Re: [Softwires] MTU problem in DS-lite

["Lee, Yiu" <Yiu_Lee at Cable.Comcast.com>]

Title: Re: [Softwires] MTU problem in DS-lite

This is *not* recommended because it will require the v4 host to know about ds-lite. This won’t work for home router model where the hosts behind the ds-lite home router won’t know about ds-lite. Besides, the v4 packet isn’t over-sized, it is the v6 encapsulation caused the oversized issue. So the tunnel points are responsible to handle the fragmentation.  

In the hosted model, the host is aware of ds-lite. The host can in fact reduce the v4 packet size to avoid fragmentation. This optimization is up to the implementation rather than mandated in the draft.


On 8/16/09 9:42 PM, "Zhen Cao" <caozhenpku at gmail.com> wrote:

> Hi Yiu,
>
> Thanks for your clarification.
>
> Do you consider another method that let the IPv4 packet inside the
> tunnel do fragmentation at a lower MTU (link-MTU - 40), so that the
> packet won't exceed the MTU after IPv6 header encapsulation. Then
> there is no need of IPv6 encapsulation and assembly.  I believe this
> is more cost efficient than IPv6 fragmentation and assembly.
>
> Thanks and regards,
> Zhen
>
> On Mon, Aug 17, 2009 at 12:34 AM, Lee, Yiu<Yiu_Lee at cable.comcast.com> wrote:
> > Hi Zhen,
> >
> > In general, the Tunnel-Entry Point and Tunnel-Exist Point should fragment
> > and reassemble the oversize datagram. This mechanism is transport protocol
> > agnostic and work for both UDP and TCP.
> >
> > For TCP, we “could” potentially avoid fragmentation by modify MSS option.
> > However, we were required by the Chairs to remove this optimization from the
> > draft in next update.
> >
> > Thanks,
> > Yiu
> >
> >
> > On 8/16/09 3:56 AM, "Zhen Cao" <caozhenpku at gmail.com> wrote:
> >
> > Hi Alain and All,
> >
> > I have a question on MTU issue in ipv4-in-ipv6 softwire. I notice
> > Sec.10.2 of DS-Lite draft has discussed the MTU problem. The draft
> > introduces one possible way of using TCP MSS option to avoid IP layer
> > fragmentation and reassembly. It is a good idea but how about the case
> > for UDP sockets? I suppose there should be a general way to handle the
> > MTU issue? Thanks for any explanation.
> >
> > Thanks and regards,
> > Zhen
> > _______________________________________________
> > Softwires mailing list
> > Softwires at ietf.org
> > https://www.ietf.org/mailman/listinfo/softwires
> >
> >

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