Chapter 2: System Models
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- latency: delay between the sending of a message by one process and
  its receipt by another.  It includes:

	- the time for the first bit to reach the destination;
	- the delay in accessing the network, with depends on the
	  network load and contention;
	- time taken by the OS at both ends;

- bandwidth: amount of information that can be transmitted in a given
  time;

- jitter: variation in the time taken to deliver a series a messages
  (relevant to multimedia data);

- clock drift rate: relative amount that a computer clock differs from
  a perfect reference clock;

- synchronous distributed system:

	- time to execute each step of a process has known lower and
	  upper bounds;

	- each message transmitted over a channel is received within a
	  known bounded time;

	- each process has a local clock whose drift rate from real
	  time has a known bound;

- hard to provide guarantees about these values;

- asynchronous distributed systems: no bounds on:

	- process execution speeds;

	- message transmission delays;

	- clock drift rates.

- Lamport's logical time: allows the order in which the messages are
  presented to be inferred withouhg recourse to clocks;

- omission failures: cases when a process or communication channel
  fails to perform actions that it is supposed to do (e.g., process
  crash, lost packets);

- a process crash is called fail-stop if other processes can detect
  certainly that the process has crashed;

- arbitrary failures: the term Byzantine failure is used to descrive
  the worst possible failure semantics, in which any type of error 
  may occur; communication channels may suffer from arbitrary
  failures;

- reliable communication:

	- validity: any message in the output buffer is eventually
	  delivered;

	- integrity: the message received is identical to the original
	  and no duplicates are sent;