Date: Tue, 12 Dec 2006 08:48:46 -0500 From: Bill Moran <wmoran@collaborativefusion.com> To: "Mike Murphree" <mike@tropo.org> Cc: freebsd-questions@freebsd.org Subject: Re: Dummynet fragmenting packets Message-ID: <20061212084846.235bfd68.wmoran@collaborativefusion.com> In-Reply-To: <60158.69.73.127.132.1165900548.squirrel@webmail.dogbark.com> References: <60158.69.73.127.132.1165900548.squirrel@webmail.dogbark.com>
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In response to "Mike Murphree" <mike@tropo.org>: > > Having an issue on a 5.3 system using ipfw and dummynet to create a > bandwidth limited and large latency pipe for a mpeg video stream. If I > pass the packets between the two NICs without routing through a dummynet > pipe, it's fine. If I route it through a pipe, it's fragmenting each > packet (client requested 1468 byte packets) into two packets, the second > packet with an offset of 1440 bytes. Does anyone have any idea why it's > doing this, and have a solution to this problem? As a general rule, fragmenting occurs when packets move between different networks with different MTUs. I.e. the originating network has a larger MTU, so the packet must be broken up in order to pass it on to the network with the smaller MTU. Now that that's out of the way, I can see 3 possibilities as to why dummynet is fragmenting packets: 1) Dummynet has the wrong information about what the MTUs are on your networks and is fragmenting the packets needlessly. 2) Dummynet is altering the packets, they become larger and then no longer fit in the MTU. 3) The endpoints are doing path-MTU-discovery, but when you put dummynet between them you somehow break PMTUD. To narrow this down, you'll need to determine what the MTUs are on each network and whether they're being respected, is the total size of the reassembled fragments the same as when the packet came in, and whether or not PMTUD is in use, and whether something in dummynet or any related filtering rules is breaking it. -- Bill Moran Collaborative Fusion Inc.
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