Application control of TCP retransmission on Linux
Solution 1
Looks like this was added in Kernel 2.6.37. Commit diff from kernel Git and Excerpt from change log below;
commit dca43c75e7e545694a9dd6288553f55c53e2a3a3 Author: Jerry Chu Date: Fri Aug 27 19:13:28 2010 +0000
tcp: Add TCP_USER_TIMEOUT socket option. This patch provides a "user timeout" support as described in RFC793. The socket option is also needed for the the local half of RFC5482 "TCP User Timeout Option". TCP_USER_TIMEOUT is a TCP level socket option that takes an unsigned int, when > 0, to specify the maximum amount of time in ms that transmitted data may remain unacknowledged before TCP will forcefully close the corresponding connection and return ETIMEDOUT to the application. If 0 is given, TCP will continue to use the system default. Increasing the user timeouts allows a TCP connection to survive extended periods without end-to-end connectivity. Decreasing the user timeouts allows applications to "fail fast" if so desired. Otherwise it may take upto 20 minutes with the current system defaults in a normal WAN environment. The socket option can be made during any state of a TCP connection, but is only effective during the synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK). Moreover, when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will overtake keepalive to determine when to close a connection due to keepalive failure. The option does not change in anyway when TCP retransmits a packet, nor when a keepalive probe will be sent. This option, like many others, will be inherited by an acceptor from its listener. Signed-off-by: H.K. Jerry Chu <[email protected]> Signed-off-by: David S. Miller <[email protected]>
Solution 2
I suggest that if the TCP_USER_TIMEOUT
socket option described by Kimvais is available, you use that. On older kernels where that socket option is not present, you could repeatedly call the SIOCOUTQ
ioctl()
to determine the size of the socket send queue - if the send queue doesn't decrease over your timeout period, that indicates that no ACKs have been received and you can close the socket.
Solution 3
After some thinking (and googling), I came to the conclusion that you can't change tcp_retries1
and tcp_retries2
value for a single socket unless you apply some sort of patch to the kernel. Is that feasible for you?
Otherways, you could use TCP_KEEPALIVE
socket option whose purpose is to check if a connection is still active (it seems to me that that's exactly what you are trying to achieve, so it has sense). Pay attention to the fact that you need to tweak its default parameter a little, because the default is to disconnect after about 2 hrs!!!
Petri Lehtinen
Coding Python, C and Javascript for fun and profit.
Updated on January 20, 2020Comments
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Petri Lehtinen over 4 years
For the impatient:
How to change the value of
/proc/sys/net/ipv4/tcp_retries2
for a single connection in Linux, usingsetsockopt()
,ioctl()
or such, or is it possible?Longer decription:
I'm developing an application that uses long polling HTTP requests. On the server side, it needs to be known when the client has closed the connection. The accuracy is not critical, but it certainly cannot be 15 minutes. Closer to a minute would do fine.
For those not familiar with the concept, a long polling HTTP request works like this:
- The client sends a request
- The server responds with HTTP headers, but leaves the response open. Chunked transfer encoding is used, allowing the server to sends bits of data as they become available.
- When all the data is sent, the server sends a "closing chunk" to signal that the response is complete.
In my application, the server sends "heartbeats" to the client every now an then (30 seconds by default). A heartbeat is just a newline character that is sent as a response chunk. This is meant to keep the line busy so that we notify the connection loss.
There's no problem when the client shuts down correctly. But when it's shut down with force (the client machine loses power, for example), a TCP reset is not sent. In this case, the server sends a heartbeat, which the client doesn't ACK. After this, the server keeps retransmitting the packet for roughly 15 minutes after giving up and reporting the failure to the application layer (our HTTP server). And 15 minutes is too long a wait in my case.
I can control the retransmission time by writing to the following files in
/proc/sys/net/ipv4/
:tcp_retries1 - INTEGER This value influences the time, after which TCP decides, that something is wrong due to unacknowledged RTO retransmissions, and reports this suspicion to the network layer. See tcp_retries2 for more details. RFC 1122 recommends at least 3 retransmissions, which is the default. tcp_retries2 - INTEGER This value influences the timeout of an alive TCP connection, when RTO retransmissions remain unacknowledged. Given a value of N, a hypothetical TCP connection following exponential backoff with an initial RTO of TCP_RTO_MIN would retransmit N times before killing the connection at the (N+1)th RTO. The default value of 15 yields a hypothetical timeout of 924.6 seconds and is a lower bound for the effective timeout. TCP will effectively time out at the first RTO which exceeds the hypothetical timeout. RFC 1122 recommends at least 100 seconds for the timeout, which corresponds to a value of at least 8.
The default value of
tcp_retries2
is indeed 8, and my experience of 15 minutes (900 seconds) of retransmission is in line with the kernel documentation quoted above.If I change the value of
tcp_retries2
to 5 for example, the connection dies much more quicker. But setting it like this affects all the connections in the system, and I'd really like to set it for this one long polling connection only.A quote from RFC 1122:
4.2.3.5 TCP Connection Failures Excessive retransmission of the same segment by TCP indicates some failure of the remote host or the Internet path. This failure may be of short or long duration. The following procedure MUST be used to handle excessive retransmissions of data segments [IP:11]: (a) There are two thresholds R1 and R2 measuring the amount of retransmission that has occurred for the same segment. R1 and R2 might be measured in time units or as a count of retransmissions. (b) When the number of transmissions of the same segment reaches or exceeds threshold R1, pass negative advice (see Section 3.3.1.4) to the IP layer, to trigger dead-gateway diagnosis. (c) When the number of transmissions of the same segment reaches a threshold R2 greater than R1, close the connection. (d) An application MUST be able to set the value for R2 for a particular connection. For example, an interactive application might set R2 to "infinity," giving the user control over when to disconnect. (e) TCP SHOULD inform the application of the delivery problem (unless such information has been disabled by the application; see Section 4.2.4.1), when R1 is reached and before R2. This will allow a remote login (User Telnet) application program to inform the user, for example.
It seems to me that
tcp_retries1
andtcp_retries2
in Linux correspond toR1
andR2
in the RFC. The RFC clearly states (in item d) that a conforming implementation MUST allow setting the value ofR2
, but I have found no way to do it usingsetsockopt()
,ioctl()
or such.Another option would be to get a notification when
R1
is exceeded (item e). This is not as good as settingR2
, though, as I thinkR1
is hit pretty soon (in a few seconds), and the value ofR1
cannot be set per connection, or at least the RFC doesn't require it. -
Jay over 9 yearsNice, any way easily in Windows ?
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Jay over 9 yearsOn a per socket basis
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Fernando Silveira over 5 yearsThe OP specificaly asked to change the value of
tcp_retries2
for a single connection, not for all TCP connections in the operating system. -
osjerick about 5 yearsThis is a very old thread, but for those still coming here I want to mention, that TCP_KEEPALIVE works only on idle connections. It means that if you have outgoing data in socket, keep-alive won't come into play, and you'll still have long timeouts unless you decrease them with tcp_retriesN or TCP_USER_TIMEOUT.
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fredk almost 3 years@Jay According to this article these settings can only be done globally on Windows.