1. Tam Vu (tvu@mail.eecis.udel.edu)
Remote Procedure Call
CISC 879 – Spring 03
Tam Vu
March 06, 03

2. Outline Introduction RPC mechanisms Design issues Case study: SUN RPC
Programming with RPC
Case study: SUN RPC

3. Introduction Problem with sockets Socket interface is straightforward
Connect
Read/write
Disconnect
Forces read/write mechanism
Not how we generally program
We usually use procedure calls
To make distributed computing look more like centralized:
I/O is not the way to go

4. Introduction 1984: Birrell & Nelson
Mechanisms to call procedures on other machines
Processes on machine A can call procedures on machine B
A is suspended
Execution continues on B
When B returns, control passed back to A
Goal: it appears to the programmer that a normal call is taking place

5. Introduction
Remote Procedure Call (RPC) is a high-level model for client-sever communication.
RPC enables clients to communicate with servers by calling procedures in a similar way to the conventional use of procedure calls in high-level languages.
Examples: File service, Authentication service.

6. The RPC model Blocking state client server request reply
Executing state
Calls procedure and wait for reply
Receives request and starts process execution
Sends reply and wait for next execution
Resumes execution

7. RPC Mechanisms
The client transfer call request (the procedure name) and the arguments to the server via client stub function
stub function
marshals arguments and places them into a message together with the remote procedure identifier.
Sends message to server and waits for call return

8. RPC Mechanisms
Server receives the call request and passes to an appropriate server stub function.
server stub function unmarshals the arguments, call the corresponding (local) service procedure.
On return, the server stub marshals the output arguments into a call return message and sends back to the client.

9. RPC Mechanisms
Client stub receives call reply, unmarshals value, returns to client code

10. RPC Mechanisms Client computer Server computer service procedure Local
stub
func.
Communication
module
Local
return
call
Client computer
Server computer
server
service
procedure
Receive
reply
Send
request
Unmarshal
results
Marshal
arguments
Select procedure
Execute procedure

11. Benefits Familiar procedure call interface
Writing applications is simplified
RPC hides all networks codes
Programmers don’t have to worry about details (sockets, port numbers, byte ordering)
RPC: presentation layer in OSI model

12. Characteristics
The called procedure is in another process which may reside in another machine.
The processes do not share address space.
Passing of parameters by reference and passing pointer values make no sense.
The called remote procedure executes within the environment of the server process.
The called procedure does not have access to the calling procedure's environment.

13. Design issues

14. Parameter passing By values By reference Trick
easy, just copy data to network message.
By reference
makes no sense without shared memory
Trick
Copy items referenced to message buffer
Ship them over
Unmarshal data at server
Pass local pointer to server stub function
Send new value back

15. Representing data No such things as incompatibility on local systems
Remote machine may have:
Different byte ordering
Different sizes of integers and other types
Different floating point representations
Different character sets
Need standard encoding to enable communication between heterogeneous systems

16. Representing data Implicit typing Explicit typing
Only values are transmitted, not data type or parameter information
E.g., Sun XDR (eXternal Data Representation)
Explicit typing
Types are transmitted with values
E.g., ISO ANS.1, XML

17. Binding How to locate host and server process?
Solution 1: use a central DB
Server sends message to central DB indicating the services it can offer
Clients contact this authority whenever they need to locate a service
Solution 2:
Client needs to know server name
Server maintains a DB of available services

18. Failures Local procedure calls do not fail
If they core dump, the entire process dies
RPC is more vulnerable to failure:
Server could generate errors
Problems in network
Server crash
Client crash while server is still running code for it
Transparency breaks here
Applications should be prepared to deal with RPC failure

19. Delivery guarantees Retry request message: Duplicate filtering :
Client retransmits the request message until either a reply or the server is assumed to have failed.
Duplicate filtering :
server filters out duplicate message.
Retransmission of replies:
Server keeps a history of reply messages to enable lost replies retransmitted without re-executing the server operations.

20. Call Semantics Semantic of local procedure calls: exactly-once
Exactly-once maybe difficult to achieve with RPC
At-least-once
The client assumes that the RP is executed at least once (on return from the RP).
Can be implemented by retransmission of the request message on time-out.
Acceptable only if the server’s operations are idempotent. That is f(x) = f(f(x)).

21. Call Semantics At-most-once
When a RPC returns, the remote procedure (RP) is assume to have been called exactly once or not at all.
Implemented by the server's filtering of duplicate requests and caching of replies.

22. Call Semantics At-most-once
This ensure the RP is called exactly once if the server does not crash during execution of the RP.
When the server crashes during the RP's execution, the partial execution may lead to erroneous results.
In this case, we want the effect that the RP has not been executed at all.

23. Call Semantics

24. More issues Performance Security
remote procedure call and return time can be significantly slower than that for local procedure call (1 - 3 orders of magnitude).
Security
Messages visible over the network
Authenticate client
Authenticate server

25. Programming with RPC
Most languages (C, C++, Java,…) have no concept of remote procedure calls
Language compilers will not generate client and server stubs
Common solution:
Use a separate compiler to generate stubs (pre-compiler)

26. Programming with RPC Interface Definition Language
Allow programmers to specify remote procedure interfaces (names, parameters, return values)
Pre-compiler can use this to generate client and server stubs
Marshalling code
Unmarshalling code
Network transport protocols
Conform to defined interface

27. Programming with RPC Client code has to be modified Server functions
Initialize RPC-related options
Transport type
Locate host/service
Handle failure of remote procedure call
Server functions
Generally need little or no modification

28. Case Studies: SUN RPC Interface definition language: XDR
a standard way of encoding data in a portable fashion between different systems;
Pre-compiler: rpcgen
A compiler that takes the definition of a remote procedure interface, and generates the client stubs and the server stubs;
Communication handling: TCP or UDP

29. References

30. Thank you!