HDLC Overview
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Alcatel-Lucent
Beta Beta
OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
HDLC FRAME FORMATS
The standard frame of the HDLC protocol handles both data and control
messages. The length of the address field is commonly 0, 8, or 16 bits, depending
on the data link layer protocol. The 8 or 16 bit control field, provides a flow control
number and defines the frame type (control or data). The exact use and structure
of this field depends upon the protocol using the frame.
Data is transmitted in the information field, which can vary in length depending
upon the protocol using the frame. Layer 2 frames are carried in the information
field. Error Control is implemented by appending a Cyclic Redundancy Check
(CRC) to the frame, which is 16 bits long in most protocols.
HDLC PROTOCOL OPERATION
HDLC is the default encapsulation on serial lines and it uses HDLC framing with
packet contents defined as follows:
• The first ("address") octet is set to 0x0F for unicast packets and 0x8Ffor broadcast
packets.
• The second ("control") octet is always 0.
The next two octets are a 16-bit "protocol code". IP at the higher-level would be
represented with the code 0x0800. Bytes after this are higher-level protocol data.
Packets with type 0x8035 carry a protocol referred to as SLARP. SLARP has two
functions: IP address determination and serial line keepalive.
For the SLARP keepalive protocol, each system sends the other a keepalive
packet at a user-configurable interval. The default interval is 10 seconds.
Both systems must use the same interval to ensure reliable operation. Each
system assigns sequence numbers to the keepalive packets it sends, starting with
zero, independent of the other system. These sequence numbers are included in
the keepalive packets that are sent to the other system. The sequence number of
the last keepalive packet received from the peer system is also included in each
keepalive packet, as assigned by the peer system. This number is called the
returned sequence number. Each system keeps track of the last returned
sequence number it has received. Immediately before sending a keepalive
packet, the system compares the sequence number of the packet which is about
to send with the returned sequence number in the last keepalive packet that is
received. If two is differ by 3 or more, it considers the serial line as failed, and will
not route further higher-level data across it until an acceptable keepalive response
is received.
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