Approaches to congestion control.
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http://douc54.cs.edinboro.edu/~bennett/class/csci475/spring2007/notes/ch20/seven.html
Approaches to congestion control.
- End to End control
- Stallings appears to call this Implicit Congestion Signaling
- For systems like TCP where there is no underlying congestion control.
- Congestion must be detected in terms of observed network behavior
- packet loss
- Delay
- TCP Decreases the window size when it detects delay
- Network Assisted congestion control
- Some form of explicit feedback is available from the underlying network.
- ICMP source quench for example.
- Also back pressure
- As upstream nodes become overfilled, they tell down stream nodes to slow down.
- Apparently, most networks don't support this.
- And this is not a transport level issue.
TCP Congestion Control
- The sender responds to perceived congestion.
- When perceived congestion is low, the sending rate increases.
- When perceived congestion is high, the sending rate decreases.
- To do this each side of a connection keeps track of a congestion control window or CongWin.
- The formula LastByteSent-LastByteAcked ≤ min{CongWin, RcvWin}
- TCP defines a loss event as
- a timeout
- The receipt of three duplicate ACKs
- When a loss even occurs, TCP declares the network to be congested.
- When no loss is occurring, TCP uses a round trip time (RTT) estimate to adjust CongWin.
- The RTT is the time between when a segment is handed to TCP and when the ACK is received.
- A sample RTT is measured periodically for one packet at a time.
- But only for segments transmitted once.
- The average RTT is computed by means of a weighted average.
- EstimatedRTT = (1-α)EstimatedRTT + &alpah;SampleRTT
- It is suggested α=0.125
- This average puts more weight on recent samples than older ones.
- This allows response to changing network conditions.
- In statistics this is called an Exponential weighted Moving Average.
- This is because the weight of old values decay exponentially fast.
- A deviation in RTT can be found in the same manner.
- DevRTT = (1-Β)DevRTT + Β |Sample RTT- Estimated RTT|
- Kurose mentions that TimeoutInterval = EstimatedRTT + 4 DevRTT
- Stallings has a nice picture of this on page 685
- TCP uses the RTT to adjust the window.
- If ACK rate is slow, (RTT high) the window is made slightly bigger.
- If ACK rate is fast (RTT low) the window is made much bigger.
- Thus TCP is called self clocking
- The TCP Congestion Control Algorithm consists of three parts.
- Additive increase, multiplicative decrease
- slow start
- reaction to timeout events.
- Additive Increase, Multiplicative Decrease
- When a decrease needs to occur, the packet that tells us to decrease has probably been delayed.
- In the case of a loss event, the window is cut in half.
- When a new ACK (not a dup) arrives the window size is increased
- Kurose suggests that the window size be increased by MSS(MSS/CongWin) each time this occurs.
- This is known as congestion avoidance
- And tends to produce a saw-toothed pattern in TCP connection.
- Slow Start
- At the beginning of a connection, TCP sets CongWin to be 1
- Since this will require a long time to ramp up, it will double the size
- Consider the sequence:
- Packet 1 is sent, CongWin = 1 (Iteration 1)
- Packet 1 is ACKed, CongWin = 2
- Packets 2 and 3 are sent, CongWin = 2 (Iteration 2)
- Packet 2 is ACKed, CongWin = 3
- Packet 3 is ACKed, CongWin = 4
- Reaction to Timeout Events
- After a triple duplicate ACK the window size is cut in half
- After a timeout,
- the window size is set to 1
- It is allowed to grow as in slow start until the window size reaches 1/2 of the level where the timeout occurs.
- Then it grows normally.
标签:control,CongWin,window,TCP,RTT,Approaches,congestion,size 来源: https://www.cnblogs.com/ztguang/p/15795791.html