Layered Protocol

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Layered Protocol

• Simple idea
• Agreements
• 0-bit/1-bit volts
• Last bit of the message
• Message has been damaged or lost
• Numbers, strings, data length

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Layered Protocol

• OSI Reference Model

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Layered Protocol

• Why layered structure?
• To reduce complexity and ease to modify
• Once the service is defined, the higher layer is
  shielded from implementation details of its lower
  layer
• Each protocol is independent of the others
• Number of layers, the name, the contents, and
  functions of each layer differs for each network

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Layered Protocol

• Protocol
• A set of rules exchanged between peer entities
  within a layer tells what a layer works
• Connection-Oriented
• Messages are delivered in order
• Connection establishment, Information transfer,
  Connection release
• Connectionless
• Each message is delivered independent of the
  others, and may be out of order

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Layered Protocol

• Typical message as it appears on the network

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Physical Layer

• Concerned with transmitting raw bits over a
  channel
• Mechanical interface connector type, meaning of
  each pin, etc.
• Electrical interface voltage level of 0 and 1,
  bit interval, etc.
• Procedural interface
• Physical transmission medium

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Data Link Layer

• Deals with error-free transmission between sender
  and receiver
• Framing special bit patters at the beginning of
  a frame
• Error control for damaged, lost, and duplicate
  frames
• Flow control

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Data Link Layer

• Discussion between a receiver and a sender

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Network Layer

• Concerned with the operation of subnet
• Routing determines an outgoing link on the best
  path to destination
• Congestion control to avoid a congestion within
  the subnet
• IP packet Connectionless protocol
• Virtual channel Connection-oriented protocol
• Virtual path group of virtual channel

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Transport Layer

• Concerned with end-to-end reliable communication
• Multiple network connection to improve throughput
• Multiplexing several transport connection onto
  same net connection
• type of service (at the time of connection
  set-up)
• in-order delivery, delivery without guarantee
  about the order, broadcasting etc.
• Flow control between hosts

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Transport Layer

• TCP (Transmission Control Protocol)
• Connection-oriented protocol
• Provides a reliable end-to-end byte stream
• Dynamically adapting to the unreliable
  internetwork and to be robust in the face of many
  kinds of failures
• UDP (Universal Datagram Protocol)
• Connectionless protocol
• Used for client-server applications

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Client-Server TCP

• Normal operation of TCP Transactional TCP

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Session and Presentation Protocol

• Session Layer concerned with controlling a
  session (or a dialogue)
• A session service can be an ordinary transport
  service, or some enhanced one.
• Dialogue control, token management, etc.
• Presentation Layer concerned with
  representation of information
• Compression
• Encryption
• Encoding the representation used inside a
  computer needs to be converted to abstract data
  structure (or net standard representation)

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Application Protocols

• Concerned with particular protocols commonly
  needed
• Virtual terminal
• FTP (File Transfer Protocol)
• HTTP (HyperText Transfer Protocol)
• Electronic mail
• Remote job entry
• Directory look up etc.

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Middleware Protocols

• Support high-level communication services
• Transfer real-time data
• Reliable multicasting services

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Remote Procedure Call

• Exist Problems
• Procedures run on different machines
• Execute in different address spaces
• Parameters and results have to be passed
• ? Both machines can crash

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Conventional Procedure Call

• Count read(fd, buf, nbytes)

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Conventional Procedure Call

• Call by value
• Copy to the stack
• Not affect the original value
• Call by reference
• Pointer to variable
• Modify the calling procedure
• Call by copy/restore
• Copy to the stack
• Overwriting the original value

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Client and Server Stubs

• Principle of RPC between a client and server
  program

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Client and Server Stubs

• 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

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Passing Value Parameters

• Parameter marshaling Packing parameter into a
  message

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Passing Value Parameters

• Little endian Big endian Invert
• Original message on the Pentium
• The message after receipt on the SPARC
• The message after being inverted. The little
  numbers in boxes indicate the address of each byte

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Passing Reference Parameters

• How are Pointers, or in general, references
  passed?
• Forbid pointers and reference parameters in
  general
• Call-by-reference have been replace by
  copy/restore
• Input Need not be copied back
• Output Need not be sent over in the first place
• We can now handle pointers

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Parameter Specification and Stub Generation

• A procedure
• The corresponding message.

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Parameter Specification and Stub Generation

• Interface Definition Language(IDL)
• An interface specified in such an IDL, is then
  subsequently compiled into a client stub and a
  server stub, along with the appropriate
  compile-time or run-time interfaces.

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Doors

• Doors in IPC machanism

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Asynchronous RPC

• The interconnection between client and server in
  a traditional RPC
• The interaction using asynchronous RPC

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Asynchronous RPC

• A client and server interacting through two
  asynchronous RPCs

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Introduction to DCE

• Distributed File Service
• Provides a transparent way of accessing any file
  in the system in the same way
• Directory Service
• Keep track of the location of all resources in
  the system
• Security Service
• Access can be restricted to authorized persons
• Distributed Time Service
• Keep clocks on the different machines globally
  synchronized

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Goals of DCE RPC

• The RPC system can automatically handle data type
  conversions between the client and the server
• Ability to hide the details, clients and servers
  are highly independent of one another
• Binding Automatically locate the correct
  server, Set up the communication

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Writing a Client and a Server
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Binding a Client to a Server

• Performing an RPC
• At-most-once operation

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Distributed Objects
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Compile-time versus Runtime Objects

• Compile-time
• Easier to build distributed application
• ex) Java
• RunTime
• Use an object adapter
• Dynamically bind

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Persistent and Transient Objects

• Persistent object
• Continues to exist
• Not dependent on its current server
• Transient object
• Server exits, the object ceases to exist

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Binding a Client to an Object
Distr_object obj_ref //Declare a systemwide
object referenceobj_ref // Initialize the
reference to a distributed objectobj_ref-gt
do_something() // Implicitly bind and invoke a
method (a) Distr_object objPref //Declare a
systemwide object referenceLocal_object
obj_ptr //Declare a pointer to local
objectsobj_ref //Initialize the reference
to a distributed objectobj_ptr
bind(obj_ref) //Explicitly bind and obtain a
pointer to the local proxyobj_ptr -gt
do_something() //Invoke a method on the local
proxy (b)

• Implicit binding using only global references
• Explicit binding using global and local references

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Implementation of Object References

• Server crashes and assign a different endpoint
  after recovery
• Location server
• Server may simultaneously support several kind
  data coming
• Implementation handle

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Static versus Dynamic Remote Method Invocations

• Remote Method Invocations (RMI)
• Similar to RPC
• Support systemwide object reference
• Static Invocation
• If interfaces change then client application must
  recompile
• Dynamic Invocation
• Invocation at runtime

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Parameter Passing
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The DCE Distributed-Object Model

• Distributed dynamic object Distributed
  named object

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DCE Remote Object Invocation

• Binding handle

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The Java Distributed-Object Model
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Java Remote Object Invocation