第十二章TCP协议.ppt

Congestion Window TCP assumes The cause of a lost segment is due to congestion in the network In flow control Sender window = Receiver window congestion ? lost segments ? retransmission ? more congestion Sender window = Min ( rwnd, cwnd) rwnd: receiver window —— a receiver-side limit cwnd: congestion window —— a sender-side limit Rwnd:接收窗口，是接收方根据其可用缓存大小准许的窗口值。接收端将此值放在TCP报文段的首部中，传送给发送端。 Cwnd:拥塞窗口，是发送端根据所感知的网络拥塞程度得出的窗口值。 前者是接收方使用的流量控制，后者是发送方使用的流量控制，限制发送方向网络注入数据的速度。 每个窗口都反映了发送方可以发送的字节数量，发送端的发送窗口是它们两个中较小的那个值。 为了进行拥塞控制，发送窗口不仅取决于接收端，还要取决于网络的拥塞情况 Congestion Control Algorithms RFC 2581 Slow start and congestion avoidance MUST be used by a TCP sender to control the amount of data that has been sent but not yet acknowledged 慢启动和加法增大，乘法减小 Fast retransmit/Fast recovery To detect and repair loss based on incoming duplicate ACKs TCP assumes 1 or 2 duplicate ACKs ? a reordering of segments 3 or more duplicate ACKs ? a segment has been lost Slow Start andCongestion Avoidance Number of transmitted segments Congestion window size ( in segments) Slow start Congestion avoidance RFC 2581 IW: the initial size of the cwnd LW: the cwnd size after a TCP sender detects loss IW = min (4*SMSS, max ( 2*SMSS, 4380 bytes ) ) LW = 1 full-sized segment, i.e., 1 SMSS sst1 = any, i.e. rwnd sst2 = max ( flightsize/2, 2*SMSS ) sst1 sst2 12.9 TCP Timer Retransmission timer To control a lost or discarded segment The waiting time for an ack of a segment Persistence timer To deal with the zero window-size advertisement The waiting time for an ack with a non-zero window size Keepalive timer To prevent a long idle connection between two TCP The waiting time for some data from a client Time-waited timer To be used during connection termination Retransmission Timer Usage When TCP sends a segment, it creates a retransmission timer for that segment If an ack is received for that segment before time-out, the timer is destroyed Otherwise the segment is retransmitted and the timer is reset Calculation Retransm