End-to-end delay or one-way delay (OWD) refers to the time taken for a packet to be transmitted across a network from source to destination. It is a common term in IP network monitoring, and differs from Round-Trip Time (RTT).
The ping utility calculates the RTT, that is, the time to go and come back to a host. This does not assure that the go and back paths are the same in terms of congestion, number of hops, or Quality of Service (QoS).
In order to avoid such problems, OWD concept comes into play. The end-to-end delay is calculated between two synchronized points A and B of an IP network, and it is the time that a packet spends in travelling across the IP network from A to B. The transmitted packets need to be identified at source and destination in order to avoid packet loss or packet reordering.
The measurement method makes obvious that this value is substantially different from the Round-Trip Time/2 value.
- dend-end= N[ dtrans+dprop+dproc+dqueue]
- dend-end= end-to-end delay
- dtrans= transmission delay
- dprop= propagation delay
- dproc= processing delay
- dqueue= Queuing delay
- N= number of links (Number of routers - 1)
Note: we have neglected queuing delays.
Each router will have its own dtrans, dprop, dproc hence this formula gives a rough estimate.
These four components are further subdivided:
- Nodal processing:
- Check bit errors
- Determine output link
- Time waiting at output link for transmission
- Depends on congestion level of router
- Transmission delay:
- R=Link bandwidth (bit/s)
- L=Packet length (bits)
- Time to send bits into link = L/R
- Propagation delay:
- d = Length of physical link
- s = Propagation speed in medium
- Propagation delay = d/s