It deals with communication primitives(send & receive) and also about RPC protocol...

1. Preetha/ME-CSE/I

2. INTRODUCTION
• High level constructs [Helps the program in
using underlying communication network]
• Two Types of Communication Models
– Message passing
– Remote Procedure Calls
Preetha/ME-CSE/I

3. Message Passing Method
• Two basic communication primitives
 SEND - Message & its Destination
 RECEIVE - Source of msg &Buffer for storing the
message
• Client-Server Computation Model
 Client sends Message to server and waits
 Server replies after computation
Preetha/ME-CSE/I

4. Design issues
Blocking vs Non blocking primitives
Nonblocking
• SEND primitive return the control to the user process as soon as the message is
copied from user buffer to kernel buffer.
• Advantage : Programs have maximum flexibility in performing
computation and communication in any order.
• Drawback : Programming becomes tricky and difficult .
Blocking
• SEND primitive does not return the control to the user process until message has
been sent or acknowledgement has been received.
• Advantage : Program behavior is predictable.
• Drawback : Lack of flexibility in programming.Preetha/ME-CSE/I

5. Cont…
Synchronous vs Asynchronous Primitives
Synchronous
• SEND primitive is blocked until corresponding RECEIVE
primitive is executed at the target computer.
• Also known as rendezvous.
Asynchronous
• Messages are buffered.
• SEND primitive does not block even if there is no
corresponding execution of the RECEIVE primitive.
• The corresponding RECEIVE primitive can be either blocking
or non-blocking.
• Drawback : buffering message is more complex , as it involves
creating , manging &destroying buffers.Preetha/ME-CSE/I

6. Details to be handled in Message Passing
• Pairing of Response with Requests
• Data Representation
• Sender should know the address of Remote
machine
• Communication and System failures
Preetha/ME-CSE/I

7. Remote procedure call (RPC)
• RPC is a protocol that one program can use to
request a service from a program located in
another computer in a network without having
to understand network details.
• RPC uses the client/server model.
• The requesting program is a client and the
service-providing program is the server.
Preetha/ME-CSE/I

8. Cont..
• The main idea of an RPC is to allow a local
computer (client) to remotely call procedures
on a remote computer (server).
• RPC is an interaction between a client and a
server.
• Client invokes procedure on sever.
• Server executes the procedure and pass the
result back to client.
• Calling process is suspended and proceeds
only after getting the result from server.
Preetha/ME-CSE/I

9. Preetha/ME-CSE/I

10. RPC design issues
• Structure
• Binding
• Parameter and Result Passing
• Error handling, semantics and Correctness
Preetha/ME-CSE/I

11. Structure
Stub : It is a piece of code used for converting parameters passed
during a RPC.
Client stub procedure :
• constructs a message containing identity of remote procedures & parameters.
• sends the message to remote server machine.
Server stub procedures :
• receives the message at remote machine side & passes the parameter to the
procedure specified.
• Upon completion, it passes the result back to the client-stub procedure ,which
returns the result to the client.
Preetha/ME-CSE/I

12. Binding
It is a process that determines the remote
procedure & the machine on which it will be
executed, upon a remote procedure invocation.
Binding server :
 Client-server model uses this server.
 It stores the server machine addresses along
with the services they provide.
Preetha/ME-CSE/I

13. Parameter and result passing
1. Stub procedures convert the parameters to remote machine
format and pack them into a buffer . After the message is
received , it must be unpacked.
Problems :Conversion becomes expensive if it has to be done
on every call.
2. Send the parameters along with a code identifying the format
used so that receiver can do the conversion.
Problems :Machine should know how to convert all the
formats that can possibly be used and poor portability
because whenever new representation is introduced into the
system, existing s/w need to be updated.
Preetha/ME-CSE/I

14. Cont..
 Each data type may have standard format in
the message .Here ,sender convert data to
standard format & receiver convert from
standard format to its local representation.
 Problem : arises if both sender and receiver
use the same internal representation.
 Another issue is passing parameter by value &
reference .Passing parameter by value is
simple but other is complicated
Preetha/ME-CSE/I

15. Error handling ,semantics & correctness
RPC fails for two reasons:
 Computer failure
 Communication failure
Semantics play a significant role in development of
programs for distributes computation.
RPC semantics :
 “At least once” semantics
 “exactly once” semantics
 “At most once” semantics
Preetha/ME-CSE/I

16.  “At least once” semantics
If RPC succeeds,it implies that atleast one execution of remote
procedure has taken place at remote machine.
Else 0,partial,one or more execution.
 “exactly once” semantics
If RPC succeeds,it implies that exactly one execution of
remote procedure has taken place at remote machine.
Else 0,partial,one execution.
 “At most once” semantics
Similar to exactly once semantics , in addition calls that do not
terminate do not produce any side effects and also referred as
Zero-or-one semantics.
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