Communication Inter-process Communication is the heart of a Distributed System
Communication is the
of a Distributed System
Basic mechanisms are:
How to Communicate?
a process in machine A wants to communicate (an item) to a process in machine B
How is this done?
Build a message
and send it through the network (cable) to the “other” side.
things have to be agreed upon!
Voltage (0/1)? For bit representation..
Correctness of messages
Representation of numbers, strings, and data items(in different machines)
Approach: OSI Layered Protocols
OSI Open System Interconnection Reference:
Layers, interfaces, and protocols in the OSI model.
Layered Protocols: as you dip into the layers you “add” more info
A typical message as it appears on the network.
Design for “
”: allows the communication of one system with another – possibly different one.
exactly “Open” ;
They are governed by a set of rules/regulations…
Protocols: formalization of these rules
Mostly concerned with the transmission of 0/1s..
Network connectors (plugs)
RS232c (standard for serial communication).
Data Link Layer: frames & checksums
Discussion between a receiver and a sender in the data link layer.
Does the “routing” of packets..
Problem: shortest path is not always available!!
Even if it is available “routing” may opt for a different path? Why?
IP Internet Protocol (main representative of Network Layer)
An IP packet (frame) can be sent without any setup.
Virtual Path is gaining popularity (in ATM networks)
Data will have to delivered to the other side
with no loss..
The job of the TL is exactly this!
TCP (Transmission Control protocol)
UDP (Universal Datagram Protocol): almost similar to IP.
RTP (Real-Time Protocol)
Example: Client-Server TCP
Normal operation of TCP.
Transactional TCP (TCP for Transactions Suite)
High(-er) Level Protocols..
Session and Presentation Layer
Provides dialog facilities, keeps track of who is talking at this time, and provides synchronization if needed.
All these are useful into long transfers as precautions can be taken for the transmission of data.
FTP (file transfer protocol)
HTTP (HyperText Transfer Protocol)
Example of Higher Level Protocols: Middleware Protocols
An adapted reference model for networked communication that may enforce the notion of “atomicity”
Communication of Processes-RPCs
[Birell & Nelson TOCS94]
When a process A in machine a calls a routine on machine B, the calling process A suspends and the execution of the called program in B takes place transparently..
Information is transported from caller to callee in the parameters and the result returns
No message passing is visible to the user.
Remote Procedure Call (RPC).
Idea: make a remote procedure call appear as a local one!
Conventional Procedure Call
Parameter passing in a local procedure call: the stack before the call to read (from
The stack while
the called procedure is active
Client and Server Stubs
works in a similar way to a usual system call (for instance read()).
The difference is that a different version (the client stub) is loaded into the frame stack.
This stub does not ask the OS to get any data on behalf of the process.
The stub packs the parameters into a single message and requests the message to be sent to the server.
When the msg arrives into the server, the OS of the server surrenders the msg to the server stub (server side equivalent).
Client and Server Stubs
Principle of RPC between a client and server program.
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
, 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
Passing parameters is always a problem..
Pass by value
Pass by reference
Call by copy/restore
Marshaling of parameters has to initially take place.
Passing Value Parameters
Steps involved in doing remote computation through RPC
Possible Problems with Passing Value Parameters
Original message on the Pentium (little Endian-number their bytes from right to left)
The message after receipt on the SPARC (big Endian-numbers the bytes from left to right)
The message after being inverted is not a general solution.
The little numbers in boxes indicate the address of each byte (need some type of deli-meter in order to understand the various representations involved)
integers and strings have to be handled
How are pointers being passed??
Generally difficult problem…
One solution is forbid passing of pointers all together (rather restrictive)
Some things can be done with arrays:
Copy the array into the message and send it over..
Call-by-reference is replaced by copy/restore..
If the stubs know whether the buffer is an Input parameter to the server or an output parameter to the client optimizations can be done.
Parameter Specification and Stub Generation
The corresponding message.
Extensions of RPCs:Doors[Hamilton & Kougiouris 94]
The interconnection between client and server in a traditional RPC
The interaction using asynchronous RPC
A client and server interacting through two asynchronous RPCs
Distributed Computing Environment(DCE)
Idea: take a group of machines (running Unix, OS2, Windows etc) add a layer of software and then be able to run distributed applications
disturbing the running of existing applications!
Some basic services offered such as:
Distributed File Service (the worldwide file system)
Distributed Time Service (why?)
Writing a Client and a Server
The steps in writing a client and a server in DCE RPC.
Binding a Client to a Server
A client can call a server as long as the server is registered and ready to accept calls!
Registration makes possible that the client can
the server and
Server location is down into two steps:
Locate the server’s machine
Locate the server (ie, the correct process on that machine!).
Binding of Clients and Servers
In order to achieve the step (2) the
for the service needs to be known.. (or
Ports are used by servers as different entry points to different procedure calls.
In DCE there is a daemon (DCE Daemon) that makes this look up between (
The server also registers with a directory machine(name of machine and IP number)
Binding a Client to a Server
Client-to-server binding in DCE.
Remote Object Invocation
Object-Oriented Technology (CORBA, DCOM) is another way to develop distributed applications.
RPCs could be applied to those frameworks as well.
Methods (operations on data)
Methods are available via interfaces
between object & interfaces is critical as far as Distributed Systems concerns.
Interface can be on one machine – Object on another.
Common organization of a remote object with client-side proxy.
Proxy = client stub ; Skeleton = Server Stub
of such distributed objects is NOT distributed!
A real-distributed object should be one that is physically distributed across multiple machines.
Implicit/Explicit Binding of a Client to an Object
An example with implicit binding using only global references
An example with explicit binding using global and local references (the client first calls a a special function that binds the object before the client can invoke the object’s methods)
Types of Remote Method Invocation (RMI)
Interfaces of an object are know when client application is being developed.
Be able to compose a methods invocation at run time!
invoke(object, method, in_params, out_params)
Example: append an integer
a file object
for which the object provides the method
Dynamic: invoke(fobject, id(append), int)
returns an identifier for the method
: when is it useful?
Parameter Passing in RMI
Passing an object by reference or by value.
Message Oriented Communication
Every host is connected to one mail-server (can think of it as being the “comm” server of the picture in the previous slide).
A client interface allows users to get access to the messages (located on the mail servers).
When a user submits an message, the host forwards the message its corresponding mail server.
Mail servers forward/delete messages.
Such a system is an example of
Mail-servers follow the Pony Express Model
has to do with the fact that the mail is stored as long as it takes to deliver it..
a message cannot be delivered
to the next server/destination is discarded (typical of the transport
layer – corresponds to store-and-forward router).
Persistent asynchronous communication
Persistent synchronous communication(msg actually delivered to the receiving site)
Transient asynchronous communication (examples are UDP, asynchronous RPCs)
Receipt-based transient synchronous communication
Delivery-based transient synchronous communication at message delivery
Response-based transient synchronous communication (RPCs & RMIs mostly adhere to this type of communication).
Socket primitives for TCP/IP.
How Berkeley SocketsWork
Connection-oriented communication pattern using sockets.
The MPI Interface
Some of the most intuitive message-passing primitives of MPI.
Four combinations for loosely-coupled communications using queues.
Message-Queuing Model: calls
Basic interface to a queue in a message-queuing system.
General Architecture of a Message-Queuing System
The relationship between queue-level addressing and network-level addressing.
General Architecture of a Message-Queuing System
The general organization of a message-queuing system with routers.
The general organization of a message broker in a message-queuing system.
Thus far, communication as long as it happens correctly is all we mind. Timing of this communication was not the issue.
Timing is important in certain types of communication …
CD quality audio transmission ( 16bit samples at 44,1kHz)
Streams - Definitions
Text with ASCII/Unicode
Images with GIF/JPEG
Audio Streams with 16bit
samples using PCM
Continuous (Representation) Media
: temporal relationship among the different data items are fundamental to interpret what the data means.
Motion requires 30-40 msec per image (if represented as a sequence of images).
: temporal relations among data items are not of essence
Representations of text, still-images, code, files, etc.
Data stream: a sequence of data units
Timing is essential into continuous data streams.
Asynchronous Transmission Mode
Data are xmited one after the other with no “timing” strings attached.
Synchronous Transmission Mode
There is a maximum end-to-end delay defined for every unit in the data stream.
Isochronous Transmission Mode
Data have to be transferred on time
There are minimum and maximum end-to-end delay (jitter)
Setting up a stream between two processes across a network.
Data Streams with Direct Connection between Source and Sink
Setting up a stream directly between two devices.
What if the sink is a multiparty?
Receivers may have different requirements
Filters have to be attached…
A flow specification.
Partridge’s Model expressed with a Token Bucket Algorithm
The principle of a token bucket algorithm.
Setting Up a Stream- RSVP Protocol
The basic organization of RSVP for resource reservation in a distributed system.
The principle of explicit synchronization on the level data units.
The principle of synchronization as supported by high-level interfaces.
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