en(4)
NAME
en − Xerox 3 Mb/s Ethernet interface
SYNTAX
device en0 at uba0 csr 0161000 vector enrint enxint encollide
DESCRIPTION
The en interface provides access to a 3 Mb/s Ethernet network. Due to limitations in the hardware, DMA transfers to and from the network must take place in the lower 64K bytes of the UNIBUS address space.
The network number is specified with a SIOCSIFADDR ioctl; the host’s address is discovered by probing the on-board Ethernet address register. No packets will be sent or accepted until a network number is supplied.
The interface software implements an exponential backoff algorithm when notified of a collision on the cable. This algorithm utilizes a 16-bit mask and the VAX-11’s interval timer in calculating a series of random backoff values. The algorithm is as follows:
1. Initialize the mask to be all 1’s.
2. If the mask is zero, 16 retries have been made and we give up.
3. Shift the mask left one bit and formulate a backoff by masking the interval timer with the mask (this is actually the two’s complement of the value).
4. Use the value calculated in step 3 to delay before retransmitting the packet.
The interface handles both Internet and PUP protocol families, with the interface address maintained in Internet format. PUP addresses are converted to Internet addresses by subsituting PUP network and host values for Internet network and local part values.
The interface normally tries to use a “trailer” encapsulation to minimize copying data on input and output. This may be disabled, on a per-interface basis, by setting the IFF_NOTRAILERS flag with an SIOCSIFFLAGS ioctl.
DIAGNOSTICS
en%d: output error. The hardware indicated an error on the previous transmission.
en%d: send error. After 16 retransmissions using the exponential backoff algorithm described above, the packet was dropped.
en%d: input error. The hardware indicated an error in reading a packet off the cable.
en%d: can’t handle af%d. The interface was handed a message with addresses formatted in an unsuitable address family; the packet was dropped.