Communication
Tran, Van Hoai
Department of Systems & Networking
Faculty of Computer Science & Engineering
HCMC University of Technology
Outline
• Communication organized in layers
• Communication models
– Remote Procedure Call
– Remote Method Invocation
– Message Oriented Middleware
– Streams
2009-2010
2
Communication is critical
“A system in which hardware or software
located at networked computers
communicate and coordinate their actions
only by message passing”.
[Coulouris]
2009-2010
3
OSI model
application
presentation
session
transport
application protocol
7
presentation protocol
6
session protocol
5
transport protocol
4
network protocol
3
Practicallydataimplemented
link protocol
2
data link
in Internet
physical protocol
1
physical
network
Network
Message in layered communication
data link layer header
network layer header
transport layer header
session layer header
presentation layer header
application layer header
data link layer trailer
Message
Application Protocols
• Session/Presentation Protocols
– Rarely used in practice
• Application Protocols
– No clear distinction (in application layer) between
• Application (ftp
program)
General-purpose
protocols
(in application layer) can be
considered as protocol
transport
protocols,
• Application-specific
(Internet
File Transfer
butProtocol)
are classified as MIDDLEWARE PROTOCOLS
• General-purpose protocol
– HTTP used for Web
– HTTP used for JAVA
Middleware protocols
• What is it ?
– more specific application
– support a variety of middleware services
• Examples
– Authentication protocol
– Authorization protocol
– Multimedia broadcasting protocol
They are independent to applications, but can be
used in many middleware services
Adapted OSI model
application
presentation
middleware
session
transport
network
data link
physical
application protocol
76
presentation protocol
middleware protocol
session protocol
6
5
5
transport protocol
4
network protocol
3
data link protocol
2
physical protocol
1
Network
• Middleware communication services provide
message-passing services
Remote Procedure Call (RPC)
• Suggested by Birrell & Nelson (1984)
• Quite simple
– Calling procedures on different machines like
conventional procedure calls
•
•
•
•
Procedure A() calls a procedure B()
A() transfers parameters, execution of A() suspended
B() performed until finish
Execution returns to A()
How conventional call implemented?
• read( fd, buf, nbytes )
Stack before
Stack after
Main program’s
local variables
stack pointer
Main program’s
local variables
• Parameters
 Call-by-value
 Call-by-reference
nbytes
buf
fd
return address
read’s local variables
Principle of RPC
•
•
•
•
•
•
•
•
•
•
Client procedure calls client stub in
normal way
Client stub builds message and calls
local OS
Client OS sends message to remote OS
Remote OS gives message to server
stub
Server stub unpack parameters and
calls server
Server works and returns result to
server stub
Server stub packs message and calls
local OS
Server OS sends message to client OS
Client OS gives message to client stub
Client stub unpacks results and returns
to client
wait for result
client
call
remote
procedure
server
return
from call
request
reply
call local procedure
and return results
time
RPC example on computation
Client machine
Server machine
Server process
Client
process
k=add(I,j)
proc: “add”
int: val(i)
int: val(j)
Implementation
of add()
Client
Server
stub
stub
Client OS
k=add(I,j)
proc: “add”
int: val(i)
int: val(j)
ServerOS
proc: “add”
int: val(i)
int: val(j)
Network
Heterogenous architectures
• Large distributed systems consist of
multiple machine types, difference in
– data type representation
•
•
•
•
IBM mainframe: EBCDIC character code
Personal computer: ASCII character code
Intel: little endian
Sparc: big endian
Passing reference parameters
• Solutions
– Forbid call-by-reference
– Call-by-copy/restore
– Eliminate copying if not necessary
• Not easy to make transparent for complex
data structures
– graph
– linked list
Let applications do
Parameter specification &
Stub generation
• Client & server must agree on representation
of simple data structures
– float in IEEE standard #754
– charaters in 16-bit unicode
– stored in little endian
Client & server uses connection-oriented protocol
• Interface is define by Interface Description
Language (IDL), to support stub generation
Extended models of RPC
• Doors: as IPC
computer
Server process
Client process
return to
calling
process
main()
{
…
fd=open(door_name, …);
door_call( fd, … );
…
Invoke registered
}
door at other
process
OS
register
door
server_door()
{
…
door_return();
}
main()
{
…
fd = door_create(…);
fattach( fd, door_name, …);
…
}
Asynchronous RPC (1)
wait for result
client
call
remote
procedure
server
return
from call
request
reply
call local procedure
and return results
time
client
wait for acceptance
call
remote
procedure
server
return
from call
request
accept request
call local procedure
time
Asynchronous RPC (2)
• Deferred synchronous RPC: combining 2
asynchronous RPC
wait for acceptance
client
call
remote
procedure
server
return
from call
request
interrupt client
return
results
acknowledge
accept request
call local procedure
call client with
asynchronous RPC
time