Subject: Re: "RFC 2461bis" issue: MTU handling

From: Fred Templin <[email-masked]>
Date: Tue, 28 Oct 2003 10:41:29 -0800

Erik Nordmark wrote:

>>One case in which data packets could be used as probe packets
>>is when IPv4 is used as an L2 media for IPv6. In this case, we could
>>send IPv6-in-IPv4 encapsulated packets with the DF bit NOT set
>>in the IPv4 header expecting that the decapsulator would send us
>>some sort of indication if it sensed fragmentation.
>>    
>>
>
>Yes, you have that option for the L2 which is known as IPv4.
>
>  
>
>>But this begs the question of a fundamental design point: do we need
>>to support sub-L2 media elements (i.e., the physical elements that sit
>>below IPv4) that neither support IPv4 fragmentation nor send IPv4
>>"frag needed" messages when they can't forward a packet? Based on
>>the 1Gbps/100Mbps Ethernet bridge example, I believe the answer
>>to this is "yes" - would you agree?
>>    
>>
>
>I think the high-order bit question for those L2s is first how important
>the problem is to solve, and then what simplying assumptions we can make.
>(such as "does the network admin control all the switches i.e. does s/he
>know whether all are jumbo frame capable").
>

I think you've already given a prime example in an earlier message - suppose
I slap a 10/100 bridge onto the network in my cubile w/o telling the network
admin about it? As another example, in large corporate networks the L2
infrastructure is often under the control of regional adminstrative domains
and sometimes these regional domains don't coordinate with one another as
well as they should.

So, I think the answer to your second question is that it would be a bit 
naive
to assume that any one network admin controls all the switches.

Fred
[email-masked]

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