Chapter 3 - Distributed Systems Essentials
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Questions and Answers

What is the main issue with timestamps on different nodes in a distributed system?

  • Clock drift can make them meaningless (correct)
  • They can be relied upon to determine order
  • They are always accurate
  • They are universally standardized

    • Clock drift does not impact the reliability of event timestamps in distributed systems.

    False

    What is the title of the book by George Coulouris that covers distributed systems?

    Distributed Systems: Concepts and Design

    The implications of clock drift will become clear when discussing __________ in detail.

    <p>distributed databases</p> Signup and view all the answers

    Match the following authors with their contributions:

    <p>George Coulouris = Distributed Systems: Concepts and Design James Kurose &amp; Keith Ross = Computer Networking: A Top-Down Approach Roy T. Fielding = Architectural Styles and the Design of Network-Based Software Architectures Michael J. Fischer = Impossibility of Distributed Consensus with One Faulty Process</p> Signup and view all the answers

    What is the maximum packet size when data is streamed over TCP?

    <p>65,535 bytes</p> Signup and view all the answers

    TCP ensures that packets arrive in the exact order they were sent.

    <p>True</p> Signup and view all the answers

    What mechanism does TCP use to acknowledge packet receipt?

    <p>Cumulative acknowledgment</p> Signup and view all the answers

    TCP trades off ______ for reliability.

    <p>efficiency</p> Signup and view all the answers

    Which protocol is described as connectionless and prioritizes raw performance?

    <p>UDP</p> Signup and view all the answers

    Match the following TCP features with their descriptions:

    <p>Checksums = Verify packet integrity Sequence Numbers = Reassemble packets in order Cumulative Acknowledgment = Confirm reception of all packets up to a given point Dynamic Flow Control = Prevent overwhelming the receiver</p> Signup and view all the answers

    UDP guarantees delivery of packets in the order they are sent.

    <p>False</p> Signup and view all the answers

    What happens if a sender does not receive an acknowledgment within a timeout period?

    <p>The packet is resent</p> Signup and view all the answers

    What is the main reason IPv4 is being replaced by IPv6?

    <p>IPv6 provides an almost infinite number of IP addresses</p> Signup and view all the answers

    IPv6 is based on a 32-bit addressing scheme.

    <p>False</p> Signup and view all the answers

    What is the role of a root DNS server?

    <p>To resolve IP addresses by referring to authoritative DNS servers.</p> Signup and view all the answers

    The local DNS server remembers the IP addresses of recently contacted ______.

    <p>hosts</p> Signup and view all the answers

    Which of the following is true about DNS servers?

    <p>They are geographically replicated</p> Signup and view all the answers

    What do the packet headers in an IP network contain?

    <p>Source and destination IP addresses</p> Signup and view all the answers

    Match the following DNS components with their functions:

    <p>Local DNS Server = Remembers recently contacted hosts Root DNS Server = Starting point for IP resolution Authoritative DNS Server = Manages name resolution for domain Geographic Replication = Prevents single points of failure</p> Signup and view all the answers

    An IPv4 address is a ______-bit addressing scheme.

    <p>32</p> Signup and view all the answers

    What is the primary purpose of an interface such as IGBank in software engineering?

    <p>To define a contract between the caller and callee</p> Signup and view all the answers

    Java Remote Method Invocation (RMI) allows for non-Java applications to communicate effectively.

    <p>False</p> Signup and view all the answers

    List one major feature of Distributed Computing Environment (DCE) RPC.

    <p>Standardized approach for client/server systems</p> Signup and view all the answers

    CORBA facilitates __________ client/server communications based on an object-oriented interface definition language.

    <p>language-neutral</p> Signup and view all the answers

    Which language supports Java Remote Method Invocation (RMI)?

    <p>Java</p> Signup and view all the answers

    Match the following technologies with their main features:

    <p>DCE RPC = Standardized approach for client/server systems CORBA = Language-neutral client/server communications using IDL Java RMI = Pure Java-based remote method invocation</p> Signup and view all the answers

    The advantages of explicit interfaces in distributed systems have been recognized since the early 1990s.

    <p>True</p> Signup and view all the answers

    What is the main advantage of using Remote Procedure Call (RPC) technologies?

    <p>They allow for explicit server interfaces and simplified network communication.</p> Signup and view all the answers

    What is the main problem described in the Two Generals' Problem?

    <p>Ensuring that messages between generals reach each other reliably</p> Signup and view all the answers

    In the Two Generals' Problem, the first general can guarantee that the second general receives the attack instructions.

    <p>False</p> Signup and view all the answers

    What is the FLP Impossibility Theorem concerned with?

    <p>Consensus on asynchronous networks in the presence of crash faults.</p> Signup and view all the answers

    To increase the likelihood of message delivery, each general in the Two Generals' Problem may send multiple __________.

    <p>messengers</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Two Generals' Problem = Communication issue in distributed systems FLP Impossibility Theorem = Consensus cannot be guaranteed in asynchronous networks Asynchronous Network = Network where message delays can occur Crash Faults = Failures where messages can be delayed but not lost</p> Signup and view all the answers

    What practical solution can increase consensus in distributed systems despite message delays?

    <p>Establishing practical bounds on message delays</p> Signup and view all the answers

    In a distributed system, it is essential for each node to be online at all times to ensure consensus.

    <p>False</p> Signup and view all the answers

    Why might messages between generals get lost or delayed?

    <p>Because messengers can be captured or killed, and networks can have unpredictable delays.</p> Signup and view all the answers

    Study Notes

    Internet Protocol

    • IPv4 is a 32-bit addressing scheme that will soon run out of addresses, due to the number of devices connecting to the internet
    • IPv6 is a 128-bit successor to IPv4 and will offer an (almost) infinite number of IP addresses
    • In July 2020 about 33% of the traffic processed by Google.com was IPv6

    DNS

    • DNS servers are organized hierarchically
    • A small number of root DNS servers are the starting point for resolving an IP address
    • When an internet browser tries to find a website, a local DNS server contacts a root DNS server
    • The root server replies with a referral to an authoritative DNS server
    • An authoritative name server manages name resolution for each top-level internet domain (.com, .org, .net, etc.)
    • The local DNS server queries the appropriate top-level domain server
    • The top-level DNS server replies with the address of the DNS server that knows about all the IP addresses for the website
    • This DNS server is queried, and it returns the actual IP address for the website

    Packet Switching

    • IP defines a packet structure that contains the data to be delivered, along with source and destination IP addresses
    • The internet is a packet-switched network, which means that every packet is individually routed across the network
    • The route each packet traverses can vary dynamically based on the conditions in the network, such as link congestion or failure
    • This means that packets may not arrive at the server in the same order they are sent from the client

    TCP

    • TCP uses a cumulative acknowledgment mechanism to ensure reliable packet delivery
    • A receiver will periodically send an acknowledgment packet that contains the highest sequence number of the packets received without gaps
    • This implicitly acknowledges all packets sent with a lower sequence number
    • If a sender doesn't receive an acknowledgment within a timeout period, the packet is resent
    • TCP also uses checksums to check packet integrity
    • TCP performs dynamic flow control to ensure a sender doesn't overwhelm a slow receiver by sending data too quickly

    UDP

    • UDP is a simple, connectionless protocol, which exposes the user's program to any unreliability of the underlying network
    • UDP offers raw performance instead of reliability

    Explicit Interfaces

    • Interfaces can be used to define explicit server interfaces that can be called across the network using the same syntax as a sequential program
    • Explicit interfaces are generally considered a good practice in software engineering
    • The benefits of explicit interfaces are well known in software engineering and are one of the foundations of object-oriented design

    Remote Procedure Call (RPC)

    • RPC technologies allow programmers to define explicit server interfaces and call them across the network
    • Some RPC technologies are:
      • Distributed Computing Environment (DCE): A standardized approach for client/server systems, primary languages were C/C++
      • Common Object Request Broker Architecture (CORBA): Facilitates language-neutral client/server communications based on an object-oriented interface definition language (IDL), primary languages include: C/C++, Java, Python, and Ada
      • Java Remote Method Invocation (RMI): A pure Java-based remote method invocation that facilitates distributed client/server systems with the same semantics as Java objects.

    The Two Generals' Problem

    • The Two Generals' Problem is a classic example of the challenges of reaching consensus in distributed systems
    • The problem illustrates the difficulties of reaching agreement in the presence of unreliable communication
    • Partial failures are analogous to losing messages and acknowledgments in a distributed system
    • Message delays are unavoidable due to asynchronous networks
    • The FLP Impossibility Theorem demonstrates an asynchronous network in the presence of crash faults (messages can be delayed but not lost), it is impossible to achieve consensus within bounded time

    Clock Drift

    • Our applications cannot rely on timestamps of events on different nodes to represent the actual order of these events
    • Clock drift makes cross-node timestamps meaningless to compare
    • The implications of clock drift will be clearer when we start to discuss distributed databases in detail

    Network Fundamentals Summary

    • Networks are asynchronous, which means that the speed of communication can vary unpredictably.
    • Message delays are unpredictable in asynchronous networks.
    • Partial failures in asynchronous networks are difficult to manage, as messages can be lost or delayed.
    • Reaching consensus on an asynchronous network in the presence of failures is a challenging problem, but not impossible with the right approach.
    • Clock drift (time discrepancies between systems) can disrupt the accuracy of timestamps for events in distributed systems.

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