kj04@chello.at wrote:
> Pedro's pirouette on the meaningless (but socially necessary) small talk
In computer communications the notion of a "protocol" describes how two
(or more) computers are supposed to communicate. The most famous of them
are the HTTP - the ground for the world-wide web, and SMTP - the basis
for e-mail. Particularly interesting are the "meaningless" elements of
those protocols:
1. keepalive: makes sure that a long-term connection is regularly
refreshed "Hey! How do you do?"
The Keep-Alive extension to HTTP, as defined by the HTTP/1.1 draft,
allows persistent connections. These long-lived HTTP sessions allow
multiple requests to be send over the same TCP connection.
2. ping: checks how long it takes to obtain a response from the other
side "Do you hear me?"
Ping uses a series of Internet Control Message Protocol (ICMP) echo
messages to determine whether a remote host is active and what the
round-trip delay currently is in communicating with it.
3. checksum: Assures that the message was transmitted correctly. "OK?"
A checksum is a form of redundancy check, a very simple measure for
protecting the integrity of data by detecting errors in data that is
sent through space (telecommunications) or time (storage). It works by
adding up the basic components of a message, typically the bytes, and
storing the resulting value. Later, anyone can perform the same
operation on the data, compare the result to the authentic checksum, and
(assuming that the sums match) conclude that the message was probably
not corrupted.
4. handshake: Initializes the communication between two computers. "Hello!"
While it is possible for a pair of end hosts to initiate connection
between themselves simultaneously, typically one end opens a socket and
listens passively for a connection from the other. This is commonly
referred to as a passive open, and it designates the server-side of a
connection. The client-side of a connection initiates an active open by
sending an initial SYN segment to the server as part of the 3-way
handshake. The server-side should respond to a valid SYN request with a
SYN/ACK. Finally, the client-side should respond to the server with an
ACK, completing the 3-way handshake and connection establishment phase.
5. timeout: Breaks down a communication channel if there has been no
response in a while; assumes that the message was not transmitted. "..."
A timer at the sending TCP will cause a timeout if an acknowledgement is
not received within a reasonable round-trip time (or RTT), and the
(presumably lost) data will then be re-transmitted.
-
If there is a functioning link, we can use them to transmit the really
important messages.
With the minimalism of engineering, these elements of protocols would
have been left out if they had no function.
Still, we can see how there is almost nothing in computing that doesn't
exist in our own human communication: Ever-increasing specificity.
--
dr. Aleks Jakulin
http://kt.ijs.si/aleks/
Department of Knowledge Technologies,
Jozef Stefan Institute, Ljubljana, Slovenia.
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Received on Sat Oct 22 14:22:40 2005
