A routing update method based on multicast routing spanning tree reconstruction

Abstract

The invention discloses a routing update method based on multicast routing generation tree reconstruction, belonging to the field of satellite communication technology. The method includes periodically transferring the multicast tree based on the law of the satellite network, transferring the local multicast forwarding table of the satellite to the neighboring satellite that is about to move to the current position in advance, realizing the overall movement of the multicast tree, avoiding the long multicast tree reconstruction process after the satellite switching; using the root node backup of the multicast tree to realize the update of the root node, achieving the effect of simultaneous root node address switching and transfer process, so that the subsequent multicast process is carried out normally; and for the multicast member terminal with uncertain mobility, a method of establishing a tunnel from the initial access satellite to the terminal is adopted to keep the normal reception of its multicast data, when the fast-moving terminal successfully joins the group under the new access satellite and generates a new optimal path, then suppressing the data forwarding of the tunnel, thereby reducing the multicast interruption time.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite communications, and in particular relates to a routing update method based on multicast routing spanning tree reconstruction. Background Art

[0002] The low earth orbit (LEO) satellite constellation network will be an important part of the next generation of communication infrastructure. The wide coverage and wireless broadcast characteristics make the satellite network very suitable for forwarding multicast services. Compared with the high earth orbit (GEO) and medium earth orbit (MEO) satellite networks, the low earth orbit (LEO) satellite network has a very small information transmission delay and can support real-time multimedia services based on IP multicast such as video conferencing.

[0003] In low-orbit satellite network scenarios, there are fewer network resources on the satellite, and there are certain constraints on local storage capacity and available bandwidth. In addition, satellite movement can cause frequent switching of satellite-to-ground links and instability of inter-satellite links. It is necessary to minimize the frequent reconstruction of multicast trees and multicast communication interruptions caused by satellite-to-ground connection switching, and to pre-adjust the affected multicast paths before the satellite enters the polar region to ensure the normal forwarding of multicast data.

[0004] Existing mobile multicast technologies include remote joining method, bidirectional tunneling method, etc. Among them, the remote joining method is to exit the multicast group under the original access multicast router and rejoin the multicast group of the new multicast router to achieve multicast tree path reconstruction after switching the multicast router, but this method has the problem of long multicast communication interruption time and serious packet loss; the bidirectional tunneling method is to maintain the transmission of multicast data with the original multicast router through a unicast tunnel, but this method will cause tunnel aggregation and non-optimal path problems. For the case where the link is disconnected due to satellite switching, a periodic fault detection method is usually used. If the period is short, more resources will be consumed. If the period is long, the link cannot be quickly restored to normal, resulting in a long-term multicast communication interruption. Therefore, in order to ensure that the multicast communication in the satellite network can be as reliable and stable as possible, the main problem is how to achieve rapid convergence of multicast interruptions in the satellite mobile multicast environment based on the periodicity and regularity of satellite movement. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a routing update method based on multicast routing spanning tree reconstruction, which can be mainly used in low-orbit satellite networks to ensure that multicast communication in satellite network scenarios can adapt to the mobility of satellite access networks and quickly update multicast paths according to network conditions. Based on the regularity and predictability of satellite movement, the multicast forwarding table corresponding to the best multicast tree generated by the multicast routing algorithm is transferred to the satellite that is about to move to the current position in advance, thereby avoiding communication interruption caused by long-term reconstruction of the multicast tree; other satellites on the satellite orbit where the root node is located are used as backup root nodes, and backup root node collection messages are sent from the root node to the opposite direction of the satellite movement direction, and all interface addresses of each backup root node are stored in sequence for root node switching; if the terminal as a multicast member has mobility, a method of using a tunnel to keep receiving multicast data from the original access satellite and sending a join message to the new access satellite at the same time is used to shield its mobility until the same multicast data is received from the new access satellite, and the original access satellite is suppressed from forwarding redundant data for multicast group members.

[0006] The present invention provides an efficient multicast tree reconstruction method for large-scale satellite network scenarios, and the specific method includes:

[0007] Based on the movement law of the satellite constellation, the multicast forwarding table corresponding to the best multicast tree generated by the multicast routing algorithm is transferred in advance to the satellite that is about to move to the current position;

[0008] The multicast tree root node backup strategy is adopted to use other satellites in the satellite orbit where the root node is located as backup root nodes. By sending backup root node collection messages from the initial root node to the opposite direction of the satellite movement direction, all interface addresses of each backup root node are stored in sequence for root node switching, and the multicast tree path for the terminal to receive multicast data is updated;

[0009] It is determined whether the stability factor of the terminal is less than a preset threshold. If it is not less than the preset threshold, the terminal is accessed by using the IGMP / MLD protocol, and its mobility is shielded by receiving multicast data from the original access satellite and sending a join message to the new access satellite at the same time. If it is less than the preset threshold, a tunnel is established from the initial access satellite to the terminal to keep the terminal's multicast data normally received. When the terminal successfully joins the multicast group under the new access satellite and generates a new multicast tree path, the multicast data forwarding of the tunnel is suppressed.

[0010] Beneficial effects of the present invention:

[0011] The present invention performs periodic handover of multicast tree paths and periodic switching of root nodes according to the regularity and periodicity of large-scale satellite movement, which can avoid long-term multicast tree reconstruction and multicast communication interruption caused by satellite movement, greatly reduce the convergence time of multicast communication recovery, and save the resource consumption of on-board routing calculation to a certain extent, and maintain the optimal multicast tree constructed for the first time, thereby improving the quality of satellite network multicast communication. The present invention shields the mobility of mobile terminals by distinguishing between fixed terminals and fast-moving terminals, using a tunnel to keep receiving multicast data from the original access satellite and simultaneously sending a join message to the new access satellite, maintaining the normal forwarding of multicast data and the optimization of the path, and better providing stable multicast communication services for mobile terminals. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A schematic diagram of a large-scale satellite multicast communication scenario according to an embodiment of the present invention;

[0013] Figure 2 A routing update method based on multicast routing spanning tree reconstruction according to an embodiment of the present invention;

[0014] Figure 3 A schematic diagram of a multicast tree handover message format according to an embodiment of the present invention;

[0015] Figure 4 A schematic diagram of a multicast tree forwarding table message sending direction based on a satellite movement direction in an embodiment of the present invention;

[0016] Figure 5 A schematic diagram of a method for sequentially collecting backup root node addresses and a collection message format in an embodiment of the present invention;

[0017] Figure 6 A schematic diagram of a path updating method used for a fast-moving terminal in an embodiment of the present invention; DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] Figure 1 : is a large-scale satellite multicast communication scenario diagram of an embodiment of the present invention, and the specific application scenario is not limited thereto; the low-orbit satellite network of the embodiment of the present invention includes multiple orbital planes, each orbital plane contains multiple satellites, each satellite establishes a connection with adjacent satellites in the same orbit as itself and adjacent satellites between orbits, and the yth satellite located on the xth orbit is denoted as Sx,y . The satellites in the first orbit and the last orbit are moving in the opposite direction to the satellites in an adjacent orbit, so it is impossible to establish a stable connection, that is, each satellite contains only three links. In addition, each satellite contains four links. When the satellite enters the polar region, the relative movement speed of the satellites between orbits is large, and a stable connection cannot be established. Therefore, the link will remain disconnected before the satellite leaves the polar region. Taking the satellite network system as the access network, the multicast tree path is constructed between satellites through the multicast routing protocol. Each satellite provides multicast services based on the IGMP protocol. Terminals on the ground or in the air can act as multicast sources to send multicast data to a specific group address, or request to receive multicast data from a specific group based on the IGMP protocol.

[0020] In the embodiment of the present invention, considering that the terminal as a multicast member may be mobile, that is, there is uncertainty in the switching of the satellite-ground connection relationship, the terminal is divided into a fixed terminal and a mobile terminal. For the fixed terminal, the multicast path for receiving multicast data is updated by using the multicast tree handover and backup root node switching methods. For the mobile terminal, not only the above two methods for the fixed terminal need to be adopted, but also the method of using a tunnel to keep receiving multicast data from the original access satellite and simultaneously sending a join message to the new access satellite needs to be adopted to shield its mobility.

[0021] Figure 2 A routing update method based on multicast routing spanning tree reconstruction according to an embodiment of the present invention is as follows: Figure 2 As shown, the method includes:

[0022] 101. Based on the movement law of the satellite constellation, the multicast forwarding table corresponding to the best multicast tree generated by the multicast routing algorithm is forwarded in advance to the satellite that is about to move to the current position;

[0023] In the embodiment of the present invention, it is necessary to first obtain a virtual topology map of the satellite network, and the construction process thereof is as follows:

[0024] First, the satellite topology under the large-scale structure is simplified into a Manhattan block network, and corresponding virtual numbers are set for the satellites according to their orbits and positions in the orbits in the topological structure. The relationship between satellites can be distinguished based on the numbers, including the relationship between adjacent satellites in the orbit and between adjacent satellites between orbits.

[0025] Secondly, according to the law of satellite operation, the dynamic topology of the satellite network is discretized using virtual topology, divided into time segments, and the corresponding satellite network link state diagram is uploaded. When the satellite enters and exits the polar region, in order to enable the satellite to forward normally according to the multicast tree, it will continuously receive topological snapshots of the satellite network from the gateway during operation, thereby triggering temporary path updates. The n moments of topological changes of the satellite network are defined as a set T = (t0, t1, t2, ..., tn ), t0 to t n Each value is a time point of a topology change, and the segmentation time interval [t i , t i+1 ] represents the time point of a certain inter-orbit link change in the satellite network, so the n corresponding snapshot slices are S = (s0, s1, s2, ..., s n );

[0026] Again, since multiple satellite links may be disconnected and restored simultaneously when the satellite enters and leaves the polar region, this period of time is discarded and s i+1 =s i+2 ; Then the satellite network topology is added to the adjacency matrix used for multicast routing calculation in advance until the satellite enters and exits the polar region to trigger the multicast routing update. Because the transition time slice s i+1 The generation of i With s i+1 There is a switching interval. In order to ensure the stability of the switching, the transition time slice s needs to be reset. i+1 The moment of change;

[0027] Finally, the maximum delay of the satellite network is calculated as follows: Assuming s i+1 The original transformation time is t c The time interval that needs to be adjusted is Δτ (Δτ<<t c ), if the data packet is at t c -Δτ, and after Δτ, it is exactly at t c The time is to reach the destination, then at this time t c If the time just passes through the link that has been disconnected, the data packet will be lost. The advance time of the snapshot can be obtained based on the maximum delay of the network at this time. Suppose the number of satellites on the same orbit plane is A, and the number of satellites in the same orbit is B. At this time, since the satellites in the same orbit will enter the north and south poles of the earth respectively, the maximum average hop number is: J max =(A+B) / 3; at the same time, the transmission time of the satellite link is T t , the satellite processing delay is T p , the maximum delay of the satellite network is T max =∑(T t +T p )*J max ; At the same time, time slice s i+1 To s i+2 There will also be a certain switching interval, but due to s i+2 The topological state has been i+1 By recording it in advance, some errors caused by switching intervals can be avoided.

[0028] In the specific implementation, the dynamic topology of the satellite network is discretized by using virtual topology, and the satellite motion cycle T is divided into several time slices [t0, t1], [t1, t2], [t2, t3], ..., [t n-1 , t n ], assuming that the time it takes for a satellite to move in its orbit is t cycle , the number of satellites in one orbit is n, and the size of each time slice is t s =t cycle / n. When the multicast tree starts to be established, the periodic exchange logic of the multicast forwarding table is triggered. After each time slice, each satellite encapsulates the locally stored multicast forwarding table in a multicast path forwarding message and sends it to the neighboring satellites in the specified direction.

[0029] Specifically, based on the Manhattan block network model, a coordinate number is set for each satellite, where the x coordinate is the orbit number and the y coordinate is the satellite number in the orbit. If the number of satellite network orbits is A and the number of satellites in each orbit is B, then the first and second parameters of the satellite number are respectively from the set Set A =(1, 2, ..., A), Set B =(1, 2, ..., B), where the first parameter represents the satellite orbit in which the satellite is located, and the second parameter represents the satellite number in the orbit in which the satellite is located, such as S 2，3 represents the satellite No. 3 in the second orbit. Then, according to the numbering parameters, we can know the adjacent relationship between different satellites, including adjacent satellites between orbits and adjacent satellites within orbits. And in a large-scale satellite network, the Manhattan distance between all adjacent satellites is basically the same. a1，b1 and S a2，b2 The Manhattan distance d between two satellites is calculated as follows:

[0030] d=|a1-a2|+|b1-b2|

[0031] That is, the path weights are equal, and the virtual topology is a square structure.

[0032] Based on the above analysis, each satellite can obtain the status of the local satellite adjacent link according to the numbering parameters and the locally maintained neighbor satellite status table. When generating a multicast tree path based on the multicast routing protocol, the path-related information is generated by generating a multicast forwarding table and stored locally, and the root node mapping table is flooded from the root node to all satellites, which together constitute the complete structure of the multicast tree to ensure the normal operation of satellite multicast.

[0033] Based on the above analysis, according to the law of satellite operation, the dynamic topology of the satellite network can be discretized by using virtual topology, and the satellite operation process can be divided into time slices; divided into multiple snapshot slices according to the time interval of the slices; the time period of the satellite entering and leaving the polar region is discarded, and the advance time of the snapshot slice is calculated according to the maximum delay of the network, which is the advance forwarding time of the multicast forwarding table corresponding to the optimal multicast tree, where the advance forwarding time is T max .

[0034] 102. Using a root node backup strategy, other satellites in the satellite orbit where the root node is located are used as backup root nodes. By sending a backup root node collection message from the initial root node to the opposite direction of the satellite movement direction, all interface addresses of each backup root node are stored in sequence for switching the root node, and the multicast tree path for the terminal to receive multicast data is updated;

[0035] In the embodiment of the present invention, in order to solve the problem that satellite movement causes frequent switching of satellite-to-ground links, thereby causing long-term interruption of multicast communication. Based on the periodicity and regularity of satellite movement, the present invention divides the period of satellite movement into time periods, and triggers the transfer of multicast forwarding tables of all global satellites when the next time period is about to be switched after each time period. The local multicast forwarding table is encapsulated in a multicast routing message and transmitted in the form of a message. The message format is as follows: Figure 3 As shown, excluding the multicast routing protocol header, the remaining data portion is the table entry of the multicast forwarding table, including the multicast addresses marking different groups, the multicast source address, the root node address, and the interface number.

[0036] The direction of multicast tree handover is determined by the direction of satellite movement in the orbit. If the satellite moves clockwise in the orbit, the handover direction is counterclockwise. The handover message sending interface can be determined by the satellite number. All satellites in the network scenario have the same sending direction, such as Figure 4 As shown. When the neighbor satellite in the specified direction receives the message, it decapsulates it and takes out all the entries in the message. If the neighbor satellite does not have a multicast forwarding table locally, it directly uses all the taken out entries as the entries of the local multicast forwarding table, otherwise it replaces all of them. After all satellites replace the local multicast table, the overall transfer of the multicast tree can be realized, and its multicast tree structure remains relatively unchanged.

[0037] The multicast tree root node backup strategy includes using other satellites on the satellite orbit where the initially selected root node is located as backup root nodes. Before the first multicast tree is constructed, a backup root node collection message is sent from the initial root node in the opposite direction of the satellite movement direction in the orbit, and all interface addresses of each backup root node are collected in sequence until the message is resent to the root node.

[0038] In the above embodiment, based on the system time period switching cycle, when the next satellite moves to the current position causing the access relationship between the satellite and the terminal to change, that is, when entering the next time period, the locally stored multicast forwarding table is forwarded to the next satellite that moves to the current position, and the same is true for all satellites in the low-orbit satellite network, thereby realizing the overall backward transfer of the multicast tree path at the same time.

[0039] It can be understood that in the present invention, if the root node is not limited to a backup root node, it means that the root node is an existing root node in the multicast tree path, and the backup root node serves as a backup node for the existing root node; and the initial root node is an existing root node in the multicast tree path that has not been updated.

[0040] While exchanging the forwarding table, the root node is switched using the active / standby switching method, thereby updating the root node of the multicast tree. The method is as follows:

[0041] 1) Before the multicast tree is established, when the only root node is determined, the root node triggers the process of collecting the interface addresses of all satellites in the orbit. The root node sends a backup root node collection message in the opposite direction of the satellite's movement. When the non-root node receives the backup root node collection message, it adds the node address to the backup root node collection message and continues to forward it in the same direction until the backup root node collects and sends it to the root node, indicating that all interface addresses of all backup root nodes in the orbit have been collected;

[0042] 2) Then the root node mapping table and the backup root node table are flooded to all satellites in the satellite network. Then when the time slice is switched, that is, when the multicast tree is handed over, the root node mapping table stored locally in each satellite is triggered to update and switch to the address corresponding to the next backup root node. This ensures the normal operation of subsequent multicast communication.

[0043] In order to allow new multicast members to join and old multicast members to leave the process normally, after all paths of the multicast tree are transferred, the root node of the multicast tree needs to be switched. Therefore, the method of switching the primary and backup root nodes is adopted. Before building the multicast tree for the first time, the satellite interface addresses of other satellites on the same orbit as the selected root node are collected. The backup root node collection message is sent from the primary root node in the opposite direction of the satellite movement. The specific method is as follows: Figure 5 As shown, each time a satellite is passed, the addresses of all its interfaces are filled in until the message is sent back to the initial root node again, at which point the collection of information of all backup root nodes is completed.

[0044] When the root node sends the multicast root node mapping table to all satellites, it also sends the backup root node address mapping table together with the root node mapping table. Then each satellite can know all the addresses of the primary and backup root nodes, and periodically switch the currently used root node mapping table to the content in a backup root node mapping table entry, thereby realizing the switching of the primary and backup root nodes.

[0045] The handover process includes three phases: network layer handover process such as mobility detection, care-of address configuration and duplicate address detection, and binding update. Link layer handover delay refers to the time it takes for a mobile terminal to leave the wireless access point of the current network and establish a link layer connection with the wireless access point in the new network, expressed as T. L The network layer needs to check the received router advertisements to determine whether the node has switched to the new network layer. Compared with the link layer, the switching delay of the network layer is much greater. The time required for the mobile terminal to detect that it has switched to the new network is T D After completing the mobile detection, the mobile terminal confirms that it has moved to the new subnet. It needs to configure a new care-of address and perform duplicate address detection to ensure the uniqueness of the address. This period of time is the care-of address configuration delay, which can be expressed as T CoA The binding update delay refers to the time it takes for a mobile terminal to send a binding update message to the original satellite after switching to a new satellite, register its new care-of address, and wait for the original satellite to return a binding confirmation message. The binding update delay can be expressed as T BU The total handover delay of the mobile terminal during the handover process includes the sum of the link layer and network layer handover delays, that is:

[0046] T=T L +T D +T CoA +T BU

[0047] When the terminal moves quickly, the access relationship between the terminal and the satellite may change frequently and uncertainly, that is, the link is unstable. Therefore, based on the periodic exchange forwarding table, the tunnel and the original access satellite data forwarding suppression method are used for the mobile terminal to reduce the delay T of the care-of address configuration. CoA and the binding update delay T BU , thereby reducing the switching delay.

[0048] 103. Determine whether the stability factor of the terminal is less than a preset threshold. If it is not less than the preset threshold, access the terminal using the IGMP / MLD protocol, receive multicast data from the original access satellite and simultaneously send a join message to the new access satellite to shield its mobility. If it is less than the preset threshold, establish a tunnel from the initial access satellite to the terminal to maintain normal reception of multicast data by the terminal. When the terminal successfully joins the multicast group under the new access satellite and generates a new multicast tree path, suppress the multicast data forwarding of the tunnel.

[0049] The access relationship between the mobile terminal and the satellite may have uncertain and frequent changes. The stability of the link is determined by calculating the stability factor fac. The value of this factor is between 0 and 1. The larger the value, the higher the stability of the link. When the stability factor is the maximum value, the relative speed between the nodes is 0. The specific method flow is as follows:

[0050] 1) First, calculate the stability factor fac. The calculation method of the stability factor is: where s is the relative speed between the satellite and the terminal, fac′ is the link stability factor between the two nodes after the last hello neighbor detection message is received and before the current hello neighbor detection message is received, and n is the number of hello messages received from the neighbor in a hello message sending cycle. Both fac′ and n are reset to 0 at the end of the hello message cycle. w is the calculation weight of fac, that is,

[0051] 2) Then, when the calculated stability factor is less than 0.5, a method is adopted to reduce the switching delay by using a tunnel to keep receiving multicast data from the original access satellite and simultaneously sending a join message to the new access satellite, wherein the join message is a message in a computer network group member management protocol. After receiving the report message, the new access satellite executes a new multicast tree branch path construction process. After receiving the report message through the tunnel, the original access satellite directly forwards the multicast data to the mobile terminal through the tunnel.

[0052] 3) Finally, after the mobile terminal receives the multicast data sent from the original access satellite, it detects whether it has received the multicast data directly from the new access satellite. Once the mobile terminal receives the multicast data from the new access satellite, it sends a custom suppression message to the original access satellite to stop data forwarding but maintain the multicast state so that the mobile terminal can quickly resume multicast communication when it returns to the original location.

[0053] The stability factor fac calculation formula obtains that when the link stability factor value between a terminal and its access satellite is less than the preset threshold of 0.5, that is, when the satellite-to-ground link is unstable, a tunnel needs to be established between the terminal and the access satellite, and a method of sending a double joining report message and suppressing forwarding of the original access satellite is adopted. However, fixed terminals are still based on periodic exchange forwarding tables, and there is no need to add this method.

[0054] The specific implementation steps are as follows: Figure 6 As shown: the mobile terminal sends a group member report message to the newly accessed satellite and the original tunnel at the same time. After receiving the report message, the newly accessed satellite executes a new multicast tree branch path construction process. After the original accessed satellite receives the report message through the tunnel, it directly forwards the multicast data to the mobile terminal through the tunnel. After the mobile terminal receives the multicast data sent from the original accessed satellite, it detects whether it has received the multicast data directly from the newly accessed satellite. Once the mobile terminal receives the multicast data from the newly accessed satellite, it sends a custom suppression type message to the original accessed satellite to stop forwarding data but maintain the multicast state. The terminal receives the multicast data based on the newly generated optimal path, that is, the end-to-end delay is shorter.

[0055] A person skilled in the art can understand that all or part of the steps in the various methods of the above embodiments can be completed by instructing the relevant hardware through a program, and the program can be stored in a computer-readable storage medium, which can include: ROM, RAM, disk or CD, etc.

[0056] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A routing update method based on multicast routing spanning tree reconstruction, characterized in that: The method comprises: Based on the movement law of the satellite constellation, the multicast forwarding table corresponding to the best multicast tree generated by the multicast routing algorithm is transferred in advance to the satellite that is about to move to the current position; The multicast tree root node backup strategy is adopted to use other satellites in the satellite orbit where the root node is located as backup root nodes. By sending backup root node collection messages from the initial root node to the opposite direction of the satellite movement direction, all interface addresses of each backup root node are stored in sequence for root node switching, and the multicast tree path for the terminal to receive multicast data is updated; Determine whether the stability factor of the terminal is less than a preset threshold value. If it is not less than the preset threshold value, use the IGMP / MLD protocol to access the terminal, receive multicast data from the original access satellite and send a join message to the new access satellite at the same time to shield its mobility; if it is less than the preset threshold value, establish a tunnel from the initial access satellite to the terminal to keep the terminal receiving multicast data normally. When the terminal successfully joins the multicast group under the new access satellite and generates a new multicast tree path, suppress the multicast data forwarding of the tunnel; Among them, the calculation formula of the terminal stability factor is expressed as: in, is the stability factor of the current access terminal, s is the relative speed between the satellite and the terminal, is the link stability factor between the two nodes after the last hello neighbor detection message is received and before the current hello neighbor detection message is received. n is the number of hello neighbor detection messages received from the neighbor in one hello neighbor detection message sending cycle. Both n and are reset to 0 at the end of the hello neighbor detection message cycle, and w is the calculation weight of fac.

2. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The calculation method of the advance forwarding time of the multicast forwarding table corresponding to the best multicast tree includes: According to the law of satellite operation, virtual topology is used to discretize the dynamic topology of the satellite network. The satellite operation process is divided into time slices and then divided into multiple snapshot slices according to the time interval. The time period when the satellite enters and leaves the polar region is discarded, and the advance time of the snapshot slice is calculated according to the maximum delay of the network, which is the advance forwarding time of the multicast forwarding table corresponding to the optimal multicast tree.

3. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The forwarding process of the multicast forwarding table corresponding to the optimal multicast tree includes triggering the periodic exchange logic of the multicast forwarding table when the multicast tree begins to be established. After each time slice, each satellite encapsulates the locally stored multicast forwarding table in a multicast path forwarding message and sends it to the neighboring satellite in the specified direction.

4. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The multicast tree root node backup strategy includes using other satellites on the satellite orbit where the initially selected root node is located as backup root nodes. Before the first multicast tree is constructed, a backup root node collection message is sent from the initial root node in the opposite direction of the satellite movement direction in the orbit, and all interface addresses of each backup root node are collected in sequence until the message is resent to the root node.

5. A routing update method based on multicast routing spanning tree reconstruction according to claim 4, characterized in that: The process of collecting backup root node messages includes sending a backup root node collection message from the root node in the opposite direction of the satellite's orbital movement when the unique root node is determined before the multicast tree begins to be established; when the non-root node receives the backup root node collection message, it adds the address of the non-root node to the backup root node collection message and continues to forward it in the same direction until the backup root node collection message is sent to the root node, and all interface addresses of all backup root nodes in the orbit are collected.

6. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The updating terminal receives the multicast tree path of multicast data by flooding the root node mapping table and the backup root node table to all satellites in the satellite network; when the multicast tree is handed over, the root node mapping table stored locally on each satellite is simultaneously triggered to update and switch to the address corresponding to the next backup root node.

7. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The establishing of a tunnel from the initial access satellite to the terminal to maintain normal reception of multicast data by the terminal includes, when the calculated stability factor is less than a preset threshold, using the tunnel to maintain receiving multicast data from the original access satellite, and simultaneously sending a join message to the new access satellite to reduce its switching delay; After receiving the report message, the newly accessed satellite executes a new multicast tree branch path construction process; After the original access satellite receives the report message through the tunnel, it directly forwards the multicast data to the terminal through the tunnel.

8. A routing update method based on multicast routing spanning tree reconstruction according to claim 1, characterized in that: The suppression of multicast data forwarding in the tunnel includes detecting whether multicast data is received directly from the new access satellite after the terminal receives multicast data sent from the original access satellite; once the terminal receives multicast data from the new access satellite, a custom suppression message is sent to the original access satellite to stop data forwarding but maintain the multicast state so that the terminal can quickly resume multicast communication when it returns to the original position.

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