Dcmtk 源码解读SCP交互过程- CEcho Scp

1 创建过程

首先，创建一个DcmStorageSCPProxy的代理类，此类继承于 OFThread。整个StoreSCP服务的链接过程，可以简单的描述为在主线程中（上图中的蓝色区域）初始化链接对象（包括创建socket）；主要的创建代码在dul.cc文件中，这里需要注意的是，在服务端绑定的ip地址的时候，INADDR_ANY表示本机所有的网卡，都要进行监听。这里和127.0.0.1还有有很大区别的，127.0.0.1表示只有本机客户端可以连接此监听端口。然后，开始监听此端口。整个过程比较简单。

static OFCondition initializeNetworkTCP(PRIVATE_NETWORKKEY ** key, void *parameter)
{struct linger sockarg;int reuse = 1;(*key)->networkSpecific.TCP.tLayer = NULL;(*key)->networkSpecific.TCP.tLayerOwned = 0;(*key)->networkSpecific.TCP.port = -1;(*key)->networkSpecific.TCP.listenSocket = -1;int sock;struct sockaddr_in server;(*key)->networkSpecific.TCP.port = *(int *) parameter;(*key)->networkSpecific.TCP.listenSocket = socket(AF_INET, SOCK_STREAM, 0); // 表示创建ip4 stream类型的socketsock = (*key)->networkSpecific.TCP.listenSocket;setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (char *) &reuse, sizeof(reuse));server.sin_family = AF_INET;server.sin_addr.s_addr = INADDR_ANY;server.sin_port = (unsigned short) htons((*key)->networkSpecific.TCP.port);bind(sock, (struct sockaddr *) & server, sizeof(server)); // 这里要注意，绑定的ip地址，INADDR_ANY表示本机所有的网卡，都要进行监听。这里和127.0.0.1还有有很大区别的，127.0.0.1表示只有本机客户端可以连接此监听端口/* Listen on the socket */listen(sock, PRV_LISTENBACKLOG);return EC_Normal;
}

2 监听线程

其次，启动监听线程（上图中绿色区域），等待SCU来连接。服务端一旦监听到SCU的连接，就会对比请求的Presetation Context，是否SCP端支持，如果支持，就会向SCU发送
A-ASSOCIATE-AC消息。这里要注意，在和客户端交互的过程中，有一个很重要的概念，就是dicom通讯中的状态机流程。在交互的过程中，有13个状态

Sta1 - Idle
Sta2 - Transport connection open
Sta3 - Awaiting local A-ASSOCIATE response primitive
Sta4 - Awaiting transport connection opening to complete
Sta5 - Awaiting A-ASSOCIATE-AC or A-ASSOCIATE-RJ PDU
Sta6 - Association established and ready for data transfer
Sta7 - Awaiting A-RELEASE-RP PDU
Sta8 - Awaiting local A-RELEASE response primitive
Sta9 - Release collision requestor side; awaiting A-RELEASE response
Sta10 - Release collision acceptor side; awaiting A-RELEASE-RP PDU
Sta11 - Release collision requestor side; awaiting A-RELEASE-RP PDU
Sta12 - Release collision acceptor side; awaiting A-RELEASE response primitive
Sta13 - Awaiting Transport Connection Close Indication

另外，还有19中事件Event，来触发这些状态的跃迁。

#define   A_ASSOCIATE_REQ_LOCAL_USER   0
#define   TRANS_CONN_CONFIRM_LOCAL_USER   1
#define   A_ASSOCIATE_AC_PDU_RCV       2
#define   A_ASSOCIATE_RJ_PDU_RCV       3
#define   TRANS_CONN_INDICATION       4
#define   A_ASSOCIATE_RQ_PDU_RCV       5
#define   A_ASSOCIATE_RESPONSE_ACCEPT   6
#define   A_ASSOCIATE_RESPONSE_REJECT   7
#define   P_DATA_REQ           8
#define   P_DATA_TF_PDU_RCV       9
#define   A_RELEASE_REQ           10
#define   A_RELEASE_RQ_PDU_RCV       11
#define   A_RELEASE_RP_PDU_RCV       12
#define   A_RELEASE_RESP           13
#define   A_ABORT_REQ           14
#define   A_ABORT_PDU_RCV           15
#define   TRANS_CONN_CLOSED       16
#define   ARTIM_TIMER_EXPIRED       17
#define   INVALID_PDU            18
#define DUL_NUMBER_OF_EVENTS       19

在图1中的绿色区域，首先是TRANS_CONN_INDICATION=4事件下，将SCP当前的连接的状态从state = 1（Idle） next state = 2（Transport connection open）的转换，马上又通过A_ASSOCIATE_RQ_PDU_RCV=5 事件，将current state = 2 （Transport connection open）next state = 3（Awaiting local A-ASSOCIATE response primitive）。然后，Scp代码通过本地的一些配置，判断当前服务是否支持SCU提交的PC，是否是合法的AE，如果所有的验证都通过后，将发送A-ASSOCIATE-AC信息，通过事件A_ASSOCIATE_RESPONSE_ACCEPT=6 将本地状态从 current state = 3 （ Awaiting local A-ASSOCIATE response primitive）转化为next state = 6（Association established and ready for data transfer）的状态。

3 创建DIMSE处理线程

ASC连接成功后，启动第三个线程，处理P-DATA-TF 格式的PDU，也就是DIMSE层来处理具体的请求（在图1中的黄色区域）。

整个过程是通过一个循环调用DIMSE_readNextPDV函数来获取PDV包。

for (last = OFFalse, bytesRead = 0, type = DUL_COMMANDPDV;type == DUL_COMMANDPDV && !last;){/* get next PDV (in detail, in order to get this PDV, a *//* PDU has to be read from the incoming socket stream) */cond = DIMSE_readNextPDV(assoc, blocking, timeout, &pdv);/* if this is the first loop iteration, get the presentation context ID which is captured in the *//* current PDV. If this is not the first loop iteration, check if the presentation context IDs in *//* the current PDV and in the last PDV are identical. If they are not, return an error. */// 如果是PDV的第一个包，将表示上下文记录下来；后续的PDV如果表达上下的ID和第一个PDV的表达上下文不同的话，也不符合规范，返回；if (pdvCount == 0){pid = pdv.presentationContextID;}else if (pdv.presentationContextID != pid){delete cmdSet;return makeDcmnetSubCondition(DIMSEC_RECEIVEFAILED, OF_error, "DIMSE Failed to receive message", subCond);}/* check if the fragment length of the current PDV is odd. This should *//* never happen (see DICOM standard (year 2000) part 7, annex F) (or *//* the corresponding section in a later version of the standard.) *///保证PDV的长度是2的整数倍  ... ... 可以查看第7章的 附件F/* if information is contained the PDVs fragment, we want to insert this information into the buffer */if (pdv.fragmentLength > 0) {cmdBuf.setBuffer(pdv.data, pdv.fragmentLength);}/* if this fragment contains the last fragment of the DIMSE command, set the end of the stream *///如果是最后一个PDV，那么，将命令buffer设置结束标志量if (pdv.lastPDV) {cmdBuf.setEos();}/* insert the information which is contained in the buffer into the DcmDataset *//* variable. Mind that DIMSE commands are always specified in the little endian *//* implicit transfer syntax. Additionally, we want to remove group length tags. */econd = cmdSet->read(cmdBuf, EXS_LittleEndianImplicit, EGL_withoutGL);if (econd != EC_Normal && econd != EC_StreamNotifyClient){delete cmdSet;return makeDcmnetSubCondition(DIMSEC_RECEIVEFAILED, OF_error, "DIMSE: receiveCommand: cmdSet->read() Failed", econd);}/* update the counter that counts how many bytes were read from the incoming socket *//* stream. This variable will only be used for dumping general information. */bytesRead += pdv.fragmentLength;/* update the following variables which will be evaluated at the beginning of each loop iteration. */last = pdv.lastPDV;type = pdv.pdvType;/* update the counter that counts how many PDVs were received on the incoming *//* socket stream. This variable will be used for determining the first *//* loop iteration and dumping general information. */pdvCount++;}

当通过DIMSE_readNextPDV函数接收到从SCU发送的A-ASSOCIATE-RELEASE-RQ的请求， SCP内部发生Event:A_RELEASE_RQ_PDU_RCV=11的事件，将 current state = 6（Association established and ready for data transfer） next state = 8 （Awaiting local A-RELEASE response primitive）状态。

最后程序调用ASC_acknowledgeRelease(assoc)和DUL_AcknowledgeRelease函数，将A-ASSOCIATE-RESPONSE发送会SCU端。这时，内部状态机发生A_RELEASE_RESP=13 事件，将将 current state = 8（Awaiting local A-RELEASE response primitive）改变为 next state = 13 （Awaiting Transport Connection Close Indication）状态。

源代码中，注意文件dulfsm.cc中的FSM_ENTRY，有限状态机类，和dcm4che中的dcm4che-net.jar的State类作用相同，这里的实现过程，设计都很巧妙，有兴趣的可以研读源代码。