Multiplex communications – Wide area network – Packet switching
Patent
1993-12-13
1995-12-19
Chin, Wellington
Multiplex communications
Wide area network
Packet switching
370 60, 370 941, H04J 326, H04L 114, H04L 1226
Patent
active
054775310
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and apparatus for testing a packet-based communications network such as a computer network.
BACKGROUND
In packet-based networks, it is often desired to test communications between two specific nodes on the network. This can generally be effected from a first one of the nodes by requesting the other node to `loop-back` a test packet sent from the first node. The first node, on receiving back the test packet, can thereby ascertain not only that communication is possible with the other node, but also the round trip time for the packet. Typically such a test would be performed by an instrument such as the HP 4982A LAN protocol analyzer (available from Hewlett-Packard Company, Palo Alto, Calif.) connected to the network at the first node.
Inter-networking protocols such as the ARPA Internet Protocol (IP) may also provide a facility for determining reachability and round trip time by use of a looped-back test packet. Thus, in IP networks the Internet Control Message Protocol CiCMP) allows control and information messages, including echo request and echo reply messages, to be passed in the data portion of IP datagrams between IP software on different hosts and gateways. Other messages allowed by ICMP include timestamp request and timestamp reply messages which permit a transit time estimate to be made in both transit directions. Certain computer operating systems such as the Hewlett-Packard HPUX operating system permit users to send ICMP echo requests using a command named "ping" with the users being able to specify the number and size of test packets for which round trip times are returned.
It may also be noted that in the ARPA Transmission Control Protocol (TCP), a similar measurement is continually made as part of the transmission control process. More particularly, the round-trip time is measured between the transmission of a packet and receipt back of an acknowledgement from the destination node; this round-trip time is averaged continually into a smoothed round-trip time estimate which is then used to control the retransmissions time-out parameter RTO.
The use of looped-back test packets has thus been restricted to determining the directly-observable round-trip time characteristic of the transmission path between two nodes.
It is an object of the present invention to permit further network characteristics to be determined from the operation of transmitting packets between two nodes, these characteristics being those which are not discernible from the transmission of a single packet between the nodes.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, there is provided a method of testing a packet-based network to ascertain characteristics of packet transmission between first and second nodes on the network, said method comprising the steps of transmitting packets between said nodes, receiving the packets at one said node, and correlating packet transmission and reception to derive correlation data indicative of a said transmission characteristic, characterised in that said packets are transmitted as a sequence in which the packets are in a predetermined relationship to each other, said correlation data being so derived as to be sensitive to this relationship whereby to enable a characteristic of transmission to be determined that is unobservable from the passage of a single packet.
Preferably, at least one of the sequence parameters comprising inter-packet spacing and packet size, varies through said sequence in accordance with said predetermined relationship.
In one test according to the invention, said sequence of packets comprises for each of a multiplicity of packet sizes, a respective plurality of packets, the packets of said sequence being transmitted in isolation from each other.
The correlation data for this test will comprise the minimum and/or the mean packet journey time for each packet size. Where the minimum journey times are derived, these are preferably graphically displayed against packet size; the sl
REFERENCES:
patent: 4551833 (1985-11-01), Turner
patent: 5101402 (1992-03-01), Chui et al.
patent: 5343465 (1994-08-01), Khalil
"Measuring a Local Network's Performance", by P. D. Amer et al, Data Communications, vol. 12, No. 4, Apr. 1983, pp. 173-182.
"Statistical Monitors for Local Area Networks", D. C. Feldmeier, 11th Conference on Local Computer Networks, Oct. 8, 1986, pp. 142-146.
"Internetworking With TCP/IP", by Douglas Comer, 2nd edition 1991, Prentice-Hall International, Inc.
McKee Neil H.
Phaal Peter
Chin Wellington
Hewlett--Packard Company
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