VII Protocol Software On A Conventional Processor

39 Pages·2004·0.06 MB·English

VII
Protocol Software On A
Conventional Processor
NSD-Agere -- Chapt. 7 1 2004
Possible Implementations Of
Protocol Software
In an application program
(cid:0)
– Easy to program
– Runs as user-level process
– No direct access to network devices
– High cost to copy data from kernel address space
– Cannot run at wire speed
NSD-Agere -- Chapt. 7 2 2004
Possible Implementations Of
Protocol Software
(continued)
In an embedded system
(cid:0)
– Special-purpose hardware device
– Dedicated to specific task
– Ideal for stand-alone system
– Software has full control
NSD-Agere -- Chapt. 7 3 2004
Possible Implementations Of
Protocol Software
(continued)
In an embedded system
(cid:0)
– Special-purpose hardware device
– Dedicated to specific task
– Ideal for stand-alone system
– Software has full control
– You will experience this in lab!
NSD-Agere -- Chapt. 7 3 2004
Possible Implementations Of
Protocol Software
(continued)
In an operating system kernel
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– More difficult to program than application
– Runs with kernel privilege
– Direct access to network devices
NSD-Agere -- Chapt. 7 4 2004
Interface To The Network
Known as Application Program Interface (API)
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Can be
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– Asynchronous
– Synchronous
Synchronous interface can use
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– Blocking
– Polling
NSD-Agere -- Chapt. 7 5 2004
Asynchronous API
Also known as event-driven
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Programmer
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– Writes set of functions
– Specifies which function to invoke for each event type
Programmer has no control over function invocation
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Functions keep state in shared memory
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Difficult to program
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Example: function f() called when packet arrives
(cid:0)
NSD-Agere -- Chapt. 7 6 2004
Synchronous API Using Blocking
Programmer
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– Writes main flow-of-control
– Explicitly invokes functions as needed
– Built-in functions block until request satisfied
Example: function wait_for_packet() blocks until packet
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arrives
Easier to program
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NSD-Agere -- Chapt. 7 7 2004
Synchronous API Using Polling
Nonblocking form of synchronous API
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Each function call returns immediately
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– Performs operation if available
– Returns error code otherwise
Example: function try_for_packet() either returns next
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packet or error code if no packet has arrived
Closer to underlying hardware
(cid:0)
NSD-Agere -- Chapt. 7 8 2004
Typical Implementations And APIs
Application program
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– Synchronous API using blocking (e.g., socket API)
– Another application thread runs while an application
blocks
Embedded systems
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– Synchronous API using polling
– CPU dedicated to one task
Operating systems
(cid:0)
– Asynchronous API
– Built on interrupt mechanism
NSD-Agere -- Chapt. 7 9 2004