A Comparison of Mechanisms for Improving TCP Performance over - - PowerPoint PPT Presentation

A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

Hari Balakrishnan Venkata N. Padmanabhan Randy H. Katz University of California at Berkeley Srinivasan Seshan IBM T. J. Watson Research Center SIGCOMM ‘96, Stanford, CA August 30, 1996

Outline

• Motivation
• Description of proposed solutions
• Objectives
• Experimental methodology
• Results and analysis
• Conclusions
• Future work

TCP over Wireless Links

• Wireless links are inherently error-prone

– fades, noise, attenuation – errors often happen in bursts

• Difficult for the TCP sender to distinguish

wireless losses from buffer overflows

– incorrect invocation of congestion control in response to wireless losses

• Error bursts often lead to coarse timeouts
• Sender retransmission is the only option

– inefficient use of network bandwidth

WaveLAN Packet Errors

0.000 0.001 0.010 0.100 1.000 50 100 150 Distance (feet) Packet ErrorRate

• Packet errors versus distance from base station
• 2 Mbps AT&T WaveLAN, 1400-byte packets
• Taken from Nguyen et al. [NKNS96]

Scenario

Sender Mobile Host D D D A A Base Station

Proposed Solutions

• Reliable link-layer protocols

– error-correcting codes: hardware implementation,

• verhead even when no losses, not very effective

with long error bursts – local retransmission

• End-to-end protocols

– selective acks, explicit loss notification

• Split-connection protocols

– separate connections for wired path and last wireless hop

Reliable Link-Layer Protocols

• Local retransmision over wireless link (LL)

– more aggressive retransmission than TCP – bandwidth not wasted on wired links

• Snoop [BSAK95] (LL-TCP-AWARE)

– base station caches unacknowledged packets – uses TCP acks to determine missing packets – local retx. with suppression of duplicate acks – per-connection state at base station is soft

• Addition of selective acks yields LL-SACK

and LL-STCP-AWARE

End-to-End Protocols

• TCP-Reno (E2E)
• SMART selective acks [KM96] (E2E-SACK)

– ack includes sequence number of data packet that triggered it – sender can determine lost transmissions provided packets are not reordered

• Explicit loss notification (E2E-ELN)

– acks contain a flag indicating wireless loss – loss recovery without congestion control

Split-Connection Protocols

• Separate connections for wired path and

wireless hop (SPLIT)

– TCP sender does not deal with wireless losses – sender at base station does retransmission – per-connection state at base station is hard – example: Indirect-TCP [BB94]

• Specialized protocol for wireless connection

(SPLIT-SACK)

– selective acks – example: [YB94]

Goals

• Evaluate the different protocols on a uniform

platform

– importance of TCP-aware link layer – usefulness of selective acks and explicit loss notifications – importance of splitting the connection to effectively shield the sender from wireless losses

• Metrics

– throughput – goodput: both on wired and wireless links

Experimental Methodology

16 Internet hops 2 Mb/s AT&T WaveLAN

Base Station Sender

1400 byte packets acks

• LAN wired path: 10 Mb/s Ethernet
• WAN wired path: 16 Internet hops across the US;

T1 bottleneck link; no congestion

• Poisson-distributed bidirectional errors (with a

mean rate of 1 every 64 KB)

• 8 KB and 32 KB socket buffers, 2 MB transfers
• Instrumented BSD/OS kernel to record TCP events

Mobile Host

Results: Link-layer

• With no losses: LAN: 1.5 Mbps, WAN: 1.35 Mbps
• TCP-awareness

– 13% better throughput in LAN, 30% in WAN

• Selective acks

– 8% higher throughput in LAN, 13% in WAN

0.2 0.4 0.6 0.8 1 1.2 1.4

LL LL-TCP-AWARE LL-SACK LL-STCP-AWARE

Throughput (Mbps)

LAN WAN

Analysis: Link-layer

10 20 30 40 50 60 70 80

20 30 40 50 60 10

• LL retx. does not prevent out-of-order packets
• TCP fast retx. causes sender window to fluctuate

Suppressing duplicate acks prevents sender from invoking fast retransmission and results in good performance

Time (sec) Congestion Window (KB)

LL-TCP-AWARE LL

Results: End-to-End

0.2 0.4 0.6 0.8 1 1.2 1.4

E2E E2E-SACK E2E-ELN Throughput (Mbps)

• E2E (TCP Reno)

– coarse timeouts reduce throughput to 50% of optimal in LAN, 25% in WAN – wireless goodput is optimal because of conservative retransmissions

• SMART and ELN: significantly better throughput

LAN WAN

Analysis: ELN

ELN helps maintain a larger average congestion window size, yielding better throughput

10 20 30 40 50 60

50 100 150 200 250

Congestion Window (KB) Time (sec) E2E E2E-ELN

Results: Split-Connection

0.2 0.4 0.6 0.8 1 1.2 1.4

SPLIT SPLIT-SACK Throughput (Mbps)

• Specialized protocol for wireless connection

(SPLIT-SACK) offers a significant benefit

• Throughput with SPLIT-SACK is 5-10% lower

than LL-TCP-AWARE

LAN WAN

Analysis: Split-Connection

Wired connection maintains a large congestion window but slow progress of wireless connection causes sender to stall

20

10 30 40 50 60

20 40 60 80 100 120

Time (sec) Congestion Window (KB) Wired connection Wireless connection

Analysis: Split-Connection

20 40 60 80 100 120

10 30 40 50 60 20

Time (sec) Advertised Window (KB)

• Time average of receiver advertised window for

wired connection is 14 KB

• Bandwidth-delay product is 25-30 KB

WAN experiment

Burst Losses

2 4 6

Length of error burst (packets)

0.4 0.8 1.2

Throughput (Mbps)

LL-TCP-AWARE

While LL-TCP-AWARE can recover from small amounts of burst loss, LL-STCP-AWARE uses SACKs to perform better

LL-STCP-AWARE

Conclusions

• Link-layer protocols

– retransmission without in-order delivery do not prevent adverse interaction with TCP – suppressing duplicate acks helps significantly

• End-to-end protocols

– SMART selective acks and ELN significantly help recovery from wireless losses

• Split-connection protocols

– slow progress of wireless connection stalls sender of wired connection

Future Work

• Use wireless error traces [NKNS96]
• Effectiveness of IETF SACKs in dealing with

wireless losses

• Other types of networks (e.g., asymmetric

bandwidth, multi-hop wireless networks)

• Different workloads (e.g., Web accesses)

Web page: http://daedalus.cs.berkeley.edu