Logical Message Synchronization

At the last network meeting, the question of logical and physical

message distinctions was raised. An argument was made in favor of

never running two logical messages together as one or more physical

messages. Another method of stating this is that a logical message

must begin on a physical message boundary. This did not, however,

solve the problem of locating the end of a logical message. A rather

poor technique was suggested by myself which consisted of using the

first partial physical message as an indication of the last physical

message of the logical message. This technique was thrown out for a

number of very valid reasons. The solution that seemed most pleasing

was the inclusion of some sort of a bit count or data type

specification to precede the logical message. Most everyone seemed to

like this even though it was stated in a very general way.

As of this writing it appears that it is desired to completely sever

the relation between physical and logical messages. This certainly is

aesthetically pleasing. However, we are now forced to view the

network as a virtually infinite bit stream with no physical

delineations. It may well do to transmit a logical header and bit

count for each message as long as there are no errors along the line.

If, however, a bit is dropped, the problem of synchronization is

compounded by the fact that we have no ability to search for the

beginning of a logical message other than brute force. An error of

this type could be introduced by faulty host or user software/hardware

as well as the imp itself. This would involve the shifting of the

message bit by bit and seeing if the data looked reasonable. This

could certainly be time-consuming as well as introducing the

possibility of false synchronism.

I can think of several solutions to the problem at the moment. None

of them seems to be very good. Upon losing synchronism, a user could

send some form of error message to the other host. The other host

could then in return cease sending and wait for a message to continue

from the troubled user. This would allow the troubled user to flush

out all waiting input. He would then be assured that the next bit

started a logical message. The problems here are in assuring

synchrony due to input/output buffering in the network and at both

hosts. How, for example, can the troubled host be assured he has all

the pending data? Once he is sure, he can then resume input assuming

all is OK.

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Another partial solution requires the original restriction that

logical messages always start on physical boundaries. A user then

merely has to examine the beginning of each physical message to see if

it fits the pattern of a logical message header. This technique is a

lot safer than examining the entire input stream as well as being

quite a bit faster.

I have not intended to suggest a solution to the problem, but merely

bring it to light. If we want to restrict logical messages to begin

on physical boundaries we must plan this early in the game. (It

probably works out that way in most cases anyway.) Other schemes can

be tried later. We must, however, face up to this problem fairly

soon.

[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Carl Alexander 7/97 ]

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