NAME
ec —
3Com 10 Mb/s Ethernet
interface
SYNOPSIS
ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0
DESCRIPTION
NOTE: This driver has not been ported from
4.4BSD yet.
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(2). The
ec interface employs the address resolution protocol
described in
arp(4) 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 negotiated
with ARP. This negotiation may be disabled, on a per-interface basis, by
setting the
IFF_NOTRAILERS
flag with an
SIOCSIFFLAGS
ioctl(2).
The interface software implements an exponential backoff algorithm when notified
of a collision on the cable. This algorithm uses a 16-bit mask and the
VAX-11's interval timer in calculating a series of random backoff values. The
algorithm is as follows:
- Initialize the mask to be all 1's.
- If the mask is zero, 16 retries have been made and we give
up.
- 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.
- 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
arp(4),
inet(4),
netintro(4)
HISTORY
The
ec driver appeared in
4.2BSD.
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.