Data sent to a remote computer often travels through one or more routers; these routers can encounter a number of problems in sending the message to its ultimate destination. Routers use Internet Control Message Protocol (ICMP) messages to notify the source IP of these problems. ICMP is also used for other diagnosis and troubleshooting functions. The most common ICMP messages are listed here. Quite a few other conditions generate ICMP messages but their frequency of occurrence is quite low.
A routing loop occurs when a datagram circulates through the same routers continuously and never reaches its destination. Suppose three routers are located in Los Angeles, San Francisco, and Denver. The Los Angeles router sends datagrams to San Francisco, which sends them to Denver, which sends them back to Los Angeles again. The datagram becomes trapped and will circulate continuously through these three routers until the TTL reaches zero. A routing loop should not occur, but occasionally it does. A routing loop sometimes occurs when a network administrator places static routing entries in a routing table.
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