Session Initiation Protocol (SIP) is a application layer control simple signalling protocol for VoIP implementations using the Redirect Mode. It is a textual client-server based protocol and provides the necessary protocol mechanisms so that the end user systems and proxy servers can provide various different services.

SIP is a technology that many cable telephony architectures want to use. Because is is simple, provides various services and can communicate between networks, the cable industry is attracted to it. As a text-based protocol it is easy to debug. It also can be used as a gateway to PSTN resources. Most cable telephony protocols are tailored for packet-based communication and only have rudimentary PSTN support. Since SIP has richer PSTN capabilities, these protocols often rely on SIP gateways to access PSTN networks.  SIP used with cable telephony is not ideal and has some problems, but as a flexible protocol, cable standard developers hope to be able to solve them.