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Network Working Group                                  M. ElieRequest for Comments #64                               UCLA                         Getting Rid of Marking      Though we realize that this improvement is perhaps somewhat lateto be implemented, we believe that there exist better solutions thanmarking and suggest a simple modification to the IMP-HOST interfacewhich would avoid it.1. The harm.      Marking was introduced to suit the sending Host because it permitsthe text of a message to start on a word boundary, however, it does notsuit the receiving Host with a different word length. Moreover,itintroduces in the message useless bits. Let us illustrate this by theexample 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 onthe network.  In order to translate this message into our EBCDICinternal code, for instance.0                        17         0                           31--------------------------            ------------------------------|        leader          |            |           leader           |--------------------------            ------------------------------|               | 0 0 0 1|            | 0 0 0 1 |                  |--------------------------            -----------                  ||                        |            |                            ||                        |            |                            ||                        |            |                            || message                |            | message                    ||                        |            |                            ||                        |            |                            ||                        |            |                            ||                        |            |                            ||                        |            |                            ||                        |            |                            |                            figure 1.1                                                                [Page 1]

RFC 64                   Getting Rid of Markingwe first have to shift the whole message. We must detect the firsl 1following the leader, and from this determine that we must shift themessage 4 bits to the left. This takes approximately 12 �sec per doubleword, 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 translatethe message in internal code.1.2 Inefficiency in transmission      More important is the inefficiency resulting from addingunnecessary bits to the message, especially if it turns out that onecharacter messages are used. Figure 1.2 shows the example of a 1character text sent by the sigma 7, which results in transmitting 112bits to carry 8 bits of information, thus leading to an efficiencyfactor of 0.07. Supression of marking would                            -----------------------------------   Sigma 7                  |           leader                |                            -----------------------------------   Message                  |00000000000000000000000000000001 |                            -----------------------------------                            | text | 000000000000000000000000 |                            -----------------------------------   16 bits of padding       | 1000000000000000 |   added by sending IMP     --------------------                                figure 1.2increase this efficiency to 0.10. For a 32 bit text (length of somecontrol 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 interfacewhich has been tentatively sketched on diagrams extracted form BBN 1822report.                                                                [Page 2]

RFC 64                   Getting Rid of Marking2.1 Host to Imp      The modification consists of adding a counter to 32, enabledas the beginning of a message, and incremented at each bit passed to theIMP; when it reaches 32 it forces a "word complete" signal asking for anew word in the shift register and resetting the word length counter;thus the unused bits in the last word of the leader are not transmittedand 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 2.12.2 Imp to Host      The modification consists of adding a counter to 32. When 32 bitshave entered the shift register form the Imp at the beginning of a newmessage, the counter allows the register to be shifted up to the pointto be full (which is detected by the word length counter) withoutentering any new bit from the Imp.                                                                [Page 3]

RFC 64                   Getting Rid of MarkingThus, the next bit of the message which is the first bit of text will beentered as the first bit of the next word (see figure 2.2).Message in receiving IMP memory  Contents of receiving Host memory (35bits)0                        15    0                                   35------------------------------   --------------------------------------|                            |   |                                    ||       leader               |   |     leader                  | 0000 |------------------------------   --------------------------------------|                            |   |                                    || message                    |   | message                            ||                            |   |                                    ||                            |   |                                    |                               figure 2.2Though the accumulated cost of useless marking bits sent over thenetwork plus computation to reshape received texts makes thismodification probably whorkwhile being considered, this decision is notof our competence and we merely wanted to suggest a better solution thenmarking.            Pages 5 and 6 contain a wire Diagram of a                    "IMP to Host"                "Host's special Interface"       [ This RFC was put into machine readable form for entry ][ into the online RFC archives by Gottfried Janik 2/98 ]                                                                [Page 4]