Ip Multicast; Tcp Protocol; Tcp Port Numbers - WAGO 750 Series Manual

142 • ETHERNET
Network Communication

5.3.3.1.2 IP Multicast

Multicast refers to a method of transmission from a point to a group, which is
a point-to-multipoint transfer or multipoint connection. The advantage of
multicast is that messages are simultaneously transferred to several users or
closed user groups via one address.
IP multicasting at Internet level is realised with the help of the Internet Group
Message Protocol IGMP; neighbouring routers use this protocol to inform
each other on membership to the group.
For distribution of multicast packets in the sub-network, IP assumes that the
datalink layer supports multicasting. In the case of Ethernet, you can provide a
packet with a multicast address in order to send the packet to several
recipients with a single send operation. Here, the common medium enables
packets to be sent simultaneously to several recipients. The stations do not
have to inform each other on who belongs to a specific multicast address –
every station physically receives every packet. The resolution of IP address to
Ethernet address is solved by the use of algorithms, IP multicast addresses are
embedded in Ethernet multicast addresses.

5.3.3.2 TCP Protocol

As the layer above the Internet protocol, TCP (Transmission Control Protocol)
guarantees the secure transport of data through the network.
TCP enables two subscribers to establish a connection for the duration of the
data transmission. Communication takes place in full-duplex mode (i.e.,
transmission between two subscribers in both directions simultaneously).
TCP provides the transmitted message with a 16-bit checksum and each data
packet with a sequence number.
The receiver checks that the packet has been correctly received on the basis of
the checksum and then sets off the sequence number. The result is known as
the acknowledgement number and is returned with the next self-sent packet as
an acknowledgement.
This ensures that the lost TCP packets are detected and resent, if necessary, in
the correct sequence.

TCP port numbers

TCP can, in addition to the IP address (network and subscriber address),
respond to a specific application (service) on the addressed subscriber. For
this the applications located on a subscriber, such as a web server, FTP server
and others are addressed via different port numbers. Well-known applications
are assigned fixed ports to which each application can refer when a connection
is built up.
Examples:
A complete list of "standardized services" is contained in the RFC 1700
(1994) specifications.
Telnet Port number: 23
HTTP Port number: 80
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP