Basic Support for IPv6 Networks Operating Outside the Context of a Basic
Service Set over IEEE Std 802.11
Moulay Ismail University of Meknes
Morocco
+212670832236
n.benamar@est.umi.ac.ma
EURECOMSophia-Antipolis06904
France
+33493008134
Jerome.Haerri@eurecom.fr
Sangmyung University
31, Sangmyeongdae-gil, Dongnam-gu
31066
Cheonan
Republic of Korea
jonghyouk@smu.ac.kr
YoGoKo1137A Avenue des Champs-BlancsCESON-SEVIGNE35510
France
thierry.ernst@yogoko.fr
Internet
IPWAVE Working GroupIPv6 over 802.11pOCBIPv6 over 802.11-OCB
This document provides methods and settings
for using IPv6 to communicate among nodes within range of one another
over a single IEEE 802.11-OCB link. Support for these methods and
settings require minimal changes to existing stacks. This document
also describes limitations associated with using these methods.
Optimizations and usage of IPv6 over more complex scenarios
are not covered in this specification and are a subject for future work.
Introduction
This document provides a baseline for using IPv6 to
communicate among nodes in range of one another over a single IEEE 802.11-OCB link
(a.k.a., 802.11p;
see Appendices ,
, and )
with minimal changes to existing stacks. Moreover, the document
identifies the limitations
of such usage. Concretely, the document describes the layering
of IPv6 networking on top of the IEEE Std 802.11 MAC layer or an IEEE Std 802.3
MAC layer with a frame translation underneath. The resulting stack is derived from IPv6
over Ethernet but operates over 802.11-OCB to provide at least P2P (point-to-point) connectivity
using IPv6 Neighbor Discovery (ND) and link-local addresses.
The IPv6 network layer operates on 802.11-OCB in the same
manner as operating on the Ethernet with the following
exceptions:
Exceptions due to the different operation of the IPv6 network
layer on 802.11 compared to the Ethernet. The operation of IP
on Ethernet is described in and .
Exceptions due to the OCB nature of 802.11-OCB compared to
802.11. This has impacts on security, privacy, subnet
structure, and movement detection. Security and
privacy recommendations are discussed in Sections and . The subnet structure is described
in . The movement
detection on OCB links is not described in this document.
Likewise, ND extensions and IP Wireless Access in Vehicular
Environments (IPWAVE) optimizations for vehicular communications are
not in scope of this document. The expectation is that further specifications will be edited to cover
more complex vehicular networking scenarios.
The reader may refer to for an overview of
problems related to running IPv6 over 802.11-OCB. It is out of scope
of this document to reiterate those problems.
Terminology
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.
The document makes uses of the following terms:
IP-OBU (Internet Protocol On-Board Unit):
An IP-OBU denotes a
computer situated in a vehicle such as a car, bicycle,
or similar. It has at least one IP interface that runs in
mode OCB of 802.11 and has an "OBU" transceiver. See
the definition of the term "OBU" in .
IP-RSU (IP Roadside Unit):
An IP-RSU is situated along the
road. It has at least two distinct IP-enabled interfaces. The
wireless PHY/MAC layer of at least one of its IP-enabled
interfaces is configured to operate in 802.11-OCB mode. An
IP-RSU communicates with the IP-OBU over an 802.11
wireless link operating in OCB mode. An IP-RSU is similar to
an Access Network Router (ANR), defined in , and a Wireless Termination Point (WTP),
defined in .
OCB (outside the context of a Basic Service Set - BSS):
This is a mode
of operation in which a station (STA) is not a member of a BSS and does
not utilize IEEE Std 802.11 authentication, association, or
data confidentiality.
802.11-OCB:
This refers to the mode specified in IEEE Std 802.11-2016 when the
MIB attribute dot11OCBActivited is 'true'.
Communication Scenarios Where IEEE 802.11-OCB Links Are Used
IEEE 802.11-OCB networks are used for vehicular
communications as 'Wireless Access in Vehicular
Environments'. In particular, we refer the reader to , which lists
some scenarios and requirements for IP in Intelligent
Transportation Systems (ITS).
The link model is the following: STA --- 802.11-OCB --- STA.
In vehicular networks, STAs can be IP-RSUs and/or IP-OBUs.
All links are assumed to be P2P, and multiple links can be on one radio
interface. While 802.11-OCB is clearly specified and a legacy IPv6
stack can operate on such links, the use of the operating environment
(vehicular networks) brings in new perspectives.
IPv6 over 802.11-OCBMaximum Transmission Unit (MTU)
The default MTU for IP packets on 802.11-OCB is inherited
from and, as such, is 1500 octets.
As noted in , every link on the Internet must have a
minimum MTU of 1280 octets and must follow the other
recommendations, especially with regard to fragmentation.
Frame Format
IP packets MUST be transmitted over 802.11-OCB media as QoS
data frames whose format is specified in an IEEE 802.11 spec
.
The IPv6 packet transmitted on 802.11-OCB is
immediately preceded by a Logical Link Control (LLC) header
and an 802.11 header. In the LLC header and in accordance
with EtherType Protocol Discrimination (EPD; see ), the value of the Type field MUST be set to
0x86DD (IPv6). The mapping to the 802.11 data service SHOULD
use a 'priority' value of 1 (QoS with a 'Background' user priority),
reserving higher priority values for safety-critical and time-sensitive
traffic, including the ones listed in .
To simplify the Application Programming Interface (API)
between the operating system and the 802.11-OCB media,
device drivers MAY implement IPv6 over Ethernet as per
and then a frame translation from 802.3 to 802.11 in order
to minimize the code changes.
Link-Local Addresses
There are several types of IPv6 addresses that may be
assigned to an 802.11-OCB interface. Among these types of
addresses, only the IPv6 link-local addresses can be formed
using an EUI-64 identifier, particularly during transition
time (the period of time before an interface starts using an address
different from the LL one).
If the IPv6 link-local address is formed using an EUI-64
identifier, then the mechanism for forming that address is
the same mechanism as that used to form an IPv6 link-local
address on Ethernet links. Moreover, regardless of whether the interface
identifier is derived from the EUI-64 identifier, its length is 64 bits,
as is the case for the Ethernet .
Stateless Autoconfiguration
The steps a host takes in deciding how to
autoconfigure its interfaces in IPv6 are described
in . This section describes
the formation of Interface Identifiers for 'Global' or 'Unique Local' IPv6 addresses. Interface Identifiers
for 'link-local' IPv6 addresses are discussed in .
The RECOMMENDED method for forming
stable Interface Identifiers (IIDs) is described in . The method of forming IIDs described in
MAY be used during
transition time, particularly for IPv6 link-local
addresses. Regardless of the method used to form the IID,
its length is 64 bits, similarly to IPv6 over Ethernet .
The bits in the IID have no specific meaning,
and the identifier should be treated as an opaque value.
The bits 'Universal' and 'Group' in the identifier of an
802.11-OCB interface, which is an IEEE link-layer address, are
significant. The details of this significance are
described in .
Semantically opaque IIDs, instead of
meaningful IIDs derived from a valid and
meaningful MAC address (), help avoid certain privacy risks (see the risks
mentioned in ). If
semantically opaque IIDs are needed, they
may be generated using the method for generating
semantically opaque IIDs with IPv6
Stateless Address Autoconfiguration given in . Typically, an opaque IID is formed starting from identifiers different
from the MAC addresses and from cryptographically strong
material. Thus, privacy-sensitive information is absent
from Interface IDs because it is impossible to calculate
back the initial value from which the Interface ID was first
generated.
Some applications that use IPv6 packets on 802.11-OCB links
(among other link types) may benefit from IPv6 addresses
whose IIDs don't change too often. It is
RECOMMENDED to use the mechanisms described in to
permit the use of stable IIDs that do not
change within one subnet prefix. A possible source for the
Net_Iface parameter is a virtual interface name or logical
interface name that is decided by a local administrator.
Address Mapping
Unicast and multicast address mapping MUST follow the
procedures specified for Ethernet interfaces described in Sections and of .
Address Mapping -- Unicast
This document is scoped for Address Resolution (AR) and Duplicate Address Detection (DAD) per .
Address Mapping -- Multicast
The multicast address mapping is performed according to
the method specified in . The meaning of the value "33-33"
mentioned there is
defined in .
Transmitting IPv6 packets to multicast destinations over
802.11 links proved to have some performance issues . These
issues may be exacerbated in OCB mode.
Future improvement to this specification should consider solutions for these problems.
Subnet Structure
When vehicles are in close range, a subnet may be formed over
802.11-OCB interfaces (not by their in-vehicle interfaces).
A Prefix List conceptual data structure () is maintained for each
802.11-OCB interface.
The IPv6 Neighbor Discovery protocol (ND) requires reflexive properties
(bidirectional connectivity), which is generally, though not always, the case for P2P OCB links.
IPv6 ND also requires transitive properties for DAD and AR, so an IPv6 subnet can be mapped
on an OCB network only if all nodes in the network share a single
physical broadcast domain. The extension to IPv6 ND operating on a
subnet that covers multiple OCB links and does not fully overlap
(i.e., non-broadcast multi-access (NBMA)) is not in scope of this document.
Finally, IPv6 ND requires permanent connectivity of all nodes in the subnet
to defend their addresses -- in other words, very stable network conditions.
The structure of this subnet is ephemeral in that it is
strongly influenced by the mobility of vehicles: the hidden
terminal effects appear, and the 802.11 networks in OCB mode may
be considered ad hoc networks with an addressing model,
as described in . On the other hand,
the structure of the internal subnets in each vehicle is
relatively stable.
As recommended in , when the timing
requirements are very strict (e.g., fast-drive-through IP-RSU
coverage), no on-link subnet prefix should be configured on
an 802.11-OCB interface. In such cases, the exclusive use
of IPv6 link-local addresses is RECOMMENDED.
Additionally, even if the timing requirements are not very
strict (e.g., the moving subnet formed by two following
vehicles is stable, a fixed IP-RSU is absent), the subnet is
disconnected from the Internet (i.e., a default route is absent),
and the addressing peers are equally qualified (that is, it is impossible
to determine whether some vehicle owns and distributes
addresses to others), the use of link-local addresses is
RECOMMENDED.
The baseline ND protocol MUST be supported over 802.11-OCB links.
Transmitting ND packets may prove to have some performance
issues, as mentioned in and
.
These issues may be exacerbated in OCB mode.
Solutions for these problems should consider the OCB mode
of operation. Future solutions to OCB should consider solutions
for avoiding broadcast. The best of current knowledge
indicates the kinds of issues that may arise with ND in
OCB mode; they are described in .
Protocols like Mobile IPv6 and
DNAv6 , which depend on timely
movement detection, might need additional tuning work to
handle the lack of link-layer notifications during handover.
This topic is left for further study.
Security Considerations
Any security mechanism at the IP layer or above that may be
implemented for the general case of IPv6 may also be implemented
for IPv6 operating over 802.11-OCB.
The OCB operation does not use existing 802.11
link-layer security mechanisms. There is no encryption
applied below the network layer running on 802.11-OCB. At
the application layer, the IEEE 1609.2 document provides security services for
certain applications to use; application-layer mechanisms are
out of scope of this document. On the other hand, a security
mechanism provided at the networking layer, such as IPsec , may provide data security protection to a
wider range of applications.
802.11-OCB does not provide any cryptographic protection because it operates outside the context of a BSS (no
Association Request/Response or Challenge messages).
Therefore, an attacker can sniff or inject traffic while within
range of a vehicle or IP-RSU (by setting an interface card's frequency to the proper range).
Also, an attacker may not adhere to the legal limits
for radio power and can use a very sensitive directional antenna;
if attackers wish to attack a given exchange, they do not necessarily
need to be in close physical proximity. Hence, such a link is less protected than
commonly used links (a wired link or the aforementioned 802.11 links with link-layer security).
Therefore, any node can join a subnet and directly communicate with any
nodes on the subset, including potentially impersonating another node. This
design allows for a number of threats outlined in .
While not widely deployed, SEND is a solution
that can address spoof-based attack vectors.
Privacy Considerations
As with all Ethernet and 802.11 interface identifiers , the identifier of an 802.11-OCB
interface may involve privacy, MAC address spoofing, and IP
hijacking risks. A vehicle embarking an IP-OBU
whose egress interface is 802.11-OCB may expose itself to
eavesdropping and subsequent correlation of data. This may
reveal data considered private by the vehicle owner; there
is a risk of being tracked. In outdoor public
environments, where vehicles typically circulate, the
privacy risks are greater than in indoor settings.
It is highly likely that attacker sniffers are deployed
along routes that listen for IEEE frames, including IP
packets, of vehicles passing by. For this reason, in 802.11-OCB deployments, there is a strong necessity to use
protection tools such as dynamically changing MAC addresses
(), semantically opaque Interface
Identifiers, and stable Interface Identifiers (). An example of a change policy is to change the MAC
address of the OCB interface each time the system boots up.
This may help mitigate privacy risks to a
certain level. Furthermore, for privacy concerns, recommends using an address-generation scheme
rather than generating addresses from a fixed link-layer address.
However, there are some specificities related to vehicles. Since roaming is an important
characteristic of moving vehicles, the use of the same Link-Local Address over time
can indicate the presence of the same vehicle in different places and thus lead to location tracking.
Hence, a vehicle should get hints about a change of environment (e.g., engine running, GPS, etc.)
and renew the IID in its LLAs.
Privacy Risks of Meaningful Information in Interface IDs
The privacy risks of using MAC addresses displayed in
Interface Identifiers are important. IPv6 packets can
be captured easily on the Internet and on-link on public
roads. For this reason, an attacker may realize many
attacks on privacy. One such attack on 802.11-OCB is to
capture, store, and correlate company ID information
present in the MAC addresses of a large number of cars (e.g., listening for
Router Advertisements or other IPv6 application data
packets, and recording the value of the source address in
these packets). Further correlation of this information
with other data captured by other means or other visual
information (e.g., car color) may constitute privacy
risks.
MAC Address and Interface ID Generation
In 802.11-OCB networks, the MAC addresses may change during
well-defined renumbering events. At the moment the MAC
address is changed on an 802.11-OCB interface, all the
Interface Identifiers of IPv6 addresses assigned to that
interface MUST change.
Implementations should use a policy dictating when the MAC address is changed on the 802.11-OCB interface.
For more information on the motivation of this policy, please refer to
the privacy discussion in .
A 'randomized' MAC address has the following
characteristics:
The "Local/Global" bit is set to "locally administered".
The "Unicast/Multicast" bit is set to "Unicast".
The 46 remaining bits are set to a random value using a
random number generator that meets the requirements of
.
To meet the randomization requirements for the 46 remaining
bits, a hash function may be used. For example, the hash function
defined in
may be used with the input of a 256-bit local secret, the 'nominal'
MAC address of the interface, and a representation of the date and
time of the renumbering event.
A randomized Interface ID has the same characteristics of a
randomized MAC address except for the length in bits.
Pseudonymization Impact on Confidentiality and Trust
Vehicle and drivers privacy relies on pseudonymization mechanisms
such as the ones described in .
This pseudonymization means that upper-layer protocols and applications
SHOULD NOT rely on layer-2 or layer-3 addresses to assume that the other participant can be trusted.
IANA Considerations
This document has no IANA actions.
ReferencesNormative References
IEEE Standard for Information
technology - Telecommunications and information exchange
between systems Local and metropolitan area networks--Specific
requirements - Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY)
Specifications
IEEEInformative References
IEEE Standard for Information Technology -
Telecommunications and Information Exchange Between Systems -
Local and Metropolitan Area Networks - Specific Requirements -
Part 11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications
IEEE
IEEE Standard for Information technology--Telecommunications and
information exchange between systems Local and metropolitan area
networks--Specific requirements Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications
IEEE
IEEE Standard for Ethernet
IEEE
IEEE Standard for Information technology - Telecommunications and
information exchange between systems - Local and metropolitan area
networks - Specific requirements, Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications,
Amendment 6: Wireless Access in Vehicular Environments
IEEESecure Hash Standard (SHS)National Institute of Standards and
Technology
IEEE Standard for Wireless Access
in Vehicular Environments--Security Services for
Applications and Management Messages
IEEE
IEEE Standard for Wireless Access
in Vehicular Environments (WAVE) -- Networking Services
IEEE
IEEE Standard for Wireless Access
in Vehicular Environments (WAVE) -- Multi-Channel
Operation
IEEE
Intelligent Transport Systems (ITS);
Security; ITS communications security architecture and
security management
Electronic Code of Federal Regulationse-CFRTitle 47, CFR 90.7 - DefinitionsElectronic Code of Federal Regulationse-CFRTitle 47, CFR 95 - PERSONAL RADIO SERVICESElectronic Code of Federal Regulationse-CFRTitle 47, Part 90 - PRIVATE LAND MOBILE RADIO SERVICES802.11p
The term "802.11p" is an earlier definition. The behavior of
"802.11p" networks is rolled in . In that document, the term "802.11p" disappears.
Instead, each 802.11p feature is conditioned by the IEEE
Management Information Base (MIB) attribute "OCBActivated"
. Whenever OCBActivated is
set to "true", the IEEE Std 802.11-OCB state is activated. For
example, an 802.11 STAtion operating outside the context of a
BSS has the OCBActivated flag set. Such a
station, when it has the flag set, uses a BSS identifier equal
to ff:ff:ff:ff:ff:ff.
Aspects Introduced by OCB Mode to 802.11
In IEEE 802.11-OCB mode, all nodes in the wireless range
can directly communicate with each other without involving
authentication or association procedures. In OCB mode, the
manner in which channels are selected and used is simplified
compared to when in BSS mode. Contrary to BSS mode, at the link
layer, it is necessary to statically set the same channel
number (or frequency) on two stations that need to communicate
with each other (in BSS mode, this channel set operation is
performed automatically during 'scanning'). The manner in
which stations set their channel number in OCB mode is not
specified in this document. Stations STA1 and STA2 can
exchange IP packets only if they are set to the same channel.
At the IP layer, they then discover each other by using the IPv6
Neighbor Discovery protocol. The allocation of a particular
channel for a particular use is defined statically in
standards authored by ETSI in Europe, the FCC in the United States of America, and
similar organizations in South Korea, Japan, and other parts of
the world.
Briefly, the IEEE 802.11-OCB mode has the following
properties:
The use by each node of a 'wildcard' BSS identifier (BSSID) (i.e., each bit
of the BSSID is set to 1).
No IEEE 802.11 beacon frames are transmitted.
No authentication is required in order to be able to communicate.
No association is needed in order to be able to communicate.
No encryption is provided in order to be able to communicate.
Flag dot11OCBActivated is set to "true".
All the nodes in the radio communication range (IP-OBU and IP-RSU)
receive all the messages transmitted (IP-OBU and IP-RSU) within the
radio communication range. The MAC CDMA function resolves any
eventual conflict(s).
The message exchange diagram in
illustrates a comparison between traditional 802.11 and 802.11
in OCB mode. The 'Data' messages can be IP packets such as
HTTP or others. Other 802.11 management and control frames
(non-IP) may be transmitted, as specified in the 802.11
standard. The names of these messages as
currently specified by the 802.11 standard are listed in .
The 802.11-OCB interface was specified in , Amendment 6: Wireless
Access in Vehicular Environments, as an amendment
to . Since then, this amendment
has been integrated into and .
In , anything qualified specifically as
"OCBActivated" or "outside the context of a basic service"
that is set to be "true" actually refers to OCB aspects
introduced to 802.11.
In order to delineate the aspects introduced by 802.11-OCB to
802.11, we refer to the earlier . The amendment is concerned with
vehicular communications, where the wireless link is similar
to that of Wireless LAN (using a PHY layer specified by
802.11a/b/g/n) but needs to cope with the high mobility
factor inherent in scenarios of communications between moving
vehicles and between vehicles and fixed infrastructure
deployed along roads. While 'p' is a letter identifying the
Amendment, just like 'a', 'b', 'g', and 'n' are, 'p' is concerned
more with MAC modifications and is slightly concerned with PHY
modifications; the others are mainly about PHY modifications.
It is possible in practice to combine a 'p' MAC with an 'a'
PHY by operating outside the context of a BSS with Orthogonal
Frequency Division
Multiplexing (OFDM) at
5.4 GHz and 5.9 GHz.
The 802.11-OCB links are specified to be as compatible as
possible with the behavior of 802.11a/b/g/n and future
generation IEEE WLAN links. From the IP perspective, an
802.11-OCB MAC layer offers practically the same interface to
IP as 802.11a/b/g/n and 802.3. A packet sent by an IP-OBU
may be received by one or multiple IP-RSUs. The link-layer
resolution is performed by using the IPv6 Neighbor Discovery
protocol.
To support this similarity statement (IPv6 is layered on top
of LLC on top of 802.11-OCB in the same way that IPv6 is
layered on top of LLC on top of 802.11a/b/g/n (for WLAN) or
on top of LLC on top of 802.3 (for Ethernet)), it is
useful to analyze the differences between the 802.11-OCB and
802.11 specifications. During this analysis, we note that
whereas 802.11-OCB lists relatively complex and numerous
changes to the MAC layer (and very few to the PHY layer),
there are only a few characteristics that may be important
for an implementation transmitting IPv6 packets on 802.11-OCB
links.
The most important 802.11-OCB aspect that influences the IPv6
functioning is the OCB characteristic; an additional, less
direct influence is the maximum bandwidth afforded by the PHY
modulation/demodulation methods and channel access specified
by 802.11-OCB. The maximum bandwidth theoretically possible
in 802.11-OCB is 54 Mbit/s (when using, for example, the
following parameters: a 20 MHz channel; modulation of 64-QAM;
a coding rate R of 3/4). With regard to IP over 802.11-OCB, in
practice, a commonly observed figure is 12 Mbit/s; this bandwidth allows
the operation of a wide range of protocols relying on IPv6.
Operation outside the context of a BSS (OCB): The 802.11-OCB links
(previously 802.11p) are operated without a BSS. This means that IEEE 802.11
beacon, Association Request/Response, Authentication
Request/Response, and similar frames are not used. The used
identifier of BSS (BSSID) always has a hexadecimal value of
0xffffffffffff (48 '1' bits, represented as MAC address
ff:ff:ff:ff:ff:ff; otherwise, the 'wildcard' BSSID), as
opposed to an arbitrary BSSID value set by an administrator
(e.g., 'My-Home-AccessPoint'). The OCB operation -- namely,
the lack of beacon-based scanning and lack of
authentication -- should be taken into account when the
Mobile IPv6 protocol and the
protocols for IP layer security
are used. The way these protocols adapt to OCB is not
described in this document.
Timing Advertisement: This is a new message defined in
802.11-OCB that does not exist in 802.11a/b/g/n. This
message is used by stations to inform other stations about
the value of time. It is similar to the time delivered
by a Global Navigation Satellite System (GNSS) (e.g., Galileo, GPS, etc.) or by a cellular
system. This message is optional for implementation.
Frequency range: This is a characteristic of the PHY layer; it has
almost no impact on the interface between MAC and IP. However, it is worth considering that the
frequency range is regulated by a regional authority
(ARCEP, ECC/CEPT based on ENs from ETSI, FCC, etc.); as
part of the regulation process, specific applications are
associated with specific frequency ranges.
In the case of 802.11-OCB, the regulator associates a set of frequency
ranges or slots within a band to the use of applications of vehicular
communications in a band known as "5.9 GHz".
The 5.9 GHz band is different from the 2.4 GHz and 5 GHz
bands used by Wireless LAN. However, as with Wireless LAN, the
operation of 802.11-OCB in 5.9 GHz bands does not require a
license in the EU (in the US, the 5.9 GHz is a licensed band of
spectrum; for the fixed infrastructure, explicit FCC authorization
is required; for an on-board device, a 'licensed-by-rule' concept
applies, where rule certification conformity is required). Technical
conditions are different from those of the "2.4 GHz"
or "5 GHz" bands. The allowed power levels and, implicitly, the
maximum allowed distance between vehicles is 33 dBm for
802.11-OCB (in Europe) compared to 20 dBm for Wireless
LAN 802.11a/b/g/n; this leads to a maximum distance of
approximately 1 km compared to approximately 50 m.
Additionally, specific conditions related to congestion
avoidance, jamming avoidance, and radar detection are
imposed on the use of DSRC (in the US) and on the use of
frequencies for Intelligent Transportation Systems (in
the EU) compared to Wireless LAN (802.11a/b/g/n).
'Half-rate' encoding: As the frequency range, this
parameter is related to PHY and thus does not have much
impact on the interface between the IP layer and the
MAC layer.
In vehicular communications using 802.11-OCB links, there
are strong privacy requirements with respect to
addressing. While the 802.11-OCB standard does not
specify anything in particular with respect to MAC
addresses, in these settings, there is a strong need
for a dynamic change of these addresses (as opposed to the
non-vehicular settings -- real wall protection -- where
fixed MAC addresses do not currently pose privacy
risks). This is further described in . A relevant function is described in
and .
Changes Needed on an 802.11a Software Driver to Become an 802.11-OCB Driver
The 802.11p amendment modifies both the 802.11 stack's
physical and MAC layers, but all the induced modifications
can be quite easily obtained by modifying an existing
802.11a ad hoc stack.
The conditions for 802.11a hardware to be compliant with 802.11-OCB are as
follows:
The PHY entity shall be an OFDM system. It must support the frequency
bands on which the regulator recommends the use of ITS
communications -- for example, using an IEEE 802.11-OCB layer of
5875 MHz to 5925 MHz in France.
The OFDM system must provide a "half-clocked" operation
using 10 MHz channel spacings.
The chip transmit spectrum mask must be compliant with the
"Transmit spectrum mask" from the IEEE 802.11p amendment
(but experimental environments do not require compliance).
The chip should be able to transmit up to 44.8 dBm when
used in the United States and up to
33 dBm in Europe; other regional conditions apply.
Changes needed on the network stack in OCB mode are as follows:
Physical layer:
Orthogonal frequency-division multiple access
The chip must use the Orthogonal Frequency Division Multiple
Access (OFDMA) encoding mode.
The chip must be set to half-mode rate mode (the
internal clock frequency is divided by two).
The chip must use dedicated channels and should allow
the use of higher emission powers. This may require
modifications to the local computer file that
describes regulatory domains rules if used by the
kernel to enforce local specific restrictions. Such
modifications to the local computer file must respect
the location-specific regulatory rules.
MAC layer:
All management frames (beacons, join, leave, and
others) emission and reception must be disabled,
except for frames of subtype Action and Timing
Advertisement (defined below).
No encryption key or method must be used.
Packet emission and reception must be performed as in
ad hoc mode using the wildcard BSSID
(ff:ff:ff:ff:ff:ff).
The functions related to joining a BSS (Association
Request/Response) and authentication
(Authentication Request/Reply, Challenge) are not
called.
The beacon interval is always set to 0 (zero).
Timing Advertisement frames, defined in the
amendment, should be supported. The upper layer
should be able to trigger such frames emission and retrieve
information contained in the received Timing
Advertisements.
Protocol Layering
A more theoretical and detailed view of layer stacking and
interfaces between the IP layer and 802.11-OCB layers is
illustrated in . The IP layer
operates on top of EtherType Protocol Discrimination
(EPD). This discrimination layer is described in . The interface between IPv6 and EPD is the LLC_SAP
(Link Layer Control Service Access Point).
Design Considerations
The networks defined by 802.11-OCB are in many ways similar to
other networks of the 802.11 family. In theory, the
transportation of IPv6 over 802.11-OCB could be very similar to
the operation of IPv6 over other networks of the 802.11
family. However, the high mobility, strong link asymmetry, and
very short connection makes the 802.11-OCB link significantly
different from other 802.11 networks. Also, automotive
applications have specific requirements for reliability,
security, and privacy, which further add to the particularity
of the 802.11-OCB link.
IEEE 802.11 Messages Transmitted in OCB Mode
At the time of writing, this is the list of
IEEE 802.11 messages that may be transmitted in OCB mode,
i.e., when dot11OCBActivated is true in a STA:
The STA may send management frames of subtype Action and,
if the STA maintains a TSF Timer, subtype Timing
Advertisement.
The STA may send control frames except those of subtype
PS-Poll, CF-End, and CF-End plus CFAck.
The STA MUST send data frames of subtype QoS
Data.
Examples of Packet Formats
This section describes an example of an IPv6 packet captured
over an IEEE 802.11-OCB link.
By way of example, we show that there is no modification in the
headers when transmitted over 802.11-OCB networks -- they are
transmitted like any other 802.11 and Ethernet packets.
We describe an experiment for capturing an IPv6 packet on an
802.11-OCB link. In the topology depicted in , the packet is an IPv6 Router Advertisement.
This packet is emitted by a router on its 802.11-OCB
interface. The packet is captured on the host using a
network protocol analyzer (e.g., Wireshark). The capture is
performed in two different modes: direct mode and monitor
mode. The topology used during the capture is depicted below.
The packet is captured on the host. The host is an IP-OBU
containing an 802.11 interface in Peripheral Component Interconnect
(PCI) Express format (an Industrial Technology Research Institute
(ITRI) product). The kernel runs the ath5k software driver with
modifications for OCB mode. The capture tool is Wireshark.
The file format for saving and analyzing is .pcap. The packet is
generated by the router, which is an IP-RSU (an ITRI
product).
During several capture operations running from a few moments
to several hours, no messages relevant to the BSSID contexts
were captured (Association Request/Response, Authentication
Req/Resp, or beacon). This shows that the operation of
802.11-OCB is outside the context of a BSSID.
Overall, the captured message is identical to a capture of
an IPv6 packet emitted on an 802.11b interface. The contents
are exactly the same.
Capture in Monitor Mode
The IPv6 RA packet captured in monitor mode is illustrated
below. The Radiotap header provides more flexibility for
reporting the characteristics of frames. The Radiotap header
is prepended by this particular stack and operating system on
the host machine to the RA packet received from the network
(the Radiotap header is not present on the air). The
implementation-dependent Radiotap header is useful for
piggybacking PHY information from the chip's registers as data
in a packet that is understandable by userland applications using
socket interfaces (the PHY interface can be, for example,
power levels, data rate, or the ratio of signal to noise).
The packet present on the air is formed by the IEEE 802.11 Data
header, Logical Link Control header, IPv6 Base header, and
ICMPv6 header.
The value of the Data Rate field in the Radiotap header is set
to 6 Mb/s. This indicates the rate at which this RA was
received.
The value of the Transmitter Address in the IEEE 802.11 Data
header is set to a 48-bit value. The value of the destination
address is 33:33:00:00:00:1 (all-nodes multicast address).
The value of the BSS ID field is ff:ff:ff:ff:ff:ff, which is
recognized by the network protocol analyzer as being
"broadcast". The Fragment number and Sequence number fields
together are set to 0x90C6.
The value of the Organization Code field in the
Logical Link Control header is set to 0x0, recognized as
"Encapsulated Ethernet". The value of the Type field is
0x86DD (hexadecimal 86DD; otherwise, #86DD), recognized
as "IPv6".
A Router Advertisement is periodically sent by the router to
multicast group address ff02::1. It is ICMP packet type
134. The IPv6 Neighbor Discovery's Router Advertisement
message contains an 8-bit field reserved for single-bit flags,
as described in .
The IPv6 header contains the link-local address of the router
(source) configured via the EUI-64 algorithm, and the destination
address is set to ff02::1.
The Ethernet Type field in the Logical Link Control header
is set to 0x86dd, which indicates that the frame transports
an IPv6 packet. In the IEEE 802.11 data, the destination
address is 33:33:00:00:00:01, which is the corresponding
multicast MAC address. The BSS ID is a broadcast address of
ff:ff:ff:ff:ff:ff. Due to the short link duration between
vehicles and the roadside infrastructure, there is no need in IEEE 802.11-OCB
to wait for the completion of association
and authentication procedures before exchanging data. IEEE
802.11-OCB enabled nodes use the wildcard BSSID (a value of
all 1s) and may start communicating as soon as they arrive
on the communication channel.
Capture in Normal Mode
The same IPv6 Router Advertisement packet described above
(monitor mode) is captured on the host in normal mode and is
depicted below.
One notices that the Radiotap header, the IEEE 802.11 Data
header, and the Logical Link Control headers are not present.
On the other hand, a new header named the Ethernet II header is
present.
The Destination and Source addresses in the Ethernet II header
contain the same values as the Receiver Address and
Transmitter Address fields present in the IEEE 802.11 Data header in
the monitor mode capture.
The value of the Type field in the Ethernet II header is
0x86DD (recognized as "IPv6"); this value is the same as
the value of the Type field in the Logical Link Control header
in the monitor mode capture.
The knowledgeable experimenter will no doubt notice the
similarity of this Ethernet II header with a capture in normal
mode on a pure Ethernet cable interface.
A frame translation is inserted on top of a pure IEEE 802.11
MAC layer in order to adapt packets before delivering the
payload data to the applications. It adapts 802.11 LLC/MAC
headers to Ethernet II headers. Specifically, this
adaptation consists of the elimination of the Radiotap,
802.11, and LLC headers and the insertion of the Ethernet
II header. In this way, IPv6 runs straight over LLC over
the 802.11-OCB MAC layer; this is further confirmed by the
use of the unique Type 0x86DD.
Extra Terminology
The following terms are defined outside the IETF. They are
used to define the main terms in the terminology section ().
DSRC (Dedicated Short Range Communication):
The US Federal Communications Commission
(FCC) Dedicated Short Range Communication (DSRC) is defined in
the Code of Federal Regulations (CFR) 47, Parts 90 and 95 .
This Code is referenced in the definitions below. At the time
of the writing of this document, the last update of this
Code was dated December 6, 2019.
Radio techniques are used to transfer data over short distances between
roadside and mobile units, between mobile units, and between portable and
mobile units to perform operations related to the improvement of traffic
flow, traffic safety, and other intelligent transportation service
applications in a variety of environments. DSRCS systems may also transmit status and
instructional messages related to the units involved.
OBU (On-Board Unit):
An
On-Board Unit is a DSRCS transceiver that is normally mounted
in or on a vehicle or may be a portable unit in some instances. An OBU can be operational while a vehicle or
person is either mobile or stationary. The OBUs receive and
contend for time to transmit on one or more radio frequency
(RF) channels. Except where specifically excluded, OBU
operation is permitted wherever vehicle operation or human
passage is permitted. The OBUs mounted in vehicles are
licensed by rule under part 95 of and
communicate with Roadside Units (RSUs) and other OBUs.
Portable OBUs are also licensed by rule under part 95 of . OBU operations in the Unlicensed National
Information Infrastructure (U-NII) Bands follow the rules in
those bands.
RSU (Roadside Unit):
A
Roadside Unit is a DSRC transceiver that is mounted along a
road or pedestrian passageway. An RSU may also be mounted on
a vehicle or may be hand carried, but it may only operate when the
vehicle or hand-carried unit is stationary. Perhaps
Furthermore, an RSU is restricted to the location where it is licensed
to operate. However, portable
or handheld RSUs are permitted to operate where they do not
interfere with a site-licensed operation. An RSU broadcasts
data to OBUs or exchanges data with OBUs in its communications
zone. An RSU also provides channel assignments and operating
instructions to OBUs in its communications zone when
required.
Neighbor Discovery (ND) Potential Issues in Wireless Links
IPv6 Neighbor Discovery (IPv6 ND) was
designed for point-to-point and transit links, such as
Ethernet, with the expectation of cheap and reliable support
for multicast from the lower layer. indicates that the operation on shared media and on
NBMA networks require additional
support, e.g., for AR and DAD, which depend on multicast. An
infrastructureless radio network such as OCB shares properties
with both shared media and NBMA networks and then adds its
own complexity, e.g., from movement and interference that
allow only transient and non-transitive reachability between
any set of peers.
The uniqueness of an address within a scoped domain is a key
pillar of IPv6 and is the basis for unicast IP communication. details the DAD method to prevent an address from
being duplicated. For a link-local address, the scope is the link,
whereas for a globally reachable address, the scope is much
larger. The underlying assumption for DAD to operate
correctly is that the node that owns an IPv6 address can reach
any other node within the scope at the time it claims its
address, which is done by sending a Neighbor Solicitation (NS) multicast message, and
can hear any future claim for that address by another party
within the scope for the duration of the address ownership.
In the case of OCB, there is a potentially a need to define a scope that is
compatible with DAD. The scope cannot be the set of nodes that a transmitter
can reach at a particular time because that set varies all the time and
does not meet the DAD requirements for a link-local address that can be
used anytime and anywhere. The generic expectation of a reliable
multicast is not ensured, and the operation of DAD and AR
as specified by cannot be
guaranteed. Moreover, multicast transmissions that rely on
broadcast are not only unreliable but are also often
detrimental to unicast traffic (see ).
Early experience indicates that it should be possible to
exchange IPv6 packets over OCB while relying on IPv6 ND alone
for DAD and AR (Address Resolution) in good conditions. In the absence
of a correct DAD operation, a node that relies only on IPv6 ND
for AR and DAD over OCB should ensure that the addresses that
it uses are unique by means other than DAD. It must be noted
that deriving an IPv6 address from a globally unique MAC
address has this property but may yield privacy issues.
provides a more recent approach to IPv6 ND, in
particular DAD. is designed to fit wireless and
otherwise constrained networks whereby multicast and/or
continuous access to the medium may not be guaranteed. ("Link-Local Addresses and Registration")
indicates that the scope of uniqueness for a link-local
address is restricted to a pair of nodes that uses it to
communicate and provides a method to assert the uniqueness
and resolve the link-layer address using a unicast exchange.
also enables a router (acting as a 6LR) to own a
prefix and act as a registrar (acting as a 6LBR) for addresses
within the associated subnet. A peer host (acting as a 6LN)
registers an address derived from that prefix and can use it
for the lifetime of the registration. The prefix is advertised
as not on-link, which means that the 6LN uses the 6LR to relay
its packets within the subnet, and participation to the subnet
is constrained to the time of reachability to the 6LR. Note
that an RSU that provides internet connectivity MAY announce a
default router preference , whereas a car that does
not provide that connectivity MUST NOT do so. This operation
presents similarities to that of an access point, but at
Layer 3. This is why is well suited for wireless in
general.
Support of may be implemented on OCB. OCB nodes
that support SHOULD support the 6LN operation in order
to act as a host and may support the 6LR and 6LBR operations
in order to act as a router and in particular to own a prefix
that can be used by hosts that are compliant with for address
autoconfiguration and registration.
Acknowledgements
The authors would like to thank for
initiating this work and for being the lead author up to draft version 43 of
this document.
The authors would like to thank for reviewing,
proofreading, and suggesting modifications for this document.
The authors would like to thank for
proofreading and suggesting modifications for this document.
The authors would like to thank for
reviewing the suggesting modifications of this document.
The authors would like to thank , , , , ,
, , , ,
, , , ,
, , , ,
, , , , , , , , , , , , , , , , ,
, , , , ,
, , , , , , , , , , and . Their
valuable comments clarified particular issues and generally
helped to improve the document.
, , and others wrote 802.11-OCB
drivers for Linux.
For the multicast discussion, the authors would like to thank
, ,
, , , and participants to discussions in network working
groups.
The authors would like to thank the participants of the
Birds-of-a-Feather "Intelligent Transportation Systems"
meetings held at IETF in 2016.
The human rights protocol considerations review was done by .
The work of was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A4A1025632).The work of was supported by EURECOM industrial members,
namely BMW Group, IABG, Monaco Telecom, Orange, SAP and Symantec. This RFC
reflects the view of the IPWAVE WG and does not necessarily reflect the
official policy or position of EURECOM industrial members.Contributors and contributed to this document.
contributed extensively regarding IPv6 handovers
between links running outside the context of a BSS (802.11-OCB
links).
contributed the idea of the use of IPv6 over
802.11-OCB for the distribution of certificates.
, , , and
provided significant feedback on the experience
of using IP messages over 802.11-OCB in initial trials.
contributed extensively to the MTU
discussion, offered the ETSI ITS perspective, and reviewed
other parts of the document.