Editing High-Level Data Link Control (section)

===Unnumbered frames===
Unnumbered frames are identified by the low two bits being 1.  With the P/F flag, that leaves 5 bits as a frame type.  Even though fewer than 32 values are in use, some types have different meanings depending on the direction they are sent: as a command or as a response.  The relationship between the '''DISC''' (disconnect) command and the '''RD''' (request disconnect) response seems clear enough, but the reason for making '''SARM''' command numerically equal to the '''DM''' response is obscure.

{| class="wikitable"
|-
!rowspan=2|Name
!rowspan=2|Command/<br>Response
!rowspan=2|Description
!rowspan=2|Info
!colspan=8|C-Field Format
|-
!7!!6!!5!!4!!3!!2!!1!!0
|-
|Set normal response mode '''SNRM'''
|align=center|C
|Set mode
|Use 3 bit sequence number
|1||0||0||align=center|P||0||0||1||1
|-
|SNRM extended '''SNRME'''
|align=center|C
|Set mode; extended
|Use 7 bit sequence number
|1||1||0||align=center|P||1||1||1||1
|-
|Set asynchronous response mode '''SARM'''
|align=center|C
|Set mode
|Use 3 bit sequence number
|0||0||0||align=center|P||1||1||1||1
|-
|SARM extended '''SARME'''
|align=center|C
|Set mode; extended
|Use 7 bit sequence number
|0||1||0||align=center|P||1||1||1||1
|-
|Set asynchronous balanced mode '''SABM'''
|align=center|C
|Set mode
|Use 3 bit sequence number
|0||0||1||align=center|P||1||1||1||1
|-
|SABM extended '''SABME'''
|align=center|C
|Set mode; extended
|Use 7 bit sequence number
|0||1||1||align=center|P||1||1||1||1
|-
|Set Mode '''SM'''
|align=center|C
|Set mode, generic
|New in ISO 13239
|1||1||0||align=center|P||0||0||1||1
|-
|Set initialization mode '''SIM'''
|align=center|C
|colspan=2|Initialize link control function in the addressed station
|0||0||0||align=center|P||0||1||1||1
|-
|Request initialization mode '''RIM'''
|align=center|R
|Initialization needed
|Request for '''SIM''' command
|0||0||0||align=center|F||0||1||1||1
|-
|Disconnect '''DISC'''
|align=center|C
|Terminate logical link connection
|Future I and S frames return '''DM'''
|0||1||0||align=center|P||0||0||1||1
|-
|Request disconnect '''RD'''
|align=center|R
|Solicitation for '''DISC''' Command
|
|0||1||0||align=center|F||0||0||1||1
|-
|Unnumbered acknowledgment '''UA'''
|align=center|R
|colspan=2|Acknowledge acceptance of one of the set-mode commands.
|0||1||1||align=center|F||0||0||1||1
|-
|Disconnect mode '''DM'''
|align=center|R
|Responder in disconnected mode
|Mode set required
|0||0||0||align=center|F||1||1||1||1
|-
|Unnumbered information '''UI'''
|align=center|C/R
|Unacknowledged data
|Has a payload
|0||0||0||align=center|P/F||0||0||1||1
|-
|UI with header check '''UIH'''
|align=center|C/R
|Unacknowledged data
|New in ISO 13239
|1||1||1||align=center|P/F||1||1||1||1
|-
|Unnumbered poll '''UP'''
|align=center|C
|colspan=2|Used to solicit control information
|0||0||1||align=center|P||0||0||1||1
|-
|Reset '''RSET'''
|align=center|C
|Used for recovery
|Resets N(R) but not N(S)
|1||0||0||align=center|P||1||1||1||1
|-
|Exchange identification '''XID'''
|align=center|C/R
|colspan=2|Used to Request/Report capabilities
|1||0||1||align=center|P/F||1||1||1||1
|-
|Test '''TEST'''
|align=center|C/R
|colspan=2|Exchange identical information fields for testing
|1||1||1||align=center|P/F||0||0||1||1
|-
|Frame reject '''FRMR'''
|align=center|R
|colspan=2| Report receipt of unacceptable frame
|1||0||0||align=center|F||0||1||1||1
|-
| Nonreserved 0 '''NR0'''
|align=center|C/R
|Not standardized
|For application use
|0||0||0||align=center|P/F||1||0||1||1
|-
| Nonreserved 1 '''NR1'''
|align=center|C/R
|Not standardized
|For application use
|1||0||0||align=center|P/F||1||0||1||1
|-
| Nonreserved 2 '''NR2'''
|align=center|C/R
|Not standardized
|For application use
|0||1||0||align=center|P/F||1||0||1||1
|-
| Nonreserved 3 '''NR3'''
|align=center|C/R
|Not standardized
|For application use
|1||1||0||align=center|P/F||1||0||1||1
|-
| Ack connectionless, seq 0 '''AC0'''
|align=center|C/R
|Not part of HDLC
|IEEE 802.2 LLC extension
|0||1||1||align=center|P/F||0||1||1||1
|-
| Ack connectionless, seq 1 '''AC1'''
|align=center|C/R
|Not part of HDLC
|IEEE 802.2 LLC extension
|1||1||1||align=center|P/F||0||1||1||1
|-
| Configure for test '''CFGR'''
|align=center|C/R
|Not part of HDLC
|Was part of SDLC
|1||1||0||align=center|P/F||0||1||1||1
|-
| Beacon '''BCN'''
|align=center|R
|Not part of HDLC
|Was part of SDLC
|1||1||1||align=center|F||1||1||1||1
|}

{| class="wikitable" style="text-align: center;"
|+ HDLC U frames, by binary encoding
!colspan=8| C-Field Format ||rowspan=2| Command ||rowspan=2| Response
|rowspan=15|
!colspan=8| C-Field Format ||rowspan=2| Command ||rowspan=2| Response
|-
! 0 || 1 || 2 || 3 || 4 || 5 || 6 || 7
! 0 || 1 || 2 || 3 || 4 || 5 || 6 || 7
|-
| 1 || 1 || 0 || 0 || P/F || 0 || 0 || 0 ||colspan=2| UI
| 1 || 1 || 1 || 0 || P/F || 0 || 1 || 0 ||colspan=2 bgcolor=lightgrey| (unused)
|-
| 1 || 1 || 0 || 0 || P/F || 0 || 0 || 1 || SNRM ||
| 1 || 1 || 1 || 0 || P/F || 0 || 1 || 1 ||colspan=2| CFGR{{Ref|Dagger1|†}}
|-
| 1 || 1 || 0 || 0 || P/F || 0 || 1 || 0 || DISC || RD
| 1 || 1 || 1 || 0 || P/F || 1 || 0 || x ||colspan=2 bgcolor=lightgrey| (unused)
|-
| 1 || 1 || 0 || 0 || P/F || 0 || 1 || 1 || SM{{Ref|Asterisk1|*}} ||
| 1 || 1 || 1 || 0 || P/F || 1 || 1 || x ||colspan=2| AC0–AC1{{Ref|Dagger2|†}}
|-
| 1 || 1 || 0 || 0 || P/F || 1 || 0 || 0 || UP ||
| 1 || 1 || 1 || 1 || P/F || 0 || 0 || 0 || SARM || DM
|-
| 1 || 1 || 0 || 0 || P/F || 1 || 0 || 1 ||colspan=2 bgcolor=lightgrey| (unused)
| 1 || 1 || 1 || 1 || P/F || 0 || 0 || 1 || RSET ||
|-
| 1 || 1 || 0 || 0 || P/F || 1 || 1 || 0 || || UA
| 1 || 1 || 1 || 1 || P/F || 0 || 1 || 0 || SARME ||
|-
| 1 || 1 || 0 || 0 || P/F || 1 || 1 || 1 ||colspan=2| TEST
| 1 || 1 || 1 || 1 || P/F || 0 || 1 || 1 || SNRME ||
|-
| 1 || 1 || 0 || 1 || P/F || 0 || x || x ||colspan=2| NR0–NR3
| 1 || 1 || 1 || 1 || P/F || 1 || 0 || 0 || SABM ||
|-
| 1 || 1 || 0 || 1 || P/F || 1 || x || x ||colspan=2 bgcolor=lightgrey| (unused)
| 1 || 1 || 1 || 1 || P/F || 1 || 0 || 1 ||colspan=2| XID
|-
| 1 || 1 || 1 || 0 || P/F || 0 || 0 || 0 || SIM || RIM
| 1 || 1 || 1 || 1 || P/F || 1 || 1 || 0 || SABME ||
|-
| 1 || 1 || 1 || 0 || P/F || 0 || 0 || 1 || || FRMR
|rowspan=2| 1 ||rowspan=2| 1 ||rowspan=2| 1 ||rowspan=2| 1 ||rowspan=2| P/F ||rowspan=2| 1 ||rowspan=2| 1 ||rowspan=2| 1 ||colspan=2| UIH{{Ref|Asterisk2|*}}
|-
|colspan=10|
| || BCN{{Ref|Dagger3|†}}
|}
: <sup>*</sup> {{Note|Asterisk1}}{{Note|Asterisk2}} ISO/IEC 13239 addition
: <sup>†</sup> {{Note|Dagger1}}{{Note|Dagger2}}{{Note|Dagger3}} Not part of HDLC

The UI, UIH, XID, TEST frames contain a payload, and can be used as both commands and responses.  The SM command and FRMR response also contain a payload.
* A UI frame contains user information, but unlike an I frame it is neither acknowledged nor retransmitted if lost.
* A UIH frame (an ISO/IEC 13239 addition) is like a UI frame, but additionally applies the frame check sequence only to a specified-length prefix of the frame; transmission errors after this prefix are not detected.
* The XID frame is used to exchange terminal capabilities.  [[Systems Network Architecture]] defined one format, but the variant defined in ISO 8885 is more commonly used.  A primary advertises its capabilities with an XID command, and a secondary returns its own capabilities in an XID response.
* The TEST frame is simply a [[ping (networking utility)|ping]] command for debugging purposes.  The payload of the TEST command is returned in the TEST response.
* The SM command (an ISO/IEC 13239 addition) is a generic "set mode" command which includes an information field (in the same ISO 8885 format as XID) specifying parameters.  This allows parameter values (like 15- and 31-bit sequence numbers) and parameters like window sizes and maximum frame sizes not expressible by the standard six mode-set commands to be negotiated.
* The FRMR response contains a description of the unacceptable frame, in a standardized format.  The first 1 or 2 bytes are a copy of the rejected control field, the next 1 or 2 contain the secondary's current send and receive sequence numbers, and the following 4 or 5 bits are error flags indicating the reason for the rejection.