Chapter 4: Interprocess Communication
---

- Layering:

	---------------------------
	  Applications, services
	---------------------------
	       RMI and RPC		Chapter 5
	---------------------------
	   Request-reply protocol
	       marshalling		Chapter 4
	External data representation
	---------------------------
		UDP / TCP		Chapter 3
	---------------------------

- synchronous communication: both send and receive are blocking;

- asynchronous communication: send is non-blocking, while both
  blocking and non-blocking receive are supported;

- instead of using ports, in the V System messages are addressed to
  processes;

- an agreed standard for the representation of data structures and
  primite values is called an external data representation;

- marshalling is the process of taking a collection of data items and
  assembling them into a form suitable for transmission in a message;
  unmarshalling is the reverse process; Ex: 
	
	- CORBA's common data representation;
	- Java's oject serialization mechanism;
	- Sun's XDR

- CORBA'S CDR:

	- can represent all data types used as arguments in remote
	  invocation in CORBA;

	- 15 basic types + composite types;

	- in CDR, multi-byte types are transmitted in the sender's
	  format;

	- the types of data items are described in CORBA IDL, which
	  provides a notation for describing the types of the
	  arguments and results of RMI methods;

	- from this definition, the CORBA interface compiler generates
	  appropriate marshalling and unmarshalling operations;

- Java object serialization:

	- to serialize an object, its class information is written
	  out, followed by the types and names of its instance variables;
	  If the instance variables belong to new classes, then their
	  class information must also be written out.  

- Remote object references:

	- when a remote method invocation is sent, the message needs
	  to specify a particular object.  A remote object reference 
	  is an identifier for a remote object valid throughout a 
	  distributed system;

	- must be unique over space and time. Ex:

		- IP || port || time || object#

- Three protocols for implementing various types of RPC:

	- the request (R) protocol;
	- the request-reply (RR) protocol;
	- the request-reply-acknowledge (RRA) protocol;

- IP multicast:

	- multicast addresses can be permanent or temporary;
	- UDP/IP multicast suffers from omission failures;
	- some applications require reliable multicast: ordered, no
	  duplicates, all-or-none semantics;