Tag Archives: Time events

8.4. Time events

Timers provide a great help in troubleshooting network problems. All values are reported in milliseconds (ms).

These timers should be used in conjunction with the session termination flags. In TCP mode with “option tcplog” set on the frontend, 3 control points are reported under the form “Tw/Tc/Tt”, and in HTTP mode, 5 control points are reported under the form “Tq/Tw/Tc/Tr/Tt” :

  • Tq: total time to get the client request (HTTP mode only). It’s the time elapsed between the moment the client connection was accepted and the moment the proxy received the last HTTP header. The value “-1” indicates that the end of headers (empty line) has never been seen. This happens when the client closes prematurely or times out.
  • Tw: total time spent in the queues waiting for a connection slot. It accounts for backend queue as well as the server queues, and depends on the queue size, and the time needed for the server to complete previous requests. The value “-1” means that the request was killed before reaching the queue, which is generally what happens with invalid or denied requests.
  • Tc: total time to establish the TCP connection to the server. It’s the time elapsed between the moment the proxy sent the connection request, and the moment it was acknowledged by the server, or between the TCP SYN packet and the matching SYN/ACK packet in return. The value “-1” means that the connection never established.
  • Tr: server response time (HTTP mode only). It’s the time elapsed between the moment the TCP connection was established to the server and the moment the server sent its complete response headers. It purely shows its request processing time, without the network overhead due to the data transmission. It is worth noting that when the client has data to send to the server, for instance during a POST request, the time already runs, and this can distort apparent response time. For this reason, it’s generally wise not to trust too much this field for POST requests initiated from clients behind an untrusted network. A value of “-1” here means that the last the response header (empty line) was never seen, most likely because the server timeout stroke before the server managed to process the request.
  • Tt: total session duration time, between the moment the proxy accepted it and the moment both ends were closed. The exception is when the “logasap” option is specified. In this case, it only equals (Tq+Tw+Tc+Tr), and is prefixed with a ‘+’ sign. From this field, we can deduce “Td”, the data transmission time, by subtracting other timers when valid :
         Td = Tt - (Tq + Tw + Tc + Tr)

Timers with “-1” values have to be excluded from this equation. In TCP mode, “Tq” and “Tr” have to be excluded too. Note that “Tt” can never be negative.

These timers provide precious indications on trouble causes. Since the TCP protocol defines retransmit delays of 3, 6, 12… seconds, we know for sure that timers close to multiples of 3s are nearly always related to lost packets due to network problems (wires, negotiation, congestion). Moreover, if “Tt” is close to a timeout value specified in the configuration, it often means that a session has been aborted on timeout.

Most common cases :

  • If “Tq” is close to 3000, a packet has probably been lost between the client and the proxy. This is very rare on local networks but might happen when clients are on far remote networks and send large requests. It may happen that values larger than usual appear here without any network cause. Sometimes, during an attack or just after a resource starvation has ended, haproxy may accept thousands of connections in a few milliseconds. The time spent accepting these connections will inevitably slightly delay processing of other connections, and it can happen that request times in the order of a few tens of milliseconds are measured after a few thousands of new connections have been accepted at once. Setting “option http-server-close” may display larger request times since “Tq” also measures the time spent waiting for additional requests.
  • If “Tc” is close to 3000, a packet has probably been lost between the server and the proxy during the server connection phase. This value should always be very low, such as 1 ms on local networks and less than a few tens of ms on remote networks.
  • If “Tr” is nearly always lower than 3000 except some rare values which seem to be the average majored by 3000, there are probably some packets lost between the proxy and the server.
  • If “Tt” is large even for small byte counts, it generally is because neither the client nor the server decides to close the connection, for instance because both have agreed on a keep-alive connection mode. In order to solve this issue, it will be needed to specify “option httpclose” on either the frontend or the backend. If the problem persists, it means that the server ignores the “close” connection mode and expects the client to close. Then it will be required to use “option forceclose”. Having the smallest possible ‘Tt’ is important when connection regulation is used with the “maxconn” option on the servers, since no new connection will be sent to the server until another one is released.
  • Other noticeable HTTP log cases (‘xx’ means any value to be ignored) :

    • Tq/Tw/Tc/Tr/+Tt The “option logasap” is present on the frontend and the log was emitted before the data phase. All the timers are valid except “Tt” which is shorter than reality.
    • -1/xx/xx/xx/Tt The client was not able to send a complete request in time or it aborted too early. Check the session termination flags then “timeout http-request” and “timeout client” settings.
    • Tq/-1/xx/xx/Tt It was not possible to process the request, maybe because servers were out of order, because the request was invalid or forbidden by ACL rules. Check the session termination flags.
    • Tq/Tw/-1/xx/Tt The connection could not establish on the server. Either it actively refused it or it timed out after Tt-(Tq+Tw) ms. Check the session termination flags, then check the “timeout connect” setting. Note that the tarpit action might return similar-looking patterns, with “Tw” equal to the time the client connection was maintained open.
    • Tq/Tw/Tc/-1/Tt The server has accepted the connection but did not return a complete response in time, or it closed its connection unexpectedly after Tt-(Tq+Tw+Tc) ms. Check the session termination flags, then check the “timeout server” setting.
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