[Bloat] Jumbo frames and LAN buffers (was: RE: Burst Loss)

Fri May 13 18:36:41 EDT 2011

Even through jumbo frames are not standardized, most new network equipment
supports them (though generally support is disabled by default). If you IPv4
route jumbo packets to a network that doesn't support them, the router will
fragment for you. Under IPv6, it is the sender's responsibility to choose an
MTU that is supported by all networks between source and destination. IPv6
routers do no fragmentation.

Although consumer products are often dumbed down, it is not difficult to
find switches with comprehensive QoS
configurability. Weighted fair queuing is a popular scheme.
Strict priority is a bit dangerous but useful for latency-critical
applications. The IEEE has just ratified a credit-based algorithm called
802.1av. What I find is missing from all but the high-end equipment is
configurability of buffering capacity and behavior. Bad buffering can burn
an otherwise competent QoS implementation.

In his talks, Jim Gettys claims that these QoS features do not fix
bufferbloat - they just move the problem elsewhere. I generally agree with
this though I find that moving the problem elsewhere is sometimes a
perfectly acceptable solution.

Kevin Gross

On Fri, May 13, 2011 at 2:21 PM, Dave Taht <dave.taht at gmail.com> wrote:

>
>
> On Fri, May 13, 2011 at 2:03 PM, Kevin Gross <kevin.gross at avanw.com>wrote:
>
>> Do we think that bufferbloat is just a WAN problem? I work on live media
>> applications for LANs and campus networks. I'm seeing what I think could be
>> characterized as bufferbloat in LAN equipment. The timescales on 1 Gb
>> Ethernet are orders of magnitude shorter and the performance problems caused
>> are in many cases a bit different but root cause and potential solutions
>> are, I'm hoping, very similar.
>>
>>
>>
>> Keeping the frame byte size small while the frame time has shrunk
>> maintains the overhead at the same level. Again, this has been a conscious
>> decision not a stubborn relic. Ethernet improvements have increased
>> bandwidth by orders of magnitude. Do we really need to increase it by a
>> couple percentage points more by reducing overhead for large payloads?
>>
>>
>>
>> The cost of that improved marginal bandwidth efficiency is a 6x increase
>> in latency. Many applications would not notice an increase from 12 us to 72
>> us for a Gigabit switch hop. But on a large network it adds up, some
>> applications are absolutely that sensitive (transaction processing, cluster
>> computing, SANs) and (I thought I'd be preaching to the choir here) there's
>> no way to ever recover the lost performance.
>>
>>
>>
>
> You are preaching to the choir here, but I note several things:
>
> Large frame sizes on 10GigE networks to other 10GigE networks is less of a
> problem than 10GigE to 10Mbit networks. I would hope/expect that frame would
> fragment in that case.
>
> Getting to where latencies are less than 10ms in the general case makes
> voip feasible again. I'm still at well over 300ms on bismark.
>
> Enabling higher speed stock market trades and live music exchange over a
> lan would be next on my list after getting below 10ms on the local
> switch/wireless interface!
>
> A lot of research points to widely enabling some form of fair queuing at
> the servers and switches to distribute the load at sane levels. (nagle, 89)
> I think few gig+e vendors are doing that in hardware, and it would be good
> to know who is and who isn't.
>
> For example, the switch I'm using on bismark has all sorts of wonderful QoS
> features such as fair queuing, but as best as I can tell they are not
> enabled, and I'm seeing buffering in the switch at well above 20ms....
>
> It is astonishing that a switch chip this capable has reached the consumer
> marketplace...
>
> http://realtek.info/pdf/rtl8366s_8366sr_datasheet_vpre-1.4_20071022.pdf
>
> And depressing that so few of it's capabilities have software to configure
> them.
>
>> Kevin Gross
>>
>>
>>
>> *From:* Dave Taht [mailto:dave.taht at gmail.com]
>> *Sent:* Friday, May 13, 2011 8:54 AM
>> *To:* rick.jones2 at hp.com
>> *Cc:* Kevin Gross; bloat at lists.bufferbloat.net
>> *Subject:* Re: [Bloat] Burst Loss
>>
>>
>>
>>
>>
>> On Fri, May 13, 2011 at 8:35 AM, Rick Jones <rick.jones2 at hp.com> wrote:
>>
>> On Thu, 2011-05-12 at 23:00 -0600, Kevin Gross wrote:
>> > One of the principal reasons jumbo frames have not been standardized
>> > is due to latency concerns. I assume this group can appreciate the
>> > IEEE holding ground on this.
>>
>> Thusfar at least, bloaters are fighting to eliminate 10s of milliseconds
>> of queuing delay.  I don't think this list is worrying about the tens of
>> microseconds difference between the transmission time of a 9000 byte
>> frame at 1 GbE vs a 1500 byte frame, or the single digit microseconds
>> difference at 10 GbE.
>>
>>
>> Heh.  With the first iteration of the bismark project I'm trying to get to
>> where I have less than 30ms latency under load and have far larger problems
>> to worry about than jumbo frames. I'll be lucky to manage 1/10th that
>> (300ms) at this point.
>>
>> Not, incidentally that I mind the idea of jumbo frames. It seems silly to
>> be saddled with default frame sizes that made sense in the 70s, and in an
>> age where we will be seeing ever more packet encapsulation, reducing the
>> header size as a ratio to data size strikes me as a very worthy goal.
>>
>> _______________________________________________
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>> Bloat at lists.bufferbloat.net
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>>
>>
>
>
> --
> Dave Täht
> SKYPE: davetaht
> US Tel: 1-239-829-5608
> http://the-edge.blogspot.com
>
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