One Way Delay

Metric Parameters

• Src, the IP address of a host
• Dst, the IP address of a host
• T, a time
• S, a ToS value
• V, the IP version field
• L, the packet lenght in bits

Metric Units

Definition

• For a real number dT, >>the *One-way-Delay* from Src to Dst at T for a given S, V and L, is dT<< means that Src sent the first bit of a packet to Dst at wire-time* T and that Dst received the last bit of that packet at wire-time T+dT.
• >>The *One-way-Delay* from Src to Dst at T for a given S, V and L, is undefined (informally, infinite)<< means that Src sent the first bit of a packet to Dst at wire-time T and that Dst did not receive that packet.

Accuracy

An very interresting point is which accuracy could be expected, when presenting a "e2e" measurement as being the sum of several mesurements. ("e2e" means either end-to-end or edge-to-edge across several networks)

Methodologies

• Arrange that Src and Dst are synchronized; that is, that they have clocks that are very closely synchronized with each other and each fairly close to the actual time.
• At the Src host, select Src and Dst IP addresses, Type of Service and IP version, and form a test packet of with these parameters. Any 'padding' portion of the packet needed only to make the test packet a given size should be filled with randomized bits to avoid a situation in which the measured delay is lower than it would otherwise be due to compression techniques along the path.
• At the Dst host, arrange to receive the packet.
• At the Src host, place a timestamp in the prepared packet, and send it towards Dst.
• If the packet arrives within a reasonable period of time, take a timestamp as soon as possible upon the receipt of the packet. By subtracting the two timestamps, an estimate of one-way delay can be computed. Error analysis of a given implementation of the method must take into account the closeness of synchronization between Src and Dst. If the delay between Src's timestamp and the actual sending of the packet is known, then the estimate could be adjusted by subtracting this amount; uncertainty in this value must be taken into account in error analysis. Similarly, if the delay between the actual receipt of the packet and Dst's timestamp is known, then the estimate could be adjusted by subtracting this amount; uncertainty in this value must be taken into account in error analysis.
• If the packet fails to arrive within a reasonable period of time, the one-way delay is taken to be undefined (informally, infinite).