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Congestion Avoidance in TCP

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Congestion Avoidance in TCP


 

  • Consequence of lack of congestion control

     

    • When a popular resource is shared without regulation the result is always over-utilization

       

    • With the introduction of TCP in 1983, users can write networking applications that require reliablity with greater ease

       

    • When more applications are available, more data and information are exchange on the Internet.

       

    • In mere 3 years time, the Internet had its first breakdown....

       

    • A classic paper by Jacobson contains the following introduction:

       

      "In October of ‘86, the Internet had the first of what became a series of congestion collapses.

      ..., the data throughput from LBL to UC Berkeley (sites separated by 400 yards and 2 IMP - i.e., routers - hops) dropped from 32 Kilo bits/sec to 40 bits/sec."

       

    • Jacobson‘s paper can be found here: click here

     

  • History of Congestion Control in TCP

     

    • There have been many (and increasingly sophisticated) congestion avoidance mechanims added to TCP since Jacobson‘s work on Congestion Control.

       

    • The Congestion Control mechanism in TCP is an ever developing process.... (it is still a research topic !)

       

    • The most popular versions of TCP - named after cities in Nevada - are:

       

       

      1. TCP Tahoe

         

        • This is the original version of TCP congestion control as implemented by Jacobson

           

        • Congestion detection mechanism is based on packet loss

           

        • Techniques used for congestion control:

           

          • Slow Start
          • Congestion Avoidance
          • Fast Retransmit

         

      2. TCP Reno

         

        • This is the most popular version of TCP congestion control mechanism today.

           

        • Techniques used for congestion control:

           

          • same as TCP Tahoe (Slow Start, Congestion Avoidance and Fast Retransmit), plus
          • Fast Recovery

         

      3. TCP Vegas

         

        • This is completely new implementation

           

        • Congestion detection mechanism is based on end-to-end delay

           

     



     

  • TCP packet size

     

    • TCP is a byte oriented protocol

       

    • However, TCP would not send every byte in a separate packet since this would result in an enormous overhead...

       

    • User data is carried inside a TCP packet which itself is carried inside a IP packet :

       

      bubuko.com,布布扣

       

       

    • Example:

       

      • If no additional options are used (no additional packet header information beside the necessary ones), and each packet carries a single byte of data, the IP packet size will be 41 bytes.

         

      • The efficiency (useful part of the packet) would be 1/41 or 2.4%

         

      • That‘s equivelent to Uncle Sam taking 97.6% in income taxes !!!

       

    • TCP will always try to send multiple bytes in a packet to improve efficient

       



       

    • Since we are dealing with congestion control, we will assume the worst case scenario which is when the TCP source is transmitting a large amount of data continuously

      In other words:

       

      We assume that every TCP source is sending at maximum data rate

      This is achieved by sending packets whose size is as large as possible

      In other words, in the analysis of TCP congestion control scheme, we always assume that:

       

      TCP always transmits packets of size equal to MSS bytes

       



       

    • Maximum Packet (Segment) Size

       

      • User/System can impose a maximum packet size used in TCP

         

      • The maximum packet size is called Maximum Segment Size or MSS

       

    • The MSS will play an important part in describing the congestion avoidance mechanism used in TCP....

     



     

  • Transmission Data Rate

     

    • The transmission data rate is indirectly dependent on the transmit window size...

      The dependency is pretty complicated and very dynamic in nature

      The following examples will derive a simple relationship between the data transmission rate and the transmit window size.

       

    • Example 1:

       

      • Suppose the sender has a lot of data to transmit (transmits continously) and the transmit window size is equal to MSS (Max Segment Size). The following will happen:

         

         

        bubuko.com,布布扣

         

         

        1. Because transmit window size is equal to MSS, the sender can send only 1 packet at a time and must stop (because he promised not to send more than MSS bytes before hearing back from the receiver on how the data were received).

           

        2. The ACK for will return in approximately RTT (round trip time) sec

           

        3. When the ACK returns, the sender sends the next packet. (If the ACK does not return for a long time, the sender will retransmit - sender assumes the packet was lost).

           

        4. The resulting transmission rate is approximately MSS/RTT bps.

       


       

    • Example 2: If window size = 2*MSS, the sender can send faster:

       

       

      bubuko.com,布布扣

       

      But do not conclude that data rate is proportional to the window size. The above examples are "idealized". Network delays, route changes and other factors can make the relationship very unpredictable and dynamic.

     



     

  • TCP Transmit Window Size

     

    • Terminology:

       

       

      Transmit Window

       

          # packets (
      MSS
        bytes in each packet) that sender can transmit without having to wait for acknowledgement

       

       

    • The size of the Transmit Window is computed using 2 windows:

       

      1. Advertised Window Size
      2. TCP‘s Congestion Window Size

       


       

    • We have learned about the Advertised Window Size previously (see: click here ):

       

      • The advertised window size is the amount of data that the receiver is willing to buffer when data arrived out of order:

         

         

        bubuko.com,布布扣

         

     



     

  • TCP‘s Congestion Window Size

     

     

    • An intuitive definition of the Congestion Window Size is:

       

      Congestion Window Size is

       

          the number of packets (of
      MSS
          bytes of data) that the sender
      believes
        that it can transmit into the network without causing congestion in the network.

       

       

    • Notice that this amount of data depends on the current network status and thus varies over time...

      In fact, it changes faster than the weather and it is just as unpredictable...

     



     

  • Relationship of the Transmit Window and the Congestion Window

     

     

    • When users send a large amount of data through the shared Internet, they must be be courteous in regard to:

       

      1. The receiving party (do not overwhelm a slower receiver)
      2. The shared transmission medium (Internet)

       

    • In other words, the sender must NOT transmit more data than:

       

      1. The receiver can handle
      2. The network links can handle

       

    • In yet other words:

       

      1. TCP Transmit Window size <= Advertised Window size,       and:

         

      2. TCP Transmit Window size <= Congestion Window size

       

     



     

  • Notations used in TCP congestion control scheme

     

     

    • Advertised Window Size (AWS) = amount of data that the receiver will buffer.

      AWS is negotiated at connection establishment and remains unchanged afterwards

       

    • Congestion Window Size (CWND) = window size imposed by the TCP congestion mechanism to avoid causing congestion in the network

      CWND changes over time !!!

       

    • Transmit Window Size (TWS) = the amount of unacknowledged data, i.e., data that TCP transmits in a burst without receiving any indication on what happened to the data.

       

    • Relationship:

       

        TWS = min (AWS, CWND)
                   

         

       



     

  • How TCP controls its transmission rate

     

    • Recall that the Advertised Window Size (AWS) is contained in the TCP header (so TCP has this information to its disposal)

       

    • The TCP congestion control algorithm will compute the value of CWND according to (implicit) signals/events (such as timeout, duplicate ACKs, see: click here ) from the network

      (We have not yet discussed HOW TCP changes the value of CWND - will come next)

       

    • From the values of AWS and CWND, TCP will computes the transmit window size as:

       

    • TWS = min (AWS, CWND)
    •  

    • Because AWS is not under TCP control (but determined by the receiver), we will leave this value out of the discussion.

      In the remainder of the discussion, we will discuss how TCP updates the value of CWND

     



     

  • TCP modes/phases of operation

     

    • The key to understand why TCP operates in the way it does is to remember that network condition changes constantly

       

      • New TCP connections can be started at any time which will reduce the avaliable network capacity for existing TCP connections

         

      • Existing TCP connections can end at any time which will increase the avaliable network capacity for the remaining TCP connections

       

    • To accomodate the uncertainty, TCP operates in two different modes/phases

       

      1. Slow Start Mode/Phase:

         

        • This is the start up mode of operation of TCP

           

        • In this mode/phase, TCP has an idea (guess) about the maximum transmission rate and TCP is trying to reach this transmission rate

           

        • Although TCP has an idea (guess) about this maximum transmission rate, TCP will NOT transmit at this rate instantaneously

          Rather, TCP will try to reach this maximum transmission rate in a piece meal fashion

           

        • In this phase, TCP will start by transmitting ONE packet and at each successfully transmission epoch, TCP will DOUBLE the number of packets (resulting in an exponential increase in number of packets in time).

         

      2. Congestion Avoidance Phase:

         

        • This is the phase that begins AFTER the start up phase

          The start up phase ends when TCP has reached the maximum transmission rate that it "believed" to be safe.

           

        • In other words, TCP is now in uncharted territory....

          Because TCP has reach the maximum safe level, it would appear that there is still some more capacity available - it would be a shame NOT to use the available capacity !!!

           

        • But ! TCP has no idea what the new maximum capacity is... so it must be careful !

           

        • In this phase, TCP will increase the number of packet much slower than in the start up phase (increase rate will not be exponential, but linear)

       

     



     

  • TCP congestion strategy: A video game analogy

     

    • What TCP is doing is somewhat the same strategy as playing a video game...

       

    • In some adventure video games, there are "danger" areas where the player get killed by some booby trap.

       

    • So how do you play such a video game ?

       

       

      1. You just walked in a trap and get killed....

         

      2. Restart the game, and play quickly upto the point where you got killed.

         

      3. From that point on, play very carefully.....

       

    • The life of TCP is like a never-ending video game:

       

      1. When TCP detects congestion (through a packet loss), it sets CWND to half of the transmit window size that it was used when the packet loss occured.

        (Because the current transmit window size causes packet loss, half of the current transmit window size is a conservative estimate of the NEW safe level to operate !)

         

      2. Then TCP will restart by transmitting using CWND = 1 (ONE packet outstanding) and increase CWND exponentially (from ONE) to the new congestion window size CWND

        This phase is the slow start phase

         

      3. When TCP reach the new congestion window size (this is the point where TCP believe it is safe), it will enter the second phase and increase the window size much less aggressively (linearly instead of exponentially)

        This phase is the congestion avoidance phase

         

      4. The congestion avoidance phase ends when TCP detects a packet loss and the cycle starts again from the top....

       



       

  • Overview of the (idealized) TCP congestion control operation:

     

     

    bubuko.com,布布扣

     

        1. When TCP starts out, it sets CWND = AWS (try to send as much data as the receiver can handle)

          If the network can handle this transmission rate, TCP will not need to do any congestion control !!! (Because the bottle neck is at the receiver...)

          The picture above shows a scenario where the network capacity is less than what the receiver can handle - i.e., the network is the bottle neck.

           

        2. At some point (in the figure, it happens when sender transmits 50 Kbps ), packets are dropped and congestion is detected.

          Because the packet drop happens at the moment when the sender was transmitting 50 Kbps , the new target congestion rate is set to 25 Kbps

           

        3. TCP increase the transmission rate exponentially until it reaches 25 Kbps

           

        4. From 25 Kbps onwards, TCP will increase the transmission rate linearly - until it discover a packet loss

          (In the figure, it happens when sender is transmitting 30 Kbps )

          Because the packet drop happens at the moment when the sender was transmitting 30 Kbps , the new target congestion rate is set to 15 Kbps

           

        5. TCP increase the transmission rate exponentially until it reaches 15 Kbps

           

        6. From 15 Kbps onwards, TCP will increase the transmission rate linearly - until it discover a packet loss

          And so on....

     

    • NOTES:

       

      • Remember: the goal of TCP is get the highest possible throughput.

         

      • This goal is not achieved by sending as fast as possible, but as much as the network can handle !!!

         

      • The available network bandwidth changes constantly.

         

      • TCP tries to determine the available capacity by remembering when the data rate at which a packet drop occured the last time and proceeds carefully starting from half way of this capacity level.

         

      • NOTE: the first time that TCP starts, it has no idea what the network capacity is and the only thing that it can do is to set the level to what the receiver can handle...

     



     

  • An overview of of techniques used in TCP Congestion control

     

    • We have just seen an high level discussion of the TCP Congestion control algorithm consisting of 2 different phase

      In the slow start phase, transmission rate increases exponentially in time.

      In the congestion avoidance phase, transmission rate increases linearly in time.

       

    • So basically, the difference between the 2 phases is the rate of increase in transmission speed.

       

    • Now it‘s time to see how the increase in transmission speed is realised.

       

    • TCP uses the following 3 mechanisms with very sexy sounding names:

       

      1. Slow Start
      2. Fast Retransmit
      3. Fast Recovery

    We will look at each mechanism separately and indicated when the mechanism is appropriate.

     


    The SLOW START Phase

     

  • The Slow Start Mechanism

     

    • During the slow start phase, TCP uses the slow start mechanism for congestion control.

       

    • Information needed to implement the slow start mechanism:

       

      • SSThresHold

         

            The window size that TCP
        believes
            to be safe

         

          • SSThresHold = AWS when TCP begins for the first time
          • SSThresHold is set to TWS/2 when TCP detects a packet loss

         

      • CWND

         

            The (current) congestion window size

        CWND and AWS will determine the transmit window size of TCP

         

       


       

    • Operation of the slow start mechanism is as follows:

       

       

      Initilization:

       

      • SSThresHold is set to AWS (when TCP first begins) or Transmit Window/2 (when TCP detects congestion)

      Slow Start:

       

      • Set CWND = MSS (i.e., ONE packet)

         

      • TCP increases CWND by MSS whenever TCP receives an NEW ACK packet (= an ACK message that TCP has never seen before)

        (NOTE:: If TCP receives a duplicate ACK, no updates are made to the CWND variable)

       


       

    • Example of TCP operation in the slow start phase:

       

       

      bubuko.com,布布扣

       

      • Initially (at time 0), CWND = 1

         

      • At time RTT (round trip time), CWND = 2

         

      • At time 2 RTT, CWND = 4

         

      • At time 3 RTT (not in figure), CWND = 8

         

      • And so on...

         

      • When you plot CWND over time, CWND will increase exponentially

       

       

     

  • When to BEGIN a new Slow Start epoch

     

    • A slow start epoch can be initiated in 2 stituations:

       

       

      1. When a TCP connection is first establish.

        In this case, SSThresHold is set to AWS

         

      2. When TCP has detected a packet loss

        In this case, SSThresHold is set to Transmit Window/2

       

     

  • When to END a Slow Start epoch

     

    • The slow start epoch can ended by 2 events:

       

       

      1. When CWND > SSThresh

         

            This is a "normal" termination.

        In this case, TCP will enter the congestion control phase:

         

        bubuko.com,布布扣

         

        TCP is now in "uncharted" territory and will increase its congestion window slower

         

      2. When TCP detects a packet loss

         

            This is an "abnormal" termination.

        In this case, TCP will re-enter

        TCP first sets SSThresHold = Transmit Window

        Then TCP resets CWND = 1 to start a new Slow Start epoch

       

       

     



     

  • A $64,000 question:

     

    • Why would TCP use a "slow start" procedure to increase CWND from ONE all the way to SSThrehHold

      Why not just set CWND to SSThrehHold and be done with it ???

       

       

    • Answer:

       

      • TCP uses timeouts to tell if packets are lossed

         

      • The timeout value used must be estimated because we don‘t know in advance how far away the receiver is located.

         

      • So TCP must maintain an estimate for the RTT to the receiver and the timeout interval is a function of the RTT

         

      • By sending packets slowly instead of in a burst, TCP can measure the RTT of packets more accurately

     

     

  • How can you call the EXPONENTIAL increase of transmission rate in "Slow Start" SLOW ???

     

    • The name "slow start" is probably one of the worst misnomer in networking...

       

    • How on earth can you call an exponential increase in window size SLOW ???

       

    • To understand the terminology, you have to look in history....

       

    • Prior to Jacobson‘s work, TCP operates as follows:

       

       

      bubuko.com,布布扣

       

       

        1. A new TCP connection first negotiate a advertised window size (AWS)

           

        2. The source immediately transmits an amount of data that is equal to the advertised window size (e.g., when a large file is transfered).

       

    • Now, compared to sending AWS bytes of data, the new way of start transmitting ONE packet first is indeeds slower...

     


    The Congestion Avoidance Phase

     

  • TCP‘s Congestion Avoidance mode

     

    • TCP enters the congestion avoidance phase when the slow start phase terminates normally

      (i.e., CWND > SSThresHold)

       


       

    • Operation of the congestion avoidance phase is as follows:

       

       

      • Ideally, TCP increases CWND by ONE packet or MSS bytes after every RTT seconds

        It is quite complex to remember how many bytes you have acknowledge...

        It is far easier to increase CWND each time you receive a NEW acknowledgement

         

      • Notice that if TCP is transmitting maximum size packets, and the congestion window is CWND, then there are approximately CWND/MSS packets sent using the transmit window

        So if we add MSS/CWND to the congestion window size, we will have effectively increase CWND by ONE after all the acknowledgement packets return (they wil return in RTT seconds)

         

      • So practically, this can be (approximately) accomplished by increasing the congestion window CWND by MSS/CWND packets or MSS * MSS/CWND bytes after TCP receives a NEW acknowledgement

        So:

         

              CWND = CWND + MSS * MSS/CWND
           

        when TCP receive a NEW acknowledgement

        (Again, when a duplicate (old) ACK is received, CWND is not updated)

         

       


      Example of TCP operation in the congestion avoidance phase:

       

      bubuko.com,布布扣

       

      • Suppose that CWND = 4 when TCP enters the congestion avoidance phase...

        TCP sends out 4 packets (each containing MSS bytes) to the receiver.

         

      • If there is no congestion, 4 NEW ACK packets will be received in approximately RTT seconds

         

      • When the first ACK packet is received, TCP updates CWND as follows:

         

          CWND = CWND + MSS * MSS/CWND        // CWND = 4 MSS  
               = 4 MSS + MSS * MSS/(4 MSS)
               = 4 MSS + MSS * 1/4
               = 4.25 MSS
        

         

         

      • When the second ACK packet is received, TCP updates CWND as follows:

         

          CWND = CWND + MSS * MSS/CWND        // CWND = 4.25 MSS  
               = 4.25 MSS + MSS * MSS/(4.25 MSS)
               = 4.25 MSS + MSS * 1/4.25
               = 4.485 MSS
        

         

         

      • When the third ACK packet is received, TCP updates CWND as follows:

         

          CWND = CWND + MSS * MSS/CWND        // CWND = 4.485 MSS  
               = 4.485 MSS + MSS * MSS/(4.485 MSS)
               = 4.485 MSS + MSS * 1/4.485
               = 4.708 MSS
        

         

         

      • When the fourth (and final) ACK packet is received, TCP updates CWND as follows:

         

          CWND = CWND + MSS * MSS/CWND        // CWND = 4.708 MSS  
               = 4.708 MSS + MSS * MSS/(4.708 MSS)
               = 4.708 MSS + MSS * 1/4.708
               = 4.92 MSS
        

         

      • So you can see that CWND is increased approximately by MSS or ONE packet after RTT seconds

        (In the slow start phase, CWND DOUBLES after each RTT seconds)

       



       

    • NOTE: the actual implementation of TCP (see Stevens - Volume 2) increases CWND during congestion avoidance slightly faster than above using to the following formula:

       

       

          CWND = CWND + MSS * MSS/CWND + MSS/8    
      

       

     



     

  • Why do TCP want to keep increasing CWND ?

     

    • Why does TCP not keep CWND constant after reaching the "safe" operation level SSThresHold ?

       

    • When TCP keeps increasing CWND, it would eventually cause a congestion !!!

      So why so foolish ???

       

    • The reason is:

       

      • TCP does not know the current network capacity... because network condition keeps changing.

         

      • The goal of TCP is to transfer data as fast as possible.

        If TCP would stop increasing CWND, it would not be true to its goal.

         

      • So during the congestion avoidance period, TCP is testing the tolerance of the network:

         

            after it has successfully transferring
        CWND
            amount of data, it adds
        one more packet
            to the congestion window:
        CWND + MSS
            and
        retest
          the network.

      (This technique is similar to kids testing their boundary by asking their parents for favors over and over again... The boundary may have moved :-))

     



     

  • When does the Congestion Avoidance phase BEGIN ?

     

    • When the Slow Start phase terminates successfully

     



     

  • When does the Congestion Avoidance phase END ?

     

    • Eventually, TCP will push the congestion window too far and cause some packet drop.

       

    • When a packet loss occurs, it can cause the sending TCP to timeout

       

    • When a timeout occurs, the congestion avoidance phase ends and TCP will begin a slow start phase:

       

       

      bubuko.com,布布扣

       

       

      1. TCP sets SSThresh = CWND/2.

        This is the new "safe" operation level...

         

      2. Then TCP set CWND = 1 x MSS (i.e., 1 packet worth of data) and increases CWND at an expontial rate towards SSThresh (the "safe" level")

 

 


Fast Retransmit


 

  • Fast Retransmit

     

    • Before the introduction of the Fast Retransmit, TCP was not "pro-active".

       

    • Example:

       

          When
      packet 14
          is lossed, and the receiver
      repeatedly
          transmits
      ACK 13
          back to the sender, the sender would
      NOT act on this signal
          :

       

      bubuko.com,布布扣

       

       

          1. Assume all packets upto packet 13 have been received and acknowledged

             

          2. When packet 14 is lossed, and packets #15, #16, #17 and #18 arrive out of order at the receiver, the receiving will send back ACK 13 to indicate that the last consecutive packet received was #13

             

          3. Prior to the introduction of Fast Retransmit, TCP does not act upon the multiple duplicate ACK messages from the receiver.

             

          4. The sender would TIMEOUT and them retransmits the lossed packet

       

      • FURTHERMORE, when TCP times out, TCP will enter the SLOW START phase

       



       

    • Fast Retransmit: using duplicate ACKs as indicators for lossed pacet

       

       

      • Clearly, when the receiver keep sending the same (duplicate) ACK, some message may be lossed

         

      • BUT, because packets can arrive OUT OF ORDER, an occasional duplicate ACK can arrive:

         

        bubuko.com,布布扣

         

         

      • So to eliminate most of these false loss indications, it is decided that when TCP receives 3 duplicate ACKs (so TCP received a total of 4 identical ACK packets), then TCP concludes that the packet is lossed and TCP retransmits the lossed packet IMMEDIATELY (without waiting for timeout):

         

        bubuko.com,布布扣

         

        • Having received 3 duplicate ACKs privides a high probability that a packet was lossed, but does not provide certainty...

       



       

    • After TCP retransmits the lossed packet, it enters the Slow Start phase (because a packet loss had occured)

       

    • Time to show TCP congestion control mechanism in action...

       



     

  • TCP Tahoe Demo

     

    • TCP Tahoe (the original version by Jacobson) incorporates the Slow Start and Congestion Avoidance mechanisms.

       

    • We will look at the operation of TCP Tahoe in this sample network:

       

      bubuko.com,布布扣

       

       

    • Here is a NS2 source file to simulate a TCP Tahoe source: click here

       

      • Right click and save the file in your directory.
      • Run program with:

         

              export PATH=/usr/local/gnu/gcc/4.1.0/bin:$PATH
              export LD_LIBRARY_PATH=/usr/local/gnu/gcc/4.1.0/lib:$LD_LIBRARY_PATH
        
              /home/cheung/NS/run-ns Tahoe.tcl
        

         

      • You should see the Network Animator window when it finish running... click PLAY to see the simulation in action

       

    • You don‘t need to run the simulation to see the animation... I have saved a copy of the animation file generated by the simulation.

      The NAM (Network Animation) output file is here: click here

       

          To see the animation, save the NAM file in your directory and use this command:
          /home/cheung/NS/bin/nam   Tahoe.nam
      

       

    • The Congestion Window CWND plot data output file is here: click here

       

          To see the plot of the CWND of TCP, save the Congestion Window CWND plot file in your directory and run
      gnuplot

      In gnuplot, issue the command:

           plot "WinFile" using 1:2 title "Flow 1" with lines 1
      

      You should see this plot:

       

      bubuko.com,布布扣

      You can see the operation of TCP Tahoe clearly from the above figure:

       

      1. At approximately time 0, TCP Tahoe starts and it is in the slow start mode: the congestion window size increases exponentially

         

      2. At approximately time 5, packet loss is detected.

        TCP marks SSThresh = 25 (approximately) and begins another slow start

         

      3. When it reaches CWND = 25 (approximately), the CWND increases linearly - here TCP Tahoe enters the congestion avoidance mode

         

      4. At approximately time 19, TCP Tahoe detects packet loss and begins a slow start.

        SSThresHold is approximately 22.

         

      5. TCP begins another slow start and so on...

     

     

     


    Fast Recovery

     

  • Added Improvement: Fast Recovery:

     

    • Fast recovery is a beautiful little improvement made to TCP that signifantly increased the TCP‘s performance level.

       

    • Research discovered that:

       

      Most Fast Retransmit actions
          occur during
      mild congestions
        situation, i.e., congestions that clears up very quickly.

       

    • Recall that TCP performs a SLOW START after TCP performs Fast Retransmit (because there was a packet loss)

       

    • Instead of perform a SLOW START (which reduces CWND down to 1 x MSS), research found that TCP can use a larger congestion window without causing network congestion !

       

       

    • Fast recovery:

       

       

      When TCP performs a fast restransmit (so TCP did not timeout):

       

      1. set SSThresh = CWND/2

         

      2. set CWND = SSThresh + 3 * MSS.

        (The rationel is that 3 duplicate ACK is worth 3 MSS bytes)

         

      3. TCP continue to use congestion avoidance (but using the new values of of SSThresh and CWND).

       

    • Example:

       

       

      bubuko.com,布布扣

       

       

     



     

  • TCP Reno Demo (Reno implements Fast Recovery)

     

    • Here is a NS2 source file to simulate a TCP Reno source: click here

       

      • Right click and save the file in your directory.
      • Run program with:

         

              export PATH=/usr/local/gnu/gcc/4.1.0/bin:$PATH
              export LD_LIBRARY_PATH=/usr/local/gnu/gcc/4.1.0/lib:$LD_LIBRARY_PATH
        
              /home/cheung/NS/run-ns Reno.tcl
        

         

      • You should see the Network Animator window when it finish running... click PLAY to see the simulation in action

       

    • You don‘t need to run the simulation to see the animation... I have saved a copy of the animation file generated by the simulation.

      The NAM (Network Animation) output file is here: click here

       

          To see the animation, save the NAM file in your directory and use this command:
          /home/cheung/NS/bin/nam   Reno.nam
      

       

    • The Congestion Window CWND plot data output file is here: click here

       

          To see the plot of the CWND of TCP, save the Congestion Window CWND plot file in your directory and run
      gnuplot

      In gnuplot, issue the command:

           plot "Reno-Window" using 1:2 title "Flow 1" with lines 1
      

      You should see this plot:

       

      bubuko.com,布布扣

      You can see that this small change in TCP Reno has resulted in a huge performance improvement:

       

        1. Again, at time 20, TCP Reno is in congestion avoidance mode

           

        2. At approximately time 27, TCP Reno detects packet loss and performs a fast retransmit and the fast recovery was successful. TCP Reno continues in the congestion avoidance mode without performing a slow start (We can see this because CWND did not start with 1)

           

        3. Again, at approximately time 40, TCP Reno detects packet loss and performs a fast retransmit and the fast recovery was successful. TCP Reno continues in the congestion avoidance mode without performing a slow start (We can see this because CWND did not start with 1)

     







     

  • Further TCP Research: the TCP Flow Synchronization problem

     

    • Do NOT think that this is the end of the story about Congestion Control on the Internet !

       

    • This material on TCP is only the tip of the iceberg...

      There are much more problems and issues with TCP after they introduced TCP Reno

       

    • For example, research has discovered that different TCP flows sharing a bottle neck link will synchronize with each other !!!

      (Here is a paper that points out the phenomenon: click here )

      Example that illustrates TCP synchronization:

       

      bubuko.com,布布扣

       

      • Source 1 (red) start transmitting at time 0.1 sec
      • Source 2 (blue) start transmitting at time 20.0 sec

       

       

    • Here is a NS2 source file to simulate 2 TCP Reno sources sharing the bottle neck link: click here

       

      • Right click and save the file in your directory.
      • Run program with:

         

              /home/cheung/NS/run-ns Reno.tcl
        

         

      • You should see the Network Animator window when it finish running... click PLAY to see the simulation in action

       

    • You need to run the simulation to see the animation because I deleted the animation file generated by the simulation.

       

    • The Congestion Window CWND plot data output files are here:

       

      To see the plot of the CWND of TCP, save the Congestion Window CWND plot file in your directory and run gnuplot

      In gnuplot, issue the command:

           plot "WinFile" using 1:2 title "Flow 1" with lines 1,           "WinFile2" using 1:2 title "Flow 2" with lines 2
      
      

      You should see this plot:

       

      bubuko.com,布布扣

       

    • The plots shows clearly that:

       

      • Flow 1 starts early
      • Flow 2 starts later and cause packet drops for both flows
      • The 2 TCP flows perform Slow Start together and reduce their windows simultaneously

         

      • Eventually, the congestion windows of both flows are synchronized !!!

      This kind of behavior is not good, because the best way to utilizate all network capacity is for one of the flow to cut back

      (But it should NOT always be the same flow, otherwise you have unfairness)

       



       

    • The research to solve this phenomenon triggered the development of "Active Queue Management (AQM)" - among them, the "Random Early Drop/Detection (RED)" queue is the best well-known representative.

       

     



     

  • Further TCP Research: RTT unfairness problem

     

    • TCP is NOT fair when different TCP connection share a bottle neck link but have different Round Trip Times (RTT)

       

    • Example that illustrates TCP unfairness when RTTs differ:

       

      bubuko.com,布布扣

       

      • Source 1 (red) start transmitting at time 0.1 sec, RTT is 2 x 150 msec
      • Source 2 (blue) start transmitting at time 20.0 sec, RTT is 2 x 640 msec

         

      • Due to higher RTT, the CWND of flow 2 will increase slower !!!

       

       

    • Here is a NS2 source file to simulate 2 TCP Reno sources sharing the bottle neck link: click here

       

      • Right click and save the file in your directory.
      • Run program with:

         

              /home/cheung/NS/run-ns Reno.tcl
        

         

      • You should see the Network Animator window when it finish running... click PLAY to see the simulation in action

        (The NAM file is too big and I deleted it...)

       

       

    • The Congestion Window CWND plot data output files are here:

       

      To see the plot of the CWND of TCP, save the Congestion Window CWND plot file in your directory and run gnuplot

      In gnuplot, issue the command:

           plot "WinFile" using 1:2 title "Flow 1" with lines 1,           "WinFile2" using 1:2 title "Flow 2" with lines 2
      
      

      You should see this plot:

       

      bubuko.com,布布扣

       

      • You can see that the average congestion window size of flow 2 is lower than flow 1 and they do not converge...

         

      • BTW, you can also see the TCP flow synchronization problem in this plot:

         

            Both flows will often perform
        slow start
          at (approximately) the same time

       

     



     

  • Further TCP Research: Gigabit networks

     

    • Another area of intense research is to adapt TCP for higher speed networks (Giga or Tera bit networks).

      In these networks, the usable window size is huge... hundreds of thousands of packets.

      TCP cannot afford the luxury to increase its window size by 1 in each RTT.

      In order to reach the fill capacity of the network, TCP must increase faster...

    • Example that illustrates TCP‘s behavior in high speed network :

       

      bubuko.com,布布扣

       

      • Source start transmitting at time 0.1 sec

       

       

    • Here is a NS2 source file to simulate TCP Reno on a high speed (Giga bit) network: click here

       

      • Right click and save the file in your directory.
      • Run program with:

         

              /home/cheung/NS/run-ns Reno.tcl
        

         

      • You should see the Network Animator window when it finish running... click PLAY to see the simulation in action

        (The NAM file is too big and I deleted it...)

       

       

    • The Congestion Window CWND plot data output files are here:

       

      To see the plot of the CWND of TCP, save the Congestion Window CWND plot file in your directory and run gnuplot

      In gnuplot, issue the command:

           plot "WinFile" using 1:2 title "Flow 1" with lines 1
      
      

      You should see this plot:

       

      bubuko.com,布布扣

       

      • You can see that TCP performed 2 unsuccessful slow starts

         

      • At approximately 16 sec, TCP performs the 3rd slow start.

         

      • This slow start terminates at approximately 18 sec.

         

      • Then TCP performs Congestion Avoidance... all the way up from CWND = 20 (approximately) to 250+ The simulated ended after 140 sec and TCP has not reached full capacity yet !!!

       

    • If TCP wants to take advantage of high speed links, it must increase the congestion window more aggressively and additively

       

     http://www.mathcs.emory.edu/~cheung/Courses/558-old/Syllabus/6-transport/TCP.html





Congestion Avoidance in TCP

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原文地址:http://www.cnblogs.com/forcheryl/p/4053288.html

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