Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

[Iljitsch van Beijnum <[e-mail]>]

On 9 feb 2010, at 20:10, Templin, Fred L wrote:

> Another concern I should have called out earlier is what
> if the IPv6 host is on a 1500 link and opens a TCP
> connection to the IPv4 server with MSS=1440. The IPv4
> server sends a 1480b IPv4 packet that gets translated
> into a 1500b IPv6 packet. The packet makes its way
> through the IPv6 network toward the IPv6 host, but
> first it hits a 1480 MTU link (e.g., and IPv6 over IPv4
> tunnel). The ICMPv6 PTB gets triggered, then translated
> into ICMPv4 fragmentation needed, then dropped by
> a filtering gateway in the IPv4 Internet.

> Black hole, right?

Indeed. Note however that the same black hole would exist if both ends are IPv6 or both ends are IPv4 and there is no translator. I don't think we can reasonably expect NAT64s to fix all possible black holes.

In practice, I think NAT64 users would have to avoid reduced MTUs between the translator and the IPv6 hosts (and I would assume that a deployment that needs NAT64 is far enough along that it doesn't need IPv6-in-IP tunneling) or artificially lower the IPv6 host MTU or the translator MTU / rewrite the MSS if a reduced MTU is present between the translator and the IPv6 hosts.