Asymmetric Bandwidth Channels Jonathan Smith Distributed Systems Laboratory

Asymmetric Bandwidth Channels Jonathan Smith Distributed Systems Laboratory University of Pennsylvania

The ABC Model B’cast Channel C Client Channel A Channel B Server

Channels A, B and C u A is low latency and B/W, duplex u B is hi latency and B/W, simplex u C is hi latency and B/W, simplex u B is unicast and C is multicast u A *may* be real-time (POTS, ATM)

Opportunity for Basic Research u CATV and DBS penetration; ADSL also! u Cost advantage for multicast u Bidirectional channels WRONG!

Applications u World-Wide Web (R/W>10) u Telerobotics u ``Snowcrash’’ shared VR u Mobile computers

Challenges u Real-time/interactivity u Integration with existing protocols (IP) u Routing algorithms u Application Transparency? u Resource multiplexing

Approach highlights u Half-duplex use of multiple paths u Resource scheduling at multicast server u Multipath routing at multicast server u Use of new ‘‘protocol boosters’’ ideas C B A

Half-duplex Paths u. ABC model: two paths u. Fast multicast (w/delay) u. Slow dedicated unicast C B u. Client: Fast path R/O u. Server: Fast path W/O A

Scheduling u Server uplink is shared u Service reflects application Qo. S u R/T: R/T server + R/T unicast u Integrated N/W, Appl. and O. S.

Multipath Routing u Classifying packets by path u Size (Control vs. Data bimodal) u Transparency to TCP, browsers, etc. u Flow control and RTT algorithms u Path differences: B/W, delay, prob(error)

A new design methodology: Protocol Boosters u Assume things are working well u Detect when they are not (policy) u Add functions (mechanism) to fix u Functions are called “protocol boosters” u An optimistic approach to transparently achieving high end-to-end performance or new capabilities

Protocol Boosters u Protocol Elements added ‘‘as-needed’’ u Example of “optimistic” design method u Useful to maintain common case Host A Host B Application Network Element Booster De. Booster Boosted Subnet Application

Examples u Implemented over IP on Free. BSD – Encryption Booster – Compression Booster u FEC Booster at Bellcore u Hardware Support: The P 4* *see http: //www. cis. upenn. edu/~boosters/boosters. html

Performance Potential: u Thruput: TCP, TCP/FEC, Hybrid * T h r u p u t * * * Bit Error Rate

Switch. Ware Module Loader u Active Bridging Caml System Input NIC LAN #1 Frame . . Loaded modules Linux Kernel Output NIC Frame LAN #2

Multipath Boosting: Receiver u Merge two streams into one: Application Receive Booster

Multipath Boosting: Sender u Create two streams from one: one Application Send Booster ?

Summary u This architecture is coming – ADSL (Asymmetric Digital Subscriber Line) – CATV (Cable Television/Cable Modem) – DBS (Digital Broadcast Satellite) u Current Approaches – Multipath Routing with Protocol Boosters – Apply to Real-time telerobotics