场景介绍
如果需要使用UDP传输较大数据，例如传输10M的图片，这突破了UDP的设计原则。UDP的设计是基于"datagram"，也就是它假设你发送的每个数据包都能包含在单一的包内。并且设定UDP数据包的最大长度受基础网络协议的限制。

UDP数据包的理论最大长度限制是 65535 bytes，这包含 8 bytes 数据包头和 65527 bytes 数据。但如果基于IPv4网络传输，则还需减去 20 bytes 的IP数据包头。
则单一的UDP数据包可传输的数据最大长度为：

则单一的UDP数据包可传输的数据最大长度为：

MaxUdpDataLength = 65535 - 8 - 20 = 65507 bytes

这就需要实现UDP包的分包传输和接收组包功能。

分包功能
复制代码 代码如下:
/// <summary>
/// UDP数据包分割器
/// </summary>
public static class UdpPacketSplitter
{
/// <summary>
/// 分割UDP数据包
/// </summary>
/// <param name="sequence">UDP数据包所持有的序号</param>
/// <param name="datagram">被分割的UDP数据包</param>
/// <param name="chunkLength">分割块的长度</param>
/// <returns>
/// 分割后的UDP数据包列表
/// </returns>
public static ICollection<UdpPacket> Split(long sequence, byte[] datagram, int chunkLength)
{
if (datagram == null)
throw new ArgumentNullException("datagram");

List<UdpPacket> packets = new List<UdpPacket>();

int chunks = datagram.Length / chunkLength;
int remainder = datagram.Length % chunkLength;
int total = chunks;
if (remainder > 0) total++;

for (int i = 1; i <= chunks; i++)
{
byte[] chunk = new byte[chunkLength];
Buffer.BlockCopy(datagram, (i - 1) * chunkLength, chunk, 0, chunkLength);
packets.Add(new UdpPacket(sequence, total, i, chunk, chunkLength));
}
if (remainder > 0)
{
int length = datagram.Length - (chunkLength * chunks);
byte[] chunk = new byte[length];
Buffer.BlockCopy(datagram, chunkLength * chunks, chunk, 0, length);
packets.Add(new UdpPacket(sequence, total, total, chunk, length));
}

return packets;
}
}

发送分包
复制代码 代码如下:
private void WorkThread()
{
while (IsRunning)
{
waiter.WaitOne();
waiter.Reset();

while (queue.Count > 0)
{
StreamPacket packet = null;
if (queue.TryDequeue(out packet))
{
RtpPacket rtpPacket = RtpPacket.FromImage(
RtpPayloadType.JPEG,
packet.SequenceNumber,
(long)Epoch.GetDateTimeTotalMillisecondsByYesterday(packet.Timestamp),
packet.Frame);

// max UDP packet length limited to 65,535 bytes
byte[] datagram = rtpPacket.ToArray();
packet.Frame.Dispose();

// split udp packet to many packets
// to reduce the size to 65507 limit by underlying IPv4 protocol
ICollection<UdpPacket> udpPackets
= UdpPacketSplitter.Split(
packet.SequenceNumber,
datagram,
65507 - UdpPacket.HeaderSize);
foreach (var udpPacket in udpPackets)
{
byte[] udpPacketDatagram = udpPacket.ToArray();
// async sending
udpClient.BeginSend(
udpPacketDatagram, udpPacketDatagram.Length,
packet.Destination.Address,
packet.Destination.Port,
SendCompleted, udpClient);
}
}
}
}
}

接收组包功能
复制代码 代码如下:
private void OnDatagramReceived(object sender, UdpDatagramReceivedEventArgs<byte[]> e)
{
try
{
UdpPacket udpPacket = UdpPacket.FromArray(e.Datagram);

if (udpPacket.Total == 1)
{
RtpPacket packet = new RtpPacket(udpPacket.Payload, udpPacket.PayloadSize);
Bitmap bitmap = packet.ToBitmap();
RaiseNewFrameEvent(
bitmap, Epoch.GetDateTimeByYesterdayTotalMilliseconds(packet.Timestamp));
}
else
{
// rearrange packets to one packet
if (packetCache.ContainsKey(udpPacket.Sequence))
{
List<UdpPacket> udpPackets = null;
if (packetCache.TryGetValue(udpPacket.Sequence, out udpPackets))
{
udpPackets.Add(udpPacket);

if (udpPackets.Count == udpPacket.Total)
{
packetCache.TryRemove(udpPacket.Sequence, out udpPackets);

udpPackets = udpPackets.OrderBy(u => u.Order).ToList();
int rtpPacketLength = udpPackets.Sum(u => u.PayloadSize);
int maxPacketLength = udpPackets.Select(u => u.PayloadSize).Max();

byte[] rtpPacket = new byte[rtpPacketLength];
foreach (var item in udpPackets)
{
Buffer.BlockCopy(
item.Payload, 0, rtpPacket,
(item.Order - 1) * maxPacketLength, item.PayloadSize);
}

RtpPacket packet = new RtpPacket(rtpPacket, rtpPacket.Length);
Bitmap bitmap = packet.ToBitmap();
RaiseNewFrameEvent(
bitmap,
Epoch.GetDateTimeByYesterdayTotalMilliseconds(packet.Timestamp));

packetCache.Clear();
}
}
}
else
{
List<UdpPacket> udpPackets = new List<UdpPacket>();
udpPackets.Add(udpPacket);
packetCache.AddOrUpdate(
udpPacket.Sequence,
udpPackets, (k, v) => { return udpPackets; });
}
}
}
catch (Exception ex)
{
RaiseVideoSourceExceptionEvent(ex.Message);
}
}