无线移动互联网：原理、技术与应用崔勇CH7WirelessTCP课件教学.pptVIP

PPT研究院 POWERPOINT ACADEMY RTO (Retransmission Timeout ) * Ssthresh: slow start thresh,慢启动门限 * Exposed station problems C cannot send to D when A is transmitting data to B * Standard TCP on BS-MH performs poorly when multiple packet losses occur per window (timeouts can occur on the BS-MH connection, stalling during the timeout interval) * RTT on slow links large, retransmissions result in large variance, pushing RTO up * TCP receiver (MN) reduces interference between TCP and link level retransmission by delaying third and subsequent dupacks. * 接受方不断发送 ACK 信息和 32 位的 acknowledge seq number， 用来通知发送方，期望接收到数据包的序号，同时也表示已经正确接收的数据。 在上述情况下，只要  * 接受方不断发送 ACK 信息和 32 位的 acknowledge seq number， 用来通知发送方，期望接收到数据包的序号，同时也表示已经正确接收的数据。 在上述情况下，只要  * Epoch: regular intervals needed to update the transmission rate, which we refer to as update epochs * * * Solutions Hide mobility from the TCP sender Make TCP adaptive to mobility Schemes Fast retransmit / fast recovery Smooth handoffs M-TCP Freeze-TCP Multicast * * Fast retransmit / fast recovery Method Artificially force fast retransmit mode after handoff Send duplicate ACK after handoff, instead of entering slow start. * * Fast retransmit / fast recovery Advantage Simple changes result in significant higher performance Requires minimal changes to existing TCP structure Disadvantages Scheme doesn’t consider fact of losses over links during handoff * * * Smooth handoffs Provide sufficient overlap between cells to avoid packet loss Buffer packets at BS Discard the packets after a short interval If handoff occurs before the interval expires, forward the packets to the new base station Prevents packet loss on handoff * M-TCP In the fast retransmit scheme sender starts transmitting soon after handoff BUT congestion window shrinks M-TCP attempts to avoid shrinkage in the congestion window K. Brown@ Sigcomm CCR’97 * M-TCP Uses TCP Persist Mode When a new ack is received with receiver’s advertised window = 0, the sender enters per