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Implementing Remote Procedure Calls

  • 51

Caller machine

Callee machine

User

call ~

RPC + Stub

send callpkt

CalICalIID, dispatcherHint, dispatcherlD, procedure, arguments \

RPC + Stub

invoke proc

Server

  • -

    ~ do call

await ack or result

~ --

.

return

/

ResultCalllD, results

send results

mtum

Fig. 3.

The packets transmitted during a simple call.

protocol (compared with the two-packet handshake required in many other protocols); receipt of a call packet from a previously unknown activity is sufficient to create the connection implicitly. When the connection is active (when there is a call being handled, or when the last result packet of the call has not yet been acknowledged), both ends maintain significant amounts of state information. However, when the connection is idle the only state information in the server machine is the entry in its table of sequence numbers. A caller has minimal state information when a connection is idle: a single machine-wide counter is sufficient. When initiating a new call, its sequence number is just the next value of this counter. This is why sequence numbers in the calls from an activity are required only to be monotonic, not sequential. When a connection is idle, no process in either machine is concerned with the connection. No communications (such as "pinging" packet exchanges) are required to maintain idle connections. We have no explicit connection termination protocol. If a connection is idle, the server machine may discard its state information after an interval, when there is no longer any danger of receiving retransmitted call packets (say, after five minutes), and it can do so without interacting with the caller machine. This scheme provides the guarantees of traditional connection-oriented protocols without the costs. Note, however, that we rely on the unique identifier we introduced when doing remote binding. Without this identifier we would be unable to detect duplicates if a server crashed and then restarted while a caller was still retrans- mitting a call packet (not very likely, but just plausible). We are also assuming that the call sequence number from an activity does not repeat even if the calling machine is restarted (otherwise a call from the restarted machine might be eliminated as a duplicate). In practice, we achieve this as a side effect of a 32-bit conversation identifier which we use in connection with secure calls. For non- secure calls, a conversation identifier may be thought of as a permanently unique identifier which distinguishes incarnations of a calling machine. The conversation identifier is passed with the call sequence number on every call. We generate conversation identifiers based on a 32-bit clock maintained by every machine (initialized from network time servers when a machine restarts).

From experience with previous systems, we anticipate that this light-weight connection management will be important in building large and busy distributed systems.

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

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