Challenge: high throughput despite loss. Roofnet has >30% loss probability. 4 Computer Networks Course (15-744), CMU, 2012.

Transcription

1 MOTIVATIONS MAC-INDEPENDENT OPPORTUNISTIC ROUTING & ENCODING S. Chachulski, M. Jennings, S. Katti, D. Katabi SIGCOMM 07 Low quality of wireless links Multipath fading, scattering, other signals, etc. Roofnet has >30% loss probability Bruno Vavala PhD Student Challenge: high throughput despite loss Inter-flow coding (COPE) v.s. Intra-flow coding () 2 TRADITIONAL ROUTING OPPORTUNISTIC ROUTING Choose the nexthop Low link quality implies low probability of receiving packet Resending may be necessary 10% 10% 20% 90% 5% 90% Nodes overhear the transmission Nodes help forwarding Packet overlapping, wasted communication p Coordination 3 4

2 EX-OR Knowledge of the complete set of inter-node loss rates (for fwd list) Agreement on the set of potential forwarders Avoid simultaneous transmission through scheduler Switch to traditional routing when 90% transfer completed No spatial reuse: one forwarder can transmit at a time 5 EX-OR Knowledge of the complete set of inter-node loss rates (for fwd list) Agreement on the set of potential forwarders Avoid simultaneous transmission through scheduler Switch to traditional routing when 90% transfer completed I ll fwd No spatial reuse: one forwarder can transmit at a time 7 EX-OR Knowledge of the complete set of inter-node loss rates (for fwd list) Agreement on the set of potential forwarders Avoid simultaneous transmission through scheduler Switch to traditional routing when 90% transfer completed No spatial reuse: one forwarder can transmit at a time 6 8

3 9 Who has which packet? Need coordination! a +b make a combination and send it c +d 11 is it familiar? a +b a +b c +d c +d 10 a +b linear system a +b { c +d c +d 12

4 { c1+...+ckpk pk c1+...+ckpk pk c1+...+ckpk pk c1+...+ckpk... p k 16

5 { { c1+...+ckpk { pk... p k fwd1 fwd2... fwd_n more1 more2... more_k For simplicity, let the coded packets be packets AT THE DESTINATION 1.Receive all packets of a batch more1 more2... more_k... p k 2.Get coded packets and code vectors 3.Solve the linear system... pk 19 V.S. EXOR HEADER header ExOR header 18 RE-CODING Coding at the source is ok, but not enough Observations Duplicates. Still need coordination? Nodes receive coded packets and code vectors Linear Coding Solution Check if a new packet is innovative (linear independence) r p j = r c_i p_i = r c_i p_i = c i p_i = p j Re-code received coded packets 20

6 RE-CODING CHALLENGES Coding at the source is ok, but not enough Observations Nodes receive coded packets and code vectors Linear Coding Solution Check if a new packet is innovative (linear independence) p 1 p 2 p 3 r p j = r c_i p_i = r c_i p_i = c i p_i = p j How many packets should be sent? When should routing stop? How to do efficient coding? Re-code received coded packets PACKETS TO SEND STOPPING RULE Greedy routing (with respect to the ETX metric) When new packet is received, if distance to destination decreases, increment credit counter (according to ETX/expected delivery) Stop when the destination receives the entire batch Reliable ACK through shortest path Flush old batch due to ACK ACK ACK Flush old batch due to timeout Decrement after each forwarding Destination ACK has priority After the source receives ACK, credits can only decrease Start with next batch 23 24

7 EFFICIENT CODING Code just innovative packets Use code vector and linear independence to find innovative packets Otherwise no useful information transmitted Less dead spots using Better that ExOR Much better than traditional routing PERFORMANCE unicast throughput Checking linear independence is expensive exploit spatial reuse But you have the code vector, use it instead of packets! MAC layer not always ready for transmission is MAC-independent, so do something else while waiting concurrent transmissions unicast throughput Pre-coding is a good idea THANK YOU!