第二人生的源码分析三十二消息解包的实现

最新推荐文章于 2022-10-24 15:59:32 发布

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最新推荐文章于 2022-10-24 15:59:32 发布

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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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第二人生的源码分析 三十二 消息解包的实现

第二人生的源码分析三十二消息解包的实现