tailieunhanh - Traffic Analysis and Design of Wireless IP Networks phần 3

đàm thoại và trực tuyến, được quy định cụ thể để thực hiện giao thông thời gian thực. Những người khác, các lớp học tương tác và nền, chủ yếu là Sau đó, xác suất mà hệ thống sẽ vẫn còn trong tình trạng tương tự là PII. Xác suất mà hệ thống sẽ để lại trạng thái của nó | Wireless Mobile Internet 61 3. When the next positive ACK arrives that acknowledges the new data then cwnd ssthresh the value from the first step . This ACK should acknowledge all the intermediate segments sent between the lost packet and the receipt of the first duplicate ACK. So here TCP is in congestion avoidance. Fast retransmissions are efficient for single packet losses but they are not sufficient for recovery from multiple losses in a single window 4 . This usually results in coarse-grained timeout before the packet is retransmitted. There are several variants of TCP depending upon the included mechanisms. We outline the most commonly used TCP implementations in the following section. TCP Implementations There are different implementations of TCP. The most used versions are Tahoe and Reno. TCP Tahoe includes slow start congestion avoidance and fast retransmission mechanisms. In Tahoe slow start follows fast retransmission. If we additionally include the fast recovery mechanism to TCP Tahoe we obtain the TCP Reno version. The mechanisms described in the previous sections are all implemented in Reno. TCP Tahoe functions well at single loss within the congestion window. But it follows the congestion by invoking slow start. TCP Reno improves the performances of the TCP stream at a single loss per window but problems occurs when multiple packets are dropped from a window of data. Such behavior at multiple dropped packets from a window is overcome by some changes implemented in latter versions of TCP such as TCP NewReno and TCP selective acknowledgments SACK . TCP NewReno makes simple changes to the Reno version to avoid waiting for the retransmit timer when multiple packets are lost from a window. It uses partial ACKs to retransmit missing packets . each duplicate ACK indicates that the following segment is lost and it is retransmitted until TCP receives a positive ACK . At all times TCP remains in fast retransmission and fast recovery phases. This way .