Hi Ron, > -----Original Message----- > From: Ronald Bonica [mailto:[email protected]] > Sent: Wednesday, July 10, 2013 10:12 AM > To: Templin, Fred L; Doug Barton > Cc: [email protected] > Subject: RE: Meta-issues: On the deprecation of the fragmentation > function > > > > > > Sure, the tunnel ingress can probe the path to the egress; such a > > probing method is already covered by SEAL. > > Most GRE implementations do this, too. > > But, if the path MTU will > > not accommodate a packet that after encapsulation is as large as > > (1280 + HLEN) there is no alternative for the ingress other than to > > start fragmenting since the ingress is not allowed to send a PTB > > message reporting a size smaller than 1280. > > I understand that you want to solve for the use-case in which a tunnel > interior link has MTU < (1280 + HLEN).
Yes; for example, a 1280 MTU tunnel crossing another 1280 MTU tunnel. > But before solving for that use- > case, we need to do a cost/benefit analysis. > > We understand the cost of solving for this use-case. The task of > reassembly is moved to the egress router. So, we need to make sure that > the egress router is large enough to handle the task of reassembly and > we need to make sure that its resources cannot be monopolized by a DoS > attack. We also have to maintain our fragmentation capability. I understand that. But a couple of points: 1) the egress will never be asked to reassemble more than (1500 + HLEN) 2) the ingress will feel the pain too and will be motivated to tune out the fragmentation > Note that some of the cost is absorbed by the owner of the egress > router. However, a portion of the cost is absorbed by the entire > community, as they deal with the operation complexity associated with > fragmentation. The community can help to get rid of fragmentation. All they have to do is configure a larger MTU on links that connect routers to other routers. The MTU should be (1500 + HLEN) or larger. > Now let's try to understand the benefit. Is there an installed base of > IPv6-capable links with MTU < (1280 + HLEN) that carry traffic between > tunnel endpoints? Is there a reason why someone might want to design a > network this way? Tunnels within tunnels, as one example. They are a common use case in operational practice today. > If we were to solve for this use-case, who would be the beneficiary? The entire Internet would benefit, since all barriers to larger MTUs would be removed. > Possibly the party deploying the MTU-challenged link? Or, asked another > way, would cost be assigned to the beneficiary? Not sure I understood that. Everyone benefits, but the tunnel endpoints doing the fragmentation have to do what is necessary to maintain the tunnel in any event. Thanks - Fred [email protected] > Ron > > > > -------------------------------------------------------------------- IETF IPv6 working group mailing list [email protected] Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
