1EN(4)                      Kernel Interfaces Manual                      EN(4)
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NAME

6       en - Xerox 3 Mb/s Ethernet interface
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SYNOPSIS

9       /sys/conf/SYSTEM:
10            NEN  en_controllers # Xerox prototype (3 Mb) Ethernet
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DESCRIPTION

13       The  en interface provides access to a 3 Mb/s Ethernet network.  Due to
14       limitations in the hardware, DMA transfers to and from the network must
15       take place in the lower 64K bytes of the UNIBUS address space, and thus
16       this must be among the first UNIBUS devices enabled after boot.
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18       Each of the host's network addresses is specified at boot time with  an
19       SIOCSIFADDR  ioctl.   The  station address is discovered by probing the
20       on-board Ethernet address register, and is used to verify the  protocol
21       addresses.  No packets will be sent or accepted until a network address
22       is supplied.
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24       The interface software implements an exponential backoff algorithm when
25       notified of a collision on the cable.  This algorithm utilizes a 16-bit
26       mask and the VAX-11's interval timer in calculating a series of  random
27       backoff values.  The algorithm is as follows:
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29       1.   Initialize the mask to be all 1's.
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31       2.   If the mask is zero, 16 retries have been made and we give up.
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33       3.   Shift the mask left one bit and formulate a backoff by masking the
34            interval timer with the mask (this is actually the  two's  comple‐
35            ment of the value).
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37       4.   Use  the value calculated in step 3 to delay before retransmitting
38            the packet.
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40       The interface handles both Internet and NS protocol families.  It  nor‐
41       mally tries to use a ``trailer'' encapsulation to minimize copying data
42       on input and output.  The use of trailers is negotiated with ARP.  This
43       negotiation  may  be disabled, on a per-interface basis, by setting the
44       IFF_NOTRAILERS flag with an SIOCSIFFLAGS ioctl.
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DIAGNOSTICS

47       en%d: output error.  The hardware indicated an error  on  the  previous
48       transmission.
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50       en%d: send error.  After 16 retransmissions using the exponential back‐
51       off algorithm described above, the packet was dropped.
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53       en%d: input error.  The hardware indicated an error in reading a packet
54       off the cable.
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56       en%d:  can't  handle  af%d.   The  interface  was handed a message with
57       addresses formatted in an unsuitable address  family;  the  packet  was
58       dropped.
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SEE ALSO

61       intro(4N), inet(4F)
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BUGS

64       The  device  has insufficient buffering to handle back to back packets.
65       This makes use in a production environment painful.
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67       The hardware does word at a time DMA without byte swapping.  To compen‐
68       sate,  byte swapping of user data must either be done by the user or by
69       the system.  A kludge to byte swap only IP packets is provided  if  the
70       ENF_SWABIPS  flag is defined in the driver and set at boot time with an
71       SIOCSIFFLAGS ioctl.
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753rd Berkeley Distribution       August 20, 1987                          EN(4)