hmm. dumb question, wearing "participant" hat...fragmentation is done end to end in IPv6; the network doesn't fragment. Ultimately, the question for the application is what packet size will work to get its data end to end, of which the tunnel is just a part of the path. The big difference in doing this end to end and on the tunnel is that on the tunnel a packet-too-large ICMP goes to the tunnel endpoint and not the originator of the message, so that a per-potential-destination route MTU could be maintained, but that only goes to the actual originator when the originator fires off a message.
It seems to me more practical to advise applications to (a) respond to ICMP packet-too-huge by reducing their MSS, and to provide some tool that would measure goodput rates such that if goodput is zero but ICMP is not getting back to follow a heuristic to Matt's pmtud. that has several effects. First, it is consistent with the end2end principal, which suggests that one should not implement at a lower layer something that already has to be supplied end to end; doing this in the tunnel is a duplicative mechanism. Second, it generalizes; on any path, there is a least MTU en route and tyerefore a packet size that will work. It is IMHO up to the application, if it wants its transmissions to work, to find that packet size among other things.
On Oct 6, 2005, at 8:46 AM, Templin, Fred L wrote:
Fred,I'm not sure I exactly understand your question, but the requirement is for a path MTU assurance mechanism that is employed *within* the tunnel to ensure that packets no larger than the tunnel MTU make it through to the other side of the tunnel, i.e., the mechanism would act only on one hop of what may be multiple hop path. The path MTU method outlined in Matt Mathis' draft is employed at packetization layers *above* the tunnel to determine the MTU of the path that may extend many hops beyond the other side of the tunnel, and I agree that the algorithm could be used for protocols other than TCP.The algorithm used by the tunnel MTU assurance mechanism might be quite similar to that described in Matt's draft, but the use case and implementation are different. In fact, it could happen that implementations of Matt's packetization layer path MTU scheme and a tunnel MTU assurance mechanism would occur in the same physical box and would act at independently of each other.Fred fred.l.templin@boeing.com-----Original Message----- From: Fred Baker [mailto:fred@cisco.com] Sent: Wednesday, October 05, 2005 5:37 PM To: Templin, Fred L Cc: v6ops@ops.ietf.org; Matt Mathis Subject: Re: Requirements for IP-in-IP Tunnel MTU Assurance ah. ok.so, dumb question of the month. are you looking for a simple protocol that can be run over such a tunnel (including not only ip/ ip, butGRE, IPSEC, etc) to determine the pmt of the tunnel? If so, would something along the lines of pmtu accomplish it? As in, it seems to me that the algrorithm is funcdamentally useful for things other than TCP.On Oct 5, 2005, at 2:05 PM, Templin, Fred L wrote:Fred,Yes, I know about Matt Mathis' work on Packetization Layer Path MTU Discovery (PLPMTUD) but that work covers packetization layers above layer 3 and is independent of the requirement to provide an assured MTU below layer 3 for IP/IP tunnels.To your question, the requirement is for an assured MTU for IP/IP tunnels, i.e., when the tunnel MTU is X bytes then upper layer protocols can be assured that packets no larger than X will traverse the tunnel under normal circumstances or a layer 3 "packet too big" message will be returned that informs upper layers of a smaller MTU. For tunnels over IPv4, if no other mechanism were provided then the only assured MTU that could be offered would be 68 bytes since RFC 791 specifies that as the minimum link MTU for IPv4. But, IPv6 needs to see an assured MTU of 1280 bytes so some form of MTU assurance for tunnels is needed.Fred fred.l.templin@boeing.com-----Original Message----- From: Fred Baker [mailto:fred@cisco.com] Sent: Wednesday, October 05, 2005 1:34 PM To: Templin, Fred L Cc: v6ops@ops.ietf.org; Matt Mathis Subject: Re: Requirements for IP-in-IP Tunnel MTU AssuranceI imagine you have read http://www.psc.edu/~mathis/MTU/pmtud/ draft- mathis-pmtud-method-00.txt. The fundamental algorithm is that the sending TCP starts from the negotiated MSS (or should transmission suddenly stop being successful, potentially due to a route change that reduces the real path MTU from the current message size it is sending) and reduces its segment size until succeeds in getting a packet through, and then periodically attempts to increase the message size in order to take advantage of capacity that might become available as routes shift. It seems that this fundamental approach would address your issues. Does it?As to the efficiency of file transfer, yes, if the header overhead is of fixed size, carrying more payload will increase efficiency. ie, if the IPv6 header is 40 bytes and the TCP header is 20, and the IPv6 MTU is 1500 bytes, the maximum payload we can carry is 1440 bytes, or 96%, and being able to switch that to a 9180 byte IPv6 MTU would change the efficiency to 99.3% and reduce the interrupt load an the two hosts by a factor of about 9180/1500. Changing from 1280 to 1500 bytes, however, is a change from 95.3% to 96% efficiency, which seems relatively minor, and switching from 1280 to 1380 bytes of payload seems even more trivial.The principal value of getting the pmtu right, it seems to me, is to first get an estimated pmtu that in fact works at all (downsize it from the negotiated MSS to a packet size that works regardless of what hiccups lie en route), and second to that, it would be really nice to minimize the interrupt load on the sending and receiving hosts in order to balance the goals of maximizing throughput and minimizing the impact on the hosts themselves. ie, if the sender and receiver are on 10/100 Ethernets and there is an IP/IP tunnel on the path, carrying a 1280 byte payload is better than carrying a 1440 byte payload because the former works and the latter doesn't, and it would be nice to be able to figure out that a 1380 byte payload would also work and be a .35% improvement in efficiency.So, question (and yes, this is a question): are you chasing a problem I don't see? What is the objective here?On Oct 5, 2005, at 9:16 AM, Templin, Fred L wrote:Fred,Please see the following draft which documents operational issues relevant to v6ops and calls for a solution: http:// www.ietf.org/internet-drafts/draft-templin-mtuassurance-00.txtPlease note that this obsoletes a previos draft called: "Requirements for Link Adaptation over IP-in-IPv4 Tunnels".Fred fred.l.templin@boeing.com-----Original Message----- From: Templin, Fred L Sent: Friday, September 30, 2005 3:40 PM To: Fred Baker Cc: v6ops@ops.ietf.org Subject: Requirements for Link Adaptation over IP-in-IPv4 Tunnels (was RE: Link Adaptation for IPv6-in-IPv4 Tunnels) Fred - please see:http://www.ietf.org/internet-drafts/draft-templin-linkadapt- reqts-00.txtFred fred.l.templin@boeing.com-----Original Message----- From: Fred Baker [mailto:fred@cisco.com] Sent: Friday, September 23, 2005 9:24 AM To: Templin, Fred L Cc: v6ops@ops.ietf.org Subject: Re: Link Adaptation for IPv6-in-IPv4 Tunnels Does it still propose a protocol or protocol change?I don't know offhand whether the charter of v6ops then precluded protocol work, and won't go into whether it was proper for [MECH] to be done in v6ops. Given the present charter, I can treat it as a requirements document that may be asking for something like the work you are proposing. But my read of the document you pointed to is that it is still proposing an incompatible change (as in "unchanged equipment will not perform the function and once the message is segmented in this fashion it must be reassembled in this fashion.I think that has to be done in a WG chartered to do non- backward-compatible changes to IPv4.On Sep 23, 2005, at 9:04 AM, Templin, Fred L wrote:Can this work be contributed as an extension to [MECH]?