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  • A.D. Birrell and B. J. Nelson

still responding, there is no upper bound on how long we will wait for results; that is, we will abort a call if there is a communication breakdown or a crash but not if the server code deadlocks or loops. This is identical to the semantics of local procedure calls.

3.2 Simple Calls

We have tried to make the per call communication particularly efficient for the situation where all of the arguments will fit in a single packet buffer, as will all of the results, and where frequent calls are being made. To make a call, the caller sends a call packet containing a call identifier (discussed below), data specifying the desired procedure (as described in connection with binding), and the argu- ments. When the callee machine receives this packet the appropriate procedure is invoked. When the procedure returns, a result packet containing the same call identifier, and the results, is sent back to the caller.

The machine that transmits a packet is responsible for retransmitting it until an acknowledgment is received, in order to compensate for lost packets. However, the result of a call is sufficient acknowledgment that the call packet was received, and a call packet is sufficient to acknowledge the result packet of the previous call made by that process. Thus in a situation where the duration of a call and the interval between calls are each less than the transmission interval, we transmit precisely two packets per ~all (one in each direction). If the call lasts longer or there is a longer interval between calls, up to two additional packets may be sent (the retransmission and an explicit acknowledgment packet); we believe this to be acceptable because in those situations it is clear that commu- nication costs are no longer the limiting factor on performance.

The call identifier serves two purposes. It allows the caller to determine that the result packet is truly the result of his current call (not, for example, a much delayed result of some previous call), and it allows the callee to eliminate duplicate call packets (caused by retransmissions, for example). The call identifier consists of the calling machine identifier (which is permanent and globally unique), a machine-relative identifier of the calling process, and a sequence number. We term the pair machine identifier, process an activity. The important property of an activity is that each activity has at most one outstanding remote call at any time--it will not initiate a new call until it has received the results of the preceding call. The call sequence number must be monotonic for each activity (but not necessarily sequential). The RPCRuntime on a callee machine maintains a table giving the sequence number of the last call invoked by each calling activity. When a call packet is received, its call identifier is looked up in this table. The call packet can be discarded as a duplicate (possibly after acknowledg- ment) unless its sequence number is greater than that given in this table. Figure 3 shows the packets transmitted in simple calls.

It is interesting to compare this arrangement with connection establishment, maintenance and termination in more heavyweight transport protocols. In our protocol, we think of a connection as the shared state information between an activity on a calling machine and the RPCRuntime package on the server machine accepting calls from that activity. We require no special connection establishment

ACM Transactions on Computer Systems, Vol. 2, No. 1, February 1984

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