DSR P2P communication is a nested protocol, which uses the steam API to send P2P packets.

The upper layer, a raw P2P packet, encapsulates the entire P2P system.
It's 1st byte is the the following enum:

enum EPacketType : uint8_t {
    ePacketType_GameData = 0,
    ePacketType_P2PProtocol = 1,
    ePacketType_Voice = 2,
    ePacketType_P2PInit = 3, //Essentially just an unencrypted P2P protocol packet

    ePacketDataType = 7,

    ePacketType_UDP = 1 << 3,
    ePacketType_TCP = 1 << 4,
    ePacketType_Encrypted = 1 << 6,

The function SteamNetworking::SendRawP2PPacketType @1410ba220 sends the packet.

The function SteamNetworking::ParseRawP2PPacketType @01410B5340 is responsible for reading this byte, then based on that either treats it as a Steam Communication packet, or a GameData Communication packet.

The raw P2PPacket also includes checksum data and encryption variables.

These are the packet types that can be contained in a RawP2PPacket:

DSR Steam Communication. It handles session connectivity, lobby creation, and connecting. It is parsed immediately upon receiving.

DSR GameData Communication. It handles sending game data such as player position, animations, and character info. Upon receiving it is put in a queue, and later grabbed from that queue and parsed.