dr0ss r0zz wrote: > > hmm I dont think the router will drop the packet..
The router must drop the packet. It can't send the packet if the MTU for the outgoing interface is smaller than the packet and the Don't Fragment (DF) bit is set in the packet, which is what we were discussing. > TCP has > something called path of MTU RFC 1191..... That's the IP MTU Path Discovery process. It's used by end hosts to discover the lowest MTU on a path so they can send packets without them being fragmented. It relies on the fact that a router discards packets and sends back "ICMP Destination Unreachable, Fragmentation Needed and the Don't Fragment Bit Was Set" messages. The end system starts with a big packet with the DF bit set. If that results in the ICMP message, the end system resends, reducing the size until it coverges on the right size that doesn't result in the ICMP message coming back. > > A quick defintion > The smallest MTU of any link on the current path between two > hosts. This may change over time since the route between two > hosts, especially on the Internet, may change over time. It is > not necessarily symmetric and can even vary for different types > of traffic from the same host. Yes, that's a good definition of the path MTU (PMTU), but it doesn't support your claim that the packets in question aren't dropped. Here's what the RFC says: "In this memo, we describe a technique for using the Don't Fragment (DF) bit in the IP header to dynamically discover the PMTU of a path. The basic idea is that a source host initially assumes that the PMTU of a path is the (known) MTU of its first hop, and sends all datagrams on that path with the DF bit set. If any of the datagrams are too large to be forwarded without fragmentation by some router along the path, that router will discard them and return ICMP Destination Unreachable messages with a code meaning "fragmentation needed and DF set". Upon receipt of such a message (henceforth called a "Datagram Too Big" message), the source host reduces its assumed PMTU for the path. The PMTU discovery process ends when the host's estimate of the PMTU is low enough that its datagrams can be delivered without fragmentation. Or, the host may elect to end the discovery process by ceasing to set the DF bit in the datagram headers; it may do so, for example, because it is willing to have datagrams fragmented in some circumstances. Normally, the host continues to set DF in all datagrams, so that if the route changes and the new PMTU is lower, it will be discovered. Unfortunately, the Datagram Too Big message, as currently specified, does not report the MTU of the hop for which the rejected datagram was too big, so the source host cannot tell exactly how much to reduce its assumed PMTU. To remedy this, we propose that a currently unused header field in the Datagram Too Big message be used to report the MTU of the constricting hop. This is the only change specified for routers in support of PMTU Discovery." _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com > Priscilla Oppenheimer wrote: > > > > JJ Angleton wrote: > > > > > > If you've set the flags in the IP header to not allow > > > fragmentation, and then you pass through in interface with a > > > smaller MTU, what happens? > > > > When a router attempts to forward an IP packet onto an > > interface where the MTU is smaller than the packet and the > > Don't Fragment bit is set, the router sends back an ICMP > Message: > > > > Type = 3 = Destination Unreachable > > Code = 4 = Fragmentation was needed and the Don't Fragment bit > > was set. > > > > The router also drops the packet. > > > > > > > > By the way, in a related question - I've looked all through > > the > > > Cisco website and can't find what the standard MTU is for > > Frame > > > Relay. > > > > I found this in RFC 3090: To avoid packet discards on the > Frame > > Relay interface, the RECOMMENDED default Frame Relay MTU is > > 1564 based on a PPP default MTU of 1500 bytes. That's the > > default. You could set it higher, probably, but why bother if > > the end stations aren't sending anything bigger? > > _______________________________ > > > > Priscilla Oppenheimer > > www.troubleshootingnetworks.com > > www.priscilla.com > > > > > > > > > > > > > > > > > > --------------------------------- > > > Do you Yahoo!? > > > U2 on LAUNCH - Exclusive medley & videos from Greatest Hits > CD > > > > > > > > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57653&t=57322 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
