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 $P$ and $Q$ 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.