udp
UDT是建立在UDP协议基础之上的应用层协议，其最终是通过UDP协议来接发数据。
最近在看udt协议的一些知识，先把包结构放上来
UDT包结构
总体结构：
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
|
Packet Header(包头) |
|
Data/Control Information Field(数据包/控制包 信息) |
数据包包头结构：
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 | Sequence Number |
|
ff |o | Message Number |
|
Time Stamp |
|
Destination Socket ID |
bit 0:
0: Data Packet(0：数据包)
1: Control Packet（1：控制包）
bit ff:
11: solo message packet（11：单一的消息包）
10: first packet of a message（10：一份消息的第一个包）
01: last packet of a message（00：一份消息的最后一个包）
bit o:
0: in order delivery not required（0：没有要求按正常序号传递）
1: in order delivery required（1：要求按正常序号传递）
控制包包头结构：
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 |
|
Additional Info |
|
Time Stamp |
|
Destination Socket ID |
bit 0:
0: Data Packet(0：数据包)
1: Control Packet（1：控制包）
bit 1~15:
0: Protocol Connection Handshake
Add. Info: Undefined
Control Info: Handshake information (see CHandShake)
1: Keep-alive
Add. Info: Undefined
Control Info: None
2: Acknowledgement (ACK)
Add. Info: The ACK sequence number
Control Info: The sequence number to which (but not include) all the previous packets have beed received
Optional: RTT
RTT Variance
advertised flow window size (number of packets)
estimated bandwidth (number of packets per second)
3: Negative Acknowledgement (NAK)
Add. Info: Undefined
Control Info: Loss list (see loss list coding below)
4: Congestion Warning
Add. Info: Undefined
Control Info: None
5: Shutdown
Add. Info: Undefined
Control Info: None
6: Acknowledgement of Acknowledement (ACK-square)
Add. Info: The ACK sequence number
Control Info: None
7: Message Drop Request
Add. Info: Message ID
Control Info: first sequence number of the message
last seqeunce number of the message
65535: Explained by bits 16 - 31
bit 16 - 31:
This space is used for future expansion or user defined control packets.
在UDT的实现中，是通过类CChannel来处理的，顾名思义，可以理解为通过UDP管道来接发数据。
来看看CChannel提供的主要方法：
void setSndBufSize(const int& size);
void setRcvBufSize(const int& size);
void open(const sockaddr* addr = NULL);
int sendto(const sockaddr* addr, CPacket& packet) const;
int recvfrom(sockaddr* addr, CPacket& packet) const;
注：CPacket即UDP包结构（数据包与控制包）
#ifndef WIN32
int m_iSocket; // socket descriptor
#else
SOCKET m_iSocket;
#endif
int m_iSndBufSize; // UDP sending buffer size
int m_iRcvBufSize; // UDP receiving buffer size
setSndBufSize与setRcvBufSize分别设置发送和接收缓冲区大小，即对m_iSndBufSize和m_iRcvBufSize赋值，主要看看open函数：
void CChannel::open(const sockaddr* addr)
{
// construct an socket
m_iSocket = socket(m_iIPversion, SOCK_DGRAM, 0);
if (NULL != addr)
{
socklen_t namelen = (AF_INET == m_iIPversion) ? sizeof(sockaddr_in) :sizeof(sockaddr_in6);
if (0 != bind(m_iSocket, addr, namelen))
//error
}
else
{
//sendto or WSASendTo will also automatically bind the socket
addrinfo hints;
addrinfo* res;
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_flags = AI_PASSIVE;
hints.ai_family = m_iIPversion;
hints.ai_socktype = SOCK_DGRAM;
if (0 != getaddrinfo(NULL, "0", &hints, &res))
//error
if (0 != bind(m_iSocket, res->ai_addr, res->ai_addrlen))
//error
freeaddrinfo(res);
}
setUDPSockOpt(); //调用TCP/IP协议栈提供的方法设置UDP选项（缓冲区大小）
}
void CChannel::setUDPSockOpt()
{
if ((0 != setsockopt(m_iSocket, SOL_SOCKET, SO_RCVBUF, (char *)&m_iRcvBufSize, sizeof(int))) ||(0 != setsockopt(m_iSocket, SOL_SOCKET, SO_SNDBUF, (char *)&m_iSndBufSize,sizeof(int))))
//error
#ifdef WIN32
DWORD ot = 1; //milliseconds
if (setsockopt(m_iSocket, SOL_SOCKET, SO_RCVTIMEO, (char *)&ot, sizeof(DWORD)) < 0)
throw CUDTException(1, 3, NET_ERROR);
#else
// Set receiving time-out value
if (setsockopt(m_iSocket, SOL_SOCKET, SO_RCVTIMEO, (char *)&tv, sizeof(timeval)) < 0)
throw CUDTException(1, 3, NET_ERROR);
#endif
}
下面看看sendto函数，该函数是把CPacket内容通过udp发送出去，分linux系与win32两种情况。
int CChannel::sendto(const sockaddr* addr, CPacket& packet) const
{
// 主机序到网络序
if (packet.getFlag())//如果是控制包
for (int i = 0, n = packet.getLength() / 4; i < n; ++ i)
*((uint32_t *)packet.m_pcData + i) = htonl(*((uint32_t *)packet.m_pcData + i));
// convert packet header into network order(包头序也需要转换)
for (int j = 0; j < 4; ++ j)
packet.m_nHeader[j] = htonl(packet.m_nHeader[j]);
#ifndef WIN32
msghdr mh;
mh.msg_name = (sockaddr*)addr;
mh.msg_namelen = (AF_INET == m_iIPversion) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
mh.msg_iov = (iovec*)packet.m_PacketVector;
mh.msg_iovlen = 2;
mh.msg_control = NULL;
mh.msg_controllen = 0;
mh.msg_flags = 0;
int res = sendmsg(m_iSocket, &mh, 0); //调用协议栈函数发送出去
#else
DWORD size = CPacket::m_iPktHdrSize + packet.getLength();
int addrsize = (AF_INET == m_iIPversion) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
int res = WSASendTo(m_iSocket, (LPWSABUF)packet.m_PacketVector, 2, &size, 0, addr, addrsize, NULL, NULL);
res = (0 == res) ? size : -1;
#endif
// 恢复序
for (int k = 0; k < 4; ++ k)
packet.m_nHeader[k] = ntohl(packet.m_nHeader[k]);
if (packet.getFlag())
for (int l = 0, n = packet.getLength() / 4; l < n; ++ l)
*((uint32_t *)packet.m_pcData + l) = ntohl(*((uint32_t *)packet.m_pcData + l));
return res;
}
recvfrom的原理也类似与sendto
int CChannel::recvfrom(sockaddr* addr, CPacket& packet) const
{
#ifndef WIN32
msghdr mh;
mh.msg_name = addr;
mh.msg_namelen = (AF_INET == m_iIPversion) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
mh.msg_iov = packet.m_PacketVector;
mh.msg_iovlen = 2;
mh.msg_control = NULL;
mh.msg_controllen = 0;
mh.msg_flags = 0;
#ifdef UNIX
fd_set set;
timeval tv;
FD_ZERO(&set);
FD_SET(m_iSocket, &set);
tv.tv_sec = 0;
tv.tv_usec = 10000;
select(m_iSocket+1, &set, NULL, &set, &tv);
#endif
int res = recvmsg(m_iSocket, &mh, 0);
#else
DWORD size = CPacket::m_iPktHdrSize + packet.getLength();
DWORD flag = 0;
int addrsize = (AF_INET == m_iIPversion) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
int res = WSARecvFrom(m_iSocket, (LPWSABUF)packet.m_PacketVector, 2, &size, &flag, addr, &addrsize, NULL, NULL);
res = (0 == res) ? size : -1;
#endif
if (res <= 0)
{
packet.setLength(-1);
return -1;
}
packet.setLength(res - CPacket::m_iPktHdrSize);
// convert back into local host order
for (int i = 0; i < 4; ++ i)
packet.m_nHeader[i] = ntohl(packet.m_nHeader[i]);
if (packet.getFlag())
for (int j = 0, n = packet.getLength() / 4; j < n; ++ j)
*((uint32_t *)packet.m_pcData + j) = ntohl(*((uint32_t *)packet.m_pcData + j));
return packet.getLength();
}
