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1.1       root        1: .\" Copyright (c) 1983 The Regents of the University of California.
                      2: .\" All rights reserved.
                      3: .\"
                      4: .\" Redistribution and use in source and binary forms are permitted provided
                      5: .\" that: (1) source distributions retain this entire copyright notice and
                      6: .\" comment, and (2) distributions including binaries display the following
                      7: .\" acknowledgement:  ``This product includes software developed by the
                      8: .\" University of California, Berkeley and its contributors'' in the
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                     11: .\" Neither the name of the University nor the names of its contributors may
                     12: .\" be used to endorse or promote products derived from this software without
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                     14: .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
                     15: .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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                     17: .\"
                     18: .\"    @(#)ec.4        6.6 (Berkeley) 6/23/90
                     19: .\"
                     20: .TH EC 4 "June 23, 1990"
                     21: .UC 5
                     22: .SH NAME
                     23: ec \- 3Com 10 Mb/s Ethernet interface
                     24: .SH SYNOPSIS
                     25: .B "device ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0"
                     26: .SH DESCRIPTION
                     27: The
                     28: .I ec
                     29: interface provides access to a 10 Mb/s Ethernet network through
                     30: a 3com controller.
                     31: .PP
                     32: The hardware has 32 kilobytes of dual-ported memory on the UNIBUS. 
                     33: This memory
                     34: is used for internal buffering by the board, and the interface code reads
                     35: the buffer contents directly through the UNIBUS.
                     36: The address of this memory is given in the \fIflags\fP field
                     37: in the configuration file.
                     38: The first interface normally has its memory at Unibus address 0.
                     39: .PP
                     40: Each of the host's network addresses
                     41: is specified at boot time with an SIOCSIFADDR
                     42: ioctl.  The
                     43: .I ec
                     44: interface employs the address resolution protocol described in
                     45: .IR arp (4)
                     46: to dynamically map between Internet and Ethernet addresses on the local
                     47: network.
                     48: .PP
                     49: The interface normally tries to use a ``trailer'' encapsulation
                     50: to minimize copying data on input and output.
                     51: The use of trailers is negotiated with ARP.
                     52: This negotiation may be disabled, on a per-interface basis,
                     53: by setting the IFF_NOTRAILERS
                     54: flag with an SIOCSIFFLAGS ioctl.
                     55: .PP
                     56: The interface software implements an exponential backoff algorithm
                     57: when notified of a collision on the cable.  This algorithm utilizes
                     58: a 16-bit mask and the VAX-11's interval timer in calculating a series
                     59: of random backoff values.  The algorithm is as follows:
                     60: .TP 5
                     61: 1.
                     62: Initialize the mask to be all 1's.
                     63: .TP 5
                     64: 2.
                     65: If the mask is zero, 16 retries have been made and we give
                     66: up.
                     67: .TP 5
                     68: 3.
                     69: Shift the mask left one bit and formulate a backoff by
                     70: masking the interval timer with the smaller of the complement of this mask
                     71: and a 5-bit mask, resulting in a pseudo-random number between 0 and 31.
                     72: This produces the number of slot times to delay,
                     73: where a slot is 51 microseconds.
                     74: .TP 5
                     75: 4.
                     76: Use the value calculated in step 3 to delay before retransmitting
                     77: the packet.
                     78: The delay is done in a software busy loop.
                     79: .SH DIAGNOSTICS
                     80: .BR "ec%d: send error" .
                     81: After 16 retransmissions using the
                     82: exponential backoff algorithm described above, the packet
                     83: was dropped.
                     84: .PP
                     85: .BR "ec%d: input error (offset=%d)" .
                     86: The hardware indicated an error
                     87: in reading a packet off the cable or an illegally sized packet.
                     88: The buffer offset value is printed for debugging purposes.
                     89: .PP
                     90: .BR "ec%d: can't handle af%d" .
                     91: The interface was handed
                     92: a message with addresses formatted in an unsuitable address
                     93: family; the packet was dropped.
                     94: .SH SEE ALSO
                     95: netintro(4), inet(4), arp(4)
                     96: .SH BUGS
                     97: The hardware is not capable of talking to itself.  The software
                     98: implements local sending and broadcast by sending such packets to the
                     99: loop interface.  This is a kludge.
                    100: .PP
                    101: Backoff delays are done in a software busy loop.  This can degrade the
                    102: system if the network experiences frequent collisions.

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