Internet is widely spread and the IP (Internet Protocol, RFC791) has provided an official form for extending protocols. There is a protocol field at byte offset 9 in the IP header. RFC1060 records allocation of these values. IP assumes that the source and destinations in the header may be interpreted without reference to the protocol field, Thus this does not suit the needs of Silk packets which presume custom routing.
The Point to Point Protocol (PPP: RFC1171 & 1172) recommends itself as a layer between the IP protocol and the modem. (IP specifies no error control except for part of the header.) It has two forms, one of which seems to be reminiscent of HDLC and the other with start-stop links. It seems to assume error control at some lower layer such as is available in some modern modems. There is a two byte “Protocol Field” specified at offset 2 of the PPP frame which is not to be confused with the IP protocol field in the IP header. Its values are specified in RFC1171. There is room for new codes. Link Control Protocol (LCP) allows “Configuration Options” to be negotiated as the link is opened. This can be used to determine if both ends deal with silk packets.
Another especially simple scheme is to interpose silk code between the link hardware and existing IP link control code. The silk code would be the first to see arriving packets. It eliminates silk packets and the older link code is unaware of the event. Similarly outgoing silk traffic can be interpolated between IP traffic. This scheme has the advantage that existing link control code need not be modified. It may, however, require yet another copying of the IP traffic.
It seems that two protocols can share the link. Each packet indicates its protocol. When both IP and Silk outbound traffic contend for the link, an externally specified priority controls.