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Link Layer-Based TCP Optimisation for Disconnecting Networks

Link Layer-Based TCP Optimisation for Disconnecting Networks. James Scott Glenford Mapp. Disconnected period. Retransmissions. 1.5s of wasted time. TCP Disconnection Problem. Networked Surfaces. Provide networking with physical surfaces Such as desks, floors, etc.

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Link Layer-Based TCP Optimisation for Disconnecting Networks

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  1. Link Layer-Based TCP Optimisation for Disconnecting Networks James Scott Glenford Mapp

  2. Disconnected period Retransmissions 1.5s of wasted time TCP Disconnection Problem

  3. Networked Surfaces • Provide networking with physical surfaces • Such as desks, floors, etc. • Using conductive pads on surface and on objects • Objects may be any electronic device, mobile or immobile • No 'plug', no special alignment required • Offers transparency of connection for users • Provides mobility for devices • Support a range of services • Ethernet-style computer networks • Slower serial buses for peripherals • Power

  4. Networked Surface Prototype

  5. {C,Disc,Rec}onnection • Connection involves a “handshaking” procedure • Executed on each of the pad “links” created • Determines which functions (ground, power, data) is performed by each pad link • Takes ~0.2s in the prototype • Disconnection done by custom link layer • Token-based link layer -> polls each object continually • Must revert pads back to a handshaking state • Takes ~0.1s in the prototype • Reconnections may be frequent • E.g. nudging notebook PC while typing • Need quick recovery (e.g. when using a remote terminal)

  6. “Smart Link Layer” Solution • Buffer single TCP packet per connection • On both sides of disconnecting link • Packet is chosen to be the “best” kick-starter, fulfilling: Highest ack, lowest unack’d seq, longest length, biggest rx window • On reconnection, retransmit packet • Applicable to many other networking scenarios • Handoff between networks (e.g. NS and 802.11) • Coverage “blackspots” in wireless networks • Advantages of solution: • Lightweight -> can be implemented in NIC hardware • Locally acting -> no need to modify correspondent hosts • External to TCP -> no need to modify TCP/IP stack

  7. Bulk Transfer Experiments • Test conditions: • Conducted on prototype NS • “Availability” of network varied by causing random disconnections • 10 file transfers of 5Mbyte between NS device and computer on fixed network

  8. Interactivity Experiments • Test conditions: • Conducted on prototype NS • “Availability” of network varied by causing random disconnections • VNC remote desktop software used to view an application which flashes a pixel 5 times per second • Measure number of updates received over a period of time

  9. Conclusions and Related Work • “Smart link layer” shown to work well • Causes immediate resumption of TCP data flow • Advantages: Lightweight, locally deployable, device-independent, graceful failure, security-preserving • Disadvantages: encrypted connections • Related work: snoop, I-TCP, etc • Differences: timescales, slow-start, heavyweight • Current IETF work: PILC, TRIGTRAN • Lots of interest in this issue

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