So basically the transport control 
protocol has a responsibility of 
compensating for the imperfections of the 
IP layer. 
Data can arrive not out of order, it can 
arrive not at all and so the TCP layer 
marks the data in a way, and stores the 
data in the source computers until it is 
acknowledged by the destination 
computers. 
And so TCP buffers this information, and 
the buffers are kept on the edge, and 
this allows the internet, to grow in 
wonderful ways.{COUGH} So, this results 
in a layer, that really provides us what 
seems to be an end to end connection. 
So we can send data in one end, a stream 
of data, just send it in, just roll it 
in, and out comes a stream of data. 
It comes out in the same order, it comes 
out reliably, and we can stop thinking 
about it. 
And again, the whole goal of this layered 
architecture is so that we can pretend 
that the complexity inside this box is 
simple. 
So it's not simple inside. 
But we, the applications, we're going to 
make use of this transport layer and 
pretend it's simple. 
Okay? 
And so we just, I put in H. 
And a H comes out and I put in an E and 
an E comes out and I put in an L and an L 
comes out, an O comes out, and it's 
hundreds of thousands of engineers and 
billions of dollars of research that 
makes it work, but it just is simple. 
It's been oversimplified. 
So now we've talked about the link layer, 
the IP layer, and the transport layer. 
And up next, we're going to talk about 
what we do once we have reliable pipes 
connecting one application to another.