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the idle buffer for output, there is a cyclic permutation of
the input, output, and free buffers.
Each of the routing messages sent and received carries a
software checksum. In addition, the input, output, and timeout
processes for routing all compute a checksum on the program
before executing it, and a reload is initiated in the event of
failure. These reliability measures are discussed in more
detail in the next section.
2.5 Failure Protocols
The network is designed to be largely invulnerable to circuit
or IMP failure as well as to outages for maintenance. Special
status and test procedures are employed to help cope with various
failures. In the normal course of events the IMP program transmits
hello bits in its routing messages. The acknowledgment
for a hello packet is an I-heard-you (IHY) bit in a returning
null packet.
A dead line is detected by the sustained absence (approximately
3.2 sec) of IHY messages on that line. No regular
packets will be routed onto a dead line, and any packets awaiting
transmission will be rerouted. Routing tables in the network
are adjusted automatically to reflect the loss. Receipt of
thirty consecutive I-heard-you packets is required (an event
which consumes at least 15 seconds) before a dead line is defined
to be alive once again.
A dead line may reflect trouble either in the communication
facilities or in the neighboring IMP itself. Normal
line errors caused by dropouts, impulse noise, or other similar
conditions should not result in a dead line, because such
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