TCP Behavior across Multihop Wireless Networks and the Wired Internet* Kaixin Xu, Sang Bae, Sungwook Lee, Mario Gerla Computer Science Department, UCLA 405 Hilgard Ave., Los Angeles, CA {xkx, sbae, swlee, gerla}@cs.ucla.edu ABSTRACT Emerging wireless ad hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. These file transfers use TCP for reliability and congestion control. However, recent experiments with ad hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are well behaved. In fact they are routinely used in most Campuses, Businesses and Research Labs. This paper is an experimental study of the unstable behavior of TCP across 802.11 ad hoc networks and the wired Internet. We investigate the fairness issues of multiple TCP flows as well as the coexistence of TCP flows and video streams in the wired/wireless scenario. Detailed analysis of the measurement results is also presented. The paper will prove very valuable to future commercial and military ad hoc networks. Categories and Subject Descriptors C.2.0 [Computer-Communication Networks]: General – data communications. General Terms Measurement, Performance. Keywords TCP Performance, Fairness, Ad Hoc Network, MANET. 1. INTRODUCTION As wireless multihop networks emerge, it will become necessary to communicate across multihop networks to servers back in the wired network. In the battlefield, for example, image files containing target profiles are downloaded from databases to mobiles in the tactical ad hoc network. At the same time, images of potential targets collected by mobiles may be sent back to processing centers in the wired Internet. In ad hoc collaborative net set up among members of a search and rescue team, one of the team members may occasionally connect to the Internet (via satellite, say) to download files from a remote server and share them with his colleagues. There may also be a need to upload files from mobiles to the Internet, for example pictures of objects requiring further processing. Another opportunity for ad hoc (multihop) and wired segment interconnection arises in wireless LANs when multihopping is invoked to overcome and bypass a LAN Access Point (AP) fault. Namely, if the AP fails, and the mobiles in the affected area cannot connect to neighbor APs directly, they may try to reach the Internet indirectly by multihopping through mobile neighbors in adjacent areas. In all the above cases there will be file transfers between wired and ad hoc wireless hosts, all running over TCP. Thus, it is vital thus to assure that TCP perform efficiently over mixed wired and multihop segments. TCP performance over wired connection is well understood. Recently, good progress has also been made on TCP over paths that include one or more wireless links operated in a point to point mode (eg, satellite links, or last hop wireless LANs). These wireless links often introduce random packet errors and loss. Care must be taken to correctly handle such loss and distinguish it from congestion loss (the latter requires the intervention of congestion control mechanisms such as TCP window reduction). Recent extensions of conventional TCP (eg, TCP Snoop [16], TCP Peach [18], TCP Westwood [17], etc) can deal with such wireless loss situations. Much more challenging is the problem of achieving good TCP performance within an ad hoc, multihop network. This has been an area of active research recently, and progress has been made in several directions. Three different types of challenges are posed to TCP design by such networks. First, as the topology changes, the path is interrupted and TCP goes into repeated, exponentially increasing time-out with severe performance impact. Efficient retransmission strategies have been proposed to overcome such problems [13][14][15]. The second problem has to do with the fact that TCP performance in ad hoc multihop environment * This work is supported in part by ONR “MINUTEMAN” project under contract N00014-01-C-0016 and TRW under a Graduate Student Fellowship. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. WoWMoM’02, September 28, 2002, Atlanta, Georgia, USA. Copyright 2002 ACM 1-58113-474-6/02/0009…$5.00depends critically on the window in use. If the window grows too large, there are too many packets (and ACKs) on the path, all competing for the same medium. Congestion builds up and causes “wastage” of the broadcast medium and severe throughput degradation [3]. This is different from wired networks, which are fairly tolerant of large windows. The third problem is due to the interaction of the 802.11 MAC layer protocol, more precisely, the hidden terminal problem and binary backoff scheme etc., with the TCP window mechanism and time out. This TCP/802.11 interaction was found to cause unfairness among competing TCP flows and (in extreme cases) “capture” of the channel by a few flows. Solutions to the second and third problems have been recently proposed [4][5]. Moving now to the wired/wireless ad hoc environment, we note that yet new challenges arise. The situation here is much more complex than that in the first
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