LLCP (Logical Link Control Protocol)

What Is LLCP?

LLCP (Logical Link Control Protocol) is the transport-layer protocol that enables peer-to-peer communication between two active NFC devices. Defined in the NFC ForumNFC ForumIndustry body developing NFC standards, specifications, and certifications since 2004View full → LLCP specification, it provides both connection-oriented and connectionless data exchange services, sitting above the NFCIP-1 physical and data link layers. LLCP is the foundation upon which higher-level protocols like SNEP operate.

Protocol Architecture

LLCP implements a layered architecture inspired by the OSI model. It provides three service classes that applications can choose from based on their reliability requirements:

The connection-oriented service provides flow control, sequence numbering, and acknowledgment, ensuring that data arrives in order and without loss. The connectionless service trades reliability for lower overhead, suitable for small messages where retransmission at the application layer is acceptable.

LLCP Frame Structure

LLCP communication uses Protocol Data Units (PDUs) with a compact header structure. Each PDU contains a destination service access point (DSAP), a source service access point (SSAP), a PDU type field, and a sequence number for connection-oriented transfers. The maximum information unit (MIU) defaults to 128 bytes but can be negotiated up to 2176 bytes during connection establishment.

Service access points (SAPs) identify the communicating applications, similar to port numbers in TCP/IP networking. Well-known SAPs are reserved for standard services: SAP 1 is the Service Discovery Protocol, and SAP 4 is reserved for SNEP.

Symmetry and Role Switching

One of LLCP's distinctive features is its symmetry mechanism. In NFC peer-to-peer modepeer-to-peer modeBidirectional data exchange between two active NFC devicesView full →, both devices alternate between initiator and target roles during data exchange. LLCP manages this role switching transparently, sending SYMM (symmetry) PDUs when a device has no data to transmit but needs to maintain the link. This ping-pong exchange continues as long as both devices remain in the RF field.

LLCP and Android Beam

LLCP was most prominently used in Android Beam, which was introduced in Android 4.0 (2011) and deprecated in Android 10 (2019). Android Beam used LLCP's connection-oriented service to establish a SNEP session, exchange NDEF messages, and then optionally hand off large file transfers to Bluetooth or Wi-Fi Direct. The deprecation of Android Beam significantly reduced the practical importance of LLCP in consumer applications.

Current Relevance

While peer-to-peer mode and LLCP are less prominent in consumer applications today, they remain important in industrial NFC deployments and in the NFC Forum certificationNFC Forum certificationInteroperabilityInteroperabilityCross-manufacturer device/tag compatibility guaranteeView full → testing program for NFC products and devicesView full → process. The NFC Forum Test Suite includes comprehensive LLCP compliance tests, and any device seeking NFC Forum Certification must demonstrate correct LLCP behavior. Understanding LLCP is also essential for developing custom NFC communication solutions that go beyond simple tag reading and writing.