-----Original Message----- From: Masataka Ohta [mailto:mohta@necom830.hpcl.titech.ac.jp] Sent: Tuesday, June 05, 2012 12:42 PM To: Templin, Fred L Cc: nanog@nanog.org Subject: Re: IPv6 day and tunnels
Templin, Fred L wrote:
I am making a general statement that applies to all tunnels everywhere.
General statement?
General statement for IPv6-in-IPv4 tunneling, yes. But inner fragmentation applies equally for *-in-* tunneling.
Even though you assume tunnel MTU 1500B
What I am after is a tunnel MTU of infinity. 1500 is the minimum packet size that MUST get through. 1501+ packets are admitted into the tunnel unconditionally in hopes that they MIGHT get through.
and tunnel overhead 20B?
The size "20" represents the size of the IPv4 encaps header. The size "40" would represent the size of an IPv6 encaps header. The size "foo" would represent the size of some other encapsulation overhead, e.g., for IPsec tunnels, IP/UDP tunnels, etc. So, let the size of the encaps header(s) be "X", substitute X for 20 everywhere and you will see that the approach is fully generally applicable.
For those, specs say that all that is required for MRU is 1500 and not 1500+20.
That is a requirement for hosts with Ethernet interface, which is, by no means, general and has nothing to do with tunnels.
RFC2460 says the MinMRU for IPv6 nodes is 1500. RFC1122 says that IPv4 hosts should reassemble as much as their connected interfaces (1500 for Ethernet). RFC1812 says the MinMRU for IPv4 routers is 576.
For the general argument on tunnels, see, for example, RFC2473 "Generic Packet Tunneling in IPv6", where there is no requirement of 1500.
Note that the RFC uses outer fragmentation:
(b) if the original IPv6 packet is equal or smaller than the IPv6 minimum link MTU, the tunnel entry-point node encapsulates the original packet, and subsequently fragments the resulting IPv6 tunnel packet into IPv6 fragments that do not exceed the Path MTU to the tunnel exit-point.
Wow - that is an interesting quote out of context. The text you quoted is describing the limiting condition to make sure that 1280 and smaller get through even if the path MTU is deficient. In that case alone, outer fragmentation is needed. My document also allows for outer fragmentation on the inner fragments. But, like the RFC4213-derived IPv6 transition mechanisms treats outer fragmentation as an anomalous condition to be avoided if possible - not a steady state operational approach. See Section 3.2 of RFC4213. Thanks - Fred fred.l.templin@boeing.com
Masataka Ohta