Date: Tue, 18 Mar 2003 12:08:28 -0800 From: "Crist J. Clark" <crist.clark@attbi.com> To: Wiktor Niesiobedzki <w@evip.pl> Cc: freebsd-ipfw@freebsd.org Subject: Re: Prioritizing empty TCP ACKs with ipfw? Message-ID: <20030318200828.GC74853@blossom.cjclark.org> In-Reply-To: <20030314224655.GA2616@mail.evip.pl> References: <20030314085636.GB64326@galgenberg.net> <el59ycqr.fsf@ID-23066.news.dfncis.de> <20030314224655.GA2616@mail.evip.pl>
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On Fri, Mar 14, 2003 at 11:46:55PM +0100, Wiktor Niesiobedzki wrote: > On Fri, Mar 14, 2003 at 08:22:36PM +0100, clemens fischer wrote: > > Ulrich Spoerlein <q@uni.de>: > > > > > I recently read this paper [1] and have to say that I am amazed. Is > > > this possible with ipfw/2 too? If so, how would one set this up? > > > > > > [1] http://www.benzedrine.cx/ackpri.html > > > > if i'm not mistaken, this is available right now with ipfw, see the > > link to luigis dummynet page in the article you cited. > > > With IPFW2 I use currently "iplen 40" option, is there any *better* way, of > selecting empty ACK packet? The amount of data in any given TCP segment is not stored explicitly in the header. It has to be calculated by, segment_data_length = ip_datagram_len - ip_header_len - tcp_offset Given that an IP header is almost always 20-bytes long and the TCP header (offset) is usually 20-bytes, your value makes sense... Unless the TCP header is carrying options, e.g. timestamps. Doing this calculation would be easy enough, but I think your solution is probably sufficient. If any change were to be made, I think changing the 'iplen' option to do "greater-than" and "less-than" checks, rather than just "equals" would be more useful in general. That way, you can catch ACKs with no data, but ones that also have a timestamp option (<53), or sack options (<53, <61, or <68, depending on how many you want to allow). -- Crist J. Clark | cjclark@alum.mit.edu | cjclark@jhu.edu http://people.freebsd.org/~cjc/ | cjc@freebsd.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-ipfw" in the body of the message
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