End-to-End Delay

End-to-End Delay

So far we have focused on the nodal delay, that is, the delay at a single router. Lets now examine the total delay from source to destination. To get a handle on this notion, 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 insignificant), 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 on each link is dPROP. The nodal delays accumulate and give an end-to-end delay,

         dend-end = N (dproc + dtrans + dprop)

where, once again, dtrans = L/R, where L is the packet size. We leave it to you to generalize this formula to the case of heterogeneous delays at the nodes and to the presence of an average queuing delay at each node.

Traceroute

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 that 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 includes the name and address of the router.

More particularly, 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 marked 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 way, the source can build again the route taken by packets flowing from source to destination, and the source can decide the round-trip delays to all the intervening routers. Traceroute in fact repeats the experiment just explained three times, so the source actually sends 3 ● N packets to the destination. RFC 1393 describes Traceroute in detail.

Here is an example of the output of the Traceroute program, where the route was being traced from the source host gaia.cs.umass.edu (at the University of Massachusetts) to the host cis.poly.edu (at Polytechnic University in Brooklyn). The output has six columns: the first column is the n value described above, that is, the number of the router along the route; the second column is the name of the router; the third column is the address of the router (of the form xxx.xxx.xxx.xxx); the last three columns are the round-trip delays for three experiments. If the source receives fewer than three messages from any given router (due to packet loss in the network), Traceroute places an asterisk just after the router number and reports fewer than three round-trip times for that router.

trace

In the trace above there are nine routers between the source and the destination. The majority of these routers have a name, and all of them have addresses. For instance, the name of Router 3 is border4-rt-gi-1-3.gw.umass.edu and its address is 128.119.2.194. Looking at the data provided for this same router, we see that in the first of the three trials the round-trip delay between the source and the router was 1.03 msec. The round-trip delays for the subsequent two trials were 0.48 and 0.45 rnsec. These round-trip delays include all of the delays just discussed, including transmission delays, propagation delays, router processing delays, and queuing delays. Because the queuing delay is changeable with time, the round-trip delay of packet n sent to a router n can sometimes be longer than the round-trip delay of packet n+1 sent to router n+1.  In fact, we observe this phenomena in the above example: the delays to Router 6 are larger than the delays to Router 7.

Want to try out Traceroute for yourself? We highly recommended that you visit http://www.traceroute.org, which provides a Web interface to an extensive list of sources for route tracing, You choose a source and supply the hostname for any destination. The Traceroute program then does  all the work. There are many free software programs that provide a graphical interface to Traceroute; one of our favorites is PingPlotter [PingPlotter 2009].

End System, Application, and Other Delays

In addition to processing, transmission, and propagation delays, there can be extra significant delays in the end systems. For example, diul-up modems introduce a modulation/encoding delay, which can be on the order of tens of milliseconds. (The modulation/encoding delays for other access technologies - including Ethernet, cable modem, and DSL - are less significant and usually unimportant.) An end system wanting to transmit a packet into a shared medium (e.g., as in a WiFi or Ethernet scenario) may purposefully delay its transmission as part of its protocol for sharing the medium with other end systems; we'll examine such protocols in detail in "The Link Layer and Local Area Networks". Another important delay is media packetization delay, which is present in Voice-over-IP (VoIP) applications. In VoIP, the sending side must first fill a packet with encoded digitized speech before passing the packet to the Internet. This time to fill a packet - called the packetization delay - can be significant and can impact the user - perceived quality of a VoIP call.


Tags

nodal delay, transmission rate, transmission delays

Copy Right

The contents available on this website are copyrighted by TechPlus unless otherwise indicated. All rights are reserved by TechPlus, and content may not be reproduced, published, or transferred in any form or by any means, except with the prior written permission of TechPlus.