Subject: MTU on a Link Local Address.

From: [email-masked]
Date: Thu, 14 Sep 2006 10:02:02 -0400

Hi all, as we all know Link Local addresses and communicating through them are a must in an IPv6 environment. Without it Neighbor Discovery will not work and so address resolution.
Our system enables one to set an MTU for and IPv6 address. So it is very much possible to set the MTU also for the Link Local IPv6 address.
The question I have for this forum is what is the MTU value that should be set for this Link Local address?

Possibilities:

1.	1280 --- the reason is that this value would guarantee a Neighbor Discovery operation since all devices must support at least this MTU.
2.	The MTU of NIC device associated with this Link Local --- All of the Neighbor Discovery messages (even the future ones) are guaranteed to be less than 1280 so I shouldn't worry about it.
3.	???

Thanks for your answers!
Shuki 

Shuki Sasson 
Principal Engineer, Network Storage Group 
EMC² 
where information lives
Fax: 508 305 9026   
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Email: [email-masked] 

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From: Mark Smith <[email-masked]>
Date: Thu, 14 Sep 2006 23:57:22 +0930

> 2.	The MTU of NIC device associated with this Link Local --- All of the Neighbor Discovery messages (even the future ones) are guaranteed to be less than 1280 so I shouldn't worry about it.

I'd think it would be the maximum size supported by all devices
attached to the segment. Remember, link local addressing may not be
just used for ND, but also any other traffic*, so anything that will
make the link work most efficiently would be ideal. Typically that
would be the largest MTU capable of being sent and received by all
devices on the link. 

Regards,
Mark.

*e.g. telnetting/ftp to a remote router at the other end of a link
where the router has no other addressing assigned other than LL.

---

From: [email-masked]
Date: Thu, 14 Sep 2006 10:41:34 -0400

Hi Mark, the setting of MTU to 1280 will not block larger packets from
coming in (this will be limited by the physical device capability) but
will force the packets sent through that IP to be fragmented to a
maximum of 1280 size packets. The only issue is that we are loosing
performance by setting it to the lowest value. With setting it to a
higher MTU we risk that Neighbor Discovery will not work properly with
devices that support 1280 only. It seems to me that one wants to make
sure that ND is always working properly...
Do you still maintain your recommendation?

Thanks,
Shuki

-----Original Message-----
From: Mark Smith
[mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 10:27 AM
To: sasson, shuki
Cc: harris, arthur; ting, dennis; viswanadha, kamakshi; [email-masked];
nathanson, daphna
Subject: Re: MTU on a Link Local Address.

On Thu, 14 Sep 2006 10:02:02 -0400
[email-masked] wrote:

> Hi all, as we all know Link Local addresses and communicating through
them are a must in an IPv6 environment. Without it Neighbor Discovery
will not work and so address resolution.
> Our system enables one to set an MTU for and IPv6 address. So it is
very much possible to set the MTU also for the Link Local IPv6 address.
> The question I have for this forum is what is the MTU value that
should be set for this Link Local address?
> 
> Possibilities:
> 
> 1.	1280 --- the reason is that this value would guarantee a
Neighbor Discovery operation since all devices must support at least
this MTU.
> 2.	The MTU of NIC device associated with this Link Local --- All of
the Neighbor Discovery messages (even the future ones) are guaranteed to
be less than 1280 so I shouldn't worry about it.

Regards,
Mark.

---

From: [email-masked]
Date: Thu, 14 Sep 2006 11:00:27 -0400

Thanks!
Shuki

-----Original Message-----
From: Suresh Krishnan [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 11:02 AM
To: sasson, shuki
Cc: [email-masked]; harris, arthur; [email-masked]; viswanadha, kamakshi; nathanson, daphna
Subject: Re: MTU on a Link Local Address.

Hi Sasson,
   It should be 2. the MTU of the NIC device on the link (assuming it is 
properly configured). You can certainly use 1280 bytes as an MTU but if 
you link can support substantially larger MTU, it is inefficient to use 
1280.

Cheers
Suresh

---

From: Suresh Krishnan <[email-masked]>
Date: Thu, 14 Sep 2006 11:02:18 -0400

Hi Sasson,

Cheers
Suresh

---

From: Mark Smith <[email-masked]>
Date: Fri, 15 Sep 2006 01:10:29 +0930

On Thu, 14 Sep 2006 10:41:34 -0400
[email-masked] wrote:

> devices that support 1280 only. 

Well, in that case, you'd have the problem that my last sentence
avoids ...

"Typically that would be the largest MTU capable of being sent and
received by all devices on the link."

In other words, the smallest Maxiumum Recieve Unit value of the any of
the devices attached to a link/segment constrains all other devices to
being able to send frames no larger than that, despite possible
capabilities otherwise. A good example being an FastEthernet interface
only capable of receiving 1500 byte frames being attached to a GigE
segment with where all the GigE devices (NICs & Switch(es)) being
capable of potentially much larger MTUs. In that case, the "link's
MTU/MRU" is 1500 bytes, no larger.

The only solution at this time to getting your GigE large MTUs back is
to put a router between the FE and GigE devices, which seems to me to
be a bit of a "sledge hammer" approach. Unfortunately at this time, the
sledge hammer is the only tool we've got available. I think it would be
nice if there was some IPv6 mechanism, probably within ND, where
sending hosts could be told "for this neighbour, or for this on-link
prefix, only send frames up to this size". Some of the feasibility of
the per-neighbour MTU was discussed quite a bit on the list a while
back. 

I wonder if the grouping together devices with the same MTU/MRU
capabilities into different on-link prefixes, and then having a
per-prefix MTU would overcome some of the per-neighbour MTU issues
identified at that time.

>It seems to me that one wants to make

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 14 Sep 2006 09:00:01 -0700

Don't forget though that tunnels are also links with link-local
addresses and there may be arbitrary physical link technologies
(with heterogeneous MTUs) on the paths between tunnel endpoints.

All links are required to support a minimum MTU of 1280 bytes for
IPv6, and this is no different for tunnels. But, larger MTUs can
be realized if the tunnel endpoints engage in a link adaptation
scheme with tunnel endpoint-based segmentation/reassembly and
dynamic segment size probing; see:

http://bgp.potaroo.net/ietf/idref/draft-templin-linkadapt/

Fred
[email-masked]

-----Original Message-----
From: Suresh Krishnan [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 8:02 AM
To: [email-masked]
Cc: [email-masked]; [email-masked]; [email-masked]; [email-masked]; [email-masked]
Subject: Re: MTU on a Link Local Address.

Hi Sasson,

Cheers
Suresh

---

From: [email-masked]
Date: Thu, 14 Sep 2006 12:06:14 -0400

Thanks for your answer!
Shuki

-----Original Message-----
From: Templin, Fred L [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 12:00 PM
To: Suresh Krishnan; sasson, shuki
Cc: harris, arthur; [email-masked]; viswanadha, kamakshi; [email-masked]; nathanson, daphna
Subject: RE: MTU on a Link Local Address.

Fred

-----Original Message-----
From: Suresh Krishnan [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 8:02 AM
To: [email-masked]
Cc: [email-masked]; [email-masked]; [email-masked]; [email-masked]; [email-masked]
Subject: Re: MTU on a Link Local Address.

Hi Sasson,

Cheers
Suresh

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 14 Sep 2006 09:05:10 -0700

> per-prefix MTU would overcome some of the per-neighbour MTU issues

Well, fe80::/10 is an on-link prefix and in some use-cases
there might be no other on-link prefixes. Were you thinking
to possibly carve more-specific prefixes out of fe80::/10?

Fred

---

From: [email-masked]
Date: Thu, 14 Sep 2006 12:06:14 -0400

Thanks for your answer!
Shuki

-----Original Message-----
From: Templin, Fred L [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 12:00 PM
To: Suresh Krishnan; sasson, shuki
Cc: harris, arthur; [email-masked]; viswanadha, kamakshi; [email-masked]; nathanson, daphna
Subject: RE: MTU on a Link Local Address.

Fred

-----Original Message-----
From: Suresh Krishnan [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 8:02 AM
To: [email-masked]
Cc: [email-masked]; [email-masked]; [email-masked]; [email-masked]; [email-masked]
Subject: Re: MTU on a Link Local Address.

Hi Sasson,

Cheers
Suresh

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 14 Sep 2006 09:05:10 -0700

Fred

---

From: Elwyn Davies <[email-masked]>
Date: Thu, 14 Sep 2006 18:11:12 +0100

> 2.	The MTU of NIC device associated with this Link Local --- All of the Neighbor Discovery messages (even the future ones) are guaranteed to be less than 1280 so I shouldn't worry about it.
Until it has heard from a router connected to a link a host should 
assume the MTU is either
1. The fixed well-known MTU associated with the type of link with which 
the link local address is associated, if it has one (e.g., Ethernet), or
2.  The guaranteed minimum MTU (1280 bytes) otherwise, where the link 
type doesn't have a fixed MTU or if it is easier to decide later

The node cannot go wrong by initially assuming an MTU of 1280 bytes if 
it prefers since the first few neighbor discovery etc packets are 
generally quite small.

Assuming there is a router connected to the link and the link is of a 
type that doesn't have a predetermined MTU (like token ring) or there 
are bridged heterogeneous link layers (or just 'cos the administrator 
thinks it is a good idea), the Router Advertisement should contain an 
MTU option.  Once this has been received, the host knows what MTU to use 
on that link.

If it turns out that this is an isolated link with no routers or the 
router doesn't provide an MTU option, the host can carry on with its 
original choice, or if it started with 1280 bytes and knows the fixed 
link type MTU it can use that value in future.

See RFC2461 (soon to be superseded by draft-ietf-ipv6-rfc2461bis-08 - 
but that doens't alter any of this).

Regards,
Elwyn

---

From: "Kayumba .H Thierry" <[email-masked]>
Date: Thu, 14 Sep 2006 19:21:09 +0000 (GMT)

hello all,
1.can you help me with theoritical concepts and terminology related to IPv6?
 
2.can you also help me with configuration's notes of transtions strategies 
(dual stack and tunneling )

Cheers
 
----- Message d'origine ----
De : Elwyn Davies <[email-masked]>
À : [email-masked]
Cc : [email-masked]; [email-masked]; [email-masked]; [email-masked]; [email-masked]
Envoyé le : Jeudi, 14 Septembre 2006, 6h11mn 12s
Objet : Re: MTU on a Link Local Address.

Regards,
Elwyn

---

From: "Kayumba .H Thierry" <[email-masked]>
Date: Thu, 14 Sep 2006 19:37:38 +0000 (GMT)

hi to all
how to test the features and the perfomance of IPv6 ?
 
Regards.

---

From: Tim Chown <[email-masked]>
Date: Fri, 15 Sep 2006 09:35:13 +0100

>    how to test the features and the perfomance of IPv6 ?

Have a look at: http://www.ipv6ready.org/, or the ETSI IPv6 Plugfests, 
next one here: http://www.v6summit.com/Plugtests/framework.htm

-- 
Tim/::1

---

From: "Su Thunder" <[email-masked]>
Date: Fri, 15 Sep 2006 14:12:33 +0000

which feature and performance of IPv6 do you want to test?
IPv6 is a huge conception containing numeral issues.

>From: "Kayumba .H Thierry" <[email-masked]>
>Reply-To: "Kayumba .H Thierry" <[email-masked]>
>To: Elwyn Davies <[email-masked]>, [email-masked]
>CC: [email-masked], 
[email-masked],[email-masked], [email-masked], 
[email-masked]
>Subject: features of IPv6
>Date: Thu, 14 Sep 2006 19:37:38 +0000 (GMT)
>
>hi to all