Re: [6bed4-devel] Tunnel MTU fixed at 1280
zeroconfig IPv6 tunnel
Status: Beta
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vanrein
From: Rick v. R. <ri...@op...> - 2011-11-30 10:04:00
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Hello Timothy, Welcome :) > > I don't see a benefit in deviating from the IPv6 protocol here and would say a minimum MTU of 1280 is required. If the MTU is too large, let the v6 layer handle the problem. > > That could lead to a relay reassembling IPv4 packets before forwarding > them, the IPv6 MTU should be limited to IPv4 MTU-overhead. Packet fragmentation at the IPv4 layer is always possible. IPv4 was dimensioned for packets up to 576 bytes (after reassembly) as per RFC 791 and may quietly drop larger ones. This deviates from current practice, so it is a bit of a theoretical problem. What this means however, is that an MTU in excess of 576 is never safe to assume; IPv6 on the other hand, requires a minimum MTU of 1280. Combining these spells disaster for tunnels -- in theory. In practice, this should not be much of a problem. Virtuall all current Internet is run over MTU 1500 networks, so fragmentation at the IPv4 level would be highly exceptional. Fragmantation can occur within IPv6, even after path discovery. This is the result of routing dynamicity. It is a risk that every IPv6 layer should take into account. But it is also true that 6bed4 increases that dynamicity. > The client > MUST perform IPv4 MTU discovery reliably, this is difficult given some > NAT routers don't issue ICMP Fragmentation Needed errors, but instead > adjust the TCP MSS. You are saying that the 6bed4 tunnel would have to perform IPv4 MTU discovery if it were to support higher MTU's for IPv6, right? Are you suggesting that 6bed4 needs to do more than merely pass the MTU discovery attempts from a using IPv6 layer? > The route from the relay to client is more difficult, IPv4 MTU discovery > by the relay is impossible, and there is a risk that packets from > different relays will have different identification fields and mixed up > on reassembly. Perhaps the client could respond to IPv4 fragments with > IPv6 Too Big. I'm afraid that IPv4-level fragmentation, which can arise as soon as a path element has an MTU below 1500, is a fact of life that cannot be avoided. I don't think this problem can be solved in the tunnel. It is one of the ways of saying "native IPv6 is better". Cheers, -Rick |