Patent Publication Number: US-11025523-B2

Title: Path detection

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase of International Patent Application Serial No. PCT/CN2018/081569 entitled “PATH DETECTION,” filed on Apr. 2, 2018. International Patent Application Serial No. PCT/CN2018/081569 claims priority to Chinese Patent Application No. 201710210833.4 filed on Mar. 31, 2017. The entire contents of each of the above-cited applications are hereby incorporated by reference in their entirety for all purposes. 
     BACKGROUND 
     To implement operation and maintenance of an Internet Protocol (IP) network, a path of a service flow through the network needs to be detected. 
     A traceroute mechanism can be used for path detection. In the traceroute mechanism, a detection packet is sent based on an IP address of a destination host (abbreviated as a destination IP address), so as to detect the path of the service flow from a source host to the destination host. 
     The detection packet in the traceroute mechanism is different from a service flow packet. For example, a port number configured for the detection packet may be different from that for the service flow packet, so as to avoid effect of the detection packet on the service flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating a method based on an example of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating packet structures based on an example of the present disclosure. 
         FIG. 3  is a schematic diagram illustrating an example of the present disclosure. 
         FIGS. 4-9  are schematic diagrams illustrating various structures of a detection packet based on respective examples of the present disclosure. 
         FIG. 10  is a schematic block diagram illustrating path detection logic based on an example of the present disclosure. 
         FIG. 11  is a schematic diagram illustrating a hardware structure of a forwarding device based on an example of the present disclosure. 
         FIG. 12  is a schematic block diagram illustrating path detection logic based on another example of the present disclosure. 
         FIG. 13  is a schematic diagram illustrating a hardware structure of a software defined network (SDN) controller based on an example of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following description is merely illustrative of some embodiments of the present disclosure but is not intended to limit the disclosure, and any modifications, equivalent substitutions or adaptations thereto within the spirit and scope of the disclosure shall be encompassed in the appended claims. 
     In the traceroute mechanism, forwarding information in a header of a detection packet for detecting a path of a service flow is different from forwarding information in a header of the service flow. Taking a user datagram protocol (UDP) port number as an example of the forwarding information, a normal UDP port number may correspond to a certain pre-defined application. In the traceroute mechanism, to avoid potential effect of the detection packet on an actual service application, a UDP port number in the header of the detection packet may be a specific UDP port number (e.g., a UDP port number greater than 30000), which dose not correspond to any service application and is different from any UDP port number for a corresponding service flow application in a header of the service flow. However, whether the detection packet or the service flow is to be forwarded, the forwarding path is selected based on the forwarding information in the header when there is an equal-cost path or a policy-based route (PRB). Since the forwarding information in the header of the detection packet is different from the forwarding information in the header of the service flow, the path finally detected based on the detection packet is different from an actual path of the service flow. 
     In examples of the present disclosure, the detection packet for detecting the path of the service flow is constructed by simulating the header of the actual service flow. Thus, when each forwarding device forwards the detection packet based on the forwarding information in the header of the detection packet, it can be ensured that forwarding behavior corresponding to the detection packet is consistent with that corresponding to the actual service flow, so that the path of the actual service flow could be precisely determined. 
       FIG. 1  is a flowchart illustrating a method based on an example of the present disclosure. The method can be applied to a forwarding device. In an example, the forwarding device may be a router or the like, which is not limited in examples of the present disclosure. 
     As shown in  FIG. 1 , the method may include procedures as follows. 
     At block  101 , a first forwarding device receives a first detection packet sent from a previous hop forwarding device on a path of a designated service flow. 
     In an example, the first forwarding device is any non-initial forwarding device on the path of the designated service flow. 
     In an example, when the previous hop forwarding device is an initial forwarding device on the path of the designated service flow, the first detection packet is a detection packet for detecting the path of the designated service flow, which is constructed by the initial forwarding device based on a control command from a SDN controller. When the initial forwarding device forwards the constructed detection packet, its device address is carried in a payload of the detection packet. 
     Forwarding information in the header of the designated service flow is carried in the control command, such that forwarding information in the header of the constructed detection packet is the same as the forwarding information in the header of the designated service flow. The SDN controller may determine the initial forwarding device on the path of the service flow as follows: calculating each path from a source IP address of the service flow to a destination IP address of the service flow based on the source IP address, the destination IP address and network topology managed by the SDN controller; sampling a service flow for an initial forwarding device on each path; and determine an initial forwarding device, through which a service flow has passed, as the initial forwarding device on the path of the service flow. 
     In another example, when the previous hop forwarding device is a non-initial forwarding device on the path of the designated service flow, the previous hop forwarding device constructs no detection packet but forwards the received detection packet to the first forwarding device. The detection packet forwarded to the first forwarding device is the first detection packet. 
     The forwarding information in the header of the first detection packet is the same as the forwarding information in the header of the designated service flow. The forwarding information in the first detection packet will be described below by way of example. 
     At block  102 , the first forwarding device sends to the SDN controller a forwarding error message in which all device addresses in a payload of the first detection packet are carried. 
     In an example, the header of the first detection packet includes a Time To Live (TTL) field, where the TTL field is set to an initial value. 
     With regard to the TTL field in the header of the first detection packet, the process at block  102  where the first forwarding device sends the forwarding error message to the SDN controller may comprise procedures as follows. 
     The TTL field in the header of the first detection packet is updated. When the updated TTL field is equal to zero, it may be confirmed that an error occurs in the forwarding of the first detection packet, and the forwarding error message is sent to the SDN controller. 
     In an example, the updating of the TTL field in the header of the first detection packet may comprise subtracting a preset value from the TTL field in the header of the first detection packet. The preset value may be equal to the initial value above. 
     In an example, the first detection packet with the TTL field in the header updated as zero may be used as the forwarding error message. 
     At block  103 , the first forwarding device adds its device address to the payload of the first detection packet to obtain a second detection packet, and forwards the second detection packet to a next hop forwarding device on the path of the designated service flow based on the forwarding information in the header of the second detection packet. 
     In an example, at block  103 , the TTL field in the header of the second detection packet is set to the initial value. Thus, when receiving the second detection packet, the next hop forwarding device on the path of the designated service flow subtracts the preset value equal to the initial value from the TTL field in the header of the second detection packet, and thus may confirm that an error occurs in the forwarding of the second detection packet. Then, the next hop forwarding device may perform the process of sending the forwarding error message to the SDN controller at block  102 . Thus, upon receiving the second detection packet, the next hop forwarding device may perform processes substantially the same as those at blocks  102  and  103 , wherein the next hop forwarding device is regarded as the first forwarding device and the second detection packet is regarded as the first detection packet. 
     It should be noted that the procedure at block  102  may be performed before or after the procedure at block  103 , which is not limited in examples of the present disclosure. 
     From block  103 , it can be seen that, when forwarding the detection packet, each forwarding device on the path of the designated service flow adds its device address to the detection packet. Moreover, referring to block  102 , all the device addresses in the payload of the first detection packet are carried in the forwarding error message. It follows that the forwarding error message, which is sent to the SDN controller by a last hop forwarding device on the path of the designated service flow, includes device addresses of all forwarding devices on the path of the designated service flow except the last hop forwarding device, and an order of the respective device addresses in the forwarding error message is the same as an order of the corresponding forwarding devices on the path of the designated service flow. Thus, the SDN controller can accurately determine the path of the designated service flow based on the forwarding error message sent by the last hop forwarding device, which will be described below in terms of a method applied to the SDN controller as shown in  FIG. 2 . 
     In an example, the header of the first detection packet further includes a checksum field which is configured as a designated check value indicating a checksum error, such that a destination host corresponding to the service flow discards the first detection packet upon receiving the first detection packet. 
     In an example, the header of the first detection packet includes an IP header and a transport layer protocol header (e.g., TCP/UDP header). The TTL field is in the IP header, and the checksum field is in the TCP/UDP header. The forwarding information in the IP header may include a source IP address, a destination IP address, a protocol type and a Differentiated Services Code Point (DSCP). The forwarding information in the TCP/UDP header may include a source port number and a destination port number. 
     In an example, based on actual requirement, a further header may be added to the constructed detection packet as mentioned above. For example, when a path of a Virtual Extensible Local Area Network (VXLAN) packet is to be detected, a VXLAN header may be added to the constructed detection packet. 
     So far, the process in  FIG. 1  ends. 
     From the example process above, it can be seen that the forwarding device may construct the detection packet for detecting the path of the service flow by simulating the header of the actual service flow. Thus, when each forwarding device forwards the detection packet based on the forwarding information in the header of the detection packet, it can be ensured that forwarding behavior corresponding to the detection packet is consistent with that corresponding to the actual service flow, so that the path of the actual service flow could be precisely determined. 
       FIG. 2  is a flowchart illustrating another method based on an example of the present disclosure, which corresponds to procedures in  FIG. 1 . The method can be applied to a SDN controller. 
     As shown in  FIG. 2 , the method may include procedures as follows. 
     At block  201 , a SDN controller receives a forwarding error message from a forwarding device. The forwarding device may be any non-initial forwarding device on the path of the designated service flow. 
     As mentioned above, the forwarding error message is sent by the forwarding device upon receiving a detection packet. 
     Before performing procedures in  FIG. 2 , the SDN controller may determine an initial forwarding device on the path of the designated service flow, and send a control command to the initial forwarding device, such that the initial forwarding device constructs the detection packet. The SDN controller may determine the initial forwarding device on the path of the designated service flow by the sampling method above. 
     In an example, all of device addresses in a payload of the detection packet are carried in the forwarding error message sent by the forwarding device. All the device addresses comprise device addresses of respective forwarding devices, through which the detection packet has passed before reaching the forwarding device sending the forwarding error message. 
     At block  202 , the SDN controller determines the path of the designated service flow based on all the device addresses carried in a received last forwarding error message. 
     In an example, the SDN controller may limit a period of time for detecting the path of the service flow. When no forwarding error message for detecting the path of the service flow is received in a set period of time, the SDN controller may determine a forwarding error message received recently as the last forwarding error message, and determine the path of the service flow based on all the device addresses carried in the last forwarding error message. 
     In an example, the SDN controller may determine the path of the service flow based on all the device addresses carried in the last forwarding error message and a device address of a determined last forwarding device on the path of the designated service flow. Herein, the SDN controller may determine the last forwarding device on the path of the service flow in a manner similar to the above method for determining the initial forwarding device on the path of the service flow. More specifically, the SDN controller may calculate each path from a source IP address of the service flow to a destination IP address of the service flow based on the source IP address, the destination IP address and the managed network topology, and sample a service flow for a last forwarding device on each path to determine the last forwarding device on the path of the service flow. 
     So far, the process in  FIG. 2  ends. 
     In the example illustrated in  FIG. 2 , at least one device address is carried in the forwarding error message, such that the SDN controller can quickly determine the actual path of the service flow, and workload of the SDN controller can be reduced. 
     The process in  FIG. 1  will be described in detail below by way of example. 
       FIG. 3  is a schematic diagram illustrating an example of the present disclosure, wherein a service flow Flow 1  is taken as an example. The Flow 1  may comprise a video packet in a format as illustrated in  FIG. 4 . 
     A SDN controller calculates each possible path of the Flow 1  based on a source IP address of the Flow 1  (e.g., an IP address of PC 1 ), a destination IP address of the Flow 1  (e.g., an IP address of PC 3 ) and a collected network topology as shown in  FIG. 3 , and samples a service flow for an initial forwarding device on each calculated path to determine Router B, for example, as an initial forwarding device on the path of the Flow 1 . 
     The SDN controller sends, to the initial forwarding device Router B on the path of the Flow  1 , a control command in which forwarding information in a header of the Flow 1  is carried. The forwarding information in the header of the Flow 1  includes forwarding information in an IP header of the Flow 1  and forwarding information in a UDP header of the Flow 1 . The forwarding information in the IP header of the Flow 1  includes, for example, a source IP address (IP 1 ), a destination IP address (IP 3 ), a protocol type and DSCP. The forwarding information in the UDP header of the Flow 1  includes, for example, a source port number and a destination port number. 
     Upon receiving the control command, the Router B constructs a detection packet for detecting a path of the Flow 1  based on the forwarding information carried in the control command. For the convenience of description, the constructed detection packet is denoted as detection packet  1 _ 1 . A TTL field in an EP header of the detection packet  1 _ 1  is set to an initial value (for example, 1). The IP header of the detection packet  1 _ 1  further includes the forwarding information in the IP header of the Flow 1 , carried in the received control command, such as the source IP address (IP 1 ), the destination IP address (IP 3 ), the protocol type and the DSCP. A checksum field in a UDP header of the detection packet  1 _ 1  is set to a designated check value (for example, 0x0) indicating a checksum error. The UDP header of the detection packet  1 _ 1  further includes the forwarding information in the UDP header of the Flow 1 , carried in the received control command, such as the source port number and the destination port number. 
     The Router B adds its IP address to the detection packet  1 _ 1 . For the convenience of description, the detection packet to which the IP address of the Router B is added is denoted as detection packet  1 _ 2 .  FIG. 6  illustrates a format of the detection packet  1 _ 2 . 
     The Router B forwards the forwarding packet  1 _ 2  based on forwarding information in an EP header of the detection packet  1 _ 2  and forwarding information in a UDP header of the detection packet  1 _ 2 . In an example, the Router B notifies to the SDN controller a message that the detection packet  1 _ 2  has been sent, such that SDN controller knows that a process of detecting a path of the Flow′ starts. 
     It is assumed that the Router B forwards the detection packet  1 _ 2  to Router A based on the forwarding information in the IP header of the detection packet  1 _ 2  and the forwarding information in the UDP header of the detection packet  1 _ 2 . 
     The Router A receives the detection packet  1 _ 2 . 
     The Router A may subtract a preset value (for example, 1) from a TTL field of the detection packet  1 _ 2  to obtain a value (for example, zero) indicating that an error occurs in the process of forwarding the detection packet  1 _ 2 . Then, the TTL field of the detection packet  1 _ 2  may become zero. For the convenience of description, the detection packet  1 _ 2  with the TTL field becoming zero is denoted as detection packet  1 _ 3 . 
     The Router A may send the detection packet  1 _ 3  as a forwarding error message to the SDN controller. 
     The Router A may update the TTL field of the detection packet  1 _ 3  as an initial value (for example, 1), and add an IP address of the Router A to the detection packet  1 _ 3 . For the convenience of description, the detection packet to which the IP address of the Router A is added is denoted as detection packet  1 _ 4 . Then, the respective IP addresses of the Routers B and A are carried in the detection packet  1 _ 4 . In an example, the IP address of the Router B and the IP address of the Router A are arranged in the detection packet  1 _ 4  in an order of addition of them to the detection packet, wherein the IP address of the Router A follows the IP address of the Router B.  FIG. 7  illustrates such a structure of the detection packet  1 _ 4 . 
     The Router A forwards the detection packet  1 _ 4  based on forwarding information in an IP header of the detection packet  1 _ 4  and forwarding information in a UDP header of the detection packet  1 _ 4 . It is assumed that the Router A forwards the detection packet  1 _ 4  to Router C based on the forwarding information in the IP header of the detection packet  1 _ 4  and the forwarding information in the UDP header of the detection packet  1 _ 4 . 
     The Router C receives the detection packet  1 _ 4 . 
     The Router C may subtract the preset value (for example, 1) from a TTL field of the detection packet  1 _ 4  to obtain a value (for example, zero) indicating that an error occurs in the process of forwarding the detection packet  1 _ 4 . Then, the TTL field of the detection packet  1 _ 4  may become zero. For the convenience of description, the detection packet  1 _ 4  with the TTL field becoming zero is denoted as detection packet  1 _ 5 . 
     The Router C may send the detection packet  1 _ 5  as a forwarding error message to the SDN controller. 
     The Router C may update the TTL field of the detection packet  1 _ 5  as the initial value (for example, 1), and add an IP address of the Router C to the detection packet  1 _ 5 . For the convenience of description, the detection packet to which the IP address of the Router C is added is denoted as detection packet  1 _ 6 . Then, the respective IP addresses of the Routers B, A and C are carried in the detection packet  1 _ 6 . In an example, the IP address of the Router B, the IP address of the Router A and the IP address of the Router C are arranged in the detection packet  1 _ 6  in an order of addition of them to the detection packet, wherein the IP address of the Router A follows the IP address of the Router B, and the IP address of the Router C follows the IP address of the Router A.  FIG. 8  illustrates such a structure of the detection packet  1 _ 6 . 
     The Router C forwards the detection packet  1 _ 6  based on forwarding information in an IP header of the detection packet  1 _ 6  and forwarding information in a UDP header of the detection packet  1 _ 6 . It is assumed that Router C forwards the detection packet  1 _ 6  to Router F based on the forwarding information in the IP header of the detection packet  1 _ 6  and the forwarding information in the UDP header of the detection packet  1 _ 6 . 
     The Router F receives the detection packet  1 _ 6 . 
     The Router F may subtract the preset value (for example, 1) from a TTL field of the detection packet  1 _ 6  to obtain a value (for example, zero) indicating that an error occurs in the process of forwarding the detection packet  1 _ 6 . Then, the TTL field of the detection packet  1 _ 6  may become zero. For the convenience of description, the detection packet  1 _ 6  with the TTL field becoming zero is denoted as detection packet  1 _ 7 . 
     The Router F may send the detection packet  1 _ 7  as a forwarding error message to the SDN controller. 
     The Router F may update the TTL field of the detection packet  1 _ 7  as the initial value (for example, 1), and add an EP address of the Router F to the detection packet  1 _ 7 . For the convenience of description, the detection packet  1 _ 7  to which the IP address of the Router F is added is denoted as detection packet  1 _ 8 . Then, the respective IP addresses of the Routers B, A, C and F are carried in the detection packet  1 _ 8 . In an example, the IP address of the Router B, the EP address of the Router A, the IP address of the Router C and the IP address of the Router F are arranged in the detection packet  1 _ 8  in an order of addition of them to the detection packet, wherein the IP address of the Router A follows the IP address of the Router B, the IP address of the Router C follows the IP address of the Router A, and the IP address of the Router F follows the IP address of the Router C.  FIG. 9  illustrates such a structure of the detection packet  1 _ 8 . 
     The Router F forwards the detection packet  1 _ 8  based on forwarding information in an IP header of the detection packet  1 _ 8  and forwarding information in a UDP header of the detection packet  1 _ 8 . It is assumed that the Router F forwards the detection packet  1 _ 8  to the destination host PC 3  based on the forwarding information in the IP header of the detection packet  1 _ 8  and the forwarding information in the UDP header of the detection packet  1 _ 8 . 
     Upon receiving the detection packet  1 _ 8 , PC 3  may parse the UDP header of the detection packet  1 _ 8  and find that the checksum field in the UDP header of the detection packet  1 _ 8  is equal to the designated check value (for example, 0x0) indicating a checksum error, thereby discarding the detection packet  1 _ 8 . So far, the process of forwarding the detection packet ends. 
     When no forwarding error message is received by the SDN controller in a set period of time, the SDN controller takes the forwarding error message received recently, i.e., the detection packet  1 _ 7 , as the last forwarding error message, and may determine the path of the Flow 1  as follows: the IP address of the Router B, the IP address of the Router A and the IP address of the Router C are carried in the last forwarding error message and arranged therein in the same order as an order of the corresponding forwarding devices on the path of the Flow 1 ; the Router F is determined as the last forwarding device on the path of the Flow 1  based on the sampling method described above; and thus it is finally calculated that the path of the Flow 1  sequentially involves the Router B, the Router A, the Router C and the Router F. 
     So far, the process in the example ends. 
     Methods of the present disclosure are described above. Devices of the present disclosure are described below. 
       FIG. 11  schematically illustrates a hardware structure of a forwarding device  1100  based on an example of the present disclosure. The forwarding device  1100  may include a processor  1101  and a machine-readable storage medium  1102 . The processor  1101  may communicate with the machine-readable storage medium  1102  via a system bus  1103 , and perform the above method of detecting a path by reading and executing machine-executable instructions corresponding to path detection logic  1000  stored in the machine-readable storage medium  1102 . 
     As used herein, the machine-readable storage medium  1102  may be any electronic, magnetic, optical, or other physical storage apparatus which may contain or store information such as executable instructions, data, and the like. For example, the machine-readable storage medium  1102  may be any of a Random Access Memory (RAM), a volatile memory, a non-volatile memory, a flash memory, a storage drive (e.g., a hard drive), a solid state drive, any type of storage disc (e.g., a compact disc (CD), a Digital Versatile Disc (DVD), etc.), and the like, or a combination thereof. 
       FIG. 10  is a schematic block diagram illustrating path detection logic  1000  based on an example of the present disclosure. The path detection logic  1000  can be applied to a forwarding device. As shown in  FIG. 10 , the path detection logic  1000  may include a receiving module  1001  and a forwarding module  1002 . 
     The receiving module  1001  is configured to: receive a first detection packet for detecting a path of a designated service flow from a previous hop forwarding device on the path of the designated service flow, wherein forwarding information in a header of the first detection packet is the same as forwarding information in a header of the designated service flow, and a device address of at least one forwarding device on the path of the designated service flow is carried in a payload of the first detection packet. 
     The forwarding module  1002  is configured to: add a device address of the present forwarding device to the payload of the first detection packet to obtain a second detection packet; forward the second detection packet to a next hop forwarding device on the path of the designated service flow based on forwarding information in a header of the second detection packet; and send to a SDN controller a forwarding error message in which the device address of the at least one forwarding device in the payload of the first detection packet is carried, such that the SDN controller determines the path of the designated service flow based on the device address carried in the forwarding error message. 
     In an example, when the previous hop forwarding device is an initial forwarding device on the path of the designated service flow, the first detection packet is constructed by the initial forwarding device based on a control command from the SDN controller, and a device address of the initial forwarding device is carried in the payload of the first detection packet. 
     In an example, the header of the first detection packet comprises a TTL field which is set to an initial value. 
     In an example, the forwarding module  1002  is configured to: update the TTL field in the header of the first detection packet by subtracting a preset value from the TTL field, wherein the preset value is equal to the initial value of the TTL field; and when the updated TTL field is equal to zero, determine that an error occurs in forwarding of the first detection packet, and forward the forwarding error message to the SDN controller. 
     In an example, the first detection packet with the TTL field updated as zero is carried in the forwarding error message; and a TTL field in the header of the second detection packet is set to the initial value. 
     In an example, the header of the first detection packet further comprises a checksum field which is configured as a designated check value indicating a checksum error, such that a destination host corresponding to the designated service flow discards the first detection packet upon receiving the first detection packet. 
     In an example, the header of the first detection packet comprises an IP header and a transport layer protocol header. The TTL field is in the IP header, and the checksum field is in the transport layer protocol header. The forwarding information in the IP header may comprise a source IP address, a destination IP address, a protocol type and a Differentiated Services Code Point (DSCP). The forwarding information in the transport layer protocol header may comprise a source port number and a destination port number. 
     So far, the description of the logic illustrated in  FIG. 10  ends. 
       FIG. 13  schematically illustrates a hardware structure of a SDN controller  1300  based on an example of the present disclosure. The SDN controller  1300  may include a processor  1301  and a machine-readable storage medium  1302 . The processor  1301  may communicate with the machine-readable storage medium  1302  via a system bus  1303 , and perform the above method of detecting a path by reading and executing machine-executable instructions corresponding to path detection logic  1200  stored in the machine-readable storage medium  1302 . 
     As used herein, the machine-readable storage medium  1302  may be any electronic, magnetic, optical, or other physical storage apparatus which may contain or store information such as executable instructions, data, and the like. For example, the machine-readable storage medium  1302  may be any of a RAM, a volatile memory, a non-volatile memory, a flash memory, a storage drive (e.g., a hard drive), a solid state drive, any type of storage disc (e.g., a CD, a DVD, etc.), and the like, or a combination thereof. 
       FIG. 12  is a schematic block diagram illustrating path detection logic  1200  based on another example of the present disclosure. The path detection logic  1200  can be applied to a SDN controller. As shown in  FIG. 12 , the path detection logic  1200  may include a receiving module  1201  and a determining module  1202 . 
     The receiving module  1201  is configured to receive a forwarding error message from a forwarding device. 
     The determining module  1202  is configured to determine a path of a designated service flow based on all device addresses carried in a received last forwarding error message. 
     In an example, the logic  1200  further includes a commanding module  1203 , which is configured to: determine an initial forwarding device on the path of the designated service flow, and send a control command to the initial forwarding device, such that the initial forwarding device constructs a detection packet. 
     So far, the description of the logic illustrated in  FIG. 12  ends. 
     Since the device embodiments substantially correspond to the method embodiments, a reference may be made to part of the descriptions of the method embodiments for the related part. The device embodiments described above are merely illustrative, where the units described as separate members may be or not be physically separated, and the members displayed as units may be or not be physical units, i.e., may be located in one place, or may be distributed to a plurality of network units. Part or all of the modules may be selected according to actual requirements to implement the objectives of the solutions in the embodiments. Those of ordinary skill in the art may understand and carry out them without creative work. 
     It shall be noted that the relational terms such as “first” and “second” used herein are merely intended to distinguish one entity or operation from another entity or operation rather than to require or imply any such actual relation or order existing between these entities or operations. Also, the term “including”, “comprising” or any variation thereof is intended to encompass non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements but also other elements not listed explicitly or those elements inherent to such a process, method, article or device. Without more limitations, an element defined by the statement “including a . . . ” shall not be precluded to include additional same elements present in a process, method, article or device including the elements. 
     The above are detailed description of a method and a device provided according to the embodiments of the present disclosure. Specific examples are used herein to set forth the principles and the implementing methods of the present disclosure, and the descriptions of the above embodiments are only intended to help understanding of the method and the core idea of the present disclosure. Meanwhile, those of ordinary skill in the art may make alterations to the specific embodiments and the scope of application in accordance with the idea of the present disclosure. In conclusion, the present description shall not be interpreted as limitation to the present disclosure.