UDP的recvfrom会返回一个完整的数据报

1.问题

在阅读Luanti服务器网络部分的代码时，发现未使用tcp连接，而是基于udp实现的可靠连接，读到如下代码，m_udpSocket.Receive后，就直接判断，若received_size < BASE_HEADER_SIZE就认为是错误了。基于本人之前很少些UDP程序，写的是TCP的，就有疑惑，这里如何保证收到的收据包肯定大于BASE_HEADER_SIZE？

		// Call Receive() to wait for incoming dataAddress sender;s32 received_size = m_connection->m_udpSocket.Receive(sender,*packetdata, packetdata.getSize());if (received_size < 0)return;if ((received_size < BASE_HEADER_SIZE) ||(readU32(&packetdata[0]) != m_connection->GetProtocolID())) {LOG(derr_con << m_connection->getDesc()<< "Receive(): Invalid incoming packet, "<< "size: " << received_size<< ", protocol: "<< ((received_size >= 4) ? readU32(&packetdata[0]) : -1)<< std::endl);return;}session_t peer_id = readPeerId(*packetdata);u8 channelnum = readChannel(*packetdata);if (channelnum > CHANNEL_COUNT - 1) {LOG(derr_con << m_connection->getDesc()<< "Receive(): Invalid channel " << (u32)channelnum << std::endl);return;}

2. 寻找答案

1. UDP 套接字（SOCK_DGRAM）
   数据报边界：UDP 是面向消息的协议，每次 recvfrom 会返回一个完整的数据报（即对端 sendto 发送的单个数据块）。

2. TCP 套接字（SOCK_STREAM）
   字节流特性：TCP 无消息边界，recvfrom 返回的数据量由内核缓冲区中的可用数据和接收缓冲区大小决定。

MSDN上的描述：

For stream-oriented sockets such as those of type SOCK_STREAM, a call
to recvfrom returns as much information as is currently available—up
to the size of the buffer specified. If the socket has been configured
for inline reception of OOB data (socket option SO_OOBINLINE) and OOB
data is yet unread, only OOB data will be returned. The application
can use the ioctlsocket or WSAIoctl SIOCATMARK command to determine
whether any more OOB data remains to be read. The from and fromlen
parameters are ignored for connection-oriented sockets.

For message-oriented sockets, data is extracted from the first
enqueued message, up to the size of the buffer specified. If the
datagram or message is larger than the buffer specified, the buffer is
filled with the first part of the datagram, and recvfrom generates the
error WSAEMSGSIZE. For unreliable protocols (for example, UDP) the
excess data is lost. For UDP if the packet received contains no data
(empty), the return value from the recvfrom function function is zero.

MSDN译文：

• 面向流的套接字（如 SOCK_STREAM 类型）相关
  对面向流的套接字调用 recvfrom 时，会返回当前可用的、最多达到指定缓冲区大小的信息量。如果套接字已配置为内联接收带外（OOB）数据（套接字选项 SO_OOBINLINE）且存在未读的带外数据，则仅返回带外数据。应用程序可以使用 ioctlsocket 或 WSAIoctl 的 SIOCATMARK 命令来确定是否还有更多带外数据有待读取。对于面向连接的套接字，from 和 fromlen 参数会被忽略。

• 面向消息的套接字（数据提取方面）相关
  对于面向消息的套接字，数据从第一个排队的消息中提取，最多提取到指定缓冲区大小的数据量。如果数据报或消息大于指定的缓冲区，则缓冲区将填充数据报的部分内容，剩下的则丢弃，并且 recvfrom 会生成 WSAEMSGSIZE 错误。对于不可靠协议（例如 UDP），多余的数据会丢失。对于 UDP，如果接收到的数据包不包含数据（为空），recvfrom 函数的返回值是零。