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Re: Question on flow control for a single file transfer

From: Roberto Peon <grmocg@gmail.com>
Date: Sun, 3 Nov 2013 20:49:29 -0800
Message-ID: <CAP+FsNenB99MhGqywpnum6RYGwNJ1kFre_ZcROA4T2Szo+EiJQ@mail.gmail.com>
To: Peter L <bizzbyster@gmail.com>
Cc: HTTP Working Group <ietf-http-wg@w3.org>, Will Chan <willchan@chromium.org>, Yoav Nir <ynir@checkpoint.com>, Martin Thomson <martin.thomson@gmail.com>
Peter, this isn't true. Servers care about things other than maximizing bdp
to a single connection, and in general care about performance across more
connections.

Speaking as someone who has some implementation experience at scale, when
the receiver asserts flow control or other policy, and the csender fails to
respect it, it will be assumed to be malicious and the connection is far
likelier to be terminated.

Resource utilization constraints exist to constrain utilization. Doing so
can impact bandwidth utilization, true, but sometimes this is exactly what
is intended as a tradeoff for some other property, e.g. maximizing
utilization of a more contended resource.

The fact that this contention is not seen by the client doesn't mean
anything. There is almost alwatlys more than one client or client
connection. I do not want to be forced to override tcp's auto buffer
estimating by imposing a max window in the implementation, as that will
impact latency later, but I also don't want huge buffers in kernel space
soaking up precious memory.
-=R
On Nov 3, 2013 8:06 PM, "Peter Lepeska" <bizzbyster@gmail.com> wrote:

> If a receiver cannot absorb any more data, it will not make a buffer
> available to TCP.
>
> Don't forget that in HTTP 1.x we don't do flow control. We leave that to
> the transport layer and this works well. Layering flow control on top of
> flow control can only result in slower flows. This slowdown is necessary
> when two or more streams are being sent at once but let's not take this hit
> in the simple case of one stream.
>
> Peter
>
> On Sunday, November 3, 2013, William Chan (陈智昌) wrote:
>
>> http://en.wikipedia.org/wiki/Flow_control_(data) says "In data
>> communications, flow control is the process of managing the rate of data
>> transmission between two nodes to prevent a fast sender from overwhelming a
>> slow receiver."
>>
>> Guesstimating BDP is only important if the receiver cares about
>> maximizing throughput. Which hopefully it does, but there's no guarantee.
>> Sometimes due to resource constraints, the receiver cannot accept that much
>> data, and it asserts flow control in this case. And senders *need* to
>> respect that. Otherwise a receiver with any sense, like a highly scalable
>> server, will terminate the connection since the peer is misbehaving.
>>
>>
>> On Sun, Nov 3, 2013 at 7:44 PM, Peter Lepeska <bizzbyster@gmail.com>wrote:
>>
>>> Sloppiness? I don't get that. The sender's job is to transmit the data
>>> as fast as possible, not to respect the receiver's best guesstimate of
>>> available bandwidth sent ½ RTT ago. In this case, the sender's job is to
>>> keep the TCP buffer full of data so it can send it when it has the
>>> opportunity to.
>>>
>>> Respecting the peer's receive window in the single file send case is
>>> harmless at best and detrimental otherwise.
>>>
>>> Peter
>>>
>>> On Sunday, November 3, 2013, William Chan (陈智昌) wrote:
>>>
>>>> I don't feel comfortable encouraging such sloppiness, I worry about
>>>> future interop. Respecting a peer's receive window isn't hard. Just do it :)
>>>>
>>>> And even though wget doesn't support upload (to my knowledge, but I'm
>>>> not an expert), a command line tool may upload, in which case it should
>>>> definitely respect the peer's receive window.
>>>> On Nov 3, 2013 6:22 PM, "Yoav Nir" <ynir@checkpoint.com> wrote:
>>>>
>>>>>
>>>>>  On Nov 3, 2013, at 1:25 PM, William Chan (陈智昌) <willchan@chromium.org>
>>>>> wrote:
>>>>>
>>>>>  It's probably understood already, but just to be clear, this is
>>>>> receiver controlled and directional. Unless you control both endpoints, you
>>>>> must implement flow control in order to respect the peer's receive windows,
>>>>> even if you disable your own receive windows. Cheers.
>>>>>
>>>>>
>>>>>  This discussion started with tools like WGET. If all you're ever
>>>>> sending is one single request equivalent to "GET xxx", you're likely fine
>>>>> not considering server receive window.
>>>>>
>>>>>  For a single file, the data that the client sends to the server
>>>>> never exceeds the default server receive window.
>>>>>
>>>>>
>>>>>
>>
Received on Monday, 4 November 2013 04:49:56 UTC

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