Satellite Link Congestion Control Method Based on Status Confirmation

Abstract

The present invention provides a satellite link congestion control method based on status confirmation. First, both communication parties enable timestamps and explicit congestion notification, and use whether the timer times out as a critical point to judge the forward and reverse congestion of the link. Then, based on whether a data packet sent by the receiver is received within the set range of the timer, the link congestion status is determined and different processing is performed. For reverse link congestion, the reverse queuing delay is calculated to optimize the actual link throughput in the Vegas algorithm congestion control mechanism, thereby realizing the adjustment of reverse link congestion. By distinguishing the location and status, i.e., the congestion degree, when the link is congested, the present invention adopts different processing mechanisms for forward and reverse congestion, prevents and controls the occurrence of congestion, eliminates the impact of reverse acknowledgment packet congestion on the forward transmission efficiency in a poor communication network environment, and improves the link throughput and the utilization rate of bandwidth resources.

Description

Technical Field

[0001] The present invention belongs to the technical field of space network data communication, and particularly relates to a satellite link congestion control method based on status confirmation. Background Art

[0002] Satellite communication is to use a satellite as a relay to process microwave signals sent by an earth space station. The communication satellite serving as a relay station is required to be fixed at a certain position in the sky above the earth and run in the same direction and period as the earth. Therefore, the communication satellite is also called a geostationary satellite. The microwave signal emitted by a geostationary satellite can cover at most about 40% of the earth's area. Thus, global communication can be achieved by arranging at most three satellites at equal intervals along the geostationary orbit. It can be seen that the satellite network has a wide coverage range, is not restricted by any terrain, has a large communication capacity, and is suitable for long-distance transmission. Therefore, it is becoming more and more popular and gradually becoming a trend. However, such characteristics of the satellite network also bring problems such as long distance, long delay, bandwidth asymmetry, and extremely high bit error rate, making packet loss and congestion very likely to occur during the communication process. Traditional ground transmission protocols cannot accurately distinguish the reasons for the change in delay and the resulting decrease in link throughput and bandwidth utilization, and cannot make reasonable and timely adjustments. Therefore, it is self-evident to optimize relevant protocols to improve the current situation of satellite communication.

[0003] With the rapid development of technology, continuous progress of technology, and increasingly high requirements for information satisfaction, the research and scale of traditional communication networks are no longer sufficient to meet the target group of users. The research on satellite communication technology is extremely urgent.

[0004] Due to the differences between space wireless networks and ground wired networks, the currently widely used ground transmission protocol TCP does not produce good results in satellite networks. The Consultative Committee for Space Data Systems (CCSDS) proposed the SCPS-TP protocol applicable to satellite networks and capable of maintaining efficient and stable data transmission based on the TCP protocol. Vegas is used for congestion control in SCPS-TP. The Vegas algorithm is an algorithm based on delay feedback and based on the change in delay. However, considering the characteristics of long delay, high bit error rate, and bandwidth asymmetry in satellite networks, the judgment of the link situation by the Vegas algorithm is very single. It blindly changes the congestion window (cwnd) through the change in the Round Trip Time (RTT), which greatly reduces the link throughput and wastes bandwidth resources. Therefore, it is necessary to develop a new satellite link congestion control method to effectively improve the communication efficiency in satellite networks. Summary of the Invention

[0005] Based on the above problems, the present invention proposes a satellite link congestion control method based on status confirmation, including:

[0006] Both communication parties enable timestamps and Explicit Congestion Notification (ECN), and use whether the timer times out as a critical point to judge the forward and reverse congestion of the link; specifically stated as:

[0007] Based on whether a data packet sent by the receiver is received within the set range of the timer, the congestion status of the link is determined and different processing is performed. If a packet is received, the forward link congestion situation is determined; otherwise, it is assumed that the reverse link is congested and further confirmation is required.

[0008] When determining the forward link congestion situation, count the number of Acknowledge character (ACK) packets carrying ECN-echo (ECE) received continuously, and judge the congestion degree of the forward link according to the reception situation of ACKs to adjust the congestion window; including:

[0009] 1) If an ACK carrying ECE is received, it is determined that the link is mildly congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 3 / 4 times the current window, that is, cwnd = 3 / 4cwnd;

[0010] 2) If two consecutive ACKs carrying ECE are received, it is determined that the link is moderately congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 2 times the current window, that is, cwnd = 1 / 2cwnd;

[0011] 3) If three consecutive ACKs carrying ECE are received, it is determined that the link is severely congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 4 times the current window, that is, cwnd = 1 / 4cwnd.

[0012] When it is assumed that the reverse link is congested, first calculate the number of data packets N existing in the reverse cache queue back , and further confirm the congestion status. If it is indeed reverse congestion, then calculate the reverse queuing delay QD back Optimize the actual link throughput in the congestion control mechanism of the Vegas algorithm to achieve the regulation of reverse link congestion; specifically stated as:

[0013] If the data packet reception time exceeds the set time threshold, then calculate the number of ACK packets N existing in the reverse cache queue back :

[0014]

[0015] In the formula, Eback Denotes the reverse link expected transmission rate, A back Denotes the reverse link actual transmission rate, RTT back Denotes the reverse time delay Denotes the minimum reverse time delay, cwnd(t) denotes the congestion window size at time t;

[0016] Set the reverse cache threshold α, according to N back And the size relationship with the threshold α, determine the link congestion situation; including:

[0017] If N back > α, it means the reverse link is congested, calculate the reverse queuing delay time QD back :

[0018] QD back = RTT back - RTTNor back

[0019] In the formula, RTTNor back Is the sum of the transmission delay calculated by the reverse acknowledgment packet according to the data block length and data transmission rate and the propagation delay calculated by the distance and signal propagation rate;

[0020] If N back ≤ α, check whether the ACK received at the previous moment carries ECE. If it does, it means the forward link is severely congested, set the value of the congestion window cwnd to 1, otherwise it is determined that the link congestion is caused by the bit error rate;

[0021] Use QD back Optimize the calculation of the actual link throughput in the Vegas algorithm congestion control mechanism; control the congestion situation of the satellite link through the optimized Vegas congestion control mechanism;

[0022] The optimized actual link throughput A' is expressed as:

[0023] A' = cwnd / (RTT - QD back )

[0024] In the formula, RTT represents the round-trip delay of the current data packet.

[0025] The beneficial effects of the present invention are:

[0026] The present invention proposes a satellite link congestion control method based on status confirmation. By distinguishing the location and status, i.e., the congestion degree, when the link is congested, different processing mechanisms are adopted for forward and reverse congestion, preventing and controlling the occurrence of congestion. When the communication network environment is poor, the influence of reverse acknowledgment packet congestion on the forward transmission efficiency is eliminated, and the link throughput and the utilization rate of bandwidth resources are improved. Description of the Drawings

[0027] Figure 1 This is a flowchart of a satellite link congestion control method based on status confirmation in the present invention.

[0028] Figure 2 This is a prediction graph of throughput results in the present invention.

[0029] Figure 3 This is a prediction graph of packet loss rate results in the present invention.

[0030] Figure 4 This is a congestion avoidance flowchart of the traditional Vegas algorithm in the present invention. Detailed Embodiment

[0031] The present invention will be further described below in conjunction with the drawings and specific embodiments.

[0032] In satellite communication, when using the traditional communication protocol TCP Vegas algorithm for congestion control, due to the inability to identify the cause and location of packet loss, the link bandwidth cannot be fully utilized, resulting in a decrease in throughput. To address this issue, the present invention proposes a satellite link congestion control method based on status confirmation, which can determine the possible location of congestion in real time during data transmission, enabling the sender to accurately judge the change in delay and timely adjust the congestion window to prevent and control the occurrence of congestion.

[0033] TCP Vegas is a typical delay feedback-based protocol. According to the change in RTT, it determines the expected throughput and actual throughput, estimates the number of packets diff in the buffer queue, judges the network condition, adjusts the congestion window, and performs congestion avoidance. Figure 4 The following is a congestion avoidance flowchart of the traditional Vegas algorithm. In the congestion control mechanism of the traditional Vegas algorithm, the actual throughput of the link A = cwnd / RTT. Since the reasons for the change in RTT are diverse and the delay caused by abnormal congestion cannot be excluded, when the forward communication is normal and congestion occurs in the reverse direction, the sender detects an increase in delay and directly reduces the value of cwnd according to the congestion avoidance mechanism, resulting in the inability to fully utilize the link bandwidth and a decrease in throughput.

[0034] In view of the deficiencies of the traditional Vegas algorithm, a satellite link congestion control method based on status confirmation proposed by the present invention has a forward and reverse congestion determination mechanism as follows: for the forward direction, it combines Explicit Congestion Notification (ECN) and the arrival of acknowledgment packets (ACK) to judge the link congestion situation. For the reverse direction, it determines link congestion by calculating whether the number of acknowledgment packets in the buffer queue exceeds a threshold. At the same time, whether a packet times out is used as the critical point for forward and reverse determination. ECN is a function that, after the communication parties negotiate to use it, routers supporting ECN set a congestion mark (Congestion Experienced, CE) in the data packet header of congested packets. When the receiver receives a packet with the CE bit, it will send an ACK with an Explicit Congestion Echo (ECE) flag to inform the sender that the packet has experienced congestion during transmission. It is an extension of the TCP / IP protocol. Its flowchart is as shown in Figure 1 shown, and the specific process is as follows:

[0035] The communication parties enable timestamps and Explicit Congestion Notification (ECN), and use whether the timer times out as the critical point to judge the forward and reverse congestion of the link; the specific description is as follows:

[0036] Based on whether a data packet sent by the receiver is received within the set range of the timer, the link congestion status is determined and different processing is performed. If a packet is received, the forward link congestion situation is judged; otherwise, it is first assumed that the reverse link is congested and further confirmation is required.

[0037] Before timeout occurs, it means that the source end can receive ACKs, excluding reverse link congestion. According to the number of consecutive ACKs carrying the ECE mark received, the congestion degree of the forward link is determined. When judging the forward link congestion situation, the number of acknowledgment packets (ACK) carrying Explicit Congestion Echo (ECE) received continuously is counted, and the congestion degree of the forward link is judged according to the reception situation of the ACKs to adjust the congestion window; including:

[0038] 1) If an ACK carrying ECE is received, it is determined that the link is lightly congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 3 / 4 times the current window, that is, cwnd = 3 / 4cwnd;

[0039] 2) If two consecutive ACKs carrying ECE are received, it is determined that the link is moderately congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 2 times the current window, that is, cwnd = 1 / 2cwnd;

[0040] 3) If three consecutive ACKs carrying ECE are received, it is determined that the link is severely congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 4 times the current window, i.e., cwnd = 1 / 4cwnd.

[0041] After the timer times out, first, it is necessary to calculate the number of data packets N existing in the reverse cache queue back to further confirm the congestion status.

[0042] When assuming reverse link congestion, by calculating the reverse queuing delay time QD back Optimize the actual link throughput in the congestion control mechanism of the Vegas algorithm to achieve the regulation of reverse link congestion; specifically expressed as:

[0043] S1: If the data packet reception time exceeds the set time threshold, that is, after a timeout occurs, immediately calculate the actual number of acknowledgment packets N existing in the reverse cache queue back :

[0044]

[0045] In the formula, E back represents the expected transmission rate of the reverse link, A back represents the actual transmission rate of the reverse link, RTT back represents the reverse delay, represents the minimum reverse delay, and cwnd(t) represents the size of the congestion window at time t;

[0046] S2: Set the reverse cache threshold α, and determine the link congestion situation according to the size relationship between N back and the threshold α; including:

[0047] If N back > α, it indicates reverse link congestion, and calculate the reverse queuing delay time QD back :

[0048] QD back = RTT back - RTTNor back

[0049] In the formula, RTTNor back is the sum of the transmission delay calculated based on the data block length and data transmission rate of the reverse acknowledgment packet and the propagation delay calculated based on the distance and signal propagation rate;

[0050] If N backIf α is less than or equal to α, check whether the ACK received at the previous moment carries ECE. If it does, it indicates severe congestion in the forward link, and set the value of the congestion window cwnd to 1. Otherwise, it is determined that the link congestion is caused by the bit error rate.

[0051] In this embodiment, the threshold α of the reverse cache is set to 2. If N back > α, it indicates reverse congestion. First, do not adjust cwnd, and immediately retransmit the lost data packet. If N back ≤ α, on the one hand, it may be severe forward congestion, and on the other hand, it may also be caused by an extremely high bit error rate. Further observe whether the recently received ACK carries ECE. If it does, there is forward congestion, and reset cwnd to 1. If not, no relevant operation is performed.

[0052] S3: Use QD back Optimize the calculation of the actual link throughput in the Vegas algorithm congestion control mechanism; control the congestion situation of the satellite link through the optimized Vegas congestion control mechanism.

[0053] The optimized actual link throughput A' is expressed as:

[0054] A' = cwnd / (RTT - QD back )

[0055] In the formula, RTT represents the transmission delay of the current data packet.

[0056] The congestion avoidance mechanism of the traditional Vegas protocol dynamically adjusts the congestion window by comparing the difference between the expected throughput and the actual throughput with the thresholds α and β. When reverse congestion occurs in the method of the present invention, the congestion avoidance mechanism of the optimized Vegas algorithm adjusts the congestion window. First, calculate the reverse queuing delay QD according to the reverse delay back , which is the difference between the current reverse transmission delay RTT back and the reverse normal transmission delay RTTNor back (the sum of the transmission delay and the propagation delay of the acknowledgment packet). Then, when calculating the actual link throughput, exclude the queuing delay caused by reverse transmission congestion on the basis of the previous total delay, avoiding improper adjustment of the congestion window due to the increase in delay caused by reverse congestion and reducing the impact on forward transmission.

[0057] To verify the effectiveness of the method of the present invention, in this embodiment, based on the OPNET simulation software, the throughput and the packet loss rate are used as evaluation indicators to verify the impact of the improved Vegas algorithm on the transmission efficiency.

[0058] Use the OPNET simulation software to build the environment required for satellite network communication. Take the satellite node as the sender and the ground workstation as the receiver, simplifying the intermediate link. The satellite uses the geostationary satellite as a reference, 36,000 kilometers from the earth's surface, and the one-way transmission delay reaches 270 ms. The forward and reverse link bandwidth ratio is set to a typical 1000:1, with a forward bandwidth of 100 Mbps and a reverse bandwidth of 100 kbps. The size of the transmitted file is 2000 bytes, and the transmission is carried out in the form of FTP data. Under the same parameters, the throughput and packet loss rate of the traditional Vegas algorithm and the method of the present invention are compared in this simulation. The simulation results are as Figure 2 and Figure 3 shown.

[0059] It can be seen from Figure 2 that the throughput of the traditional Vegas algorithm increases rapidly in the early stage and is more stable during the transmission process. After congestion occurs, under the conditions of long delay and bandwidth asymmetry in the satellite network, the throughput rate of the method of the present invention still increases to a certain extent compared with the traditional Vegas algorithm, making full use of the link bandwidth.

[0060] It can be seen from Figure 3 that at the beginning of the communication, the packet loss rate of the traditional Vegas algorithm increases rapidly. This is because the traditional algorithm cannot adapt to the long delay and high error rate of the satellite network and does not pay attention to the importance of data integrity. The increase in the packet loss rate obtained by the method of the present invention is not so drastic, mainly because the improved Vegas algorithm distinguishes the congestion state, optimizes the communication link, and makes the calculation of the actual link throughput more accurate by excluding the influence of reverse congestion. Therefore, the packet loss rate gradually stabilizes after fluctuations in the later stage.

[0061] The simulation experiment proves that the method of the present invention has different processing mechanisms for forward congestion and reverse congestion respectively. It can not only prevent and control the occurrence of forward congestion in time, but also reduce the influence of reverse congestion on the transmission efficiency of the forward link, basically eliminating the pseudo-congestion caused by packet loss due to error codes, and is more suitable for satellite networks.

Claims

1. A satellite link congestion control method based on status confirmation, characterized in that Including: Both communication parties start the timestamp and display congestion notification, and use whether the timer times out as a critical point to judge the forward and reverse congestion of the link; Based on whether a data packet sent by the receiver is received within the set range of the timer, the link congestion state is determined and different processing is performed. If a packet is received, the forward link congestion situation is determined; otherwise, it is assumed that the reverse link is congested; When determining the forward link congestion situation, count the number of acknowledgment packets ACKs carrying the explicit congestion notification echo ECE received continuously, and judge the congestion degree of the forward link according to the reception situation of the ACKs to adjust the congestion window; When assuming reverse link congestion, calculate the number of packets N present in the reverse cache queue back to further confirm the congestion status. If reverse congestion is determined, calculate the reverse queuing delay QD back Optimize the actual link throughput in the Vegas algorithm congestion control mechanism to achieve the regulation of reverse link congestion.

2. The satellite link congestion control method based on status confirmation according to claim 1, wherein, The judgment of the congestion degree of the forward link according to the reception situation of the ACKs to adjust the congestion window is specifically described as: If an ACK carrying an ECE is received, it is determined that the link is mildly congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 3 / 4 times the current window; If two consecutive ACKs carrying an ECE are received, it is determined that the link is moderately congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 2 times the current window; If three consecutive ACKs carrying an ECE are received, it is determined that the link is severely congested. When the congestion window cwnd is less than the threshold ssthresh, the congestion window is adjusted to 1 / 4 times the current window.

3. A satellite link congestion control method based on status confirmation according to claim 1, characterized in that, Calculate the number of data packets N existing in the reverse cache queue back to further confirm the congestion status, which is specifically expressed as: If the data packet reception time exceeds the set time threshold, calculate the number N of data packets existing in the reverse cache queue back : Where, E back represents the reverse link expected transmission rate, A back represents the reverse link actual transmission rate, RTT back represents the reverse delay, represents the minimum reverse delay, and cwnd(t) represents the congestion window size at time t; Set the reverse cache threshold α, according to N back Based on the size relationship with the threshold α, determine the link congestion situation; specifically expressed as: If N back > α, it indicates reverse link congestion, and calculate the reverse queuing delay time QD back : QD back = RTT back - RTTNor back wherein, RTTNor back is the sum of the transmission delay calculated by the reverse acknowledgment packet based on the data block length and data transmission rate and the propagation delay calculated based on the distance and signal propagation rate; If N back ≤ α, check whether the ACK received at the previous moment carries ECE. If it does, it means that the forward link is severely congested, and set the value of the congestion window cwnd to 1. Otherwise, it is determined that the link congestion is caused by the bit error rate.

4. A satellite link congestion control method based on status confirmation according to claim 3, characterized in that, The calculation formula for the optimized actual link throughput A' is: A' = cwnd(RTT - QD back ) In the formula, RTT represents the round-trip delay of the current data packet.

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