Now let us consider the usefulness of a common proposal, namely, that the
communication system provide, internally, a guarantee of reliable data transmission.
It might accomplish this guarantee by providing selective redundancy in
the form of packet checksums, sequence number checking, and internal retry
mechanisms, for example. With sufficient care, the probability of undetected bit
errors can be reduced to any desirable level. The question is whether or not this
attempt to be helpful on the part of the communication system is useful to the
careful file transfer application.