This counter tracks the number of HDLC header bytes received over the satellite interface, where the MIB defines that header as the sum of the Control field, HDLC address, Flow ID, and CRC. This counter is mainly useful as a protocol-overhead metric -- comparing its growth to the payload-bytes counter shows what fraction of received satellite bandwidth is consumed by HDLC framing overhead versus actual data, which matters when trying to account for the gap between the provisioned data rate and the effective throughput seen by end applications. On a link carrying many small packets, this counter can grow surprisingly close in magnitude to the payload byte count, since fixed per-frame HDLC overhead eats a larger proportional share of small frames than large ones.