Network Working Group M. Elie
Request for Comments #64 UCLA
Getting Rid of Marking
Though we realize that this improvement is perhaps somewhat late
to be implemented, we believe that there exist better solutions than
marking and suggest a simple modification to the IMP-HOST interface
which would avoid it.
Marking was introduced to suit the sending Host because it permits
the text of a message to start on a word boundary, however, it does not
suit the receiving Host with a different word length. Moreover,it
introduces in the message useless bits. Let us illustrate this by the
example of our Sigma 7, a 32 bit machine.
Suppose we receive a message from an 18 bit machine (figure 1.1)
coded in 8 bit ASCII characters which will eventually become standard on
the network. In order to translate this message into our EBCDIC
internal code, for instance.
0 17 0 31
-------------------------- ------------------------------
| leader | | leader |
-------------------------- ------------------------------
| | 0 0 0 1| | 0 0 0 1 | |
-------------------------- ----------- |
| | | |
| | | |
| | | |
| message | | message |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
figure 1.1
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RFC 64 Getting Rid of Marking
we first have to shift the whole message. We must detect the firsl 1
following the leader, and from this determine that we must shift the
message 4 bits to the left. This takes approximately 12 µsec per double
word, which makes 1,5 msec per full regular message. This is not huge,
but still it is about one-third of the time it will take to translate
the message in internal code.
More important is the inefficiency resulting from adding
unnecessary bits to the message, especially if it turns out that one
character messages are used. Figure 1.2 shows the example of a 1
character text sent by the sigma 7, which results in transmitting 112
bits to carry 8 bits of information, thus leading to an efficiency
factor of 0.07. Supression of marking would
-----------------------------------
Sigma 7 | leader |
-----------------------------------