End to end delay

Measurement

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 most common method by which OWDs are calculated between two points A and B of an IP network is to first synchronize their clocks; A records a timestamp on the packet and sends it to B, which notes the receiving time and calculates the OWD as their difference. The transmitted packets need to be identified at source and destination in order to avoid packet loss or packet reordering. This method however suffers several limitations, such as requiring intensive cooperation between both parties, and the accuracy of the measured delay is subject to the synchronization precision. On the other hand, the Minimum-Pairs protocol is an example by which several cooperating entities, A, B, and C, could measure OWDs between one of them and a fourth less cooperative one (e.g., between B and X).

The measurement method makes obvious that this value is substantially different from the Round-Trip Time/2 value. Accordingly, new OWD-estimation protocols must evaluate the expected precision before it can be practical to use in practice.

Delay components

d_end-end= N[ d_trans+d_prop+d_proc+d_queue]

where

d_end-end= end-to-end delay d_trans= transmission delay d_prop= propagation delay d_proc= processing delay d_queue= Queuing delay N= number of links (Number of routers - 1)

Note: we have neglected queuing delays.

Each router will have its own d_trans, d_prop, d_proc hence this formula gives a rough estimate.

These four components are further subdivided:

1. Nodal processing:
   1. Check bit errors
   2. Determine output link
2. Queuing:
   1. Time waiting at output link for transmission
   2. Depends on congestion level of router
3. Transmission delay:
   1. R=Link bandwidth (bit/s)
   2. L=Packet length (bits)
   3. Time to send bits into link = L/R
4. Propagation delay:
   1. d = Length of physical link
   2. s = Propagation speed in medium
   3. Propagation delay = d/s