End to End Delay
Our discussion up to this point has focussed on the nodal delay, that is, the delay at a single router. Let’s now consider the total delay from source to destination. To get a handle on this concept, suppose there are N-1 routers between the source host and the destination host. Let’s also suppose for the moment that the network is uncongested ( so that queuing delays are negligible), the processing delay at each router and at the source host is dproc, the transmission rate out of each router and out of the source host is R bits/sec, and the propagation in each link is dprop. The nodal delays accumulate and give end-to-end delay,
dend-to-end = N (dproc + dtrans + dprop)
Where, once again, dtrans = L/R, where L is the packet size.
To get a hands-on feel for end-to-end delay in a computer network, we can make use of the Traceroute program. Traceroute is a simple program that can run in any internet host. When the user specifies a destination hostname, the program in the source host sends multiple, special packets toward the destination. As these packets work their way toward the destination, they pass through a series of routers. When a router receives one of these special packets, it sends back to the source a short message that contains the name and address of the router.
More specifically, suppose there are N-1 routers between the source and the destination. Then the source will send N special packets into the network, with each packet addressed to the ultimate destination. These N special packets are marked 1 through N, with the first packet market 1 and the last packet marked N. When the nth router receives the nth packet marked n, the router does not forward the packet toward its destination, but instead sends a message back to the source. When the destination host receives the Nth packet, it too returns a message back to the source. The source records the time that elapses between when it sends a packet and when it receives the corresponding return message; it also records the name and address of the router ( or the destination host) that returns the message. In this manner, the source can reconstruct the route taken by packets flowing from source to destination, and the source can determine the round-trip delays to all the intervening routers. Traceroute actually repeats the experiment just described three times, so the source actually send 3xN packets to the destination.