Enabling Internet “Suspend/Resume”

with Session Continuations

Alex C. Snoeren

MIT Laboratory for Computer Science(with Hari Balakrishnan, Frans Kaashoek, and Jon Salz)

Challenges of “Suspend/Resume”

What happens to the “other” end?

How does the session resume?

It’s a state management problem.

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

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

Using Continuations

User level

App

Kernel

SL

• 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

An Example: SSH Session

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

SSHd

shell

emacs

Xapp

serverclient

ssh

Conserving Session Resources

SSHd

emacs

SocketBuffers

NetworkPorts

OpenFiles

Kernel

Resources dedicated toactive session

>>Resources dedicated to

suspended session

Release systemresources as well

Continuation generation is recursive!

SSH Continuation

• Don’t suspend until it’s 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

Continuation Efficiency

Anecdotal evidence of size and speed

SSHd FTPd0

200

400

600

800

1000

1200

1400

1600

1800

2000

Mem

ory

Usa

ge (

KB

)

0

50

100

150

200

250

300

Res

tart

Lat

ency

(m

sec)

SSHd FTPd

System Resources

Shared Pages

Non-Shared Pages

Startup Latency

Session Overhead

SessionContinuation

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…