为什么SChannel TLS将消息限制为32kb

我正在使用SChannel构建客户机/服务器程序。我想做的事情之一就是进行文件共享。我发现了一些使用Schannel进行通信的客户端程序的示例代码，我想知道为什么消息的最大大小是32kb。下面是执行接收的示例函数

int tls_handshake(tls_ctx *c, tls_session *s) {
    DWORD         flags_in, flags_out;
    SecBuffer     ib[2], ob[1];
    SecBufferDesc in, out;
    int           len;

    // send initial hello
    if (!tls_hello(c, s)) {
      return 0;
    }

    flags_in = ISC_REQ_REPLAY_DETECT   |
               ISC_REQ_CONFIDENTIALITY |
               ISC_RET_EXTENDED_ERROR  |
               ISC_REQ_ALLOCATE_MEMORY |
               ISC_REQ_MANUAL_CRED_VALIDATION;

    c->ss     = SEC_I_CONTINUE_NEEDED;
    s->buflen = 0;

    while (c->ss == SEC_I_CONTINUE_NEEDED ||
           c->ss == SEC_E_INCOMPLETE_MESSAGE ||
           c->ss == SEC_I_INCOMPLETE_CREDENTIALS)
    {
      if (c->ss  == SEC_E_INCOMPLETE_MESSAGE)
      {
        // receive data from server
        len = recv(s->sck, &s->buf[s->buflen], s->maxlen - s->buflen, 0);

        // socket error?
        if (len  == SOCKET_ERROR) {
          c->ss  =  SEC_E_INTERNAL_ERROR;
          break;
        // server disconnected?
        } else if (len==0) {
          c->ss = SEC_E_INTERNAL_ERROR;
          break;
        }
        // increase buffer position
        s->buflen += len;
      }

      // inspect what we've received
      //tls_hex_dump(s->buf, s->buflen);

      // input data
      ib[0].pvBuffer   = s->buf;
      ib[0].cbBuffer   = s->buflen;
      ib[0].BufferType = SECBUFFER_TOKEN;

      // empty buffer
      ib[1].pvBuffer   = NULL;
      ib[1].cbBuffer   = 0;
      ib[1].BufferType = SECBUFFER_VERSION;

      in.cBuffers      = 2;
      in.pBuffers      = ib;
      in.ulVersion     = SECBUFFER_VERSION;

      // output from schannel
      ob[0].pvBuffer   = NULL;
      ob[0].cbBuffer   = 0;
      ob[0].BufferType = SECBUFFER_VERSION;

      out.cBuffers     = 1;
      out.pBuffers     = ob;
      out.ulVersion    = SECBUFFER_VERSION;

      c->ss = c->sspi->
        InitializeSecurityContextA(
        &s->cc, &s->ctx, NULL, flags_in, 0,
        SECURITY_NATIVE_DREP, &in, 0, NULL,
        &out, &flags_out, NULL);

      // what have we got so far?
      if (c->ss == SEC_E_OK ||
          c->ss == SEC_I_CONTINUE_NEEDED ||
          (FAILED(c->ss) && (flags_out & ISC_RET_EXTENDED_ERROR)))
      {
        // response for server?
        if (ob[0].cbBuffer != 0 && ob[0].pvBuffer) {
          // send response
          tls_send(s->sck, ob[0].pvBuffer, ob[0].cbBuffer);
          // free response
          c->sspi->FreeContextBuffer(ob[0].pvBuffer);
          ob[0].pvBuffer = NULL;
        }
      }
      // incomplete message? continue reading
      if (c->ss==SEC_E_INCOMPLETE_MESSAGE) continue;

      // completed handshake?
      if (c->ss==SEC_E_OK) {
        s->established = 1;

        // If the "extra" buffer contains data, this is encrypted application
        // protocol layer stuff and needs to be saved. The application layer
        // will decrypt it later with DecryptMessage.
        if (ib[1].BufferType == SECBUFFER_EXTRA) {
          DEBUG_PRINT("  [ we have extra data after handshake.\n");
          memmove(s->pExtra.pvBuffer,
              &s->buf[(s->buflen - ib[1].cbBuffer)], ib[1].cbBuffer);

          s->pExtra.cbBuffer   = ib[1].cbBuffer;
          s->pExtra.BufferType = SECBUFFER_TOKEN;
        } else {
          // no extra data encountered
          s->pExtra.pvBuffer   = NULL;
          s->pExtra.cbBuffer   = 0;
          s->pExtra.BufferType = SECBUFFER_EMPTY;
        }
        break;
      }
      // some other error
      if(FAILED(c->ss)) break;

      // Copy any leftover data from the "extra" buffer, and go around again.
      if(ib[1].BufferType == SECBUFFER_EXTRA) {
        memmove(s->buf, &s->buf[(s->buflen - ib[1].cbBuffer)], ib[1].cbBuffer);
        s->buflen = ib[1].cbBuffer;
        DEBUG_PRINT("  [ we have %i bytes of extra data.\n", s->buflen);

        tls_hex_dump(s->buf, s->buflen);

      } else {
        s->buflen = 0;
      }
    }
    return c->ss==SEC_E_OK ? 1 : 0;
}

代码来自我在这里找到的Github：

在他定义的一个头文件中

#define TLS_MAX_BUFSIZ      32768

---

我在其他地方也读到，这是TLS的一个限制。有可能提高这个限制吗？如果我需要得到更多，会发生什么？像一个大文件一样？

所有基于TCP的协议（包括TLS）都是基于帧的：无论您拥有多少数据，都会将其拆分为给定数量的数据包，这样实际上您可以发送无限量的数据，并且不会因每个数据包的大小而发生任何变化（性能除外）。TLS 1.3说“记录层将信息块分割成TLSPlaintext记录，其中包含2^14字节或更少的数据块。”如何读取发送的数据包的正确字节数？通常在我的包里我会有一个长度。但由于这是调用recv，并且数据是加密的，所以它不知道要recv多少。