Re: [v4tov6transition] [Softwires] 6a44 MTU issues

["Templin, Fred L" <[e-mail]>]

Hi Remi, 

> -----Original Message-----
> From: Rémi Després [mailto:[e-mail]] 
> Sent: Thursday, October 14, 2010 12:09 AM
> To: Templin, Fred L
> Cc: Softwires; [e-mail]
> Subject: Re: [Softwires] [v4tov6transition] 6a44 MTU issues
> 
> 
> Le 13 oct. 2010 à 22:04, Templin, Fred L a écrit :
> >>>> ...
> >>>> If IPv6 packets longer than 1280 would be accepted by 6a44 
> >>>> servers, hosts could receive them in fragmented IPv4 datagrams.
> >>> 
> >>> Or they might be reassembled in the NAT(s) in front of
> >>> the host.
> >> They would indeed.
> >> But they would then be forwarded across the customer network.
> >> There, they may somewhere be fragmented to fit into fragments 
> >> shorter than the ISP MTU + 28.
> >> This happens for example if the ISP IPv6 MTU would be 1500-28 
> >> and that of some link in the customer site would be 1500-40 
> >> (say, for an IPv4 in IPv6 tunnel). 
> >> Right?
> > 
> > The fact of life is that we have the ISP managed network
> > domain and the end user network unmanaged network domain,
> > whether the ISP controls the CPE or not. The managed domain
> > should be well-behaved, but any manner of MTU irregularities
> > is possible within the unmanaged domain. For example, I can
> > login to my Linksys home router and manually set the MTU to
> > any value I want.
> > 
> > Anything that can be configured can be mis-configured, and
> > the ISP has no control of any misconfigurations that might
> > occur in the end user network. As far as the ISP can tell,
> > the end user network is just a black hole that silently
> > consumes packets. 
> 
> The point isn't about misconfigurations.
> It is that stateless tunnels (i.e. without SEAL ...) are 
> legitimate within customer sites.
> They can lead some intra-site IPv4 PMTUs that are less than 
> 1500, and even less than 1500 - 28.

No; the point is that (without a deterministic MTU discovery
scheme) you can't control the few end user networks that set
a 1308 MTU so the vast majority of end user networks that
set a 1500+ MTU are made to suffer.

Fred
[e-mail]

> Then, because the 6a44 design privileges robustness, it is 
> better to avoid more constraints on intra-site PMTUs than 
> those that are absolutely mandatory.
> In order to statelessly support IPv6/UPD/IPV4 packets 
> intra-site IPv4 PMTUs of 1280 + 8 + 20 = 1308 octets IS the 
> obligation (one that should be satisfied with all realistic 
> stateless tunnels in customer sites).
> 
> RD
> 
> 
> 
>