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Re: Question on flow control for a single file transfer

From: Michael Sweet <msweet@apple.com>
Date: Mon, 04 Nov 2013 11:04:57 -0500
Cc: "William Chan (陈智昌)" <willchan@chromium.org>, Yoav Nir <ynir@checkpoint.com>, "<ietf-http-wg@w3.org>" <ietf-http-wg@w3.org>, Martin Thomson <martin.thomson@gmail.com>
Message-id: <05BBA7E4-6975-4DF7-A5C2-F915D3E4C882@apple.com>
To: Peter Lepeska <bizzbyster@gmail.com>
Peter,

On Nov 3, 2013, at 11:04 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.

The problem with relying on TCP-based flow control is that you are forcing retransmissions and log-jamming all access to the other end.  If instead you send your file in chunks sized to the receiver’s capabilities then you can either a) do other useful work or b) go to sleep until the receiver tells you it can accept more data.  Add a small amount of rate-tracking code on the sending side and you should be able to keep the receiver window near full.


> 
> 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.
> 
> 
> 

____________________________________________________________
Michael Sweet, Senior Printing System Engineer, PWG Chair
Received on Monday, 4 November 2013 16:05:45 UTC

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