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EN(4)		       BSD/vax Kernel Interfaces Manual			 EN(4)

NAME
     en — Xerox 3 Mb/s Ethernet interface

SYNOPSIS
     device en0 at uba0 csr 161000 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, 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(2).  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 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 retransmit‐
		ting the packet.

     The interface handles both Internet and NS protocol families.  It nor‐
     mally 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 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.

SEE ALSO
     netintro(4), inet(4)

HISTORY
     The en driver appeared in 4.2BSD.

BUGS
     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 compen‐
     sate, 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.

4.2 Berkeley Distribution	 June 5, 1993	     4.2 Berkeley Distribution
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