Proposed is an operating method for a first device (100) in a wireless communication system. The method may comprise the steps of: transmitting a physical sidelink control channel (PSCCH) to a second device (200); transmitting, to the second device (200), a physical sidelink shared channel (PSSCH) on the basis of the PSCCH; determining a physical sidelink feedback channel (PSFCH) resource related to the PSSCH on the basis of an index of a slot related to the PSSCH and an index of a subchannel; and declaring sidelink (SL) radio link failure (RLF) for a connection between the first device (100) and the second device (200), on the basis that a discontinuous transmission (DTX) counter reaches a maximum DTX-related threshold.