Enabling Internet

1
Enabling Internet Suspend/Resume with Session
Continuations

• Alex C. Snoeren
• MIT Laboratory for Computer Science
• (with Hari Balakrishnan, Frans Kaashoek, and Jon
  Salz)

2
Challenges of Suspend/Resume
How does the session resume?
What happens to the other end?
?
?
Its a state management problem.
3
Suspend/Resume Tasks

• Intelligent disconnection handling
• Buffer otherwise lost communications
• Emulate remote services locally
• Release resources while disconnected
• Graceful resumption handling
• Reallocate resources and restore state
• Adapt to new network conditions
• Indicate how to resume processing

4
Motivating Continuations

• Observation complete context inappropriate
• Some previous state irrelevant, or, even worse,
• Invalidated due to change in conditions
• Similar problem in programming languages
• Block when state and context is complex
• Pass continuation if state and context is small
• Continuations can request blocking behavior

5
Using Continuations

• Expand session notion
• Align with application
• Annotate state, resources,
• associated computation
• Include system state
• Provide synchronization and preservation
  assistance
• Shared attribute/value store
• Persist local system IPC, file descriptors
• System invokes continuation at session resumption
• Generated in response to disconnection
  notification

App
User level
SL
Kernel
6
An Example SSH Session

• Remote log in / port forwarding
• Provides secure remote communication
• Data compressed and encrypted as a stream

7
Conserving Session Resources
Resources dedicated to active session
emacs
gtgt Resources dedicated to suspended session
SSHd
Socket Buffers
Open Files
Release system resources as well
Network Ports
Kernel
Continuation generation is recursive!
8
SSH Continuation

• Dont suspend until its convenient
• Process pending data, deliver to app or network
• Only a minimum of state to preserve
• Auth, crypto, and compression state
• Preserve IPC to child processes
• Notify child processes of disconnection
• Tunneled apps share connectivity fate

Added 250 LOC in an afternoon
9
Continuation Efficiency
SSHd
FTPd
SSHd
FTPd
Anecdotal evidence of size and speed
10
Conclusion Future Directions

• Continuations enable suspend/resume
• Integrates well with session model
• Support intelligent application adaptation
• Enable system resource conservation
• Useful for mobility across hosts?
• Continuations eliminate dependencies
• An area for future exploration