TRANSPORT LAYER PROTOCOLS FOR MOBILE AD HOC NETWORKS in VS .NET

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the TCP sender distinguish wireless-induced packet losses from congestion-induced packet losses. When the average queue size of a node exceeds a certain threshold, the node marks the so-called CW ag in all of its outgoing segments. When a sender receives a DUPACK with the CW ag set, it knows that the network is in the congestion state. Otherwise (if the CW ag of the DUPACK is not set), the DUPACK is assumed to be caused by wireless errors. TCP-Jersey uses the estimated bandwidth to adjust cwnd and ssthresh only when it receives an ACK or three DUPACKs with the CW ag set. Explicit Retransmit. TCP-Jersey also modi es the traditional fast retransmit algorithm. Instead of halving cwnd upon the reception of three DUPACKs, TCP-Jersey keeps cwnd unchanged (as long as the DUPACKs are not due to congestion); cwnd is in fact adjusted separately by the rate control procedure if necessary. TCP-Jersey has strengths that are similar to that of TCP-Westwood because both of them use the idea of bandwidth estimation. However, TCP-Jersey could perform better in some cases than TCP-Westwood [16] because TCP-Jersey uses explicit congestion warnings from intermediate nodes to notify the sender of possible congestion instances in the network. TCP-Jersey uses this extra information to distinguish wireless packet losses from congestion-related packet losses. In addition, TCP-Jersey triggers the rate control procedure upon the reception of ACKs (or DUPACKs) with the CW ag set, and thus it can proactively avoid congestion more rapidly. Besides, TCP-Jersey has a more robust bandwidth estimator since the estimation does not involve the use of the minimum observed RTT as in TCP-Vegas and TCP-Westwood. Therefore, its estimation is not directly affected by route changes. However, similar to conventional TCP, TCP-Jersey does not have a mechanism to handle the effects of route failures. Moreover, TCP-Jersey requires the use of the explicit feedback information from intermediate nodes. Deploying TCP-Jersey is more dif cult, since it relies on the cooperation of all (or at least many) nodes. 9.4.2 Determining Route Failure and Wireless Error As mentioned in Section 9.3, conventional TCP does not have any mechanisms to handle route failures and wireless errors. In effect, TCP performance degrades signi cantly due to these factors. There is a need for having some effective means to determine these events accurately so as to allow TCP to react appropriately. The representative schemes that determine route failures and wireless errors and distinguish these effects from congestion for improving the TCP performance include Explicit Link Failure Noti cation (ELFN) [1] and ADTCP [19].5 Explicit Link Failure Noti cation (ELFN). ELFN [1] is a simple scheme that provides link failure information to the TCP sender to assist the sender in
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are schemes called TCP-Feedback [20] and ATCP [21], which employ ideas that are very similar to that of ELFN and ADTCP, respectively. Thus, the discussions of these scheme are omitted.
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distinguishing packet losses that are caused by link failures from those that are caused by congestion. Explicit Link Failure Noti cation. When a link failure occurs, the upstream node of the failed link will send a host unreachable ICMP message to the TCP sender. The sender, upon receiving this message, disables its retransmission timers and enters the standby mode. It then uses a periodic probe message to determine whether the route has been restored. When an acknowledgment is received (implying that the route is reestablished), the TCP sender restores its retransmission timers and invokes the previous states (prior to the failure). In this way, the TCP sender can avoid the slow-start phase after a route failure. ELFN is a simple yet ef cient scheme for improving TCP performance. ELFN prevents the TCP sender from invoking congestion control unnecessarily when it detects that a packet loss is caused by a link failure as opposed to congestion. Besides, its periodic probe mechanism allows the TCP sender to actively determine when the route is restored so that the normal transmission can be resumed quickly to avoid an underutilization of the available bandwidth. However, ELFN requires intermediate nodes to assist in detecting and notifying the TCP sender of link failures, and this complicates its implementation and deployment. Furthermore, ELFN uses the previously stored states to resume transmission after a link failure, which may not be appropriate since the previously stored states may not re ect the characteristics of the new route. Another problem with ELFN is that only the TCP sender that triggers the ELFN message is noti ed of the failure; that is, all the other TCP senders that use the same link may not know that their path has failed until they trigger the ELFN message by themselves. ADTCP. ADTCP [19] is an end-to-end scheme that uses multiple end-to-end metrics to determine the cause of packet losses and allow the TCP sender to react appropriately. Classi cation of Connection States. ADTCP casts a connection into one of the following states: (1) congestion, (2) channel error, (3) route change, and (4) disconnection. ADTCP avoids an erroneous execution of congestion control mechanism when it determines that the packet loss is not caused by network congestion. Multiple Metrics. ADTCP uses a multimetric technique to identify the reason for packet losses. The metrics under consideration are: r Interpacket delay difference (IDD) is the delay difference between consecutive packet arrivals. r Short-term throughput (STT) is the throughput during a short interval of observation. r Packet out-of-order delivery ratio (POR) is the ratio of the number of out-oforder packets to the total number of received packets during a short interval of observation.
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