https://bugzilla.novell.com/show_bug.cgi?id=211867 Summary: SuSE Linux IP stack sets Don't Fragment flag on UDP datagrams. Product: SUSE Linux 10.1 Version: Final Platform: x86 OS/Version: SuSE Linux 10.1 Status: NEW Severity: Major Priority: P5 - None Component: Network AssignedTo: bnc-team-screening@forge.provo.novell.com ReportedBy: fdjongh@novell.com QAContact: qa@suse.de The Don't Fragment flag is meant for Path MTU discovery. Unlike TCP, UDP does not support Path MTU Discovery, since UDP does not packetize data into segments. Section 6.1 on page 9 of RFC 1191 reads the following about this topic: "We do not want the IP layer to simply set the DF bit in every packet, since it is possible that a packetization layer, perhaps a UDP application outside the kernel, is unable to change its datagram size." However, the SuSE Linux TCP/IP stack sets the DF bit in every datagram, unless Path MTU discovery is disabled by the sysctl parameter 'ip_no_pmtu_disc'. Of course, disabling Path MTU Discovery is not a solution, because Path MTU Discovery is very useful for performance of TCP connections, since it avoids fragmentation and reassembly. Especially fragmentation can put a heavy load on routers. The IP stack should only set the DF bit on datagrams that carry TCP segments, unless a UDP application specifically requests to set it with socket initialization. From the ip manual page in section 7 I understand that the IP stack allows applications to choose whether they want Path MTU Discovery per IP_MTU_DISCOVER socket option. As this manual page also reads that, in case 'ip_no_pmtu_disc' is false, the IP stack is supposed to only enable Path MTU Discovery with SOCK_STREAM (TCP) sockets, and to disable it on all others, I have no doubt that the actual behaviour of the IP stack is not in conformance with the intended, documented behaviour and RFC 1191. Please note that setting the DF flag with UDP datagrams will cause connectivity problems on UDP connections that cross a network path with links that support a smaller MTU than the local segment of the SuSE Linux host, if the concerning UDP application does not support Path MTU Discovery. Most UDP applications do not support Path MTU Discovery. Although I report this bug for SuSE Linux 10.1, the same defect is in every SuSE Linux distribution. -- Configure bugmail: https://bugzilla.novell.com/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are on the CC list for the bug, or are watching someone who is.