EC(4) BSD/vax Kernel Interfaces Manual EC(4)NAMEec — 3Com 10 Mb/s Ethernet interface
device ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0
The ec interface provides access to a 10 Mb/s Ethernet network through a
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 Eth‐
ernet 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 set‐
ting 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 retransmit‐
ting 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 read‐
ing a packet off the cable or an illegally sized packet. The buffer off‐
set 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
SEE ALSOnetintro(4), inet(4), arp(4)HISTORY
The ec driver appeared in 4.2BSD.
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 June 5, 1993 4.2 Berkeley Distribution