Understanding Networked Applications:
A First Course
Chapter 12
by
David G. Messerschmitt
Understanding Networked Applications:
A First Course
Communication services
by
David G. Messerschmitt
Example communication services
• Messages
• Queuing and multiplexing
• Message with reply
– Remote method invocation
• Conversation
• Broadcast
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A First Course
Some objectives
• Allow modules to interact in the same way
across different hosts as on a single host
• Make it easier to develop applications by
capturing common generic needs
• Support a range of applications with distinct
needs
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Message
• The smallest unit of information meaningful
to the sender and recipient
– Recipient must be ready and listening
• Directs or informs the recipient
– Pure push model
• Employs a send-receive protocol
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Supporting deferred applications
• Queuing
– Queuing allows a message to be sent even
though the recipient is not waiting
– The message sits in a queue somewhere in the
infrastructure until it is actively retrieved by the
recipient
– Combines send-receive and request-reply
protocols
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A First Course
Participants in message with
queuing
Sender
Recipient
Send/
receive
Queue
Sender can
send message
whenever it
wants
Request/
response
Recipient can
access message
whenever it wants
Infrastructure
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A First Course
Multiplexing
• Multiplexing
– Allows recipient to receive messages from two
or more senders
– How are the senders identified?
– Presumes queuing: what if two messages are
sent simultaneously?
– Supported by Message Oriented Middleware
(MOM)
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Multiplexing and queuing
Sender1
Recipient
Sender2
Mux
&
queue
Recipient deals with
one message at a time
even if coming from
many senders
Infrastructure
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A First Course
Example application: workflow
Receiving
Administrative Purchasing
assistants
Workers
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What message is not
• Delivery may not be guaranteed
• No shared context of messages from same
sender to recipient
– Application is free to create that context on its
own
– Messages may not be delivered in the same
order as sent
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A First Course
Message with reply
• Service couples two messages:
– Request and coupled response message
– Sender freed of burden of associating response
• Requests information or a service
– Pure pull model
• Immediate
– Recipient presumes sender is waiting for reply
and responds as quickly as possible
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A First Course
RMI
method: parameters return_values
Method invocation
An action object-oriented
programming is called a
method
method: parameters return_values
Remote method invocation (RMI)
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A First Course
Sender is blocked waiting for response
method: parameters -> return_values
Client object
Server object
method,parameters
Blocked: can’t do
anything else
Computes
method()
returns
Time
= Blocked waiting for response
Understanding Networked Applications
= Working on request
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A First Course
Comparison
Sender
Recipient
Sender
Send/receive
Queue
Recipient
Send/receive
Message
waiting
Request/
response
Request/
response
= Work on message or request
= Blocked waiting for response
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Conversation
• Two-way exchange of messages
• Messages in one direction are influenced by
messages in the other
• To maintain conversation
– Messages should be delivered reliably
– Messages should be delivered in the same order
as sent
– No coupling of replies with messages
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Session protocol
A session typically
provides
guaranteed
ordered delivery to
maintain proper
context
Establish
Conversation
Disestablish
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Some advantages of sessions
• Aids application by managing a shared
context for a conversation
– Identification, ordering, reliable delivery
• Reduces overhead
– Example: only one name server query
• Security (Chapter 8)
• Quality of service (Chapter 11)
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A First Course
Examples
• What are examples of applications that
would naturally leverage:
–
–
–
–
Messages?
Queuing and multiplexing?
Message with reply?
Conversation?
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Multimedia record-storeplayback
Audio
coder
Audio
decoder
Record
Storage
Playback
One (big)
Storage
message
Video
coder
Understanding Networked Applications
Video
decoder
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A First Course
Streaming multimedia
Audio
coder
Audio
decoder
Video
coder
Video
decoder
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A First Course
A session supports streaming
multimedia
Accumulate audio in message
Send
message
Time
Receive
message
Playback audio in
message
Delay
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Delay and latency
Message latency = time from
sending of message until it is
received in its entirety
Time
Delay = time elapsing between
an event (or series of events) and
its (their) representation
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Relationship of delay and latency
Latency
Time
Delay
Delay = latency (of network) + message accumulation time
If delay is important, keep the messages short!
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