第二人生的源码分析(二十八)UDP发送数据的可靠性控制

来源：互联网发布：java菜单功能管理系统编辑：程序博客网时间：2024/05/14 13:49

学习过网络编程的人，应该都知道UDP是“不可靠”的协议。不知道你有没有想过UDP为什么不可靠，不可靠在那些方面。其实，UDP和TCP协议一样都是建立在不可靠的IP协议之上，UDP的不可靠是指它不具备流量控制，不具备数据包顺序达到，不具备验证数据包是否丢失。那么在第二人生里使用UDP协议又是怎么样来实现可靠的数据传送的呢？现在就来分析下面这段代码：

#001 // This can be called from signal handlers,

#002 // so should should not use llinfos.

#003 S32 LLMessageSystem::sendMessage(const LLHost &host)

#004 {

#005 if (! mMessageBuilder->isBuilt())

#006 {

#007 mSendSize = mMessageBuilder->buildMessage(

#008 mSendBuffer,

#009 MAX_BUFFER_SIZE,

#010 0);

#011 }

#012

如果消息还没有打包，就进行打包。

#013 if (!(host.isOk())) // if port and ip are zero, don't bother trying to send the message

#014 {

#015 return 0;

#016 }

#017

判断服务器是否可用。

下面查找服务器与客户端的UDP通信环路。

#018 LLCircuitData *cdp = mCircuitInfo.findCircuit(host);

#019 if (!cdp)

#020 {

#021 // this is a new circuit!

#022 // are we protected?

#023 if (mbProtected)

#024 {

#025 // yup! don't send packets to an unknown circuit

#026 if(mVerboseLog)

#027 {

#028 llinfos << "MSG: -> " << host << "/tUNKNOWN CIRCUIT:/t"

#029 << mMessageBuilder->getMessageName() << llendl;

#030 }

#031 llwarns << "sendMessage - Trying to send "

#032 << mMessageBuilder->getMessageName() << " on unknown circuit "

#033 << host << llendl;

#034 return 0;

#035 }

#036 else

#037 {

#038 // nope, open the new circuit

#039

#040 cdp = mCircuitInfo.addCircuitData(host, 0);

#041 }

#042 }

#043 else

#044 {

#045 // this is an old circuit. . . is it still alive?

#046 if (!cdp->isAlive())

#047 {

#048 // nope. don't send to dead circuits

#049 if(mVerboseLog)

#050 {

#051 llinfos << "MSG: -> " << host << "/tDEAD CIRCUIT/t/t"

#052 << mMessageBuilder->getMessageName() << llendl;

#053 }

#054 llwarns << "sendMessage - Trying to send message "

#055 << mMessageBuilder->getMessageName() << " to dead circuit "

#056 << host << llendl;

#057 return 0;

#058 }

#059 }

#060

下面判断是否通过HTTP协议发送数据，还是通过UDP协议来发送。

#061 // NOTE: babbage: LLSD message -> HTTP, template message -> UDP

#062 if(mMessageBuilder == mLLSDMessageBuilder)

#063 {

#064 LLSD message = mLLSDMessageBuilder->getMessage();

#065

#066 const LLHTTPSender& sender = LLHTTPSender::getSender(host);

#067 sender.send(

#068 host,

#069 mLLSDMessageBuilder->getMessageName(),

#070 message,

#071 createResponder(mLLSDMessageBuilder->getMessageName()));

#072

#073 mSendReliable = FALSE;

#074 mReliablePacketParams.clear();

#075 return 1;

#076 }

#077

#078 // zero out the flags and packetid. Subtract 1 here so that we do

#079 // not overwrite the offset if it was set set in buildMessage().

#080 memset(mSendBuffer, 0, LL_PACKET_ID_SIZE - 1);

#081

下面开创建数据包的ID。

#082 // add the send id to the front of the message

#083 cdp->nextPacketOutID();

#084

#085 // Packet ID size is always 4

#086 *((S32*)&mSendBuffer[PHL_PACKET_ID]) = htonl(cdp->getPacketOutID());

#087

#088 // Compress the message, which will usually reduce its size.

#089 U8 * buf_ptr = (U8 *)mSendBuffer;

#090 U32 buffer_length = mSendSize;

#091 mMessageBuilder->compressMessage(buf_ptr, buffer_length);

#092

上面进行数据压缩的工作。

#093 if (buffer_length > 1500)

#094 {

#095 if((mMessageBuilder->getMessageName() != _PREHASH_ChildAgentUpdate)

#096 && (mMessageBuilder->getMessageName() != _PREHASH_SendXferPacket))

#097 {

#098 llwarns << "sendMessage - Trying to send "

#099 << ((buffer_length > 4000) ? "EXTRA " : "")

#100 << "BIG message " << mMessageBuilder->getMessageName() << " - "

#101 << buffer_length << llendl;

#102 }

#103 }

下面把数据放到可靠的连接里发送数据。

#104 if (mSendReliable)

#105 {

#106 buf_ptr[0] |= LL_RELIABLE_FLAG;

#107

#108 if (!cdp->getUnackedPacketCount())

#109 {

#110 // We are adding the first packed onto the unacked packet list(s)

#111 // Add this circuit to the list of circuits with unacked packets

#112 mCircuitInfo.mUnackedCircuitMap[cdp->mHost] = cdp;

#113 }

#114

#115 cdp->addReliablePacket(mSocket,buf_ptr,buffer_length, &mReliablePacketParams);

#116 mReliablePacketsOut++;

#117 }

#118

把收到服务器的包标识ID放到发送的数据后面返回给服务器。

#119 // tack packet acks onto the end of this message

#120 S32 space_left = (MTUBYTES - buffer_length) / sizeof(TPACKETID); // space left for packet ids

#121 S32 ack_count = (S32)cdp->mAcks.size();

#122 BOOL is_ack_appended = FALSE;

#123 std::vector<TPACKETID> acks;

#124 if((space_left > 0) && (ack_count > 0) &&

#125 (mMessageBuilder->getMessageName() != _PREHASH_PacketAck))

#126 {

#127 buf_ptr[0] |= LL_ACK_FLAG;

#128 S32 append_ack_count = llmin(space_left, ack_count);

#129 const S32 MAX_ACKS = 250;

#130 append_ack_count = llmin(append_ack_count, MAX_ACKS);

#131 std::vector<TPACKETID>::iterator iter = cdp->mAcks.begin();

#132 std::vector<TPACKETID>::iterator last = cdp->mAcks.begin();

#133 last += append_ack_count;

#134 TPACKETID packet_id;

#135 for( ; iter != last ; ++iter)

#136 {

#137 // grab the next packet id.

#138 packet_id = (*iter);

#139 if(mVerboseLog)

#140 {

#141 acks.push_back(packet_id);

#142 }

#143

#144 // put it on the end of the buffer

#145 packet_id = htonl(packet_id);

#146

#147 if((S32)(buffer_length + sizeof(TPACKETID)) < MAX_BUFFER_SIZE)

#148 {

#149 memcpy(&buf_ptr[buffer_length], &packet_id, sizeof(TPACKETID)); /* Flawfinder: ignore */

#150 // Do the accounting

#151 buffer_length += sizeof(TPACKETID);

#152 }

#153 else

#154 {

#155 // Just reporting error is likely not enough. Need to

#156 // check how to abort or error out gracefully from

#157 // this function. XXXTBD

#158 // *NOTE: Actually hitting this error would indicate

#159 // the calculation above for space_left, ack_count,

#160 // append_acout_count is incorrect or that

#161 // MAX_BUFFER_SIZE has fallen below MTU which is bad

#162 // and probably programmer error.

#163 llerrs << "Buffer packing failed due to size.." << llendl;

#164 }

#165 }

#166

#167 // clean up the source

#168 cdp->mAcks.erase(cdp->mAcks.begin(), last);

#169

#170 // tack the count in the final byte

#171 U8 count = (U8)append_ack_count;

#172 buf_ptr[buffer_length++] = count;

#173 is_ack_appended = TRUE;

#174 }

#175

下面调用函数sendPacket按流量控制发送数据给服务器。

#176 BOOL success;

#177 success = mPacketRing.sendPacket(mSocket, (char *)buf_ptr, buffer_length, host);

#178

#179 if (!success)

#180 {

#181 mSendPacketFailureCount++;

#182 }

#183 else

#184 {

#185 // mCircuitInfo already points to the correct circuit data

#186 cdp->addBytesOut( buffer_length );

#187 }

#188

#189 if(mVerboseLog)

#190 {

#191 std::ostringstream str;

#192 str << "MSG: -> " << host;

#193 char buffer[MAX_STRING]; /* Flawfinder: ignore */

#194 snprintf(buffer, MAX_STRING, "/t%6d/t%6d/t%6d ", mSendSize, buffer_length, cdp->getPacketOutID()); /* Flawfinder: ignore */

#195 str << buffer

#196 << mMessageBuilder->getMessageName()

#197 << (mSendReliable ? " reliable " : "");

#198 if(is_ack_appended)

#199 {

#200 str << "/tACKS:/t";

#201 std::ostream_iterator<TPACKETID> append(str, " ");

#202 std::copy(acks.begin(), acks.end(), append);

#203 }

#204 llinfos << str.str() << llendl;

#205 }

#206

#207 /*lldebugst(LLERR_MESSAGE) << "MessageSent at: " << (S32)totalTime()

#208 << "," << mMessageBuilder->getMessageName()

#209 << " to " << host

#210 << llendl;*/

#211

#212 mPacketsOut++;

#213 mBytesOut += buffer_length;

#214

#215 mSendReliable = FALSE;

#216 mReliablePacketParams.clear();

#217 return buffer_length;

#218 }

在上面这个函数里，先调用函数buildMessage把消息打包，然后调用host.isOk()判断服务器是否可用，接着查看是否通HTTP来传送。然后再看是否需要可靠性地发送数据，如果需要就把这个消息加到可靠性发送的队列里。在每次发送数据之前，还需要判断是否从服务器收到回应包的ID，如果有，就把这些ID放到发送数据包的后面，一起发送回去给服务器，这样就让服务器知道客户端已经收到什么数据。也就是说有确认机制，就可以把数据控制得可靠了。最后调用mPacketRing.sendPacket函数来通UDP来发送数据给服务器。