BGP 在两台路由器之间建立 BGP 对等体的过程中。有以下交流。让我有点困惑的是，在每个 UPDATE Message 之后，同一个数据包中有两个 KEEPALIVE 消息。知道为什么吗？

**Source       Destination  Proto  Length  Message**
ROUTER_A --> ROUTER_B      BGP    99     OPEN Msg
ROUTER_B --> ROUTER_A      BGP    118    OPEN Msg, KEEPALIVE Msg
ROUTER_A --> ROUTER_B      BGP    73     KEEPALIVE Msg
ROUTER_A --> ROUTER_B      BGP    106    UPDATE Msg
ROUTER_A --> ROUTER_B      BGP    92     KEEPALIVE Msg, KEEPALIVE Msg

Border Gateway Protocol - KEEPALIVE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 19
    Type: KEEPALIVE Message (4)
Border Gateway Protocol - KEEPALIVE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 19
    Type: KEEPALIVE Message (4)

ROUTER_B --> ROUTER_A      BGP    106    UPDATE Msg
ROUTER_B --> ROUTER_A      BGP    92     KEEPALIVE Msg, KEEPALIVE Msg

Border Gateway Protocol - KEEPALIVE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 19
    Type: KEEPALIVE Message (4)
Border Gateway Protocol - KEEPALIVE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 19
    Type: KEEPALIVE Message (4)