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



NAME
     ec - 3Com 10 Mb/s Ethernet interface

SYNOPSIS
     /sys/conf/SYSTEM:
	  NEC  _e_c__c_o_n_t_r_o_l_l_e_r_s # 3Com Ethernet

DESCRIPTION
     The _e_c interface provides access to a 10 Mb/s Ethernet net-
     work 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 _f_l_a_g_s 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 _e_c interface employs
     the address resolution protocol described in _a_r_p(4P) to
     dynamically map between Internet and Ethernet addresses on
     the local network.

     The interface normally tries to use a ``trailer'' encapsula-
     tion 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.

     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 smaller of the com-
	  plement 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.




Printed 11/26/99	 August 20, 1987			1






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



DIAGNOSTICS
     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 debug-
     ging purposes.

     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.

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

BUGS
     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 network experiences frequent col-
     lisions.






























Printed 11/26/99	 August 20, 1987			2