How is frame count (FCnt) desyncronisation managed? (1.0.4+)

stevencellist · May 7, 2025, 7:58am

max_fcnt_gap nor maxfcntgap yields any results on the Chirpstack repository, so then I tried gap which led me straight to

github.com/chirpstack/chirpstack

chirpstack/src/storage/device.rs

188ef3d8f


      
          fn get_full_f_cnt_up(next_expected_full_fcnt: u32, truncated_f_cnt: u32) -> u32 {
              // Handle re-transmission.
              if truncated_f_cnt == (((next_expected_full_fcnt % (1 << 16)) as u16).wrapping_sub(1)) as u32 {
                  return next_expected_full_fcnt - 1;
              }
          
              let gap = ((truncated_f_cnt as u16).wrapping_sub((next_expected_full_fcnt % (1 << 16)) as u16))
                  as u32;
          
              next_expected_full_fcnt.wrapping_add(gap)
          }

With a bit of inference and a clarification on wrapping_sub() and wrapping_add(), it is clear that the gap has a max value of (1 << 16) which is added to the FCnt value.