Skip site navigation (1)Skip section navigation (2)
Date:      Tue, 31 Jul 2001 04:34:31 -0700
From:      "Ted Mittelstaedt" <>
To:        "Ron Klinkien" <>, "kesu" <>
Cc:        <freebsd-questions@FreeBSD.ORG>
Subject:   RE: Need help limiting bandwith ARP uses over cable modem.
Message-ID:  <000001c119b4$c405b2c0$>
In-Reply-To: <00de01c119b1$8e29e2a0$>

Next in thread | Previous in thread | Raw E-Mail | Index | Archive | Help
Hi Ron,

  Thanks for pointing that out, I wasn't paying attention, the comment I made
regarding that was an off-the-cuff remark.  It is obvious that the poster is
positive that arping is his problem and is fixated on that, and I already
recognized the futility of arguing that so I didn't really intend to adresss
his rather long justification.  It's completely moot anyway because until he
tests it he is not going to know if his hypothesis is correct or not.  The
bandwidth comment really isn't relevant to the rest of the response.

Did you understand the rest of the discussion regarding subnetting?  It's
not an easy part of TCP/IP for a lot of people to grasp and like all these
responses it's very short.  Hopefully it's not short enough to be completely
impossible to understand.  If it works for him it will save him a lot of work
as well as proving if the arp traffic decrease that results will make any
difference or not.

Otherwise the only thing that I can think might help him is building another
FreeBSD box that's a filtering bridge.

Ted Mittelstaedt                             
Author of:                           The FreeBSD Corporate Networker's Guide
Book website:                

>-----Original Message-----
>From: Ron Klinkien []
>Sent: Tuesday, July 31, 2001 4:09 AM
>To: Ted Mittelstaedt; kesu
>Cc: freebsd-questions@FreeBSD.ORG
>Subject: Re: Need help limiting bandwith ARP uses over cable modem.
>If i'm correct you wrote a FreeBSD book
>so you should know Kilobits and bytes are different...
>----- Original Message -----
>From: "Ted Mittelstaedt" <>
>To: "kesu" <>
>Cc: <freebsd-questions@FreeBSD.ORG>
>Sent: Tuesday, July 31, 2001 8:23 AM
>Subject: RE: Need help limiting bandwith ARP uses over cable modem.
>> >-----Original Message-----
>> >From: owner-freebsd-questions@FreeBSD.ORG
>> >[mailto:owner-freebsd-questions@FreeBSD.ORG]On Behalf Of kesu
>> >Sent: Monday, July 30, 2001 5:34 PM
>> >To: Ted Mittelstaedt
>> >> >
>> >> >My incoming bandwith is 560 killobits/second my outgoing bandwith is
>> >> >killobits/second now surely you can see how receieving a saturating
>> >> >of packets on the inbound is going to generate _more_ outbound packets
>> >> >than my bandwith limitation.
>> >>
>> >> No, actually I cannot.  If your RECIEVING a larger volume of data than
>> >> sending then while you may be generating a lot of packets, those are
>> >> packets.
>> >
>> >I have anylized the situation, and in fact the ARP packets are causing
>> >problem.  I can use the output of tcpdump to accuratle measure the
>> >bandwith used by ARP packets and it easily exceeds my tiny 16-KilloBytes
>> >second upstream cap.
>> Wait just a second here.  In your previous posting you said your upstream
>> bandwidth is 128k.  Now your saying it's 16k.
>> If you have a 560k downstream and a 16k upstream then I can well believe
>> having a problem.
>> >
>> >The packet loss becomes user noticable when the modem resets. (as in
>> Obviously.  But, how often is it resetting?
>> >
>> >But the ARP packets are pointless, as it isn't a lan, and version 1.1
>> >causes the ARP to be selective, only sharing between local computers when
>> >a connection attempt is actually made between the two. DOCSIS 1.1
>> >compliant modems are going to make cable modems a much more user friendly
>> >experience, since the reduction in ARP traffic will benefit everyone.
>> >
>> I'll believe it when I see it.  The IP-over-Cable people have been
>> really going through some rediculous gyrations to try to get piss-poor and
>> ignorant networking design to work on oversubscribed segments.
>> You said earlier that your provider runs a full /24 subnet, ie: 254
>> per network.  You also said your downstream is 500K.  That means that your
>> provider is pushing 127 Megabits into just your segment, right?  Is my
>> multiplication wrong?  Do you see a problem here?  I do.
>> >ise _extremely_ annoying.  Since the modem has a tendancy to reset
>> >around once a week, this is very annoying to me.
>> >
>> Your upset over a 2 minute network outage that happens ONCE A WEEK?
>> >crash on me every day (windows.)  Furthermore, why did you assume I use
>> >@home?
>> Because frankly I cannot imagine a small ISP to be this stupid.  @Home is
>> a different story, I fully expect them to be this stupid.
>> You already said the solution to your problem earlier although you
>> missed it.  That is, your subnet mask is too big.  This is something
>> that your ISP should have done by subnetting instead of throwing everyone
>on a
>> single /24.  If they had cut up that /24 into 4 /26's then you wouldn't be
>> seeing the kind of ARP traffic your seeing.
>> This does lead into a trick that you can _try_ there is no guarentee that
>> will work, though.  That is you can still adjust your own mask.  Let me
>> explain:
>> Suppose that your ISP's gateway is with a subnet mask of
>> Your IP numbers are,, and and you
>> course have a subnet mask of
>> Now, when your system gets an arp request from, say,, it
>> that this number is on it's network.  So, it responds.
>> However, what happens if you change your OWN subnet mask to
>> Well, your system will still think that the gateway at is on
>> subnet, and thus it can still reach it.  It will still think that
>> is on it subnet and can reach that.
>> But, it will think that packets from all IP numbers from
>> are NOT on it's own subnet, and thus it should
>> respond to them with a broadcast.
>> Now, if your doing network-style broadcasts then this may not work because
>> your ISP's gateway may not pick up your own ARP's and thus time you out.
>> if it does it's a crude but effective hack that will get your machine to
>> ignore the majority of traffic on the subnet.
>> Frankly your ISP should have been slapped for putting an entire /24 on a
>> shared media like this, but what's done is done.
>> Good luck,
>> Ted Mittelstaedt
>> Author of:                           The FreeBSD Corporate Networker's
>> Book website:
>> To Unsubscribe: send mail to
>> with "unsubscribe freebsd-questions" in the body of the message

To Unsubscribe: send mail to
with "unsubscribe freebsd-questions" in the body of the message

Want to link to this message? Use this URL: <$c405b2c0$1401a8c0>