Hardware Reference
In-Depth Information
Sessions vs. Messages
So far, most of the communication in this topic has involved opening a dedicated con-
nection between two points for the duration of the conversation. This is session-based
communication. Sometimes you want to make a network in which objects can talk to
each other more freely, switching conversational partners on the fly, or even address-
ing the whole group if the occasion warrants. For this, you need a more message-based
protocol.
Sessions Versus Messages
In Chapter 5, you learned about the Transmission Control
Protocol, TCP, which is used for much of the communica-
tion on the Internet. To use TCP, your device has to request
a connection to another device. The other device opens
a network port, and the connection is established. Once
the connection is made, information is exchanged; then,
the connection is closed. The whole request-connect-
converse-disconnect sequence constitutes a
session
. If
you want to talk to multiple devices, you have to open and
maintain multiple sessions. Sessions characterize TCP
communications.
other object on a given subnet. For example, if your
address is 192.168.1.45, and you send a UDP message
to 192.168.1.255, everybody on your subnet receives
the message. Because this is such a handy thing to
do, a special address is reserved for this purpose:
255.255.255.255. This is the limited broadcast address—it
goes only to addresses on the same LAN, and does not
require you to know your subnet address. This address is
useful for tasks such as finding out who's on the subnet.
The advantage of UDP is that data moves faster because
there's no error-checking. It's also easier to switch the end
device that you're addressing on the fly. The disadvantage
is that it's less reliable byte-for-byte, as dropped packets
aren't resent. UDP is useful for streams of data where
there's a lot of redundant information, like video or audio.
If a packet is dropped in a video or audio stream, you may
notice a blip, but you can still make sense of the image or
sound.
Not all network communication is session-based. There's
another protocol used on the Internet, called the
User
Datagram Protocol
, or
UDP
. With UDP, you compose a
message, give it an address, send it, and forget about it.
Unlike the session-based TCP, UDP communication is all
about messages. UDP messages are called
datagrams
.
Each datagram is given a destination address and
is sent on its merry way. There is no two-way socket
connection between the sender and receiver. It's the
receiver's responsibility to decide what to do if some of
the datagram packets don't arrive, or if they arrive in the
wrong order.
The relationship between TCP and UDP is similar to the
relationship between Bluetooth and 802.15.4. Bluetooth
devices have to establish a session to each other to com-
municate, whereas 802.15.4 radios (like the XBee radios
in Chapter 6) communicate simply by sending addressed
messages out to the network without waiting for a result.
Like TCP, Bluetooth is reliable for byte-critical applications,
but it's less flexible in its pairings than 802.15.4.
X
Because UDP doesn't rely on a dedicated one-to-one
connection between sender and receiver, it's possible
to send a broadcast UDP message that goes to every
