1. CMSC621: Advanced Operating Systems Advanced Operating Systems
Nilanjan Banerjee
Associate Professor, University of Maryland
Baltimore County
Slide Credit: Dr. Ying Lu
Advanced Operating Systems

2. Client-Server Communication
Assume that you are developing a client-server application:
How to let the two processes (client and server) located on two machines communicate with each other?
Socket programming: using functions like connect(sd, (struct sockaddr *)&sin, sizeof(sin)), write(sd, buf, strlen(buf)) etc.

3. Remote Procedure Calls (RPC)
Avoid explicit message exchange between processes
Basic idea is to allow a process on a machine to call procedures on a remote machine
Make a remote procedure possibly look like a local one
Original paper on RPC:
A. Birrell, B Nelson, “Implementing Remote Procedure Calls”, ACM Symposium on Operating System Principles, 1984

4. Conventional Procedure Call
How are parameters passed in a local procedure call
E.g., #include <sys/types.h>
#include <unistd.h>
...
char buf[20];
size_t nbytes;
ssize_t bytes_read;
int fd;
nbytes = sizeof(buf);
bytes_read = read(fd, buf, nbytes);

5. Conventional Procedure Call
Figure 4-5. (a) Parameter passing in a local procedure call: the stack before the call to read. (b) The stack while the called procedure is active.

6. Remote Procedure Calls (RPC)
How are parameter passed in a remote procedure call, while making it look like a local procedure call?

7. Client and Server Stubs
Principle of RPC between a client and server program.

8. Steps of a Remote Procedure Call
Client procedure calls client stub in normal way
Client stub builds message, calls local OS
Client's OS sends message to remote OS
Remote OS gives message to server stub
Server stub unpacks parameters, calls server
Server does work, returns result to the stub
Server stub packs it in message, calls local OS
Server's OS sends message to client's OS
Client's OS gives message to client stub
Stub unpacks result, returns to client

9. Passing Value Parameters (1)
2-8
Steps involved in doing remote computation through RPC

10. Passing Value Parameters (2)
The usual contrast is between most versus least significant byte first, called big-endian and little-endian respectively.

11. Passing Value Parameters (3)
Original message on the Pentium (little-endian)
The message after receipt on the SPARC (big-endian)
Note: the little numbers in boxes indicate the address of each byte

12. Passing Value Parameters (3)
Original message on the Pentium (little-endian)
The message after receipt on the SPARC (big-endian)
The message after being inverted (integer 5, string: “LLIJ”)
Note: the little numbers in boxes indicate the address of each byte

13. Passing reference parameters
What is Call By Value and Call By Refernce?
Example: call foo(int, int * ) or read(fd, buf, nbytes)
Call by copy/restore
The dreaded “pointer problem”
Linked list
Complex graph a b a’ b’
foo(a, &b )
Call foo(a, &b’ )
Copy value a and contents of loc b
into a’ and loc b’
Return Copy contents of loc b’ into b
Machine A
Machine B

14. Marshalling
Values must match cross the network
Machine formats differ
Integer byte order
Little-endian or big-endian
Floating point format
IEEE 754 or not
Marshalling  transferring data structure used in remote procedure call from one address space to another.
Define a “network format”, for example following XDR (eXternal Data Representation) standard

15. RPC: The basic mechanism
Client calls a local procedure on the client stub
The client stub acts as a proxy and marshalls the call and the args.
The client stub send this to the remote system (via TCP/UDP)
The server stub unmarshalls the call and args from the client
The server stub calls the actual procedure on the server
The server stub marshalls the reply and sends it back to the client
Client process
Server process
Client
routines
Server
routines 1 5
Client stub
Server
stub 2 4
RPC
runtime
RPC
runtime
Process
Process
kernel
kernel 3 6
Network
routines
Network
routines
Source: R. Stevens, Unix Network Programming (IPC) Vol 2, 1998

16. Binding a Client to a Server (1)
Registration of a server makes it possible for a client to locate the server and bind to it.
Server location is done in two steps:
Locate the server’s machine.
Locate the server on that machine.

17. Binding a Client to a Server (2)
Figure Client-to-server binding in DCE.

18. Asynchronous RPC (1)
2-12
The interconnection between client and server in a traditional RPC
The interaction using asynchronous RPC

19. A client and server interacting through two asynchronous RPCs
2-13
One-Way RPC: The client invokes a remote procedure but it does not block or wait until it receives a return.

20. Client and server fail independently
LPC v.s. RPC
Global variables
Client and server fail independently
RPC: requires code to deal with server crashes

21. Semantics of remote procedure calls
When Things Go Wrong
Semantics of remote procedure calls
Local procedure call: exactly once
How many times a remote procedure call may be called?
A remote procedure call may be called:
0 time: server crashed or server process died before executing server code
1 time: everything worked well
1 or more: due to excess latency or lost reply from server, client retransmitted
Exactly once may be difficult to achieve with RPC

22. Most RPC systems will offer either:
RPC Semantics
Most RPC systems will offer either:
at least once semantics
or at most once semantics
Understand application:
Illustrate some applications that “at least once” is suitable?
Idempotent functions: may be run any number of times without harm
Illustrate some applications that “at most once” is suitable?
At-most-once call semantics are for those RPC applications which require a guarantee that multiple invocations of the same RPC call by a client will not be processed on the server. An example of such an application is inventory control. Consider several point-of-sale (POS) workstations and a server which maintains inventory records. Each POS workstation makes an RPC call to the server when an item is sold. The call causes a count of the number of items left in the inventory to be decremented on the server. If a call indicating that five of the item X was sold is processed on the server more than once, then the inventory record for item X will be in error.