1. Message Based Communication
TANENBAUM-Chapter 2

2. Message Oriented Communication
RPC and RMI type firmware is not always desirable or even practical. They need both senders and receivers to be invoked.
In such cases messaging is the way to go!
Subject of concern in communication
Persistence
Transient
Asynchronous
Synchronous

3. Persistence and Synchronicity in Communication (1)
2-20
General organization of a communication system in which hosts are connected through a network and messages are passed and queued in an asynchronous manner.
Thus, persistence in comm is achieved.

4. Example: Persistence and Synchronicity in Communication (2)
Persistent communication of letters back in the days of the Pony Express.

5. Persistence and Synchronicity in Communication (3)
2-22.1
Persistent asynchronous communication
Persistent synchronous communication

6. Persistence and Synchronicity in Communication (4)
2-22.2
Transient asynchronous communication
Receipt-based transient synchronous communication

7. Persistence and Synchronicity in Communication (5)
Delivery-based transient synchronous communication at message delivery
Response-based transient synchronous communication

8. Message Based Transient Communication: Berkely Sockets
Messaging through transport level sockets
Sockets form an interface to transport layer
Socket is an operating system abstraction over the end-to-end communication protocol
UNIX derivative OSs are the first in providing socket interface to TCP of TCP/IP protocol suit
So called, well-known port numbers together with IP numbers are used to bind to a socket…

9. Socket primitives for TCP/IP.
Berkeley Sockets (1)
Primitive
Meaning
Socket
Create a new communication endpoint
Bind
Attach a local address to a socket
Listen
Announce willingness to accept connections. By allocating enough memory
Accept
Block caller until a connection request arrives
Connect
Actively attempt to establish a connection
Send
Send some data over the connection
Receive
Receive some data over the connection
Close
Release the connection
Socket primitives for TCP/IP.

10. Connection-oriented communication pattern using sockets.
Berkeley Sockets (2)
Connection-oriented communication pattern using sockets.

11. Message-oriented Transient Communication: MPI
Message passing middleware standard
MPI is developed to provide a transport layer independency or more transparency
MPI provides mainly transient communication, could be synch or asynch
MPI library can be tailored to high speed communication, usually provided as proprietary libraries
MPI is developed for parallel application, where each group and a process with the group are assigned identifiers

12. The Message-Passing Interface (MPI)
Primitive
Meaning
MPI_bsend
Append outgoing message to a local send buffer
MPI_send
Send a message and wait until copied to local or remote buffer
MPI_ssend
Send a message and wait until receipt starts
MPI_sendrecv
Send a message and wait for reply
MPI_isend
Pass reference to outgoing message, and continue
MPI_issend
Pass reference to outgoing message, and wait until receipt starts
MPI_recv
Receive a message; block if there are none
MPI_irecv
Check if there is an incoming message, but do not block
Examples for some of the most intuitive message-passing primitives of MPI. There are over 100 MPI functions, used especially to improve communication efficiency

13. Message Queing Model
Targeted to support non time critical persistent asynchronous transmission
Each receiving application has its own private queue, to which messages are delivered, with no guarantee for when they will be processed
The sender and receiver queues need to be globally identified
Queues are managed by queue managers
So called message brokers can be used to transfer a message into the format acceptable by the end receiver…

14. Message-Queuing Model (1)
2-26
Four combinations for loosely-coupled communications with respect to queues.

15. Message-Queuing Model (2)
Primitive
Meaning
Put
Append a message to a specified queue
Get
Block until the specified queue is nonempty, and remove the first message
Poll
Check a specified queue for messages, and remove the first. Never block.
Notify
Install a handler to be called when a message is put into the specified queue.
Basic interface to a queue in a message-queuing system.

16. General Architecture of a Message-Queuing System (1)
Mapping queue-level names to network locations, same as in DNS . Normally queues are managed by queue managers. Static mapping is easier dynamic mapping is more complex.

17. General Architecture of a Message-Queuing System (2)
2-29
The general organization of a message-queuing system with routers, that know about network topology, where every queue manager needs to know the nearest router, malticasting can also be done.

18. Message Brokers
2-30
The general organization of a message broker in a message-queuing system, used to convert one message format to another. Message broker is an application level message converter. A database should be used to allow rule based format conversions.

19. Message Brokers (cont.)
General message queues are aimed to support persistent communication for the end user.
Although systems behaves similarly they are not to guarantee message delivery, message priorities, logging facilitis, multicasting, load balancing, fault tolerance, etc.

20. Example: IBM MQSeries 2-31
General organization of IBM's MQSeries message-queuing system. a MCA-Message Channel Agent manages two ends of a message channel,
This is primarily aimed at mainframe database applications
Message channels between QM are implemented in TCP connection, and managed by Message Ch Agent-MCA.

21. Properties of MCA in IBM MQseries
Attribute
Description
Transport type
Determines the transport protocol to be used
FIFO delivery
Indicates that messages are to be delivered in the order they are sent
Message length
Maximum length of a single message
Setup retry count
Specifies maximum number of retries to start up the remote MCA
Delivery retries
Maximum times MCA will try to put received message into queue
Some attributes associated with message channel agents-MCA. Sending and receiving MCAs must be compatible in terms of rules of communication between them

22. Message Transfer in IBM MQSeries(1)
The general organization of an MQSeries queuing network using routing tables and aliases.
A message header, in addition to the send queue address, has to have the address of the queue manager-QM and the destination queue under that QM

23. Message Transfer in MQSeries(2) Primitives available in an IBM MQSeries MQI
Description
MQopen
Open a (possibly remote) queue
MQclose
Close a queue
MQput
Put a message into an opened queue
MQget
Get a message from a (local) queue
Message Queue Interface-MQI is a simple programming interface for the applications.
MQSeries provides these facilities to applications when the messages have arrived

24. Stream Oriented Communication(1)
In digital transmission of video and voice the timing is important. This depend humans’ physical hearing and vision abilities… this is continuous media (CM) domain… The transmission has to be in streams rather than self contained messages.
So, which facilities should a distributed system provide for such CM streams transmission:
Temporal relationship between different items of CM is crucial for correct interpretation of such data

25. Stream Oriented Communication(2)
CM is supported by data streams, because of existence of temporal relationship between the data items.
Temporal (time dependent) data transmission is generally provided by so called data streams.
Asynchronous tr mode: no limits are defined
Synch tr mode: max end-to-end delay is defined
Isochronous tr mode: max and min end-to-end delay is defined
Complex streams have more than one sub stream, e.g., stereo audio can have two audio streams…
Streams can be multicast between one source and a number of sinks and filtered if needed…

26. Setting up a stream between two processes across a network.
Data Stream (1)
Setting up a stream between two processes across a network.

27. Setting up a stream directly between two devices.
Data Stream (2)
2-35.2
Setting up a stream directly between two devices.

28. An example of multicasting a stream to several receivers.
Data Stream (3)
An example of multicasting a stream to several receivers.

29. Stream Comm: Quality of service (QoS)
QoS for CM tr can be defined in terms of timeliness, volume, and reliability
In addition to time requirements, in an application, using so called bucket algorithm, service requirements may include
loss sensitivity of data items
Minimum delay noticed by the receiver
Max delay variation
Quality of guarantee
QoS requires allocation of resources, usually, in terms of processing capacity, bandwidth, buffers…
There is no best model for specifying QoS parameters
Resource reSerVation Protocol is a transports level for routers to tune to the QoS requirements, by the servers.

30. Characteristics of the Input
Specifying QoS (1)
Characteristics of the Input
Service Required
maximum data unit size (bytes)
Token bucket rate (bytes/sec)
Token bucket size (bytes)
Maximum transmission rate (bytes/sec)
Loss sensitivity (bytes)
Loss interval (sec)
Burst loss sensitivity (data units)
Minimum delay noticed (sec)
Maximum delay variation (sec)
Quality of guarantee
A flow specification.

31. Specifying QoS (2): Example method
The principle of a token bucket algorithm. Irregular data stream is regulated at the bucket output.

32. Setting Up a Stream: RSVP-Resource reSerVation Protocol
The basic organization of RSVP-Resource reSerVation Protocol- for resource reservation in a distributed system, to achieve the required QoS.

33. Synchronization Example (1)
In multimedia systems this is very important, as more than one stream may come together to meet the application requirements
For example
Two voice streams for stereo voice applications
Presentations slides with voice streams
Video stream with voice stream: lip synchronization!
Synchronization at the lowest level is conducted by the operating system, for example switching between video frames and voice frames,
The low level communication channels must be able meet such requirements

34. Synchronization Example (2)
The principle of explicit synchronization on the level data unit: video and voice

35. Synchronization using Middleware (3)
2-41
The principle of synchronization as supported by high-level interfaces. Different streams are first converted into discrete stream packets, with synchronization information, which can then be multiplexed into one stream as in MPEG-2 standard.