Thoughts on increasing MTUs on the internet

Thu Apr 12 15:56:54 UTC 2007

On Apr 12, 2007, at 10:04 AM, Gian Constantine wrote:

> I agree. The throughput gains are small. You're talking about a  
> difference between a 4% header overhead versus a 1% header overhead  
> (for TCP).

One of the "benefits" of larger MTU is that, during the additive  
increase phase, or after recovering from congestion, you reach full  
speed sooner --  it does also mean that if you do reach congestion,  
you throw away more data, and, because of the length of flows, are  
probably more likely to cause congestion...

>
> One could argue a decreased pps impact on intermediate systems, but  
> when factoring in the existing packet size distribution on the  
> Internet and the perceived adjustment seen by a migration to 4470  
> MTU support, the gains remain small.t

>
> Development costs and the OpEx costs of implementation and support  
> will, likely, always outweigh the gains.
>
> Gian Anthony Constantine
>
>
> On Apr 12, 2007, at 7:50 AM, Saku Ytti wrote:
>
>>
>> On (2007-04-12 11:20 +0200), Iljitsch van Beijnum wrote:
>>
>>> What do you guys think about a mechanism that allows hosts and
>>> routers on a subnet to automatically discover the MTU they can use
>>> towards other systems on the same subnet, so that:
>>> 1. It's no longer necessary to limit the subnet MTU to that of the
>>> least capable system
>>>
>>> 2. It's no longer necessary to manage 1500 byte+ MTUs manually
>>
>> To me this sounds adding complexity for rather small pay-off. And
>> then we'd have to ask IXP people, would the enable this feature
>> if it was available? If so, why don't they offer high MTU VLAN
>> today?
>> And in the end, pay-off of larger MTU is quite small, perhaps
>> some interrupts are saved but not sure how relevant that is
>> in poll() based NIC drivers. Of course bigger pay-off
>> would be that users could use tunneling and still offer 1500
>> to LAN.
>>
>> IXP peeps, why are you not offering high MTU VLAN option?
>> From my point of view, this is biggest reason why we today
>> generally don't have higher end-to-end MTU.
>> I know that some IXPs do, eg. NetNOD but generally it's
>> not offered even though many users would opt to use it.
>>
>> Thanks,
>> -- 
>>   ++ytti
>

--
Some people are like Slinkies......Not really good for anything but  
they still bring a smile to your face when you push them down the  
stairs.

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