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

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

On 9 feb 2010, at 19:09, Dan Wing wrote:

> Which is why tweaking the TCP MSS is a helpful workaround
> to blocked ICMP PTB.

There very little need to do anything to the MSS option, see my previous message.

Only in the case where the IPv6 host supports an MTU larger than 1500 but the translator doesn't, there is a possible optimization by having the translator rewrite the MSS to the maximum that the translator supports so the translator wouldn't have to send too bigs and there is less risk of PMTUD black holes. However, it's not clear to me that this case is worth the trouble.

> But tweaking TCP MSS restricts us to 
> the lowest common denominator (that is, limiting TCP MSS 
> means we can't expect to use larger-than-1500 frames).

Obviously you would be advertising the minimum of what the IPv6 host and the translator can handle and not simply hardcode 1500. If both do more than 1500 bytes then you advertise more than 1500 bytes and you get to use more than 1500 bytes. Note that this won't trigger any black holes because the IPv4 hosts will send 1500-byte packets. (The intersection between users with broken PMTUD setups and users with an MTU larger than 1500 is rather small.)