Four Round Trips Before the First Useful Byte, and Every One of Them Is the Speed of Light Doing Its Job
Last timeTurning a Name Into a Number
Setting up a connection is an exchange of messages that must happen before any request is sent. Counting them explains why distance beats bandwidth for most pages.
The three messages
The address is known. Nothing useful can be sent yet, because the two machines
have never spoken and neither knows anything about the other.
Why it takes three
The usual question is why two messages do not suffice, and the answer is a
small piece of reasoning worth doing properly.
Suppose only two messages: the client proposes, the server acknowledges. At the
end of that, the client has evidence that the server received something, since
it got a reply. The server has no such evidence about itself. It sent an
acknowledgement and does not know whether it arrived. If it did not, the server
is holding a connection the client knows nothing about, and will hold it until
something times out.
Each side needs one piece of evidence: that something it sent was received.
That requires an acknowledgement travelling in each direction, and since the
middle message can carry both the acknowledgement one way and the proposal the
other, three is the minimum. It is not a convention and it is not historical
accident; two cannot do it.
The starting numbers matter for the same reason. Both sides agree on where
counting begins so that every later message can be identified by position,
which is what makes loss detectable at all, and that agreement is what the
exchange produces.
The second handshake
Everything above establishes a reliable channel. It does not establish a
private one, and almost everything is private now, so there is a second
negotiation on top.
The lesson stops here
3 more paragraphs to go
You have read the opening. The rest of the argument, the problems that check whether it landed, and the lines worth keeping at the end all come with a plan.
The first lesson of every course in the library reads the whole way through, free, so you can see exactly what the rest of them are.
See the planThe contentsThis is the reading half
Starting the course gives you your own copy of it. Every idea on every page has problems standing under it, marked with a reason rather than a tick, and any sentence you do not believe can be opened and argued with. None of that can happen on a page nobody owns.
The contents