Link Manager Protocol (LMP)

General Description

The Link Manager Protocol (LMP) is used to control and negotiate all aspects of the operation of the Bluetooth connection between two devices. This includes the set-up and control of logical transports and logical links, and for control of physical links. The Link Manager Protocol is used to communicate between the Link Managers (LM) on the two devices which are connected by the ACL logical transport.

General Rules

LMP messages are exchanged over the ACL-C logical link that is carried on the default ACL logical transport (see Bluetooth Specification, Baseband Section 4.4 for details). The ACL-C logical link is distinguished from the ACL-U (which carries L2CAP and user data) by the Logical Link Identifier (LLID) field carried in the payload header of variable-length packets. The ACL-C has a higher priority than other traffic.

LMP messages are carried on the ACL-C logical link, which does not guarantee a time to deliver or acknowledge packets. LMP procedures take account of this when synchronizing state changes in the two devices. For example, criteria are defined that specify when a logical transport address (LT_ADDR) may be re-used after it becomes available due to a device leaving the piconet or entering the park state. Other LMP procedures, such as hold or role switch include the Bluetooth clock as a parameter in order to define a fixed synchronization point. The transitions into and out of sniff mode are protected with a transition mode.

The LMP operates in terms of transactions. A transaction is a connected set of message exchanges which achieve a particular purpose.

Device Features

All features added after the 1.1 specification have associated LMP feature bits. Support of these features may be made “mandatory” by the qualification process but the LM still considers them to be optional since it must interoperate with older devices which do not support them.