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

• 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

• 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!

• 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
Don’t prescribe a naming scheme!

 Proxies provide transparency
 Applications oblivious to changes
 Must highly engineer and place well
 End host support more flexible
 Can integrate with other resources
 Simplifies trust model
 Can be equally transparent
WAP Proxy

Location Query
(DNS Lookup)
Naming Service
(Dynamic DNS)
Session Initiation
Session Migration
Correspondent
Host
Location Update
(DNS Update)
Mobile Host foo.bar.edu

Generality
User space
Application
Session Layer
Transport Protocol
Network Layer
Link Layer
Kernel
Transparency

• 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

• 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

User space Kernel vic vat wb
Application
Resources
Distill
Discard
Buffer
System Resources

Session Creation
Application
Session Callbacks
Connectivity
Monitor
Session Layer
Connectivity Updates
Transport Protocol
Network Layer
Link Layer
Policy
Engine
Policy Decisions

• 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
TCP TCP

• 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

• 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

• 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.

• 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