Link Layer Acknowledgment in HDLC

Various styles of acknowledgement are used by link layer protocols to confirm the correct transfer of PDUs. The most common is that used by HDLC. In this case, the remote receiver returns frames back to the local node to indicate which I-frames have been successfully received, or to be more exact which frame the remote receiver next expects to receive.

Many systems employ a cumulative acknowledgment scheme, utilising the fact that I-frames are sent in order of increasing send sequence number (N(S)). In this scheme, each acknowledgement which is received acknowledges all I-frames up to the indicated frame. (In HDLC the number indicates the next expected I-frame). This reduces the volume of acknowledgment data, and also reduces the impact of a lost acknowledgment (due to a frame from the remote node being corrupted).

Acknowledgment Procedure for HDLC (Simplex Data Flow) using continuous acknowledgments

In the figure above, the frame RR(7) confirms the correct reception of both frames I(5) and I(6).) Since there is normally a continuous flow of acknowledgement data, it is usually efficient to piggy-back the acknowledgement data with I-frames traveling from the remote node (only possible when the link is carrying full duplex traffic).

See also HDLC window.