From mboxrd@z Thu Jan 1 00:00:00 1970 From: "David S. Miller" Subject: Re: [ROUTE] PMTU only works on half the time Date: Tue, 2 Dec 2003 16:07:38 -0800 Sender: netdev-bounce@oss.sgi.com Message-ID: <20031202160738.439f9f0d.davem@redhat.com> References: <20031201155005.1c515793.davem@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: herbert@gondor.apana.org.au, netdev@oss.sgi.com Return-path: To: Julian Anastasov In-Reply-To: Errors-to: netdev-bounce@oss.sgi.com List-Id: netdev.vger.kernel.org On Wed, 3 Dec 2003 01:40:06 +0200 (EET) Julian Anastasov wrote: > Please review and edit if needed. Thanks Julian, I will review your patch today. But I want to mention something about this. TOS checks I think are pretty unreliable. TOS has meaning within a network realm, right? This means that if we make a packet with TOS X, once it leaves our realm the TOS value may be arbitrarily changed so that the TOS in the packet has meaning within that new realm. If a PMTU or other routing ICMP gets sent back to us after TOS changes have been made, there is absolutely no way to reliably match the routes properly for update. Therefore TOS based routes cannot handle ICMP messages reliably in the global internet. In fact, it is possible and even likely to apply an ICMP for the wrong TOS variant of a given route when this TOS rewriting occurs.