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

Hi Paul,

On May 28, 2008, at 2:05 PM, Paul Wells wrote:

> 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.

I happen to remember the discussion of this problem on the OSPF list  
and this was not the primary motivation. There were lots of problems  
with bridged heterogeneous LANs with mismatched MTUs (ethernet, FDDI,  
token ring, and the worst of all technologies - ATM emulated  
LANs :^).  Adjacencies would come up fine initially but the exchange  
process would hang indefinitely when they were restarted due to the  
router with the larger MTU having a larger database and trying to use  
full DD packets. Unfortunately, the OSPF list was hosted on a server  
at Microsoft Corporation in those days and I don't have access to  
archives. Here is some text from RFC 2178, appendix G:

G.9 Detecting interface MTU mismatches

    When two neighboring routers have a different interface MTU for  
their
    common network segment, serious problems can ensue: large packets  
are
    prevented from being successfully transferred from one router to the
    other, impairing OSPF's flooding algorithm and possibly creating
    "black holes" for user data traffic.

    This memo provides a fix for the interface MTU mismatch problem by
    advertising the interface MTU in Database Description packets.  
When a
    router receives a Database description packet advertising an MTU
    larger than the router can receive, the router drops the Database
    Description packet. This prevents an adjacency from forming, telling
    OSPF flooding and user data traffic to avoid the connection between
    the two routers. For more information, see Sections 10.6, 10.8, and
    A.3.3.

On the other hand, once the MTU checking was implemented, I believe  
data plane MTU consistency has been purported as a benefit. If we  
used the IPv4 MTU in the IPv4 address database exchanges, we could  
still have an IPv6 MTU mismatch. One could depend on the unicast IPv6  
address family for this checking but, heretofore, we've kept the  
instances independent.

Thanks,
Acee

>
> 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
>>> _______________________________________________
>>> OSPF mailing list
>>> OSPF@ietf.org
>>> https://www.ietf.org/mailman/listinfo/ospf
>> _______________________________________________
>> OSPF mailing list
>> OSPF@ietf.org
>> https://www.ietf.org/mailman/listinfo/ospf