A Comparison of Mechanisms for Improving TCP Performance over Wireless Links
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1 A Comparison of Mechanisms for Improving TCP Performance over Wireless Links Hari Balakrishnan Venkata N. Padmanabhan Srinivasan Seshan Randy H. Katz Daedalus Group, Department of EECS, University of California at Berkeley
2 Outline of Talk Motivation Description of protocols Experimental methodology Results and explanations Conclusions and future work
3 Problems with TCP over Wireless Links TCP: reliable byte-stream protocol with cumulative acknowledgments and retransmissions. Packet losses due to wireless bit-errors mistaken for congestion losses. Bulk losses cause coarse-granularity timeouts. Variable bandwidths and delays make transport protocol adaptation hard. Handoffs often cause packet loss and variable delays, resulting in coarse timeouts for connections.
4 Link-layer protocols Proposed Solutions Locally optimized solutions. Transport-aware link protocols. End-to-end protocols Vanilla TCP (TCP Reno). Selective acknowledgments based on SMART scheme. Explicit Loss Notifications (ELN) to make sender aware of non-congestion losses. Split-connection protocols Attempt isolation of source from wireless link by splitting TCP connection at base station.
5 Objectives To evaluate and compare performance of protocols: Best combination of mechanisms in each protocol class. Importance of TCP-awareness for link-layer protocols. Usefulness of selective acknowledgments and explicit loss notifications. Effectiveness of split connections. Performance metrics: throughput: number of bytes/transfer time (Mbps). goodput : number of useful bytes/total number of bytes sent over link (%age). Context: bulk data transfer to a mobile host connected over wired links and one wireless hop.
6 Main Results A reliable link-layer protocol with some TCPawareness provides very good performance. Splitting TCP connections is not essential for good performance; using unmodified TCP over wireless hop does not improve performance much. Selective acknowledgments and explicit loss notifications are very effective in recovering from wireless losses.
7 Protocols: TCP-Aware Link-layer Base Station Sender Snoop agent at base station. Caches TCP segments going to mobile. Local retransmissions by observing duplicate acks and timeouts. No extra messaging for good performance. Mobile Host
8 5 Protocols: TCP-Aware Link-layer Base Station Sender Mobile Host
9 5 Protocols: TCP-Aware Link-layer Base Station Sender Mobile Host
10 6 Protocols: TCP-Aware Link-layer Base Station Sender Mobile Host
11 Protocols: TCP-Aware Link-layer Base Station 4 3 Sender ack 0 Mobile Host 2
12 Protocols: TCP-Aware Link-layer Base Station 5 Sender ack 0 ack ack 0 Mobile Host
13 Protocols: TCP-Aware Link-layer Base Station Sender Suppress Duplicate Acks ack Mobile Host
14 Protocols: TCP-Aware Link-layer Base Station Sender Mobile Host
15 Protocols: TCP-Aware Link-layer Base Station 6 Sender
16 Protocols: TCP-Aware Link-layer Base Station Sender ack Mobile Host
17 Protocols: TCP-Aware Link-layer 6 Base Station Sender ack 5 ack Mobile Host
18 Protocols: Link-layer Link-layer protocol (LL) same as TCP-aware link layer, but without suppressing duplicate acknowledgments. LL retransmits packets on observing TCP duplicate acknowledgments and on last-hop timeouts. No extra protocol messaging. Out-of-order packet delivery. LL-SACK: uses selective acknowledgments over wireless link. LL-OPT: TCP-aware and also uses SACKs.
19 Protocols: End-to-End TCP Reno: cumulative acknowledgments and coarse-granularity timeouts. Reno+SACK: SMART-based selective acknowledgments. Reno+ELN: Explicit Loss Notifications (ELN) generated at receiver when wireless losses occur and propagated to sender.
20 SMART-based Selective Acknowledgments 6 5 Router 4 Sender In addition to cumulative acknowledgment, send information about packet that caused the acknowledgment. Enables sender to construct bit-mask of lost packets. Assume no reordering of packets in network and retransmit packet when first SMART acknowledgment arrives. Not very robust if acknowledgments get lost. 3 2 Mobile Host
21 SMART-based Selective Acknowledgments Router 6 Sender ack 2 (smart 4) Mobile Host
22 SMART-based Selective Acknowledgments 3 retransmission Router Sender ack 2 (smart 4) ack 2 (smart 6) Mobile Host
23 Description of Protocols: Split-Connections Base Station Sender TCP TCP 2 Isolates sender from wireless link by splitting TCP connection at base station. Mobile Host Violates end-to-end semantics of TCP acknowledgments. Hard state at base station complicates handoffs and increases handoff latencies. SPLIT-SACK: Use selective acknowledgments over wireless connection.
24 Experimental Methodology Sender 400 byte packets 6 Internet hops Base Station acks 2 Mb/s AT&T WaveLAN LAN experiments with source on same 0 Mb/s Ethernet as base station. WAN experiments between IBM (NY) and UCB in the absence of congestion. Poisson-distributed bi-directional bit-errors ( every 64 KB). Instrumented kernel to record timeouts, retransmissions, changes in congestion window, etc.
25 Experimental Results: Link-layer Throughput (Mbps) LAN LAN WAN 32 KB8 KB32 KB LAN LANWAN 32 KB8 KB32 KB LAN LANWAN 32 KB8 KB32 KB LAN LAN WAN 32 KB8 KB32 KB LL LL-TCP-AWARE LL-SACK LL-OPT LAN performance almost the same for all LL protocols. Simple link-layer reliable protocols could adversely impact TCP performance. Transport-aware link protocols perform well over lossy links.
26 Benefits of TCP-Awareness Congestion Window (bytes) LL-TCP-AWARE (suppress duplicate acks) LL (no duplicate ack suppression) Time (sec) 30% improvement for LL-TCP-AWARE: congestion window fluctuates rapidly for LL (no coarse timeouts occur). Connection bandwidth-delay product more than KB. Suppressing duplicate acknowledgments and TCP-awareness leads to better utilization of link bandwidth and performance
27 Experimental Results: End-to-End.4 Throughput (Mbps) LAN LAN WAN 32 KB 8 KB 32 KB LAN LAN 32 KB 8 KB TCP Reno Reno + SMART LAN LAN WAN 32 KB 8 KB 32 KB Reno + ELN Coarse timeouts impair throughput (50% of optimal in LAN, 25% in WAN); goodputs always 97.5%. Selective Acknowledgments and Explicit Loss Notifications significantly improve performance.
28 Benefits of ELN Congestion Window (bytes) With ELN Without ELN Time (sec) Congestion window does not vary as rapidly with ELN, leading to a 00% improvement in throughput (using a simple message)
29 Experimental Results: Split-Connections Throughput (Mbps) LAN 32 KB LAN 8 KB SPLIT WAN 32 KB LAN 32 KB LAN 8 KB WAN 32 KB SPLIT-SACK SPLIT-SACK significantly better than SPLIT alone. Performance of SPLIT-SACK 5-0% less than LL-TCP-AWARE. Splitting the connection is not essential for good performance
30 Split-Connection Congestion Window Congestion Window (bytes) Wired connection Wireless connection Time (sec) Wired connection does not shrink congestion window but wireless connection times out often, causing sender to stall
31 Burst Losses Throughput (Mbps) Burst Length (packets) LL-TCP-AWARE LL-OPT While LL-TCP-AWARE can recover from small amounts of burst loss, LL-OPT uses SACKs to perform better loss recovery
32 Conclusions A reliable link-layer protocol with some TCPawareness provides very good performance. Splitting TCP connections is not essential for good performance; using unmodified TCP over wireless hop does not improve performance much. Selective acknowledgments and explicit loss notifications are very effective in recovering from wireless losses.
33 Future Work Evaluate performance of IETF SACK proposal, especially over wireless and satellite networks. Performance and protocol improvements for multihop packet radio networks (large variations). Improved reliable transport protocols for asymmetric (and possibly lossy) connections. More sophisticated link-layer protocols.
34 Snoop Performance Improvement.6.4 Throughput (Mbits/s) Snoop Regular TCP /Bit-error Rate ( error every x bits)
35 Fixed Host FH Internet Large Round-Trip Variations Ethernet Radios ER PT PT Poletop Radios PT GW ER PT PT Mobile Host MH Modem PR ER PT PT Large round-trip time variations due to variable latencies and contention
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