Unusual use of IP Broadcast Address
pos...@isi.edu
----- Begin Included Message -----
Date: Wed, 12 Jan 1994 10:47:28 -0500
From: etk...@stowe.mitre.org (Wendy Etkind)
To: Pos...@ISI.EDU
Subject: rfc1020
Mr. Postel:
In rfc1020 the internet address 255.255.255.255, the broadcast
address, is reserved for you. Granted this is a special address,
would it be illegal for me to route the broadcast address to a
specific network?
For example, for purposes of my network protocol development I have
reserved the subnets 129.83.14 and 129.83.15 for my personal use.
These subnets are within the MITRE beford domain (MITRE bedford
has all of 129.83).
I am developing a network protocol for the government (a tactical
protocol called TACO2 which is based heavily on MIT's netblt and
includes standard IP, slip and a subset of hdlc). I have this
protocol (TACO2) running on both Suns and pcs. I have ttya set to
129.83.14 and ttyb set to 129.83.15. I would like to transmit to a
pc which is attached to my tty port. The tty to pc connection is
the tactical connection. Usually there are satellites or HF radios
in between the tty port and the pc.
Since I will not always know the address of the pc I would like to
address it as 255.255.255.255. Of course I would like the pc
attached to ttya to be on the 129.83.14 subnet but this may not
be possible. These pcs will never be on the internet so people have
been using their own addresses.
I have a modified version of "route" which will route the broadcast
address to my 129.83.14 gateway (the tty port). Using this private
route I can transmit to the broadcast address and the resulting
packets go out the tty port rather than to all address (and does
not go out the ethernet).
Before I release this version of route and let my users send to the
broadcast address I want to make sure that this is legal as far
as standard internet numbering schemes are concerned. While my pcs
will never be on the internet, I expect at some point, that the Sun
will act as a tactical to stragic gateway. The Sun could be on the
internet (or the milnet perhaps) and the Sun's tty ports would
be attached to the tactical pcs. I will use tcp/ip for normal
(ethernet) internet traffic and TACO2 for the tactical (tty) traffic.
If I have the Sun on the internet then I want to make sure that I am
doing legal things with respect to the broadcast address, especially
if I am considering redirecting it.
Thank you for your time,
Wendy Etkind
Wendy Etkind (etk...@mbunix.mitre.org or linus!mbunix!etkind)
Member of the Technical Staff: Imaging Systems, Mitre Corp.
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Bob Braden
I don't think I fully comprehend the situation, but it seems to me
that if you want to send to a host whose address you don't know
but which is known to be on subnet 129.83.14, you should use the
directed broadcast address 129.83.14.255. Using 255.255.255.255
does not seem correct.
Bob Braden
Steve Deering
It is legal for a host or router to send packets addressed to the Limited
Broadcast Address (255.255.255.255) on any of its attached subnets; in fact,
that is the preferred address to use for IP broadcasting on any subnet
(see Host Requirements RFC 1122, section 3.3.6). Your two serial lines
are simply two additional subnets attached to your Sun.
One issue that arises is the following: if an application on a multihomed
host or a router originates a packet destined to 255.255.255.255, which
interface should it be sent over? In my opinion, the service interface to
the IP module should provide a means for the application to specify the
outgoing interface (it might be implicit in the application's choice of IP
source address or explicit as, say, a separate parameter to the Send()
operation or a control operation performed before the Send() operation).
Unfortunately, BSD-derived Unixes do not provide any such means, but
rather limit the use of 255.255.255.255 to one interface only
(configurable through the ifconfig command). If you are stuck with
such an implementation, the best you can do is configure one of the
serial lines to use the 255.255.25.255 broadcast address, and configure
the Ethernet and the other serial line to use Directed Broadcast addresses,
i.e., addresses of the form <net><subnet><-1>.
Steve Deering
P.S. I suggest that you consider using IP multicast rather than IP broadcast;
the service interface to IP multicast does include the ability to
choose the outgoing interface.
Steve Deering
Bob,
Wendy said that her PCs don't necessarily know that they are on subnet
129.83.14, so they wouldn't be able to recognize 129.83.14.255 as a
broadcast address they should accept.
Upon re-reading her message, I realize that I may also have misunderstood
what she was proposing when I wrote my response. The references to a
modified version of routed seem to imply that the "broadcast" packets will
be originated from machines other than the Sun that has the serial links to
the PCs. If that's true, what she is proposing is significantly at odds
with the IP architecture (treating an all-ones broadcast as a directed
broadcast to a remote subnet known as a special case by the routers). IP
multicast would satisfy her requirement without bending the architecture.
Steve
Bob Braden
*> From dee...@parc.xerox.com Thu Jan 13 10:48:09 1994
*> To: etk...@stowe.mitre.org
*> Cc: ietf-...@ISI.EDU
*> Subject: Re: Unusual use of IP Broadcast Address
*> In-Reply-To: postel's message of Wed, 12 Jan 94 17:21:02 -0800.
*> <1994011301...@zen.isi.edu>
*> Date: Thu, 13 Jan 1994 10:43:46 PST
*> Sender: Steve Deering <dee...@parc.xerox.com>
*> From: Steve Deering <dee...@parc.xerox.com>
*> Content-Length: 1538
*> X-Lines: 29
*>
*> Wendy,
*>
*> It is legal for a host or router to send packets addressed to the Limited
*> Broadcast Address (255.255.255.255) on any of its attached subnets; in fact,
*> that is the preferred address to use for IP broadcasting on any subnet
*> (see Host Requirements RFC 1122, section 3.3.6). Your two serial lines
*> are simply two additional subnets attached to your Sun.
*>
I just reviewed 3.3.6, and Steve is correct [I still don't like it! :-) ]
Bob
Bob Braden
----- Begin Included Message -----
From etk...@stowe.mitre.org Thu Jan 13 10:48:53 1994
Date: Thu, 13 Jan 1994 13:44:57 -0500
From: etk...@stowe.mitre.org (Wendy Etkind)
To: bra...@ISI.EDU
Subject: Re: Unusual use of IP Broadcast Address
Cc: pos...@ISI.EDU
Content-Length: 3094
X-Lines: 78
Bob:
The problem is that my users have (without my guidance) chosen
the address 127.0.0.1 for their local address (some users have chosen
other addresses, we have no coherent addressing scheme yet).
For my development my Sun's address is 129.83.7.8 (a legally assigned
address on MITRE bedford's chunk of the net). I also have reserved for
my development subnets 129.83.14 and 129.83.15.
The problem is trying to send packets to a pc whose address is 127.0.0.1,
or some other random address (this is a point-to-point problem only).
I can send packets to this pc by using the address 255.255.255.255.
In order to use this address I have written a modified version of
route which routes packets to address 255.255.255.255 to my 129.83.14
gateway (my tty port).
Jon Postel has recommended that I try to have all my users move
to the 192.0.2 subnet and I can have my tty port set to 192.0.2
and send broadcasts to 192.0.2.255 safely.
The main problem is that there are approximately 150,000 - 200,000
users of my software (I don't mean to brag, these are accurate
estimates), most of these users use the address 127.0.0.1 and
most of these users always send to the address 255.255.255.255
so that they will always be able to talk to the pc on the other
side of the connection.
Changing addresses could mean a flood of incompatibility and hardship.
I am still left with the operational preferance of always sending
to the broadcast address. Until I can release Sun software that will
do this most users will not use Suns.
I would like to get everyone playing by internet legal rules as I
move the software onto Sun's acting as strategic to tactical
gateways. A Sun in this position really must play by the rules
even if the Sun's local ethernet never gets connected to the actual
internet.
I hope this clarifies my problem a bit.
Wendy
Wendy Etkind (etk...@mbunix.mitre.org or linus!mbunix!etkind)
Member of the Technical Staff: Imaging Systems, Mitre Corp.
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From: bra...@ISI.EDU (Bob Braden)
Message-Id: <1994011318...@zephyr.isi.edu>
To: ietf-...@ISI.EDU, pos...@ISI.EDU
Subject: Re: Unusual use of IP Broadcast Address
Cc: etk...@stowe.mitre.org
Status: R
I don't think I fully comprehend the situation, but it seems to me
that if you want to send to a host whose address you don't know
but which is known to be on subnet 129.83.14, you should use the
directed broadcast address 129.83.14.255. Using 255.255.255.255
does not seem correct.
Bob Braden
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----- Begin Included Message -----
From bra...@ISI.EDU Thu Jan 13 11:11:06 1994
Date: Thu, 13 Jan 1994 11:11:04 -0800
From: bra...@ISI.EDU (Bob Braden)
To: bra...@ISI.EDU, etk...@stowe.mitre.org
Subject: Re: Unusual use of IP Broadcast Address
Cc: pos...@ISI.EDU
Content-Length: 622
X-Lines: 18
Wendy,
Thanks for the explanation. Reading between the lines, I assume that
these PCs recognize both the loopback address and the broadcast address
as themselves, but no other IP source addresses. If the choice is
between sending to the loopback address or the broadcast address,
surely the broadcast address is the right answer.
But PLEASE make your gateway an effective firewall between the Internet
conventions, should be be connected to Milnet someday, and the private
conventions you are forced to use.
And congratulations on having 200K PC running your software! I don't
think I ever got beyond O(100).
Bob
Bob Braden
*>
*> Bob,
*>
*> Wendy said that her PCs don't necessarily know that they are on subnet
*> 129.83.14, so they wouldn't be able to recognize 129.83.14.255 as a
*> broadcast address they should accept.
*>
*> Upon re-reading her message, I realize that I may also have misunderstood
*> what she was proposing when I wrote my response. The references to a
*> modified version of routed seem to imply that the "broadcast" packets will
*> be originated from machines other than the Sun that has the serial links to
*> the PCs. If that's true, what she is proposing is significantly at odds
*> with the IP architecture (treating an all-ones broadcast as a directed
*> broadcast to a remote subnet known as a special case by the routers). IP
*> multicast would satisfy her requirement without bending the architecture.
*>
*> Steve
*>
*>
Nice try, but her machines don't recognize IP multicast addresses, either.
Bob
Steve Deering
How do you know? At least one commercial IP stack for PCs (the one from
FTP Software) supports IP multicast.
And even if her PC's don't support IP multicast, wouldn't it be preferable
to remedy that omission than to hack the Suns to forward all-ones broadcasts?
Steve