rmnet driver is used for supporting the Multiplexing and aggregation Protocol (MAP). This protocol is used by all recent chipsets using Qualcomm Technologies, Inc. modems.
This driver can be used to register onto any physical network device in IP mode. Physical transports include USB, HSIC, PCIe and IP accelerator.
Multiplexing allows for creation of logical netdevices (rmnet devices) to handle multiple private data networks (PDN) like a default internet, tethering, multimedia messaging service (MMS) or IP media subsystem (IMS). Hardware sends packets with MAP headers to rmnet. Based on the multiplexer id, rmnet routes to the appropriate PDN after removing the MAP header.
Aggregation is required to achieve high data rates. This involves hardware sending aggregated bunch of MAP frames. rmnet driver will de-aggregate these MAP frames and send them to appropriate PDN’s.
2. Packet format¶
- MAP packet (data / control)
MAP header has the same endianness of the IP packet.
Bit 0 1 2-7 8 - 15 16 - 31 Function Command / Data Reserved Pad Multiplexer ID Payload length Bit 32 - x Function Raw Bytes
Command (1)/ Data (0) bit value is to indicate if the packet is a MAP command or data packet. Control packet is used for transport level flow control. Data packets are standard IP packets.
Reserved bits are usually zeroed out and to be ignored by receiver.
Padding is number of bytes to be added for 4 byte alignment if required by hardware.
Multiplexer ID is to indicate the PDN on which data has to be sent.
Payload length includes the padding length but does not include MAP header length.
MAP packet (command specific):
Bit 0 1 2-7 8 - 15 16 - 31 Function Command Reserved Pad Multiplexer ID Payload length Bit 32 - 39 40 - 45 46 - 47 48 - 63 Function Command name Reserved Command Type Reserved Bit 64 - 95 Function Transaction ID Bit 96 - 127 Function Command data
Command 1 indicates disabling flow while 2 is enabling flow
|0||for MAP command request|
|1||is to acknowledge the receipt of a command|
|2||is for unsupported commands|
|3||is for error during processing of commands|
Aggregation is multiple MAP packets (can be data or command) delivered to rmnet in a single linear skb. rmnet will process the individual packets and either ACK the MAP command or deliver the IP packet to the network stack as needed
MAP header|IP Packet|Optional padding|MAP header|IP Packet|Optional padding….
MAP header|IP Packet|Optional padding|MAP header|Command Packet|Optional pad…
3. Userspace configuration¶
rmnet userspace configuration is done through netlink library librmnetctl and command line utility rmnetcli. Utility is hosted in codeaurora forum git. The driver uses rtnl_link_ops for communication.