HACK: HIERARCHICAL ACKS FOR EFFICIENT WIRELESS MEDIUM UTILIZATION
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1 HACK: HIERARCHICAL ACKS FOR EFFICIENT WIRELESS MEDIUM UTILIZATION LYNNE SALAMEH, ASTRIT ZHUSHI, MARK HANDLEY, KYLE JAMIESON, BRAD KARP.! UNIVERSITY COLLEGE LONDON
2 WIFI MOSTLY USED FOR TCP DOWNLOADS Server Access Point (AP) Client TCP Download
3 WIFI MOSTLY USED FOR TCP DOWNLOADS Server Access Point (AP) Client TCP ACKs
4 WIFI MOSTLY USED FOR TCP DOWNLOADS Link Layer ACKs Server Access Point (AP) Client
5 WIFI MEDIUM ACQUISITION INCURS OVERHEAD Backoff Preamble 1500 bytes Preamble SIFS Data DIFS 800 μs Rate =15 Mbps Link-Layer ACK * Diagram to Scale
6 WIFI MEDIUM ACQUISITION INCURS OVERHEAD Data ACK Rate =15 Mbps ~80% Utilization
7 WIFI MEDIUM ACQUISITION INCURS HIGH OVERHEAD ACK Rate = 60 Mbps ~51% Utilization
8 WIFI MEDIUM ACQUISITION INCURS HIGH OVERHEAD ACK Rate = 150 Mbps ~29% Utilization
9 WIFI MEDIUM ACQUISITION INCURS HIGH OVERHEAD ACK Rate = 600 Mbps ~9% Utilization
10 AGGREGATION INCREASES UTILIZATION AT HIGH RATES Aggregate Block ACK Rate = 600 Mbps ~80% Utilization
11 TCP DOUBLES MEDIUM ACQUISITIONS AP Client Acquire TCP Data Aggregate Block ACK TCP ACKs Aggregate Acquire * Not to Scale Time Block ACK
12 TCP DOUBLES MEDIUM ACQUISITIONS AP Client TCP Data Aggregate At 600 Mbps, ~61% utilization. Block ACK TCP ACKs Aggregate Time Block ACK
13 DO WE NEED TO ACQUIRE THE MEDIUM FOR FEEDBACK? AP Client TCP ACKs cumulative: no need for Block ACK. Encapsulate TCP ACK in Link-Layer ACK. Time TCP Data Aggregate Block ACK TCP ACKs Aggregate Block ACK
14 DO WE NEED TO ACQUIRE THE MEDIUM FOR FEEDBACK? AP Client TCP Data Aggregate TCP ACKs cumulative: robust to packet loss. Encapsulate TCP ACK in Link-Layer ACK. Block ACK + Encapsulated TCP ACKs Time
15 HACK: HIERARCHICAL ACKNOWLEDGEMENTS
16 GOALS Coexistence with stock Simple NIC changes. No TCP changes. Robustness to loss.
17 BLOCK ACKS MUST MEET HARD DEADLINE AP Client Need to send Block ACK within SIFS. TCP ACKs not ready in time. Can t wait longer: other senders might jump in. Time TCP Data Aggregate Block ACK SIFS (16μs) TCP ACKs
18 TCP ACKS DON T HAVE A HARD DEADLINE AP Client TCP Data 1, 2 TCP ACKs can afford to wait. Append them to the next Link-Layer ACK. Block ACK TCP Data 3, 4 TCP ACKs 1 & 2 Time Block ACK + TCP ACK 1, 2
19 CONSECUTIVE TCP ACKS ARE REDUNDANT Urgent Ptr Checksum 1 Offset + Flags Window Size ACK Number Sequence Number Dest. Port Source Port TCP ACK Header (20 Bytes)
20 CONSECUTIVE TCP ACKS ARE REDUNDANT Urgent Ptr Checksum 1 Offset + Flags Window Size ACK Number Sequence Number Dest. Port Source Port TCP ACK 1
21 CONSECUTIVE TCP ACKS ARE REDUNDANT Urgent Ptr Check- sum 1 2 Offset + Flags Window Size Size + K ACK Number Number + N Sequence Number Dest. Port Source Port TCP ACK 1 Urgent Ptr Offset + Flags Sequence Number Dest. Port Source Port TCP ACK 2
22 CONSECUTIVE TCP ACKS ARE REDUNDANT Urgent Ptr Checksum 1 Offset + Flags Window Size ACK Number Sequence Number Dest. Port Source Port TCP ACK 1 K N TCP ACK 2
23 HACK USES ROHC FOR EFFICIENT COMPRESSION Urgent Ptr Checksum 1 Offset + Flags Window Size ACK Number Sequence Number Dest. Port Source Port TCP ACK 1 HACK uses Robust Header Compression (ROHC, RFC 6846) K N ROHC shrinks a TCP ACK to about 5 bytes TCP ACK 2
24 TO HACK OR NOT TO HACK? AP Client Tx Queue TCP Data 1 & 2 Block ACK? TCP ACKs 1 & 2 From Network Stack
25 TO HACK OR NOT TO HACK? AP Client More Data 4 3 Tx Queue 4 Tx Queue 3 TCP Data 1 & 2 Block ACK TCP Data 3 & 4 TCP ACKs 1 & 2 From Network Stack Block ACK + TCP ACKs 1, 2
26 NIC NEEDS NO TCP KNOWLEDGE AP Client TCP ACKs NETWORK STACK NETWORK STACK Compress More Data? DRIVER Opaque Blob DRIVER NIC NIC
27 NIC NEEDS NO TCP KNOWLEDGE AP Client TCP ACKs NETWORK STACK NETWORK STACK Compress More Data? DRIVER DRIVER NIC NIC Block ACK
28 NIC NEEDS NO TCP KNOWLEDGE AP Client TCP ACKs NETWORK STACK NETWORK STACK Decompress DRIVER NIC DRIVER NIC
29 WHAT ABOUT LOSS OF BLOCK ACKS? AP Client TCP can tolerate loss of ACKs. ROHC cannot: endpoints should remain in sync. HACK should be robust against loss. Details in paper. TCP Data Aggregate Block ACK + Encapsulated TCP ACKs Time
30 EVALUATION Microsoft s Software Radio (SoRa): a (no aggregation). ns-3 simulator n (aggregation).
31 TCP GOODPUT EXPERIMENT SoRa NS a n 54 Mbps 150 Mbps High Quality Link 1, 2 Clients Up to 10 Clients Measure steady state aggregate goodput
32 HACK IMPROVES TCP GOODPUT SoRa (802.11a) ns-3 (802.11n) 1 client 2 clients # clients Mean Aggregate Goodput (Mbps) TCP/Wifi Mean Aggregate Goodput (Mbps) TCP/HACK % Improvement % Improvement
33 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) SNR (db) 15 Mbps
34 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) Mbps 15 Mbps SNR (db)
35 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) Mbps 30 Mbps 15 Mbps SNR (db)
36 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
37 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) Mbps 60 Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
38 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES 120 Average Goodput (Mbps) Mbps 90 Mbps 60 Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
39 HACK IMPROVES GOODPUT ACROSS 140 FULL RANGE OF LINK RATES Average Goodput (Mbps) Mbps 120 Mbps 90 Mbps 60 Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
40 HACK IMPROVES GOODPUT ACROSS FULL RANGE OF LINK RATES Average Goodput (Mbps) Mbps 135 Mbps 120 Mbps 90 Mbps 60 Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
41 HACK IMPROVES GOODPUT ACROSS FULL RANGE OF LINK RATES Average Goodput (Mbps) Mbps 135 Mbps 120 Mbps 90 Mbps 60 Mbps 45 Mbps 30 Mbps 15 Mbps SNR (db)
42 HACK IMPROVES GOODPUT ACROSS FULL RANGE OF LINK RATES TCP/HACK TCP/802.11n Average Goodput (Mbps) SNR (db)
43 HACK IMPROVES GOODPUT ACROSS FULL RANGE OF LINK RATES TCP/HACK TCP/802.11n Average Goodput (Mbps) TCP/HACK outperforms TCP/802.11n, even in lossy environments SNR (db)
44 CONCLUSIONS TCP/HACK reduces medium acquisitions. Increases goodput by: ~29-32% with no aggregation ~11-20% with aggregation Practical to deploy on real NICs.
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