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Dead Connection Detection (DCD) is a feature of SQL*Net 2.1 and later, includingOracle Net8 and Oracle NET. DCD detects when a partner in a SQL*Net V2 client/serveror server/server connection has terminated unexpectedly, and flags the dead session so PMON can release the resources associated with it.
死连接检测是net 2.1之后的一个特性。包括NET8和net. 服务器端检测到客户端异常中断，PMON进程会释放分配给该会话的资源。

 

DCD is intended primarily for environments in which clients power down their systems without disconnecting from their Oracle sessions, a problem characteristic of networks with PC clients.
死链接检测主要的产生环境是在客户端在没有正常断开连接后断开连接，问题出在客户端的网络上。

DCD is initiated on the server when a connection is established. At this time SQL*Net reads the SQL*Net parameter files and sets a timer to generate an alarm.  The timer interval is set by providing a non-zero value in minutes for the SQLNET.EXPIRE_TIME parameter in the sqlnet.ora file.
死连接检测被初始化在服务器端。SQL*NET读取参数文件的设置。时间间隔设置SQLNET.EXPIRE_TIME为非零参数再sqlnet.ora文件里。

When the timer expires, SQL*Net on the server sends a "probe" packet to the client. (In the case of a database link, the destination of the link constitutes the server side of the connection.)  The probe is essentially an empty SQL*Net packet and does not represent any form of SQL*Net level data, but it creates data traffic on the underlying protocol.
当超过设置时间，SQL*NET发送一个"探索"包到客户端。对于一个数据库链接,链接的目的地构成连接的服务器端。"探索"本质上是个空包。不会表示任何SQL*NET级别的数据，但是它创建了数据流在底层的协议。

 

If the client end of the connection is still active, the probe is discarded, and the timer mechanism is reset.  If the client has terminated abnormally, the server will receive an error from the send call issued for the probe, and SQL*Net on the server will signal the operating system to release the connection's resources.
如果客户端是ACTIVE,包被丢弃，探索时间被重置，如果客户端异常终止，服务器端将会接受一个错误。服务端标记操作系统释放链接资源。

 

On Unix servers, the sqlnet.ora file must be in either $TNS_ADMIN or $ORACLE_HOME/network/admin. Neither /etc nor /var/opt/oracle alone is valid. 

 

It should be also be noted that in SQL*Net 2.1.x, an active orphan process (one processing a query, for example) will not be killed until the query completes. In SQL*Net 2.2, orphaned resources will be released regardless of 

activity.

This is a server feature only.  The client may be running any supported SQL*Net V2 release.

 

 

THE FUNCTION OF THE PROTOCOL STACK 

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While Dead Connection Detection is set at the SQL*Net level, it relies heavily on the underlying protocol stack for it's successful execution. For example,you might set SQLNET.EXPIRE_TIME=1 in the sqlnet.ora file, but it is unlikely that an orphaned server process will be cleaned up immediately upon expiration of that interval. 

 

TCP/IP, for example, is a connection-oriented protocol, and as such, the protocol will implement some level of packet timeout and retransmission in an effort to guarantee the safe and sequenced order of data packets. If a timely acknowledgement is not received in response to the probe packet, the TCP/IP stack will retransmit the packet some number of times before timing out. After TCP/IP gives up, then SQL*Net receives notification that the probe failed.

 

The time that it takes TCP/IP to timeout is dependent on the TCP/IP stack, and timeouts of many minutes are entirely common.  This has been an area of concern for many customers, as many retransmissions at the protocol layer causes what could be a significant lag between the expiration of the DCD interval and the time when the orphaned process is actually killed. 

 

The easiest way to determine if the protocol stack is causing such a delay involves testing different DCD intervals.

TESTING THE PROTOCOL STACK 

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Set the SQLNET.EXPIRE_TIME parameter to 1 minute and note the time required toclean up an orphaned server process.  Then set SQLNET.EXPIRE_TIME to 5 minutes and again observe the time required to clean up the shadow. If the TCP/IP timeout is the reason the server resources do not get released, the time to clean up the shadow should increase by about 4 minutes.

If the TCP/IP retransmission timeout is indeed the problem, the Operating System kernel can be tuned to reduce the interval for and number of packet retransmissions (on many Unix platforms, the file /usr/include/netinet/tcp_timer.h contains the configuration parameters). 

 

Reducing the interval and number of retransmissions may impact other system components, since in effect you are shrinking the window allowed forconnections to process data, possibly resulting in inadvertent loss of connections during periods of heavy system load.  Slower connections from

remote sites may be impacted by this change.

 

Kernel parameters that may affect retransmission include but are not limited to TCP_TTL, TCPTV_PERSMIN, TCPTV_MAX, and TCP_LINGERTIME. 

 

*** To avoid disrupting other system processes, it is important to contact the appropriate vendor for assistance in tuning the operating system kernel or protocol stack. *** 

 

 

MONITORING DEAD CONNECTION DETECTION 

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The best way to determine if DCD is enabled and functioning properly is to generate a server trace and search the file for the DCD probe packet. To generate a server trace, set TRACE_LEVEL_SERVER=16 and TRACE_DIRECTORY_SERVER= in sqlnet.ora on the server (note the locationof the sqlnet.ora file).  The resulting trace file will have a filename ofsvr_.trc and will be located in the specified directory. 

 

Is DCD Enabled? 

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For pre-Oracle8i versions, enable level 16 SQL*Net server tracing and search the resultant server trace file for an entry like the following:

  osntns: Enabling dead connection detection (1 min) 

 The timer interval listed should match the value of SQLNET.EXPIRE_TIME.

For Oracle8i onwards, you should see the following:

  nstimini: entry 

  nstimig: entry 

  nstimig: normal exit 

  nstimini: initializing NLTM in asynchronous mode 

  nstimini: normal exit 

  nstimstart: entry

 

Is DCD Working? 

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Search the server trace file for DCD probe packets. They will appear in the form of empty data packets, as follows: 

 

  nstimexp: entry 

  nstimexp: timer expired at 05-OCT-95 12:15:05 

  nsdo: entry 

  nsdo: cid=0, opcode=67, *bl=0, *what=1, uflgs=0x2, cflgs=0x3 

  nsdo: nsctx: state=8, flg=0x621c, mvd=0 

  nsdo: gtn=93, gtc=93, ptn=10, ptc=2048 

  nsdoacts: entry 

  nsdofls: entry 

  nsdofls: DATA flags: 0x0 

  nsdofls: sending NSPTDA packet 

  nspsend: entry 

  nspsend: plen=10, type=6 

  nttwr: entry 

  nttwr: socket 4 had bytes written=10 

  nttwr: exit 

  nspsend: 10 bytes to transport 

  nspsend:packet dump 

  nspsend:00 0A 00 00 06 00 00 00  |........| 

  nspsend:00 00 00 00 00 00 00 00  |........| 

  nspsend: normal exit 

  nsdofls: exit (0) 

  nsdoacts: flushing transport 

  nttctl: entry 

  nsdoacts: normal exit 

  nsdo: normal exit 

  nstimexp: normal exit

The entry:

  nspsend:00 0A 00 00 06 00 00 00  |........| 

  nspsend:00 00 00 00 00 00 00 00  |........| 

 

represents the probe packet.  Note that DCD packets are 10 bytes long when they are issued to the protocol stack. Once the protocol header and trailer bytes for the underlying protocols have been added, the packet could be approximately 70 bytes long.

 

If DCD is enabled, you will see these probe packets written to the trace file when the timer expires.  If the server is a UNIX system, it might be useful to establish a connection and tail the trace file: 

  tail -f svr_.trc 

The time elapsed after each probe packet is written to the server trace should match the SQLNET.EXPIRE_TIME value.

Note: from version 9.2.0.4.0 onwards, DCD probe packets are no longer traced in SQL*Net trace files, however DCD packets can be observed using other forms of tracing, such as network sniffer tracing.

KNOWN PROBLEMS OR LIMITATIONS 

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- Of the few reported problems, perhaps the most significant is DCD's poor performance on Windows NT.  Dead connections are cleaned up only when the server is rebooted and the database is restarted.  Exactly how well DCD works on NT depends on the client's protocol implementation. SQL*Net v2.3 has improved the performance over earlier releases. 

 

  This has been logged as port-specific Bug#303578. 

 

- On SCO Unix, a problem was reported in which server processes spin, consuming large amounts of CPU, once the DCD timer expires. The problem is due to improper signal handling and can be eliminated by disabling DCD.

 

  This is port-specific Bug#293264

- Orphaned resources are not released if only the client application is terminated. Only after the client PC has been rebooted does DCD release these resources. For example, if a Windows application is killed yet Windows remains running, the probe packet may be received and discarded as if the connection is still active.  As it currently stands, it appears that DCD detects dead client

machines, but not dead client processes.

 

  This is logged as generic Bug#280848. 

 

- The SQL*Net V2 implementation on MVS does not use the generic DCD mechanism,and therefore the SQLNET.EXPIRE_TIME parameter does not apply. The KEEPALIVE function of IBM's TCP/IP is used instead. This was implemented prior to development of DCD. 

 

  This is documented in port-specific Bug#301318. 

 

- DCD relies heavily on issuing probe packets during any phase of the connection.This is not be possible with some protocols which run half-duplex. Hence, DCD is not enabled on protocols like APPC/LU6.2. 

   This is not a bug, but is rather the intended design. 

 - Local connections using BEQ protocol adapters are not supported with DCD. 
Local connections using the IPC protocol adapters are supported with DCD.

-BUG#1388806 : On Windows NT, DCD FAILS AFTER 16 CONNECTIONS

 A FINAL NOTE...

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On most OS'es (even more recent versions of Windows) if a process exits abnormally or is killed by an administrator, the OS will still gracefully clean up resources associated with that process including the network connection(s).  It will tell the server on the other end that it is closing 

the network connection. DCD is still useful for times when there are problems with the physical network (e.g. ethernet cable falls off the machine) or if the OS kernel panics and crashes (e.g. blue screen of death) before it can close the network connections.  It may have another side benefit with certain load balancing hardware, that may prematurely abort connections it thinks have

been idle too long, by sending a dummy packet to the client periodically.

 

Under no circumstances should you rely 100% on Dead Connection Detection.  It was developed to handle clients that have abnormally exited. Clients should always exit their applications gracefully. It is the responsibility of the application developer to make this possible. DCD is intended only to clean up after abnormal events.

 

DCD is much more resource-intensive than similar mechanisms at the protocol level, so if you depend on DCD to clean up all dead processes, that will put an undue load on the server. 

 

Clearly it is advantageous to exit applications cleanly in the first place.