ss entry contained in its routing table. If no match is found, the gateway discards the datagram and sends an ICMP (see next subsection) message back to the datagram source. 12.3.3.2 Internet Control Message Protocol (ICMP). ICMP is used as an adjunct to IP when there is an error in datagram processing. ICMP uses the basic support of IP as if it were a higher-level protocol.; however, ICMP is actually an integral part of and is implemented by every IP module. ICMP messages are sent in several situations: for example, when a datagram cannot reach its destination, when a gateway (router) does not have the buffering capacity to forward a datagram, and when the gateway (router) can direct the host to send traffic on a shorter route. ICMP messages typically report errors in the processing of datagrams. To avoid the possibility of infinite regress of messages about messages, and SO on, no ICMP messages are sent about ICMP messages. There are eight distinct ICMP messages: 1. Destination unreachable message 2. Time exceeded message 3. Parameter problem message 4. Source quench message 5. Redirect message 6. Echo or echo reply message 7. Timestamp or timestamp reply message 8. Information request or information reply message 12.3.3.3 IP Summary. The IP provides connectionless service, meaning that there is no call setup phase prior to the exchange of traffic. There are no flow control or error control capabilities incorporated in IP. These are left to the next higher layer, the transmission control protocol (TCP). The IP is transparent to subnetworks connecting at lower layers; thus different types of networks can attach to an IP gateway or router. To compensate for these deficiencies in IP, the TCP (transmission control protocol) was developed an upper layer to IP. It should be noted that TCP/IP can be found in both the LAN and WAN environments. 12.3.4 The Transmission Control Protocol (TCP) 12.3.4.1 TCP Defined. TCP (Refs. 3, 9) was designed to provide reliable commu- nication between pairs of processes in logically dis