EN(4)                                                                    EN(4)



NAME
       en - Xerox 3 Mb/s Ethernet interface

SYNOPSIS
       device en0 at uba0 csr 161000 vector enrint enxint encollide

DESCRIPTION
       The  _e_n 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 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  comple‐
            ment 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  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 back‐
       off 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
       intro(4N), inet(4F)

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
       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.



4.2 Berkeley Distribution        May 16, 1986                            EN(4)