EN(4)		    UNIX Programmer's Manual		    EN(4)



NAME
     en - Xerox 3 Mb/s Ethernet interface

SYNOPSIS
     /sys/conf/SYSTEM:
	  NEN  _e_n__c_o_n_t_r_o_l_l_e_r_s # Xerox prototype (3 Mb) Ethernet

DESCRIPTION
     The _e_n interface provides access to a 3 Mb/s Ethernet net-
     work.  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 regis-
     ter, and is used to verify the protocol addresses.  No pack-
     ets will be sent or accepted until a network address is sup-
     plied.

     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 actu-
	  ally 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 fami-
     lies.  It 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.

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



Printed 11/26/99	 August 20, 1987			1






EN(4)		    UNIX Programmer's Manual		    EN(4)



     was dropped.

     en%d: input error.  The hardware indicated an error in read-
     ing 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.

SEE ALSO
     intro(4N), inet(4F)

BUGS
     The device has insufficient buffering to handle back to back
     packets.  This makes use in a production environment pain-
     ful.

     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 SIOCSIF-
     FLAGS ioctl.
































Printed 11/26/99	 August 20, 1987			2