EC(4)							     Kernel Interfaces Manual							     EC(4)

NAME

       ec - 3Com 10 Mb/s Ethernet interface

SYNOPSIS

       /sys/conf/SYSTEM:
	    NEC  ec_controllers # 3Com Ethernet

DESCRIPTION

       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 inter-
       face code reads the buffer contents directly through the UNIBUS.  The address of this memory is given in the flags field in the	configura-
       tion 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 dynamically 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 nego-
       tiated  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 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.

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 debugging 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 collisions.

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