Subject: Re: [16ng] Re: I-D ACTION:draft-ietf-16ng-ipv6-over-ipv6cs-00.txt

From: Alexandru Petrescu <[email-masked]>
Date: Tue, 17 Oct 2006 16:29:26 +0200

Soininen Jonne (Nokia-NET/Espoo) wrote:
> Alex,
> 
> 
> On 10/17/06 4:30 PM, "ext Alexandru Petrescu" 
> <[email-masked]> wrote:
> 
>>> Haven't seen this happening in practise.. Dunno if the asymmetry
>>> e.g. in GPRS ul/dl timeslots numbers should reflect in any way to
>>> MTU sizes.. I doubt not.
>> Probably yes timeslot-MTU relationship, I have no practical way to
>> check.
> 
> There should be no relationship with the MTU to the timeslots. The
> timeslots are a "lower-layer" issue. The convergence sub-layer should
> hide these things from IP.
> 
> How did you find this issue? I would also be really interested in,
> which network this occurred. Please, feel free to contact me also
> off-list if you don't feel like sharing this info with everybody for
> whatever reason.

The timeslot potential reason of MTU asymmetry was suggested by Jouni.

How I found the MTU variance: have an IPv6 moving network behind an MR
connected to GPRS, and an IPv6 webserver on an LFN streaming MP3 towards
a client outside the UMTS network.  The MR sends IPv6-in-IPv6-in-IPv4 to
the UMTS network first-hop router.  MR's IPv4 stack receives
'packet-too-big' from the first-hop router, so it fragments the IPv4
sent packet, to a size smaller than the size of packets it receives.

In mobile WiMax or 802.16e - do we know whether the MS wireless link is
symmetric or asymmetric with respect to MTU?  Worth mentioning in the
draft?

Alex

---

From: "Soininen Jonne (Nokia-NET/Espoo)" <[email-masked]>
Date: Tue, 17 Oct 2006 17:58:37 +0300

Alex,

On 10/17/06 5:29 PM, "ext Alexandru Petrescu"
<[email-masked]> wrote:

[snip]
> The timeslot potential reason of MTU asymmetry was suggested by Jouni.

There is a remote possibility. However, this is not how GPRS was designed
and implemented in the implementations I know...

> sent packet, to a size smaller than the size of packets it receives.

Again, your packet-too-big seems to come _after_ the packet has traveled
over the GPRS network. Thus, the smaller MTU seems to be in the network
_behind_ the first-hop-router. I don't know if the packets travel a
different route to your client than coming down from the client.

If there would be a problem with the MTU of your GPRS link the packet would
not even travel to the first-hop-router (it would not fit the link MTU and
your MR would send a packet-too-big to your web server). You can check the
MTU of your GPRS interface on the MR.

Anyways, you case does not show there is an MTU-asymmetric link, but that
your packets are too big to a link in the network. It can easily happen as
you have multiple layers of tunneling.

> symmetric or asymmetric with respect to MTU?  Worth mentioning in the

It is symmetric. Anyways it does not matter. Every interface is set an MTU
(Maximum Transmission Unit) - to my understanding you don't set the
receiving side as the sending router/host is supposed to check that the
packet fits to the limits of the layer 2.

Thus, I don't think this has to be mentioned - it generally isn't.

Cheers,

Jonne.

-- 
Jonne Soininen
Nokia

Tel: +358 40 527 46 34
E-mail: [email-masked]