Date: Sun, 22 Jul 2007 10:49:39 +0300 From: Artyom Viklenko <artem@aws-net.org.ua> To: Stephen.Clark@seclark.us Cc: freebsd-net@freebsd.org, Artem Belevich <fbsdlist@src.cx>, Julian Elischer <julian@elischer.org> Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <46A30C13.1030909@aws-net.org.ua> In-Reply-To: <46A2342C.1030205@seclark.us> References: <200707150237.l6F2bAgZ011098@redrock.karels.net> <469E0FFF.8070802@seclark.us> <20070720172021.8EA3D13C4B3@mx1.freebsd.org> <46A10063.9010902@elischer.org> <46A10860.50804@es.net> <ed91d4a80707201534q6e32a9c5j8803ec96fa7f6515@mail.gmail.com> <46A1BDDE.5080403@aws-net.org.ua> <46A2342C.1030205@seclark.us>
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Stephen Clark wrote:
> Artyom Viklenko wrote:
>
>> Artem Belevich wrote:
>>
>>
>>> Here's one example where MTU!=MRU would be useful.
>>>
>>> Think of asymmetric bandwith-limited ADSL links. Lower MTU would allow
>>> lower TX latency for high priority packets when upstream is saturated,
>>> yet large MRU on the downstream would be great for downloads.
>>>
>>> Right now with 6.2 one has to trade off lower latency for faster
>>> download.
>>>
>>> --Artem
>>>
>>
>> You can prioritize small packets with ACKs, for example, by other
>> techniques - ALTQ one of them.
>> Unconditional lovering MTU even on ADSL tend to loss throughtput.
>>
>> And let's think about TCP MSS. When TCP connection establishes,
>> TCP stack uses MTU as measure to choose MSS.
>>
>> Any two hosts, connected to single Layer2 network MUST use
>> same MTU. Any other cases lead to hard-to-solve problems.
>>
>> This is all IMHO. But I would not like to see different
>> MTU and MRU on my Ethernet interfaces! :)
>>
>>
>>
> Yes but the mss is what the endpoints in the connection know about their
> own mtu's,
> at this point there is no knowledge of the mtu/mru's of intermediate
> routers.
>
> Steve
>
Why? When two endpoints negotiated tcp connections they do know
about remote mss - maximum segment that remote side can receive.
PMTU is little bit anothr problem of misconfiguration on firewalls and
intermediate routers. Very common situation when users block whole ICMP
protocol on their router/host while connects to Internet via PPPoE/PPTP.
--
Sincerely yours,
Artyom Viklenko.
-------------------------------------------------------
artem@aws-net.org.ua | http://www.aws-net.org.ua/~artem
FreeBSD: The Power to Serve - http://www.freebsd.org
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