Networking
Out-of-band networking means the ability for the tasks managed by a companion core to send and receive network packets from the out-of-band Dovetail stage, i.e. guaranteeing ultra-low latency. Dovetail brings in the necessary hooks and features into the Linux network stack in order for a companion core to implement its own. This section details two aspects of the support Dovetail provides for out-of-band networking:
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the kernel API available to NIC driver writers for enabling a complete [out-of-band I/O path]({{ relref “#dovetail-net-full-oob” }}) between applications and the network interface controller.
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the kernel hooks into the in-band network stack a companion core can rely on in order to implement its own (out-of-band) stack, such as the EVL network stack. Those hooks allow the companion core to interpose on key events related to networking in the Linux kernel. They live in several areas of the Linux network stack, namely the buffer management, socket interface and device handling.
Input diversion and out-of-band network ports
At the device level, out-of-band networking requires:
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the ability to redirect the input/ingress data flow received from NIC drivers to the companion core so that it may pick the packets which should be delivered to out-of-band tasks, leaving the rest to normal handling by the in-band/regular network stack. This is what Dovetail calls input diversion, this is a per-device (aka netdev) property which can be turned on or off dynamically. Which user-level interface should be used to perform that switch is defined by the companion core, not Dovetail. Likewise, the companion core implements the mechanism for selecting the packets which Dovetail submits via the netif_deliver_oob() hook.
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the ability to define network devices as I/O endpoints for applications to receive and send packets from the out-of-band stage. Dovetail calls such devices out-of-band network ports.
These are separate properties. A virtual network device such as a VLAN device could me made to act as an out-of-band port for applications, but would still need the underlying real network device it sits on to divert its input to the companion core. Conversely, a real network device can divert input and could be made an out-of-band port as well, but that is not a requirement. The rules for accepting packets from the diverted input flow in the out-of-band network stack are exclusively defined by the companion core, which may apply whatever filter it sees fit. Typically, the EVL network stack can use either VLAN tagging or eBPF filtering for that purpose.
Full or partial support for out-of-band networking
Dovetail does not require NIC drivers to enable the out-of-band operating mode. Indeed, one could use an unmodified stock driver with an out-of-band network stack, at the expense of lower latency figures and bounded execution times. In this case, the real-time I/O path would be restricted to the execution of the network stack implemented by the companion core, deferring the actual I/O operations to the in-band stage from which unmodified drivers would handle them. On the other hand, one must provide out-of-band support in the NIC driver in order to achieve complete, end-to-end real-time support between the application and the transmission hardware.
