EC(4)                                                                    EC(4)



NAME
       ec - 3Com 10 Mb/s Ethernet interface

SYNOPSIS
       device ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0

DESCRIPTION
       The  _e_c 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 _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’’ encapsulation to min‐
       imize copying data on input and output.  The use of trailers is negoti‐
       ated 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 back‐
       off 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  imple‐
       ments  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.



4.2 Berkeley Distribution        May 16, 1986                            EC(4)