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Using LabVIEW with TCP/IP and UDP - Tutorial - Developer Zone - National Instruments
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Using LabVIEW with TCP/IP and UDP
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Internet Protocol (IP), User Datagram Protocol (UDP), and Transmission Control Protocol (TCP) are basic tools for
network communication. The name TCP/IP comes from two of the best-known protocols of the Internet protocol suite,
the Transmission Control Protocol and the Internet Protocol.
With TCP/IP you can communicate over single networks or interconnected networks. The individual networks can be
separated by large geographical distances. TCP/IP routes data from one network or Internet computer to another.
Because TCP/IP is available on most computers, it can transfer information among diverse systems.
LabVIEW and TCP/IP
You can use the TCP/IP protocols with LabVIEW on all platforms. LabVIEW includes TCP and UDP VIs and functions
for creating client or server VIs.
Table of Contents:
z
IP
z
UDP
z
TCP
IP
IP performs the low-level service of moving data between computers. IP packages data into components called
datagrams. A datagram contains, among other things, the data and a header that indicates the source and destination
addresses. IP determines the correct path for the datagram to take across the network or Internet and sends the data to
the specified destination.
IP cannot guarantee delivery. In fact, IP might deliver a single datagram more than once if the datagram is duplicated in
transmission. Programs rarely use IP but use TCP or UDP instead.
UDP
UDP provides simple, low-level communication among processes on computers. Processes communicate by sending
datagrams to a destination computer or port. A port is the location where you send data. IP handles the computer-to-
computer delivery. Once the datagram reaches the destination computer, UDP moves the datagram to its destination
port. If the destination port is not open, UDP discards the datagram. UDP shares the same delivery problems of IP.
Use UDP in applications where reliability is not critical. For example, an application might transmit informative data to a
destination frequently enough that a few lost segments of data are not problematic.
Using UDP
Because UDP is not a connection-based protocol as TCP is, you do not need to establish a connection with a
destination before you send or receive data. Instead, you specify the destination for the data when you send each
datagram. Operating systems do not report transmission errors.
Use the UDP Open function to open a UDP socket on a port. The number of simultaneously open UDP ports depends
on the operating system. The UDP Open function returns a network connection refnum that uniquely identifies the UDP
socket. Use this connection ID to refer to this socket in subsequent VI calls.
Use the UDP Write function to send data to a destination, and use the UDP Read function to read that data. Each write
operation requires a destination address and port. Each read operation contains the source address and port. UDP
preserves the datagram bytes that you specified for each command you send.
In theory, you can send datagrams of any size; however, you typically would not use UDP to send large datagrams
because it is not as reliable as TCP.
When you finish all communications on a port, use the UDP Close function to free system resources.
Using LabVIEW with TCP/IP and UDP - Tutorial - Developer Zone - National Instruments
UDP Multicast
Using the UDP functions, you can communicate to a single client (singlecast) or to all computers on the subnet through
a broadcast. If you want to communicate to multiple specific computers, you have to configure the UDP functions to
iterate through a list of clients. Using this technique creates duplicate network traffic because LabVIEW sends a
separate copy of the data to each client and maintains a list of clients interested in receiving the data.
Use multicasting to communicate between a single sender and multiple clients on a network without requiring the
sender to maintain a list of clients or send multiple copies of the data to each client. To receive data broadcast by a
multicast sender, all clients must join a multicast group. The sender does not have to join a group to send data. The
sender specifies a multicast IP address, which defines a multicast group. Multicast IP addresses are in the
224.0.0.0
to
239.255.255.255
range. When a client wants to join a multicast group it subscribes to the multicast IP address of
the group. Once subscribed to a multicast group, the client receives data sent to the multicast IP address.
To multicast in LabVIEW, use the polymorphic UDP Multicast Open VI located on the
UDP
palette to open connections
capable of reading, writing, or reading and writing UDP data. Specify the time-to-live (TTL) for writing data, the multicast
address for reading data, and the multicast port number for reading and writing data. The default TTL is
1
, which means
LabVIEW sends the datagram only to the local subnet. When a router receives a multicast datagram, it decrements the
datagram TTL. If the TTL is greater than
1
, the router forwards the datagram to other routers. The following table lists
what action occurs to a multicast datagram when you specify a value for the
time-to-live
parameter.
0
Datagram remains on the host computer.
1
Datagram sent to every client on the same local subnet that subscribes to that IP address. Hubs/repeaters and
bridges/switches forward the datagram. Routers do not forward the datagram if the TTL is
1
.
Note:
If you specify a value greater than
1
, the datagram is sent and routers forward it through TTL-1 layers.
Refer to the UDP Multicast Receiver VI and the UDP Multicast Sender VI located in the
examples\comm\UDP.llb
for
examples of using UDP multicasting.
TCP
CP ensures reliable transmission across networks, delivering data in sequence without errors, loss, or duplication. TCP
retransmits the datagram until it receives an acknowledgment.
Setup
Before using TCP/IP, confirm that you have the correct setup, which varies depending on the platform you use.
z
Windows and UNIX
TCP/IP is built in. You do not need to use a third-party product to communicate using
TCP/IP. If your network is configured properly, LabVIEW requires no additional setup.
z
Mac OS
LabVIEW networking requires Open Transport, included in Mac OS 7.5 and later.
Using TCP
TCP is a connection-based protocol, which means that sites must establish a connection before transferring data. TCP
permits multiple, simultaneous connections.
You initiate a connection by waiting for an incoming connection or by actively seeking a connection with a specified
address. In establishing TCP connections, you have to specify the address and a port at that address. A number
between 0 and 65,535 represents a port. UNIX reserves port numbers less than 1,024 for privileged applications.
Different ports at a given address identify different services at that address.
Use the TCP Open Connection function to actively establish a connection with a specific address and port. If the
connection is successful, the function returns a network connection refnum that uniquely identifies that connection. Use
this connection ID to refer to the connection in subsequent VI calls.
You can use the following methods to wait for an incoming connection:
z
Use the TCP Listen VI to create a listener and wait for an accepted TCP connection at a specified port. If the
connection is successful, the VI returns a connection ID, the address, and the port of the remote TCP client.
z
Use the TCP Create Listener function to create a listener and use the Wait on Listener function to listen for and
accept new connections. The Wait on Listener function returns the same listener ID you wired to the function and
the connection ID for a connection. When you finish waiting for new connections, use the TCP Close function to
close a listener. You cannot read from or write to a listener.
The advantage of using the second method is that you can use the TCP Close function to cancel a listen operation,
which is useful when you want to listen for a connection without using a timeout, but you want to cancel the listen when
Using LabVIEW with TCP/IP and UDP - Tutorial - Developer Zone - National Instruments
another condition becomes true. You can close the listen VI at any time.
When you establish a connection, use the TCP Read function and the TCP Write function to read and write data to the
remote application.
Use the TCP Close Connection function to close the connection to the remote application. If unread data remains, and
the connection closes, you might lose data. Use a higher level protocol for your computer to determine when to close
the connection. Once a connection is closed, you cannot read from it or write to it again.
TCP Versus UDP
TCP is the best method to use if you want reliable data transmission. UDP is a connectionless protocol with higher
performance, but it does not ensure reliable data transmission.
TCP Client Example
The following steps are a generalized description of how to use the TCP functions to create a TCP client.
1. Use the TCP Open Connection function to open a connection to a server. You must specify the Internet address of
the server and the port for the server.
The address identifies a computer on the network. The port identifies a communication channel on the computer that the
server uses to listen for communication requests. When you create a TCP server, you specify the port that you want the
server to use for communication.
2. Use the TCP Write function to send the command to a server.
3. Use the TCP Read function to read results from the server.
With the TCP Read function, you must specify the number of characters you want to read.
Use the following methods to handle different sized commands:
z
Precede the command and the result with a fixed
size
parameter that specifies the size of the command or
result. In this case, read the
size
parameter and then read the number of characters the
size
parameter
specifies. This method is the most flexible.
z
Make each command and result a fixed size. When a command is smaller than the size you specify, you
can pad it to the fixed size. This method is the most efficient.
z
Follow each command and result with a specific terminating character. Read data in small units until you
reach the terminating character.
4. Use the TCP Close Connection function to close the connection to the server.
Timeouts and Errors
When you design a network application, consider carefully what should happen if something fails. For example, if the
server crashes, determine how each client VI handles it.
One solution is to make sure that each client VI has a timeout. If something fails to produce results after a certain
amount of time, the client continues execution. In continuing, the client can try to reestablish the connection or report the
error. If necessary, the client VI can shut down the application.
TCP Server Example
The following steps are a generalized description of how to use the TCP functions to create a TCP server.
1. Use the TCP Listen VI to wait for a connection. You must specify the port. This port must be the same port that the
client attempts to access. Refer to the
TCP Client Example
section of this document for more information about using
the TCP Listen VI.
2. If a connection is established, read from that port to retrieve a command. As discussed in the
TCP Client Example
section of this document, you must decide the format for commands. If a
size
parameter precedes commands, read the
length field first and then read the amount of data the length field indicates are remaining.
3. Use the TCP Write function to return results. As discussed in the
TCP Client Example
section of this document, the
data must be in a form that the client can accept.
4. Use the TCP Close Connection function to close the connection.
TCP and UDP Examples
Refer to the
examples\comm\TCP.llb
and the
examples\comm\UDP.llb
for examples of using the TCP and UDP
VIs and functions.
Using LabVIEW with TCP/IP and UDP - Tutorial - Developer Zone - National Instruments
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