Foundations

Layered Protocols

  • Due to the absence of shared memory, all communication in distributed systems is based on sending and receiving (low level) messages.

  • Processes PP and QQ have to agree on the meaning of the bits being sent.

The OSI Reference Model

  • 7 layers:

    1. Physical: Data cables, Cat6

    2. Data Link: Switching, MAC addresses, Ethernet

    3. Network: IP addresses, Routing

    4. Transport: TCP/UDP

    5. Session: Session management

    6. Presentation: WMV, JPEG, MOV

    7. Application: HTTP, SMTP

  • Mnemonic:

    • Please Do Not Throw Sausage Pizza Away

Middleware Protocols

  • 4 layers:

    1. Hardware

    2. OS

    3. Middleware

    4. Application

  • Example:

    • DNS

Types of Communication

  • Persistent communication

    • A message that has been submitted for transmission is stored by the communication middleware as long as it takes to deliver it to the receiver.

    • E.g., electronic mail system

  • Transient communication

    • A message is stored by the communication system only as long as the sending and receiving application are executing.

  • Asynchronous communication

    • A sender continues immediately after it has submitted its message for transmission.

  • Synchronous communication

    • The sender is blocked until its request is known to be accepted.

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