Exposing SNP instead of NII and UNDI has some advantages and some disadvantages over using NII and UNDI. SNP-based network drivers are never UEFI Runtime Drivers, so such drivers do not have to worry about meeting the UEFI Runtime Driver requirements. This allows an SNP driver to be compiled for all the CPU architectures supported by the UEFI Specification including EBC. SNP may be required for some nonstandard network interface controllers that cannot meet the UNDI requirements.

When a network driver exposes SNP directly the system firmware layers MNP on top of SNP and does not use its internal SNP driver as part of this network stack.

The following figure shows a possible network stack based on a network driver producing SNP. The inclusion of Load File Protocol is not guaranteed here, but is a choice made by the system firmware.

Figure 30-SNP Based Network Stack

The implementation of the Simple Network Protocol is typically found in the file SimpleNetwork.c. Appendix A contains a template for a SimpleNetwork.c file for a UEFI Driver. The list of tasks to implement the Simple Network Protocol is as follows:

• Add global variable for the EFI_SIMPLE_NETWORK_PROTOCOL instance to SimpleNetworkProtocol.c.

• Create child handle in Driver Binding Protocol Start() and install the Simple Network Protocol and the Device Path Protocol. Also allocate and initialize an EFI_SIMPLE_NETWORK_MODE structure in the Simple Network Protocol.

• Implement the Simple Network Protocol services in SimpleNetwork.c.

• Create an Exit Boot Services Event to disable DMA when packets are received.

The following example shows the protocol interface structure for the Simple Network Protocol for reference. This protocol is composed of 13 services, an EFI_EVENT that can be used to poll when a packet has been received, and a Mode structure that contains details on the attributes and capabilities of the network interface controller.

Example 240-Simple Network Protocol

typedef struct _EFI_SIMPLE_NETWORK_PROTOCOL EFI_SIMPLE_NETWORK_PROTOCOL;
​
///
/// The EFI_SIMPLE_NETWORK_PROTOCOL protocol is used to initialize access
/// to a network adapter. Once the network adapter initializes,
/// the EFI_SIMPLE_NETWORK_PROTOCOL protocol provides services that
/// allow packets to be transmitted and received.
///
struct _EFI_SIMPLE_NETWORK_PROTOCOL { ///
  /// Revision of the EFI_SIMPLE_NETWORK_PROTOCOL. All future revisions must
  /// be backwards compatible. If a future version is not backwards compatible
  /// it is not the same GUID.
  ///
  UINT64 Revision;
  EFI_SIMPLE_NETWORK_START Start;
  EFI_SIMPLE_NETWORK_STOP Stop;
  EFI_SIMPLE_NETWORK_INITIALIZE Initialize;
  EFI_SIMPLE_NETWORK_RESET Reset;
  EFI_SIMPLE_NETWORK_SHUTDOWN Shutdown;
  EFI_SIMPLE_NETWORK_RECEIVE_FILTERS ReceiveFilters;
  EFI_SIMPLE_NETWORK_STATION_ADDRESS StationAddress;
  EFI_SIMPLE_NETWORK_STATISTICS Statistics;
  EFI_SIMPLE_NETWORK_MCAST_IP_TO_MAC MCastIpToMac;
  EFI_SIMPLE_NETWORK_NVDATA NvData;
  EFI_SIMPLE_NETWORK_GET_STATUS GetStatus;
  EFI_SIMPLE_NETWORK_TRANSMIT Transmit;
  EFI_SIMPLE_NETWORK_RECEIVE Receive;
  ///
  /// Event used with WaitForEvent() to wait for a packet to be received.
  ///
  EFI_EVENT WaitForPacket;
  ///
  /// Pointer to the EFI_SIMPLE_NETWORK_MODE data for the device.
  ///
  EFI_SIMPLE_NETWORK_MODE *Mode;
};
​
extern EFI_GUID gEfiSimpleNetworkProtocolGuid;