从UDP接收到数据后，就会组装成一个完整的数据包，然后检验整个数据包是否有效，并且还处理收到回应的UDP包标识，这样构造一个完整的可靠性连接。具体处理代码如下：
#001 BOOL LLMessageSystem::checkMessages( S64 frame_count )
#002 {
#003
// Pump
#004
BOOL valid_packet = FALSE;
#005
mMessageReader = mTemplateMessageReader;
#006
#007
LLTransferTargetVFile::updateQueue();
#008
下面保存第一次收到消息的时间。
#009
if (!mNumMessageCounts)
#010
{
#011
// This is the first message being handled after a resetReceiveCounts,
#012
// we must be starting the message processing loop. Reset the timers.
#013
mCurrentMessageTimeSeconds = totalTime() * SEC_PER_USEC;
#014
mMessageCountTime = getMessageTimeSeconds();
#015
}
#016
下面开始循环处理所有收到的消息包。
#017
// loop until either no packets or a valid packet
#018
// i.e., burn through packets from unregistered circuits
#019
S32 receive_size = 0;
#020
do
#021
{
#022
clearReceiveState();
#023
#024
BOOL recv_reliable = FALSE;
#025
BOOL recv_resent = FALSE;
#026
S32 acks = 0;
#027
S32 true_rcv_size = 0;
#028
#029
U8* buffer = mTrueReceiveBuffer;
#030
从底层环路里接收数据包。
#031
mTrueReceiveSize = mPacketRing.receivePacket(mSocket, (char *)mTrueReceiveBuffer);
#032
// If you want to dump all received packets into SecondLife.log, uncomment this
#033
//dumpPacketToLog();
#034
获取数据包的大小和发送的服务器地址。
#035
receive_size = mTrueReceiveSize;
#036
mLastSender = mPacketRing.getLastSender();
#037
下面检验数据包的长度是否有效，如果无效就处理下一个数据包。
#038
if (receive_size < (S32) LL_MINIMUM_VALID_PACKET_SIZE)
#039
{
#040
// A receive size of zero is OK, that means that there are no more packets available.
#041
// Ones that are non-zero but below the minimum packet size are worrisome.
#042
if (receive_size > 0)
#043
{
#044
llwarns << "Invalid (too short) packet discarded " << receive_size << llendl;
#045
callExceptionFunc(MX_PACKET_TOO_SHORT);
#046
}
#047
// no data in packet receive buffer
#048
valid_packet = FALSE;
#049
}
#050
else
#051
{
#052
LLHost host;
#053
LLCircuitData* cdp;
#054
判断这个数据包后面是否包含发送数据包的回应ID，这样可以处理可靠性的包发送，不像包丢失。
#055
// note if packet acks are appended.
#056
if(buffer[0] & LL_ACK_FLAG)
#057
{
#058
acks += buffer[--receive_size];
#059
true_rcv_size = receive_size;
#060
if(receive_size >= ((S32)(acks * sizeof(TPACKETID) + LL_MINIMUM_VALID_PACKET_SIZE)))
#061
{
#062
receive_size -= acks * sizeof(TPACKETID);
#063
}
#064
else
#065
{
#066
// mal-formed packet. ignore it and continue with
#067
// the next one
#068
llwarns << "Malformed packet received. Packet size "
#069
<< receive_size << " with invalid no. of acks " << acks
#070
<< llendl;
#071
valid_packet = FALSE;
#072
continue;
#073
}
#074
}
#075
下面解压缩包数据。
#076
// process the message as normal
#077
mIncomingCompressedSize = zeroCodeExpand(&buffer, &receive_size);
#078
mCurrentRecvPacketID = ntohl(*((U32*)(&buffer[1])));
#079
host = getSender();
#080
找到这个HOST的环路，然后让这个环路管理类处理。
#081
const bool resetPacketId = true;
#082
cdp = findCircuit(host, resetPacketId);
#083
#084
// At this point, cdp is now a pointer to the circuit that
#085
// this message came in on if it's valid, and NULL if the
#086
// circuit was bogus.
#087
处理那些服务器已经收到回应的ID。
#088
if(cdp && (acks > 0) && ((S32)(acks * sizeof(TPACKETID)) < (true_rcv_size)))
#089
{
#090
TPACKETID packet_id;
#091
U32 mem_id=0;
#092
for(S32 i = 0; i < acks; ++i)
#093
{
#094
true_rcv_size -= sizeof(TPACKETID);
#095
memcpy(&mem_id, &mTrueReceiveBuffer[true_rcv_size], /* Flawfinder: ignore*/
#096
sizeof(TPACKETID));
#097
packet_id = ntohl(mem_id);
#098
//llinfos << "got ack: " << packet_id << llendl;
#099
cdp->ackReliablePacket(packet_id);
#100
}
#101
if (!cdp->getUnackedPacketCount())
#102
{
#103
// Remove this circuit from the list of circuits with unacked packets
#104
mCircuitInfo.mUnackedCircuitMap.erase(cdp->mHost);
#105
}
#106
}
#107
查看这个包是否需要确认的处理的。
#108
if (buffer[0] & LL_RELIABLE_FLAG)
#109
{
#110
recv_reliable = TRUE;
#111
}
#112
if (buffer[0] & LL_RESENT_FLAG)
#113
{
#114
recv_resent = TRUE;
#115
if (cdp && cdp->isDuplicateResend(mCurrentRecvPacketID))
#116
{
#117
// We need to ACK here to suppress
#118
// further resends of packets we've
#119
// already seen.
#120
if (recv_reliable)
#121
{
#122
//mAckList.addData(new LLPacketAck(host, mCurrentRecvPacketID));
#123
// ***************************************
#124
// TESTING CODE
#125
//if(mCircuitInfo.mCurrentCircuit->mHost != host)
#126
//{
#127
// llwarns << "DISCARDED PACKET HOST MISMATCH! HOST: "
#128
// << host << " CIRCUIT: "
#129
// << mCircuitInfo.mCurrentCircuit-
#130 >mHost
#131
// << llendl;
#132
//}
#133
// ***************************************
#134
//mCircuitInfo.mCurrentCircuit->mAcks.put(mCurrentRecvPacketID);
#135
cdp->collectRAck(mCurrentRecvPacketID);
#136
}
上面把收到服务器的包ID也保存到下一次回应包队列里，准备返回去给服务器确认。
#137
#138
//llinfos << "Discarding duplicate resend from " << host << llendl;
#139
if(mVerboseLog)
#140
{
#141
std::ostringstream str;
#142
str << "MSG: <- " << host;
#143
char buffer[MAX_STRING]; /* Flawfinder: ignore*/
#144
snprintf(buffer, MAX_STRING, "/t%6d/t%6d/t%6d ", receive_size,
#145 (mIncomingCompressedSize ? mIncomingCompressedSize : receive_size), mCurrentRecvPacketID); /* Flawfinder: ignore */
#146
str << buffer << "(unknown)"
#147
<< (recv_reliable ? " reliable" : "")
#148
<< " resent "
#149
<< ((acks > 0) ? "acks" : "")
#150
<< " DISCARD DUPLICATE";
#151
llinfos << str.str() << llendl;
#152
}
#153
mPacketsIn++;
#154
valid_packet = FALSE;
#155
continue;
#156
}
#157
}
#158
通过上面这段代码的学习，可以看到UDP的可靠性，就是通过自己的包ID来作确认的。主要通过包的长度来查看包是否完整，然后通过包的标识来查看这个数据包后面是否有服务器收到的包ID发回来确认。因为有些数据是经过压缩的，所以在这里也调用函数zeroCodeExpand来解压。
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