Cisco HDLC (also known as cHDLC) is an extension to the High-Level Data Link Control protocol created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International Organization for Standardization (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. High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). ... ... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... In the field of telecommunications, a communications protocol is the set of standard rules for data representation, signalling, authentication and error detection required to send information over a communications channel. ... The International Organization for Standardization (ISO) is an international standard-setting body composed of representatives from national standards bodies. ...

Framing

Cisco HDLC frames 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 multiprotocol support. Thus, cHDLC frames contain a field for identifying the network protocol being encapsulated. In telecommunications, a frame is a packet which has been encoded for transmission over a particular link. ... In networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. ...

SLARP

The Cisco HDLC extensions also include the definition of SLARP or the Serial Line Address Resolution Protocol (ARP). SLARP is used to provide dynamic address assignment between two endpoints across a serial line, a keep-alive mechanism for ensuring the availability of a given link, and finally a method for determining link health. In computer networking, the Address Resolution Protocol (ARP) is the method for finding a hosts hardware address when only its network layer address is known. ... A male DE-9 serial port on the rear panel of a PC. In computing, a serial port consists of an interface on a computer system through which information transfers in or out one bit at a time (contrast parallel port). ...

The keep-alive mechanism works in a request/response fashion between the two serial line endpoints. Sequence numbers carried inside the SLARP frames are incremented for each keep-alive sent, and as such the health of the link can be determined by each endpoint by keeping track of the sequence. If a certain count of sequence numbers are missed, the keep-alive frames are deemed to be lost, and an alert is pushed up through higher layer protocols to say that the link has most likely been lost.

Structure

cHDLC Frame Structure

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

• 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).

The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ...

SLARP Address Request/Response Frame Structure

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

Currently only 3 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.

• The Op-Code will be 0x00 for address requests and 0x01 for address responses.
• The Address and Mask fields are used to contain a 4-octet IP address and mask.
• The 1-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.

See also

• Point-to-Point Protocol, a non-proprietary Internet Standard defined by RFC 1661 and RFC 16622 that solves the problems Cisco HDLC solves as well as many other problems.