NetFort Advertising

Monitoring Network Round Trip Time

Network Switch

Guide For Monitoring Network Round Trip Time

Keeping with our philosophy of trying to make maximum use of the information we get from a span port the engineering team have been busy adding the ability the track TCP connection round trip time to the LANGuardian. In the posting below I explain what network round trip time is and how knowing this value can help you manage your network.

The network round trip time (RTT) is a measurement of the time between a packet being sent and an acknowledgement being received for that packet, ie the sum of the path latencies in the forward and reverse directions for the packet. The network RTT can be influenced by some of the following factors

  • Queueing
    • Number of packets queuing at each router between the source and the destination.
  • Network Topology
    • Routing Topology, ie distance between routers.

Typically the ping command is used to measure this value on a IP network, however this approach has two disadvantages.

  • Modern networks differentiate the type of treatment they give to different protocols using Quality of Service (QoS) techniques. This may mean that a VOIP connection would experience a very different type of service than a ping packet that uses the ICMP protocol.
  • Some networks will block ICMP traffic making it impossible to use ping as a measurement of network RTT.

An alternative approach to monitoring the network RTT is to monitor the time taken for a TCP packet to be acknowledged. In using this approach the latency of live production protocols can be monitored. To get an accurate estimate of the network RTT this value must be measured during the TCP connection setup time prior to any application level delays occur.

The LANGuardian measures this time by recording the following two attributes for each TCP connection:

  • Timestamp of the servers SYN-ACK Packet
  • Timestamp of the clients ACK Packet

The network RTT is derived by subtracting those two values from each other.

For example in the screenshot below a TCP hand shake is shown

net 2

When this TCP connection was processed by the LANGuardian it outputted the RTT value at 64 microseconds

net 3

The Importance of Network Round Trip Time

  • The lower the network RTT the higher the end to end TCP performance.
  • For protocols where interactive communication occurs such as VOIP, Virtual Desktopping and gaming understanding where the delay is occurring between the client and server is crucial, ie is it the network or the application.