A cHDLC interface on a Nokia router transmits a SLARP address resolution reply packet in response to a received SLARP address resolution request packet from peers. The cHDLC interface does not transmit SLARP address resolution request packets.
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 sent to the other system. Also included in each keepalive packet is the sequence number of the last keepalive packet received from the other system, as assigned by the other 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, it compares the sequence number of the packet it is about to send with the returned sequence number in the last keepalive packet it has received. If the two differ by 3 or more, it considers the line to have failed, and does not route higher-level data across it until an acceptable keepalive response is received.
There is interaction between the SLARP address resolution protocol and the SLARP keepalive protocol. When one end of a serial line receives a SLARP address resolution request packet, it assumes that the other end has restarted its serial interface and resets its keepalive sequence numbers. In addition to responding to the address resolution request, it acts as if the other end had sent it a keepalive packet with a sequence number of zero, and a returned sequence number the same as the returned sequence number of the last real keepalive packet it received from the other end.