## 5 Case Studies on the OMF-Enabled Testbed
### Case Study 1: Round-Trip Time Measurements  
In this section, the accuracy of a passive RTT measurement tool, named the RTT-tool [26], is validated using the results of two active ping tools as reference values. More specifically, the measurements of the passive TCP-based RTT-tool are compared with the results of two active ICMP-based and UDP-based approaches which, hereafter, are referred to as icmpping and udpping, respectively. As the names of the tools suggest, icmpping uses ICMP Echo Request and ICMP Echo Response packets, whereas udpping is based on sending and receiving UDP request and UDP response datagrams, respectively. The RTT is the total time taken for a packet to travel from the sender to the receiver and back to the sender. It is expressed as:  
\[RTT=2\cdot\left(t_{\text{queue}}+t_{\text{propagation}}+t_{\text{transmission}}+t_{ \text{forwarding}}\right) \tag{1}\]  
In Equation (1), \(t_{\text{queue}}\) represents the time spent waiting in queues due to congestion or scheduling delays, while \(t_{\text{propagation}}\) is the time required for the signal to propagate through the medium, such as fiber optics or copper wires. The term \(t_{\text{transmission}}\) accounts for the time needed to push the packet bits onto the link, depending on the link bandwidth, and \(t_{\text{forwarding}}\) refers to the time taken by intermediate devices like routers to process and forward the packet. Together, these components determine the total RTT under symmetric and additive delays. The passive RTT-tool implements the SYN/ACK (SA) methodology, which measures the RTT during the setup of a TCP connection. Establishing a TCP connection consists of a sequence of three segments: SYN, SYN/ACK, and ACK, collectively called the three way handshake (TWH). The passive RTT-tool, placed anywhere on the path between the client and server, measures the RTT by observing these three consecutive segments. The arrival times of the initial SYN segment (\(t_{\text{SYN}}\)) and the ACK segment (\(t_{\text{ACK}}\)) following the SYN/ACK segment (\(t_{\text{SYN/ACK}}\)) are recorded. Subsequently, the end-to-end RTT can be determined using the equation:  
\[RTT_{\text{passive}}=t_{\text{ACK}}-t_{\text{SYN}} \tag{2}\]  
In Equation (2), \(t_{\text{SYN}}\) is the timestamp when the SYN segment is sent, \(t_{\text{ACK}}\) is the timestamp when the ACK segment is received, and \(RTT_{\text{passive}}\) represents the calculated round-trip time based on the passive methodology. Although \(t_{\text{SYN/ACK}}\) is part of the process, it is not directly used in the final calculation. Accurate measurement requires that none of the three segments are lost and that the client or server sends the segments immediately [27]. To validate the results of the passive RTT-tool against the ones of the active icmpping and udpping tools, a similar experimental methodology used in [27] was implemented in an experiment description file using the testbed setup and configuration illustrated in Figure 2(a). Particularly, as the SA methodology examines the TWH phase of a TCP connection, the active ping measurements, the results of which are used for reference purposes, are collected just before the connection establishment. The Apache web server operated on research node R illustrated in Figure 2(a) forms the measurement target. An available HTTP file, larger than 10 KB, is provided by the web server. First, to actively measure the RTTs of the path which will represent the reference values for the SA estimations, the prepared experiment description file first triggers the active ping measurements from research node S to research node R. Immediately after the RTT for the path has actively been measured by icmpping and udpping, the experiment description file causes the client to download the HTTP file from the target research node R using the GNU Wget utility over the same path. After the transfer is over, the RTTs of the HTTP-TCP transfer for the same path is passively estimated by the RTT-tool.