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SAE J2735-Draft-Rev28 [issued: 11-10-08]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
11.2
Persistence of the temporary MAC ID field
The MAC address used by OBUs is randomly generated at various times according to a timer, or vehicle
start-up, or possibly other events. This random MAC address is called the Temporary ID in DSRC
messages. The reason for having a non-permanent MAC address, and avoiding any other long-term
identification that is publicly available, is to preserve privacy through anonymity. The MAC value for a
mobile OBU device (unlike a typical wireless or wired 802 device) will therefore periodically change to a
new random value to ensure the overall anonymity of the vehicle. Because this value is used as a means to
identify the local vehicles that are interacting during an encounter, it is used in the message set.
Annex A Message Framework
Introduction
This annex is intended as a guide for message framework issues.
Message Common Header
A message common header element is a data element in a message that is common to all messages. It is
part of the transmitted message, is not changed by any lower levels and is required and used by a receiving
application or applications. The only identified message common header element in this standard is the
message ID included as the first word of content in all messages.
Application Programming Interface
An Application Programming Interface (API) is required to process common management information not
included in a message (Application Protocol Data Unit). This message related information is not
transmitted as part of the message set. An API for J2735 purposes is either information provided by an
application which is required by the application’s lower layers or is information required by an application
and provided by the application’s lower layers. The mechanism of communication is not considered in
scope for J2735 and may or may not be provided by other standards. Any J2735 API should include the
transmitted power level and the message priority.
Temporary ID
The Temporary ID, an element of the Basic Safety Message, might occasionally need to be changed for
purposes of privacy. The Temporary ID value should be chosen randomly and is separate from any other
identifier in the vehicle. Thus any relationship it might have with any other identifier is out of scope for the
J2735 standard. A vehicle should refrain from changing the temporary vehicle ID when event flags are
set. A mechanism for when and how the Temporary ID changes and how the changes are coordinated
between layers is yet to be determined. The change mechanism itself is considered out of scope for the
J2735 standard.
PSC/PSID
The PSC/PSID is an example of information shared by application and its lower layers. It is considered out
of scope for the J2735 standard.