I think we are using the same terminology to refer to 2 different things. After a connection is made:

A        B
SYN------>
<--------SYN+ACK
ACK------>

[download]

Then the ACK flag (or bit) is kept high for all subsequent packets. I used the term ACK to mean specifically the packet sent in response to a SYN+ACK packet, I did not mean to use it to refer to any packet with the ACK flag set.

You are correct that data is allowed to be sent during the 3 way handshaking provided that it remains in a buffer until the connection is established (from RFC 793). So the data does (should) not be passed up the stack until the connection is made.

I currently do not have access to a computer with the libpcap library but this code should help get you started towards your goal.

use Net::RawIP;

$ack_with_data = Net::RawIP->new({
    ip => {
            saddr => '111.111.111.111',
            daddr => '111.111.111.112'
           },
    });
    tcp => {
            source     => 111,
            dest       => 111,
            ack        => 1,
            #insert the correct seq number and ack seq number
            seq        => 11111,
            ack_seq    => 11111,
            #insert http data here (i am not very familiar with http)
            data     => "GET /asdlfk HTTP/1.1\nHost: asd.asd.com\n",
            },
    });
$ack_with_data->send;

[download]

Disclaimer: This code might be missing some things. I was not able to test it but it should help you see how to form a packet and send it. The specific fields will need to be correctly set and I might be missing a few.

In reply to Re^3: Piggybacking data on TCP ACK by zek152
in thread Piggybacking data on TCP ACK by rrekhi

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