Packet Guide to Voice over IP

Chapter 7. Skinny Client Control Protocol

The Skinny Client Control Protocol (SCCP), or Skinny, is a Cisco proprietary signaling protocol for VoIP devices. As a signaling protocol, it is used for registration of endpoints; call messages such as off-hook, on-hook, addressing (dial numbers); and controlling phone display. As the name suggests it is a lightweight protocol and, if you have the right dissector, is straightforward to read and understand. When combined with the light weight Cisco call manger called Call Manager Express (CME), small VoIP deployments supporting a number of features and phones can be set up quickly.

Unlike most of the protocols we study in the Packet Guide series, the Skinny Client Control Protocol (SCCP) is proprietary. So the protocol documentation is not as easy to come by as that for signaling protocols such as H.323 or the Session Initiation Protocol. The massive Cisco website and the complexity of the Cisco IOS can lead to some difficulties in trying to understand even basic operation. It is included in this book because of the large number of installed devices using SCCP and the popularity of Cisco as a platform. To quote from a recent VoIP conference:

Millions of installed Cisco phones would argue for the continued use of Skinny as a protocol.

Like most VoIP signaling protocols, SCCP has a clearly defined set of messages and behaviors that can be observed and repeated. However, SCCP also implements several methods that are nonstandard. This means that ...