Casio EG-800U - Cassiopeia - Win CE 3.0 150 MHz Specifications download pdf (Page 6)

SLAA137A

6 MSP430 Internet Connectivity

2.4 Internet Control Message Protocol

The internet control message protocol (ICMP) provides a mechanism for reporting problems and

generating diagnostic messages, e.g., when a datagram cannot reach its destination, when a

gateway does not have the buffering capacity to forward a datagram, or when the gateway

recommends a shorter route. Thus the datagram service can achieve a better communication

reliability. The ICMP uses the basic support of IP. However, it is an integral part of the IP layer.

Information about this protocol can be found in Reference [3].

The only two messages that are of interest for this application are the ECHO and the ECHO

REPLY messages. They are mainly used by the operating-system command-line utility PING. It

sends an ECHO message to another host which answers with an ECHO-REPLY message and

also sends back the received data. The PING tool draws information about the round-trip time

(RTT) and the reliability of a network.

2.5 Transmission Control Protocol

The transmission control protocol (TCP) is a transport-layer protocol from the TCP/IP network

stack. It is a highly reliable, connection-oriented host-to-host protocol for use in a packet-

switched network. It is the most established transport layer for commonly used Internet protocols

(e.g. HTTP, SMTP, FTP, telnet). A simple, potentially unreliable datagram service of a lower

level protocol (such as IP) is enough for TCP to provide its service. To provide this service, the

following mechanisms are implemented:

• Basic data transfer: TCP splits a continuous stream of bytes in segments and sends them

out as IP-frames.

• Reliability: Recovery from damaged, lost or duplicated data is achieved by assigning a

sequence number to each byte and some of the special flags. The sending TCP also

requires an acknowledgement from the receiving TCP. If this is not received within a

timeout period, data is sent again.

• Flow control: Each time a TCP receives a frame, it tells the other TCP how many bytes it is

allowed to send before further permission is needed.

• Multiplexing: The TCP introduces port numbers that allow multiplexing of IP addresses. Any

combination of an IP address and a port-number is called a socket. A unique TCP

connection is determined by a pair of sockets.

• Connections: Before data transfer can take place, a connection between the host and the

client must be established. This is done by using a three-way handshake. During this

handshake, the sequence numbers are synchronized. Afterwards the normal data transfer

can begin.

A TCP session runs through different states from establishing to closing. The state changes

occur as a reaction to different events. These events can be user function calls as well as

receiving segments or time-controlled actions. For detailed information about the TCP state

machine and how to work with it, see RFC 793 [4] and RFC 1122 [6].

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Casio EG-800U - Cassiopeia - Win CE 3.0 150 MHz Specifications Page 6