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EC(4)											    EC(4)

       ec - 3Com 10 Mb/s Ethernet interface

	    NEC  ec_controllers # 3Com Ethernet

       The ec interface provides access to a 10 Mb/s Ethernet network through a 3com controller.

       The  hardware  has  32 kilobytes of dual-ported memory on the UNIBUS.  This memory is used
       for internal buffering by the board, and the interface  code  reads  the  buffer  contents
       directly  through  the  UNIBUS.	The address of this memory is given in the flags field in
       the configuration file.	The first interface normally has its memory at Unibus address 0.

       Each of the host's network addresses is specified at boot time with an SIOCSIFADDR  ioctl.
       The  ec	interface employs the address resolution protocol described in arp(4P) to dynami-
       cally map between Internet and Ethernet addresses on the local network.

       The interface 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  SIOCSIF-
       FLAGS ioctl.

       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 smaller of the complement of this mask and a  5-bit  mask,  resulting  in	a
	    pseudo-random  number  between  0  and 31.	This produces the number of slot times to
	    delay, where a slot is 51 microseconds.

       4.   Use the value calculated in step 3 to delay before retransmitting  the  packet.   The
	    delay is done in a software busy loop.

       ec%d:  send  error.   After  16	retransmissions  using	the exponential backoff algorithm
       described above, the packet was dropped.

       ec%d: input error (offset=%d).  The hardware indicated an error in reading  a  packet  off
       the  cable or an illegally sized packet.  The buffer offset value is printed for debugging

       ec%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), arp(4P)

       The  hardware  is not capable of talking to itself.  The software implements local sending
       and broadcast by sending such packets to the loop interface.  This is a kludge.

       Backoff delays are done in a software busy loop.  This can degrade the system if the  net-
       work experiences frequent collisions.

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