Ok, I have a tinsy problem.
I am trying to interact with an existing server (I have NO CONTROL over the server) it is a temporary tcp stream connection, only meant to send and receive a single command (think of it like a web server, but it isn't)
Now, I need to get an update from this server once every second (sometimes even more frequently).
My problem is, this is what is showing up in my netstat...
tcp 0 0 localhost.localdo:34373 localhost.localdo:20000 TIME_WAIT
tcp 0 0 localhost.localdo:34372 localhost.localdo:20000 TIME_WAIT
tcp 0 0 localhost.localdo:34375 localhost.localdo:20000 TIME_WAIT
tcp 0 0 localhost.localdo:34374 localhost.localdo:20000 TIME_WAIT
tcp 0 0 localhost.localdo:34369 localhost.localdo:20000 TIME_WAIT
And this goes on and on and on.
I am closing my end of the connection, and I'm fairly sure the server is closing its end of the connection. What can I do to prevent this time wait issue?
Here is the meat of the section of code that might be causing the problem.
I am sending the request. I am then waiting for a response (up to a certain period) (check sock does a select to see if there is data to be read) then I am returning the results.
do {
Invalid = CheckSocket(sock);
Counter++;
} while ((Counter<5) && (Invalid == 1));
if (Invalid)
sprintf(IBuffer,"Server is Busy, please try again later.\n");
else
if (read(sock, IBuffer,( sizeof(unsigned char)*FEEDBACK_BUFFER_LENG
< 0) {
perror("receiveing on stream socket");
close(sock);
exit(1);
}
//************************
// Close this connection, since the stream isn't persistent
//************************
close(sock);
Now, my program is operating normally, but those TIME_WAIT's are stacking up. is there any way to not have those happen, or to reduce the length of time of the wait so it won't stack up so fast?
This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.
Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.
If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.
Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.
Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.
Access the answers to your technology questions today.
30-day free trial. Register in 60 seconds.
Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.
Try it out and discover for yourself.
30-day free trial. Register in 60 seconds.
Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.
This is what Rich Stevens says about TIME_WAIT:
"There are two reasons for the TIME_WAIT state:
1. to implement TCP's full-duplex connection termination
reliably
2. to allow old duplicate segments to expire in the network
"
That means, we have to accept going through this state
of a TCP connection. Otherwise, it may not work correctly.
The duration of TIME_WAIT state is implementation-dependent (recommended value is 4 minutes, BSD has 1 minute).
http://www.faqs.org/faqs/u
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 (andrew@erlenstar.demon.co
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
====== ======
ESTABLISHED ESTABLISHED
(client closes)
ESTABLISHED ESTABLISHED
<CTL=FIN+ACK><SEQ=Sc><ACK=
FIN_WAIT_1
<<-------- <CTL=ACK><SEQ=Ss><ACK=Sc+1
FIN_WAIT_2 CLOSE_WAIT
<<-------- <CTL=FIN+ACK><SEQ=Ss><ACK=
LAST_ACK
<CTL=ACK>,<SEQ=Sc+1><ACK=S
TIME_WAIT CLOSED
(2*msl elapses...)
CLOSED
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.
o 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.
o 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.
o 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.
I had read both of those things on the net, I was just hoping there was a way. I have found ways to reduce the length of time by recompiling the kernel, but that would only work for me, and isn't a valid option for everyone.
Personally, if it were me, I'd make it a long term socket. Right now, you connect, send you request, and get a reply that is no more then about 2040, but most reponses are in the range of 50 bytes.
I am currently begging the author of the server to convert the program to a long term stream or UDP (I don't think delivery guarantee is vital for this app to be honest, but if it is, making the stream long term would be a better way to go. testing with 5 clients hitting the server for data every 1.5 seconds is piling up >140 TIME_WAITs, which may not be a lot, but this is only 5 clients, with 1 request; by the time it is done, that 1 request could really be 3 to 5 requests in that span of time - each request requiring a new connection to the server piling up the TIME_WAITs even higher, if there were 50 clients making 5 requests, I figure that could pile up to 35000 TIME_WAIT's, and that just seems a waste for what the server is doing)
Well, it seems to me that there is a way (again,
according to Stevens).
If you set SO_LINGER option using the struct linger
{
int l_onoff;
int l_linger;
}
where l_onoff set to nonzero and l_linger is zero,
"TCP aborts the connection when it is closed.
That is, TCP discards any data still remaining in the
socket send buffer and sends an RST to the peer, not
the normal four-packet connection termination
sequence... This avoids TCP TIME_WAIT state...
Some implementations, notably Solaris 2.x where x <= 5,
do not implement this feature of SO_LINGER"
Business Accounts
Answer for Membership
by: navigatorPosted on 2002-05-15 at 21:51:20ID: 7012930
btw - I know the TIME_WAIT state is normal, my problem is that they are piling up too fast. I have NO control over the server I am querying, so there is no way for me to change it from a temporary stream to a long term stream.
Basically, what I am asking, is there anything I can do on the client end to reduce or remove the TIME_WAIT? I've tested it with 5 clients hitting the server at the average 1.5 second interval, and the system resources are dwindling fast.