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transmitted, multiplying by 10 or more the effective delay seen
by the source. Therefore, the following transmission ordering
scheme has been instituted: priority packets which have never
been transmitted are sent first; next sent are any regular
packets which have never been transmitted; finally, if there
are no new packets to send, previously transmitted packets which
are unacknowledged are sent. Of course, unacknowledged packets
are periodically retransmitted even when there is a continuous
stream of new traffic.
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2.3.2 Physical Circuit Protocol
Each packet is individually routed from IMP to IMP through
the network toward the destination. At each IMP along the way,
the transmitting hardware generates initial and terminal framing
characters and checksum digits that are shipped with the packet
and are used for error detection by the receiving hardware of
the next IMP. The format of a packet on an inter-IMP channel is
shown in Figure 2-2.
Errors in transmission can affect a packet by destroying the
framing and/or by modifying the data content. If the framing
is disturbed in any way, the packet either will not be recognized
or will be rejected by the receiver. In addition, the check digits
provide protection against errors that affect only the data. The
check digits can detect all patterns of four or fewer errors
occurring within a packet, and any single error burst of a length
less than twenty-four bits. An overwhelming majority of all other
possible errors (all but about one in 2 24 ) is also detected. Thus,
the mean time between undetected errors in the subnet should be on
the order of years.
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