HDLC
The basis of HDLC is laid out in a series of ISO/IEC joint specifications. Bill (“Chops”) Westfield initially described Cisco’s multiprotocol specifications in an email to a public mailing list in 1990. It was subsequently quoted several times on other mailing lists and is readily available from web search engines. Two standards form the basis of the portions of HDLC relevant to data communications:
- ISO/IEC 3309
“Information technology—Telecommunications and information exchange between systems—High-level data link control (HDLC) procedures—Frame structure”
This document describes the basic HDLC frame structure and the frame check sequence, but does not describe how the address and control fields are used.
- ISO/IEC 4335
“Information technology—Telecommunications and information exchange between systems—High-level data link control (HDLC) procedures—Elements of procedures”
This specification describes how the various frame types use the control field.
Ethernet Type Codes
Cisco HDLC makes use of Ethernet type codes to identify higher-layer protocols. The IEEE is the official keeper of Ethernet type codes; their list of “EtherType Field Public Assignments” can be found at http://standards.ieee.org/regauth/ethertype/type-pub.html. IANA also maintains a list of assigned Ethernet types at http://www.iana.org/assignments/ethernet-numbers/.
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