Tip of the Month

September 2001

Relay Delay? No Problem With Two Analyzers...

It often pays to have more than one set of eyes (analyzers) in helping to pinpoint delays in your network. Use this simple methodology to approximate the round-trip packet delay added by a switch, router, frame relay cloud, or between any two points in your network infrastructure:

1. Use two protocol analyzers to capture packets on either side of the device or portion of your network under test. The higher the timestamp accuracy and resolution on the analyzer (such as that offered by EtherPeek), the better. The analyzers do NOT have to be time synchronized!

2. Match an exact command/response packet pair in both analyzers.

3. Take the difference in timestamps between a command and the corresponding reply packet (either by subtracting the absolute time by hand or by filtering on the command/reply pair and observing the delta time) in the first analyzer and subtract from that the difference in timestamps between the same in the second analyzer.

For example, in a capture from two analyzers on either side of a router, analyzer 1 command packet "x" timestamp = 10.00121 seconds, analyzer 1 reply packet "y" timestamp = 10.01491. Analyzer 2 command "x" timestamp = 45.10002 seconds, analyzer 2 reply "y" timestamp = 45.11202.

Approximate round trip delay through the router = (10.01491 - 10.00121) - (45.11202 - 45.10002) = 0.00170 seconds or 1.70 milliseconds. (Keep in mind that a packet is usually timestamped after it is received; thus, the "delay" time also includes the transmission time of the reply packets.) Note that the overall round trip delay as measured by analyzer 1 is 13.7 milliseconds.

Short packets such as small ping packets (echo and echo reply) work well. Do this for several samples and you will get a pretty good idea of the average delay added by the device or devices/transmission media that you are measuring.

WILDPACKETS ACADEMY COURSES

• WP-100 Foundations Of Network Protocol Analysis
• WP-101 Network Troubleshooting Methods Using EtherPeek
• WP-102 Full-Duplex and Switched Ethernet Analysis
• WP-103 TCP/IP Protocol Analysis Methods
• WP-104 Advanced TCP/IP Protocol Analysis
• WP-105 AppleTalk and Mac OS/X Network Analysis
• WP-106 802.11 Wireless Network Analysis Using AiroPeek
• WP-106A 802.11 WLAN Analysis Using AiroPeek + AiroPeek Orientation
• WP-107 LAN/WAN Special Topics

For more information on WildPackets Academy, to download a course catalogue, to peruse the training schedule, or to register for any of our courses, please visit http://www.wildpackets.com/services/academy/overview.