ec.4

早期freebsd实现

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.\"     @(#)ec.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt EC 4 vax
.Os BSD 4.2
.Sh NAME
.Nm ec
.Nd 3Com 10 Mb/s Ethernet interface
.Sh SYNOPSIS
.Cd "device ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0"
.Sh DESCRIPTION
The
.Nm ec
interface provides access to a 10 Mb/s Ethernet network through
a 3com controller.
.Pp
The hardware has 32 kilobytes of dual-ported memory on the
.Tn UNIBUS .
This memory
is used for internal buffering by the board, and the interface code reads
the buffer contents directly through the
.Tn UNIBUS .
The address of this memory is given in the
.Ar flags
field
in the configuration file.
The first interface normally has its memory at Unibus address 0.
.Pp
Each of the host's network addresses
is specified at boot time with an
.Dv SIOCSIFADDR
.Xr ioctl 2 .
The
.Nm ec
interface employs the address resolution protocol described in
.Xr arp 4
to dynamically map between Internet and Ethernet addresses on the local
network.
.Pp
The interface normally tries to use a
.Dq trailer
encapsulation
to minimize copying data on input and output.
The use of trailers is negotiated with
.Tn ARP .
This negotiation may be disabled, on a per-interface basis,
by setting the
.Dv IFF_NOTRAILERS
flag with an
.Dv SIOCSIFFLAGS
.Xr ioctl .
.Pp
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
.Tn VAX-11 Ns 's
interval timer in calculating a series
of random backoff values.  The algorithm is as follows:
.Bl -enum -offset indent
.It
Initialize the mask to be all 1's.
.It
If the mask is zero, 16 retries have been made and we give
up.
.It
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.
.It
Use the value calculated in step 3 to delay before retransmitting
the packet.
The delay is done in a software busy loop.
.El
.Sh DIAGNOSTICS
.Bl -diag
.It ec%d: send error.
After 16 retransmissions using the
exponential backoff algorithm described above, the packet
was dropped.
.Pp
.It 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.
.Pp
.It 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.
.El
.Sh SEE ALSO
.Xr netintro 4 ,
.Xr inet 4 ,
.Xr arp 4
.Sh HISTORY
The
.Nm
driver appeared in
.Bx 4.2 .
.Sh 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.
.Pp
Backoff delays are done in a software busy loop.  This can degrade the
system if the network experiences frequent collisions.