multilink – a mobile node’s ability to coordinate
multiple diverse underlying data links as a single logical unit
(i.e., the OMNI interface) to achieve the required communications
performance and reliability objectives.Multinet – the ability to span the OMNI link across
multiple diverse network administrative segments while maintaining
seamless end-to-end communications between mobile nodes and
correspondents such as air traffic controllers, fleet
administrators, etc.Mobility – a mobile node’s ability to change network
points of attachment (e.g., moving between wireless base stations)
which may result in an underlying interface address change, but
without disruptions to ongoing communication sessions with peers
over the OMNI link.Multicast – the ability to send a single network
transmission that reaches multiple nodes belonging to the same
interest group, but without disturbing other nodes not subscribed to
the interest group.Multihop – a mobile node vehicle-to-vehicle relaying
capability useful when multiple forwarding hops between vehicles may
be necessary to “reach back” to an infrastructure access
point connection to the OMNI link.MTU assurance – the ability to deliver packets of various
robust sizes between peers without loss due to a link size
restriction, and to dynamically adjust packets sizes to achieve the
optimal performance for each independent traffic flow.This document specifies the transmission of IP packets and MN/MS
control messages over OMNI interfaces. The OMNI interface supports
either IP protocol version (i.e., IPv4 or IPv6
) as the network layer in the data plane, while
using IPv6 ND messaging as the control plane independently of the data
plane IP protocol(s). The OAL operates as a sublayer between L3 and L2
based on IPv6 encapsulation as discussed in the
following sections.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. Additionally, this document assumes
the following IPv6 ND message types: Router Solicitation (RS), Router
Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement
(NA) and Redirect. Nodes that implement IPv6 ND maintain per-neighbor
state in Neighbor Cache Entries (NCEs). Each NCE is indexed by the
neighbor's Link-Local Address (LLA), which must also match the
Unique-Local Address (ULA) used for encapsulation.The Protocol Constants defined in Section 10 of 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 (with Link-Local scope assumed).The term "IP" is used to refer collectively to either Internet
Protocol version (i.e., IPv4 or IPv6 ) when a specification at the layer in question
applies equally to either version.The following terms are defined within the scope of this
document:an end system with a mobile
router that has one or more distinct upstream data link connections
grouped together into one or more logical units. 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.a simple or complex
downstream-attached mobile network that travels with the MN as a
single logical unit. The IP addresses assigned to EUN devices remain
stable even if the MN's upstream data link connections change.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.an entity in
the MS (either singular or aggregate) that coordinates the mobility
events of one or more MN.an aggregated
IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32,
192.0.2.0/24, etc.) assigned to the OMNI link and from which
more-specific Mobile Network Prefixes (MNPs) are delegated. OMNI
link administrators typically obtain MSPs from an Internet address
registry, however private-use prefixes can alternatively be used
subject to certain limitations (see: ). OMNI
links that connect to the global Internet advertise their MSPs to
their interdomain routing peers.a longer IP
prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56,
192.0.2.8/30, etc.) and assigned to a MN. MNs sub-delegate the MNP
to devices located in EUNs. Note that OMNI link Relay nodes may also
service non-MNP routes (i.e., GUA prefixes not covered by an MSP)
but that these correspond to fixed correspondent nodes and not MNs.
Other than this distinction, MNP and non-MNP routes are treated
exactly the same by the OMNI routing system.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 is
assumed, and sometimes referred to as "protected spectrum". Private
enterprise networks and ground domain aviation service networks may
provide multiple secured IP hops between the MN's point of
connection and the nearest Access Router.a router in the ANET for
connecting MNs to correspondents in outside Internetworks. The AR
may be located on the same physical link as the MN, or may be
located multiple IP hops away. In the latter case, the MN uses
encapsulation to communicate with the AR as though it were on the
same physical link.a MN's attachment to a link in
an ANET.a connected network
region with a coherent IP addressing plan that provides transit
forwarding services between ANETs and nodes that connect directly to
the open INET via unprotected media. No physical and/or data link
level security is assumed, therefore security must be applied by
upper layers. The global public Internet itself is an example.a node's attachment to a link
in an INET.a "wildcard" term used when a given
specification applies equally to both ANET and INET cases.a Non-Broadcast, Multiple Access
(NBMA) virtual overlay configured over one or more INETs and their
connected ANETs. An OMNI link may 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.a node's attachment to an OMNI
link, and configured over one or more underlying *NET interfaces. If
there are multiple OMNI links in an OMNI domain, a separate OMNI
interface is configured for each link.an OMNI interface
sublayer service whereby original IP packets admitted into the
interface are wrapped in an IPv6 header and subject to fragmentation
and reassembly. The OAL is also responsible for generating
MTU-related control messages as necessary, and for providing
addressing context for spanning multiple segments of a bridged OMNI
link.a whole IP packet or
fragment admitted into the OMNI interface by the network layer prior
to OAL encapsulation and fragmentation, or an IP packet delivered to
the network layer by the OMNI interface following OAL decapsulation
and reassembly.an original IP packet encapsulated
in OAL headers and trailers, which is then submitted for OAL
fragmentation and reassembly.a portion of an OAL packet
following fragmentation but prior to *NET encapsulation, or
following *NET encapsulation but prior to OAL reassembly.an OAL packet that does
not require fragmentation is always encapsulated as an "atomic
fragment" with a Fragment Header with Fragment Offset and More
Fragments both set to 0, but with a valid Identification value.an encapsulated OAL
fragment following *NET encapsulation or prior to *NET
decapsulation. OAL sources and destinations exchange carrier packets
over underlying interfaces, and may be separated by one or more OAL
intermediate nodes. OAL intermediate nodes may perform
re-encapsulation on carrier packets by removing the *NET headers of
the first hop network and replacing them with new *NET headers for
the next hop network.an OMNI interface acts as an OAL
source when it encapsulates original IP packets to form OAL packets,
then performs OAL fragmentation and *NET encapsulation to create
carrier packets.an OMNI interface acts as an
OAL destination when it decapsulates carrier packets, then performs
OAL reassembly and decapsulation to derive the original IP
packet.an OMNI interface acts
as an OAL intermediate node when it removes the *NET headers of
carrier packets received on a first segment, then re-encapsulates
the carrier packets in new *NET headers and forwards them into the
next segment.an IPv6 Neighbor Discovery option
providing multilink parameters for the OMNI interface as specified
in .an
IPv6 Link Local Address that embeds the most significant 64 bits of
an MNP in the lower 64 bits of fe80::/64, as specified in .an
IPv6 Unique-Local Address derived from an MNP-LLA.an
IPv6 Link Local Address that embeds a 32-bit
administratively-assigned identification value in the lower 32 bits
of fe80::/96, as specified in .an
IPv6 Unique-Local Address derived from an ADM-LLA.an OMNI interface's manner of
managing diverse underlying interface connections to data links as a
single logical unit. The OMNI interface provides a single unified
interface to upper layers, while underlying interface selections are
performed on a per-packet basis considering traffic selectors 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.an OAL intermediate node's manner of
bridging multiple diverse IP Internetworks and/or private enterprise
networks at the OAL layer below IP. Through intermediate node
concatenation of bridged network segments in this way, multiple
diverse Internetworks (such as the global public IPv4 and IPv6
Internets) can serve as transit segments in a bridged path for
forwarding IP packets end-to-end. This bridging capability provide
benefits such as supporting IPv4/IPv6 transition and coexistence,
joining multiple diverse operator networks into a cooperative single
service network, etc.an iterative relaying of IP packets
between MNs over an OMNI underlying interface technology (such as
omnidirectional wireless) without support of fixed infrastructure.
Multihop services entail node-to-node relaying within a
Mobile/Vehicular Ad-hoc Network (MANET/VANET) for MN-to-MN
communications and/or for "range extension" where MNs within range
of communications infrastructure elements provide forwarding
services for other MNs.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", "IP layer", etc.a *NET interface over
which an OMNI interface is configured. The OMNI interface is seen as
a L3 interface by the IP layer, and each underlying interface is
seen as a L2 interface by the OMNI interface. The underlying
interface either connects directly to the physical communications
media or coordinates with another node where the physical media is
hosted.Each
MSE and AR is assigned a unique 32-bit Identification (MSID) (see:
). IDs are assigned according to
MS-specific guidelines (e.g., see: ).A means for
ensuring fault tolerance through redundancy by connecting multiple
affiliated OMNI interfaces to independent routing topologies (i.e.,
multiple independent OMNI links).A means for
selecting underlying interface(s) for packet transmission and
reception within a single OMNI interface.The set of all SBM/PBM OMNI links
that collectively provides services for a common set of MSPs. Each
OMNI domain consists of a set of affiliated OMNI links that all
configure the same ::/48 ULA prefix with a unique 16-bit Subnet ID
as discussed in .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
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 virtual interface configured over one or more
underlying interfaces, which may be physical (e.g., an aeronautical
radio link, etc.) or virtual (e.g., an Internet or higher-layer
"tunnel"). The OMNI interface architectural layering model is the same
as in , and augmented as
shown in . The IP layer therefore sees the OMNI
interface as a single L3 interface nexus for multiple underlying
interfaces that appear as L2 communication channels in the
architecture.| |
Address Binding | +----------------------------+
+---->| IP (L3) |
IP Address +---->| |
Binding | +----------------------------+
+---->| OMNI Interface |
Logical-to- +---->| (OMNI Adaptation Layer) |
Physical | +----------------------------+
Interface +---->| L2 | L2 | | L2 |
Binding |(IF#1)|(IF#2)| ..... |(IF#n)|
+------+------+ +------+
| L1 | L1 | | L1 |
| | | | |
+------+------+ +------+
]]>Each underlying interface provides an L2/L1 abstraction according to
one of the following models:INET interfaces connect to an INET either natively or through one
or several IPv4 Network Address Translators (NATs). Native INET
interfaces have global IP addresses that are reachable from any INET
correspondent. NATed INET interfaces typically have private IP
addresses and connect to a private network behind one or more NATs
that provide INET access.ANET interfaces connect to a protected ANET that is separated
from the open INET by an AR acting as a proxy. The ANET interface
may be either on the same L2 link segment as the AR, or separated
from the AR by multiple IP hops.VPNed interfaces use security encapsulation over a *NET to a
Virtual Private Network (VPN) gateway. Other than the link-layer
encapsulation format, VPNed interfaces behave the same as for Direct
interfaces.Direct (aka "point-to-point") interfaces connect directly to a
peer without crossing any *NET paths. An example is a line-of-sight
link between a remote pilot and an unmanned aircraft.The OMNI interface forwards original IP packets from the
network layer (L3) using the OMNI Adaptation Layer (OAL) (see: ) as an encapsulation and fragmentation sublayer
service. This "OAL source" then further encapsulates the resulting OAL
packets/fragments in *NET headers to create OAL carrier packets for
transmission over underlying interfaces (L2/L1). The target OMNI
interface receives the carrier packets from underlying interfaces
(L1/L2) and discards the *NET headers. If the resulting OAL
packets/fragments are addressed to itself, the OMNI interface acts as an
"OAL destination" and performs reassembly if necessary, discards the OAL
encapsulation, and delivers the original IP packet to the network layer
(L3). If the OAL fragments are addressed to another node, the OMNI
interface instead acts as an "OAL intermediate node" by re-encapsulating
in new *NET headers and forwarding the new carrier packets over an
underlying interface without reassembling or discarding the OAL
encapsulation. The OAL source and OAL destination are seen as
"neighbors" on the OMNI link, while OAL intermediate nodes are seen as
"bridges" capable of multinet concatenation.The OMNI interface can send/receive original IP packets to/from
underlying interfaces while including/omitting various encapsulations
including OAL, UDP, IP and L2. The network layer can also access the
underlying interfaces directly while bypassing the OMNI interface
entirely when necessary. This architectural flexibility may be
beneficial for underlying interfaces (e.g., some aviation data links)
for which encapsulation overhead may be a primary consideration. OMNI
interfaces that send original IP packets directly over underlying
interfaces without invoking the OAL can only reach peers located on the
same OMNI link segment. However, an ANET proxy that receives the
original IP packet can forward it further by performing OAL
encapsulation with source set to its own address and destination set to
the OAL destination corresponding to the final destination (i.e., even
if the OAL destination is on a different OMNI link segment).Original IP packets sent directly over underlying interfaces are
subject to the same path MTU related issues as for any Internetworking
path, and do not include per-packet identifications that can be used for
data origin verification and/or link-layer retransmissions. Original IP
packets presented directly to an underlying interface that exceed the
underlying network path MTU are dropped with an ordinary ICMPv6 Packet
Too Big (PTB) message returned. These PTB messages are subject to loss
the same as for any non-OMNI IP interface.The OMNI interface encapsulation/decapsulation layering possibilities
are shown in below. Imaginary vertical
lines drawn between the Network Layer and Underlying interfaces in the
figure denote the encapsulation/decapsulation layering combinations
possible. Common combinations include NULL (i.e., direct access to
underlying interfaces with or without using the OMNI interface),
OMNI/IP, OMNI/UDP/IP, OMNI/UDP/IP/L2, OMNI/OAL/UDP/IP, OMNI/OAL/UDP/L2,
etc.The OMNI/OAL model gives rise to a number of opportunities:MNs receive a MNP from the MS, and coordinate with the MS through
IPv6 ND message exchanges. The MN uses the MNP to construct a unique
Link-Local Address (MNP-LLA) through the algorithmic derivation
specified in and assigns the LLA to
the OMNI interface. Since MNP-LLAs are uniquely derived from an MNP,
no Duplicate Address Detection (DAD) or Multicast Listener Discovery
(MLD) messaging is necessary.since Temporary ULAs are statistically unique, they can be used
without DAD, e.g. for MN-to-MN communications until an MNP-LLA is
obtained.underlying interfaces on the same L2 link segment as an AR do not
require any L3 addresses (i.e., not even link-local) in environments
where communications are coordinated entirely over the OMNI
interface.as underlying interface properties change (e.g., link quality,
cost, availability, etc.), any active interface can be used to
update the profiles of multiple additional interfaces in a single
message. This allows for timely adaptation and service continuity
under dynamically changing conditions.coordinating underlying interfaces in this way allows them to be
represented in a unified MS profile with provisions for mobility and
multilink operations.exposing a single virtual interface abstraction to the IPv6 layer
allows for multilink operation (including QoS based link selection,
packet replication, load balancing, etc.) at L2 while still
permitting L3 traffic shaping based on, e.g., DSCP, flow label,
etc.the OMNI interface allows inter-INET traversal when nodes located
in different INETs need to communicate with one another. This mode
of operation would not be possible via direct communications over
the underlying interfaces themselves.the OAL supports lossless and adaptive path MTU mitigations not
available for communications directly over the underlying interfaces
themselves. The OAL supports "packing" of multiple IP payload
packets within a single OAL packet.the OAL applies per-packet identification values that allow for
link-layer reliability and data origin authentication.L3 sees the OMNI interface as a point of connection to the OMNI
link; if there are multiple OMNI links (i.e., multiple MS's), L3
will see multiple OMNI interfaces.Multiple independent OMNI interfaces can be used for increased
fault tolerance through Safety-Based Multilink (SBM), with
Performance-Based Multilink (PBM) applied within each interface.Other opportunities are discussed in .
Note that even when the OMNI virtual interface is present, applications
can still access underlying interfaces either through the network
protocol stack using an Internet socket or directly using a raw socket.
This allows for intra-network (or point-to-point) communications without
invoking the OMNI interface and/or OAL. For example, when an IPv6 OMNI
interface is configured over an underlying IPv4 interface, applications
can still invoke IPv4 intra-network communications as long as the
communicating endpoints are not subject to mobility dynamics. depicts the architectural model for a MN
with an attached EUN connecting to the MS via multiple independent
*NETs. When an underlying interface becomes active, the MN's OMNI
interface sends IPv6 ND messages without encapsulation if the first-hop
Access Router (AR) is on the same underlying link; otherwise, the
interface uses IP-in-IP encapsulation. The IPv6 ND messages traverse the
ground domain *NETs until they reach an AR (AR#1, AR#2, ..., AR#n),
which then coordinates with an INET Mobility Service Endpoint (MSE#1,
MSE#2, ..., MSE#m) and returns an IPv6 ND message response to the MN.
The Hop Limit in IPv6 ND messages is not decremented due to
encapsulation; hence, the OMNI interface appears to be attached to an
ordinary link..-(::EUN:::)
+--------------+ `-(::::)-'
|OMNI interface|
+----+----+----+
+--------|IF#1|IF#2|IF#n|------ +
/ +----+----+----+ \
/ | \
/ | \
v v v
(:::)-. (:::)-. (:::)-.
.-(::*NET:::) .-(::*NET:::) .-(::*NET:::)
`-(::::)-' `-(::::)-' `-(::::)-'
+----+ +----+ +----+
... |AR#1| .......... |AR#2| ......... |AR#n| ...
. +-|--+ +-|--+ +-|--+ .
. | | |
. v v v .
. <----- INET Encapsulation -----> .
. .
. +-----+ (:::)-. .
. |MSE#2| .-(::::::::) +-----+ .
. +-----+ .-(::: INET :::)-. |MSE#m| .
. (::::: Routing ::::) +-----+ .
. `-(::: System :::)-' .
. +-----+ `-(:::::::-' .
. |MSE#1| +-----+ +-----+ .
. +-----+ |MSE#3| |MSE#4| .
. +-----+ +-----+ .
. .
. .
. <----- Worldwide Connected Internetwork ----> .
...........................................................
]]>After the initial IPv6 ND message exchange, the MN (and/or any nodes
on its attached EUNs) can send and receive original IP packets over the
OMNI interface. OMNI interface multilink services will forward the
packets via ARs in the correct underlying *NETs. 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.The OMNI interface observes the link nature of tunnels, including the
Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and the
role of fragmentation and reassembly . The OMNI interface is configured
over one or more underlying interfaces as discussed in , where the interfaces (and their associated *NET
paths) may have diverse MTUs. OMNI interface considerations for
accommodating original IP packets of various sizes are discussed in the
following sections.IPv6 underlying interfaces are REQUIRED to configure a minimum MTU of
1280 bytes and a minimum MRU of 1500 bytes .
Therefore, the minimum IPv6 path MTU is 1280 bytes since routers on the
path are not permitted to perform network fragmentation even though the
destination is required to reassemble more. The network therefore MUST
forward original IP packets of at least 1280 bytes without generating an
IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message . (While the source can apply "source fragmentation"
for locally-generated IPv6 packets up to 1500 bytes and larger still if
it knows the destination configures a larger MRU, this does not affect
the minimum IPv6 path MTU.)IPv4 underlying interfaces are REQUIRED to configure a minimum MTU of
68 bytes and a minimum MRU of 576 bytes . Therefore, when the Don't
Fragment (DF) bit in the IPv4 header is set to 0 the minimum IPv4 path
MTU is 576 bytes since routers on the path support network fragmentation
and the destination is required to reassemble at least that much. The
OMNI interface therefore MUST set DF to 0 in the IPv4 encapsulation
headers of carrier packets that are no larger than 576 bytes, and SHOULD
set DF to 1 in larger carrier packets unless it has a way to determine
the encapsulation destination MRU and has carefully considered the
issues discussed in .The OMNI interface configures an MTU and MRU of 9180 bytes ; the size is therefore not a reflection of the
underlying interface or *NET path MTUs, but rather determines the
largest original IP packet the OAL (and/or underlying interface) can
forward or reassemble. For each OAL destination (i.e., for each OMNI
link neighbor), the OAL source may discover "hard" or "soft" Reassembly
Limit values smaller than the MRU based on receipt of IPv6 ND messages
with OMNI Reassembly Limit sub-options (see: ).
The OMNI interface employs the OAL as an encapsulation sublayer service
to transform original IP packets into OAL packets/fragments, and the OAL
in turn uses *NET encapsulation to forward carrier packets over the
underlying interfaces (see: ).When an OMNI interface forwards an original IP packet from the
network layer for transmission over one or more underlying interfaces,
the OMNI Adaptation Layer (OAL) acting as the OAL source drops the
packet and returns a PTB message if the packet exceeds the MRU and/or
the hard Reassembly Limit for the intended OAL destination. Otherwise,
the OAL source applies encapsulation to form OAL packets subject to
fragmentation producing OAL fragments suitable for *NET encapsulation
and transmission as carrier packets over underlying interfaces as
described in .These carrier packets travel over one or more underlying networks
bridged by OAL intermediate nodes, which re-encapsulate by removing the
*NET headers of the first underlying network and appending *NET headers
appropriate for the next underlying network in succession. (This process
supports the multinet concatenation capability needed for joining
multiple diverse networks.) After re-encapsulation by zero or more OAL
intermediate nodes, the carrier packets arrive at the OAL
destination.When the OAL destination receives the carrier packets, it discards
the *NET headers and reassembles the resulting OAL fragments into an OAL
packet as described in . The OAL destination then
decapsulates the OAL packet to obtain the original IP packet, which it
then delivers to the network layer.The OAL presents an OMNI sublayer abstraction similar to ATM
Adaptation Layer 5 (AAL5). Unlike AAL5 which performs segmentation and
reassembly with fixed-length 53 octet cells over ATM networks, however,
the OAL uses IPv6 encapsulation, fragmentation and reassembly with
larger variable-length cells over heterogeneous underlying networks.
Detailed operations of the OAL are specified in the following
sections.When the network layer forwards an original IP packet into the OMNI
interface, the OAL source inserts an IPv6 encapsulation header but
does not decrement the Hop Limit/TTL of the original IP packet since
encapsulation occurs at a layer below IP forwarding . The OAL source copies the "Type of Service/Traffic
Class" and "Congestion Experienced" values in the original packet's IP header into the
corresponding fields in the OAL header, then sets the OAL header "Flow
Label" as specified in . The OAL source
finally sets the OAL header IPv6 Hop Limit to a conservative value
sufficient to enable loop-free forwarding over multiple concatenated
OMNI link segments and sets the Payload Length to the length of the
original IP packet.The OAL next selects source and destination addresses for the IPv6
header of the resulting OAL packet. MN OMNI interfaces set the OAL
IPv6 header source address to a Unique Local Address (ULA) based on
the Mobile Network Prefix (MNP-ULA), while AR and MSE OMNI interfaces
set the source address to an Administrative ULA (ADM-ULA) (see: ). When a MN OMNI interface does not (yet) have
an MNP-ULA, it can use a Temporary ULA and/or Host Identity Tag (HIT)
instead (see: ).When the OAL source forwards an original IP packet toward a final
destination via an ANET underlying interface, it sets the OAL IPv6
header source address to its own ULA and sets the destination to
either the Administrative ULA (ADM-ULA) of the ANET peer or the Mobile
Network Prefix ULA (MNP-ULA) corresponding to the final destination
(see below). The OAL source then fragments the OAL packet if
necessary, encapsulates the OAL fragments in any ANET headers and
sends the resulting carrier packets to the ANET peer which either
reassembles before forwarding if the OAL destination is its own ULA or
forwards the fragments toward the true OAL destination without first
reassembling otherwise.When the OAL source forwards an original IP packet toward a final
destination via an INET underlying interface, it sets the OAL IPv6
header source address to its own ULA and sets the destination to the
ULA of an OAL destination node on the final *NET segment. The OAL
source then fragments the OAL packet if necessary, encapsulates the
OAL fragments in any *NET headers and sends the resulting carrier
packets toward the OAL destination on the final segment OMNI node
which reassembles before forwarding the original IP packets toward the
final destination.Following OAL IPv6 encapsulation and address selection, the OAL
source next appends a 2 octet trailing Checksum (initialized to 0) at
the end of the original IP packet while incrementing the OAL header
IPv6 Payload Length field to reflect the addition of the trailer. The
format of the resulting OAL packet following encapsulation is shown in
:The OAL source next selects a 32-bit Identification value for the
packet as specified in then calculates the
checksum per the 8-bit Fletcher algorithm specified in . The OAL source calculates the checksum over the
entire OAL packet beginning with a pseudo-header of the IPv6 header
similar to that found in Section 8.1 of and
extending to the end of the (0-initialized) checksum trailer. The OAL
IPv6 pseudo-header is formed as shown in :After calculating the checksum, the OAL source writes the results
over the (0-initialized) trailing checksum octets. The OAL source then
inserts a single OMNI Routing Header (ORH) if necessary (see: ) while incrementing Payload Length to
reflect the addition of the ORH, where the late addition of the ORH is
not covered by the checksum. (Alternatively, the OAL source can defer
ORH insertion until after fragmentation, then manually insert an
identical copy of the ORH between the IPv6 header and Fragment Header
of each fragment while resetting the IPv6 Payload Length and Next
Header fields accordingly.)The OAL source next fragments the OAL packet if necessary while
assuming the IPv4 minimum path MTU (i.e., 576 bytes) as the worst case
for OAL fragmentation regardless of the underlying interface IP
protocol version since IPv6/IPv4 protocol translation and/or
IPv6-in-IPv4 encapsulation may occur in any *NET path. By always
assuming the IPv4 minimum even for IPv6 underlying interfaces, the OAL
source may produce smaller fragments with additional encapsulation
overhead but will always interoperate and never run the risk of loss
due to an MTU restriction or due to presenting an underlying interface
with a carrier packet that exceeds its MRU. Additionally, the OAL path
could traverse multiple *NET "segments" with intermediate OAL
forwarding nodes performing re-encapsulation where the *NET
encapsulation of the previous segment is replaced by the *NET
encapsulation of the next segment which may be based on a different IP
protocol version and/or encapsulation sizes.The OAL source therefore assumes a default minimum path MTU of 576
bytes at each *NET segment for the purpose of generating OAL fragments
for *NET encapsulation and transmission as carrier packets. In the
worst case, each successive *NET segment may re-encapsulate with
either a 20 byte IPv4 or 40 byte IPv6 header, an 8 byte UDP header and
in some cases an IP security encapsulation (40 bytes maximum assumed).
Any *NET segment may also insert a maximum-length (40 byte) ORH as an
extension to the existing 40 byte OAL IPv6 header plus 8 byte Fragment
Header if an ORH was not already present. Assuming therefore an
absolute worst case of (40 + 40 + 8) = 88 bytes for *NET encapsulation
plus (40 + 40 + 8) = 88 bytes for OAL encapsulation leaves (576 - 88 -
88) = 400 bytes to accommodate a portion of the original IP
packet/fragment. The OAL source therefore sets a minimum Maximum
Payload Size (MPS) of 400 bytes as the basis for the minimum-sized OAL
fragment that can be assured of traversing all segments without loss
due to an MTU/MRU restriction. The Maximum Fragment Size (MFS) for OAL
fragmentation is therefore determined by the MPS plus the size of the
OAL encapsulation headers. (Note that the OAL source includes the 2
octet trailer as part of the payload during fragmentation, and the OAL
destination regards it as ordinary payload until reassembly and
checksum verification are complete.)The OAL source SHOULD maintain "path MPS" values for individual OAL
destinations initialized to the minimum MPS and increased to larger
values (up to the OMNI interface MTU) if better information is known
or discovered. For example, when *NET peers share a common underlying
link or a fixed path with a known larger MTU, the OAL source can base
path MPS on this larger size (i.e., instead of 576 bytes) as long as
the *NET peer reassembles before re-encapsulating and forwarding
(while re-fragmenting if necessary). Also, if the OAL source has a way
of knowing the maximum *NET encapsulation size for all segments along
the path it may be able to increase path MPS to reserve additional
room for payload data. The OAL source must include the uncompressed
OAL header size in its path MPS calculation, since a full header could
be included at any time.The OAL source can also actively probe individual OAL destinations
to discover larger path MPS values using packetization layer probes in
a similar fashion as ,
but care must be taken to avoid setting static values for dynamically
changing paths leading to black holes. The probe involves sending an
OAL packet larger than the current path MPS and receiving a small
acknowledgement message in response (with the possible receipt of
link-layer error message in case the probe was lost). For this
purpose, the OAL source can send an NS message with one or more OMNI
options with large PadN sub-options (see: )
in order to receive a small NA response from the OAL destination.
While observing the minimum MPS will always result in robust and
secure behavior, the OAL source should optimize path MPS values when
more efficient utilization may result in better performance (e.g. for
wireless aviation data links). (If so, the OAL source should maintain
separate path MPS values for each (source, target) underlying
interface pair for the same OAL destination, since each underlying
interface pair may support a different path MPS.)When the OAL source performs fragmentation, it SHOULD produce the
minimum number of non-overlapping fragments under current MPS
constraints, where each non-final fragment MUST be at least as large
as the minimum MPS, while the final fragment MAY be smaller. The OAL
source also converts all original IP packets no larger than the
current MPS into "atomic fragments" by including a Fragment Header
with Fragment Offset and More Fragments both set to 0.For each fragment produced, the OAL source writes an ordinal number
for the fragment into the Reserved field in the IPv6 Fragment Header.
In particular, the OAL source writes the ordinal number '0' for the
first fragment, '1' for the second fragment, '2' for the third
fragment, etc. up to and including the final fragment. Since the
minMPS is 400 and the MTU is 9180, at most 23 fragments will be
produced for each OAL packet.The OAL source finally encapsulates the fragments in *NET headers
to form carrier packets and forwards them over an underlying
interface, while retaining the fragments and their ordinal numbers
(i.e., #0, #1, #2, etc.) for a link persistence period in case
link-layer retransmission is requested (see: ).
The formats of OAL fragments and carrier packets are shown in .During *NET encapsulation, the OAL source first encapsulates each
OAL fragment in a UDP header as the first *NET encapsulation sublayer
if NAT traversal, packet filtering middlebox traversal and/or OAL
header compression are necessary. The OAL source then appends any
additional encapsulation sublayer headers necessary and presents the
*NET packet to an underlying interface (see: ).When a UDP header is included, the OAL source next sets the UDP
source port to a constant value that it will use in each successive
carrier packet it sends to the next OAL hop. For packets sent to an
MSE, the OAL source sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a MN peer,
the source sets the UDP destination port to the cached port value for
this peer. The OAL source then sets the UDP length to the total length
of the OAL fragment in correspondence with the OAL header Payload
Length (i.e., the UDP length and IPv6 Payload Length must agree). The
OAL source finally sets the UDP checksum to 0 since the only fields not
already covered by the OAL checksum or underlying *NET CRCs are the
Fragment Header fields, and any corruption in those fields will be
garbage collected by the reassembly algorithm (however, see for additional considerations). The UDP
encapsulation header is often used in association with IP
encapsulation, but may also be used between neighbors on a shared
physical link with a true L2 header format such as for transmission
over IEEE 802 Ethernet links. This document therefore requests a new
Ether Type code assignment TBD1 in the IANA 'ieee-802-numbers'
registry for direct User Datagram Protocol (UDP) encapsulation over
IEEE 802 Ethernet links (see: ).For *NET encapsulations over IP, the OAL source next copies the
"Type of Service/Traffic Class" and
"Congestion Experienced" values in the OAL
IPv6 header into the corresponding fields in the *NET IP header, then
(for IPv6) sets the *NET IPv6 header "Flow Label" as specified in
. The OAL source then sets the *NET IP TTL/Hop
Limit the same as for any *NET host, i.e., it does not copy the Hop
Limit value from the OAL header. For carrier packets undergoing OAL
intermediate node re-encapsulation, the node decrements the OAL IPv6
header Hop Limit and discards the carrier packet if the value reaches
0. The node then copies the "Type of Service/Traffic Class" and
"Congestion Experienced" values from the previous hop *NET
encapsulation header into the OAL IPv6 header before setting the next
hop *NET IP encapsulation header values the same as specified for the
OAL source above.Following *NET encapsulation/re-encapsulation, the OAL source sends
the resulting carrier packets over one or more underlying interfaces.
The underlying interfaces often connect directly to physical media on
the local platform (e.g., a laptop computer with WiFi, etc.), but in
some configurations the physical media may be hosted on a separate
Local Area Network (LAN) node. In that case, the OMNI interface can
establish a Layer-2 VLAN or a point-to-point tunnel (at a layer below
the underlying interface) to the node hosting the physical media. The
OMNI interface may also apply encapsulation at the underlying
interface layer (e.g., as for a tunnel virtual interface) such that
carrier packets would appear "double-encapsulated" on the LAN; the
node hosting the physical media in turn removes the LAN encapsulation
prior to transmission or inserts it following reception. Finally, the
underlying interface must monitor the node hosting the physical media
(e.g., through periodic keepalives) so that it can convey
up/down/status information to the OMNI interface.When an OMNI interface receives a carrier packet from an underlying
interface, the OAL destination discards the *NET encapsulation headers
and examines the OAL header of the enclosed OAL fragment. If the OAL
fragment is addressed to a different node, the OAL destination
re-encapsulates and forwards as discussed below. If the OAL fragment
is addressed to itself, the OAL destination accepts or drops the
fragment based on the (Source, Destination, Identification)-tuple. The
OAL destination next drops all non-final OAL fragments smaller than
the minimum MPS and all fragments that would overlap or leave "holes"
smaller than the minimum MPS with respect to other fragments already
received. The OAL destination records the ordinal number of each
accepted fragment of the same OAL packet (i.e., as Frag #0, Frag #1,
Frag #2, etc.) and admits them into the reassembly cache.When reassembly is complete, the OAL destination removes the ORH if
present while decrementing Payload Length to reflect the removal of
the ORH. The OAL destination next verifies the resulting OAL packet's
checksum and discards the packet if the checksum is incorrect. If the
OAL packet was accepted, the OAL destination then removes the OAL
header/trailer, then delivers the original IP packet to the network
layer. Note that link layers include a CRC-32 integrity check which
provides effective hop-by-hop error detection in the underlying
network for payload sizes up to the OMNI interface MTU , but that some hops may traverse intermediate layers
such as tunnels over IPv4 that do not include integrity checks. The
trailing Fletcher checksum therefore allows the OAL destination to
detect OAL packet splicing errors due to reassembly misassociations
and/or to verify the integrity of OAL packets whose fragments may have
traversed unprotected underlying network hops .
The Fletcher checksum algorithm also provides diversity with respect
to both lower layer CRCs and upper layer Internet checksums as part of
a complimentary multi-layer integrity assurance architecture.When the OAL source and destination are on the same *NET segment,
carrier packet header compression is possible. When the OAL source and
destination exchange initial IPv6 ND messages as discussed in the
following Sections, each caches the observed *NET UDP source port and
source IP (or L2) address associated with the OAL IPv6 source address
found in the full-length OAL IPv6 header. After the initial IPv6 ND
message exchange, the OAL source can apply OAL Header Compression for
subsequent carrier packets to significantly reduce encapsulation
overhead.When the OAL source determines that header compression state has
been established (i.e., following an IPv6 ND message exchange), it can
begin sending OAL fragments with significant portions of the IPv6
header, Fragment Header and OMNI Routing Header (ORH) omitted. For OAL
first-fragments (including atomic fragments), the OAL uses OMNI
Compressed Header - Type 0 (OCH-0) as shown in :In this format, the UDP header appears in its entirety in
the first 8 octets, followed by the first 4 octets of the IPv6 header
and with the remainder omitted. The Version field is set to 0 (to
distinguish OCH-0 from both OCH-1 and a true IP protocol version
number) and is followed by an uncompressed Traffic Class and Flow
Label. This compressed IPv6 header is then followed by a compressed
IPv6 Fragment Header with the Fragment Offset field and two Reserved
bits omitted (since these fields always encode the value 0 in
first-fragments), and with the (M)ore Fragments bit relocated to the
least significant bit of the first Reserved field. The Reserved field
must be set to 0 on transmission and ignored on reception, and the
other fields are set the same as for uncompressed IPv6 fragmentation.
The compressed ORH includes a single omIndex octet that encodes an
underlying interface index for the target Client (or 0 if the target
underlying interface is unspecified). The OCH-0 header is then
followed by the OAL fragment body, and the UDP length field is reduced
by the difference in length between the compressed headers and
full-length (IPv6, Fragment, ORH) headers. The OCH-0 format applies
only for first fragments, which are always regarded as ordinal
fragment 0 even though the OCH-0 does not include an explicit Ordinal
field.For OAL non-first fragments (i.e., those with non-zero Fragment
Offsets), the OAL uses OMNI Compressed Header - Type 1 (OCH-1) as
shown in :In this format, the UDP header appears in its entirety in
the first 8 octets, but all IPv6 header fields except for Version are
omitted. The Version field is set to 1 (to distinguish OCH-1 from both
OCH-0 and a true IP protocol version number) and is followed by a
compressed IPv6 Fragment Header that includes a 5-bit Ordinal number
for this fragment and with other unnecessary fields compressed or
omitted. (R)eserved is set to 0, and (M)ore Fragments/Fragment Offset
are copied from the uncompressed fragment header. The compressed ORH
includes a single omIndex octet that encodes an underlying interface
index for the target Client (or 0 if the target underlying interface
is unspecified). The OCH-1 header is then followed by the OAL fragment
body, while the UDP length field is reduced by the difference in
length between the compressed headers and full-length (IPv6, Fragment,
ORH) headers. The OCH-1 format applies only for non-first fragments,
therefore Ordinal is set to a value beginning with 1 for the first
non-first fragment and monotonically incremented for each successive
non-first fragment up to and including the final fragment.When the OAL destination receives a carrier packet with an OCH, it
first determines the OAL IPv6 source and destination addresses by
examining the UDP source port and L2 source address, then determines
the length by examining the UDP length. The OAL destination then
examines the Version field immediately following the UDP header. If
Version encodes the value 0, the OAL destination processes the
remainder of the header as an OCH-0, then reconstitutes the full-sized
IPv6 and Fragment Headers and adds this OAL fragment to the reassembly
buffer if the fragment is acceptable. If Version encodes the value 1,
the OAL destination instead processes the remainder of the header as
an OCH-1, then reconstitutes the full-sized IPv6 and Fragment Headers.
Note that, since OCH-1 does not include Traffic Class, Flow Label or
Next Header information, the OAL destination writes the value 0 into
those fields when it reconstitutes the full headers. The values will
be correctly populated during reassembly after an OAL first fragment
with an OCH-0 or uncompressed OAL header arrives.When the OAL destination is an LHS Proxy/Server, it examines the
destination address after re-constituting the OAL header. If the
destination address is its own ADM-ULA, the Proxy/Server submits the
resulting OAL fragment for local reassembly. Following reassembly, the
Proxy/Server re-encapsulates the OAL packet (while re-fragmenting if
necessary) and forwards the packet/fragments to the Client underlying
interface identified by omIndex. If the destination address is the
MNP-ULA of one of its Clients, the Proxy/Server instead forwards the
OAL fragment via the Client underlying interface identified by
omIndex. If the header compression state and/or destination address
are not recognized, the Proxy/Server instead drops the packet.When the OAL destination is the Client, it examines the destination
address after re-constituting the OAL header. If the destination
address is its own MNP-ULA, the Client submits the resulting OAL
fragment for local reassembly. Otherwise, the Client drops the
packet.Note: OAL header compression does not interfere with checksum
calculation and verification, which must be applied according to the
full OAL pseudo-header per even when
compression is used. Carrier packets may further include uncompressed
headers at any time even after header compression state has been
established.The OAL encapsulates each original IP packet as an OAL packet then
performs fragmentation to produce one or more carrier packets with the
same 32-bit Identification value. In environments were spoofing is not
considered a threat, OAL nodes can send OAL packets beginning with a
random initial Identification value and incremented (modulo 2**32) for
each successive packet. In other environments, OAL nodes should
maintain explicit per-neighbor send and receive windows to exclude
spurious carrier packets that might clutter the reassembly cache. OAL
neighbors maintain windows using TCP-like synchronization with Identification sequence numbers beginning with
an unpredictable initial value and
incremented (modulo 2 *32) for each successive OAL packet.OAL neighbors exchange IPv6 ND messages with OMNI options that
include TCP-like information fields to manage streams of OAL packets
instead of streams of octets. As a link-layer service, the OAL
provides low-persistence best-effort retransmission with no
mitigations for duplication, reordering or deterministic delivery.
Since the service model is best-effort and only control message
sequence numbers are acknowledged, OAL nodes can select unpredictable
new initial sequence numbers outside of the current window without
delaying for the Maximum Segment Lifetime (MSL).OAL neighbors maintain current and previous window state in IPv6 ND
neighbor cache entries (NCEs) to support dynamic rollover to a new
window while still sending OAL packets and accepting carrier packets
from the previous windows. Each NCE is indexed by the neighbor's LLA,
which must also match the ULA used for OAL encapsulation. OAL
neighbors synchronize windows through asymmetric and/or symmetric IPv6
ND message exchanges. When an OAL neighbor receives an IPv6 ND message
with new window information, it resets the previous window state based
on the current window then resets the current window based on new
and/or pending information.The IPv6 ND message OMNI option header includes TCP-like
information fields including Sequence Number, Acknowledgement Number,
Window and flags (see: ). OAL neighbors
maintain the following TCP-like state variables in the NCE:OAL neighbors "OAL A" and "OAL B" exchange IPv6 ND messages per
with OMNI options that include TCP-like
information fields. When OAL A synchronizes with OAL B, it maintains
both a current and previous SND.WND beginning with a new unpredictable
ISS and monotonically increments SND.NXT for each successive OAL
packet transmission. OAL A initiates synchronization by including the
new ISS in the Sequence Number of an authentic IPv6 ND NS/RS message
with the SYN flag set and with Window set to M as a tentative receive
window size while creating a NCE in the INCOMPLETE state if necessary.
OAL A caches the new ISS as pending, uses the new ISS as the
Identification for OAL encapsulation, then sends the resulting OAL
packet to OAL B and waits up to RetransTimer milliseconds to receive a
solicited NA/RA ACK response (retransmitting up to MAX_UNICAST_SOLICIT
times if necessary).When OAL B receives the carrier packets containing the NS/RS SYN,
it creates a NCE in the STALE state if necessary, resets its RCV
variables, caches the tentative (send) window size M, and selects a
(receive) window size N (up to 2^24) to indicate the number of OAL
packets it is willing to accept under the current RCV.WND. (The
RCV.WND should be large enough to minimize control message overhead
yet small enough to provide an effective filter for spurious carrier
packets.) OAL B then prepares a solicited NA/RA message with the ACK
flag set, with the Acknowledgement Number set to OAL A's next sequence
number, and with Window set to N. Since OAL B does not assert an ISS
of its own, it uses OAL A's IRS as the Identification for OAL
encapsulation then sends the resulting OAL packet to OAL A.When OAL A receives the carrier packets containing the solicited
NA/RA, it notes that their Identification matches its pending ISS. OAL
A then sets the NCE state to REACHABLE and resets its SND variables
based on the Window size and Acknowledgement Number (which must
include the sequence number following the pending ISS). OAL A can then
begin sending OAL packets to OAL B with Identification values within
the (new) current SND.WND for up to ReachableTime milliseconds or
until the NCE is updated by a new IPv6 ND message exchange. This
implies that OAL A must send a new NS/RS SYN message before sending
more than N OAL packets within the current SND.WND, i.e., even if
ReachableTime is not nearing expiration.After OAL B returns the solicited NA/RA, it accepts carrier packets
received from OAL A within either the current or previous RCV.WND as
well as any new authentic NS/RS SYN messages received from OAL A even
if outside the windows. IPv6 ND messages used for window
synchronization must therefore fit within a single carrier packet
(i.e., within current MPS constraints), since the carrier packets of
fragmented IPv6 ND messages with out-of-window Identification values
could be part of a DoS attack and should not be admitted into the
reassembly cache. OAL B discards all other carrier packets received
from OAL A with out-of-window Identifications.OAL neighbors can employ asymmetric window synchronization as
described above using two independent [(NS/RS SYN) -> (NA/RA ACK)]
exchanges (i.e., a four-message exchange), or they can employ
symmetric window synchronization using a modified version of the TCP
three-way handshake as follows:OAL A prepares an NS/RS SYN message with an unpredictable ISS
not within the current SND.WND and with Window set to M as a
tentative receive window size. OAL A caches the new ISS and Window
size as pending information, uses the pending ISS as the
Identification for OAL encapsulation, then sends the resulting OAL
packet to OAL B and waits up to RetransTimer milliseconds to
receive a solicited NA/RA ACK response (retransmitting up to
MAX_UNICAST_SOLICIT times if necessary).OAL B receives the carrier packets containing the NS/RS SYN,
then resets its RCV variables based on the Sequence Number while
caching OAL A's tentative receive Window size M and a new
unpredictable ISS outside of its current window as pending
information. OAL B then prepares a solicited NA/RA response with
Sequence Number set to the pending ISS and Acknowledgement Number
set to OAL A's next sequence number. OAL B then sets both the SYN
and ACK flags, sets Window to N and sets the OPT flag according to
whether an explicit NS ACK is optional or mandatory. OAL B then
uses the pending ISS as the Identification for OAL encapsulation,
sends the resulting OAL packet to OAL A and waits up to
RetransTimer milliseconds to receive an acknowledgement
(retransmitting up to MAX_UNICAST_SOLICIT times if necessary).OAL A receives the carrier packets containing the NA/RA
SYN/ACK, then resets its SND variables based on the
Acknowledgement Number (which must include the sequence number
following the pending ISS) and OAL B's advertised Window N. OAL A
then resets its RCV variables based on the Sequence Number and
marks the NCE as REACHABLE. If the OPT flag is clear, OAL A next
prepares an immediate solicited NA message with the ACK flag set,
the Acknowledgement Number set to OAL B's next sequence number,
with Window set a value that may be the same as or different than
M, and with the OAL encapsulation Identification to SND.NXT, then
sends the resulting OAL packet to OAL B. If the OPT flag is set
and OAL A has OAL packets queued to send to OAL B, it can
optionally begin sending their carrier packets under the (new)
current SND.WND as implicit acknowledgements instead of returning
an explicit NA ACK. In that case, the tentative Window size M
becomes the current receive window size.OAL B receives the implicit/explicit acknowledgement(s) then
resets its SND state based on the pending/advertised values and
marks the NCE as REACHABLE. If OAL B receives an explicit
acknowledgement, it uses the advertised Window size and abandons
the tentative size. (Note that OAL B sets the OPT flag in the NA
SYN/ACK to assert that it will interpret timely receipt of carrier
packets within the (new) current window as an implicit
acknowledgement. Potential benefits include reduced delays and
control message overhead, but use case analysis is outside the
scope of this specification.)Following synchronization, OAL A and OAL B hold updated NCEs and
can exchange OAL packets with Identifications set to SND.NXT while the
state remains REACHABLE and there is available window capacity. Either
neighbor may at any time send a new NS/RS SYN to assert a new ISS. For
example, if OAL A's current SND.WND for OAL B is nearing exhaustion
and/or ReachableTime is nearing expiration, OAL A continues to send
OAL packets under the current SND.WND while also sending an NS/RS SYN
with a new unpredictable ISS. When OAL B receives the NS/RS SYN, it
resets its RCV variables and may optionally return either an
asymmetric NA/RA ACK or a symmetric NA/AR SYN/ACK to also assert a new
ISS. While sending IPv6 ND SYNs, both neighbors continue to send OAL
packets with Identifications set to the current SND.NXT then reset the
SND variables after an acknowledgement is received.While the optimal symmetric exchange is efficient, anomalous
conditions such as receipt of old duplicate SYNs can cause confusion
for the algorithm as discussed in Section 3.4 of . For this reason, the OMNI option header includes
an RST flag which OAL nodes set in solicited NA responses to ACKs
received with incorrect acknowledgement numbers. The RST procedures
(and subsequent synchronization recovery) are conducted exactly as
specified in .OAL nodes may set the PNG ("ping") flag in IPv6 ND advertisement
messages when a reachability confirmation is needed. (OAL nodes
therefore most often set the PNG flag in (unsolicited) advertisement
messages and ignore it in solicitation messages.) When an OAL node
receives a PNG, it returns a solicited NA ACK with the PNG message
Identification in the Acknowledgment, but without updating RCV state
variables. OAL nodes return unicast solicited NA ACKs even for
multicast PNG destination addresses, since OMNI link multicast is
based on unicast emulation. OAL nodes may also send unsolicited NA
messages to request selective retransmissions (see: ).OAL nodes that employ the window synchronization procedures
described above observe the following requirements:OAL nodes MUST select new unpredictable ISS values that are
outside of the current SND.WND.OAL nodes MUST set the initial NS SYN message Window field to a
tentative value to be used only if no concluding NA ACK is
sent.OAL nodes that receive NA/RA messages with the PNG and/or SYN
flag set MUST NOT set the PNG and/or SYN flag in solicited NA
responses.OAL nodes that send NA/RA messages with the PNG and/or SYN flag
set MUST ignore solicited NA responses with the PNG and/or SYN
flag set.OAL nodes MUST send ND messages with authentication signatures
while using unpredictable Identification values until window
synchronization is complete.When an OAL node sends an RS SYN to the All-Routers multicast
address, it may receive multiple unicast RA ACK or SYN/ACK replies -
each with a distinct LLA source address. The OAL node then creates a
separate NCE for each distinct neighbor and completes window
synchronization through independent message exchanges with each
neighbor. The fact that all neighbors receive the same ISS in the
original RS SYN is not a matter for concern, as further window
synchronization will be conducted on a per-neighbor basis.Note: Although the OAL employs TCP-like window synchronization and
supports solicited NA ACK responses to NA/RA SYNs and PNGs, all other
aspects of the IPv6 ND protocol (e.g., control message exchanges, NCE
state management, timers, retransmission limits, etc.) are honored
exactly per .Note: Recipients of OAL-encapsulated IPv6 ND messages index the NCE
based on the ULA source address, which also determines the carrier
packet Identification window. However, IPv6 ND messages may contain an
LLA source address that does not match the ULA source address when the
recipient acts as an ND proxy.When the OAL source sends carrier packets to an OAL destination, it
should cache recently sent packets in case best-effort selective
retransmission is requested. The OAL destination in turn maintains a
checklist for the (Source, Destination, Identification)-tuple of
recently received carrier packets and notes the ordinal numbers of OAL
packet fragments already received (i.e., as Frag #0, Frag #1, Frag #2,
etc.). The timeframe for maintaining the OAL source and destination
caches determines the link persistence (see: ).If the OAL destination notices some fragments missing after most
other fragments within the same link persistence timeframe have
already arrived, it may send a uNA message to the OAL source. The OAL
destination creates a uNA message with an OMNI option containing an
authentication sub-option to provide authentication (if the OAL source
is on an open Internetwork) and one or more Fragmentation Report
sub-options that include a list of (Identification, Bitmap)-tuples for
fragments received and missing from this OAL source (see: ). The OAL destination signs the message if an
authentication sub-option is included, performs OAL encapsulation
(with the its own address as the OAL source and the source address of
the message that prompted the uNA as the OAL destination) and sends
the message to the OAL source.When the OAL source receives the uNA message, it authenticates the
message using the authentication sub-option (if present) then examines
the Fragmentation Report. For each (Source, Destination,
Identification)-tuple, the OAL source determines whether it still
holds the corresponding carrier packets in its cache and retransmits
any for which the Bitmap indicates a loss event. For example, if the
Bitmap indicates that ordinal fragments #3, #7, #10 and #13 from the
same OAL packet are missing the OAL source only retransmits carrier
packets containing those fragments and no others. When the OAL
destination receives the retransmitted carrier packets, it admits the
enclosed fragments into the reassembly cache and updates its
checklist. If some fragments are still missing, the OAL destination
may repeat the request in a small number of additional uNAs within the
link persistence timeframe.Note that the OAL provides a link-layer low persistence Automatic
Repeat Request (ARQ) service based on Selective Repeat (SR) capability
consistent with and Section 8.1 of . The service provides the benefit of timely
best-effort link-layer retransmissions which may reduce packet loss
and avoid some unnecessary end-to-end delays.When the OMNI interface forwards original IP packets from the
network layer, it invokes the OAL and returns internally-generated
ICMPv4 Fragmentation Needed or ICMPv6 Path
MTU Discovery (PMTUD) Packet Too Big (PTB)
messages as necessary. This document refers to both of these
ICMPv4/ICMPv6 message types simply as "PTBs", and introduces a
distinction between PTB "hard" and "soft" errors as discussed
below.Ordinary PTB messages with ICMPv4 header "unused" field or ICMPv6
header Code field value 0 are hard errors that always indicate that a
packet has been dropped due to a real MTU restriction. In particular,
the OAL source drops the packet and returns a PTB hard error if the
packet exceeds the OAL destination MRU. However, the OMNI interface
can also forward large original IP packets via OAL encapsulation and
fragmentation while at the same time returning PTB soft error messages
(subject to rate limiting) if it deems the original IP packet too
large according to factors such as link performance characteristics,
reassembly congestion, etc. This ensures that the path MTU is adaptive
and reflects the current path used for a given data flow. The OMNI
interface can therefore continuously forward packets without loss
while returning PTB soft error messages recommending a smaller size if
necessary. Original sources that receive the soft errors in turn
reduce the size of the packets they send (i.e., the same as for hard
errors), but can soon resume sending larger packets if the soft errors
subside.An OAL source sends PTB soft error messages by setting the ICMPv4
header "unused" field or ICMPv6 header Code field to the value 1 if a
original IP packet was deemed lost (e.g., due to reassembly timeout)
or to the value 2 otherwise. The OAL source sets the PTB destination
address to the original IP packet source, and sets the source address
to one of its OMNI interface unicast/anycast addresses that is
routable from the perspective of the original source. The OAL source
then sets the MTU field to a value smaller than the original packet
size but no smaller than 576 for ICMPv4 or 1280 for ICMPv6, writes the
leading portion of the original IP packet into the "packet in error"
field, and returns the PTB soft error to the original source. When the
original source receives the PTB soft error, it temporarily reduces
the size of the packets it sends the same as for hard errors but may
seek to increase future packet sizes dynamically while no further soft
errors are arriving. (If the original source does not recognize the
soft error code, it regards the PTB the same as a hard error but
should heed the retransmission advice given in suggesting retransmission based on normal
packetization layer retransmission timers.)An OAL destination may experience reassembly cache congestion, and
can return uNA messages to the OAL source that originated the
fragments (subject to rate limiting) to advertise reduced hard/soft
Reassembly Limits and/or to report individual reassembly failures. The
OAL destination creates a uNA message with an OMNI option containing
an authentication message sub-option (if the OAL source is on an open
Internetwork) followed optionally by at most one hard and one soft
Reassembly Limit sub-options with reduced hard/soft values, and with
one of them optionally including the leading portion an OAL first
fragment containing the header of an original IP packet whose source
must be notified (see: ). The OAL
destination encapsulates the leading portion of the OAL first fragment
(beginning with the OAL header) in the "OAL First Fragment" field of
sub-option, signs the message if an authentication sub-option is
included, performs OAL encapsulation (with the its own address as the
OAL source and the source address of the message that prompted the uNA
as the OAL destination) and sends the message to the OAL source.When the OAL source receives the uNA message, it records the new
hard/soft Reassembly Limit values for this OAL destination if the OMNI
option includes Reassembly Limit sub-options. If a hard or soft
Reassembly Limit sub-option includes an OAL First Fragment, the OAL
source next sends a corresponding network layer PTB hard or soft error
to the original source to recommend a smaller size. For hard errors,
the OAL source sets the PTB Code field to 0. For soft errors, the OAL
source sets the PTB Code field to 1 if the L flag in the Reassembly
Limit sub-option is 1; otherwise, the OAL source sets the Code field
to 2. The OAL source crafts the PTB by extracting the leading portion
of the original IP packet from the OAL First Fragment field (i.e., not
including the OAL header) and writes it in the "packet in error" field
of a PTB with destination set to the original IP packet source and
source set to one of its OMNI interface unicast/anycast addresses that
is routable from the perspective of the original source. For future
transmissions, if the original IP packet is larger than the hard
Reassembly Limit for this OAL destination the OAL source drops the
packet and returns a PTB hard error with MTU set to the hard
Reassembly Limit. If the packet is no larger than the current hard
Reassembly Limit but larger than the current soft limit, the OAL
source can also return a PTB soft error (subject to rate limiting)
with Code set to 2 and MTU set to the current soft limit while still
forwarding the packet to the OMNI destination.Original sources that receive PTB soft errors can dynamically tune
the size of the original IP packets they to send to produce the best
possible throughput and latency, with the understanding that these
parameters may change over time due to factors such as congestion,
mobility, network path changes, etc. The receipt or absence of soft
errors should be seen as hints of when increasing or decreasing packet
sizes may be beneficial. The OMNI interface supports continuous
transmission and reception of packets of various sizes in the face of
dynamically changing network conditions. Moreover, since PTB soft
errors do not indicate a hard limit, original sources that receive
soft errors can begin sending larger packets without waiting for the
recommended 10 minutes specified for PTB hard errors . The OMNI interface
therefore provides an adaptive service that accommodates MTU diversity
especially well-suited for dynamic multilink environments.In light of the above, OAL sources, destinations and intermediate
nodes observe the following normative requirements:OAL sources MUST NOT send OAL fragments including original IP
packets larger than the OMNI interface MTU or the OAL destination
hard Reassembly Limit, i.e., whether or not fragmentation is
needed.OAL sources MUST NOT fragment original IP packets smaller than
the minimum MPS minus the trailer size, but must instead
encapsulate them as atomic fragments.OAL sources MUST produce non-final fragments with payloads no
smaller than the minimum MPS during fragmentation.OAL sources MUST NOT send OAL fragments that include any
extension headers other than a single ORH and a single Fragment
Header.OAL intermediate nodes SHOULD and OAL destinations MUST
unconditionally drop any OAL fragments with offset and length that
would cause the reassembled packet to exceed the OMNI interface
MRU and/or OAL destination hard Reassembly Limit.OAL intermediate nodes SHOULD and OAL destinations MUST
unconditionally drop any non-final OAL fragments with payloads
smaller than the minimum MPS.OAL intermediate nodes SHOULD and OAL destinations MUST
unconditionally drop OAL fragments that include any extension
headers other than a single ORH and a single Fragment Header.OAL destinations MUST drop any new OAL fragments with Offset
and Payload length that would overlap with other fragments and/or
leave holes smaller than the minimum MPS between fragments that
have already been received.Note: Under the minimum MPS, ordinary 1500 byte original IP packets
would require at most 4 OAL fragments, with each non-final fragment
containing 400 payload bytes and the final fragment containing 302
payload bytes (i.e., the final 300 bytes of the original IP packet
plus the 2 octet trailer). Likewise, maximum-length 9180 byte original
IP packets would require at most 23 fragments. For all packet sizes,
the likelihood of successful reassembly may improve when the OMNI
interface sends all fragments of the same fragmented OAL packet
consecutively over the same underlying interface pair instead of
spread across multiple underlying interface pairs. Finally, an assured
minimum/path MPS allows continuous operation over all paths including
those that traverse bridged L2 media with dissimilar MTUs.Note: Certain legacy network hardware of the past millennium was
unable to accept packet "bursts" resulting from an IP fragmentation
event - even to the point that the hardware would reset itself when
presented with a burst. This does not seem to be a common problem in
the modern era, where fragmentation and reassembly can be readily
demonstrated at line rate (e.g., using tools such as 'iperf3') even
over fast links on ordinary hardware platforms. Even so, the OAL
source could impose an inter-fragment delay while the OAL destination
is reporting reassembly congestion (see: ) and
decrease the delay when reassembly congestion subsides.As discussed in Section 3.7 of , there are
four basic threats concerning IPv6 fragmentation; each of which is
addressed by effective mitigations as follows:Overlapping fragment attacks - reassembly of overlapping
fragments is forbidden by ; therefore,
this threat does not apply to the OAL.Resource exhaustion attacks - this threat is mitigated by
providing a sufficiently large OAL reassembly cache and
instituting “fast discard” of incomplete reassemblies
that may be part of a buffer exhaustion attack. The reassembly
cache should be sufficiently large so that a sustained attack does
not cause excessive loss of good reassemblies but not so large
that (timer-based) data structure management becomes
computationally expensive. The cache should also be indexed based
on the arrival underlying interface such that congestion
experienced over a first underlying interface does not cause
discard of incomplete reassemblies for uncongested underlying
interfaces.Attacks based on predictable fragment identification values -
in environments where spoofing is possible, this threat is
mitigated through the use of Identification windows per . By maintaining windows of acceptable
Identifications beginning with unpredictable values, OAL neighbors
can quickly discard spurious carrier packets that might otherwise
clutter the reassembly cache. The OAL additionally provides an
integrity check to detect corruption that may be caused by
spurious fragments received with in-window Identification
values.Evasion of Network Intrusion Detection Systems (NIDS) - since
the OAL source employs a robust MPS, network-based firewalls can
inspect and drop OAL fragments containing malicious data thereby
disabling reassembly by the OAL destination. However, since OAL
fragments may take different paths through the network (some of
which may not employ a firewall) each OAL destination must also
employ a firewall.IPv4 includes a 16-bit Identification (IP ID) field with only
65535 unique values such that at high data rates the field could wrap
and apply to new carrier packets while the fragments of old packets
using the same IP ID are still alive in the network . Since carrier packets sent via an IPv4 path with
DF=0 are normally no larger than 576 bytes, IPv4 fragmentation is
possible only at small-MTU links in the path which should support data
rates low enough for safe reassembly . (IPv4
carrier packets larger than 576 bytes with DF=0 may incur high data
rate reassembly errors in the path, with the OAL destination checksum
providing a last-resort integrity verification.) Since IPv6 provides a
32-bit Identification value, IP ID wraparound at high data rates is
not a concern for IPv6 fragmentation.Fragmentation security concerns for large IPv6 ND messages are
documented in . These concerns are addressed
when the OMNI interface employs the OAL instead of directly
fragmenting the IPv6 ND message itself. For this reason, OMNI
interfaces MUST NOT send IPv6 ND messages larger than the OMNI
interface MTU, and MUST employ OAL encapsulation and fragmentation for
IPv6 ND messages larger than the current MPS for this OAL
destination.Unless the path is secured at the network-layer or below (i.e., in
environments where spoofing is possible), OMNI interfaces MUST NOT
send ordinary carrier packets with Identification values outside the
current window and MUST secure IPv6 ND messages used for address
resolution or window state synchronization. OAL destinations SHOULD
therefore discard without reassembling any out-of-window OAL fragments
received over an unsecured path.By default, the OAL source includes a 40-byte IPv6 encapsulation
header for each original IP packet during OAL encapsulation. The OAL
source also calculates and appends a 2 octet trailing checksum then
performs fragmentation such that a copy of the 40-byte IPv6 header
plus an 8-byte IPv6 Fragment Header is included in each OAL fragment
(when an ORH is added, the OAL encapsulation headers become larger
still). However, these encapsulations may represent excessive overhead
in some environments. OAL header compression can dramatically reduce
the amount of encapsulation overhead, however a complimentary
technique known as "packing" (see: ) is also supported so that
multiple original IP packets and/or control messages can be included
within a single OAL "super-packet".When the OAL source has multiple original IP packets to send to the
same OAL destination with total length no larger than the OAL
destination MRU, it can concatenate them into a super-packet
encapsulated in a single OAL header and trailing checksum. Within the
OAL super-packet, the IP header of the first original IP packet (iHa)
followed by its data (iDa) is concatenated immediately following the
OAL header, then the IP header of the next original packet (iHb)
followed by its data (iDb) is concatenated immediately following the
first original packet, etc. with the trailing checksum included last.
The OAL super-packet format is transposed from and shown in :
+-----+-----+
| iHa | iDa |
+-----+-----+
|
| +-----+-----+
| | iHb | iDb |
| +-----+-----+
| |
| | +-----+-----+
| | | iHc | iDc |
| | +-----+-----+
| | |
v v v
+----------+-----+-----+-----+-----+-----+-----+----+
| OAL Hdr | iHa | iDa | iHb | iDb | iHc | iDc |Csum|
+----------+-----+-----+-----+-----+-----+-----+----+
<--- OAL "Super-Packet" with single OAL Hdr/Csum --->
]]>When the OAL source prepares a super-packet, it applies OAL
fragmentation and *NET encapsulation then sends the carrier packets to
the OAL destination. When the OAL destination receives the
super-packet it reassembles if necessary, verifies and removes the
trailing checksum, then regards the remaining OAL header Payload
Length as the sum of the lengths of all payload packets. The OAL
destination then selectively extracts each original IP packet (e.g.,
by setting pointers into the super-packet buffer and maintaining a
reference count, by copying each packet into a separate buffer, etc.)
and forwards each packet to the network layer. During extraction, the
OAL determines the IP protocol version of each successive original IP
packet 'j' by examining the four most-significant bits of iH(j), and
determines the length of the packet by examining the rest of iH(j)
according to the IP protocol version.The OMNI interface forwards original IP packets from the network
layer by first invoking the OAL to create OAL packets/fragments if
necessary, then including any *NET encapsulations and finally engaging
the native frame format of the underlying interface. For example, for
Ethernet-compatible interfaces the frame format is specified in , for aeronautical radio interfaces the frame format
is specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical
Manual), for various forms of tunnels the frame format is found in the
appropriate tunneling specification, etc.See for a map of the various *NET
layering combinations possible. For any layering combination, the final
layer (e.g., UDP, IP, Ethernet, etc.) must have an assigned number and
frame format representation that is compatible with the selected
underlying interface.OMNI nodes are assigned OMNI interface IPv6 Link-Local Addresses
(LLAs) through pre-service administrative actions. "MNP-LLAs" embed the
MNP assigned to the mobile node, while "ADM-LLAs" include an
administratively-unique ID that is guaranteed to be unique on the link.
LLAs are configured as follows:IPv6 MNP-LLAs encode the most-significant 64 bits of a MNP within
the least-significant 64 bits of the IPv6 link-local prefix
fe80::/64, i.e., in the LLA "interface identifier" portion. The
prefix length for the LLA is determined by adding 64 to the MNP
prefix length. For example, for the MNP 2001:db8:1000:2000::/56 the
corresponding MNP-LLA is fe80::2001:db8:1000:2000/120. Non-MNP
routes are also represented the same as for MNP-LLAs, but include a
GUA prefix that is not properly covered by the MSP.IPv4-compatible MNP-LLAs are constructed as fe80::ffff:[IPv4],
i.e., the interface identifier consists of 16 '0' bits, followed by
16 '1' bits, followed by a 32bit IPv4 address/prefix. The prefix
length for the LLA is determined by adding 96 to the MNP prefix
length. For example, the IPv4-Compatible MN OMNI LLA for
192.0.2.0/24 is fe80::ffff:192.0.2.0/120 (also written as
fe80::ffff:c000:0200/120).ADM-LLAs are assigned to ARs and MSEs and MUST be managed for
uniqueness. The lower 32 bits of the LLA includes a unique integer
"MSID" value between 0x00000001 and 0xfeffffff, e.g., as in fe80::1,
fe80::2, fe80::3, etc., fe80::feffffff. The ADM-LLA prefix length is
determined by adding 96 to the MSID prefix length. For example, if
the prefix length for MSID 0x10012001 is 16 then the ADM-LLA prefix
length is set to 112 and the LLA is written as fe80::1001:2001/112.
The "zero" address for each ADM-LLA prefix is the Subnet-Router
anycast address for that prefix ; for
example, the Subnet-Router anycast address for fe80::1001:2001/112
is simply fe80::1001:2000. The MSID range 0xff000000 through
0xffffffff is reserved for future use.Since the prefix 0000::/8 is "Reserved by the IETF" , no MNPs can be allocated from that block ensuring
that there is no possibility for overlap between the different MNP- and
ADM-LLA constructs discussed above.Since MNP-LLAs are based on the distribution of administratively
assured unique MNPs, and since ADM-LLAs are guaranteed unique through
administrative assignment, OMNI interfaces set the autoconfiguration
variable DupAddrDetectTransmits to 0 .Note: If future protocol extensions relax the 64-bit boundary in IPv6
addressing, the additional prefix bits of an MNP could be encoded in
bits 16 through 63 of the MNP-LLA. (The most-significant 64 bits would
therefore still be in bits 64-127, and the remaining bits would appear
in bits 16 through 48.) However, the analysis provided in suggests that the 64-bit boundary will remain in the
IPv6 architecture for the foreseeable future.Note: Even though this document honors the 64-bit boundary in IPv6
addressing, it specifies prefix lengths longer than /64 for routing
purposes. This effectively extends IPv6 routing determination into the
interface identifier portion of the IPv6 address, but it does not
redefine the 64-bit boundary. Modern routing protocol implementations
honor IPv6 prefixes of all lengths, up to and including /128.OMNI domains use IPv6 Unique-Local Addresses (ULAs) as the source and
destination addresses in OAL packet IPv6 encapsulation headers. ULAs are
only routable within the scope of a an OMNI domain, and are derived from
the IPv6 Unique Local Address prefix fc00::/7 followed by the L bit set
to 1 (i.e., as fd00::/8) followed by a 40-bit pseudo-random Global ID to
produce the prefix [ULA]::/48, which is then followed by a 16-bit Subnet
ID then finally followed by a 64 bit Interface ID as specified in
Section 3 of . All nodes in the same OMNI domain
configure the same 40-bit Global ID as the OMNI domain identifier. The
statistic uniqueness of the 40-bit pseudo-random Global ID allows
different OMNI domains to be joined together in the future without
requiring renumbering.Each OMNI link instance is identified by a value between 0x0000 and
0xfeff in bits 48-63 of [ULA]::/48; the values 0xff00 through 0xfffe are
reserved for future use, and the value 0xffff denotes the presence of a
Temporary ULA (see below). For example, OMNI ULAs associated with
instance 0 are configured from the prefix [ULA]:0000::/64, instance 1
from [ULA]:0001::/64, instance 2 from [ULA]:0002::/64, etc. ULAs and
their associated prefix lengths are configured in correspondence with
LLAs through stateless prefix translation where "MNP-ULAs" are assigned
in correspondence to MNP-LLAs and "ADM-ULAs" are assigned in
correspondence to ADM-LLAs. For example, for OMNI link instance
[ULA]:1010::/64:the MNP-ULA corresponding to the MNP-LLA fe80::2001:db8:1:2 with
a 56-bit MNP length is derived by copying the lower 64 bits of the
LLA into the lower 64 bits of the ULA as [ULA]:1010:2001:db8:1:2/120
(where, the ULA prefix length becomes 64 plus the IPv6 MNP
length).the MNP-ULA corresponding to fe80::ffff:192.0.2.0 with a 28-bit
MNP length is derived by simply writing the LLA interface ID into
the lower 64 bits as [ULA]:1010:0:ffff:192.0.2.0/124 (where, the ULA
prefix length is 64 plus 32 plus the IPv4 MNP length).the ADM-ULA corresponding to fe80::1000/112 is simply
[ULA]:1010::1000/112.the ADM-ULA corresponding to fe80::/128 is simply
[ULA]:1010::/128.etc.Each OMNI interface assigns the Anycast ADM-ULA specific to the OMNI
link instance. For example, the OMNI interface connected to instance 3
assigns the Anycast address [ULA]:0003::/128. Routers that configure
OMNI interfaces advertise the OMNI service prefix (e.g.,
[ULA]:0003::/64) into the local routing system so that applications can
direct traffic according to SBM requirements.The ULA presents an IPv6 address format that is routable within the
OMNI routing system and can be used to convey link-scoped IPv6 ND
messages across multiple hops using IPv6 encapsulation . 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 OAL encapsulation is omitted
whenever possible to conserve bandwidth (see: ).Each OMNI link can be subdivided into "segments" that often
correspond to different administrative domains or physical partitions.
OMNI nodes can use IPv6 Segment Routing when
necessary to support efficient forwarding to destinations located in
other OMNI link segments. A full discussion of Segment Routing over the
OMNI link appears in .Temporary ULAs are constructed per based on
the prefix [ULA]:ffff::/64 and used by MNs when they have no other
addresses. Temporary ULAs can be used for MN-to-MN communications
outside the context of any supporting OMNI link infrastructure, and can
also be used as an initial address while the MN is in the process of
procuring an MNP. Temporary ULAs are not routable within the OMNI
routing system, and are therefore useful only for OMNI link "edge"
communications. Temporary ULAs employ optimistic DAD principles since they are probabilistically unique.Note: IPv6 ULAs taken from the prefix fc00::/7 followed by the L bit
set to 0 (i.e., as fc00::/8) are never used for OMNI OAL addressing,
however the range could be used for MSP/MNP addressing under certain
limiting conditions (see: ).OMNI domains use IP Global Unicast Address (GUA) prefixes as Mobility Service Prefixes (MSPs) from which Mobile
Network Prefixes (MNP) are delegated to Mobile Nodes (MNs). Fixed
correspondent node networks reachable from the OMNI domain are
represented by non-MNP GUA prefixes that are not derived from the MSP,
but are treated in all other ways the same as for MNPs.For IPv6, GUA prefixes are assigned by IANA
and/or an associated regional assigned numbers authority such that the
OMNI domain can be interconnected to the global IPv6 Internet without
causing inconsistencies in the routing system. An OMNI domain could
instead use ULAs with the 'L' bit set to 0 (i.e., from the prefix
fc00::/8), however this would require
IPv6 NAT if the domain were ever connected to the global IPv6
Internet.For IPv4, GUA prefixes are assigned by IANA
and/or an associated regional assigned numbers authority such that the
OMNI domain can be interconnected to the global IPv4 Internet without
causing routing inconsistencies. An OMNI domain could instead use
private IPv4 prefixes (e.g., 10.0.0.0/8, etc.) ,
however this would require IPv4 NAT if the domain were ever connected to
the global IPv4 Internet.OMNI MNs and MSEs that connect over open Internetworks include a
unique node identification value for themselves in the OMNI options of
their IPv6 ND messages (see: ). One useful
identification value alternative is the Host Identity Tag (HIT) as
specified in , while Hierarchical HITs (HHITs)
may provide an alternative more
appropriate for certain domains such as the Unmanned (Air) Traffic
Management (UTM) service for Unmanned Air Systems (UAS). Another
alternative is the Universally Unique IDentifier (UUID) which can be self-generated by a node without
supporting infrastructure with very low probability of collision.When a MN is truly outside the context of any infrastructure, it may
have no MNP information at all. In that case, the MN can use an IPv6
temporary ULA or (H)HIT as an IPv6 source/destination address for
sustained communications in Vehicle-to-Vehicle (V2V) and (multihop)
Vehicle-to-Infrastructure (V2I) scenarios. The MN can also propagate the
ULA/(H)HIT into the multihop routing tables of (collective)
Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using only the vehicles
themselves as communications relays.When a MN connects to ARs over (non-multihop) protected-spectrum
ANETs, an alternate form of node identification (e.g., MAC address,
serial number, airframe identification value, VIN, etc.) may be
sufficient. The MN can then include OMNI "Node Identification"
sub-options (see: ) in IPv6 ND messages should the
need to transmit identification information over the network arise.OMNI interfaces maintain a neighbor cache for tracking per-neighbor
state and use the link-local address format specified in . IPv6 Neighbor Discovery (ND) messages sent over OMNI interfaces without
encapsulation observe the native underlying interface Source/Target
Link-Layer Address Option (S/TLLAO) format (e.g., for Ethernet the
S/TLLAO is specified in ). IPv6 ND messages sent
over OMNI interfaces using encapsulation do not include S/TLLAOs, but
instead include a new option type that encodes encapsulation addresses,
interface attributes and other OMNI link information. (Note that OMNI
interface IPv6 ND messages sent without encapsulation may include both
OMNI options and S/TLLAOs, but the information conveyed in each is
mutually exclusive.) Hence, this document does not define an S/TLLAO
format but instead defines a new option type termed the "OMNI option"
designed for these 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. 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 manage their
dynamically-changing multilink profiles by including OMNI options in
IPv6 ND messages as discussed in the following subsections.The first OMNI option appearing in an IPv6 ND message is formatted
as shown in :In this format:Type is set to TBD2.Length is set to the number of 8 octet blocks in the option.
The value 0 is invalid, while the values 1 through 255 (i.e., 8
through 2040 octets, respectively) indicate the total length of
the OMNI option.Preflen is an 8 bit field that determines the length of prefix
associated with an LLA. Values 0 through 128 specify a valid
prefix length (all other values are invalid). For IPv6 ND messages
sent from a MN to the MS, Preflen applies to the IPv6 source LLA
and provides the length that the MN is requesting or asserting to
the MS. For IPv6 ND messages sent from the MS to the MN, Preflen
applies to the IPv6 destination LLA and indicates the length that
the MS is granting to the MN. For IPv6 ND messages sent between MS
endpoints, Preflen provides the length associated with the
source/target MN that is subject of the ND message.S/T-omIndex is an 8 bit field that includes an omIndex value
for the source or target underlying interface for this IPv6 ND
message. MN OMNI interfaces MUST number each distinct underlying
interface with an omIndex value between '1' and '255' that
represents a MN-specific 8-bit mapping for the actual ifIndex
value assigned by network management . The
value '0' denotes either an AR's INET interface or
"unspecified".The remaining header fields before the Sub-Options begin are
modeled from the Transmission Control Protocol (TCP) header
specified in Section 3.1 of and include a
32 bit Sequence Number followed by a 32 bit Acknowledgement Number
followed by 8 flags bits followed by a 24-bit Window. The (SYN,
ACK, RST) flags are used when TCP-like window synchronization is
used, while the TCP (URG, PSH, FIN) flags are never used and
therefore omitted. The (OPT, PNG) flags are OMNI-specific, and the
remaining flags are Reserved. Together, these fields support the
asymmetric and symmetric OAL window synchronization services
specified in .Sub-Options is a Variable-length field such that the complete
OMNI Option is an integer multiple of 8 octets long. Sub-Options
contains zero or more sub-options as specified in .The OMNI option is included in all OMNI interface IPv6 ND
messages; the option is processed by receiving interfaces that
recognize it and otherwise ignored. If multiple OMNI option instances
appear in the same IPv6 ND message, only the first option includes the
OMNI header fields before the Sub-Options while all others are coded
as follows:The OMNI interface processes the Sub-Options of all OMNI
option instances received in the same IPv6 ND message in the
consecutive order in which they appear. The OMNI option(s) included in
each IPv6 ND message may include full or partial information for the
neighbor. The union of the information in the most recently received
OMNI options is therefore retained and aged/removed in conjunction
with the corresponding NCE.Each OMNI option includes a Sub-Options block containing zero or
more individual sub-options. Each consecutive sub-option is
concatenated immediately after its predecessor. All sub-options except
Pad1 (see below) are in type-length-value (TLV) format encoded as
follows: Sub-Type is a 5-bit field that encodes the Sub-Option type.
Sub-options defined in this document are:Sub-Types 14-29 are available for future assignment for
major protocol functions. Sub-Type 31 is reserved by IANA.Sub-Length is an 11-bit field that encodes the length of the
Sub-Option Data in octets.Sub-Option Data is a block of data with format determined by
Sub-Type and length determined by Sub-Length.During transmission, the OMNI interface codes Sub-Type and
Sub-Length together in network byte order in 2 consecutive octets,
where Sub-Option Data may be up to 2040 octets minus the length of the
OMNI option header octets preceding the Sub-Options. This allows ample
space for coding large objects (e.g., ASCII strings, domain names,
protocol messages, security codes, etc.), while a single OMNI option
is limited to 2040 octets the same as for any IPv6 ND option. The OMNI
interface codes initial sub-options in a first OMNI option instance
and subsequent sub-options in additional instances in the same IPv6 ND
message in the intended order of processing. The OMNI interface can
then code any remaining sub-options in additional IPv6 ND messages if
necessary. Implementations must observe these size limits and refrain
from sending IPv6 ND messages larger than the OMNI interface MTU.During reception, the OMNI interface processes the OMNI option
Sub-Options while skipping over and ignoring any unrecognized
sub-options. The OMNI interface processes the Sub-Options of all OMNI
option instances in the consecutive order in which they appear in the
IPv6 ND message, beginning with the first instance and continuing
through any additional instances to the end of the message. If an
individual sub-option length would cause processing to exceed the OMNI
option total length, the OMNI interface accepts any sub-options
already processed and ignores the final sub-option. The interface then
processes any remaining OMNI options in the same fashion to the end of
the IPv6 ND message.Note: large objects that exceed the Sub-Option Data limit are not
supported under the current specification; if this proves to be
limiting in practice, future specifications may define support for
fragmenting large objects across multiple OMNI options within the same
IPv6 ND message.The following sub-option types and formats are defined in this
document:Sub-Type is set to 0. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Type is followed by 3 'x' bits, set to any value on
transmission (typically all-zeros) and ignored on receipt. Pad1
therefore consists of 1 octet with the most significant 5 bits
set to 0, and with no Sub-Length or Sub-Option Data fields
following.Sub-Type is set to 1. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N that encodes the number of padding
octets that follow.Sub-Option Data consists of N octets, set to any value on
transmission (typically all-zeros) and ignored on receipt.Interface Attributes (Type 1) and (Type 2) were defined in and have been moved to
historic status. Their sub-option types (2 and 3) are reserved for
future use.Interface Attributes (Type 3) was never defined; the number was
skipped to bring (Type 4) into agreement with the corresponding
sub-option Type value.The Interface Attributes (Type 4) sub-option provides L2
forwarding information for the multilink conceptual sending
algorithm discussed in . The L2 information
is used for selecting among potentially multiple candidate
underlying interfaces that can be used to forward carrier packets to
the neighbor based on factors such as traffic selectors and link
quality. Interface Attributes (Type 4) further includes link-layer
address information to be used for either OAL encapsulation or
direct UDP/IP encapsulation (when OAL encapsulation can be
avoided).Interface Attributes (Type 4) must be honored by all
implementations. Throughout the remainder of this specification,
when the term "Interface Attributes" appears without a "Type"
designation the below format is indicated:Sub-Type is set to 4. If multiple instances with different
omIndex values appear in OMNI options of the same message all
are processed. If multiple instances with the same omIndex value
appear, the Traffic Selectors of all are processed while the
remaining information is processed only for the first instance
and ignored in all other instances.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. All fields beginning with omIndex up to
and including TS Format are always present, while the 'A' and
'T' flags determine the remaining Sub-Option Data format.Sub-Option Data contains an "Interface Attributes (Type 4)"
option encoded as follows:omIndex is a 1-octet value corresponding to a specific
underlying interface the same as specified above for the
OMNI option S/T-omIndex field. The OMNI options of a same
message may include multiple Interface Attributes
sub-options, with each distinct omIndex value pertaining to
a different underlying interface. The OMNI option will often
include an Interface Attributes sub-option with the same
omIndex value that appears in the S/T-omIndex. In that case,
the actual encapsulation address of the received IPv6 ND
message should be compared with the L2ADDR encoded in the
sub-option (see below); if the addresses are different the
presence of a NAT is indicated.TS Format is a 1-octet field that encodes a Traffic
Selector version per when T is 1.
If TS Format encodes the value 1, the Traffic Selector
includes IPv4 information. If it encodes the value 2, the
Traffic Selector includes IPv6 information. If it encodes
the value 0, the Traffic Selector field is omitted.omType is set to an 8-bit integer value corresponding to
the underlying interface identified by omIndex. The value
represents an OMNI interface-specific 8-bit mapping for the
actual IANA ifType value registered in the 'IANAifType-MIB'
registry [http://www.iana.org].Provider ID is set to an OMNI interface-specific 8-bit ID
value for the network service provider associated with this
omIndex.Link encodes a 4-bit link metric. The value '0' means the
link is DOWN, and the remaining values mean the link is UP
with metric ranging from '1' ("lowest") to '15'
("highest").Resvd is 4-bit field reserved for future use, set to 0 on
transmit and ignored on receipt.The following address-related fields appear next in
consecutive order:FMT - a 3-bit "Forward/Mode/Type" code corresponding
to the included Link Layer Address as follows:When the most significant bit (i.e.,
"FMT-Forward") is clear, the Proxy/Server must
reassemble. When the bit is set, the Proxy/Server
must forward the fragments to the Client (while
changing the OAL destination address) without
reassembling.When the next most significant bit (i.e.,
"FMT-Mode") is clear, L2ADDR is the address of the
Proxy/Server and the Client must be reached through
the Proxy/Server. When the bit is set, the Client
can be reached on the open *NET where it may be
located behind one or more NATs and L2ADDR is either
the address of the Proxy/Server (when FMT-Forward is
set) or the native INET address of the Client itself
(when FMT-Forward is clear).The least significant bit (i.e., "FMT-Type")
determines the IP address version encoded in L2ADDR.
If FMT-Type is clear, L2ADDR includes a 4-octet IPv4
address. If FMT-Type is set, L2ADDR includes a
16-octet IPv6 address.SRT - a 5-bit Segment Routing Topology prefix length
value that (when added to 96) determines the prefix
length to apply to the ULA formed from concatenating
[ULA*]::/96 with the 32 bit LHS MSID value that follows.
For example, the value 16 corresponds to the prefix
length 112.LHS - the 32 bit MSID of the Last Hop Proxy/Server on
the path to the target. When SRT and LHS are both set to
0, the LHS is considered unspecified in this IPv6 ND
message. When SRT is set to 0 and LHS is non-zero, the
prefix length is set to 128. SRT and LHS together
provide guidance to the OMNI interface forwarding
algorithm. Specifically, if SRT/LHS is located in the
local OMNI link segment then the OMNI interface can
encapsulate according to FMT/L2ADDR (following any
necessary NAT traversal messaging); else, it must
forward according to the OMNI link spanning tree. See
for further
discussion.Link Layer Address (L2ADDR) - identifies the
link-layer address (i.e., the encapsulation address) of
the source/target according to FMT. The UDP Port Number
appears in the first 2 octets and the IP address appears
in the next 4 octets for IPv4 or 16 octets for IPv6. The
Port Number and IP address are recorded in network byte
order, and in ones-compliment "obfuscated" form per
. The OMNI interface forwarding
algorithm uses FMT/L2ADDR to determine the encapsulation
address for forwarding when SRT/LHS is located in the
local OMNI link segment.When TS Format is non-zero, the remainder of the
sub-option includes a traffic selector formatted per beginning with the "Flags (A-N)" field,
and with the Traffic Selector IP protocol version coded in
the TS Format field. Note that each Interface Attributes
sub-option includes at most one IPv4 or IPv6 Traffic
Selector block. If a single interface identified by omIndex
requires traffic selectors for multiple IP protocol
versions, or if a traffic selector block would exceed the
space available in a single Interface Attributes sub-option,
the remaining information is coded in additional sub-options
all having the same omIndex in the following format:Sub-Type is set to 5. If multiple instances appear in OMNI
options of the same message all are processed. Only the first
MAX_MSID values processed (whether in a single instance or
multiple) are retained and all other MSIDs are ignored.Sub-Length is set to 4n, with n representing the number of
MSIDs included.A list of n 4 octet MSIDs is included in the following 4n
octets. The Anycast MSID value '0' in an RS message MS-Register
sub-option requests the recipient to return the MSID of a nearby
MSE in a corresponding RA response.Sub-Type is set to 6. If multiple instances appear in OMNI
options of the same message all are processed. Only the first
MAX_MSID values processed (whether in a single instance or
multiple) are retained and all other MSIDs are ignored.Sub-Length is set to 4n, with n representing the number of
MSIDs included.A list of n 4 octet MSIDs is included in the following 4n
octets. The Anycast MSID value '0' is ignored in MS-Release
sub-options, i.e., only non-zero values are processed.Sub-Type is set to 7. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.Geo Type is a 1 octet field that encodes a type designator
that determines the format and contents of the Geo Coordinates
field that follows. The following types are currently
defined:0 - NULL, i.e., the Geo Coordinates field is
zero-length.A set of Geo Coordinates of length up to the remaining
available space for this OMNI option. New formats to be
specified in future documents and may include attributes such as
latitude/longitude, altitude, heading, speed, etc.The Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
sub-option may be included in the OMNI options of RS messages sent
by MNs and RA messages returned by MSEs. ARs that act as proxys to
forward RS/RA messages between MNs and MSEs also forward DHCPv6
Sub-Options unchanged and do not process DHCPv6 sub-options
themselves. Note that DHCPv6 messages do not include a Checksum
field, but integrity is protected by the IPv6 ND message
Checksum.Sub-Type is set to 8. If multiple instances appear in OMNI
options of the same message the first is processed and all
others are ignored.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The 'msg-type' and 'transaction-id'
fields are always present; hence, the length of the DHCPv6
options is limited by the remaining available space for this
OMNI option.'msg-type' and 'transaction-id' are coded according to
Section 8 of .A set of DHCPv6 options coded according to Section 21 of
follows.The Host Identity Protocol (HIP) Message sub-option may be
included in the OMNI options of RS messages sent by MNs and RA
messages returned by ARs. ARs that act as proxys authenticate and
remove HIP messages in RS messages they forward from a MN to an MSE.
ARs that act as proxys insert and sign HIP messages in the RA
messages they forward from an MSE to a MN.The HIP message sub-option should be included in any OMNI IPv6 ND
message that traverses an open Internetwork, i.e., where link-layer
authentication is not already assured by lower layers. Sub-Type is set to 9. If multiple instances appear in OMNI
options of the same message the first is processed and all
others are ignored.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the HIP parameters. The length of the
entire HIP message is therefore limited by the remaining
available space for this OMNI option.The HIP message is coded per Section 5 of , except that the OMNI "Sub-Type" and
"Sub-Length" fields replace the first 2 octets of the HIP
message header (i.e., the Next Header and Header Length fields).
Also, since the IPv6 ND message header already includes a
Checksum the HIP message Checksum field is replaced by a
Reserved field set to 0 on transmission and ignored on
reception.The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message
sub-option may be included in the OMNI options of IPv6 ND messages
sent by MNs and MSEs. PIM-SM messages are formatted as specified in
Section 4.9 of , with the exception that the
Checksum field is replaced by a Reserved field (set to 0) since the
IPv6 ND message header already includes a Checksum. The PIM-SM
message sub-option format is shown in :Sub-Type is set to 10. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the PIM-SM message. The length of the
entire PIM-SM message is therefore limited by the remaining
available space for this OMNI option.The PIM-SM message is coded exactly as specified in Section
4.9 of , except that the Checksum field
is replaced by a Reserved field set to 0 on transmission and
ignored on reception. The "PIM Ver" field MUST encode the value
2, and the "Type" field encodes the PIM message type. (See
Section 4.9 of for a list of PIM-SM
message types and formats.)The Reassembly Limit sub-option may be included in the OMNI
options of IPv6 ND messages. The message consists of a 14-bit
Reassembly Limit value, followed by two flag bits (H, L) optionally
followed by an (N-2)-octet leading portion of an OAL First Fragment
that triggered the message.Sub-Type is set to 11. If multiple instances appear in OMNI
options of the same message the first occurring "hard" and
"soft" Reassembly Limit values are accepted, and any additional
Reassembly Limit values are ignored.Sub-Length is set to 2 if no OAL First Fragment is included,
or to a value N greater than 2 if an OAL First Fragment is
included.A 15-bit Reassembly Limit follows, and includes a value
between 1500 and 9180. If any other value is included, the
sub-option is ignored. The value indicates the hard or soft
limit for original IP packets that the source of the message is
currently willing to reassemble; the source may increase or
decrease the hard or soft limit at any time through the
transmission of new IPv6 ND messages. Until the first IPv6 ND
message with a Reassembly Limit sub-option arrives, OMNI nodes
assume initial default hard/soft limits of 9180 (I.e., the OMNI
interface MRU). After IPv6 ND messages with Reassembly Limit
sub-options arrive, the OMNI node retains the most recent
hard/soft limit values until new IPv6 ND messages with different
values arrive.The 'H' flag is set to 1 if the Reassembly Limit is a "Hard"
limit, and set to 0 if the Reassembly Limit is a "Soft"
limit.If N is greater than 2, the remainder of the Reassembly Limit
sub-option encodes the leading portion of an OAL First Fragment
that prompted this IPv6 ND message. The first fragment is
included beginning with the OAL IPv6 header, and continuing with
as much of the fragment payload as possible without causing the
IPv6 ND message to exceed the minimum IPv6 MTU.The Fragmentation Report may be included in the OMNI options of
uNA messages sent from an OAL destination to an OAL source. The
message consists of (N / 8)-many (Identification, Bitmap)-tuples
which include the Identification values of OAL fragments received
plus a Bitmap marking the ordinal positions of individual fragments
received and fragments missing.Sub-Type is set to 12. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the sub-option. If N is not an integral
multiple of 8 octets, the sub-option is ignored. The length of
the entire sub-option should not cause the entire IPv6 ND
message to exceed the minimum MPS.Identification (i) includes the IPv6 Identification value
found in the Fragment Header of a received OAL fragment. (Only
those Identification values included represent fragments for
which loss was unambiguously observed; any Identification values
not included correspond to fragments that were either received
in their entirety or may still be in transit.)Bitmap (i) includes an ordinal checklist of fragments, with
each bit set to 1 for a fragment received or 0 for a fragment
missing. For example, for a 20-fragment fragmented OAL packet
with ordinal fragments #3, #10, #13 and #17 missing and all
other fragments received, the bitmap would encode:(Note that loss of an OAL atomic fragment is
indicated by a Bitmap(i) with all bits set to 0.)Sub-Type is set to 13. If multiple instances appear in OMNI
options of the same IPv6 ND message the first instance of a
specific ID-Type is processed and all other instances of the
same ID-Type are ignored. (Note therefore that it is possible
for a single IPv6 ND message to convey multiple Node
Identifications - each having a different ID-Type.)Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The ID-Type field is always present;
hence, the maximum Node Identification Value length is limited
by the remaining available space in this OMNI option.ID-Type is a 1 octet field that encodes the type of the Node
Identification Value. The following ID-Type values are currently
defined:0 - Universally Unique IDentifier (UUID) . Indicates that Node Identification Value
contains a 16 octet UUID.1 - Host Identity Tag (HIT) .
Indicates that Node Identification Value contains a 16 octet
HIT.2 - Hierarchical HIT (HHIT) . Indicates that Node
Identification Value contains a 16 octet HHIT.3 - Network Access Identifier (NAI) . Indicates that Node Identification Value
contains an N-1 octet NAI.4 - Fully-Qualified Domain Name (FQDN) . Indicates that Node Identification Value
contains an N-1 octet FQDN.5 - 252 - Unassigned.253-254 - Reserved for experimentation, as recommended in
.255 - reserved by IANA.Node Identification Value is an (N - 1) octet field encoded
according to the appropriate the "ID-Type" reference above.When a Node Identification Value is used for DHCPv6 messaging
purposes, it is encoded as a DHCP Unique IDentifier (DUID) using the
"DUID-EN for OMNI" format with enterprise number 45282 (see: ) as shown in :In this format, the ID-Type and Node Identification Value fields
are coded exactly as in following the 6
octet DUID-EN header, and the entire "DUID-EN for OMNI" is included
in a DHCPv6 message per .Sub-Type is set to 14. If multiple instances appear in OMNI
options of the same IPv6 ND message all are processedSub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.RFC4443 Error Message Body is an N-octet field encoding the
body of an ICMPv6 Error Message per Section 2.1 of . ICMPv6 error messages are processed exactly
per the standard, while ICMPv6 informational messages must not
be included and are ignored if received. OMNI interfaces include
as much of the ICMPv6 error message body in the sub-option as
possible without causing the IPv6 ND message to exceed the
minimum IPv6 MTU.Since the Sub-Type field is only 5 bits in length, future
specifications of major protocol functions may exhaust the remaining
Sub-Type values available for assignment. This document therefore
defines Sub-Type 30 as an "extension", meaning that the actual
Sub-Option type is determined by examining a 1 octet
"Extension-Type" field immediately following the Sub-Length field.
The Sub-Type Extension is formatted as shown in :Sub-Type is set to 30. If multiple instances appear in OMNI
options of the same message all are processed, where each
individual extension defines its own policy for processing
multiple of that type.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The Extension-Type field is always
present, and the maximum Extension-Type Body length is limited
by the remaining available space in this OMNI option.Extension-Type contains a 1 octet Sub-Type Extension value
between 0 and 255.Extension-Type Body contains an N-1 octet block with format
defined by the given extension specification.Extension-Type values 2 through 252 are available for
assignment by future specifications, which must also define the
format of the Extension-Type Body and its processing rules.
Extension-Type values 253 and 254 are reserved for experimentation,
as recommended in , and value 255 is
reserved by IANA. Extension-Type values 0 and 1 are defined in the
following subsections:Sub-Type is set to 30.Sub-Length is set to N that encodes the number of
Sub-Option Data octets that follow. The Extension-Type and
Header Type fields are always present, and the Header Option
Value is limited by the remaining available space in this OMNI
option.Extension-Type is set to 0. Each instance encodes exactly
one header option per Section 5.1.1 of , with the leading '0' octet omitted and the
following octet coded as Header Type. If multiple instances of
the same Header Type appear in OMNI options of the same
message the first instance is processed and all others are
ignored.Header Type and Header Option Value are coded exactly as
specified in Section 5.1.1 of ; the
following types are currently defined:0 - Origin Indication (IPv4) - value coded per Section
5.1.1 of .1 - Authentication Encapsulation - value coded per
Section 5.1.1 of .2 - Origin Indication (IPv6) - value coded per Section
5.1.1 of , except that the address
is a 16-octet IPv6 address instead of a 4-octet IPv4
address.Header Type values 3 through 252 are available for
assignment by future specifications, which must also define
the format of the Header Option Value and its processing
rules. Header Type values 253 and 254 are reserved for
experimentation, as recommended in ,
and value 255 is Reserved by IANA.Sub-Type is set to 30.Sub-Length is set to N that encodes the number of
Sub-Option Data octets that follow. The Extension-Type and
Trailer Type fields are always present, and the maximum-length
Trailer Option Value is limited by the remaining available
space in this OMNI option.Extension-Type is set to 1. Each instance encodes exactly
one trailer option per Section 4 of .
If multiple instances of the same Trailer Type appear in OMNI
options of the same message the first instance is processed
and all others ignored.Trailer Type and Trailer Option Value are coded exactly as
specified in Section 4 of ; the
following Trailer Types are currently defined:0 - Unassigned1 - Nonce Trailer - value coded per Section 4.2 of
.2 - Unassigned3 - Alternate Address Trailer (IPv4) - value coded per
Section 4.3 of .4 - Neighbor Discovery Option Trailer - value coded per
Section 4.4 of .5 - Random Port Trailer - value coded per Section 4.5
of .6 - Alternate Address Trailer (IPv6) - value coded per
Section 4.3 of , except that each
address is a 16-octet IPv6 address instead of a 4-octet
IPv4 address.Trailer Type values 7 through 252 are available for
assignment by future specifications, which must also define
the format of the Trailer Option Value and its processing
rules. Trailer Type values 253 and 254 are reserved for
experimentation, as recommended in ,
and value 255 is Reserved by IANA.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 while using the L2 address of the AR as the L2
address for all multicast packets.The MN uses Multicast Listener Discovery (MLDv2) to coordinate with the AR, and *NET L2 elements use
MLD snooping .Since the OMNI link model is NBMA, OMNI links support link-scoped
multicast through iterative unicast transmissions to individual
multicast group members (i.e., unicast/multicast emulation).The MN's IPv6 layer selects the outbound OMNI interface according to
SBM considerations when forwarding original IP 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. Each OMNI interface maintains default
routes via ARs discovered as discussed in , and
may configure more-specific routes discovered through means outside the
scope of this specification.After an original IP packet enters the OMNI interface, one or more
outbound underlying interfaces are selected based on PBM traffic
attributes, and one or more neighbor underlying interfaces are selected
based on the receipt of Interface Attributes sub-options in IPv6 ND
messages (see: ). Underlying interface selection
for the node's own local interfaces are based on traffic selectors,
cost, performance, message size, etc. Both node-local and neighbor
underlying interface traffic selectors may also be configured to
indicate replication for increased reliability at the expense of packet
duplication. The set of all Interface Attributes received in IPv6 ND
messages determines the multilink forwarding profile for selecting the
neighbor's underlying interfaces.When the OMNI interface sends an original IP packet over a selected
outbound underlying interface, the OAL employs encapsulation and
fragmentation as discussed in , then performs
*NET encapsulation as determined by the L2 address information received
in Interface Attributes. The OAL also performs encapsulation when the
nearest AR is located multiple hops away as discussed in . (Note that the OAL MAY employ packing when multiple
original IP packets and/or control messages are available for forwarding
to the same OAL destination.)OMNI interface multilink service designers MUST observe the BCP
guidance in Section 15 in terms of implications
for reordering when original IP 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
ULA (e.g., [ULA]:0002::, [ULA]:1000::, [ULA]:7345::, etc.). 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 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 ULA is written
into an original IP packet's 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
packet to the MN's mobile router entity, and the Anycast ULA
identifies the OMNI interface to be used for transmission to the next
hop. When the MN receives the packet, 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 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).After an AR has registered an MNP for a MN (see: ), the AR will forward packets destined to an address
within the MNP to the MN. The MN will under normal circumstances then
forward the packet to the correct destination within its internal
networks.If at some later time the MN loses state (e.g., after a reboot), it
may begin returning packets destined to an MNP address to the AR as
its default router. The AR therefore must drop any packets originating
from the MN and destined to an address within the MN's registered MNP.
To do so, the AR institutes the following check:if the IP destination address belongs to a neighbor on the same
OMNI interface, and if the link-layer source address is the same
as one of the neighbor's link-layer addresses, drop the
packet.MNs interface with the MS by sending RS messages with OMNI options
under the assumption that one or more AR on the *NET will process the
message and respond. The MN then configures default routes for the OMNI
interface via the discovered ARs as the next hop. The manner in which
the *NET ensures AR coordination is link-specific and outside the scope
of this document (however, considerations for *NETs that do not provide
ARs that recognize the OMNI option are discussed in ).For each underlying interface, the MN sends an RS message with an
OMNI option to coordinate with MSEs identified by MSID values. Example
MSID discovery methods are given in and include
data link login parameters, name service lookups, static configuration,
a static "hosts" file, etc. When the AR receives an RS', it selects a
nearby MSE (which may be itself) and returns an RA with the selected
MSID in an MS-Register sub-option. The AR selects only a single nearby
MSE while also soliciting the MSEs corresponding to any non-zero
MSIDs.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's OMNI interface sends initial RS messages over an UP
underlying interface with its MNP-LLA as the source (or with the
unspecified address (::) as the source if it does not yet have an
MNP-LLA) and with destination set to link-scoped All-Routers multicast
(ff02::2) . The OMNI interface includes an OMNI
option per with a Preflen assertion,
Interface Attributes appropriate for underlying interfaces,
MS-Register/Release sub-options containing MSID values, Reassembly
Limits, an authentication sub-option and with any other necessary OMNI
sub-options (e.g., a Node Identification sub-option as an identity for
the MN). The OMNI interface then sets the S/T-omIndex field to the index
of the underlying interface over which the RS message is sent.The OMNI interface then sends the RS over the underlying interface
using OAL encapsulation and fragmentation if necessary. If OAL
encapsulation is used for RS messages sent over an INET interface, the
entire RS message must appear within a single carrier packet so that it
can be authenticated without requiring reassembly. The OMNI interface
selects an Identification value (see: ), sets the
OAL source address to the ULA corresponding to the RS source (or a
Temporary ULA if the RS source is the unspecified address (::)) and sets
the OAL destination to site-scoped All-Routers multicast (ff05::2) then
sends the message.ARs process IPv6 ND messages with OMNI options and act as an MSE
themselves and/or as a proxy for other MSEs. ARs receive RS messages and
create a NCE for the MN, then prepare to act as an MSE themselves and/or
coordinate with any MSEs named in the Register/Release lists in a manner
outside the scope of this document. When an MSE processes the OMNI
information, it first validates the prefix registration information then
injects/withdraws the MNP in the routing/mapping system and
caches/discards the new Preflen, MNP and Interface Attributes. The MSE
then informs the AR of registration success/failure, and the AR returns
an RA message to the MN with an OMNI option per .The AR's OMNI interface returns the RA message via the same
underlying interface of the MN over which the RS was received, and with
destination address set to the MNP-LLA (i.e., unicast), with source
address set to its own LLA, and with an OMNI option with S/T-omIndex set
to the value included in the RS. The OMNI option also includes a Preflen
confirmation, Interface Attributes, MS-Register/Release and any other
necessary OMNI sub-options (e.g., a Node Identification sub-option as an
identity for the AR). The RA also includes any information for the link,
including RA Cur Hop Limit, M and O flags, Router Lifetime, Reachable
Time and Retrans Timer values, and includes any necessary options such
as:PIOs with (A; L=0) that include MSPs for the link .RIOs with more-specific routes.an MTU option that specifies the maximum acceptable packet size
for this underlying interface.The AR prepares the RA using OAL encapsulation/fragmentation with an
Identification value selected per , with source
set to the ULA corresponding to the RA source and with destination set
to the ULA corresponding to the RA destination. The AR then sends the
initial RA message to the MN and MAY later send additional periodic
and/or event-driven unsolicited RA messages per . In that case, the S/T-omIndex field in the OMNI
option of the unsolicited RA message identifies the target underlying
interface of the destination MN.The AR can combine the information from multiple MSEs by sending one
or more "aggregate" RAs to the MN in order conserve *NET bandwidth. Each
aggregate RA includes an OMNI option with MS-Register/Release
sub-options with the MSEs represented by the aggregate. If an aggregate
is sent, the RA message contents must consistently represent the
combined information advertised by all represented MSEs. Note that since
the AR uses its own ADM-LLA as the RA source address, the MN determines
the addresses of the represented MSEs by examining the
MS-Register/Release OMNI sub-options. Note also that the AR must return
any MSE RA messages that set window synchronization flags directly to
the MN, i.e., and without including them in an aggregate.When the MN receives the RA message, it creates an OMNI interface NCE
for each MSID that has confirmed MNP registration via the L2 address of
this AR. If the MN connects to multiple *NETs, it records the additional
L2 AR addresses in each MSID NCE (i.e., as multilink neighbors). The MN
then configures a default route via the MSE that returned the RA
message, and assigns the Subnet Router Anycast address corresponding to
the MNP (e.g., 2001:db8:1:2::) to the OMNI interface. The MN then
manages its underlying interfaces according to their states as
follows:When an underlying interface transitions to UP, the MN sends an
RS over the underlying interface with an OMNI option. The OMNI
option contains at least one Interface Attribute sub-option with
values specific to this underlying interface, and may contain
additional Interface Attributes specific to other underlying
interfaces. The option also includes any MS-Register/Release
sub-options.When an underlying interface transitions to DOWN, the MN sends an
RS or unsolicited NA message over any UP underlying interface with
an OMNI option containing an Interface Attribute sub-option for the
DOWN underlying interface with Link set to '0'. The MN sends
isolated unsolicited NAs when reliability is not thought to be a
concern (e.g., if redundant transmissions are sent on multiple
underlying interfaces), or may instead set the SYN flag in the OMNI
header to trigger a reliable solicited NA reply.When the Router Lifetime for a specific AR nears expiration, the
MN sends an RS over the underlying interface to receive a fresh RA.
If no RA is received, the MN can send RS messages to an alternate
MSID in case the current MSID has failed. If no RS messages are
received even after trying to contact alternate MSIDs, the MN marks
the underlying interface as DOWN.When a MN wishes to release from one or more current MSIDs, it
sends an RS or unsolicited NA message over any UP underlying
interfaces with an OMNI option with a Release MSID. Each MSID then
withdraws the MNP from the routing/mapping system and informs the AR
that the release was successful.When all of a MNs underlying interfaces have transitioned to DOWN
(or if the prefix registration lifetime expires), any associated
MSEs withdraw the MNP the same as if they had received a message
with a release indication.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 an UP
underlying interface (i.e., even after attempting to contact alternate
MSEs), 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 or independently of 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. Still other
deployments may elect to administratively disable the ordinary RS/RA
messaging used by the IPv6 layer over the OMNI interface, since they are
not required to drive the internal RS/RA processing. (Note that this
same logic applies to IPv4 implementations that employ ICMP-based Router
Discovery per .)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.Note: if an AR acting as a proxy forwards a MN's RS message to
another node acting as an MSE using UDP/IP encapsulation, it must use a
distinct UDP source port number for each MN. This allows the MSE to
distinguish different MNs behind the same AR at the link-layer, whereas
the link-layer addresses would otherwise be indistinguishable.Note: when an AR acting as an MSE returns an RA to an INET Client, it
includes an OMNI option with an Interface Attributes sub-option with
omIndex set to 0 and with SRT, FMT, LHS and L2ADDR information for its
INET interface. This provides the Client with partition prefix context
regarding the local OMNI link segment.In environments where Identification window synchronization is
necessary, the RS/RA exchanges discussed above observe the procedures
specified in . In the asymmetric case, the
initial RS/RA exchange establishes only the MN's send window and
AR/MSE's receive window such that a corresponding NS/NA exchange would
be needed in the reverse direction. In the symmetric case, the MN
returns an explicit/implicit acknowledgement response to the RA to
symmetrically establish the send/receive windows of both parties.The initial RS/RA exchange between a MN and an MSE over a first
underlying interface must invoke window synchronization, while
subsequent RS/RA exchanges performed over additional underlying
interfaces within ReachableTime and with in-window Identification
values need not also invoke window synchronization. Following the
initial exchange, future window (re)synchronizations can occur over
any underlying interface, i.e., and not necessarily only over the one
used for the initial exchange.When a MN sends an RS SYN that includes an OMNI MS-Register
sub-option with multiple MSIDs, it may receive multiple RA SYN/ACKs -
each with their own synchronization parameters. The resulting
"multi-three-way" handshake would require the MN to establish separate
NCE SND/RCV state and return explicit/implicit acknowledgements for
each responding MSE.On some *NETs, a MN may be located multiple IP hops away from the
nearest AR. Forwarding through IP multihop *NETs is conducted through
the application of a routing protocol (e.g., a MANET/VANET routing
protocol over omni-directional wireless interfaces, an inter-domain
routing protocol in an enterprise network, etc.). These *NETs could be
either IPv6-enabled or IPv4-only, while IPv4-only *NETs could be
either multicast-capable or unicast-only (note that for IPv4-only
*NETs the following procedures apply for both single-hop and multihop
cases).A MN located potentially multiple *NET hops away from the nearest
AR prepares an RS message with source address set to its MNP-LLA (or
to the unspecified address (::) if it does not yet have an MNP-LLA),
and with destination set to link-scoped All-Routers multicast the same
as discussed above. The OMNI interface then employs OAL encapsulation
and fragmentation, and sets the OAL source address to the ULA
corresponding to the RS source (or to a Temporary ULA if the RS source
was the unspecified address (::)) and sets the OAL destination to
site-scoped All-Routers multicast (ff05::2). For IPv6-enabled *NETs,
the MN then encapsulates the message in UDP/IPv6 headers with source
address set to the underlying interface address (or to the ULA that
would be used for OAL encapsulation if the underlying interface does
not yet have an address) and sets the destination to either a unicast
or anycast address of an AR. For IPv4-only *NETs, the MN instead
encapsulates the RS message in UDP/IPv4 headers with source address
set to the IPv4 address of the underlying interface and with
destination address set to either the unicast IPv4 address of an AR
or an IPv4 anycast address reserved for OMNI.
The MN then sends the encapsulated RS message via the *NET interface,
where it will be forwarded by zero or more intermediate *NET hops.When an intermediate *NET hop that participates in the routing
protocol receives the encapsulated RS, it forwards the message
according to its routing tables (note that an intermediate node could
be a fixed infrastructure element or another MN). This process repeats
iteratively until the RS message is received by a penultimate *NET hop
within single-hop communications range of an AR, which forwards the
message to the AR.When the AR receives the message, it decapsulates the RS (while
performing OAL reassembly, if necessary) and coordinates with the MS
the same as for an ordinary link-local RS, since the network layer Hop
Limit will not have been decremented by the multihop forwarding
process. The AR then prepares an RA message with source address set to
its own ADM-LLA and destination address set to the LLA of the original
MN. The AR then performs OAL encapsulation and fragmentation, with OAL
source set to its own ADM-ULA and destination set to the ULA
corresponding to the RA source. The AR then encapsulates the message
in UDP/IPv4 or UDP/IPv6 headers with source address set to its own
address and with destination set to the encapsulation source of the
RS.The AR then forwards the message to a *NET node within
communications range, which forwards the message according to its
routing tables to an intermediate node. The multihop forwarding
process within the *NET 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 the AR as an on-link neighbor.Note: An alternate approach to multihop forwarding via IPv6
encapsulation would be for the MN and AR to statelessly translate the
IPv6 LLAs into ULAs and forward the RS/RA messages without
encapsulation. This would violate the
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 mandates 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 .Note: An alternate approach to multihop forwarding via IPv4
encapsulation would be to employ IPv6/IPv4 protocol translation.
However, for IPv6 ND messages the LLAs would be truncated due to
translation and the OMNI Router and Prefix Discovery services would
not be able to function. The use of IPv4 encapsulation is therefore
indicated.Note: An IPv4 anycast address for OMNI in IPv4 networks could be
part of a new IPv4 /24 prefix allocation, but this may be difficult to
obtain given IPv4 address exhaustion. An alternative would be to
re-purpose the prefix 192.88.99.0 which has been set aside from its
former use by .OMNI links maintain a constant value "MAX_MSID" selected to provide
MNs with an acceptable level of MSE redundancy while minimizing
control message amplification. It is RECOMMENDED that MAX_MSID be set
to the default value 5; if a different value is chosen, it should be
set uniformly by all nodes on the OMNI link.When a MN sends an RS message with an OMNI option via an underlying
interface to an AR, the MN must convey its knowledge of its
currently-associated MSEs. Initially, the MN will have no associated
MSEs and should therefore send its initial RS messages to the
link-scoped All-Routers multicast address. The AR will then return an
RA message with source address set to the ADM-LLA of the selected MSE
(which may be the address of the AR itself).As the MN activates additional underlying interfaces, it can
optionally include an MS-Register sub-option with MSIDs for MSEs
discovered from previous RS/RA exchanges. The MN will thus eventually
begin to learn and manage its currently active set of MSEs, and can
register with new MSEs or release from former MSEs with each
successive RS/RA exchange. As the MN's MSE constituency grows, it
alone is responsible for including or omitting MSIDs in the
MS-Register/Release lists it sends in RS messages. The inclusion or
omission of MSIDs determines the MN's interface to the MS and defines
the manner in which MSEs will respond. The only limiting factor is
that the MN should include no more than MAX_MSID values in each list
per each IPv6 ND message, and should avoid duplication of entries in
each list unless it wants to increase likelihood of control message
delivery.When an AR receives an RS message sent by a MN with an OMNI option,
the option will contain zero or more MS-Register and MS-Release
sub-options containing MSIDs. After processing the OMNI option, the AR
will have a list of zero or more MS-Register MSIDs and a list of zero
or more of MS-Release MSIDs. The AR then processes the lists as
follows:For each list, retain the first MAX_MSID values in the list and
discard any additional MSIDs (i.e., even if there are duplicates
within a list).Next, for each MSID in the MS-Register list, remove all
matching MSIDs from the MS-Release list.Next, proceed as follows:If the AR's own MSID appears in the MS-Register list, send
an RA message directly back to the MN and send a proxy copy of
the RS message to each additional MSID in the MS-Register list
with the MS-Register/Release lists omitted. Then, send an
unsolicited NA (uNA) message to each MSID in the MS-Release
list with the MS-Register/Release lists omitted and with an
OMNI option with S/T-omIndex set to 0.Otherwise, send a proxy copy of the RS message to each
additional MSID in the MS-Register list with the MS-Register
list omitted. For the first MSID, include the original
MS-Release list; for all other MSIDs, omit the MS-Release
list.Each proxy copy of the RS message will include an OMNI option
and OAL encapsulation header with the ADM-ULA of the AR as the source
and the ADM-ULA of the Register MSE as the destination. When the
Register MSE receives the proxy RS message, if the message includes an
MS-Release list the MSE sends a uNA message to each additional MSID in
the Release list with an OMNI option with S/T-omIndex set to 0. The
Register MSE then sends an RA message back to the (Proxy) AR wrapped
in an OAL encapsulation header with source and destination addresses
reversed, and with RA destination set to the MNP-LLA of the MN. When
the AR receives this RA message, it sends a proxy copy of the RA to
the MN.Each uNA message (whether sent by the first-hop AR or by a Register
MSE) will include an OMNI option and an OAL encapsulation header with
the ADM-ULA of the Register MSE as the source and the ADM-ULA of the
Release MSE as the destination. The uNA informs the Release MSE that
its previous relationship with the MN has been released and that the
source of the uNA message is now registered. The Release MSE must then
note that the subject MN of the uNA message is now "departed", and
forward any subsequent packets destined to the MN to the Register
MSE.Note: It is not an error for the MS-Register/Release lists to
include duplicate entries. If duplicates occur within a list, the AR
will generate multiple proxy RS and/or uNA messages - one for each
copy of the duplicate entries.Note: the MN is responsible for honoring the window synchronization
protocol for each responding MSE when it sends a single RS message
with synchronization parameters and an MS-Register option with
multiple MSIDs. Each responding MSE will cache identical RCV state
information based on the single RS message, then responds with its own
unique SND parameters.When a MN is not pre-provisioned with an MNP-LLA (or, when the MN
requires additional MNP delegations), it requests the MSE to select
MNPs on its behalf and set up the correct routing state within the MS.
The DHCPv6 service supports this
requirement.When an MN needs to have the MSE select MNPs, it sends an RS
message with source set to the unspecified address (::) if it has no
MNP_LLAs. If the MN requires only a single MNP delegation, it can then
include a Node Identification sub-option in the OMNI option and set
Preflen to the length of the desired MNP. If the MN requires multiple
MNP delegations and/or more complex DHCPv6 services, it instead
includes a DHCPv6 Message sub-option containing a Client Identifier,
one or more IA_PD options and a Rapid Commit option then sets the
'msg-type' field to "Solicit", and includes a 3 octet
'transaction-id'. The MN then sets the RS destination to All-Routers
multicast and sends the message using OAL encapsulation and
fragmentation if necessary as discussed above.When the MSE receives the RS message, it performs OAL reassembly if
necessary. Next, if the RS source is the unspecified address (::)
and/or the OMNI option includes a DHCPv6 message sub-option, the MSE
acts as a "Proxy DHCPv6 Client" in a message exchange with the
locally-resident DHCPv6 server. If the RS did not contain a DHCPv6
message sub-option, the MSE generates a DHCPv6 Solicit message on
behalf of the MN using an IA_PD option with the prefix length set to
the OMNI header Preflen value and with a Client Identifier formed from
the OMNI option Node Identification sub-option; otherwise, the MSE
uses the DHCPv6 Solicit message contained in the OMNI option. The MSE
then sends the DHCPv6 message to the DHCPv6 Server, which delegates
MNPs and returns a DHCPv6 Reply message with PD parameters. (If the
MSE wishes to defer creation of MN state until the DHCPv6 Reply is
received, it can instead act as a Lightweight DHCPv6 Relay Agent per
by encapsulating the DHCPv6 message in a
Relay-forward/reply exchange with Relay Message and Interface ID
options. In the process, the MSE packs any state information needed to
return an RA to the MN in the Relay-forward Interface ID option so
that the information will be echoed back in the Relay-reply.)When the MSE receives the DHCPv6 Reply, it adds routes to the
routing system and creates MNP-LLAs based on the delegated MNPs. The
MSE then sends an RA back to the MN with the DHCPv6 Reply message
included in an OMNI DHCPv6 message sub-option if and only if the RS
message had included an explicit DHCPv6 Solicit. If the RS message
source was the unspecified address (::), the MSE includes one of the
(newly-created) MNP-LLAs as the RA destination address and sets the
OMNI option Preflen accordingly; otherwise, the MSE includes the RS
source address as the RA destination address. The MSE then sets the RA
source address to its own ADM-LLA then performs OAL encapsulation and
fragmentation and sends the RA to the MN. When the MN receives the RA,
it reassembles and discards the OAL encapsulation, then creates a
default route, assigns Subnet Router Anycast addresses and uses the RA
destination address as its primary MNP-LLA. The MN will then use this
primary MNP-LLA as the source address of any IPv6 ND messages it sends
as long as it retains ownership of the MNP.Note: After a MN performs a DHCPv6-based prefix registration
exchange with a first MSE, it would need to repeat the exchange with
each additional MSE it registers with. In that case, the MN supplies
the MNP delegation information received from the first MSE when it
engages the additional MSEs.If the *NET 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 *NET 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 *NET
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 *NET 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.*NETs SHOULD deploy ARs in Virtual Router Redundancy Protocol (VRRP)
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) 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 *NET 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 *NET. If an MSE fails, ARs can quickly inform MNs of
the outage by sending multicast RA messages on the *NET interface. The
AR sends RA messages to MNs via the *NET interface with an OMNI option
with a Release ID for the failed MSE, and with destination address set
to All-Nodes multicast (ff02::1) .The AR SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated
by small delays . Any MNs on the *NET 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 *NET 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 ADM-LLA as the source, the MNP-LLA as the
destination 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 . In that case, the MN engages the *NET according to
the legacy IPv6 link model and without the OMNI extensions specified in
this document.If the *NET link model is multiple access, there must be assurance
that address duplication cannot corrupt the neighbor caches of other
nodes on the link. When the MN sends an RS message on a multiple access
*NET link with an 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 *NET links to ensure
isolation for MN / AR communications is through L2 address mappings as
discussed in . This arrangement imparts a
(virtual) point-to-point link model over the (physical) multiple access
link.OMNI interfaces configured over IPv6-enabled underlying interfaces on
an open Internetwork without an OMNI-aware first-hop AR receive RA
messages that do not include an OMNI option, while OMNI interfaces
configured over IPv4-only underlying interfaces do not receive any
(IPv6) RA messages at all (although they may receive IPv4 RA messages
). OMNI interfaces that receive RA messages
without an OMNI option configure addresses, on-link prefixes, etc. on
the underlying interface that received the RA according to standard IPv6
ND and address resolution conventions . OMNI interfaces configured over IPv4-only underlying
interfaces configure IPv4 address information on the underlying
interfaces using mechanisms such as DHCPv4 .OMNI interfaces configured over underlying interfaces that connect to
an open Internetwork can apply security services such as VPNs to connect
to an MSE, or can establish a direct link to an MSE through some other
means (see ). In environments where an explicit
VPN or direct link may be impractical, OMNI interfaces can instead use
UDP/IP encapsulation while including authentication signatures in IPv6
ND messages.OMNI interfaces use UDP service port number 8060 (see: and Section 3.6 of ) according to the simple UDP/IP
encapsulation format specified in for both IPv4
and IPv6 underlying interfaces. OMNI interfaces do not include the
UDP/IP header/trailer extensions specified in , but may include them as OMNI
sub-options instead when necessary. Since the OAL includes an integrity
check over the OAL packet, OAL sources selectively disable UDP checksums
for OAL packets that do not require UDP/IP address integrity, but enable
UDP checksums for others including non-OAL packets, IPv6 ND messages
used to establish link-layer addresses, etc. If the OAL source discovers
that packets with UDP checksums disabled are being dropped in the path
it should enable UDP checksums in future packets. Further considerations
for UDP encapsulation checksums are found in .For MN-to-MSE (e.g., "Vehicle-to-Infrastructure (V2I)") neighbor
exchanges, the source must include an OMNI option with an authentication
sub-option in all IPv6 ND messages. The source can apply HIP security
services per using the IPv6 ND message OMNI
option as a "shipping container" to convey an authentication signature
in a (unidirectional) HIP "Notify" message. For MN-to-MN (e.g.,
"Vehicle-to-Vehicle (V2V)") neighbor exchanges, two MNs can exchange HIP
"Initiator/Responder" messages coded in OMNI options of multiple IPv6
NS/NA messages for mutual authentication according to the HIP protocol.
(Note: a simple Hashed Message Authentication Code (HMAC) such as
specified in can be used as an alternate
authentication service in some environments.)When HIP authentication is used, the IPv6 ND message source should
include an OMNI option with a HIP message containing a valid
authentication signature. When the source prepares the HIP message, it
includes its own (H)HIT as the Sender's HIT and the neighbor's (H)HIT if
known as the Receiver's HIT (otherwise 0). Before calculating the HIP
signature, the source sets both the ICMPv6 Checksum field and HIP
signature fields to 0. The source then calculates the HIP authentication
signature over the full length of the IPv6 ND message beginning with the
ICMPv6 message header and extending over all included IPv6 ND message
options including the OMNI option itself. The source next writes the
authentication signature into the HIP signature field, then calculates
the ICMPv6 message checksum and writes the value into the ICMPv6
Checksum field.After establishing a VPN or preparing for UDP/IP encapsulation, OMNI
interfaces send RS/RA messages for MN/MSE coordination (see: ) and NS/NA messages for route optimization and
mobility management (see: ). These
control plane messages must be authenticated while data plane messages
are delivered the same as for ordinary best-effort traffic with source
address and/or Identification window-based data origin verification.
Data plane communications via OMNI interfaces that connect over open
Internetworks without an explicit VPN should therefore employ transport-
or higher-layer security to ensure integrity and/or confidentiality.OMNI interfaces configured over open Internetworks are often located
behind NATs. The OMNI interface accommodates NAT traversal using UDP/IP
encapsulation and the mechanisms discussed in . To support NAT determination, MSEs
include an Origin Indication sub-option in RA messages sent in response
to RS messages received from a Client via UDP/IP encapsulation.Note: Following the initial IPv6 ND message exchange, OMNI interfaces
configured over open Internetworks maintain neighbor relationships by
transmitting periodic IPv6 ND messages with OMNI options that include
HIP "Update" and/or "Notify" messages. When HMAC authentication is used
instead of HIP, the MN and MSE exchange all IPv6 ND messages with HMAC
signatures included based on a shared-secret.Note: The HMAC and/or HIP message authentication sub-options appear in the OMNI option,
which may occur anywhere within the IPv6 ND message body. When a node
that inserts an authentication sub-option generates the authentication
signature, it calculates the signature over the entire length of the
IPv6 ND message but with the sub-option authentication field itself set
to 0. The node then writes the resulting signature into the
authentication field then continues to prepare the message for
transmission. For this reason, if an IPv6 ND message includes multiple
authentication sub-options, the first sub-option is consulted and any
additional sub-options are ignored.Note: OMNI interfaces on open Internetworks should employ the
Identification window synchronization mechanisms specified in in order to reject spurious carrier packets that might
otherwise clutter the reassembly cache. This is especially important in
environments where carrier packet spoofing is a threat.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.The prefix delegation services discussed in
allows OMNI MNs that desire time-varying MNPs to obtain short-lived
prefixes to send RS messages with source set to the unspecified address
(::) and/or with an OMNI option with DHCPv6 Option sub-options. 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,
but may present limits for the durations of ongoing sessions that would
prefer to use a constant address.MNs that generate (H)HITs but do not have pre-assigned MNPs can
request MNP delegations by issuing IPv6 ND messages that use the (H)HIT
instead of a Temporary ULA. In particular, when a MN creates an RS
message it can set the source to the unspecified address (::) and
destination to link-scoped All-Routers multicast. The IPv6 ND message
includes an OMNI option with a HIP message sub-option, and need not
include a Node Identification sub-option since the MN's HIT appears in
the HIP message. The MN then encapsulates the message in an IPv6 header
with the (H)HIT as the source address and with destination set to either
a unicast or anycast ADM-ULA. The MN then sends the message to the MSE
as specified in .When the MSE receives the message, it notes that the RS source was
the unspecified address (::), then examines the RS encapsulation source
address to determine that the source is a (H)HIT and not a Temporary
ULA. The MSE next invokes the DHCPv6 protocol to request an MNP prefix
delegation while using the HIT as the Client Identifier, then prepares
an RA message with source address set to its own ADM-LLA and destination
set to the MNP-LLA corresponding to the delegated MNP. The MSE next
includes an OMNI option with a HIP message sub-option and any DHCPv6
prefix delegation parameters. The MSE then finally encapsulates the RA
in an IPv6 header with source address set to its own ADM-ULA and
destination set to the (H)HIT from the RS encapsulation source address,
then returns the encapsulated RA to the MN.MNs can also use (H)HITs and/or Temporary ULAs for direct MN-to-MN
communications outside the context of any OMNI link supporting
infrastructure. When two MNs encounter one another they can use their
(H)HITs and/or Temporary ULAs as original IPv6 packet source and
destination addresses to support direct communications. MNs can also
inject their (H)HITs and/or Temporary ULAs into a MANET/VANET routing
protocol to enable multihop communications. MNs can further exchange
IPv6 ND messages (such as NS/NA) using their (H)HITs and/or Temporary
ULAs as source and destination addresses. Note that the HIP security
protocols for establishing secure neighbor relationships are based on
(H)HITs. IPv6 ND messages that use Temporary ULAs instead use the HMAC
authentication service specified in .Lastly, when MNs are within the coverage range of OMNI link
infrastructure a case could be made for injecting (H)HITs and/or
Temporary ULAs into the global MS routing system. For example, when the
MN sends an RS to a MSE it could include a request to inject the (H)HIT
/ Temporary ULA into the routing system instead of requesting an MNP
prefix delegation. This would potentially enable OMNI link-wide
communications using only (H)HITs or Temporary ULAs, and not MNPs. This
document notes the opportunity, but makes no recommendation.OMNI MNs use LLAs only for link-scoped communications on the OMNI
link. Typically, MNs use LLAs as source/destination IPv6 addresses of
IPv6 ND messages, but may also use them for addressing ordinary original
IP packets exchanged with an OMNI link neighbor.OMNI MNs use MNP-ULAs as source/destination IPv6 addresses in the
encapsulation headers of OAL packets. OMNI MNs use Temporary ULAs for
OAL addressing when an MNP-ULA is not available, or as
source/destination IPv6 addresses for communications within a
MANET/VANET local area. OMNI MNs use HITs instead of Temporary ULAs when
operation outside the context of a specific ULA domain and/or source
address attestation is necessary.OMNI MNs use MNP-based GUAs as original IP packet source and
destination addresses for communications with Internet destinations when
they are within range of OMNI link supporting infrastructure that can
inject the MNP into the routing system.An OAL destination or intermediate node may need to return
ICMPv6-like error messages (e.g., Destination Unreachable, Packet Too
Big, Time Exceeded, etc.) to an OAL source.
Since ICMPv6 error messages do not themselves include authentication
codes, the OAL includes the error message as an OMNI ICMPv6 Error
sub-option in an IPv6 ND uNA message. The OAL also includes a HIP
message sub-option if the uNA requires an authentication signature.The following IANA actions are requested in accordance with and :The IANA is instructed to allocate an official Ethertype number
TBD1 from the 'ieee-802-numbers' registry for User Datagram Protocol
(UDP) encapsulation on Ethernet networks. Guidance is found in (registration procedure is Expert Review).The IANA is instructed to allocate an official Type number TBD2
from the "IPv6 Neighbor Discovery Option Formats" registry for the
OMNI option (registration procedure is RFC required). Implementations
set Type to 253 as an interim value .The IANA is instructed to allocate one Ethernet unicast address
TBD3 (suggested value '00-52-14') in the 'ethernet-numbers' registry
under "IANA Unicast 48-bit MAC Addresses" (registration procedure is
Expert Review). The registration should appear as follows:The IANA is instructed to assign two new Code values in the "ICMPv6
Code Fields: Type 2 - Packet Too Big" registry (registration procedure
is Standards Action or IESG Approval). The registry should appear as
follows:(Note: this registry also to be used to define values for
setting the "unused" field of ICMPv4 "Destination Unreachable -
Fragmentation Needed" messages.)The OMNI option defines a 5-bit Sub-Type field, for which IANA is
instructed to create and maintain a new registry entitled "OMNI Option
Sub-Type Values". Initial values are given below (registration
procedure is RFC required):The OMNI Geo Coordinates sub-option (see: )
contains an 8-bit Type field, for which IANA is instructed to create
and maintain a new registry entitled "OMNI Geo Coordinates Type
Values". Initial values are given below (registration procedure is RFC
required):The OMNI Node Identification sub-option (see: ) contains an 8-bit ID-Type field, for which IANA is
instructed to create and maintain a new registry entitled "OMNI Node
Identification ID-Type Values". Initial values are given below
(registration procedure is RFC required):The OMNI option defines an 8-bit Extension-Type field for Sub-Type
30 (Sub-Type Extension), for which IANA is instructed to create and
maintain a new registry entitled "OMNI Option Sub-Type Extension
Values". Initial values are given below (registration procedure is RFC
required):The OMNI Sub-Type Extension "RFC4380 UDP/IP Header Option" defines
an 8-bit Header Type field, for which IANA is instructed to create and
maintain a new registry entitled "OMNI RFC4380 UDP/IP Header Option".
Initial registry values are given below (registration procedure is RFC
required):The OMNI Sub-Type Extension for "RFC6081 UDP/IP Trailer Option"
defines an 8-bit Trailer Type field, for which IANA is instructed to
create and maintain a new registry entitled "OMNI RFC6081 UDP/IP
Trailer Option". Initial registry values are given below (registration
procedure is RFC required):The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
together with reclaims the UDP
port number "8060" for 'aero' as the service port for UDP/IP
encapsulation. (Note that, although was not
widely implemented or deployed, any messages coded to that
specification can be easily distinguished and ignored since they use
an invalid ICMPv6 message type number '0'.) The IANA is therefore
instructed to update the reference for UDP port number "8060" from
"RFC6706" to "RFCXXXX" (i.e., this document).The IANA has assigned a 4 octet Private Enterprise Number (PEN)
code "45282" in the "enterprise-numbers" registry. This document is
the normative reference for using this code in DHCP Unique IDentifiers
based on Enterprise Numbers ("DUID-EN for OMNI Interfaces") (see:
). The IANA is therefore instructed to change
the enterprise designation for PEN code "45282" from "LinkUp Networks"
to "Overlay Multilink Network Interface (OMNI)".The IANA has assigned the ifType code "301 - omni - Overlay
Multilink Network Interface (OMNI)" in accordance with Section 6 of
. The registration appears under the IANA
"Structure of Management Information (SMI) Numbers (MIB Module
Registrations) - Interface Types (ifType)" registry.No further IANA actions are required.Security considerations for IPv4 , IPv6 and IPv6 Neighbor Discovery
apply. OMNI interface IPv6 ND messages SHOULD include Nonce and
Timestamp options when transaction confirmation
and/or time synchronization is needed. (Note however that when OAL
encapsulation is used the (echoed) OAL Identification value can provide
sufficient transaction confirmation.)MN OMNI interfaces configured over secured ANET interfaces inherit
the physical and/or link-layer security properties (i.e., "protected
spectrum") of the connected ANETs. MN 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, the security services specified in
and/or can be
employed. While the OMNI link protects control plane messaging,
applications must still employ end-to-end transport- or higher-layer
security services to protect the data plane.Strong network layer security for control plane messages and
forwarding path integrity for data plane messages between MSEs MUST be
supported. In one example, the AERO service constructs a spanning tree between MSEs
and secures the links in the spanning tree with network layer security
mechanisms such as IPsec or WireGuard. Control
plane messages are then constrained to travel only over the secured
spanning tree paths and are therefore protected from attack or
eavesdropping. Since data plane messages can travel over route optimized
paths that do not strictly follow the spanning tree, however, end-to-end
transport- or higher-layer security services are still required.
Additionally, the OAL Identification value provides a first level of
data origin authentication that mitigates off-path spoofing.Identity-based key verification infrastructure services such as iPSK
may be necessary for verifying the identities claimed by MNs. This
requirement should be harmonized with the manner in which (H)HITs are
attested in a given operational environment.Security considerations for specific access network interface types
are covered under the corresponding IP-over-(foo) specification (e.g.,
, , etc.).Security considerations for IPv6 fragmentation and reassembly are
discussed in . In environments where spoofing is
considered a threat, OMNI nodes SHOULD employ Identification window
synchronization and OAL destinations SHOULD configure an
(end-system-based) firewall.AERO/OMNI Release-3.2 was tagged on March 30, 2021, and is undergoing
internal testing. Additional internal releases expected within the
coming months, with first public release expected end of 1H2021.This document does not itself update other RFCs, but suggests that
the following could be updated through future IETF initiatives:Updates can be through, e.g., standards action, the errata
process, etc. as appropriate.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:
Stuart Card, Michael Matyas, Robert Moskowitz, Madhu Niraula, Greg
Saccone, Stephane Tamalet, Eric Vyncke. Pavel Drasil, Zdenek Jaron and
Michal Skorepa are especially recognized for their many helpful ideas
and suggestions. Madhuri Madhava Badgandi, Sean Dickson, Don Dillenburg,
Joe Dudkowski, Vijayasarathy Rajagopalan, Ron Sackman and Katherine Tran
are acknowledged for their hard work on the implementation and technical
insights that led to improvements for the spec.Discussions on the IETF 6man and atn mailing lists during the fall of
2020 suggested additional points to consider. The authors gratefully
acknowledge the list members who contributed valuable insights through
those discussions. Eric Vyncke and Erik Kline were the intarea ADs,
while Bob Hinden and Ole Troan were the 6man WG chairs at the time the
document was developed; they are all gratefully acknowledged for their
many helpful insights. Many of the ideas in this document have further
built on IETF experiences beginning as early as Y2K, with insights from
colleagues including Brian Carpenter, Ralph Droms, Christian Huitema,
Thomas Narten, Dave Thaler, Joe Touch, and many others who deserve
recognition.Early observations on IP fragmentation performance implications were
noted in the 1986 Digital Equipment Corporation (DEC) "qe reset"
investigation, where fragment bursts from NFS UDP traffic triggered
hardware resets resulting in communication failures. Jeff Chase, Fred
Glover and Chet Juzsczak of the Ultrix Engineering Group led the
investigation, and determined that setting a smaller NFS mount block
size reduced the amount of fragmentation and suppressed the resets.
Early observations on L2 media MTU issues were noted in the 1988 DEC
FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde
represented architectural considerations for FDDI networking in general
including FDDI/Ethernet bridging. Jeff Mogul (who led the IETF Path MTU
Discovery working group) and other DEC colleagues who supported these
early investigations are also acknowledged.Throughout the 1990's and into the 2000's, many colleagues supported
and encouraged continuation of the work. Beginning with the DEC Project
Sequoia effort at the University of California, Berkeley, then moving to
the DEC research lab offices in Palo Alto CA, then to the NASA Ames
Research Center, then to SRI in Menlo Park, CA, then to Nokia in
Mountain View, CA and finally to the Boeing Company in 2005 the work saw
continuous advancement through the encouragement of many. Those who
offered their support and encouragement are gratefully acknowledged.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
Mobility Vision Lab (MVL) program.Error Characteristics of Fiber Distributed Data Interface
(FDDI), IEEE Transactions on CommunicationsPerformance of Checksums and CRC's Over Real Data, IEEE/ACM
Transactions on Networking, Vol. 6, No. 5The OMNI Interface - An IPv6 Air/Ground Interface for Civil
Aviation, IETF Liaison Statement #1676,
https://datatracker.ietf.org/liaison/1676/ICAO Document 9896 (Manual on the Aeronautical
Telecommunication Network (ATN) using Internet Protocol Suite (IPS)
Standards and Protocol), Draft Edition 3 (work-in-progress)IPv4 Address Space Registry,
https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtmlIPv6 Global Unicast Address Assignments,
https://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtmlThe OAL Checksum Algorithm adopts the 8-bit Fletcher Checksum
Algorithm specified in Appendix I of as also
analyzed in . declared
historic for the reason that the algorithms had
never seen widespread use with TCP, however this document adopts the
8-bit Fletcher algorithm for a different purpose. Quoting from Appendix
I of , the OAL Checksum Algorithm proceeds as
follows:"The 8-bit Fletcher Checksum Algorithm is calculated over a
sequence of data octets (call them D[1] through D[N]) by maintaining
2 unsigned 1's-complement 8-bit accumulators A and B whose contents
are initially zero, and performing the following loop where i ranges
from 1 to N:A := A + D[i]B := B + AIt can be shown that at the end of the loop A will contain
the 8-bit 1's complement sum of all octets in the datagram, and that
B will contain (N)D[1] + (N-1)D[2] + ... + D[N]."To calculate the OAL checksum, the above algorithm is applied over
the N-octet concatenation of the OAL pseudo-header, the encapsulated IP
packet and the two-octet trailing checksum field initialized to 0.
Specifically, the algorithm is first applied over the 40 octets of the
OAL pseudo-header as data octets D[1] through D[40], then continues over
the entire length of the original IP packet as data octets D[41] through
D[N-2] and finally concludes with the two trailing 0 octets as data
octets D[N-1] and D[N].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" , 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 , 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 *NET. 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 *NET links, ARs can maintain an
OMNI-specific unicast L2 address ("MSADDR"). For Ethernet-compatible
*NETs, this specification reserves one Ethernet unicast address TBD3
(see: ). For non-Ethernet statically-addressed
*NETs, MSADDR is reserved per the assigned numbers authority for the
*NET addressing space. For still other *NETs, 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) .<< RFC Editor - remove prior to publication >>Differences from draft-templin-6man-omni-20 to
draft-templin-6man-omni-21:Final editorial review pass resulting in multiple changes.
Document now submit for final approval (with reference to rfcdiff
from previous version).Differences from draft-templin-6man-omni-19 to
draft-templin-6man-omni-20:Final editorial review pass resulting in multiple changes.
Document now submit for final approval (with reference to rfcdiff
from previous version).Differences from draft-templin-6man-omni-18 to
draft-templin-6man-omni-19:Final editorial review pass resulting in multiple changes.
Document now submit for final approval (with reference to rfcdiff
from previous version).Differences from draft-templin-6man-omni-17 to
draft-templin-6man-omni-18:Final editorial review pass resulting in multiple changes.
Document now submit for final approval (with reference to rfcdiff
from previous version).Differences from draft-templin-6man-omni-16 to
draft-templin-6man-omni-17:Final editorial review pass resulting in multiple changes.
Document now submit for final approval (with reference to rfcdiff
from previous version).Differences from draft-templin-6man-omni-15 to
draft-templin-6man-omni-16:Final editorial review pass resulting in multiple changes.
Document now submit for final approval.Differences from draft-templin-6man-omni-14 to
draft-templin-6man-omni-15:Text restructuring to remove ambiguities, eliminate extraneous
text and improve readability.Clarified that the OMNI link model is NBMA and that link-scoped
multicast is through iterative unicast.Differences from draft-templin-6man-omni-13 to
draft-templin-6man-omni-14:Brought back the optional two-message exchange feature.Added TCP RST flag and new (OPT, PNG) flags to the OMNI option
header.Require the OAL node that initiates the symmetric connection to
include its (future) receive window size in the initial SYN.Require OAL nodes to select new ISS values that are outside of
the current SND.WND.Text clarifications for improved readability.Differences from draft-templin-6man-omni-12 to
draft-templin-6man-omni-13:Complete revision of OAL Identification Window Maintenance
section to incorporate well-known protocol conventions and
terminology.Differences from draft-templin-6man-omni-11 to
draft-templin-6man-omni-12:Expanded on details of symmetric window synchronization.Differences from draft-templin-6man-omni-10 to
draft-templin-6man-omni-11:Included an Ordinal Number field in the Compressed Header format
for non-final fragmentsClarified that the window coordination protocol is based on the
IPv6 ND connectionless protocol using TCP constructs, and not based
on the TCP connection-oriented protocol.Removed unneeded fields from the OMNI option header.Differences from draft-templin-6man-omni-09 to
draft-templin-6man-omni-10:Fixed sizing considerations for OMNI option fields.Updated handling of multiple OMNI options in the same IPv6 ND
message. Only the first option includes the header, while all other
options include only sub-options.Differences from draft-templin-6man-omni-08 to
draft-templin-6man-omni-09:Included reference to RFC3366 and updated section on Fragment
Retransmission.Added "ordinal number" marking in Fragment Header reserved
field.Differences from draft-templin-6man-omni-07 to
draft-templin-6man-omni-08:Included TCP state variables; window scaleDifferences from draft-templin-6man-omni-06 to
draft-templin-6man-omni-07:Moved Interface Attributes, Type 1 and Type 2 to historic
status.Incorporated Traffic Selector into Interface Attributes, Type
4.Differences from draft-templin-6man-omni-05 to
draft-templin-6man-omni-06:Adopted TCP as an OAL packet-based connection-oriented
protocol.Three-Way handshake for establishing symmetric send/receive
windowsWindow length specified, plus "current" and "previous"
windowsNew appendix on checksum algorithm, with citations changedSecurity architecture considerations.More details on HIP message signatures.Require firewalls at OAL destinations.Removed "equal-length" requirement for OAL non-final
fragments.Differences from draft-templin-6man-omni-04 to
draft-templin-6man-omni-05:Change to S/T-omIndex definition.Differences from draft-templin-6man-omni-03 to
draft-templin-6man-omni-04:Changed reference citations to "draft-templin-6man-aero".Included introductory description of the "6M's".Included new OMNI sub-option for PIM-SM.Differences from draft-templin-6man-omni-02 to
draft-templin-6man-omni-03:Added citation of RFC8726.Differences from draft-templin-6man-omni-01 to
draft-templin-6man-omni-02:Updated IANA registration policies for OMNI registries.Differences from draft-templin-6man-omni-00 to
draft-templin-6man-omni-01:Changed intended document status to Informational, and removed
documents from "updates" category.Updated implementation status.Minor edits to HIP message specifications.Clarified OAL and *NET IP header field settings during
encapsulation and re-encapsulation.Differences from earlier versions to
draft-templin-6man-omni-00:Established working baseline reference.