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 祍ec 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
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.
Pages 5 and 6 contain a wire Diagram of a
"IMP to Host"
"Host's special Interface"
[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Gottfried Janik 2/98 ]