Editing High-Level Data Link Control (section)

===S-frames (control)===
Supervisory Frames, or 'S-frames', are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send.  S-frames in HDLC '''do not''' have information fields, although some HDLC-derived protocols use information fields for "multi-selective reject".

The S-frame control field includes a leading "10" indicating that it is an S-frame.  This is followed by a 2-bit type, a poll/final bit, and a 3-bit sequence number.  (Or a 4-bit padding field followed by a 7-bit sequence number.)

The first (least significant) 2 bits mean it is an S-frame.  All S frames include a P/F bit and a receive sequence number as described above.  Except for the interpretation of the P/F field, there is no difference between a command S frame and a response S frame; when P/F is 0, the two forms are exactly equivalent.

====Receive Ready (RR)====
* Bit value = 00  (0x00 to match above table type field bit order<ref>{{Cite web|url=http://www.euclideanspace.com/coms/protocol/x25/link/f_types/index.htm#rr|title = X.25 frame types - Martin Baker}}</ref>)
* Indicate that the sender is ready to receive more data (cancels the effect of a previous RNR).
* Send this packet if you need to send a packet but have no I frame to send.
* A primary station can send this with the P-bit set to solicit data from a secondary station.
* A secondary terminal can use this with the F-bit set to respond to a poll if it has no data to send.

====Reject (REJ)====
* Bit value = 01  (0x08 to match above table type field bit order<ref>{{Cite web|url=http://www.euclideanspace.com/coms/protocol/x25/link/f_types/index.htm#rej|title = X.25 frame types - Martin Baker}}</ref>)
* Requests immediate retransmission starting with N(R).
* Sent in response to an observed sequence number gap; e.g. after seeing I1/I2/I3/I5, send REJ4.
* Optional to generate; a working implementation may use only RR.

====Receive Not Ready (RNR)====
* Bit value = 10  (0x04 to match above table type field bit order<ref>{{Cite web|url=http://www.euclideanspace.com/coms/protocol/x25/link/f_types/index.htm#rnr|title = X.25 frame types - Martin Baker}}</ref>)
* Acknowledge some packets but request no more be sent until further notice.
* Can be used like RR with P bit set to solicit the status of a secondary station
* Can be used like RR with F bit set to respond to a poll if the station is busy.

====Selective Reject (SREJ)====
* Bit value = 11  (0x0c to match above table type field bit order)
* Requests retransmission of only the frame N(R).
* Not supported by all HDLC variants.
* Optional to generate; a working implementation may use only RR, or only RR and REJ.