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1996-07-10
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PCbridge: a inexpensive Ethernet Bridge
Vance Morrison
What is bridge?
A bridge is a device that spans two or more networks (almost always
ethernet), forwards packets between the networks so that it appears
that the networks are a single network. Bridges differ from repeaters
because they gather information about where hosts on the networks are
and ONLY forward messages when necessary. Since local traffic stays
local, a bridged network can handle more traffic than a single larger
network. Bridges differ from routers (like PCroute), because they
only depend on the ethernet addressing scheme and not on the higher
level protocol (IP, DECNET, SNA etc), that handles internetwork routing.
What is PCbridge?
PCbridge is a simple program written for an IBM-PC that will turn
a PC with appropriate interface cards into a ethernet bridge. It
has the capability to act as a standard 2 way ethernet-ethernet
or ethernet-starlan bridge as well as a remote bridge
(ethernet-serial-ethernet) or a n-way bridge. With this software
it is possible to assemble a bridge for under $1000, that will
outperform bridges costing $2000 or more.
What does PCbridge Support?
PCbridge supports
Ethernet interfaces with the Western Digital WD8003E, WD8003EBT
WD8013EBT cards, and the 3Com 3c507 card .
Starlan cards with the WD8003S and WD8003SH cards.
Serial Interfaces with the standard 8250 IBM COM ports at
baud rates up to 56.7K baud, as well as support for the
newer 16550AF chips with built in FIFO buffers (these
newer COM boards are HIGHLY recommended since they can
reduce the interrupt overhead by a factor of 10). COM
boards with the 16550AF chips are now widely available
from PC vendors.
What does PCbridge NOT support?
Although it performs the bridging function very well, it is not
a 'smart' bridge. In particular it does NOT include any spanning
tree algorithm for detecting and breaking loops. This does not
mean PCbridge can't be used in a bridged network with loops, it simply
means that other 'smarter' bridges must be used at strategic places
that will perform this function. In addition PCbridge does NOT support
SNMP or any other network management protocol.
How fast is PCbridge?
Bridges have two metrics associated with them. First is the number
of packets a second it can forward, and the other how many packets a second
it can filter. The second metric is interesting because it can't be
easily be measured directly, after all rock connected to two pieces of
ethernet will filter (drop) as many packets per second as you like. The
meaning of the second metric is to determine just how fast the router
can examine ethernet packets if it does not need to forward them.
To measure this I placed a counter in the main loop that was incremented
every time a packet was examined (from either ethernet segment). Then I
modified the code slightly so that the code ALWAYS thought it got a packet
that it should drop. Thus by running the code for a time and determining
how many packets it examined I could come up with a very accurate filtering
rate.
The forwarding rate was measured by looping back the bridge and
injecting some test packets. These packets would then circulate
repeatedly. A count of the number of packets forwarded was kept and
this number was used to determine the packet forwarding rate.
RATES for the WD8003E card
Packet Filtering Packet Forwarding (60byte)
rate (PPS) rate (PPS)
-------------------------------------------------------------
4.77Mhz XT 4080 PPS 1677 PPS
10Mhz XT (est) 8000 PPS 3000 PPS
16Mhz AT 26214 PPS 5832 PPS
RATES for the 3Com 3c507 card
Packet Filtering Packet Forwarding (60byte)
rate (PPS) rate (PPS)
-------------------------------------------------------------
12Mhz AT 20000 PPS 11000 PPS
While packet filtering is independent of packet size, packet forwarding
is dependent on the size of the packet. The packets I used where the
smallest ethernet packet (60 bytes).
I have no data on the performance of a WD8013EBT card, but I expect
it to perform similiarly to the 3c507.
For comparison purposes a low end Retix bridge costs $2000 (actually
more if you want thin ethernet) and has a forwarding rate of 6000 PPS
and a filtering rate of 10,000 PPS. A 12Mhz AT with 3c507 cards
(costing about $1000) has approximately double the performance of
such a bridge at about half the cost. Something to think about.
------------------------------------------------------------------------
Which ethernet card is for me?
Note that all the ethernet cards have on-board input buffers that can
keep up with the full 10Mbit/sec bit rate of ethernet. The bridge
will ONLY start dropping packets when this input queue has filled.
The queue sizes are as follows
WD8003E 6.5K bytes
WD8003EBT 30.5K bytes
WD8013EBT 14.5K bytes
3c507 56.5K bytes
Note that may file serving programs (Novell and NFS for example) like
to send data in fast 8K blocks. Because the WD8003E card has only a 6.5K
buffer, this will cause packets to be lost if the WD8003E card is used
in such an environment.
The WD8013EBT and 3c507 cards have the additional advantage of having
a 16bit data path. Thus these cards can transfer data to other cards
twice as fast as the other cards. The only disadvantage to these cards
is that they are more expensive and can not be used in a XT. I machine
with the 16 bit data bus is needed.
Thus the recommended configuration is a cheap AT clone with 3c507 or
WD8013EBT cards. If an XT must be used, or packet forwarding is not
a issue, then WD8003EBT cards are the second choice. Only if no file
serving traffic is expected (like in a remote bridge), the WD8003E can
be used (but is still not recommended).
Note that if you HAVE to run NFS though a bridge with WD8003E cards it
should be possible to ask NFS to send data in smaller 4K blocks.
On UNIX systems this is done via the 'rsize' and 'wsize' parameters
in fstab. Since this has to be done for every NFS hosts, this is
clearly a choice of last resort, but I thought I would mention it.
--------------------------------------------------------------------------
Disclaimer -
I have tested PCbridge quite a bit, but we do not use it here
in a production environment. Thus I cannot guarantee that it will
work well for your particular situation. It should though.
---------------------------------------------------------------------------
Questions, comments, and bug reports to:
morrison@accuvax.nwu.edu
Vance Morrison