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.