netdev - Re: Piggyback the final ACK of the three way TCP connection establishment with the data

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Date:	Fri, 23 Mar 2012 12:03:36 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Vincent Li <vincent.mc.li@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: Piggyback the final ACK of the three way TCP connection
 establishment with the data

Hi Vincent

On Fri, Mar 23, 2012 at 8:38 AM, Vincent Li <vincent.mc.li@...il.com> wrote:
> On Thu, Mar 22, 2012 at 4:17 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Thu, 2012-03-22 at 16:13 -0700, Eric Dumazet wrote:
>>
>>> A third possibility (reading the code) if you use non blocking IO, is to
>>> send() a message right after connect()
>>>
>>>
>>
>> Or use the auto connect on sendto() more probably...
>>
>> That combines the connect() and send()
>>
>
> thanks,
>
> I got the TCP_QUICKACK working, but not the non blocking IO,
> basically, what I did is
>
>  if((sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0){
>    perror("Can't create TCP socket");
>    return 0;
>  }
>
>  val = Fcntl(sock, F_GETFL, 0);
>  Fcntl(sock, F_SETFL, val | O_NONBLOCK);
>
>
>  ip = get_ip(host);
>  fprintf(stderr, "IP is %s\n", ip);
>  remote = (struct sockaddr_in *)malloc(sizeof(struct sockaddr_in *));
>  remote->sin_family = AF_INET;
>  tmpres = inet_pton(AF_INET, ip, (void *)(&(remote->sin_addr.s_addr)));
>  if( tmpres < 0)
>  {
>    perror("Can't set remote->sin_addr.s_addr");
>    return 0;
>  }else if(tmpres == 0)
>  {
>    fprintf(stderr, "%s is not a valid IP address\n", ip);
>    return 0;
>  }
>  remote->sin_port = htons(PORT);
>
> char *query = "GET / HTTP/1.0\r\nHost: 127.0.0.1\r\n\r\n";
>
> tmpres = sendto(sock, query, strlen(query), 0, (struct sockaddr
> *)remote, sizeof(struct sockaddr));
>
>  maybe I mis-interpret you.
>
> Vincent
AFAICT the feature that Eric refers to is TCP Fast Open that I am
still testing and have not yet submit to netdev.

But one way to achieve that currently is doing a non-blocking connect
then a blocking write.

I just tried this on 2.6 machine to a remote server

fcntl(sd, F_SETFL, fcntl(sd, F_GETFL, 0) | O_NONBLOCK);
connect(sd, ...);
fcntl(sd, F_SETFL, fcntl(sd, F_GETFL, 0) & ~O_NONBLOCK);
sendto(sd, buf, buf, 0, ...);

The key is that the socket has to be in progress of connecting
when application calls write/sendto(2). If you are testing on loopback,
the connect might finish before sendto so this won't work.

HTH
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