COMMUNICATION SYSTEM, RETRANSMISSION CONTROL METHOD, RECEIVING DEVICE, AND TRANSMITTING DEVICE

Abstract

This communication system carries out communication on the basis of the TCP. The communication system comprises a transmission-side device and a reception-side device. A route switching time period is a time period in which a communication route between the transmission-side device and the reception-side device switches from a first communication route to a second communication route. If the reception-side device detects packet loss of a first packet in the route switching time period, transmission of a retransmission request relating to the first packet to the transmission-side device is prohibited during a first time interval from the detection of the packet loss. If the first packet is not received during the first time interval, the reception-side device transmits the retransmission request relating to the first packet to the transmission-side device.

Description

[Technical field]

[0001] The present invention relates to retransmission control in a communication system that performs communication based on TCP (Transmission Control Protocol). [Background technology]

[0002] In recent years, satellite communication networks have come to be used for various purposes. In order to perform appropriate communication control in such satellite communication, it is considered to use TCP, which has high reliability. One of the TCP functions for ensuring communication reliability is known as "retransmission control."

[0003] Figures 1 and 2 are conceptual diagrams for explaining retransmission control in TCP. The sender assigns a sequence number to the packet to be sent to the receiver. The receiver can detect (recognize) a "missing" packet by checking the sequence number of the packet received from the sender. This "missing" packet is called a "missing packet" or "packet loss."

[0004] In the following explanation, for simplicity, a packet with sequence number i will be called "packet i." In the example shown in Figures 1 and 2, the receiving side receives packets N+1 and onwards without having received packet N yet. This causes the receiving side to detect the loss of packet N. When the receiving side detects the loss of packet N, it sends a "retransmission request" to the transmitting side, requesting the retransmission of packet N.

[0005] The transmitting side receives a retransmission request for packet N from the receiving side. If the transmitting side receives a retransmission request for the same packet N three times, the transmitting side retransmits packet N to the receiving side. Alternatively, if the transmitting side does not receive an acknowledgement response (hereinafter referred to as "ACK") for packet N within a predetermined retransmission timeout after transmitting packet N, the transmitting side retransmits packet N to the receiving side. In the example shown in FIG. 1, the transmitting side retransmits packet N to the receiving side after receiving a retransmission request for the same packet N three times.

[0006] In the example shown in Figure 2, the packet arrival order has changed, and packet N arrives at the receiving side later than packets N+1 and N+2. Upon receiving packet N, the receiving side sends an ACK to the transmitting side to notify the receiving side of packet N. The transmitting side receives an ACK for packet N before receiving three retransmission requests for packet N. Therefore, the transmitting side determines that retransmission of packet N is unnecessary.

[0007] When TCP receives an ACK for a transmitted packet, it increases the number of transmitted packets. On the other hand, when a packet retransmission occurs, TCP reduces the number of transmitted packets to avoid congestion in the network. Therefore, in a high-latency environment with a long round trip time (RTT), such as a satellite communication network, the number of transmitted packets does not increase easily, and the throughput of the entire system decreases.

[0008] Non-Patent Document 1 discloses a retransmission control method for TCP in a high-latency environment. This retransmission control method flexibly sets the reduction amount of the number of transmitted packets when a packet retransmission occurs according to the environment. Specifically, in a high-latency environment, the reduction amount of the number of transmitted packets is suppressed compared to a low-latency environment. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] Ikeda, Nishiyama, and Kato, “A Study on Retransmission Control of Network-Assisted Transport Protocol in Long-Latency Wireless Environments”, IEICE Technical Report, SAT2010-28, July 2010. Summary of the Invention [Problem to be solved by the invention]

[0010] The inventor of the present application recognized the following problem regarding the situation where the communication route between the sending side and the receiving side is switched. FIG. 3 shows the situation where the communication route is switched between the sending timing of packet N and the sending timing of packet N+1. In particular, the switching occurs from a communication route with a large delay to a communication route with a small delay. In this case, packet N arrives at the receiving side with a significant delay from packet N+1. However, the sending side has already received a retransmission request for packet N three times, and retransmits packet N to the receiving side. Since the receiving side can receive the original packet N even if it is delayed, the retransmission of packet N can be said to be an "unnecessary retransmission". Even when such an unnecessary packet retransmission occurs, TCP reduces the number of transmitted packets. As a result, the throughput of the entire system decreases.

[0011] An object of the present invention is to provide a technique capable of suppressing a decrease in throughput of the entire system that accompanies switching of a communication route in a communication system that performs communication based on TCP. [Means for solving the problem]

[0012] The first aspect relates to a communication system that performs communication based on TCP (Transmission Control Protocol). The communication system comprises a sending device and a receiving device. The route switching time period is a time period during which the communication route between the transmitting device and the receiving device is switched from the first communication route to the second communication route. When the receiving device detects a loss of a first packet during the route switching time period, the receiving device prohibits the sending device from transmitting a retransmission request for the first packet for a first time period from the detection of the loss of the packet. If the receiving device does not receive the first packet within a first period of time, the receiving device sends a retransmission request for the first packet to the transmitting device.

[0013] The second aspect relates to a retransmission control method in a communication system in which communication is performed between a transmitting device and a receiving device based on TCP. The route switching time period is a time period during which the communication route between the transmitting device and the receiving device is switched from the first communication route to the second communication route. The retransmission control method includes a process of, when a packet loss of a first packet is detected in a receiving device during a route switching time period, prohibiting the receiving device from sending a retransmission request for the first packet to a transmitting device for a first time period from the detection of the packet loss. The retransmission control method includes a process of transmitting a retransmission request for the first packet from the receiving device to the transmitting device when the receiving device does not receive the first packet within a first period of time.

[0014] The third aspect relates to a receiving device that communicates with a sending device based on TCP. The receiving device includes a controller that performs retransmission control. The route switching time period is a time period during which the communication route between the transmitting device and the receiving device is switched from the first communication route to the second communication route. When the controller detects a loss of a first packet during the route switching time period, the controller prohibits the sending side device from transmitting a retransmission request for the first packet for a first time period from the detection of the loss of the packet. The controller sends a retransmission request for the first packet to the sending device if the controller does not receive the first packet within a first period of time.

[0015] The fourth aspect relates to a sending device that communicates with a receiving device based on TCP. The transmitting device includes a controller that performs retransmission control. The route switching time period is a time period during which the communication route between the transmitting device and the receiving device is switched from the first communication route to the second communication route. The controller sets a retransmission timeout longer than the default value for packets transmitted to the receiving device during the route switching period. Effect of the Invention

[0016] According to the present invention, even if a receiving device detects a packet loss during a route switching period, the receiving device does not immediately transmit a retransmission request to the transmitting device. Therefore, unnecessary packet retransmissions due to switching of communication routes are suppressed. This makes it possible to suppress a decrease in the throughput of the entire system. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a conceptual diagram for explaining retransmission control in TCP. [Diagram 2] FIG. 1 is a conceptual diagram for explaining retransmission control in TCP. [Diagram 3] FIG. 1 is a conceptual diagram for explaining a problem. [Figure 4] 1 is a block diagram illustrating a schematic configuration of a communication system according to an embodiment of the present invention. [Diagram 5] 1 is a schematic diagram illustrating an example of a communication system according to an embodiment of the present invention. [Figure 6] FIG. 2 is a schematic diagram illustrating another example of a communication system according to an embodiment of the present invention. [Figure 7] FIG. 11 is a schematic diagram illustrating yet another example of a communication system according to an embodiment of the present invention. [Figure 8] FIG. 2 is a conceptual diagram for explaining switching of a communication route in an embodiment of the present invention. [Figure 9] FIG. 2 is a schematic diagram for explaining an example of switching of a communication route in an embodiment of the present invention. [Figure 10]FIG. 11 is a schematic diagram for explaining another example of switching of a communication route in the embodiment of the present invention. [Figure 11] FIG. 11 is a schematic diagram for explaining still another example of switching of a communication route in the embodiment of the present invention. [Figure 12] FIG. 2 is a conceptual diagram for explaining an overview of retransmission control according to an embodiment of the present invention. [Figure 13] 5 is a flowchart showing retransmission control in a transmitting side device according to an embodiment of the present invention. [Figure 14] 5 is a flowchart showing retransmission control in a receiving side device according to an embodiment of the present invention. [Figure 15] FIG. 11 is a conceptual diagram for explaining a first modified example of retransmission control according to an embodiment of the present invention. [Figure 16] 13 is a flowchart showing a second modified example of retransmission control in a transmitting side device according to an embodiment of the present invention. [Figure 17] 13 is a flowchart showing a second modified example of retransmission control in a receiving side device according to an embodiment of the present invention. [Figure 18] 2 is a block diagram showing an example of the configuration of a transmitting device and a receiving device according to the embodiment of the present invention; [Figure 19] 2 is a block diagram showing an example of the configuration of each of controllers of a transmitting device and a receiving device according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] An embodiment of the present invention will be described with reference to the accompanying drawings.

[0019] 1.Communication Systems FIG. 4 is a block diagram showing a schematic configuration of a communication system 1 according to this embodiment. The communication system 1 includes a transmitting device TX, a receiving device RX, and a plurality of node stations ND. The node station ND relays communication. The transmitting device TX and the receiving device RX communicate with each other via at least one node station ND. When the node station ND changes, the communication route between the transmitting device TX and the receiving device RX also changes. Note that the communication in this embodiment also includes communication between the node stations ND. In other words, the transmitting device TX and the receiving device RX are concepts that also include the node station ND.

[0020] FIG. 5 is a schematic diagram showing an example of a communication system 1 according to the present embodiment. The communication system 1 includes a terminal station 10, a satellite ground station 20, and a plurality of satellite relay stations 30. Examples of the satellite relay stations 30 include geostationary satellites, medium-earth orbit satellites, low-earth orbit satellites, and high-altitude pseudo satellites. Each satellite relay station 30 corresponds to a node station ND. A satellite communication network is constructed by establishing links between the plurality of satellite relay stations 30. The terminal station 10-A and the terminal station 10-B perform wireless communication with each other via at least one node station ND (satellite relay station 30). The satellite ground station 20 performs wireless communication with the terminal station 10-A or the terminal station 10-B via at least one node station ND (satellite relay station 30). Communication may also be performed between the satellite relay stations 30. The transmitting device TX is any one of the terminal station 10, the satellite ground station 20, and the satellite relay station 30. The receiving device RX is any one of the terminal station 10, the satellite ground station 20, and the satellite relay station 30.

[0021] 6 is a schematic diagram showing another example of the communication system 1 according to the present embodiment. The communication system 1 includes a terminal station 10 and a plurality of cellular base stations 40. Each cellular base station 40 corresponds to a node station ND. The terminal station 10-A and the terminal station 10-B communicate with each other via at least one node station ND (the cellular base station 40). The transmitting device TX is either the terminal station 10 or the cellular base station 40. The receiving device RX is either the terminal station 10 or the cellular base station 40.

[0022] FIG. 7 is a schematic diagram showing yet another example of the communication system 1 according to the present embodiment. The communication system 1 includes a terminal station 10, a satellite earth station 20, a plurality of satellite relay stations 30, and a plurality of cellular base stations 40. Each of the satellite relay stations 30 and each of the cellular base stations 40 corresponds to a node station ND. The terminal station 10-A and the terminal station 10-B communicate with each other via at least one node station ND. The satellite earth station 20 communicates with the terminal station 10-A or the terminal station 10-B via at least one node station ND. The transmitting device TX is any one of the terminal station 10, the satellite earth station 20, the satellite relay station 30, and the cellular base station 40. The receiving device RX is any one of the terminal station 10, the satellite earth station 20, the satellite relay station 30, and the cellular base station 40.

[0023] The communication system 1 according to this embodiment performs communication based on TCP. That is, the transmitting device TX and the receiving device RX communicate with each other based on TCP. The transmitting device TX assigns a sequence number to a packet to be transmitted to the receiving device RX. Packet i represents a packet with sequence number i. Upon receiving packet i, the receiving device RX transmits an ACK (acknowledgement) to the transmitting device TX to notify the reception of packet i.

[0024] Furthermore, the receiving device RX can detect (recognize) missing packets (packet loss) by checking the sequence number of the received packet. When the receiving device RX detects that packet i has been lost, it sends a "retransmission request" to the transmitting device TX, requesting a retransmission of packet i. When the transmitting device TX receives a retransmission request for the same packet i three times, it retransmits packet i to the receiving device RX. Alternatively, when the transmitting device TX does not receive an ACK for packet i within the retransmission timeout after transmitting packet i, it retransmits packet i to the receiving device RX.

[0025] 2. Switching of communication routes FIG. 8 is a conceptual diagram for explaining switching of a communication route CR between a transmitting device TX and a receiving device RX. The communication route CR between a transmitting device TX and a receiving device RX switches from a first communication route CR-1 to a second communication route CR-2. The first communication route CR-1 before the switching passes through at least a node station ND-1. The second communication route CR-2 after the switching passes through at least a node station ND-2 different from the node station ND-1. The first communication route CR-1 and the second communication route CR-2 passing through different node stations ND may have different delays. For example, the delay in the second communication route CR-2 is smaller than the delay in the first communication route CR-1.

[0026] FIG. 9 is a schematic diagram for explaining an example of switching of the communication route CR in the communication system 1 as shown in FIG. 5. In the example shown in FIG. 9, the satellite ground station 20 is a transmitting device TX, and the terminal station 10 is a receiving device RX. The satellite relay station 30, which is a node station ND, includes a geostationary satellite 30-1 and low-earth orbit satellites 30-2A and 30-2B. The first communication route CR-1 passes through the geostationary satellite 30-1 and the low-earth orbit satellite 30-2B. On the other hand, the second communication route CR-2 passes only through the low-earth orbit satellites 30-2A and 30-2B without passing through the geostationary satellite 30-1. The geostationary satellite 30-1 is located at an altitude of 36,000 km above the equator, whereas the low-earth orbit satellite 30-2 is located at an altitude of 2,000 km or less. Therefore, the delay in the second communication route CR-2 is smaller than the delay in the first communication route CR-1.

[0027] FIG. 10 is a schematic diagram for explaining an example of switching of the communication route CR in the communication system 1 as shown in FIG. 6. In the example shown in FIG. 10, the terminal station 10-A is a transmitting device TX, and the terminal station 10-B is a receiving device RX. The cellular base station 40, which is a node station ND, includes a cellular base station 40-1 having a relatively large communication area 50-1 and a cellular base station 40-2 having a relatively small communication area 50-2. In the first communication route CR-1, the terminal station 10-A is directly connected to the distant cellular base station 40-1. On the other hand, in the second communication route CR-2, the terminal station 10-A is connected to the nearby cellular base station 40-2, and the cellular base stations 40-1 and 40-2 are connected by wire. Therefore, the delay in the second communication route CR-2 is smaller than the delay in the first communication route CR-1.

[0028] Fig. 11 is a schematic diagram for explaining an example of switching of a communication route CR in the communication system 1 as shown in Fig. 7. In the example shown in Fig. 11, the terminal station 10-A is a transmitting device TX, and the terminal station 10-B is a receiving device RX. The first communication route CR-1 passes through the satellite relay station 30. On the other hand, the second communication route CR-2 passes only through the cellular base station 40 without passing through the satellite relay station 30. Therefore, the delay in the second communication route CR-2 is smaller than the delay in the first communication route CR-1.

[0029] In this way, when the delay in the second communication route CR-2 is smaller than the delay in the first communication route CR-1, the problem shown in Fig. 3 above may occur. That is, unnecessary packet retransmission occurs when the communication route CR is switched, and the throughput of the entire system may decrease. Therefore, the retransmission control according to this embodiment is designed to suppress unnecessary packet retransmission caused by the switching of the communication route CR. The retransmission control according to this embodiment will be described in detail below.

[0030] 3. Retransmission Control 3-1. Overview 12 is a conceptual diagram for explaining an overview of retransmission control according to this embodiment. The communication route CR switches between the transmission timing of packet N and the transmission timing of packet N+1. The communication route CR when packet N is transmitted is the first communication route CR-1 with high latency. The communication route CR when packets N+1 and onward are transmitted is the second communication route CR-2 with low latency. Therefore, packet N arrives at the receiving device RX with a significant delay from packet N+1.

[0031] The "route switching time period PSW" is a time period during which the communication route CR between the transmitting device TX and the receiving device RX is switched. More specifically, the route switching time period PSW is a fixed period including the timing at which the communication route CR is switched (route switching timing). For example, when the node station ND includes a satellite relay station 30, the timing at which the communication route CR is switched can be predicted based on the satellite orbit information of the satellite relay station 30. In this case, the route switching time period PSW is a fixed period including the predicted route switching timing. As another example, the switching of the node station ND via which the communication route CR is passed, that is, the switching of the communication route CR can be detected based on the handover information. In this case, the route switching time period PSW is a fixed period including the detected route switching timing. The route switching time period PSW may be a fixed period after the detected route switching timing.

[0032] The receiving device RX detects the packet loss of packet N during the route switching time period PSW. However, even if the receiving device RX detects the packet loss, it does not immediately transmit a retransmission request for packet N. The receiving device RX waits for the arrival of packet N without transmitting a retransmission request until the "waiting time Tw" has elapsed since the detection of the packet loss. In other words, the receiving device RX prohibits the sending of a retransmission request for packet N to the sending device TX during the "waiting time Tw" from the detection of the packet loss. If the receiving device RX receives packet N during the waiting time Tw, it sends an ACK (acknowledgement) to the sending device TX to notify the reception of packet N. If the receiving device RX does not receive packet N even after the waiting time Tw has elapsed, it sends a retransmission request for packet N to the sending device TX for the first time.

[0033] In this way, according to this embodiment, when the receiving side device RX detects a packet loss of packet N in the route switching time period PSW, it prohibits the receiving side device RX from transmitting a retransmission request for packet N to the transmitting side device TX during the waiting time Tw from the detection of the packet loss. This prevents the receiving side device RX from transmitting a retransmission request unnecessarily until it receives packet N. This prevents the transmitting side device TX from receiving a retransmission request for packet N three times. As a result, the transmitting side device TX is prevented from retransmitting packet N unnecessarily. In other words, it is possible to prevent a decrease in the throughput of the entire system due to switching of the communication route CR.

[0034] The transmitting device TX may retransmit a packet based on the retransmission timeout RTO. Specifically, if an ACK for packet i is not received within the retransmission timeout RTO after transmitting packet i, the transmitting device TX retransmits packet i to the receiving device RX. In order to prevent unnecessary packet retransmission, the transmitting device TX may set a longer retransmission timeout RTO for a packet N transmitted in the route switching time period PSW. For example, the transmitting device TX sets the retransmission timeout RTO for a packet N transmitted in the route switching time period PSW as in the following formula (1).

[0035] Equation (1): RTO = RTOd + RTOa

[0036] RTOd is the default value of the retransmission timeout RTO. For packets sent during periods other than the route switching time period PSW, the retransmission timeout RTO is set to the default value RTOd. RTOa is the "additional time" added to a packet N sent during the route switching time period PSW.

[0037] In this way, the transmitting device TX may set the retransmission timeout RTO for the packet N transmitted to the receiving device RX during the route switching time period PSW to be longer than the default value RTOd by the additional time RTOa. This further prevents the transmitting device TX from unnecessarily retransmitting the packet N. This makes it possible to further prevent a decrease in the throughput of the entire system due to switching of the communication route CR.

[0038] In addition, the waiting time Tw in the receiving device RX and the additional time RTOa in the transmitting device TX may be a predetermined value (constant value) or may be a variable value. The waiting time Tw and the additional time RTOa may be the same or different. The waiting time Tw and the additional time RTOa may be set to the same value by the same method.

[0039] 3-2. Processing flow on the transmitting device 13 is a flowchart showing retransmission control in the transmitting device TX according to this embodiment. Here, packet i transmitted from the transmitting device TX to the receiving device RX is considered.

[0040] In step S11, the transmitting device TX judges whether the packet i was transmitted in the route switching time period PSW. As described above, the route switching time period PSW is a certain period including the timing (route switching timing) when the communication route CR is switched. For example, the transmitting device TX predicts the route switching timing based on the satellite orbit information of the satellite relay station 30, and grasps the route switching time period PSW based on the predicted route switching timing. The satellite orbit information is provided to the transmitting device TX from the satellite relay station 30 or the satellite management system. As another example, the transmitting device TX may detect the route switching timing based on handover information, and grasp the route switching time period PSW based on the detected route switching timing. The handover information is acquired by the transmitting device TX, or provided from another device.

[0041] If the packet i is not transmitted during the route switching time period PSW (step S11; No), the process proceeds to step S12. In step S12, the transmitting device TX sets the retransmission timeout RTO for the packet i to a default value RTOd (RTO=RTOd). Then, the process proceeds to step S14.

[0042] On the other hand, if the packet i was transmitted during the route switching time period PSW (step S11; Yes), the process proceeds to step S13. In step S13, the transmitting device TX sets the retransmission timeout RTO for the packet i to the sum of the default value RTOd and the additional time RTOa (RTO=RTOd+RTOa). After that, the process proceeds to step S14.

[0043] In step S14, the transmitting device TX judges whether or not an ACK for the packet i is received within the retransmission timeout RTO after transmitting the packet i. If the transmitting device TX receives an ACK for the packet i within the retransmission timeout RTO (step S14; Yes), the transmission process for the packet i is terminated.

[0044] On the other hand, if the transmitting device TX does not receive an ACK for packet i within the retransmission timeout RTO (step S14; No), the process proceeds to step S15. In step S15, the transmitting device TX retransmits packet i to the receiving device RX. After that, the process returns to step S11.

[0045] In step S11, it is not necessarily necessary to determine the magnitude of delay between the first communication route CR-1 and the second communication route CR-2. When the first communication route CR-1 has a high delay and the second communication route CR-2 has a low delay, the effect of suppressing unnecessary packet retransmission as described above can be obtained. On the other hand, when the first communication route CR-1 has a low delay and the second communication route CR-2 has a high delay, the packet arrival order is unlikely to be changed in the first place, and packets are unlikely to be dropped. Therefore, the transmitting device TX has a high probability of successfully receiving an ACK, and is unlikely to retransmit packets regardless of the setting of the retransmission timeout RTO.

[0046] 3-3. Processing flow on the receiving device FIG. 14 is a flowchart showing retransmission control in the receiving side device RX according to the present embodiment.

[0047] In step S21, the receiving device RX determines whether a packet i has been lost. If no packet loss is detected (step S21; No), the process in the current cycle ends. On the other hand, if a packet loss is detected (step S21; Yes), the process proceeds to step S22.

[0048] In step S22, the receiving device RX determines whether a packet loss occurred during the route switching time period PSW. The method of acquiring the route switching time period PSW is the same as that in step S11 by the transmitting device TX described above.

[0049] If the packet loss occurs outside the route switching time period PSW (step S22; No), the process proceeds to step S23. In step S23, the receiving device RX transmits a retransmission request for packet i to the transmitting device TX.

[0050] On the other hand, if a packet loss occurs during the route switching time period PSW (step S22; Yes), the process proceeds to step S24. In step S24, the receiving side device RX determines whether or not the missing packet i has been received.

[0051] If packet i has been received (step S24; Yes), the process proceeds to step S25. In step S25, the receiving device RX transmits an ACK to the transmitting device TX to notify the reception of packet i.

[0052] On the other hand, if packet i has not yet been received (step S24; No), the process proceeds to step S26. In step S26, the receiving device RX determines whether or not the waiting time Tw has elapsed since the detection of the packet loss. If the waiting time Tw has not elapsed (step S26; No), the process returns to step S24.

[0053] If the waiting time Tw has elapsed since the detection of the missing packet (step S26; Yes), the process proceeds to step S27. In step S27, the receiving device RX transmits a retransmission request for packet i to the transmitting device TX.

[0054] 3-4. First modified example In the first modification, the difference in propagation time between the first communication route CR-1 and the second communication route CR-2 is taken into consideration. The propagation time T1 along the first communication route CR-1 and the propagation time T2 along the second communication route CR-2 are expressed by the following formula (2).

[0055] Formula (2): T1=T L ×N L1 +T S ×N S1 +T TX1 +T RX1 T2=T L ×N L2 +T S ×N S2 +T TX2 +T RX2

[0056] T L is the propagation time in the link between nodes. N L1 is the propagation time T L N is the total number of links between the nodes. L2 is the propagation time T L is the total number of links between the nodes. S is the processing time at the node station ND. S1 is the processing time T S is the total number of nodes ND. S2 is the processing time T S is the total number of nodes ND in T TX1 is the propagation time in the link between the transmitting device TX and the node station ND of the first communication route CR-1. TX2 is the propagation time in the link between the transmitting device TX and the node station ND of the second communication route CR-2. RX1 is the propagation time in the link between the receiving device RX and the node station ND of the first communication route CR-1. RX2 is the propagation time in the link between the receiving device RX and the node station ND of the second communication route CR-2.

[0057] Fig. 15 shows an example of each propagation time. The propagation time T1 along the first communication route CR-1 is 230 ms (= 70 ms x 1 + 20 ms x 1 + 30 ms x 1 + 100 ms + 10 ms). The propagation time T2 along the second communication route CR-2 is 130 ms (= 50 ms x 1 + 30 ms x 2 + 10 ms + 10 ms). The difference between the propagation time T1 and the propagation time T2 is 100 ms. Each value may be a predetermined value (fixed value) or a measured value.

[0058] The waiting time Tw in the receiving device RX is determined based on the difference in propagation time between the first communication route CR-1 and the second communication route CR-2. For example, the waiting time Tw is set to the difference in propagation time. The waiting time Tw may be determined by the receiving device RX or by another device. In the latter case, the waiting time Tw determined by the other device is provided to the receiving device RX.

[0059] Similarly, the additional time RTOa in the transmitting device TX is determined based on the difference in propagation time between the first communication route CR-1 and the second communication route CR-2. For example, the additional time RTOa is set to the difference in propagation time. The additional time RTOa may be determined by the transmitting device TX or by another device. In the latter case, the additional time RTOa determined by the other device is provided to the transmitting device TX.

[0060] The waiting time Tw in the receiving device RX and the additional time RTOa in the transmitting device TX may be the same. In this case, the waiting time Tw and the additional time RTOa determined in any device are shared by each device that requires them.

[0061] According to the first modification, the waiting time Tw in the receiving device RX and the additional time RTOa in the transmitting device TX are set to appropriate values ​​that are neither too much nor too little, which makes it possible to efficiently suppress a decrease in the throughput of the entire system.

[0062] 3-5. Second modified example Fig. 16 is a flow chart showing a second modified example of retransmission control in the transmitting side device TX. Step S11 in the above-mentioned Fig. 13 is replaced with step S11A. The rest is the same as in Fig. 13.

[0063] In step S11A, the transmitting device TX judges whether the packet i is transmitted in a route switching time period PSW in which the delay of the communication route CR is small. Whether the delay of the communication route CR is small, that is, whether the delay of the second communication route CR-2 is smaller than the delay of the first communication route CR-1, can be judged based on the above-mentioned propagation times T1 and T2. If the packet i is transmitted in a route switching time period PSW in which the delay of the communication route CR is small (step S11A; Yes), the process proceeds to step S13. Otherwise (step S11A: No), the process proceeds to step S12.

[0064] Fig. 17 is a flowchart showing a second modified example of retransmission control in the receiving device RX. Step S22 in the above-mentioned Fig. 14 is replaced with step S22A. The rest is the same as in Fig. 14.

[0065] In step S22A, the receiving device RX determines whether or not the packet loss occurred in a route switching time period PSW in which the delay of the communication route CR is small. Whether or not the delay of the communication route CR is small, that is, whether or not the delay of the second communication route CR-2 is smaller than the delay of the first communication route CR-1, can be determined based on the above-mentioned propagation times T1 and T2. If the packet loss occurred in a route switching time period PSW in which the delay of the communication route CR is small (step S22A; Yes), the process proceeds to step S24. Otherwise (step S22A: No), the process proceeds to step S23.

[0066] 4. Example configurations of transmitting and receiving devices 18 is a block diagram showing an example of the configuration of each of the transmitting device TX and the receiving device RX according to this embodiment. Each of the transmitting device TX and the receiving device RX includes a controller 100, one or more modem units 200-1 to 200-k, one or more RF units 300-1 to 300-k, and one or more antenna units 400-1 to 400-k, where k is an integer of 1 or more.

[0067] The controller 100 outputs a transmission signal to the modem unit 200. The modem unit 200 modulates the transmission signal and outputs the modulated transmission signal to the RF unit 300. The RF unit 300 transmits the transmission signal via the antenna 400. The RF unit 300 also receives a signal via the antenna 400 and outputs the received signal to the modem unit 200. The modem unit 200 demodulates the received signal and outputs the demodulated received signal to the controller 100.

[0068] 19 is a block diagram showing an example of the configuration of the controller 100. The controller 100 includes an I / O interface 110, one or more processors 120 (hereinafter simply referred to as "processors 120"), and one or more storage devices 130 (hereinafter simply referred to as "storage devices 130").

[0069] The processor 120 performs various information processing. For example, the processor 120 includes a CPU (Central Processing Unit). The processor 120 performs communication control based on TCP. For example, the processor 120 performs the above-mentioned retransmission control.

[0070] The storage device 130 stores various types of information necessary for processing by the processor 120. Examples of the storage device 130 include a volatile memory, a non-volatile memory, a hard disk drive (HDD), and a solid state drive (SSD).

[0071] The control program 140 is a computer program executed by the processor 120. The functions of the controller 100 (processor 120) are realized by the processor 120 executing the control program 140. The control program 140 is stored in the storage device 130. The control program 140 may be recorded in a computer-readable recording medium. The control program 140 may be provided to the controller 100 via a network.

[0072] The satellite orbit information 150 is information relating to the orbit of each satellite relay station 30. The satellite orbit information 150 is provided from the satellite relay station 30 or the satellite management system. The processor 120 predicts the route switching timing based on the satellite orbit information 150 of the satellite relay station 30, and can grasp the route switching time zone PSW based on the predicted route switching timing.

[0073] The handover information 160 is information related to handover during communication. The handover information is acquired by the transmitting device TX or the receiving device RX. Alternatively, the handover information may be provided from a communication management system. The processor 120 can detect route switching timing based on the handover information and grasp the route switching time zone PSW based on the detected route switching timing.

[0074] The propagation / processing time information 170 is information indicating the propagation time and processing time as exemplified in FIG. 15. Each value may be a predetermined value (fixed value) or a measured value. For example, the propagation time is measured in each of the transmitting device TX, the receiving device RX, and the node station ND, and is shared by each device. The processing time in each node station ND is provided by each node station ND. The processor 120 can execute the processing related to the first modified example or the second modified example described above based on the propagation / processing time information 170.

[0075] The controller 100 may be realized using hardware such as an application specific integrated circuit (ASIC), a programmable logic device (PLD), or a field programmable gate array (FPGA). [Explanation of symbols]

[0076] 1. Communication Systems 10 Terminal Station 20 Satellite ground station 30 Satellite Relay Station 40 Cellular Base Station 100 Controller 110 I / O Interface 120 processors 130 Storage device 140 Control Program 150 Satellite orbit information 160 Handover Information 170 Propagation / Processing Time Information CR Communication Route CR-1 First Communication Route CR-2 Second Communication Route ND Node Station PSW route switching time RTO Retransmission Timeout RTOa additional time RTOd Default value RX Receiver TX sending device Tw Waiting time

Claims

1. A communication system that performs communication based on TCP (Transmission Control Protocol), A transmitting device; Receiving device and Equipped with a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The receiving device is when detecting a packet loss of a first packet during the route switching time period, prohibiting transmission of a retransmission request for the first packet to the transmitting device for a first time period from the detection of the packet loss; sending the retransmission request for the first packet to the transmitting device if the first packet is not received within the first time period; The transmitting device sets a retransmission timeout for a second packet transmitted to the receiving device during the route switching time period to a second time longer than a default value. Communication systems.

2. 2. The communication system according to claim 1, The second time is determined based on a difference in propagation time between the first communication route and the second communication route. Communication systems.

3. A communication system that communicates based on TCP (Transmission Control Protocol), A transmitting device; Receiving device and Equipped with a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The receiving device is when detecting a packet loss of a first packet during the route switching time period, prohibiting transmission of a retransmission request for the first packet to the transmitting device for a first time period from the detection of the packet loss; sending the retransmission request for the first packet to the transmitting device if the first packet is not received within the first time period; The first time is determined based on a difference in propagation time between the first communication route and the second communication route. Communication systems.

4. A communication system according to any one of claims 1 to 3, A delay in the second communication route is smaller than a delay in the first communication route. Communication systems.

5. A retransmission control method in a communication system in which communication is performed between a transmitting device and a receiving device based on TCP (Transmission Control Protocol), comprising: a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The retransmission control method includes: when a packet loss of a first packet is detected in the receiving device during the route switching time period, prohibiting the receiving device from transmitting a retransmission request for the first packet for a first time period from the detection of the packet loss; transmitting, if the receiving device does not receive the first packet during the first time period, a retransmission request for the first packet from the receiving device to the transmitting device; a process of setting a retransmission timeout for a second packet transmitted to the receiving device during the route switching time period in the transmitting device to be longer than a default value by a second time; Includes Retransmission control method.

6. A retransmission control method in a communication system in which communication is performed between a transmitting device and a receiving device based on TCP (Transmission Control Protocol), comprising: a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The retransmission control method includes: when a packet loss of a first packet is detected in the receiving device during the route switching time period, prohibiting the receiving device from transmitting a retransmission request for the first packet for a first time period from the detection of the packet loss; if the receiving device does not receive the first packet during the first time period, transmitting the retransmission request for the first packet from the receiving device to the transmitting device; Including, The first time is determined based on a difference in propagation time between the first communication route and the second communication route. Retransmission control method.

7. A receiving device that communicates with a transmitting device based on TCP (Transmission Control Protocol), A controller for performing retransmission control is provided, a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The controller: when detecting a packet loss of a first packet during the route switching time period, prohibiting transmission of a retransmission request for the first packet to the transmitting device for a first time period from the detection of the packet loss; sending the retransmission request for the first packet to the transmitting device if the first packet is not received within the first time period; The first time is determined based on a difference in propagation time between the first communication route and the second communication route. The receiving device.

8. A transmitting device that communicates with a receiving device based on TCP (Transmission Control Protocol), A controller for performing retransmission control is provided, a route switching time period is a time period during which a communication route between the transmitting device and the receiving device is switched from a first communication route to a second communication route; The controller sets a retransmission timeout longer than a default value for packets transmitted to the receiving device during the route switching time period. Sending device.

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