Patent Publication Number: US-2022240332-A1

Title: First base station, second base station, method, program, and recording medium

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a first base station, a second base station, a method, a program, and a recording medium. 
     Background Art 
     NPL 1 discloses dual connectivity, in which packet data is transmitted and received between one user terminal (hereinafter referred to as a User Equipment or a UE) and two base stations. 
     In dual connectivity, a Split Bearer, in which transmission and reception of data between one Packet Data Convergence Protocol (PDCP) layer and two Radio Link Control (RLC) layers is performed, is defined. 
     In the PDCP layer, downlink data is appropriately transmitted to RLC layer processing units respectively included in two different nodes in a radio communication network, and thus information for controlling a downlink data flow such as a receivable buffer size or a receivable data rate of the RLC layer processing units needs to be taken into consideration. When such is not taken into consideration, this can lead to service deterioration due to discarding of a buffer or low-rate transmission in the RLC layer processing units. 
     In view of this, NPL 1 discloses a configuration of including the above-described receivable buffer size (“Desired buffer size for the data radio bearer: 4 OCTETs” described in NPL 1) or receivable data rate (“Desired Data Rate: 4 OCTETs” described in NPL 1) in a DL Data Delivery Status transmitted from the RLC layer to the PDCP layer. 
     NPL 2 discloses that when a terminal apparatus enters a cell area of a secondary node under an environment of a Master Node (MN) Terminated Master Cell Group (MCG) Bearer, for example, dual connectivity communication between the master node and the secondary node is available. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] 3GPP TS 38.425 V15.5.0 (2019 March) 
         [NPL 2] 3GPP TS 37.340 V15.6.0 (2019 June) 
       
    
     SUMMARY 
     Technical Problem 
     However, in the techniques disclosed in NPLs 1 and 2 described above, for example, when reconfiguration related to the secondary node, such as Addition of the secondary node, is performed, transmission of downlink data to the RLC layer cannot be started with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     The example object of the present invention is to provide a first base station, a second base station, a method, a program, and a recording medium that can start transmission of downlink data to the RLC layer with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     Solution to Problem 
     According to one aspect of the present invention, a first base station includes: an information obtaining unit configured to obtain information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and a communication processing unit configured to transmit, to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, the information for controlling the downlink data flow, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a second base station includes: an execution unit configured to execute processing for a Packet Data Convergence Protocol (PDCP) layer; and a communication processing unit configured to receive, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC layer, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a method includes: obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a method includes: executing processing for a Packet Data Convergence Protocol (PDCP) layer; and receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a program causes a processor to execute: obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a program causes a processor to execute: executing processing for a Packet Data Convergence Protocol (PDCP) layer; and receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a recording medium is a non-transitory computer readable recording medium storing a program that causes a processor to execute: obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     According to one aspect of the present invention, a recording medium is a non-transitory computer readable recording medium storing a program that causes a processor to execute: executing processing for a Packet Data Convergence Protocol (PDCP) layer; and receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     Advantageous Effects of Invention 
     According to the present invention, transmission of downlink data can be started with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. Note that, according to the present invention, instead of or together with the above effects, other effects may be exerted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating an example of a schematic configuration of a system  1   a  according to an example embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating an example of a schematic configuration of a base station  100  according to a first example embodiment; 
         FIG. 3  is a block diagram illustrating an example of a schematic configuration of a base station  200  according to the first example embodiment; 
         FIG. 4  is a diagram illustrating a schematic configuration of a downlink data flow in an SN Terminated Split Bearer; 
         FIG. 5  is a sequence diagram illustrating an example of processing of an SgNB Addition Procedure in an LLS configuration; 
         FIG. 6  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in a first specific example; 
         FIG. 7  is a diagram illustrating a schematic configuration of a downlink data flow in an MN Terminated Split Bearer; 
         FIG. 8  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in a second specific example; 
         FIG. 9  is an explanatory diagram illustrating an example of a schematic configuration of a system  1   b  according to a first example alteration including a secondary node of a High Layer Split (HLS) configuration; 
         FIG. 10  is a block diagram illustrating an example of a schematic configuration of the base station  200  according to the first example alteration; 
         FIG. 11  is a sequence diagram illustrating an example of processing of the SgNB Addition Procedure in the HLS configuration; 
         FIG. 12  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in SN Terminated Split Bearer option; 
         FIG. 13  is an explanatory diagram illustrating an example of a schematic configuration of a system  1   c  according to the first example alteration including the secondary node of the HLS configuration; 
         FIG. 14  is a block diagram illustrating an example of a schematic configuration of the base station  200  according to a second example alteration; 
         FIG. 15  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in the second example alteration; and 
         FIG. 16  is an explanatory diagram illustrating an example of a schematic configuration of a system  2  according to the second example embodiment; 
     
    
    
     DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     Hereinafter, example embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that, in the Specification and drawings, elements to which similar descriptions are applicable are denoted by the same reference signs, and overlapping descriptions may hence be omitted. 
     Descriptions will be given in the following order.
         1. Overview of Example Embodiment of Present Invention   2. First Example Embodiment
           2.1. Configuration of System  1   a  according to First Example Embodiment   2.2. Configuration of Base Station  100     2.3. Configuration of Base Station  200     2.4. Technical Features   2.5. Example Alterations   
           3. Second Example Embodiment
           3.1. Configuration of System  2     3.2. Configuration of First Base Station  500     3.3. Configuration of Second Base Station  600     3.4. Technical Features   
           4. Other Example Embodiments       

     1. Overview of Example Embodiment of Present Invention 
     First, an overview of example embodiments of the present invention will be described. 
     (1) Technical Issues 
     NPL 1 discloses dual connectivity, in which packet data is transmitted and received between one user terminal (hereinafter referred to as a User Equipment or a UE) and two base stations. 
     In dual connectivity, a Split Bearer, in which transmission and reception of data between one Packet Data Convergence Protocol (PDCP) layer and two Radio Link Control (RLC) layers is performed, is defined. 
     In the PDCP layer, downlink data is appropriately transmitted to RLC layer processing units respectively included in two different nodes in a radio communication network, and thus information for controlling a downlink data flow such as a receivable buffer size or a receivable data rate of the RLC layer processing units needs to be taken into consideration. When such is not taken into consideration, this can lead to service deterioration due to discarding of a buffer or low-rate transmission in the RLC layer processing units. 
     In view of this, NPL 1 discloses a configuration of including the above-described receivable buffer size (“Desired buffer size for the data radio bearer: 4 OCTETs” described in NPL 1) or receivable data rate (“Desired Data Rate: 4 OCTETs” described in NPL 1) in a DL Data Delivery Status transmitted from the RLC layer to the PDCP layer. 
     NPL 2 discloses that when a terminal apparatus enters a cell area of a secondary node under an environment of a Master Node (MN) Terminated Master Cell Group (MCG) Bearer, for example, dual connectivity communication between the master node and the secondary node is available. 
     However, in the techniques disclosed in NPLs 1 and 2 described above, for example, when reconfiguration related to the secondary node, such as Addition of the secondary node, is performed, transmission of downlink data to the RLC layer cannot be started with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     Specifically, there is a problem that information of the receivable buffer size (Desired buffer size for the data radio bearer) or the receivable data rate (Desired Data Rate) on the RLC layer side is not included in a message of SgNB Reconfiguration Complete in a sequence for constructing dual connectivity configuration of Secondary Node (SN) Terminated Split Bearer Option as disclosed in NPL 2 described above, and thus the PDCP layer processing unit included in the SgNB cannot transmit downlink data with an optimal data amount until receiving the DL Data Delivery Status. 
     In view of this, the example object according to one aspect of the present disclosure is to start transmission of downlink data to the RLC layer with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     (2) Technical Features 
     According to one aspect of the present invention, for example, a first base station is configured to obtain information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus, and is configured to transmit, to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, the information for controlling the downlink data flow, before the first base station detects a correct random access channel access. 
     With this configuration, for example, transmission of downlink data can be started with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     Note that the above-described technical features are concrete examples of the example embodiments of the present invention, and the example embodiments of the present invention are, of course, not limited to the above-described technical features. 
     2. First Example Embodiment 
     Next, with reference to  FIG. 1  to  FIG. 13 , the first example embodiment will be described. 
     &lt;2.1. Configuration of System  1   a  According to First Example Embodiment&gt; 
     With reference to  FIG. 1 , an example of a configuration of a system  1   a  according to the first example embodiment will be described.  FIG. 1  is an explanatory diagram illustrating an example of a schematic configuration of the system  1   a  according to an example embodiment of the present invention. With reference to  FIG. 1 , the system  1   a  includes base stations  100  and  200 , a terminal apparatus  300 , a core network  400 , an X2 interface  30 , and S1 interfaces  3   a  and  3   b . For example, the base stations  100  and  200  perform communication with each other via the X2 interface  30 . The base station  100  performs communication with the core network  400  via the S1 interface  3   a . The base station  200  performs communication with the core network  400  via the S1 interface  3   b.    
     The system  1  is, for example, a system conforming to Third Generation Partnership Project (3GPP) standards/specifications. More specifically, for example, the system  1  may be a system conforming to LTE/LTE-Advanced and/or System Architecture Evolution (SAE) standards/specifications. Alternatively, the system  1  may be a system conforming to fifth-generation (5G)/New Radio (NR) standards/specifications. The system  1  is, of course, not limited to these examples. 
     (1) Base Station  100   
     The base station  100  is a node of a Radio Access Network (RAN), and performs radio communication with a terminal apparatus (for example, the terminal apparatus  300 ) located in a coverage area  10 . For example, the base station  100  may function as a master node of dual connectivity for the terminal apparatus  300 . 
     For example, the base station  100  may be an evolved Node B (eNB), or a generation Node B (gNB) in 5G. The base station  100  may include a plurality of units (or a plurality of nodes). The plurality of units (or the plurality of nodes) may include a first unit (or a first node) configured to perform higher protocol layer processing and a second unit (or a second node) configured to perform lower protocol layer processing. As an example, the first unit may be referred to as a center/central unit (CU), and the second unit may be referred to as a distributed unit (DU) or an access unit (AU). As another example, the first unit may be referred to as a digital unit (DU), and the second unit may be referred to as a radio unit (RU) or a remote unit (RU). The digital unit (DU) may be a base band unit (BBU), and the RU may be a remote radio head (RRH) or a remote radio unit (RRU). The terms for the first unit (or the first node) and the second unit (or the second node) are, of course, not limited to these examples. Alternatively, the base station  100  may be a single unit (or a single node). In this case, the base station  100  may be one of the plurality of units (e.g., either one of the first unit and the second unit) or may be connected to another unit of the plurality of units (e.g., the other one of the first unit and the second unit). 
     (2) Base Station  200   
     The base station  200  is a node of a Radio Access Network (RAN), and performs radio communication with a terminal apparatus (for example, the terminal apparatus  300 ) located in a coverage area  20 . For example, the base station  100  may function as a secondary node of dual connectivity for the terminal apparatus  300 . 
     For example, the base station  200  may be an evolved Node B (eNB), or a generation Node B (gNB) in 5G. The base station  200  may include a plurality of units (or a plurality of nodes). The plurality of units (or the plurality of nodes) may include a first unit (or a first node) configured to perform higher protocol layer processing and a second unit (or a second node) configured to perform lower protocol layer processing. As an example, the first unit may be referred to as a center/central unit (CU), and the second unit may be referred to as a distributed unit (DU) or an access unit (AU). As another example, the first unit may be referred to as a digital unit (DU), and the second unit may be referred to as a radio unit (RU) or a remote unit (RU). The digital unit (DU) may be a base band unit (BBU), and the RU may be a remote radio head (RRH) or a remote radio unit (RRU). The terms for the first unit (or the first node) and the second unit (or the second node) are, of course, not limited to these examples. Alternatively, the base station  200  may be a single unit (or a single node). In this case, the base station  200  may be one of the plurality of units (e.g., either one of the first unit and the second unit) or may be connected to another unit of the plurality of units (e.g., the other one of the first unit and the second unit). 
     (3) Terminal Apparatus  300   
     The terminal apparatus  300  performs radio communication with the base station. For example, the terminal apparatus  300  performs radio communication with the base station  100  when the terminal apparatus  300  is located in the coverage area  10  of the base station  100 , and performs radio communication with the base station  200  when the terminal apparatus  300  is located in the coverage area  20  of the base station  200 . For example, the terminal apparatus  300  is a User Equipment (UE). 
     (4) Core Network  400   
     The core network  400  includes a Mobility Management Entity (MME)  410 , a Serving Gateway (S-GW)  420 , and the like that perform control related to the base station  100  and the base station  200 . 
     &lt;2.2. Configuration of Base Station  100 &gt; 
     Next, with reference to  FIG. 2 , an example of a configuration of the base station  100  according to the first example embodiment will be described.  FIG. 2  is a block diagram illustrating an example of a schematic configuration of the base station  100  according to the first example embodiment. With reference to  FIG. 2 , the base station  100  includes a radio communication unit  110 , a network communication unit  120 , a storage unit  130 , and a processing unit  140 . 
     (1) Radio Communication Unit  110   
     The radio communication unit  110  wirelessly transmits and receives a signal. For example, the radio communication unit  110  receives a signal from the terminal apparatus, and transmits a signal to the terminal apparatus. The radio communication unit  110  is, for example, a Radio Unit (RU) that performs processing for the physical layer. 
     (2) Network Communication Unit  120   
     The network communication unit  120  receives a signal from a network, and transmits a signal to a network. 
     (3) Storage Unit  130   
     The storage unit  130  temporarily or permanently stores programs (instructions) and parameters for operations of the base station  100  as well as various data. The program includes one or more instructions for operations of the base station  100 . 
     (4) Processing Unit  140   
     The processing unit  140  provides various functions of the base station  100 . The processing unit  140  is a Lower-Layer Split—Central Unit (LLS-CU) that performs processing for a layer higher than the physical layer. Specifically, the processing unit  140  includes a layer 3 function unit  140   a  and a layer 2 function unit  140   b . The layer 3 function unit  140   a  is, for example, a function unit that performs processing of the Radio Resource Control (RRC) and the X2-Application (AP) protocol, and includes an information obtaining unit  141  and a communication processing unit  143 . The layer 3 function unit  140   a  is a function unit that performs processing in the Packet Data Convergence Protocol (PDCP) layer and the Radio Link Control (RLC) layer, and includes a PDCP layer processing unit  145  and an RLC layer processing unit  147 . Note that the processing unit  140  may further include another constituent element (for example, a Medium Access Control (MAC) layer processing unit or the like) other than these constituent elements. In other words, the processing unit  140  may also perform operations other than the operations of these constituent elements. Specific operations of the information obtaining unit  141 , the communication processing unit  143 , the PDCP layer processing unit  145 , and the RLC layer processing unit  147  will be described later in detail. 
     (5) Implementation Example 
     The radio communication unit  110  may be implemented with an antenna, a radio frequency (RF) circuit, and the like, and the antenna may be a directional antenna. The network communication unit  120  may be implemented with a network adapter and/or a network interface card, and the like. The storage unit  130  may be implemented with a memory (e.g., a nonvolatile memory and/or a volatile memory) and/or a hard disk, and the like. The processing unit  140  may be implemented with one or more processors, such as a baseband (BB) processor and/or a different kind of processor. The information obtaining unit  141 , the communication processing unit  143 , the PDCP layer processing unit  145 , and the RLC layer processing unit  147  may be implemented with the same processor, or may be implemented with different processors separately. The memory (storage unit  130 ) may be included in the one or more processors or may be provided outside the one or more processors. 
     The base station  100  may include a memory configured to store a program (instructions) and one or more processors that can execute the program (instructions). The one or more processors may execute the program, and perform operations of the processing unit  140  (operations of the information obtaining unit  141 , the communication processing unit  143 , the PDCP layer processing unit  145 , and/or the RLC layer processing unit  147 ). The program may be a program for causing the processor(s) to execute the operations of the processing unit  140  (operations of the information obtaining unit  141 , the communication processing unit  143 , the PDCP layer processing unit  145 , and/or the RLC layer processing unit  147 ). 
     Note that the base station  100  may be virtual. In other words, the base station  100  may be implemented as a virtual machine. In this case, the base station  100  (the virtual machine) may operate as a physical machine (hardware) including a processor, a memory, and the like, and a virtual machine on a hypervisor. 
     &lt;2.3. Configuration of Base Station  200 &gt; 
     Next, with reference to  FIG. 3 , an example of a configuration of the base station  200  according to the first example embodiment will be described.  FIG. 3  is a block diagram illustrating an example of a schematic configuration of the base station  200  according to the first example embodiment. With reference to  FIG. 3 , the base station  200  includes a radio communication unit  210 , a network communication unit  220 , a storage unit  230 , and a processing unit  240 . 
     (1) Radio Communication Unit  210   
     The radio communication unit  210  wirelessly transmits and receives a signal. For example, the radio communication unit  210  receives a signal from the terminal apparatus, and transmits a signal to the terminal apparatus. The radio communication unit  110  is, for example, a radio unit that performs processing for the physical layer. 
     (2) Network Communication Unit  220   
     The network communication unit  220  receives a signal from a network, and transmits a signal to a network. 
     (3) Storage Unit  230   
     The storage unit  230  temporarily or permanently stores programs (instructions) and parameters for operations of the base station  200  as well as various data. The program includes one or more instructions for operations of the base station  200 . 
     (4) Processing Unit  240   
     The processing unit  240  provides various functions of the base station  200 . The processing unit  240  is a Lower-Layer Split—Central Unit (LLS-CU) that performs processing for a layer higher than the physical layer. Specifically, the processing unit  240  includes a layer 3 function unit  240   a  and a layer 2 function unit  240   b . The layer 3 function unit  240   a  is, for example, a function unit that performs processing of the RRC and the X2-AP protocol, and includes an information obtaining unit  241  and a communication processing unit  243 . The layer 3 function unit  240   a  is a function unit that performs processing in the PDCP layer and the RLC layer, and includes a PDCP layer processing unit  245  and an RLC layer processing unit  247 . Note that the processing unit  240  may further include another constituent element (for example, a MAC layer processing unit or the like) other than these constituent elements. In other words, the processing unit  240  may also perform operations other than the operations of these constituent elements. Specific operations of the information obtaining unit  241 , the communication processing unit  243 , the PDCP layer processing unit  245 , and the RLC layer processing unit  247  will be described later in detail. 
     (5) Implementation Example 
     The radio communication unit  210  may be implemented with an antenna, a radio frequency (RF) circuit, and the like, and the antenna may be a directional antenna. The network communication unit  220  may be implemented with a network adapter and/or a network interface card, and the like. The storage unit  230  may be implemented with a memory (e.g., a nonvolatile memory and/or a volatile memory) and/or a hard disk, and the like. The processing unit  240  may be implemented with one or more processors, such as a baseband (BB) processor and/or a different kind of processor. The information obtaining unit  241 , the communication processing unit  243 , the PDCP layer processing unit  245 , and the RLC layer processing unit  247  may be implemented with the same processor, or may be implemented with different processors separately. The memory (storage unit  230 ) may be included in the one or more processors or may be provided outside the one or more processors. 
     The base station  200  may include a memory configured to store a program (instructions) and one or more processors that can execute the program (instructions). The one or more processors may execute the program, and perform operations of the processing unit  240  (operations of the information obtaining unit  241 , the communication processing unit  243 , the PDCP layer processing unit  245 , and/or the RLC layer processing unit  247 ). The program may be a program for causing the processor(s) to execute the operations of the processing unit  240  (operations of the information obtaining unit  241 , the communication processing unit  243 , the PDCP layer processing unit  245 , and/or the RLC layer processing unit  247 ). 
     Note that the base station  200  may be virtual. In other words, the base station  200  may be implemented as a virtual machine. In this case, the base station  200  (the virtual machine) may operate as a physical machine (hardware) including a processor, a memory, and the like, and a virtual machine on a hypervisor. 
     &lt;2.4. Technical Features&gt; 
     With reference to  FIG. 4  to  FIG. 8 , technical features of the first example embodiment will be described. 
     According to the first example embodiment, a first base station (for example, the information obtaining unit  141  of the base station  100 ) obtains information for controlling a downlink data flow in an RLC layer for a terminal apparatus (for example, the terminal apparatus  300 ). The first base station (for example, the communication processing unit  143  of the base station  100 ) transmits, to a second base station (for example, the base station  200 ) performing processing for a PDCP layer for the terminal apparatus (for example, the terminal apparatus  300 ), the information for controlling the downlink data flow, before the first base station (for example, the base station  100 ) detects a correct random access channel access between the terminal apparatus (for example, the terminal apparatus  300 ) and the second base station (for example, the base station  200 ). 
     Specifically, the random access channel access is carried out by the terminal apparatus (for example, the terminal apparatus  300 ) for a bearer for the second base station (for example, the base station  200 ). Here, the bearer for the second base station (for example, the base station  200 ) is a bearer that requires radio resources of a group of serving cells (for example, a secondary cell group) associated with the second base station (for example, the base station  200 ). 
     The second base station (for example, the PDCP layer processing unit  245  of the base station  200 ) executes processing for the PDCP layer. The second base station (communication processing unit  243  of the base station  200 ) receives the information for controlling the downlink data flow in the RLC from the first base station (for example, the base station  100 ) performing processing in the RLC layer for the terminal apparatus (for example, the terminal apparatus  300 ), before the first base station (for example, the base station  100 ) detects a correct random access channel access between the terminal apparatus (for example, the terminal apparatus  300 ) and the second base station (for example, the base station  200 ). 
     The first base station does not necessarily correspond to the base station  100 , and for example, the first base station may be the base station  200 . In this case, the second base station may correspond to, for example, the base station  100 . 
     (1) Information for Controlling Downlink Data Flow 
     For example, the information for controlling the downlink data flow may include information related to a buffer size for downlink data in the RLC layer. The information for controlling the downlink data flow may include information related to a data rate for the downlink data in the RLC layer. 
     Specifically, the information for controlling the downlink data flow may be a DL Data Delivery Status defined in NPL 1 described above. In this case, the buffer size for the downlink data in the RLC layer may correspond to a “Desired buffer size for the data radio bearer” defined in NPL 1. The data rate for the downlink data in the RLC layer may correspond to a “Desired Data Rate” defined in NPL 1. 
     (2) Transmission Timing for Information for Controlling Downlink Data Flow 
     The timing is a timing of transmitting the information for controlling the downlink data flow to the second base station (for example, the base station  200 ) before the first base station (for example, the base station  100 ) detects a correct random access channel access by the terminal apparatus (for example, the terminal apparatus  300 ). 
     (3) Specific Examples 
     (3-1) First Specific Example 
     In a first specific example, the first base station is a master node (for example, the base station  100 ) for dual connectivity for the terminal apparatus  300 , and the second base station is a secondary node (for example, the base station  200 ) for dual connectivity for the terminal apparatus  300 . 
     In the first specific example, a Secondary Node (SN) Terminated Split Bearer for dual connectivity is configured.  FIG. 4  is a diagram illustrating a schematic configuration of a downlink data flow in the SN Terminated Split Bearer. 
     As illustrated in  FIG. 4 , in the SN Terminated Split Bearer for dual connectivity, the terminal apparatus  300  performs communication simultaneously with the master node (base station  100 ) and the secondary node (base station  200 ). Specifically, the secondary node (for example, the PDCP layer processing unit  245  of the base station  200 ) performs PDCP layer processing, and distributes downlink data to the master node (RLC layer processing unit  147  of the base station  100 ) and the secondary node (RLC layer processing unit  147  of the base station  200 ). 
     The secondary node (PDCP layer processing unit  245  of the base station  200 ) receives a DL Data Delivery Status message from the master node (communication processing unit  143  of the base station  100 ), and can thereby check a load state of the RLC layer processing unit on the master node side (RLC layer processing unit  147  of the base station  100 ). In other words, the secondary node (PDCP layer processing unit  245  of the base station  200 ) can refer to a DL Data Delivery Status message from the master node (base station  100 ), to adjust a data amount to be distributed to each of the master node (RLC layer processing unit  147  of the base station  100 ) and the secondary node (RLC layer processing unit  247  of the base station  200 ). 
     Transmission Timing of DL Data Delivery Status Message 
     In the first specific example, the master node (communication processing unit  143  of the base station  100 ) transmits a secondary node reconfiguration complete message to the secondary node (base station  200 ) during a secondary node addition procedure for the terminal apparatus  300 . Here, the information for controlling the downlink data flow is included in the secondary node reconfiguration complete message. 
     The secondary node addition procedure for the terminal apparatus  300  may correspond to, for example, an SgNB Addition Procedure for dual connectivity of SN Terminated Split Bearer Option of a Low Layer Split (LLS) configuration. 
       FIG. 5  is a sequence diagram illustrating an example of processing of the SgNB Addition Procedure in the LLS configuration. With reference to  FIG. 5 , the “UE” corresponds to the terminal apparatus  300 , the “MN” corresponds to the base station  100 , the “SN” corresponds to the base station  200 , and the “S-GW” and “MME” each correspond to a network node in the core network  400 . In the sequence as illustrated in  FIG. 5 , the secondary node reconfiguration complete message corresponds to a message of SgNB Reconfiguration Complete. The random access channel access is carried out in the Random Access Procedure that is performed after SgNB Reconfiguration Complete. 
     For example, as a reference example, in the technique described in NPL 1 or the like, after the secondary node receives the message of SgNB Reconfiguration Complete, a PDCP processing unit in the secondary node transmits DL user Data for polling to the master node MN in order to receive a DL Data Delivery Status, and can thereby receive the DL Data Delivery Status. 
     In contrast, in the first specific example, as illustrated in  FIG. 6 , a part of processing of the SgNB Addition Procedure is performed.  FIG. 6  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in the first specific example. 
     First, after receiving an SgNB Addition Request from the base station  100  (MN) and before transmitting an SgNB Addition Request ACK to the base station  100  (MN), the layer 3 function unit  240   a  (information obtaining unit  241 ) of the base station  200  (SN) obtains information related to a buffer size and a data rate receivable in the RLC layer under the base station  200  (SN) from the layer 2 function unit  240   b  (RLC layer processing unit  247 ) (S 601 ). 
     Next, after receiving the SgNB Addition Request ACK from the base station  200  (SN), the layer 3 function unit  140   a  of the base station  100  (MN) receives a message of RRC Reconfiguration Complete (S 603 ). Next, the layer 3 function unit  140   a  (information obtaining unit  141 ) of the base station  100  (MN) obtains information related to a buffer size and a data rate receivable in the RLC layer under the base station  100  (MN) from the layer 2 function unit  140   b  (S 605 ). Next, the layer 2 function unit  140   b  (communication processing unit  143 ) of the base station  100  (MN) transmits a message of SgNB Reconfiguration Complete including these pieced of obtained information to the base station  200  (SN) (S 607 ). 
     Next, the layer 3 function unit  240   a  (communication processing unit  243 ) of the base station  200  (SN) notifies the layer 2 function unit  240   b  (PDCP layer processing unit  245 ) of the buffer size and the data rate receivable in the RLC layer of the base station  100  (MN) included in the message of SgNB Reconfiguration Complete (S 609 ). Next, the PDCP layer processing unit  245  of the base station  200  (SN) starts transmission of downlink data to the terminal apparatus  300 , according to the buffer size and the data rate receivable in the RLC layer in the base station  100  (ML) and the base station  200  (SN) (S 611 ). 
     As described above, according to the processing illustrated in  FIG. 6  described above, for example, in comparison to the reference example described above, transmission of downlink data can be started earlier and so that a data amount to the RLC layer is appropriate. 
     (3-2) Second Specific Example 
     In a second specific example, the first base station is a secondary node (for example, the base station  200 ) for dual connectivity for the terminal apparatus  300 , and the second base station is a master node (for example, the base station  100 ) for dual connectivity for the terminal apparatus  300 . 
     In the second specific example, a Master Node (MN) Terminated Split Bearer for dual connectivity is configured.  FIG. 7  is a diagram illustrating a schematic configuration of a downlink data flow in the MN Terminated Split Bearer. 
     As illustrated in  FIG. 7 , in the MN Terminated Split Bearer for dual connectivity, the terminal apparatus  300  performs communication simultaneously with the master node (base station  100 ) and the secondary node (base station  200 ). Specifically, the master node (PDCP layer processing unit  145  of the base station  100 ) performs PDCP layer processing, and distributes downlink data to the master node (RLC layer processing unit  147  of the base station  100 ) and the secondary node (RLC layer processing unit  147  of the base station  200 ). 
     The master node (PDCP layer processing unit  145  of the base station  100 ) receives a DL Data Delivery Status message from the secondary node (communication processing unit  243  of the base station  200 ), and can thereby check a load state of the RLC layer processing unit on the secondary side (RLC layer processing unit  247  of the base station  200 ). In other words, the master node (PDCP layer processing unit  145  of the base station  100 ) can refer¥ to a DL Data Delivery Status message from the secondary node (base station  200 ), to adjust a data amount to be distributed to each of the master node (RLC layer processing unit  147  of the base station  100 ) and the secondary node (RLC layer processing unit  247  of the base station  200 ). 
     Transmission Timing of DL Data Delivery Status Message 
     In the second specific example, the secondary node (communication processing unit  243  of the base station  200 ) transmits, to the master node (base station  100 ), a response message to a secondary node addition request message from the master node (base station  100 ) during the secondary node addition procedure for terminal apparatus  300 . Here, the information for controlling the downlink data flow is included in the response message to the secondary node addition request message. 
     The secondary node addition procedure for the terminal apparatus  300  corresponds to, for example, an SgNB Addition Procedure for dual connectivity of Master Node (MN) Terminated Split Bearer Option of the LLS configuration. For example, in the sequence as illustrated in  FIG. 5  described above, the secondary node addition request message corresponds to a message of SgNB Addition Request. For example, in the sequence as illustrated in  FIG. 5 , the response message to the secondary node addition request message corresponds to a message of SgNB Addition Request Acknowledge. The random access channel access is carried out in the Random Access Procedure that is performed after SgNB Addition Request Acknowledge. 
       FIG. 8  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in the second specific example. 
     First, the layer 3 function unit  240   a  of the base station  200  (SN) determines whether MN terminated or SN terminated is to be configured, according to the SgNB Addition Request received from the base station  100  (MN). Here, when it is determined that MN terminated is to be configured, the layer 3 function unit  240   a  (information obtaining unit  241 ) of the base station  200  (SN) obtains information related to a buffer size and a data rate receivable in the RLC layer from the layer 2 function unit  240   b  (RLC layer processing unit  247 ) (S 801 ). 
     Next, the layer 3 function unit  240   a  (communication processing unit  243 ) of the base station  200  (SN) transmits an SgNB Addition Request ACK including the obtained information to the base station  100  (MN) (S 803 ). 
     Next, after receiving the SgNB Addition Request ACK from the base station  200  (SN), the layer 3 function unit  140   a  of the base station  100  (MN) receives a message of RRC Reconfiguration Complete (S 805 ). Next, the layer 3 function unit  140   a  (information obtaining unit  141 ) of the base station  100  (MN) obtains information related to a buffer size and a data rate receivable in the RLC layer under the base station  100  (MN) from the layer 2 function unit  140   b  (S 807 ). Subsequently, the base station  100  (MN) transmits a message of SgNB Reconfiguration Complete to the base station  200  (SN). 
     As described above, according to the processing illustrated in  FIG. 8  described above, the information related to the buffer size and the data rate receivable in the RLC layer on the base station  200  side is added to the SgNB Addition Request ACK. Thus, after completion of the random access channel access between the base station  200  and the terminal apparatus  300 , the base station  100  can start transmission of downlink data so that downlink data of the RLC layer is appropriately distributed to respective base stations  100  and  200 . 
     &lt;2.5. Example Alterations&gt; 
     According to the first example embodiment, the specific examples described above are not restrictive, and for example, a High Layer Split (HLS) may be adopted, in which the secondary node is separated into units different from each other between the PDCP layer and the RLC layer. 
     (1) First Example Alteration 
       FIG. 9  is an explanatory diagram illustrating an example of a schematic configuration of a system  1   b  according to a first example alteration including the secondary node of the High Layer Split (HLS) configuration. 
     With reference to  FIG. 9 , in the system  1   b  according to the first example alteration, the base station  200  corresponds to a central unit included in a secondary node  20   b  of the High Layer Split (HLS) configuration. The base station  200  performs communication with a distributed unit  21   b  included in the secondary node  20   b  via an F1 Interface  25 . 
       FIG. 10  is a block diagram illustrating an example of a schematic configuration of the base station  200  according to the first example alteration. The base station  200  according to the first example alteration is different from that in the example of the configuration illustrated in  FIG. 3  described above mainly in that the base station  200  does not include the radio communication unit and also that the layer 2 function unit  240   b  does not include the RLC layer processing unit. Note that the configuration of the base station  100  according to the first example alteration is similar to the example of the configuration illustrated in  FIG. 2  described above. 
     In the first example alteration, the first base station is a master node (base station  100 ) for dual connectivity for the terminal apparatus  300 , and the second base station is a central unit (base station  200 ) included in the secondary node  20   b  for dual connectivity for the terminal apparatus  300 . In the first example alteration, the Secondary Node (SN) Terminated Split Bearer for dual connectivity is configured. 
     In the first example alteration, the base station  100  (communication processing unit  143 ) transmits a secondary node reconfiguration complete message to the central unit (base station  200 ) included in the secondary node  20   b  during the secondary node addition procedure for the terminal apparatus  300 . Here, the information for controlling the downlink data flow is included in the secondary node reconfiguration complete message. 
     The secondary node addition procedure for the terminal apparatus  300  corresponds to, for example, an SgNB Addition Procedure for dual connectivity of SN Terminated Split Bearer Option of the High Layer Split (HLS) configuration. 
       FIG. 11  is a sequence diagram illustrating an example of processing of the SgNB Addition Procedure in the HLS configuration. With reference to  FIG. 11 , the “UE” corresponds to the terminal apparatus  300 , an “MeNB” corresponds to the base station  100 , a “gNB-CU” corresponds to the base station  200 , a “gNB-DU” corresponds to the distributed unit  21   b , and the “S-GW” and “MME” each correspond to a network node in the core network  400 . The secondary node reconfiguration complete message corresponds to a message of SgNB Reconfiguration Complete. The random access channel access is carried out in the Random Access Procedure that is performed after SgNB Reconfiguration Complete. 
     In the sequence as illustrated in  FIG. 11 , the base station  100  (communication processing unit  143 ) transmits a secondary node reconfiguration complete message to the base station  200  during the secondary node addition procedure for the terminal apparatus  300 . Here, the information for controlling the downlink data flow is included in the secondary node reconfiguration complete message. 
       FIG. 12  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in SN Terminated Split Bearer option. 
     First, the layer 3 function unit of the distributed unit  21   b  (gNB-DU) receives a message of UE context setup Request from the base station  200  (gNB-CU) in response to a message of SgNB Addition Request from the base station  100 , and then the layer 3 function unit obtains a buffer size and a data rate receivable in the RLC layer under the distributed unit  21   b  (gNB-DU) from the layer 2 function unit of the distributed unit  21   b  (gNB-DU) (S 1201 ). 
     Next, the layer 3 function unit of the distributed unit  21   b  (gNB-DU) transmits a message of UE context setup Response including the obtained information to the base station  200  (gNB-CU) (S 1203 ). 
     Next, the base station  100  (MN) receives a message of SgNB Addition Request ACK from the base station  200  (gNB-CU) in response to the message of UE context setup Request, and then the base station  100  (MN) receives a message of RRC Reconfiguration Complete (S 1205 ). The layer 3 function unit  140   a  (information obtaining unit  141 ) of the base station  100  (MN) obtains information related to a buffer size and a data rate receivable in the RLC layer under the base station  100  (MN) from the layer 2 function unit  140   b  (RLC layer processing unit  147 ) (S 1207 ). Next, the layer 3 function unit  140   a  (communication processing unit  143 ) of the base station  100  (MN) transmits a message of SgNB Reconfiguration Complete including the information to the gNB-CU (base station  200 ) (S 1209 ). 
     Next, the layer 3 function unit  240   a  (communication processing unit  243 ) of the base station  200  (gNB-CU) notifies the layer 2 function unit  240   b  (PDCP layer processing unit  245 ) of the buffer size and the data rate receivable in the RLC layer of the base station  100  (MN) obtained in the message of SgNB Reconfiguration Complete (S 1211 ). Next, the PDCP layer processing unit  245  of the base station  200  (gNB-CU) starts transmission of downlink data to the terminal apparatus  300 , according to the buffer size and the data rate receivable in the RLC layer in the base station  100  (ML) and the base station  200  (gNB-CU) (S 1213 ). 
     According to the processing illustrated in  FIG. 12  described above, transmission of downlink data can be started earlier and so that a data amount to the RLC layer is appropriate. 
     (2) Second Example Alteration 
       FIG. 13  is an explanatory diagram illustrating an example of a schematic configuration of a system  1   c  according to a second example alteration including the secondary node of the HLS configuration. 
     With reference to  FIG. 13 , in the system  1   c  according to the second example alteration, the base station  200  corresponds to a distributed unit included in a secondary node  20   c  of the High Layer Split (HLS) configuration. The base station  200  performs communication with a central unit  22   c  included in the secondary node  20   c  via the F1 Interface  25 . 
       FIG. 14  is a block diagram illustrating an example of a schematic configuration of the base station  200  according to the second example alteration. The base station  200  according to the second example alteration is different from that in the example of the configuration illustrated in  FIG. 3  described above mainly in that the layer 2 function unit  240   b  does not include the PDCP layer processing unit. Note that the configuration of the base station  100  according to the second example alteration is similar to the example of the configuration illustrated in  FIG. 2  described above. 
     In the second example alteration, the first base station is a distributed unit (base station  200 ) included in the secondary node for dual connectivity for the terminal apparatus  300 , and the second base station is a master node (base station  100 ) for dual connectivity for the terminal apparatus  300 . In other words, in the second example alteration, the MN Terminated Split Bearer for dual connectivity is configured. 
     In the second example alteration, the base station  200  (communication processing unit  243 ) transmits, to the base station  100 , a response message to the secondary node addition request message from the base station  100  via the central unit  22   c  included in the secondary node  20   c , during the secondary node addition procedure for the terminal apparatus  300 . Here, the information for controlling the downlink data flow is included in the response message to the secondary node addition request message. 
     The secondary node addition procedure for the terminal apparatus  300  corresponds to, for example, an SgNB Addition Procedure for dual connectivity of MN Terminated Split Bearer Option of the HLS configuration. For example, in the sequence illustrated in  FIG. 11 , the secondary node addition request message corresponds to a message of SgNB Addition Request. For example, in the sequence as illustrated in  FIG. 11 , the response message to the secondary node addition request message corresponds to a message of UE context setup Request. The random access channel access is carried out in the Random Access Procedure that is performed after UE context setup Request. 
       FIG. 15  is a sequence diagram illustrating a part of processing of the SgNB Addition Procedure in the second example alteration. 
     First, the layer 3 function unit  240   a  (information obtaining unit  241 ) of the base station  200  (gNB-DU) receives a message of UE context setup Request from the central unit  22   c  (gNB-CU) in response to a message of SgNB Addition Request from the base station  100 , and then the layer 3 function unit  240   a  obtains information related to obtaining of a buffer size and a data rate receivable in the RLC layer under the base station  200  (gNB-DU) from the layer 2 function unit  240   b  (RLC layer processing unit  247 ) (S 1501 ). 
     Next, the layer 3 function unit  240   a  (communication processing unit  243 ) of the base station  200  (gNB-DU) transmits a message of SgNB Addition Request ACK including the obtained information to the central unit  22   c  (gNB-CU) (S 1503 ). 
     Next, the layer 3 function unit of the central unit  22   c  (gNB-CU) transmits, to the base station  100  (MN), a message of SgNB Addition Request ACK including the buffer size and data rate information receivable in the RLC layer under the base station  200  (gNB-DU) obtained in the message of UE context setup Response (S 1505 ). 
     Next, after receiving the message of SgNB Addition Request ACK from the central unit  22   c  (gNB-CU), the base station  100  (MN) receives a message of RRC Reconfiguration Complete (S 1507 ). Next, the layer 3 function unit  140   a  (information obtaining unit  141 ) of the base station  100  (MN) obtains information related to a buffer size and a data rate receivable in the RLC layer under the base station  100  from the layer 2 function unit  140   b  (RLC layer processing unit  147 ) (S 1509 ). Next, the base station  100  (MN) transmits a message of SgNB Reconfiguration Complete to the central unit  22   c  of the secondary node  20   c.    
     As described above, according to the processing illustrated in  FIG. 15  described above, the information related to obtaining of the buffer size and the data rate receivable in the RLC layer of the distributed unit (base station  200 ) included in the secondary node  20   c  is added to the message of UE context setup Response. Thus, immediately after completion of the random access channel access between the secondary node  20   c  and the terminal apparatus  300 , the layer 3 function unit  140   b  (PDCP layer processing unit  145 ) of the base station  100  can perform transmission of downlink data. 
     3. Second Example Embodiment 
     Next, with reference to  FIG. 16 , a second example embodiment of the present invention will be described. The above-described first example embodiment is a concrete example embodiment, whereas the second example embodiment is a more generalized example embodiment. 
     &lt;3.1. Configuration of System  2 &gt; 
     With reference to  FIG. 16 , an example of a configuration of a system  2  according to the second example embodiment will be described. 
       FIG. 16  is an explanatory diagram illustrating an example of a schematic configuration of the system  2  according to the second example embodiment. With reference to  FIG. 16 , the system  2  includes a first base station  500  and a second base station  600 . 
     &lt;3.2. Configuration of First Base Station  500 &gt; 
     With reference to  FIG. 16 , the first base station  500  includes an information obtaining unit  511  and a communication processing unit  513 . Specific operations of the information obtaining unit  511  and the communication processing unit  513  will be described later. 
     The information obtaining unit  511  and the communication processing unit  513  may be implemented with one or more processors, such as a baseband (BB) processor and/or a different kind of processor, a memory (e.g., a nonvolatile memory and/or a volatile memory), and/or a hard disk. The information obtaining unit  511  and the communication processing unit  513  may be implemented with the same processor or may be implemented with separate processors. The memory may be included in the one or more processors or may be provided outside the one or more processors. 
     The first base station  500  may include a memory configured to store a program (instructions) and one or more processors that can execute the program (instructions). The one or more processors may execute the program and thereby perform operations of the information obtaining unit  511  and the communication processing unit  513 . The program may be a program for causing the processor(s) to execute the operations of the information obtaining unit  511  and the communication processing unit  513 . 
     Note that the first base station  500  may be virtual. In other words, the first base station  500  may be implemented as a virtual machine. In this case, the first base station  500  (the virtual machine) may operate as a physical machine (hardware) including a processor, a memory, and the like, and a virtual machine on a hypervisor. 
     &lt;3.3. Configuration of Second Base Station  600 &gt; 
     With reference to  FIG. 16 , the second base station  600  includes a PDCP layer processing unit  611  and a communication processing unit  613 . Specific operations of the PDCP layer processing unit  611  and the communication processing unit  613  will be described later. 
     The PDCP layer processing unit  611  and the communication processing unit  613  may be implemented with one or more processors, such as a baseband (BB) processor and/or a different kind of processor, a memory (e.g., a nonvolatile memory and/or a volatile memory), and/or a hard disk. The PDCP layer processing unit  611  and the communication processing unit  613  may be implemented with the same processor or may be implemented with separate processors. The memory may be included in the one or more processors or may be provided outside the one or more processors. 
     The second base station  600  may include a memory configured to store a program (instructions) and one or more processors that can execute the program (instructions). The one or more processors may execute the program and thereby perform operations of the PDCP layer processing unit  611  and the communication processing unit  613 . The program may be a program for causing the processor(s) to execute the operations of the PDCP layer processing unit  611  and the communication processing unit  613 . 
     Note that the second base station  600  may be virtual. In other words, the second base station  600  may be implemented as a virtual machine. In this case, the second base station  600  (the virtual machine) may operate as a physical machine (hardware) including a processor, a memory, and the like, and a virtual machine on a hypervisor. 
     &lt;3.4. Technical Features&gt; 
     Technical features of the second example embodiment will be described. 
     According to the second example embodiment, the first base station  500  (information obtaining unit  511 ) obtains information for controlling a downlink data flow in an RLC layer for a terminal apparatus. The first base station  500  (communication processing unit  513 ) transmits, to the second base station  600  performing processing for a PDCP layer for the terminal apparatus, the information for controlling the downlink data flow, before the first base station  500  detects a correct random access channel access. 
     The second base station  600  (PDCP layer processing unit  611 ) executes processing for the PDCP layer. The second base station  600  (communication processing unit  613 ) receives the information for controlling the downlink data flow in the RLC from the first base station  500  performing processing in the RLC layer for the terminal apparatus, before the first base station  500  detects a correct random access channel access. 
     Relationship with First Example Embodiment 
     As an example, the first base station  500  and the second base station  600  according to the second example embodiment are the base station  100  and the base station  200  according to the first example embodiment, respectively. In this case, the descriptions of the first example embodiment may be applicable to the second example embodiment as well. 
     Note that the second example embodiment is not limited to this example. 
     The second example embodiment has been described above. According to the second example embodiment, transmission of downlink data to the RLC layer can be started with an appropriate data amount from the second base station  600  performing PDCP layer processing to the first base station  500  performing RLC layer processing. 
     4. Other Example Embodiments 
     Descriptions have been given above of the example embodiments of the present invention. However, the present invention is not limited to these example embodiments. It should be understood by those of ordinary skill in the art that these example embodiments are merely examples and that various alterations are possible without departing from the scope and the spirit of the present invention. 
     For example, the steps in the processing described in the Specification may not necessarily be executed in time series in the order described in the corresponding sequence diagram. For example, the steps in the processing may be executed in an order different from that described in the corresponding sequence diagram or may be executed in parallel. Some of the steps in the processing may be deleted, or more steps may be added to the processing. 
     An apparatus including constituent elements (for example, the information obtaining unit, the communication processing unit, the PDCP layer processing unit, and/or the RLC layer processing unit) of the base station described in the Specification (for example, one or more apparatuses (or units) among a plurality of apparatuses (or units) constituting the base station, or a module for one of the plurality of apparatuses (or units)) may be provided. Moreover, methods including processing of the constituent elements may be provided, and programs for causing a processor to execute processing of the constituent elements may be provided. Moreover, non-transitory computer readable recording media (non-transitory computer readable media) having recorded thereon the programs may be provided. It is apparent that such apparatuses, modules, methods, programs, and non-transitory computer readable recording media are also included in the present invention. 
     The whole or part of the example embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
     (Supplementary Note 1) 
     A first base station comprising: 
     an information obtaining unit configured to obtain information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and 
     a communication processing unit configured to transmit, to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, the information for controlling the downlink data flow, before the first base station detects a correct random access channel access. 
     (Supplementary Note 2) 
     The first base station according to supplementary note 1, wherein 
     the information for controlling the downlink data flow includes information related to a buffer size for downlink data in the RLC layer. 
     (Supplementary Note 3) 
     The first base station according to supplementary note 1 or 2, wherein 
     the information for controlling the downlink data flow includes information related to a data rate for downlink data in the RLC layer. 
     (Supplementary Note 4) 
     The first base station according to any one of supplementary notes 1 to 3, wherein 
     the random access channel access is performed by the terminal apparatus for a bearer for the second base station. 
     (Supplementary Note 5) 
     The first base station according to any one of supplementary notes 1 to 4, wherein 
     the first base station is a master node for dual connectivity for the terminal apparatus, and 
     the second base station is a secondary node for the dual connectivity for the terminal apparatus. 
     (Supplementary Note 6) 
     The first base station according to supplementary note 5, wherein 
     the communication processing unit is configured to transmit, to the second base station, a Secondary Node Reconfiguration Complete message during a Secondary Node Addition procedure for the terminal apparatus, and 
     the information for controlling the downlink data flow is included in the Secondary Node Reconfiguration Complete message. 
     (Supplementary Note 7) 
     The first base station according to any one of supplementary notes 1 to 4, wherein 
     the first base station is a secondary node for dual connectivity for the terminal apparatus, and 
     the second base station is a master node for the dual connectivity for the terminal apparatus. 
     (Supplementary Note 8) 
     The first base station according to supplementary note 7, wherein 
     the communication processing unit is configured to transmit, to the second base station, a response message to a Secondary Node Addition Request message from the second base station during a secondary node addition procedure for the terminal apparatus, and 
     the information for controlling the downlink data flow is included in the response message to the Secondary Node Addition Request message. 
     (Supplementary Note 9) 
     The first base station according to any one of supplementary notes 1 to 4, wherein 
     the first base station is a master node for dual connectivity for the terminal apparatus, and 
     the second base station is a central unit included in a secondary node for the dual connectivity for the terminal apparatus. 
     (Supplementary Note 10) 
     The first base station according to supplementary note 9, wherein 
     the communication processing unit is configured to transmit, to the second base station, a Secondary Node Reconfiguration Complete message during a Secondary Node Addition procedure for the terminal apparatus, and 
     the information for controlling the downlink data flow is included in the Secondary Node Reconfiguration Complete message. 
     (Supplementary Note 11) 
     The first base station according to any one of supplementary notes 1 to 4, wherein 
     the first base station is a distributed unit included in a secondary node for dual connectivity for the terminal apparatus, and 
     the second base station is a master node for the dual connectivity for the terminal apparatus. 
     (Supplementary Note 12) 
     The first base station according to supplementary note 11, wherein 
     the communication processing unit is configured to transmit, to the second base station, a response message to a Secondary Node Addition Request message from the second base station via a central unit included in the secondary node, during a secondary node addition procedure for the terminal apparatus, and 
     the information for controlling the downlink data flow is included in the response message to the Secondary Node Addition Request message. 
     (Supplementary Note 13) 
     A second base station comprising: 
     an execution unit configured to execute processing for a Packet Data Convergence Protocol (PDCP) layer; and 
     a communication processing unit configured to receive, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     (Supplementary Note 14) 
     A method comprising: 
     obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and 
     transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     (Supplementary Note 15) 
     A method comprising: 
     executing processing for a Packet Data Convergence Protocol (PDCP) layer; and 
     receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     (Supplementary Note 16) 
     A program that causes a processor to execute: 
     obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and 
     transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     (Supplementary Note 17) 
     A program that causes a processor to execute: 
     executing processing for a Packet Data Convergence Protocol (PDCP) layer; and 
     receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     (Supplementary Note 18) 
     A non-transitory computer readable recording medium storing a program that causes a processor to execute: 
     obtaining information for controlling a downlink data flow in a Radio Link Control (RLC) layer for a terminal apparatus; and 
     transmitting the information for controlling the downlink data flow to a second base station performing processing for a Packet Data Convergence Protocol (PDCP) layer for the terminal apparatus, before the first base station detects a correct random access channel access. 
     (Supplementary Note 19) 
     A non-transitory computer readable recording medium storing a program that causes a processor to execute: 
     executing processing for a Packet Data Convergence Protocol (PDCP) layer; and 
     receiving, from a first base station performing processing in a Radio Link Control (RLC) layer for the terminal apparatus, information for controlling a downlink data flow in the RLC, before the first base station detects a correct random access channel access. 
     This application claims priority based on JP 2019-146620 filed on Aug. 8, 2019, the entire disclosure of which is incorporated herein. 
     INDUSTRIAL APPLICABILITY 
     In a mobile communication system, transmission of downlink data can be started with an appropriate data amount from a base station that performs PDCP layer processing to another base station that performs RLC layer processing. 
     REFERENCE SIGNS LIST 
     
         
           1   a ,  1   b ,  1   c ,  2  System 
           100 ,  200  Base Station 
           141 ,  241 ,  511  Information Obtaining Unit 
           143 ,  243 ,  513 ,  613  Communication Processing Unit 
           145 ,  245 ,  611  PDCP Layer Processing Unit 
           147 ,  247  RLC Layer Processing Unit 
           300  Terminal Apparatus 
           500  First Base Station 
           600  Second Base Station