Reconsidering Internet Mobility

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ReconsideringInternet Mobility

• Alex C. Snoeren,
• Hari Balakrishnan, M. Frans Kaashoek
• MIT Laboratory for Computer Science

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A Model of Mobility

• Hosts change attachment point
• Move from home, to office, to
• Use multiple physical technologies
• Mobility events are rare
• When compared with packet RTTs
• Long Running Applications
• SSH Tunnels
• Streaming apps

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Services Required

• Locate the mobile host or service
• Preserve communication
• Support changes in network attachment
• Expect and support disconnection
• Gracefully detect lack of connectivity
• Conserve resources during disconnection
• Reconnect quickly and efficiently
• Keep the application informed!

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Naming is not Enough

• Naming can abstract location details
• E.g., Mobile IP uses home IP address
• But specifies both timing and granularity
• Mobility is more than a naming problem
• Apps need different semantics (IP, DNS)
• Handle resolution failures (disconnection)
• Lookup granularity is application specific
• Dont prescribe a naming scheme!

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The Problem with Proxies

• Proxies provide transparency
• Applications oblivious to changes
• Must highly engineer and place well

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Migrate Architecture
Correspondent Host
xxx.xxx.xxx.xxx
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Positioning Mobility Services
Application
Generality
User space
Session Layer
Kernel
Transport Protocol
Network Layer
Link Layer
Transparency
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Migrate Approach

• Provide services in a Session Layer
• Applications define what a session is
• A single RPC-like packet exchange
• A bundle of semantically-related streams
• Or any combination in between
• Manage sessions as a group
• Resources can be associated with network
  activity, and allocated accordingly
• Support is strictly optional

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Session Layer Services

• Provide robust communication across changes of
  end-point addresses
• Resynchronize transport protocols
• Detect and handle disconnectivity
• Buffer, discard, or distill connections
• Manage related system resources
• Memory, timers, files, devices
• Marshall and store session state

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A Conference Session
User space
Kernel
Session Layer
Application Resources
System Resources
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Migrate Framework
Application
Session Layer
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Robust Communication

• Handle disconnection
• Block or buffer sockets
• of reliable connections
• Proactively discard
• unreliable datagrams
• Restart transport layers?
• Rebind network endpoints
• Replay buffered data

Application
Session Layer
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Disconnection and Hibernation

• Monitor network connectivity
• Exploit transport/network layer signals
• Support application hints
• Interface with network monitoring agents
• Support application-dependent behavior
• Default handlers can spool to disk
• Virtualize remote services
• Invoke application-specific handlers

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Resource Management

• Notify application to hibernate
• Blocked apps can be swapped out
• Shared files and devices can be released
• System resources can be reallocated
• Similar to Resource Containers
• Timers, locks, semaphores released
• Buffer space drained/paged

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State Management

• Enable state migration
• Associate application and system state
• Transfer from kernel to app, host to host
• Context management
• State may depend on path characteristics
• Support transcoding, different transport
  protocol, etc.

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Summary

• Mobility services are session oriented
• Often several related network activities
• Naming neither necessary nor sufficient
• Applications have different needs
• Resource management is key
• Especially with large numbers of hibernated
  sessions
• Sessions enable state management

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The Session API

• Session creation
• Applications specify resources and end points
  (e.g. TCP connections, UDP port/address pairs)
• Connectivity Monitor
• Status can be signaled from above (application),
  below (transport stack), or separate monitor
• Callback Register
• Applications can register to be notified on
  disconnection, reconnection, and/or changes in
  network attachment point

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Supporting Disconnection

• Current approaches
• Generally do not address disconnection
• Proxies often assumed always connected
• Our approach
• Handle unexpected disconnections
• Many disconnections are unplanned

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Reconnection

• Hosts contact correspondent peers
• Lost hosts may force naming lookup
• Session state is resynchronized
• Transport sessions may need to be restarted
• Application processing resumed
• Notification of reconnection delivered
• System resources are reallocated

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Enabling graceful reconnection

• Current approaches
• Obscure network characteristics
• Hide path dynamics resource availability
• Our approach
• Notify higher layers of changes
• Enable application adaptation
• Eliminate lower-layer dependence
• Enable rebinding of network endpoints

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A Model of Mobility
Move from home, to office, to coffee shop, across
multiple access technologies, all without
restarting applications.
Periods between movement gtgt Round Trip Times