...
- Propagation delay(PD): A time required to send a single bit from one hop to another along the path.
- Transmission delay(TD): A time required to transmit whole packet from a hop along the outgoing link.
- Queing delay(QD): A time required by the packet to wait in the buffer before being ready for transmission into an outgoing link.
Total Total delay is the sum of all PD,TD and QD along the path. Round trip time(RTT) is equal to 2 * total delay, considering reverse path takes the same route and delays. Note: PD is normally taken as 0, compared to other delays.
Lets Lets say, we have to find minimum buffer sizes along the path. Practically, it is very difficult to measure the exact buffer size of each hop along the path. We can somehow easily estimate the queing delay along the path using ping and traceroute tools. From queing delay, we can estimate congestion, buffer overflow,incoming and outgoing data rate. If we take the difference of incoming and outgoing datarate that will give us the queue occupancy. From queue occupancy, we can roughly estimate the buffer size of that specific router. In the following sections, I am going to explain how we can measure the average queing delay using ping, and queing delay at each and every hop along the path using traceroute.
...
Lets say we have two hops(R1,R2) between source A and destination B. Traceroute measure three different RTts from A to R1 and R2 respectively. and lets denote those rtts by (RTT1, RTT2, RTT3) for R1 and (RTT1, RTT2,RTT3) for R2.
Now get the Max and min rtt from the two sets separately. and denote it minRTT1, maxRTT1 for R1 and minRTT2 and maxRTT2 for R2.
It is simple to find queing delay at R1 which is:
Queing Delay at R1 (QD1) = maxRTT1 - minRTT1
...
Infact queing delay is variable, and three RTTs are not enough to measure a good estimate of queing delay. In this case, we need to measure the analysis of like arround 10-100 RTTs to get a sufficient good estimate of queing delay. The more RTTs we have in a set for a router, the more better estimate we can fetch.
Once we have the queing delay from source to router A, we can easily find the tramission delay. Once we have transmission delay we can figure out the Incoming data rate of that router.
We know that RTT from A to R1 = 2(PdA + Td1 + Qd1), lets consider Pd, is 0 compare to Td and Qd then
RTT from A to R1 = 2(Td1 + Qd1)
Td1 = RTT/2 - Qd1
RTT from A to R2 = 2(Td1 + Qd1) + 2(Td2 + Qd2)
= 2(Td1 + Td2 + Qd1 + Qd2)
Td2 = RTT/2 -Td1 - Qd1 - Qd2
In order to get a good estimate of TD we calculate the average RTT for each hop avgRTT1 = avg(RTT1, RTT2, RTT3) for router R1.
Hence the transmission delay for nth hop is given below:
| Panel |
|---|
Td1 = avgRT1/2 - Qd1 |