If the USB peripheral involves running the eCos TCP/IP stack and that
stack needs to use USB-ethernet as a transport layer (or as one of the
transports), then the USB-ethernet package can provide a suitable
network device driver. It is still necessary for higher-level code to
perform appropriate initialization by calling usbs_eth_init, but
after that it will be the TCP/IP stack rather than application code
that transmits or receives ethernet frames.
Not all peripherals involving the USB-ethernet package will require a TCP/IP stack. Hence the provision of the network device is controlled by a configuration option CYGPKG_USBS_ETHDRV. By default this will be enabled if the TCP/IP package CYGPKG_NET is loaded, and disabled otherwise.
There are a number of other configuration options related to the network device. CYGFUN_USBS_ETHDRV_STATISTICS determines whether or not the package will maintain statistics, mainly intended for SNMP: by default this will be enabled if the SNMP support package CYGPKG_SNMPAGENT is loaded, and disabled otherwise. The name of the ethernet device is controlled by CYGDATA_USBS_ETHDRV_NAME, and has a default value of either eth0 or eth1 depending on whether or not there is another network device driver present in the configuration.
Usually eCos network device drivers default to using DHCP for obtaining necessary information such as IP addresses. This is not appropriate for USB-ethernet devices. On the host-side the USB-ethernet network device will not exist until the USB peripheral has been plugged in and communication has been established. Therefore any DHCP daemon on the host would not be listening on that network device at the point that eCos requests its IP and other information. A related issue is that the use of DHCP would imply the presence of a DHCP daemon on every affected host machine, as opposed to a single daemon (plus backups) for the network as a whole. For these reasons the USB-ethernet package precludes the use of DHCP as a way of setting the IP address, instead requiring alternatives such as manual configuration.