Sound Control Protocol Digital 6000 (EM 6000 | L 6000)

SSC Developer‘s guide for Digital 6000 | 15/57
SSC Transport Layer Adaptations
6. SSC Transport Layer Adaptations
The SSC data format as defined in the previous sections can be transported by different transport
protocols, or stored in persistent files. This section specifies what transports are supported, and how
the specific features of transport layers shall be applied to transporting SSC Messages.
6.1 UDP/IP
UDP/IP is the standard transport for all devices with an Ethernet interface or another interface typi-
cally used for internet connectivity. All those device MUST implement the UDP/IP transport for SSC.
The device implements UDP over IPv4.
One UDP datagram is used to transport one SSC Message. If the SSC Message is really large (e.g., a
complete device configuration), IP fragmentation might fail, if a restricted device does not implement
IP re-assembly properly. In that case, the SSC Server should break up the message into multiple SSC
Method Calls instead. If atomic execution is relevant, SSC time tags may be used.
The UDP port number to used by the SSC Server should normally be discovered by the SSC Client by
means of the server discovery protocol. The default port number is 45.
Rationale: No other standard UDP service is expected to use 45. The IANA reservation for a "Message
Passing Service" is historic, and SSC is actually passing messages itself. Sennheiser was founded in
1945.
6.2 SSC Server Discovery
Networked devices implement DNS-SD (Apple Bonjour) as discovery protocol.
The DNS Service-Type is specified as "_ssc".
Because all networked SSC Servers must implement SSC-over-UDP, they MUST all publish a DNS-SD
service under "_ssc. _udp". Those servers that additionally support TCP MUST publish another DNS-
SD service under "_ssc._tcp".
The DNS-SD service instance name must be identical to the device name accessible as /device/
name. DNS-SD automatic name collision resolution SHOULD be performed, and the resulting name
changes MUST be reflected back into /device/name and the persistent device configuration. The
renaming rules MAY be tailored to suit product specific requirements.
The DNS-SD service registration includes the port numbers used. SSC Clients SHOULD NOT rely on
default ports.
The DNS-SD hostname SHOULD NOT be presented to the user. It may contain a unique identification
part (e.g., derived from the device MAC or serial), to avoid name collisions and automatic renaming.
Additional information about the SSC Server may be provided with a DNS-SD TXT-record.
The following properties are currently defined for the TXT record:
• txtvers Version of the TXT record format. Currently "1".
• version SSC-Version provided by the SSC Server.