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First, the theoretical basis
1.1 Congestion
What is
- Too many hosts, the speed is too fast, too much data transmission
- To the network
- Beyond the network processing capability
- It leads to a lot of data packets for forwarding the network congestion intermediate device (such as a router) queue
- Network performance is significantly decreased phenomenon
as a result of
- Data packets through the network growth delay
- Because the queue is full are discarded resulting in a large number of packets
1.2 Congestion Control
What is
- Rational management, specification, adjustment to the network
- The number of hosts to send data, the data transmission rate or the amount of
- Avoid or eliminate existing congestion
method
TCP's congestion control can be drawn without having to belong to the state of congestion feedback
Two, TCP congestion control
What is
End angle, detecting whether the network blocked, if so, immediately down the data transmission rate
method
Of the AIMD (Additive increase, multiplicative decrease)
when the network is not congested, the window size plus growth, when the network is congested, the multiplicative decrease the window size, the following specific description
2.1TCP congestion control algorithm
(A) slow start algorithm to avoid congestion
as the picture shows:
Understand the basic concept map
RTT: Defines a segment to receive a confirmation of this time segment time interval from the transmission side of the round trip time
CongWin variables:CongWin transmitting end congestion window variables, in bytes, represents a case where not received, the number of bytes that can be transmitted continuously, with the size of the dynamic change of network congestion level receiver acknowledges
Thershold: Congestion window threshold Thershold, a turning point in the slow start and congestion avoidance
Explain the process of drawing algorithm
Slow start phase
Heuristic method, the MSS initially CongWin set (maximum segment size), within the RTT, each receive an ACK, CongWin a plus the MSS, when the congestion window is increased to the threshold thershold, into the
Congestion avoidance phase
When this slow growth phase window size, this window is confirmed for all segments, a window size increase of the MSS (whether the ACK number received), this time CongWin linear growth (plus growth), occurs when the timer times out ( network congestion)
How to solve the network congestion
CongWin congestion window value becomes 1 (multiplicative decrease), to enter the slow start phase, when the congestion window threshold value becomes half Thershold network congestion window size (FIG Thershold update value 12)
Incomprehensible point
RTT is generally a long time, all the congestion window can send all segments
So in the slow start phase, received several several ACK window size growth over the RTT time interval, the general said RTT interval is doubled, as
to avoid congestion phase, regardless received within a time interval RTT several ACK, only the window size is increased by one
(B) the fast retransmit and fast recovery algorithm
Why has this algorithm?
Repeat three times to distinguish between the timer expires and confirm the situation
mentioned in the blog post, and repeated three times to confirm the timer expires ACK are considered lost, but here and be considered as network congestion, and the sender can receive a confirmation receiving end, in fact and true network congestion is not the same, so there is this algorithm
Repeat three times to confirm the way
Although seq = segment 200 is not overtime, but after three duplicate acknowledgments will still retransmission
Fast retransmission and fast recovery algorithm of FIG.
And on a different algorithm is:
Upon receipt of three duplicate ACK, congestion window CongWin will be directly transferred to the congestion window threshold Thershold, then enter the congestion avoidance phase, which is fast recovery, the recovery value is CongWin
Third, the difference between flow control and congestion control
End data flow control, congestion control of the rate control global network
If network congestion, then the recipient has another big cache, the data can not be received
Implement two controls are sent by dynamically changing the window size, the transmission window size takes the minimum two control
