Specifications

126 AVM Access Server – Glossary

HDLC (High-level Data Link Control)

A communications protocol standardized by ISO for data packets over

serial lines. HDLC is actually a structured set of standards which define

the means by which dissimilar devices can communicate over data net-

works. HDLC is a bit-oriented and hence code-independent data link pro-

tocol for point-to-point and point-to-multipoint connections. HDLC is al-

so standardized by ITU-T (ITU = International Telecommunication Union;

ITU-T = ITU Telecommunication Standardization Sector). HDLC defines

frames in which the data blocks from the network layer are encapsulated

for transport over the physical link. According to DIN 66221, an HDLC

frame consists of the start-of-frame flag, the address field, the control

field, the data field, the frame check sequence (FCS), and the end-of-

frame flag. HDLC is used in full-duplex mode, and provides for the ac-

knowledgment of several frames at a time (usually eight). The number of

frames transmitted before acknowledgment is called the window size.

Header

Data packets are generally transmitted beginning with a header which

contains the source and destination addresses and identifies the proto-

col used to interpret the packet. Header information is often repetitive

and thus can be compressed over some links, such as ISDN lines, to in-

crease the speed of data communication and so save time and costs.

HMAC (Keyed-Hash Message Authentication Code)

A message authentication code (MAC) generated using a keyed hash

function. Any hash algorithm can be used. HMAC signatures are used

in all IPsec authentication functions.

ICMP (Internet Control Message Protocol)

ICMP is part of the IP (Internet Protocol) suite. It is situated at Layer 3

(the Network Layer) of the OSI reference model, alongside IP itself.

ICMP uses the IP packet structure in a similar way to higher-layer proto-

cols, however.

ICMP is a component of every IP implementation, and transports only

error and diagnostic information for IP. A well-known service based on

ICMP is the program “ping”.

IKE (Internet Key Exchange)

A protocol in the IPsec suite used to negotiate secure connection pa-

rameters. IKE is described in RFC 2490.