TIME_WAIT problem Notes (change)

Transfer from http://wiki.apache.org/HttpComponents/FrequentlyAskedConnectionManagementQuestions
1. Connections in TIME_WAIT State
After running your HTTP application, you use the netstat command and detect a lot of connections in state TIME_WAIT. Now you wonder why these connections are not cleaned up.

1.1. What is the TIME_WAIT State?
The TIME_WAIT state is a protection mechanism in TCP. The side that closes a socket connection orderly will keep the connection in state TIME_WAIT for some time, typically between 1 and 4 minutes. This happens after the connection is closed. It does not indicate a cleanup problem. The TIME_WAIT state protects against loss of data and data corruption. It is there to help you. For technical details, have a look at the Unix Socket FAQ, section 2.7.

1.2. Some Connections Go To TIME_WAIT, Others Not
If a connection is orderly closed by your application, it will go to the TIME_WAIT state. If a connection is orderly closed by the server, the server keeps it in TIME_WAIT and your client doesn't. If a connection is reset or otherwise dropped by your application in a non-orderly fashion, it will not go to TIME_WAIT.

Unfortunately, it will not always be obvious to you whether a connection is closed orderly or not. This is because connections are pooled and kept open for re-use by default. HttpClient 3.x, HttpClient 4, and also the standard Java HttpURLConnection do that for you. Most applications will simply execute requests, then read from the response stream, and finally close that stream.
Closing the response stream is not the same thing as closing the connection! Closing the response stream returns the connection to the pool, but it will be kept open if possible. This saves a lot of time if you send another request to the same host within a few seconds, or even minutes.

Connection pools have a limited number of connections. A pool may have 5 connections, or 100, or maybe only 1. When you send a request to a host, and there is no open connection to that host in the pool, a new connection needs to be opened. But if the pool is already full, an open connection has to be closed before a new one can be opened. In this case, the old connection will be closed orderly and go to the TIME_WAIT state.
When your application exits and the JVM terminates, the open connections in the pools will not be closed orderly. They are reset or cancelled, without going to TIME_WAIT. To avoid this, you should call the shutdown method of the connection pools your application is using before exiting. The standard Java HttpURLConnection has no public method to shutdown it's connection pool.

1.3. Running Out Of Ports
Some applications open and orderly close a lot of connections within a short time, for example when load-testing a server. A connection in state TIME_WAIT will prevent that port number from being re-used for another connection. That is not an error, it is the purpose of TIME_WAIT.

TCP is configured at the operating system level, not through Java. Your first action should be to increase the number of ephemeral ports on the machine. Windows in particular has a rather low default for the ephemeral ports. The PerformanceWiki has tuning tips for the common operating systems, have a look at the respective Network section.
Only if increasing the number of ephemeral ports does not solve your problem, you should consider decreasing the duration of the TIME_WAIT state. You probably have to reduce the maximum lifetime of IP packets, as the duration of TIME_WAIT is typically twice that timespan to allow for a round-trip delay. Be aware that this will affect all applications running on the machine. Don't ask us how to do it, we're not the experts for network tuning.

There are some ways to deal with the problem at the application level. One way is to send a "Connection: close" header with each request. That will tell the server to close the connection, so it goes to TIME_WAIT on the other side. Of course this also disables the keep-alive feature of connection pooling and thereby degrades performance. If you are running load tests against a server, the untypical behavior of your application may distort the test results. [[BR] Another way is to not orderly close connections. There is a trick to set SO_LINGER to a special value, which will cause the connection to be reset instead of orderly closed. Note that the HttpClient API will not support that directly, you'll have to extend or modify some classes to implement this hack.
Yet another way is to re-use ports that are still blocked by a connection in TIME_WAIT. You can do that by specifying the SO_REUSEADDR option when opening a socket. Java 1.4 introduced the method Socket.setReuseAddress for this purpose. You will have to extend or modify some classes of HttpClient for this too, but at least it's not a hack.

1.4. Further Reading
Unix Socket FAQ


Discussion on the HttpClient mailing list in December 2007


netstat command line tool

http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-2.html # ss2.7

2.7 Please explain the TIME_WAIT state.
Remember that TCP guarantees all data transmitted will be delivered, if at all possible. When you close a socket, the server goes into a TIME_WAIT state, just to be really really sure that all the data has gone through. When a socket is closed, both sides agree by sending messages to each other that they will send no more data. This, it seemed to me was good enough, and after the handshaking is done, the socket should be closed. The problem is two-fold. First, there is no way to be sure that the last ack was communicated successfully. Second, there may be "wandering duplicates" left on the net that must be dealt with if they are delivered.

Andrew Gierth (andrewg@microlise.co.uk) helped to explain the closing sequence in the following usenet posting:

Assume that a connection is in ESTABLISHED state, and the client is about to do an orderly release. The client's sequence no. Is Sc, and the server's is Ss. The pipe is empty in both directions.

Client Server
====== ======
(Client closes)
<CTL=FIN+ACK> <SEQ=Sc> <ACK=Ss> ------->>
<<-------- <CTL=ACK> <SEQ=Ss> <ACK=Sc+1>
<<-------- <CTL=FIN+ACK> <SEQ=Ss> <ACK=Sc+1> (Server closes)
<CTL=ACK>, <SEQ=Sc+1> <ACK=Ss+1> ------->>
(2 * msl elapses ...)
Note: the +1 on the sequence numbers is because the FIN counts as one byte of data. (The above diagram is equivalent to fig. 13 from RFC 793).

Now consider what happens if the last of those packets is dropped in the network. The client has done with the connection; it has no more data or control info to send, and never will have. But the server does not know whether the client received all the data correctly; that's what the last ACK segment is for. Now the server may or may not care whether the client got the data, but that is not an issue for TCP; TCP is a reliable rotocol, and must distinguish between an orderly connection close where all data is transferred, and a connection abort where data may or may not have been lost.

So, if that last packet is dropped, the server will retransmit it (it is, after all, an unacknowledged segment) and will expect to see a suitable ACK segment in reply. If the client went straight to CLOSED, the only possible response to that retransmit would be a RST, which would indicate to the server that data had been lost, when in fact it had not been.

(Bear in mind that the server's FIN segment may, additionally, contain data.)

DISCLAIMER: This is my interpretation of the RFCs (I have read all the TCP-related ones I could find), but I have not attempted to examine implementation source code or trace actual connections in order to verify it. I am satisfied that the logic is correct, though.

More commentarty from Vic:

The second issue was addressed by Richard Stevens (rstevens@noao.edu, author of "Unix Network Programming", see 1.5 Where can I get source code for the book [book title]?). I have put together quotes from some of his postings and email which explain this. I have brought together paragraphs from different postings, and have made as few changes as possible.

From Richard Stevens (rstevens@noao.edu):

If the duration of the TIME_WAIT state were just to handle TCP's full-duplex close, then the time would be much smaller, and it would be some function of the current RTO (retransmission timeout), not the MSL (the packet lifetime).

A couple of points about the TIME_WAIT state.

The end that sends the first FIN goes into the TIME_WAIT state, because that is the end that sends the final ACK. If the other end's FIN is lost, or if the final ACK is lost, having the end that sends the first FIN maintain state about the connection guarantees that it has enough information to retransmit the final ACK.
Realize that TCP sequence numbers wrap around after 2 ** 32 bytes have been transferred. Assume a connection between A.1500 (host A, port 1500) and B.2000. During the connection one segment is lost and retransmitted. But the segment is not really lost, it is held by some intermediate router and then re-injected into the network. (This is called a "wandering duplicate".) But in the time between the packet being lost & retransmitted, and then reappearing, the connection is closed (without any problems) and then another connection is established between the same host, same port (that is, A.1500 and B.2000; this is called another "incarnation" of the connection). But the sequence numbers chosen for the new incarnation just happen to overlap with the sequence number of the wandering duplicate that is about to reappear. (This is indeed possible, given the way sequence numbers are chosen for TCP connections.) Bingo, you are about to deliver the data from the wandering duplicate (the previous incarnation of the connection) to the new incarnation of the connection. To avoid this, you do not allow the same incarnation of the connection to be reestablished until the TIME_WAIT state terminates. Even the TIME_WAIT state doesn't complete solve the second problem, given what is called TIME_WAIT assassination. RFC 1337 has more details.
The reason that the duration of the TIME_WAIT state is 2 * MSL is that the maximum amount of time a packet can wander around a network is assumed to be MSL seconds. The factor of 2 is for the round-trip. The recommended value for MSL is 120 seconds, but Berkeley-derived implementations normally use 30 seconds instead. This means a TIME_WAIT delay between 1 and 4 minutes. Solaris 2.x does indeed use the recommended MSL of 120 seconds.
A wandering duplicate is a packet that appeared to be lost and was retransmitted. But it wasn't really lost ... some router had problems, held on to the packet for a while (order of seconds, could be a minute if the TTL is large enough) and then re-injects the packet back into the network. But by the time it reappears, the application that sent it originally has already retransmitted the data contained in that packet.

Because of these potential problems with TIME_WAIT assassinations, one should not avoid the TIME_WAIT state by setting the SO_LINGER option to send an RST instead of the normal TCP connection termination (FIN / ACK / FIN / ACK). The TIME_WAIT state is there for a reason ; it's your friend and it's there to help you :-)

I have a long discussion of just this topic in my just-released "TCP / IP Illustrated, Volume 3". The TIME_WAIT state is indeed, one of the most misunderstood features of TCP.

I'm currently rewriting "Unix Network Programming" (see 1.5 Where can I get source code for the book [book title]?). And will include lots more on this topic, as it is often confusing and misunderstood.

An additional note from Andrew:

Closing a socket: if SO_LINGER has not been called on a socket, then close () is not supposed to discard data. This is true on SVR4.2 (and, apparently, on all non-SVR4 systems) but apparently not on SVR4; the use of either shutdown () or SO_LINGER seems to be required to guarantee delivery of all data.

分类:CPP 时间:2010-05-15 人气:312
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