Subject: [Softwires] MTU problem in DS-lite

From: Zhen Cao <[email-masked]>
Date: Sun, 16 Aug 2009 15:56:08 +0800

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

---

From: "Lee, Yiu" <[email-masked]>
Date: Sun, 16 Aug 2009 12:34:15 -0400

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 ³could² 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" <[email-masked]> wrote:

> _______________________________________________
> Softwires mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/softwires
>

---

From: Zhen Cao <[email-masked]>
Date: Mon, 17 Aug 2009 09:42:16 +0800

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<[email-masked]> wrote:
> For TCP, we “could” potentially avoid fragmentation by modify MSS option.

---

From: "Lee, Yiu" <[email-masked]>
Date: Sun, 16 Aug 2009 22:29:09 -0400

This is *not* recommended because it will require the v4 host to know about
ds-lite. This won¹t work for home router model where the hosts behind the
ds-lite home router won¹t know about ds-lite. Besides, the v4 packet isn¹t
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" <[email-masked]> wrote:

>> > However, we were required by the Chairs to remove this optimization from >>
the

---

From: "Durand, Alain" <[email-masked]>
Date: Mon, 17 Aug 2009 08:43:20 -0400

> MTU issue? Thanks for any explanation.

Zhen,

The Œgeneric¹ way is as described in the draft & RFC2473, ie encapsulate the
IPv4 datagram into an IPv6 datagram that happens to be too big to be
transmitted and the fragment the IPv6 datagram. Reverse the procedure at the
end of the tunnel.

   - Alain.

---

From: Zhen Cao <[email-masked]>
Date: Mon, 17 Aug 2009 21:48:36 +0800

Hi Yiu,

Thanks for your message. In this sense, any methods except
fragmentation and assembly are optimization and up to implementation.
But it is not a bad idea to introduce some suggestion for
implementation in the draft, so keeping the text for TSS option and
others is also reasonable.

Best regards,
Zhen

On Mon, Aug 17, 2009 at 10:29 AM, Lee, Yiu<[email-masked]> wrote:
> 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

---

From: Zhen Cao <[email-masked]>
Date: Mon, 17 Aug 2009 21:49:29 +0800

Got it. Thanks, Alain.

Best Regards,
Zhen

On Mon, Aug 17, 2009 at 8:43 PM, Durand,
Alain<[email-masked]> wrote:
> The ‘generic’ way is as described in the draft & RFC2473, ie encapsulate the

---

From: "Lee, Yiu" <[email-masked]>
Date: Mon, 17 Aug 2009 09:52:27 -0400

We have many discussions in the IETF-75 meeting about optimizing
fragmentation. In the end, the group agreed that this draft isn¹t the best
place to discuss the optimization. For fragmentation problem shares among
all the tunneling protocols, this is not unique to ds-lite technology. The
chairs suggested we should produce another draft which only discusses
fragmentation strategy for tunnel protocols.