pw_router#

The pw_router module provides transport-agnostic classes for routing packets over network links.

Common router interfaces#

PacketParser#

To work with arbitrary packet formats, routers require a common interface for extracting relevant packet data, such as the destination. This interface is pw::router::PacketParser, defined in pw_router/packet_parser.h, which must be implemented for the packet framing format used by the network.

Egress#

The Egress class is a virtual interface for sending packet data over a network link. Egress implementations provide a single SendPacket function, which takes the raw packet data and transmits it.

Egresses are invoked with the PacketParser used to process the packet. This allows additional information to be extracted from each packet to assist with transmitting decisions. For example, if packets in a project include a priority, egresses may use it to provide quality-of-service by dropping certain packets under heavy load.

Status MyEgress::SendPacket(
    ConstByteSpan packet, const PacketParser& parser) override {
  // Downcast the base PacketParser to the custom implementation that was
  // passed into RoutePacket().
  const CustomPacketParser& custom_parser =
      static_cast<const CustomPacketParser&>(parser);

  // Custom packet fields can now be accessed if necessary.
  if (custom_parser.priority() == MyPacketPriority::kHigh) {
    return SendHighPriorityPacket(packet);
  }

  return SendStandardPriorityPacket(packet);
}

Some common egress implementations are provided upstream in Pigweed.

StaticRouter#

pw::router::StaticRouter is a router with a static table of address to egress mappings. Routes in a static router never change; packets with the same address are always sent through the same egress. If links are unavailable, packets will be dropped.

Static routers are suitable for basic networks with persistent links.

Usage example#

namespace {

// Define the router egresses.
UartEgress uart_egress;
BluetoothEgress ble_egress;

// Define the routing table.
constexpr pw::router::StaticRouter::Route routes[] = {{1, uart_egress},
                                                      {7, ble_egress}};
pw::router::StaticRouter router(routes);

}  // namespace

void ProcessPacket(pw::ConstByteSpan packet) {
  HdlcFrameParser hdlc_parser;
  router.RoutePacket(packet, hdlc_parser);
}

Size report#

The following size report shows the cost of a StaticRouter with a simple PacketParser implementation and a single route using an EgressFunction.

Label

Segment

Before

Delta

After

Static router with a single route

FLASH
RAM
20,840
672
+1,340
+88
22,180
760

Zephyr#

To enable pw_router.* for Zephyr add CONFIG_PIGWEED_ROUTER=y to the project’s configuration. This will enable the Kconfig menu for the following:

  • pw_router.static_router which can be enabled via CONFIG_PIGWEED_ROUTER_STATIC_ROUTER=y.

  • pw_router.egress which can be enabled via CONFIG_PIGWEED_ROUTER_EGRESS=y.

  • pw_router.packet_parser which can be enabled via CONFIG_PIGWEED_ROUTER_PACKET_PARSER=y.

  • pw_router.egress_function which can be enabled via CONFIG_PIGWEED_ROUTER_EGRESS_FUNCTION=y.