Editing High-Level Data Link Control (section)

====Information transfer====
These frames may be used as part of normal information transfer.

; Unnumbered information (UI)
: This frame (command or response) communicates user data, but without acknowledgement or retransmission in case of error.
; UI with header check (UIH)
: This frame (command or response), a ISO/IEC 13239 addition and rarely used, is like UI but also excludes CRC protection.  Only a configurable-length  prefix ("header") of the frame is covered by the CRC polynomial; errors in the rest of the frame are not detected.
; Unnumbered poll (UP) command
: This command solicits a response from the secondary.  With the poll bit set, it acts like any other poll frame, without the acknowledgement that must be included in I or S frame.  With the poll bit clear, it has a special meaning in normal response mode: the secondary ''may'' respond, even though it has not received the poll bit.  This is rarely used in HDLC, but was used in the original IBM SDLC as a substitute for the lack of asynchronous response mode; where the communication channel could handle simultaneous responses, the primary would periodically send UP to the broadcast address to collect any pending responses.