Application control of TCP retransmission on Linux

Question

For the impatient:

How to change the value of /proc/sys/net/ipv4/tcp_retries2 for a single connection in Linux, using setsockopt(), 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 and tcp_retries2 in Linux correspond to R1 and R2 in the RFC. The RFC clearly states (in item d) that a conforming implementation MUST allow setting the value of R2, but I have found no way to do it using setsockopt(), ioctl() or such.

Another option would be to get a notification when R1 is exceeded (item e). This is not as good as setting R2, though, as I think R1 is hit pretty soon (in a few seconds), and the value of R1 cannot be set per connection, or at least the RFC doesn't require it.

Answer 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]> 

Answer 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.