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Long Thin Networks

By Montenegro, G.

Description
Technical Reference Publication

Excerpt
Introduction: Optimized wireless networking is one of the major hurdles that Mobile Computing must solve if it is to enable ubiquitous access to networking resources. However, current data networking protocols have been optimized primarily for wired networks. Wireless environments have very different characteristics in terms of latency, jitter, and error rate as compared to wired networks. Accordingly, traditional protocols are ill-suited to this medium. Mobile Wireless networks can be grouped in W-LANs (for example, 802.11 compliant networks) and W-WANs (for example, CDPD [CDPD], Ricochet, CDMA [CDMA], PHS, DoCoMo, GSM [GSM] to name a few). W-WANs present the most serious challenge, given that the length of the wireless link (expressed as the delay*bandwidth product) is typically 4 to 5 times as long as that of its W-LAN counterparts. For example, for an 802.11 network, assuming the delay (round-trip time) is about 3 ms. and the bandwidth is 1.5 Mbps, the delay*bandwidth product is 4500 bits. For a W-WAN such as Ricochet, a typical round-trip time may be around 500 ms. (the best is about 230 ms.), and the sustained bandwidth is about 24 Kbps. This yields a delay*bandwidth product roughly equal to 1.5 KB. In the near future, 3rd Generation wireless services will offer 384Kbps and more. Assuming a 200 ms round-trip, the delay*bandwidth product in this case is 76.8 Kbits (9.6 KB). This value is larger than the default 8KB buffer space used by many TCP implementations. This means that, whereas for W-LANs the default buffer space is enough, future W-WANs will operate inefficiently (that is, they will not be able to fill the pipe) unless they override the default value. A 3rd Generation wireless service offering 2 Mbps with 200-millisecond latency requires a 50 KB buffer.

Table of Contents
Table of Contents 1 Introduction ................................................. 3 1.1 Network Architecture .................................... 5 1.2 Assumptions about the Radio Link ........................ 6 2 Should it be IP or Not? ..................................... 7 2.1 Underlying Network Error Characteristics ................ 7 2.2 Non-IP Alternatives ..................................... 8 2.2.1 WAP ................................................ 8 2.2.2 Deploying Non-IP Alternatives ...................... 9 2.3 IP-based Considerations ................................. 9 2.3.1 Choosing the MTU [Stevens94, RFC1144] .............. 9 2.3.2 Path MTU Discovery [RFC1191] ....................... 10 2.3.3 Non-TCP Proposals .................................. 10 3 The Case for TCP ............................................. 11 4 Candidate Optimizations ...................................... 12 4.1 TCP: Current Mechanisms ................................. 12 4.1.1 Slow Start and Congestion Avoidance ................ 12 4.1.2 Fast Retransmit and Fast Recovery .................. 12 4.2 Connection Setup with T/TCP [RFC1397, RFC1644] .......... 14 4.3 Slow Start Proposals .................................... 14 4.3.1 Larger Initial Window .............................. 14 4.3.2 Growing the Window during Slow Start ............... 15 4.3.2.1 ACK Counting .................................. 15 4.3.2.2 ACK-every-segment ............................. 16 4.3.3 Terminating Slow Start ............................. 17 4.3.4 Generating ACKs during Slow Start .................. 17 4.4 ACK Spacing ............................................. 17 4.5 Delayed Duplicate Acknowlegements ....................... 18 4.6 Selective Acknowledgements [RFC2018] .................... 18 4.7 Detecting Corruption Loss ............................... 19 4.7.1 Without Explicit Notification ...................... 19 4.7.2 With Explicit Notifications ........................ 20 4.8 Active Queue Management ................................. 21 4.9 Scheduling Algorithms ................................... 21 4.10 Split TCP and Performance-Enhancing Proxies (PEPs) ..... 22 4.10.1 Split TCP Approaches .............................. 23 4.10.2 Application Level Proxies ......................... 26 4.10.3 Snoop and its Derivatives ......................... 27 4.10.4 PEPs to handle Periods of Disconnection ........... 29 4.11 Header Compression Alternatives ........................ 30 4.12 Payload Compression .................................... 31 4.13 TCP Control Block Interdependence [Touch97] ............ 32 5 Summary of Recommended Optimizations ......................... 33 6 Conclusion ................................................... 35 7 Acknowledgements ............................................. 35 8 Security Considerations ...................................... 35

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Book Id: WPLBN0000692978
Format Type: PDF eBook
File Size: 113.01 KB.
Reproduction Date: 2005
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Title: Long Thin Networks  
Author: Montenegro, G.
Volume:
Language: English
Subject: Management, Fine Arts, Finance
Collection: Technical eBooks Collection
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