The ec interface provides access to a 10 Mb/s Ethernet network through a 3Com controller.
The hardware has 32 kilobytes of dual-ported memory on the UNIBUS. This memory is used for internal buffering by the board, and the interface code reads the buffer contents directly through the UNIBUS. The address of this memory is given in the flags field in the configuration file. The first interface normally has its memory at UNIBUS address 0.
Each of the host's network addresses
is specified at boot time with an
SIOCSIFADDR
ioctl(2).
The
ec
interface employs the address resolution protocol described in
arp(4)
to dynamically map between Internet and Ethernet addresses on the local
network.
The interface normally tries to use a
``trailer''
encapsulation
to minimize copying data on input and output.
The use of trailers is negotiated with
ARP.
This negotiation may be disabled, on a per-interface basis,
by setting the
IFF_NOTRAILERS
flag with an
SIOCSIFFLAGS
ioctl(2).
The interface software implements an exponential backoff algorithm when notified of a collision on the cable. This algorithm uses a 16-bit mask and the VAX-11's interval timer in calculating a series of random backoff values. The algorithm is as follows:
Backoff delays are done in a software busy loop. This can degrade the system if the network experiences frequent collisions.