Re: [OSPF] What is the use of MTU field in DD packet

Hi Acee,

I disagree about the "original intent" of the MTU field. As I see 
it, it's function is to prevent an OSPF adjacency from forming 
over a link where the endpoints disagree about the link MTU. We do 
this primarily to prevent the data plane from using a link that 
will drop packets sent to a system with an MTU smaller than ours.

While OSPFv3 certainly needs to know the IPv6 link MTU when 
building it's packets, this information should be available 
locally without reference to the MTU field in the DBD packet.

So, I would argue that in af-alt the MTU in the DBD packet should 
be for the address family we are routing, not IPv6 in all cases.

Regards,
Paul

Acee Lindem wrote:
> Hi Prasanna,
> 
> On May 28, 2008, at 8:18 AM, Prasanna Kumar A.S wrote:
> 
>> Hi
>>   I just wanted to understand what the primary use of exchanging  
>> MTU in
>> DD packets and doing MTU-check is? Is it only for the control plane or
>> is it for the DATA-plane?
> 
> Control-plane - when sending DD, LSR, and LSU packets, OSPF will  
> attempt to send as many LSA headers or complete LSAs as will fit in a  
> maximum sized packet.
> 
> 
>> Why I am getting this doubt is, in draft-ietf-ospf-af-alt-06.txt  
>> doesn't
>> specify which MTU we should use while exchanging the DD packet for the
>> ipv4-unicast or ipv4-mutlticast Address-family, is it ipv6-mtu or
>> ipv4-mtu?
> 
> We have this clarified in the an update which we post soon. Since  
> this is OSPFv3 which using IPv6 for transport, you always use the  
> IPv6 MTU.
> 
> Thanks,
> Acee
> 
> 
>>
>> Regards
>> Prasanna
>> _______________________________________________
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>> OSPF@ietf.org
>> https://www.ietf.org/mailman/listinfo/ospf
> 
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