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r Packet loss ratio (PLR) is the ratio of the number of missing packets within the receiving window during a short interval of observation. These metrics can be measured end-to-end, without the need for an intervention from the intermediate nodes. Identifying the Connection States. When a packet loss occurs, ADTCP determines the connection state as follows: If IDD is high and STT is low, the connection is in the congestion state. In contrast, if the connection is not congested but POR is high, the connection is in the route change state. Similarly, if the connection is not congested but PLR is high, the connection is in the channel error state. The disconnected state occurs if STT reaches zero. ADTCP improves the performance of TCP by identifying the current connection state, thus allowing TCP to react to packet loss more appropriately. ADTCP does not require any feedback from the intermediate nodes. It identi es the connection state by using end-to-end measurements only, which allows for the easy deployment of the scheme. However, the measurement of IDD requires the use of the TCP timestamp option or the sender to record the sending time of all the outstanding segments, which increases the overhead of the protocol. Furthermore, the determination of connection states requires certain metric thresholds to be carefully de ned in order for ADTCP to operate ef ciently. 9.4.3 Reducing Contention In Section 9.3, the adverse effects of the inter ow and intra ow contention problems on the TCP performance have been discussed. The reason for this is that conventional TCP does not operate its congestion window at an appropriate level. The representative schemes that reduce contention and the effects thereof include Congestion Window Limit [12] and Link RED and Adaptive Pacing [4]. These schemes are described below. Congestion Window Limit (CWL). Congestion Window Limit (CWL) [12] is a simple technique that mitigates the congestion window overshoot problem by restricting the maximum congestion window size. This scheme can reduce contention and improve the TCP performance. Adaptive Maximum Congestion Window Size. A study [4] has shown that a maximum of 1/4 spatial reuse can be achieved for a one-way ow along a chain of nodes. This study implies that there is a limit on the maximum sending rate of a TCP source, and in turn implies a limit on the maximum size of the congestion window. Chen et al. [12] have shown that the best throughput of TCP is actually achieved when the congestion window is set to approximately 1/5 of the round-trip hop count (RTHC). This approximation of 1/5 can be easily explained by considering also an increase
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in contention (and thus a reduced spatial reuse) contributed by the reverse ACK packets along the same path. To this end, an adaptive CWL scheme is proposed to adjust the maximum congestion window size to approximately 1/5 of the RTHC of the ow. CWL is a simple scheme that adjusts the maximum congestion window size dynamically so that the sending rate will not exceed the maximum spatial reuse of the channel. By this, both the intra ow and inter ow contention problems are reduced. However, CWL must be used with routing protocols that are aware of the path length. Moreover, the 1/5 window limit is set by considering a single ow. For a multiplicity of ows that compete, it is unclear that this will hold always. The factor could depend on density and the number of competing connections.
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Link RED (LRED) and Adaptive Pacing (AP). LRED and AP [4] are techniques that allow TCP to react preemptively link overload by adaptively delaying certain packet transmissions to reduce contention. Link RED. LRED works by maintaining an average number of retries for recent packet transmissions. If the average retry attempt value exceeds the minimum threshold value (minth ), LRED will mark the outgoing packets with a probability depending on the value. TCP will then reduce its sending rate and thereby, to some extent, avoid packet loss. Adaptive Pacing. AP works by distributing traf c among intermediate nodes in a more balanced way. The basic idea is to let some nodes wait, in addition to the normal backoff period, for an extra amount of time equal to a packet transmission time when necessary. This additional waiting period helps reduce the contention related drops caused by the exposed receivers. AP is used in coordination with LRED. When LRED starts to mark packets (the average retry value exceeds minth ), AP will then increase the backoff time of the pending transmission by an interval equal to the transmission time of the previous packet. LRED provides an early sign of network overload which helps TCP improve the inter ow fairness. When LRED is used in conjunction with AP, they improve the spatial reuse by reducing contention and thus improve the TCP performance. However, LRED requires the MAC layer to maintain an average transmission retry attempt value. It also requires a RED-like algorithm to be implemented at the MAC layer, and this complicates the implementation and the deployment of the scheme. 9.4.4 Detecting Spurious Retransmissions Spurious retransmission is a consequence of TCP s inability to tolerate and handle sudden delay spikes experienced by packet transmissions. The representative schemes that are classi ed into this category include TCP-Eifel [13] and Forward RTO-Recovery [22].
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