Windows 10 and above support the SIO_TCP_INFO control code for querying the statistics for a socket handle including the number of retransmissions:

• Win10: https://learn.microsoft.com/en-us/windows/win32/api/mstcpip/ns-mstcpip-tcp_info_v0
• Win11: https://learn.microsoft.com/en-us/windows/win32/api/mstcpip/ns-mstcpip-tcp_info_v1

typedef struct _TCP_INFO_v0 {
  TCPSTATE State;
  ULONG    Mss;
  ULONG64  ConnectionTimeMs;
  BOOLEAN  TimestampsEnabled;
  ULONG    RttUs;
  ULONG    MinRttUs;
  ULONG    BytesInFlight;
  ULONG    Cwnd;
  ULONG    SndWnd;
  ULONG    RcvWnd;
  ULONG    RcvBuf;
  ULONG64  BytesOut;
  ULONG64  BytesIn;
  ULONG    BytesReordered;
  ULONG    BytesRetrans;
  ULONG    FastRetrans;
  ULONG    DupAcksIn;
  ULONG    TimeoutEpisodes;
  UCHAR    SynRetrans;
} TCP_INFO_v0, *PTCP_INFO_v0;

TCP_INFO_v0 tcpInfo;
ULONG version = 0; // Specify 0 to retrieve the v0 version of this structure.
ULONG bytesReturned;

WSAIoctl(
   SocketHandle, 
   SIO_TCP_INFO, 
   &version, 
   sizeof(ULONG), 
   &tcpInfo, 
   sizeof(TCP_INFO_v0),
   &bytesReturned, 
   NULL, 
   NULL
   );