The only error checking in
the IP protocol uses a checksum in order to insure the header information has integrity. The checksum is
a simple mathematical function of the bits in the header. Each receiver recomputes the checksum to make
sure the checksum in the header matches. If the header is corrupted, the receiver simply discards the datagram!
IP provides no checking whatever of the data in the datagram, so even if the header is intact, the data may
be corrupted.
Also, it??™s possible that a data link, router, or a computer will fail at a time when datagrams are being sent.
Such failures will result in loss of the datagrams, and nothing in the IP protocol will do anything to recover
such losses. Further, since failures in the network can result in dynamic changes to routes between networks,
datagrams sent later may actually arrive sooner than datagrams sent earlier.
Most of the time these problems don??™t occur. In fact, experiments over wired networks show amazing reliability
most of the time. Error-testing devices often measure the bit error rate of networks in bits per billion sent! That??™s
on the order of one to several errors per hour on a busy network. Nevertheless, to insure that datagrams all
arrive, uncorrupted, in order, without duplication of datagrams, requires a higher-level protocol designed to
CHAP.
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