JP4429982B2 - Congestion control method and communication terminal thereof - Google Patents

Description

  The present invention relates to a technique for calculating how much a window size is increased in accordance with a congestion degree of a network in a congestion control algorithm implemented in a transport layer.

  The terminals currently used on the Internet implement TCP (Transmission Control Protocol) as a transport protocol. The congestion control algorithm in TCP can be divided into a slow start stage and a congestion avoidance stage (Non-Patent Document 1). Here, only the algorithm in the congestion avoidance stage related to the present invention will be described.

  When a transmitting terminal using TCP transmits a packet, it assigns a sequence number to each packet. When receiving the packet correctly, the receiving terminal transmits a reception confirmation packet to which the sequence number of the packet expected to be received next is given. FIG. 1 is a diagram for explaining a sequence number assigned to a packet, in which a rectangle represents a packet, and a number therein represents a sequence number. In the example of FIG. 1, when a packet with a sequence number 100 or 101 is received, the receiving terminal transmits a reception confirmation packet to which the sequence number 101 or 102 of the packet expected to be received next is given.

  The transmitting terminal manages a variable called a congestion window (cwnd), and can transmit a packet by the value of cwnd without arrival of a reception confirmation packet from the receiving terminal. For example, when the value of cwnd is 100, the transmitting terminal can transmit 100 packets without arrival of an acknowledgment packet. When the transmitting terminal receives cwnd number of acknowledgment packets, the transmitting terminal determines that the network is not congested and increases the value of cwnd by one. That is, in the previous example, when the reception confirmation packet for 100 packets is received, the value of cwnd is increased by 1 to 101. On the other hand, when the reception confirmation packet having the same sequence number of the packet expected to be received next is continuously received three times, it is determined that the network is in a congested state, the value of cwnd is halved, and the packet of that sequence number Will be resent. FIG. 2 is a diagram for explaining the conditions under which a packet is retransmitted. In FIG. 2, the packet with the sequence number 101 is discarded in the network, and the transmitting terminal receives the reception confirmation packet with the sequence number 101 that is expected to be received next three times, so the packet with the sequence number 101 is retransmitted. is doing.

  The above is the basic operation of the congestion control algorithm in TCP. Recently, it has been reported that when this round-trip propagation delay time is long, a gigabit-level throughput cannot be realized, and a new CUBIC-TCP is proposed. (Non-Patent Document 2). In CUBIC-TCP, if reception confirmation packets having the same sequence number of the next packet expected to be received are received three times in succession, it is determined that the network is in a congested state, and the value of cwnd is reduced by a factor of b. . On the other hand, if it is determined that the network is not in a congested state when it has not been received three times in succession, and the window size cwnd (t )

Set to. Here, C is a constant, K is ((b * W_max) / C) ^1/3 , and W_max is the value of cwnd when it is determined that the previous congestion state. “*” Represents multiplication. FIG. 3 shows the window size when the time t has elapsed since the window size decreased and then increased again in CUBIC-TCP.

  CUBIC-TCP increases the increase rate of cwnd and lowers the decrease rate compared to conventional TCP, so that a gigabit-level throughput can be realized even when the round-trip delay time is long.

  However, CUBIC-TCP has various problems. One of the problems is that when a flow is communicating with a large window size and another flow starts communicating, the window size of the flow that has started communication does not increase easily. The results of a simulation to confirm this problem are shown below. FIG. 4 shows a simulation model. There are two transmission terminals and two reception terminals, the link bandwidth is 600 Mbps, the propagation delay time between the transmission and reception terminals is 50 milliseconds, and the packet size is 1500 bytes. The transmission terminal always has a packet to be transmitted, and the first transmission terminal starts communication at time 0 seconds, and the second transmission terminal starts communication at time 100 seconds. FIG. 5 shows changes in the congestion window size over time. In FIG. 5, the vertical axis represents the window size, and the horizontal axis represents time. From FIG. 5, the window size of the flow that started communication in 100 seconds (dotted line in FIG. 5) is equal to the window size of the flow that has already started communication (solid line in FIG. 5) even when the time reaches 300 seconds. It can be confirmed that it is not. In other words, CUBIC-TCP has a problem that it takes a considerable time to obtain a wide-band throughput for a flow for which communication is newly started.

  In addition to CUBIC-TCP, a method called TCP Westwood + has been proposed (Non-Patent Document 3). In Westwood +, when the packet is transmitted, the current time now1 is written in the packet, and when the receiving terminal receives the packet, it reads now1 written in the packet, and when receiving the reception confirmation packet for the received packet, it confirms reception of now1. When the reception terminal receives the reception confirmation packet, it calculates the value obtained by subtracting now1 from the current time, stores the minimum value RTT_min, and uses it from the reception confirmation packet received within the round trip delay time. The possible bandwidth ABE is calculated as follows.

Here, last_t is the time when the last ABE was calculated, d is the number of packets confirmed to be received during (now-last_t), and α is a constant.

  In Westwood +, when the reception confirmation packet having the same sequence number of the next packet expected to be received is continuously received three times, it is determined that the network is in a congestion state, and the value of cwnd is reduced to RTT_min * ABE. On the other hand, if it is not received three times in succession, it is determined that the network is not in a congested state, and the window size is increased using a method similar to the conventional TCP.

  In this way, Westwood + determines the window size reduction width based on the value RTT_min * ABE when reducing the window size when it is determined that the network is in a congested state. However, when increasing the window size, the value of ABE is not used.

Murayama et al., “Transport Protocol”, Iwanami Shoten, April 6, 2001, P174-176 I. Rhee, L. Xu, “CUBIC: A New TCP Friendly High-Speed TCP Variant”, PFLDnet 2005, Feb. 2005, [online], Internet <http://www.ens-Iyon.fr/LIP/RESO / pfIdnet2005 / ＞ L. A. Grieco, S. Nascolo, “Performance Evaluation and Comparion of Westwood +, New Reno. And Vegas TCP Congestion Control”, ACM SIGCOMM Computer Communication Review, PP.25-38, April 2004

  In the congestion control method of the CUBIC-TCP, when determining the value of the window size cwnd (t) at the time when t time has elapsed since the window size was decreased and started to increase again, the values RTT_min * ABE and RTT_max * ABE are set as follows. The purpose is to solve various problems of CUBIC-TCP, for example, the problem that the time until a newly started communication flow obtains a broadband throughput becomes longer by determining the value based on the value. It is said.

In the present invention, the values RTT_min * ABE and RTT_max * ABE are calculated, and the increase width of the window size is determined based on these values.
That is, the present invention includes a step in which the transmitting terminal writes the current time now1 in the packet at the time of packet transmission, a step in which the receiving terminal reads now1 written in the packet when the packet is received, and the reception When the terminal transmits a reception confirmation packet for the received packet, a step of writing “now1” into the reception confirmation packet; and when the transmission terminal receives the reception confirmation packet, the terminal confirms the reception confirmation packet from the current time “now”. Calculating a value obtained by subtracting written “now1”, storing a minimum round-trip delay time RTT_min that is the minimum value, and a maximum round-trip delay time RTT_max that is the maximum value; Minimum round-trip delay time RTT_min has elapsed from the time ABE was calculated Calculating the available bandwidth ABE based on the number of acknowledged packets received during time, and when the transmitting terminal determines that the network is not congested, and increases the window size, RTT_min * based on the ABE and RTT_max * ABE, possess determining the window size, the transmitting terminal, in the step of determining the window size, currently, after the window size is reduced, from the start to increase again Assuming that time t has elapsed, the window size cwnd (t) is set to C (t−K) ³ + RTT_max * ABE, where C is a constant and K is (ABE * (RTT_max−RTT_min) / C) ^1/3 . Is determined).

  The differences between the present invention and the prior art are as follows. In the present invention, when the value of the window size cwnd (t) at the time when t time has elapsed after the window size is decreased and started to increase again, the value is determined based on the values RTT_min * ABE and RTT_max * ABE. ing. However, those values are not used in the conventional CUBIC-TCP. Westwood + uses the value RTT_min * ABE when decreasing the window size, but does not use those values when increasing the window size.

  By using the present invention, it is possible to improve the performance of CUBIC-TCP capable of realizing a gigabit level throughput. For example, the time required to obtain a fair bandwidth can be shortened.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 6 shows a configuration diagram of the transmission terminal in the embodiment, and FIG. 7 shows a configuration diagram of the reception terminal. As shown in FIG. 6A, the transmission terminal 600 includes a current time writing unit 601, a time reading unit 602, a round trip delay time calculating unit 603, a sequence number reading unit 604, an available bandwidth calculating unit 605, and a window size determining unit. 610. As shown in FIG. 6B, the window size determination unit 610 includes a size increase / decrease determination unit 611, a size increase unit 612, and a size decrease unit 613. As shown in FIG. 7, the receiving terminal 700 includes a time reading unit 701 and an information writing unit 702. 6 and 7 show only the parts necessary for explaining the present embodiment. Means that the transmitting terminal and the receiving terminal of the present embodiment are normally required as the transmitting terminal and the receiving terminal are shown. It is natural to have.

  When transmitting the packet A, the transmission terminal 600 writes the current time now1 in the current time writing unit 601 and then transmits the packet A. When receiving the packet A, the receiving terminal 700 reads the time information now1 written in the packet A by the time reading unit 701 and notifies the information writing unit 702 of the value. When receiving terminal 700 transmits a reception confirmation packet for packet A, information writing unit 702 writes time information now1 written in packet A and transmits packet B. When the transmission terminal 600 receives the packet B, the time reading unit 602 reads the time information now1 written in the packet B, and reads the current time now (the time when the packet B was received) and the read now1 (the time when the packet A was transmitted) ) To the round-trip delay time calculation unit 603. The round-trip delay time calculation unit 603 calculates a minimum round-trip delay time (RTT_min) and a maximum round-trip delay time (RTT_max) according to the flowchart shown in FIG. The transmission terminal 600 also reads the sequence number of the packet expected to be received next, which is given to the packet by the sequence number reading unit 604, and notifies the available bandwidth calculation unit 605 and the window size determination unit 610 of the result. The available bandwidth calculation unit 605 calculates an available bandwidth (ABE) according to the flowchart shown in FIG. The window size determination unit 610 includes a size increase / decrease determination unit 611, a size increase unit 612, and a size decrease unit 613. The size increase / decrease determination unit 611 determines whether to increase or decrease the window size according to the flowchart illustrated in FIG. The size increasing unit 612 increases the window size according to the flowchart shown in FIG. The size reducing unit 613 reduces the window size according to the flowchart shown in FIG.

  FIG. 8 shows an algorithm (flow chart) in the round-trip delay time calculation unit 603. The round-trip delay time calculation unit 603 obtains the maximum round-trip delay time (RTT_max) and the minimum round-trip delay time (RTT_min) as follows, and stores them in the storage device. First, the difference between the current time now and the now1 read from the packet received by the time reading unit 602, that is, the round-trip delay time T = now-now1 of the packet is calculated (step 801). When T> RTT_max, that is, when the round-trip delay time of the packet is larger than the maximum round-trip delay time stored so far (YES in step 802), the value of T is substituted into RTT_max, and the round-trip delay time of the packet Is stored as the maximum round-trip delay time (step 803). If T <RTT_mix, that is, if the round trip delay time of the packet is smaller than the previously stored minimum round trip delay time (YES in step 804), the value of T is substituted into RTT_min, and the round trip delay time of the packet Is stored as the minimum round-trip delay time (step 805).

  FIG. 9 shows an algorithm (flow chart) in the available bandwidth calculation unit 605. In FIG. 9, last_t_ is the time when the previous ABE was calculated. X is a number notified from the sequence number reading unit 604. C is a number notified last time from the sequence number reading unit 604. d is the number of acknowledged packets. Here, the packet that has been acknowledged refers to a packet that has been acknowledged by the receiving terminal 700. The reception confirmation packet having the sequence number X indicates that the reception terminal 700 has correctly received (X-C) packets by the packet that caused the reception confirmation packet to be sent. Therefore, d obtained by adding them is the number of packets confirmed to be received during that time. α is a constant. The ABE equation in step 904 of FIG. 9 is the same as the TCP Westwood + ABE equation described in the background section, and the overall algorithm of FIG. 9 is also the same as the TCP Westwood + algorithm. Therefore, the constant α in FIG. 9 is the same as the constant α in TCP Westwood +, and 0.9 is used as the recommended value of constant α in TCP Westwood +. Therefore, the recommended value of the constant α in FIG. is there.

  The available bandwidth calculator 605 calculates the available bandwidth (ABE) as follows. First, the available bandwidth calculation unit 605 calculates the difference between the current time now and the time last_t_ at which the previous ABE was calculated, that is, the time T = now-last_t_ from the time at which the previous ABE was calculated to the current time ( Step 901). Next, d + (X−C) is substituted for the confirmed number of packets d (step 902). Since d = 0 is set when the previous ABE was calculated (step 905), d is the number of acknowledged packets counted since the previous ABE was calculated. If T ≧ RTT_min, that is, if the time from the previous ABE calculation time to the current time is equal to or greater than the minimum round-trip delay time (YES in step 903), the available bandwidth ABE = α * ABE + (1−α) * d / T is calculated (step 904). Substituting 0 for d resets d (the number of packets that have been acknowledged), substitutes the current time now for last_t_, and sets last_t_ (the time when the previous ABE was confirmed) (step 905). By substituting the value of X into C, C (the number notified last time from the sequence number reading unit 604) is updated (step 906), and the process ends. On the other hand, if NO in step 903, the value of X is substituted into C to update C (the number notified last time from sequence number reading unit 604) (step 906), and the process ends.

  As can be seen from the flowchart in FIG. 9, when T ≧ RTT_min in step 903, ABE is calculated in step 904, so that the minimum round trip delay time RTT_min has elapsed from the time when the available bandwidth ABE was calculated last time. Therefore, the available bandwidth ABE is calculated based on the number of acknowledged packets received at the same time.

Next, an algorithm (flow chart) in the window size determination unit 610 will be described in detail.
FIG. 10A shows an algorithm (flow chart) in the size increase / decrease determination unit 611. C is a number previously notified from the sequence number reading unit 604, and num is the number of times the same sequence number is received. As illustrated in FIG. 10A, when the number is notified from the sequence number reading unit 604 (step 1011), the size increase / decrease determination unit 611 compares the value X with the value C recorded in the memory (step 1012). ). If X and C are not equal (NO in step 1012), it means that reception packets having the same sequence number have not been received continuously, so num (number of times the same sequence number has been received) is set to 1. C (the number notified last from the sequence number reading unit 604) is updated to the value of X (step 1013), the values of num and C are stored in the memory, and the process proceeds to the size increasing unit 612 (FIG. 10-2). If X and C are equal (YES in step 1012), it means that the reception confirmation packets having the same sequence number are continuously received, so num stored in the memory (the number of times the sequence number having the same number is received). 1 is added to the value of (step 1014). Then, if reception confirmation packets having num = 3, that is, the same sequence number are received three times in succession (in the case of YES at step 1015), num is updated to 1 and C is updated to the value of X. The value of C is stored in the memory (step 1016), and the process proceeds to the size reduction unit 613 (FIG. 10-3). If num is not 3 (NO in step 1015), the value of num is stored in the memory, and the process proceeds to the size increasing unit 612 (FIG. 10-2).

FIG. 10-2 shows an algorithm (flow chart) in the size increasing unit 612. t is the time since the window size was decreased and started increasing again, t_dec is the time when the window size was decreased last time, ABE is the value notified from the available bandwidth calculator 605, and W_max and W_min are The value notified from the round-trip delay time calculation unit 603, and C is a constant. The constant C in FIG. 10-2 can be the same as the constant C in the equation for calculating cwnd (t) in CUBIC-TCP described in the background art section. Since the recommended value of the constant C in CUBIC-TCP is 0.4, the recommended value of the constant C in FIG. 10-2 is also 0.4. As shown in FIG. 10-2, the size increasing unit 612 calculates a value t based on the time t_dec when the previous window size stored in the memory is decreased (step 1021). The value K = (ABE * (RTT_max−RTT_min)) based on ABE notified from the available bandwidth calculator 605, RTT_min and RTT_max notified from the round-trip delay time calculator 603, and C stored in the memory. / C) Calculate ^1/3 . The window size cwnd (t) = C (t−K) ³ + RTT_max * ABE is calculated from these values, and a new window size is determined (step 1022).

  FIG. 10C shows an algorithm (flow chart) in the size reduction unit 613 of the window size determination unit 610. As shown in FIG. 10-3, the size reduction unit 613 sets the window size cwnd = RTT_min * ABE based on the ABE notified from the available bandwidth calculation unit 605 and the RTT_min notified from the round trip delay time calculation unit 603. Calculate and determine a new window size (step 1031). Then, the time when the window size is decreased is substituted into t_dec stored in the memory of the window increasing unit (1032).

  Hereinafter, the operation of this embodiment will be described with reference to the example shown in FIG. As shown in FIG. 11, the transmitting terminal 600 transmits packets 1 to 3 to the receiving terminal 700 at times 10.000, 10.001, and 10.002 seconds, respectively, and from the reception confirmation packet 1 from the receiving terminal 700. 3 are received at times 10.101, 10.102, and 10.103 seconds, respectively. The reception confirmation packets 1 to 3 have 10.000, 10.001, 10.002 seconds as time information now1, respectively, and 100, 101, 102 as the sequence numbers of packets expected to be received next. And Immediately before receiving the reception confirmation packet 1, RTT_max (maximum round trip delay time) in FIG. 8 is 0.2 seconds, RTT_min (minimum round trip delay time) is 0.1 seconds, and last_t_ in FIG. 9 (time when the previous ABE was calculated). Is 10.003 seconds, d (number of packets acknowledged) is 100, C (number previously notified from the sequence number reading unit 604) is 99, α (constant) is 0.9, and num in FIG. (Number of times the same sequence number is received) is 1, C (number previously notified from the sequence number reading unit 604) is 99, and t_dec in FIG. 10-2 (previous time when the window size is decreased) is 9 seconds. , C (constant) is 0.4, and ABE (value notified from the available bandwidth calculator 605) is 1000.

  When the transmission terminal 600 receives the packet 1 reception confirmation packet, the time reading unit 602 reads the time information now1 written in the packet. Then, it notifies now1 (= 10.0000) and the current time now (= 10.101) to the round-trip delay time calculation unit 603. The round-trip delay time unit 603 calculates T (step 801 in FIG. 8). T is 0.101 (= 10.101-10.000), and this value is not smaller than RTT_min (= 0.1) (NO in step 804) and is not larger than RTT_max (= 0.2). (NO in step 802), the values of RTT_max and RTT_min are not updated. The sequence number reading unit 604 reads the sequence number of the packet expected to be received next given to the packet. The value is 100, and the available bandwidth calculation unit 605 and the window size determination unit 610 are notified of the value. The available bandwidth calculation unit 605 calculates T and d (steps 901 and 902 in FIG. 9). T is 0.098 (= 10.101-10.003), and d is 101 (= 100 + (100-99)). Since T (= 0.098) is not greater than or equal to RTT_min (= 0.1) (NO in step 903), no new ABE is calculated. The value C is updated to 100 (step 906). In the window size determination unit 610, first, the size increase / decrease determination unit 611 determines whether or not to increase the window size. Since X is 100 and C is 99 this time (NO in step 1012 of FIG. 10-1), num is updated to 1, C is updated to 100 (step 1013), and the process proceeds to the size increasing unit 612. In the size increasing unit, t is first calculated (step 1021 in FIG. 10-2). t is 1.101 (= 10.101-9). Next, K is calculated.

となる（ステップ１０２２）。 So the window size is

(Step 1022).

  When the transmission terminal 600 receives the packet 2 reception confirmation packet, the time reading unit 602 reads the time information now1 written in the packet. Then, now1 (= 10.0001) and the current time now (= 10.102) are notified to the round trip delay time calculation unit 603. The round-trip delay time unit 603 calculates T (step 801 in FIG. 8). T is 0.101 (= 10.102-10.001), and this value is not smaller than RTT_min (= 0.1) (NO in step 804) and is not larger than RTT_max (= 0.2). (NO in step 802), the values of RTT_max and RTT_min are not updated. The sequence number reading unit 604 reads the sequence number of the packet expected to be received next given to the packet. The value is 101, and the value is notified to the available bandwidth calculation unit 605 and the window size determination unit 610. The available bandwidth calculation unit 605 calculates T and d (steps 901 and 902 in FIG. 9). T is 0.099 (= 10.102-10.003), and d is 102 (= 101 + (101-100)). Since T (= 0.099) is not greater than or equal to RTT_min (= 0.1) (NO in step 903), no new ABE is calculated. The value C is updated to 101 (step 906). In the window size determination unit 610, first, the size increase / decrease determination unit 611 determines whether or not to increase the window size. Since X is 101 and C is 100 (NO in step 1012 in FIG. 10A), num is updated to 1, C is updated to 101 (step 1013), and the process proceeds to the size increasing unit 612. The size increasing unit 612 first calculates t (1021 in FIG. 10-1). t is 1.102 (= 10.102-9). Next, K is calculated.

となる（ステップ１０２２）。 So the window size is

(Step 1022).

  When the transmission terminal 600 receives the packet 3 reception confirmation packet, the time reading unit 602 reads the time information now1 written in the packet. Then, now1 (= 10.0002) and the current time now (= 10.103) are notified to the round trip delay time calculation unit 603. The round-trip delay time unit 603 calculates T (step 801 in FIG. 8). T is 0.101 (= 10.103-10.002), and this value is not smaller than RTT_min (= 0.1) (NO in step 804) and is not larger than RTT_max (= 0.2). (NO in step 802), the values of RTT_max and RTT_min are not updated. The sequence number reading unit 604 reads the sequence number of the packet expected to be received next given to the packet. The value is 102, and the value is notified to the available bandwidth calculation unit 605 and the window size determination unit 610. The available bandwidth calculation unit 605 calculates T and d (steps 901 and 902 in FIG. 9). T is 0.1 (= 10.103-10.003) and d is 103 (= 102 + (102-101)). Since T (= 0.1) is RTT_min (= 0.1) or more (YES in step 903), a new ABE is calculated (step 904).

d is updated to 0, last_t is updated to 10.103, and C is updated to 102 (steps 905 and 906). In the window size determination unit 610, first, the size increase / decrease determination unit 611 determines whether or not to increase the window size. Since X is 102 and C is 101 (step 1012 in FIG. 10A), num is updated to 1, C is updated to 102 (step 1013), and the process proceeds to the size increasing unit 612. The size increasing unit 612 first calculates t (step 1021 in FIG. 10-2). t is 1.103 (= 10.103-9). Next, K is calculated.

となる（ステップ１０２２）。 So the window size is

(Step 1022).

  Next, the result of having performed the simulation which confirms the effect of a present Example is shown. The simulation conditions are exactly the same as those described in the background art (see FIGS. 4 and 5). FIG. 12 shows changes in the congestion window size over time. Comparing FIG. 5 and FIG. 12, when this embodiment is used, the time until the window size of the flow that has started communication in 100 seconds becomes equal to the window size of the flow that has already started communication can be greatly reduced. Can be confirmed.

  In the embodiment described above, each unit of the transmission terminal and the reception terminal can be realized by a program stored in a computer and a storage device. The variable name used in the program is not limited to the variable name in this embodiment, and any variable name can be used.

  As mentioned above, the invention made by the present inventor has been specifically described based on the above embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Of course.

Sequence number given to the packet Conditions for packet retransmission Window size when t time has passed since the window size decreased and then increased again Simulation model Simulation results when using the conventional method System configuration of sending terminal System configuration of receiving terminal Algorithm in the round-trip delay time calculation unit 603 (flow chart) Algorithm (Flowchart) in Available Bandwidth Calculation Unit 605 Algorithm in the size increase / decrease determination unit 611 (flow chart) Algorithm in the size increase unit 612 (flow chart) Algorithm in the size reduction unit 613 (flow chart) Flow of packet transmission / reception in the embodiment Simulation results when using examples

Explanation of symbols

600 ... transmitting terminal, 601 ... current time writing unit, 602 ... time reading unit, 603 ... round-trip delay time calculating unit, 604 ... sequence number reading unit, 605 ... usable bandwidth calculating unit, 610 ... window size determining unit, 611 ... Size increase / decrease determination unit, 612 ... Size increase unit, 613 ... Size decrease unit, 700 ... Receiving terminal, 701 ... Time reading unit, 702 ... Information writing unit

Claims (2)

In the congestion control method of decreasing the window size when it is determined that the network is congested, and increasing the window size when it is determined that it is not,
The transmitting terminal writes the current time now1 in the packet at the time of packet transmission;
When the receiving terminal receives the packet, reads the now1 written in the packet;
When the receiving terminal transmits a reception confirmation packet for the received packet, writing now1 into the reception confirmation packet;
When the transmission terminal receives the reception confirmation packet, it calculates a value obtained by subtracting now1 written in the reception confirmation packet from the current time now, and the minimum round-trip delay time RTT_min, which is the minimum value, and Storing a maximum round trip delay time RTT_max which is a maximum value;
Calculating the available bandwidth ABE based on the number of acknowledged packets received during the time that the minimum round-trip delay time RTT_min has elapsed since the time when the transmitting terminal calculated the available bandwidth ABE last time;
Determining a window size based on RTT_min * ABE and RTT_max * ABE when the transmitting terminal determines that the network size is not congested and increases the window size;
I have a,
In the step of determining the window size by the transmitting terminal, if t time has elapsed since the window size decreased and then increased again, the window size cwnd (t) is set to C (t−K ^{) 3 + RTT_max * ABE, (} C is a constant, K is (ABE * (RTT_max-rTT_min) / C) congestion control method characterized by determining which is the ^1/3). In a communication terminal that performs congestion control that decreases the window size when it is determined that the network is in a congestion state and increases the window size when it is determined that the network is not,
A current time writing unit for writing the current time now1 to the packet at the time of packet transmission;
When the reception confirmation packet of the packet is received, a value obtained by subtracting now1 written in the reception confirmation packet from the current time now is calculated, and the minimum round-trip delay time RTT_min that is the minimum value and the maximum value are calculated. A round trip time calculation unit for storing a certain maximum round trip delay time RTT_max;
An available bandwidth calculation unit for calculating the available bandwidth ABE based on the number of acknowledged packets received during the time when the minimum round-trip delay time RTT_min has elapsed from the time when the previous available bandwidth ABE was calculated;
A window size determination unit for determining a window size based on RTT_min * ABE and RTT_max * ABE when increasing the window size when it is determined that the network is not in a congestion state;
Equipped with a,
Assuming that t time has elapsed since the window size determination unit started to increase again after the window size has decreased, the window size cwnd (t) is set to C (t−K) ³ + RTT_max * ABE, ( communication terminal C is a constant, K is characterized that you determined (ABE * (RTT_max-rTT_min) / C) is ^1/3).

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