Swim Lane Diagrams

Swim lane diagrams are a visual method for modeling interactions between participants in a system or protocol. They are particularly useful for understanding message flows over time, or when modelling human + system interaction.

• Each participant (e.g., client, server) is represented by a vertical “lane”
• Time flows downward along each lane
• Messages are shown as horizontal arrows between participants
• Each lane is labeled to identify the participant

This layout resembles swimming lanes, hence the name.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: SYN
    Server->>Client: SYN-ACK
    Client->>Server: ACK
    Client->>Server: Data

Swim lane diagrams complement data flow diagrams by emphasising who communicates with whom and in what order, making them especially useful for analysing protocols and interaction-driven threats.

Key Features

• Message labeling:

  • Each interaction should include details about the message contents
  • For complex systems, a legend/key can be used to simplify representation

• Computation and state:

  • Internal processing or state changes should be annotated along the participant’s lane

• Implicit trust boundaries:

  • Each participant is typically a separate entity (e.g., different machines)
  • This naturally introduces trust boundaries between lanes

Extensions

• Swim lanes can be extended to include human participants, not just systems

• This approach, called “ceremonies” (by Carl Ellison), helps model:

  • What users know
  • What users do
  • Human interactions in security-sensitive processes