1.5.6. Layer 3 Services

In addition to the Layer 4 handler various other services are based on Layer 3 that are addressed via protocol numbers (ServiceId field in the router header). Each of these services uses two protocol numbers – one for the requesting the service, the second one for responses. The two numbers differ in the lowest-order bit: for requests this bit is set, for responses the bit is deleted.

When defining such a service certain specifications should be followed. A service packet always consists of a fixed header and user data. In order to ensure expandability, the first two bytes specify the length of the header ( = user data offset). Extensions must be designed such that older services can handle more recent packets and vice versa.

In order to enable communication between different systems, the byte order is always little-endian. In addition, all fields are based on natural alignment. Structures, particularly user data, always start at 4-byte boundaries (filler byte must be used if necessary).

The following service groups are defined:

Req/Reply	Associated service
1 / 2	Address service
3 / 4	Name service (name resolution, network scan, …)
5 / 6	Network variables
63 / 64	Channel management (Layer 4)