Popular Understanding TCP handshake number is three times

After understanding, it should be said that at least three times to ensure reliable transmission.


See an online post http://www.cnblogs.com/TechZi/archive/2011/10/18/2216751.html
That said, "I Google the answer to the question found that the network on the" three handshake "process is described in detail, but for why the need to" three handshake "to establish a connection is not a good answer. I can only resort to books. ”
There is Shehid and another book explanation, in fact, is too written, not popular, but see behind the Google Forum to see a let me very satisfied with the answer.


Https://groups.google.com/forum/#!topic/pongba/kF6O7-MFxM0/discussion




This is the basic problem in communication, see two Generals ' problem:
Http://en.wikipedia.org/wiki/Two_Generals ' _problem
It's easy to see why three handshakes are necessary.
The essence of this problem is that, with a channel that is not completely reliable, it requires at least a few messages to be transmitted, and people on both sides of the channel can agree on a problem. For TCP, there is no initial
Serial number requirement, want to agree to start outgoing data on both sides, need at least 3 times the exchange of messages:
0 Plays: Obviously not.
1 Plays: A->b, A don't know if B agrees
2 Plays: A->b, B->a B does not know if a has received its own message, because the channel is not completely reliable
3 Plays: A->b, B->a, a->b. Both sides receive an ACK, which means that each has an understanding of each other's intentions, thus allowing the simplest question of whether to start communication
Reach agreement.


In simple terms, it is 0 times obviously not, 1 times B may not receive, 2 times, b do not know whether a received, in fact, more accurately, is B reply not necessarily to a,a is not afraid to send, because not sure B is ready, 3 times, B->a said B ready
The third time a->b a received B reply, both sides know what the other side to do, a know B ready to receive, B know a ready to send, the third a->b not to also matter, because the system is concerned about a try to send the third time, this should be TCP-IP have a variable
Recorded the third time indeed sent, as to the receipt does not matter.

Of course, 4 times can be, but as long as 3 times can be confirmed, so why not reduce the cost, TCP requests are also very resource-intensive.

From the HTTP process, first DNS to find IP, Should be GFW blocked the domain name has google.com URL, and then faster return a fake IP, although the real can be found, but always more slowly than the fake to the client, so was the wall, not to tcpip this step, this is the second step, to find the IP can tcpip, of course, GFW may also be directly shielded Ip

Popular Understanding TCP handshake number is three times