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.
1. The harm.
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.
1.1 Inefficiency in Computation
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
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.
1.2 Inefficiency in transmission
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 |
-----------------------------------
Message |00000000000000000000000000000001 |
-----------------------------------
| text | 000000000000000000000000 |
-----------------------------------
16 bits of padding | 1000000000000000 |
added by sending IMP --------------------
figure 1.2
increase this efficiency to 0.10. For a 32 bit text (length of some control commands), it would increase the efficiency form 0.28 to 0.4. For one packet messages, the efficiency would still be increased by 3%.
2. A remedy.
This is a suggested modification of the Host-Imp users interface which has been tentatively sketched on diagrams extracted form BBN 1822 report.
2.1 Host to Imp
The modification consists of adding a counter to 32, enabled as the beginning of a message, and incremented at each bit passed to the IMP; when it reaches 32 it forces a "word complete" signal asking for a new word in the shift register and resetting the word length counter; thus the unused bits in the last word of the leader are not transmitted and the message starts with the next word (see figure 2.1) 0 23 ------------------------------------------ | leader | | ---------------------- | | XXXXXXXXXXXXXXXX | <- contents of |----------------------------------------- sending Host memory | | (24 bits) | Message | | |
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Corresponding message in the sending IMP memory
0 15 -------------------------------- | | | | | leader | | | -------------------------------- | | | message | | |
figure 2.1
2.2 Imp to Host
The modification consists of adding a counter to 32. When 32 bits have entered the shift register form the Imp at the beginning of a new message, the counter allows the register to be shifted up to the point to be full (which is detected by the word length counter) without entering any new bit from the Imp.
Thus, the next bit of the message which is the first bit of text will be
entered as the first bit of the next word (see figure 2.2).
Message in receiving IMP memory Contents of receiving Host memory (35
bits)
0 15 0 35 ------------------------------ -------------------------------------- | | | | | leader | | leader | 0000 | ------------------------------ -------------------------------------- | | | | | message | | message | | | | | | | | |
figure 2.2
Though the accumulated cost of useless marking bits sent over the network plus computation to reshape received texts makes this modification probably whorkwhile being considered, this decision is not of our competence and we merely wanted to suggest a better solution then marking.
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Pages 5 and 6 contain a wire Diagram of a
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"IMP to Host" "Host's special Interface" -