SD-Originated Keep-Alive Format
After the Keep-Alive exchange has been negotiated for a TCP connection, the SD, transmits a periodic ping at a frequency specified by the proprietary parameter (rkeep in the example negotiation). As shown below, the ping consists of carriage return (CR) and line feed (LF) characters.
CRLFCRLF
CR = %x0D
LF = %x0A
pings are transmitted between SIP messages, and cannot be sent in the middle of SIP message. If the TCP connection is secured by TLS, pings are transmitted within the TLS encrypted stream.