Re: [v6ops] proposed TCP MSS text for rfc6204bis

[Warren Kumari <[e-mail]>]

On May 22, 2012, at 11:42 AM, Rémi Després wrote:

> Hi Warren,
> 
> Le 2012-05-21 à 16:49, Warren Kumari a écrit :
> 
>> [ Top posting / meta questions ]
>> 
>> So, both my memory and my google-foo are failing me…
>> 
>> Can anyone remember *why* the v6 in MTU is 1280 (and please don't say "Because the RFC says so!"…)
> 
> In the excellent French book in which I learned IPv6 in 2003 (livre.g6.asso.fr/index.php/Format_du_paquet_IPv6), here is what I learned:
> "Choice of 1280 as minimum MTU is to permit IPv6 tunneling. 1500 octets is the generally admitted link MTU, that imposed by Ethernet." 
> 
> As this permits multiple tunneling layers without fragmentation on the most typical links, and limits to a moderate ratio waste of bandwidth on 1500-octet remote links, this is IMHO a valuable choice.

Yes,  I get that it is to accommodate tunnel overhead (and still fit in an Ethernet frame),  I was more wondering why the overhead was chosen as 220 bytes and not 216 or 247 or 196.324.

I received this off-list (including with permissions):

"Hi Warren,
    Off-list since I cannot find a definitive reference at this point.  The 1280 number was codified in RFC 2460 based on a back of the envelope calculation by Steve Deering either before or during the Fall 1997 IETF meeting.  Starting with the Ethernet MTU of 1500, he subtracted the size of : ethernet header, IPv6 header, IPv4 header (tunneling), and several possible extension headers.  That put the number at 1280 (with some rounding to a 64-bit boundary)."


W


> 
> RD
> 
> 
> 
>> I have some niggling voice in the back of my head making 3GPP noises, but…
>> 
>> W
>> 
>> On May 20, 2012, at 10:04 AM, Mark Andrews wrote:
>> 
>>> 
>>> In message <[e-mail]>, Brian E Carpenter writes:
>>>> On 2012-05-19 22:26, Randy Bush wrote:
>>>>>> If you want to send packets of arbitrary size, in any environment
>>>>>> where PMTUD is impossible or fails, won't you need to always include
>>>>>> a fragmentation header in every packet greater than 1280?
>>>>> 
>>>>> see discussion of jumbo frames, commonly 4k or 9k, between consenting
>>>>> adults on known links
>>>> 
>>>> Yes indeed, but that isn't the general case. Across the open Internet,
>>>> I think we have the situation I described.
>>>> 
>>>> On 2012-05-19 21:16, Gert Doering wrote:
>>>> 
>>>>>>> UDP packets larger than 1280 bytes
>>>>>>> Don't do that!
>>>>> 
>>>>> Tell that to the DNS people.  They seem to really like not-using-TCP.
>>>> 
>>>> Yes, but I understand that DNSSEC more or less dooms that plan anyway.
>>>> 
>>>> However, I thinks it's true that the only fail-safe solution is to
>>>> include a frag header if you need to send UDP >1280.
>>>> 
>>>> Brian
>>> 
>>> For DNS we just fragment at 1280 using IPV6_USE_MIN_MTU.   We were
>>> thinking about this back in 1998 (draft-ietf-ipngwg-bsd-frag-00.txt)
>>> which was rolled into the advanced socket api.  It took a few more
>>> years than I would have liked to become RFC and for implementations
>>> to be available.  EDNS was already being developed back then and
>>> it was obvious that PMTUD wouldn't work for large nameservers even
>>> if they got the ICMPv6 PTBs.
>>> 
>>> For DNS there is little to be gained by trying to send any bigger
>>> packets.
>>> 
>>> YMMV for other UDP based protocols.
>>> 
>>> Mark
>>> 
>>>>  Brian
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>>> -- 
>>> Mark Andrews, ISC
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>> 
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