From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glen Turner Subject: DF Bit set on UDP traffic -- bug or feature? Date: Thu, 20 Aug 2009 09:22:45 +1000 Message-ID: <4A8C8945.70602@gdt.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: netdev@vger.kernel.org Return-path: Received: from eth6445.sa.adsl.internode.on.net ([150.101.30.44]:44460 "EHLO aix.gdt.id.au" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753253AbZHTAAd (ORCPT ); Wed, 19 Aug 2009 20:00:33 -0400 Received: from thrace.aarnet.edu.au ([202.158.213.132]) (authenticated bits=0) by aix.gdt.id.au (8.14.2/8.14.2) with ESMTP id n7JNMkPB004033 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Thu, 20 Aug 2009 08:52:48 +0930 Sender: netdev-owner@vger.kernel.org List-ID: Hi folks, Is it a bug that the DF bit is set on UDP traffic when TCP Path MTU Discovery is active, but yet when a returning ICMP DF Set But Fragmentation Required response is received the kernel does not attempt fragmentation of the outgoing UDP-carrying IP packets? This has practical consequences for large RADIUS packets, such as in the RADIUS/TLS/UDP protocol "DTLS". They are not fragmented by the network and they are not fragmented by the kernel, and thus the packets fail to pass from the application to the end-user. Can I humbly suggest that when the kernel does not implement its own fragmentation strategy (as it does with TCP Path MTU Discovery) that the DF bit not be set by the kernel? Thanks very much, Glen -- Glen Turner, Network Engineer, Australia's Academic & Research Network www.aarnet.edu.au +61 8 8303 3936