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EN(4)											    EN(4)

       en - Xerox 3 Mb/s Ethernet interface

	    NEN  en_controllers # Xerox prototype (3 Mb) Ethernet

       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,  and thus this must be among the first UNIBUS devices enabled
       after boot.

       Each of the host's network addresses is specified at boot time with an SIOCSIFADDR  ioctl.
       The  station  address is discovered by probing the on-board Ethernet address register, and
       is used to verify the protocol addresses.  No packets will be sent  or  accepted  until	a
       network address is supplied.

       The interface software implements an exponential backoff algorithm when notified of a col-
       lision 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 NS protocol families.  It normally tries to use	a
       ``trailer'' encapsulation to minimize copying data on input and output.	The use of trail-
       ers is negotiated with ARP.  This negotiation may be disabled, on a  per-interface  basis,
       by setting the IFF_NOTRAILERS flag with an SIOCSIFFLAGS ioctl.

       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.

       intro(4N), inet(4F)

       The device has insufficient buffering to handle back to back packets.  This makes use in a
       production environment painful.

       The hardware does word at a time DMA without byte swapping.  To compensate, byte  swapping
       of user data must either be done by the user or by the system.  A kludge to byte swap only
       IP packets is provided if the ENF_SWABIPS flag is defined in the driver and  set  at  boot
       time with an SIOCSIFFLAGS ioctl.

3rd Berkeley Distribution		 August 20, 1987				    EN(4)
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