Cisco HDLC is an extension to the High-Level Data Link Control

High-Level Data Link Control

High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization...

 network protocol created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer

Data link layer

The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....

 protocol

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

 that was originally developed by the International Organization for Standardization

International Organization for Standardization

The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

 (ISO). Often described as being a proprietary extension, the workings of cHDLC have in fact been openly and widely distributed, and the protocol has been implemented by many different network equipment vendors.

Framing

Cisco HDLC frames

Data frame

In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...

 make use of an alternative framing structure to the standard ISO HDLC. One of the primary reasons for the creation of cHDLC was to address the HDLC protocol's inability to provide multi-protocol support. Thus, cHDLC frames contain a field for identifying the network protocol being encapsulated.

cHDLC frame structure

The following table describes the make up of a cHDLC frame on the wire.

Address	Control	Protocol Code	Information	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits	Variable length, 0 or more bits, in multiples of 8	16 bits	8 bits

• The Address field is used to specify the type of packet contained in the cHDLC frame; 0x0F for Unicast and 0x8F for Broadcast packets.
• The Control field is always set to zero (0x00).
• The Protocol Code field is used to specify the protocol type encapsulated within the cHDLC frame (e.g. 0x0800 for Internet Protocol
  Internet Protocol
  The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
  
  ).

SLARP address request–response frame structure

A SLARP frame is designated by a specific cHDLC Protocol Code field value of 0x8035.

Currently only three types of SLARP frame are defined: address requests (0x00), address replies (0x01), and keep-alive frames (0x02).

The following table shows the makeup of a SLARP cHDLC address request–response frame.

Address	Control	Protocol Code	SLARP Op-Code	Address	Mask	Reserved	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits (0x8035)	32 bits	32 bits	32 bits	8 bits	16 bits	8 bits

• The op-code will be 0x00 for address requests and 0x01 for address responses.
• The Address and Mask fields are used to contain a four-octet IP address and mask.
• The one-byte Reserved field is currently unused and is always set to 0xFF.

SLARP Keep-Alive frame structure

The following table shows the makeup of a SLARP cHDLC keep-alive frame.

Address	Control	Protocol Code	SLARP Op-Code	Sequence Number (Sender)	Sequence Number (Last Received)	Reserved	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits (0x8035)	32 bits	32 bits	32 bits	8 bits	16 bits	8 bits

See also

• Point-to-Point Protocol
  Point-to-Point Protocol
  In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
  
  , an Internet Standard defined by RFC 1661 and RFC 1662 that solves the problems Cisco HDLC solves as well as many other problems.
• Various documents 1, 2, 3 documenting the Cisco HDLC frame format