udt的包结构
1. 数据包，基本结构如下：
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| Packet Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Application Data Payload ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
第1位为0表示是数据包，后续31位为序号，表示范围是0-(2^31 - 1)
udt-java里数据包的类DataPacket，实际的属性如下：
// 包序号， 4字节的低31位表示
private long packetSequenceNumber;
// 消息号， 4字节
private long messageNumber;
// 时间戳， 4字节
private long timeStamp;
// 对等端链接的ID， 4字节
private long destinationID;
// 实际数据长度（字节）
private int dataLength;
// 实际的数据
private byte[] data ;
2. 控制包，基本结构如下：
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|type | Reserved | ACK Seq. No. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Control Information Field ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
第1位为1表示是控制包，接下来的三位表示控制包类型。
UDT的ACK包使用子序号，每个ACK/ACK2包使用一个16位的序列号，使用16-31位表示，能够
表示的序号范围是0 - (2^16 - 1)。其他的控制包无子序号。
udt-java中所有控制包的基类ControlPacket，都存在的header(16字节）
// 4字节
System.arraycopy(PacketUtil.encodeControlPacketType(controlPacketType), 0, res, 0, 4);
// 4字节，额外信息
System.arraycopy(PacketUtil.encode(getAdditionalInfo()), 0, res, 4, 4);
// 4字节，时间戳
System.arraycopy(PacketUtil.encode(timeStamp), 0, res, 8, 4);
// 4字节，对端的链接的id
System.arraycopy(PacketUtil.encode(destinationID), 0, res, 12, 4);
udt-java中的控制包类型的定义：
public static enum ControlPacketType {
CONNECTION_HANDSHAKE,
KEEP_ALIVE,
ACK,
NAK,
UNUNSED_1,
SHUTDOWN,
ACK2,
MESSAGE_DROP_REQUEST,
UNUNSED_2,
UNUNSED_3,
UNUNSED_4,
UNUNSED_5,
UNUNSED_6,
UNUNSED_7,
UNUNSED_8,
USER_DEFINED,
}
控制包的类型及具体的包结构如下：
TYPE 000: Protocol Connection Handshake 握手控制包
Control Info:
1) 32 bits: UDT version
2) 32 bits: initial sequence number
3) 32 bits: MSS (bytes)
4) 32 bits: maximum flow window size (bytes)
udt-java中握手包ConnectionHandshake类的主要属性：
// udt版本 4字节
private long udtVersion=4;
// socket的类型，4字节
private long socketType= SOCKET_TYPE_DGRAM; //stream or dgram
// 初始序列号 4字节
private long initialSeqNo = 0;
// 包大小-MSS大小 4字节
private long packetSize;
// 流量窗口大小 4字节
private long maxFlowWndSize;
// 链接类型 4字节
private long connectionType = CONNECTION_TYPE_REGULAR;//regular or rendezvous mode
// socket id 4字节
private long socketID;
// cookie信息
private long cookie=0;
//address of the UDP socket
private InetAddress address;
TYPE 001: Keep-alive
Control Info: None
udt-java中的KeepAlive类
TYPE 010: Acknowledgement (ACK)
bits 16-31: ACK sequence number
Control Info:
1) 32 bits: The packet sequence number to which (excluding)
all the previous packets have been received
2) 32 bits: RTT (microseconds)
3) 32 bits: RTT variance, or RTTVar (microseconds)
4) 32 bits: Flow window size (number of packets)
5) 32 bits: Estimated link capacity (number of packets per
second)
udt-java中Acknowledgement类：
//the ack sequence number子序列
private long ackSequenceNumber ;
//the packet sequence number to which all the previous packets have been received (excluding)
// 接收到的包的序号
private long ackNumber ;
//round-trip time in microseconds(RTT)
private long roundTripTime;
// RTT variance
private long roundTripTimeVariance;
//Available buffer size (in bytes)
private long bufferSize;
//packet receivind rate in number of packets per second
private long pktArrivalSpeed;
//estimated link capacity in number of packets per second
private long estimatedLinkCapacity;
TYPE 011: Negative Acknowledgement (NAK)
Control Info:
32-bit integer array of compressed loss information
udt-java中的NegativeAcknowledgement类，丢包的控制包：
//after decoding this contains the lost sequence numbers
List<Integer>lostSequenceNumbers;
TYPE 100: Reserved.
This type of control message is reserved for congestion
warning that can be sent from the receiver to the sender. A
congestion warning can be triggered by ECN, or a
measurement of increasing trend in the packet delay.
TYPE 101: Shutdown
Control Info: None
udt-java中的Shutdown类
TYPE 110: Acknowledgement of Acknowledgement (ACK2)
bits 16-31: ACK sequence number
Control Info: None
对确认包的确认，udt-java中的类Acknowledgment2，包含的属性
//the ack sequence number
private long ackSequenceNumber ;
TYPE 111: Explained by bits 4 - 15, reserved for future use.
3. udt-java中的packet的解析类PacketFactory
public static UDTPacket createPacket(byte[]encodedData,int length)throws IOException{
// 判断是否是控制包
boolean isControl=(encodedData[0]&128) !=0 ;
// 解析控制包
if(isControl)return createControlPacket(encodedData,length);
// 返回数据包
return new DataPacket(encodedData,length);
}