計(jì)算機(jī)網(wǎng)絡(luò)概論上機(jī)實(shí)習(xí)ipv4協(xié)議轉(zhuǎn)發(fā)實(shí)驗(yàn)

IPv4分組收發(fā)實(shí)驗(yàn)的基礎(chǔ)上增IPv4分組，根據(jù)其信息進(jìn)行如下幾類處理():externvoidexternvoidfwd_DiscardPkt(char?pBuffer,inttype)externexternvoidfwd_SendtoLower(char?pBuffer,intlength,unsigned其中，pBufferIPv4數(shù)據(jù)包的指針；length(+內(nèi)容)，以字節(jié)計(jì)；nexthop是轉(zhuǎn)發(fā)的下一跳地址。fwd_LocalRev:將IPexternexternvoidfwd_LocalRev(char?pBuffer,intlength)其中，pBufferIPv4數(shù)據(jù)包的指針，length是數(shù)據(jù)包總長(zhǎng)度，以字節(jié)typedefstructstud_route_msg{typedefstructstud_route_msg{unsignedintdest;unsignedintmaxlen;unsignedint2345表?xiàng)l目。此處采用C++STL中的map容器，其本身由一種平衡檢索二叉樹樹map<map<int,int>下一跳地址nexthop。???insert()方法將其路由表中。條目，則調(diào)用fwd_DiscardPkt()拋棄該包。校驗(yàn)和的計(jì)算方法與前次實(shí)驗(yàn)相同。需要注意的是，由于重新計(jì)算校驗(yàn)和，需把原校驗(yàn)和字段，再對(duì)整個(gè)報(bào)頭計(jì)算校驗(yàn)和。如果將原校驗(yàn)和字段也加入，網(wǎng)絡(luò)字節(jié)序的；而本機(jī)獲取的地址及路由表項(xiàng)內(nèi)容是以主機(jī)字節(jié)序的。)目等錯(cuò)誤，函數(shù)返回1。在本次實(shí)習(xí)中，對(duì)IPv4協(xié)議的轉(zhuǎn)發(fā)流程細(xì)節(jié)更加熟悉，對(duì)路由表數(shù)SectionSection lude lude usingstd::map;externvoidfwd_LocalRcv(char?pBuffer,intlength)externvoidfwd_SendtoLower(char?pBuffer,intlength,unsignedintexternvoidfwd_DiscardPkt(char?pBuffer,inttype)externunsignedintgetIpv4Address()/?RouteTable<DstAddr,nexthop>map<int,int> theRouteTablevoidstud_Route_Init/?AddinganewentrytotheRouteTablevoidstud_route_add(stud_route_msgintDstAddr=(ntohl(proute?>dest))&(0xFFFFFFFF<<(32?htonl(proute?>intnexthop=ntohl(RouteTable.insert(pair<int,int>(DstAddr,nexthop))/?Dealingwiththereceptionandforwardingintstud_fwd_deal(char?pBuffer,intlengthintIHL=pBuffer[0]&intTTL=(int)pBufferintDstAddr=ntohl(?(unsigned?)(&pBuffer[16]))/?Thishostisthedestination,sendtotheupperclassif(DstAddr23456789return}/?TimeOut,discardingthepacket(TTL<=fwd_DiscardPkt(pBuffer,return}/?SearchtheDstAddrintheRouteTablemap<int,int>::i iter=RouteTable.find/?DstnotfoundintheRouteTable,discardingthepacket(iter==fwd_DiscardPkt(pBuffer,return}/?Routerforwardingthepacketchar?buffer=newchar[lengthmemcpy(buffer,pBuffer,length)/?TTL?1buffer[8]=(char)(TTL?/?CalculatingandfillingintheHeaderChecksumunsignedintHeaderCheckSum=memset(&buffer[10],0,sizeof(short))for(inti=0;i<4?IHL;i+=HeaderCheckSum+=((buffer[i]&0xFF)<<8)+(buffer[i+1]&HeaderCheckSum+=(HeaderCheckSum>>HeaderCheckSum=buffer[10]=(char)((unsignedshort)HeaderCheckSum>>buffer[11]=(char)((unsignedshort)HeaderCheckSum&/?Forwardingfwd_SendtoLower(buffer,length,(?iter).return}

return/?TimeOut,discardingthepacket77

if(TTL<=fwd_DiscardPkt(pBuffer,return}/?SearchtheDstAddrintheRouteTablemap<int,int>:: iter=RouteTable.find/?DstnotfoundintheRouteTable,discardingthepacketif(iter==fwd_DiscardPkt(pBuffer,return}/?Routerforwardingthepacket?/char?buffer=newchar[length];memcpy(buffer,pBuffer,length);/?TTL?1buffer[8]=(char)(TTL?/?CalculatingandfillingintheHeaderChecksum?/unsignedintHeaderCheckSum=0;memset(&buffer[10],0,sizeof(short));for(inti=0;i<4?IHL;i+=HeaderCheckSum+=((buffer[i]&0xFF)<<8)+(buffer[i+1]&0xFF);HeaderCheckSum+=(HeaderCheckSum>>16);HeaderCheckSum=buffer[10]=(char)