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

["Dan Wing" <[e-mail]>]

 

> -----Original Message-----
> From: Iljitsch van Beijnum [mailto:[e-mail]] 
> Sent: Tuesday, February 09, 2010 10:18 AM
> To: Dan Wing
> Cc: 'Templin, Fred L'; [e-mail]
> Subject: Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
> 
> 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.

There are long-standing desires to MTUs larger than 1500
with IPv6, under the theory that IPv6 Did Things Correctly.

-d


> > 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.)=