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Description  |
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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 slope of a straight line placed through the points on
this graphical display will give an indication of the network bandwidth
whilst the intercept of the same straight line on the time axis will
provide an indication of propagation delay. Of course, the minimum journey
times could be processed computationally in order to derive the same
network characteristics. Where the correlation data includes both the
minimum and the mean journey times, graphical display of both against
packet size enables the mean queuing time for packets transmitted between
the nodes to be derived, this queuing time being the difference between
the minimum and mean times for any particular packet size; again, the
queuing time could be computed rather than graphically derived. Where the
correlation data comprises the mean journey times, graphical display of
these times against packet size can provide an indication of the network's
internal packet size since if the network splits up an original sequence
packet into smaller packets, this will be reflected in an increased
journey time which should show up as a step function on the graphical
display. Again, the networks internal packet size could be derived by
computation from the correlation data.
In another test according to the invention, the sequence of packets
transmitted from the first node comprises a succession of isolated packet
bursts each made up of a plurality of packets transmitted immediately one
after another, the correlation data being so derived as to indicate for
each packet position within said bursts as transmitted, the average packet
loss rate. If the minimum buffer size in the transmission path between the
nodes under consideration is less than the number of packets included in
each burst, the loss rate of packets should increase significantly as the
buffer size is exceeded by packets occurring later in each burst. This
test therefore enables the minimum internal buffer size to be determined.
In a further test according to the invention the sequence of packets
transmitted comprises at least one isolated packet burst made up of a
plurality of packets that are transmitted immediately one after another
and are of decreasing size through the packet; in this case, the
correlation dam is derived so as to indicate any difference in packet
sequencing between the transmitted and received bursts. Such re-sequencing
may be present where the network includes devices that prioritise short
packets.
BRIEF DESCRIPTION OF THE DRAWINGS
Three network tests according to the invention, and test apparatus
embodying the invention, will now be particularly described, by way of
non-limiting example, with reference to the accompanying diagrammatic
drawings, in which:
FIG. 1 is a diagram illustrating use of the test apparatus to ascertain
characteristics of a network between two given network nodes;
FIG. 2 is a diagram illustrating a main test sequence program of the test
apparatus and associated data structures.
FIG. 3A illustrated a first test sequence of packets for use in carrying
out a first one of the network tests;
FIG. 3B illustrates a second test sequence of packets for use in the first
network test;
FIG. 4 is a graph of minimum packet delay against packet size, derived by
the first network test;
FIG. 5 is a graph of mean packet delay against packet size, derived by the
first network test;
FIG. 6 illustrates a test sequence of packet bursts for use in a second one
of the network tests;
FIG. 7 is a graph derived by the second network test and from which the
minimum network path buffer size can be determined;
FIG. 8 illustrates a test sequence of packet bursts for use in a third one
of the network tests; and
FIG. 9 is a graph derived by the third network test and from which any
packet re-sequencing can be identified.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 illustrates a packet-based communication network 10 to which three
stations, 11, 12 and 13 are connected at respective network nodes 17, 18
and 19.
The network 10 will generally be made up of one or more links operating to
the same or different protocols and will typically include a number of
queuing devices such as bridges and gateways, generally depicted by the
buffer 9 in the FIG. 1 network. These devices serve to queue packets
received at the devices pending appropriate processing of the packets (for
example routing to the appropriate network link). The different links of
the network will typically operate at different speeds with different
internal packet sizes.
The stations 11, 12 and 13 interact with the network 10 through respective
protocol stacks 14, 15 and 16, which will generally be provided by
communications software running on a processor of the associated station
(this processor may be either a dedicated communications processor or a
central processor of the station, running in a multi-tasking mode). The
protocol stacks may be of any form suitable for the particular links to
which the corresponding stations are connected. Implementations of such
protocol stacks are well known in the art and will therefore not be
described in detail; however, these stacks may include, by way of example,
the TCP/IP protocol suite, that is, the Transmission Control
Protocol/Internet Protocol suite (for a further description of TCP/IP see,
for example, "Internetworking with TCP/IP", by Douglas E. Comer, second
edition 1991, Prentice-Hall International, Inc). The lower protocol levels
may be in accordance with the IEEE 802.2 link level protocol and
appropriate ones of the IEEE 802.3/4/5/6 physical layer protocol
standards. Although only three protocol layers have been illustrated for
each stack, 14, 15 and 16 in FIG. 1, this is purely done to simplify
illustration and more or less protocol layers may be present. As will
become clear below, in relation to the network characteristic measurements
to be described, at least pan of each protocol stack is within the network
10.
Station 11 is a test station incorporating the test apparatus embodying the
present invention. The test station 11 is operative to assess the
characteristics of the transmission path between the test station and a
remote station (hereinafter, the target station) by sending out sequences
of test packets to the target station. In the preferred implementation of
the invention, the target station is arranged to loop back the test
packets to the station 11; in FIG. 1, station 12 constitutes such a target
station effecting test packet loop-back (for example, stations 11 and 12
may both operate the IP protocol and the test packets transmitted by the
station 11 may either be echo request or timestamp request messages under
the ICMP protocol to which station 12 will respond accordingly). The
station 11 on receiving back the test packet sequence, correlates the
transmission and reception of the test packets in order to derive
characteristics of data transmission across the network 10 between the
test and target stations 11 and 12. Station 11 is, for example,
constituted by a portable instrument which may be moved between different
nodes on the network to measure transmission characteristics between
various nodes.
Station 11 comprises a program-controlled processor 20 operative to run a
main test sequence program 30, an input device 21 such as a keyboard, a
graphical output device 22 and a test sequence store 23 containing
sequence specifications for a number of different test packet sequences.
Each test sequence specification specifies sequence parameters such as
packet size, packet identity, inter packet gap and packet ordering. In
response to user input through the device 21, the processor 20 is
operative to run the test sequence program 30 to transmit a test packet
sequence corresponding to a selected one of the specifications held in
store 23, to a user specified target station for loop back to the test
station 11. In transmitting packets to the target station, the test
sequence program 30 utilizes the services provided by the protocol stack
14 to send and receive packets over the network 10. The basic service
provided by the protocol stack 14 is to transmit a test packet of
specified length and including specified packet D to a specified remote
station with a loop back request indication in the packet header. Upon the
packet being returned to the protocol stack 14, an interrupt is generated
to the processor 20.
The main test sequence program is operative to carry out any required
timing of packet transmission and receipt, or if such a facility is
provided by the protocol stack (as with timestamp request messages of the
ICMP protocol), to arrange for the stack to provide appropriate timing. In
addition, the test sequence program will correlate data on the transmitted
and received packets, analyze this data, and output the analysis results
to the user via the display 22.
The test sequence program 30 will now be considered in more detail with
reference to FIG. 2. As noted above, the program 30 is intended to send
out a sequence of test packets to a specified remote station for loop back
with the parameters of the test sequence being in accordance with a
selected one of a plurality of test packet sequence specifications held in
the store 23. Each test packet sequence specification can be represented
as a list 50 containing for each of a series of test packets, a packet ID
number, the length of the packet in bytes and the interval that is to
follow the packet before the next packet in the sequence is transmitted.
The list of test packets is terminated by an entry giving a packet ID of
"999".
In addition to the test packet sequence list 50, the test sequence program
also makes use of a data structure in the form of an event list 51. This
event list lists in time order the transmission and reception of test
packets by the station 11. For each event the list contains an indication
of whether or not the event concerned is the transmission or reception of
a packet CrX/RX in list 51 of FIG. 2), an indication of the ID number of
the packet concerned, and the time of the relevant event (packet
transmission or reception). Transmission events are entered in the event
list 51 by the main test sequence program 30, whilst reception events are
entered into the list 51 by an interrupt service routine 44, initiated by
the interrupt produced by the protocol stack 14 whenever a packet is
received.
The general operation of the test sequence program 30 will now be
described:
Upon initiation of the test sequence program (block 31 ) the user is asked
to enter the identity of the test sequence it is wished to run and also to
identify the remote station to which the test packets are to be sent for
loop back (block 32). Next, the test sequence program 30 retrieves from
the relevant test packet sequence fist 50 the parameters of the first test
packet of that sequence (block 33). Thereafter the program 30 controls the
transmission of a test packet to the specified remote station using the
protocol stack 14. The transmitted test packet will include a packet ID
number of 1 and be of a length specified in table 50 (block 34). As soon
as the protocol stack 14 has been instructed to transmit the test packet,
an entry is made in the event list 51 recording the fact that packet
number 1 has been transmitted at a particular time (block 35).
Next, the program 30 times an interval corresponding to that set out in the
list 50 for packet just transmitted, this interval being the desired delay
before the next packet is sent (block 36). At the end of this time out,
the parameters of the next packet to be transmitted are fetched from the
list 50 (block 37); except in the case where the ID number of this next
packet is "999", the program now loops back to block 34 to send this next
packet.
The loop constituted by blocks 34 to 38 is thereafter repeated until all
the test packets of the current test sequence have been sent. In due
course, the end-of-sequence packet with its ID number set to "999" is
encountered and at this point the program passes from block 38 to block
39. Block 39 concerns the analysis of the event list and the display of
results to the user. Since the process of analysis and display of results
will be dependent on the test packet sequence concerned, a separate
analysis and display routine 45 will be called by program 30 in dependence
on the test sequence concerned. After the analysis and display sub-routine
has terminated, the main test sequence program 30 itself terminates (block
40).
Execution of the main test sequence program 30 is interrupted each time a
packet is received by the protocol stack. When this occurs the interrupt
service routine 44 is run (block 41) and simply involves an entry being
made in the event list to record the receipt back of a test packet, the
entry identifying the test packet concerned and its time of receipt (block
42). Thereafter the interrupt service routine is terminated (block 43) and
the control is returned to the main program.
The time stamping of packet transmission and reception as recorded in the
event list is effected from a common clock run by the processor 20, this
being true whether the time stamping is effected externally of the
protocol stack 14 or as part of the services provided by the stack (in the
latter case, the transmit timestamp of a test packet may be entered in the
event list at the same time as the receipt timestamp, rather than earlier,
as both will generally be available together from the stack on receipt
back of the test packet).
The timing of the inter-packet gap, in block 36 of the main program
generally does not require significant accuracy, so that this timing can
be done in software with any extension of the interval due to the running
of the interrupt service routine 44, being small enough to be ignored. As
will become clear below, the main purpose of timing an inter-packet gap is
that in certain cases adjacent packets are required to influence the
network independently of each other and therefore need to be isolated by a
sufficient time interval.
Influence of Network on Packet
When a test packet is sent out over the network to a particular remote
station and looped back to the transmitting station, its round trip
journey time will depend on a number of factors. More particularly, the
round trip journey time will depend on packet length because timing is
normally effected between start of packet transmission and conclusion of
packet reception, so that the longer a packet is, the greater its round
trip journey time. Furthermore, the journey time will also depend on the
protocol stack level at which the timing takes place--if timing is
effected at one of the lower protocol stack levels, then the journey time
will be less than if the timing is effected at a higher level. Similarly,
the round trip journey time will be affected by the level in the protocol
stack at which loop back is effected at the remote station; generally it
is possible to effect loop back both at the link level layer and at the
transport layer or higher and, again, the higher the loop back layer, the
greater the journey time. It is for this reason that in FIG. 1, the
protocol stacks 14, 15 and 16 have been shown as at least partially within
the boundary of the network 10. Another factor affecting the round-trip
journey time is the speed of each link taken by the packet. A still
further factor is the amount of queuing experienced by the packet as it
passes through queuing devices such as bridges and gateways. The internal
packet size used by the links in the network may also effect round-trip
journey time, because if a packet as sent must be split up into several
smaller packets for transmission over one or more of the links before
being reassembled, then there is a greater likelihood that the greater
number of reduced-sized packets will be subject to some delay. Another
factor that may influence journey time is the length of the packet as
compared to other packets on the network because certain devices on the
network may be arranged to give priority to short packets and this could
significantly influence the round-trip journey time of a test packet. A
test packet may also be subject to other influences such as being lost or
being duplicated.
Whilst the basic round-trip journey time of a test packet can be directly
measured, the underlying network parameters that may influence journey
time cannot be detected from the passage of single test packet across the
network and it is for this reason that the present apparatus is arranged
to send out sequences of test packets.
First Test Method
Turning now to a consideration in detail of a first test carried out by the
apparatus of the present invention, reference is made to FIGS. 3A and 3B
which illustrate two possible test packet sequences for use in this test.
More particularly, in FIG. 3A a first test sequence is illustrated in
which twenty test packets are sent comprising four packets for each of
five different sizes ranging from 500 bytes to 2,500 bytes. Each test
packet is separated from its neighbours by a time interval "t". The time
interval "t" is chosen such that each packet is isolated from its
neighbours in the sense that any effect on the network caused by one
packet has disappeared by the time the next packet is transmitted.
The test sequence shown in FIG. 3B is similar to that shown in FIG. 3A and
includes the same test packets but arranged somewhat differently into four
sets of five differently sized packets. However, again the packets are
each isolated from each other by a time interval "t".
Either of the test packet sequences shown in FIGS. 3A and 3B may be used in
carrying out the first test. The test packet sequence list 50 illustrated
in FIG. 2 shows the list entries for the first few packets of the FIG. 3A
sequence.
To carry out the first test, the test sequence program 30 is run and the
user chooses either the test sequence of FIG. 3A or of FIG. 3B and
identifies the remote station concerned. Thereafter, the program 30
transmits the test sequence and, together with the interrupt service
routine 44, builds up the event list 51 indicating the time of
transmission and receipt of the various test packets of the selected test
sequence. Once all the packets have been transmitted, the test program 30
calls the appropriate analysis and display sub-routine. For the first
test, this sub-routine is operative to analyze the event list and provide
a further list of packet identity number against round-trip time, this
information being readily derived by subtracting the time of transmission
of a packet from its time of receipt, as recorded in the event list. Then
for each packet size involved in the test packet sequence, the sub-routine
derives a minimum and mean round trip time and these values are
graphically displayed against packet size on the output device 22.
FIG. 4 shows the graph of minimum delay against packet size for typical
test results. As can be seen a straight line 70 has been plotted through
the minimum delay values. The intercept of the line 70 on the minimum
delay axis gives the overall propagation delay between the test station
and the remote station 12 (that is, the delay that a zero length packet
would experience without queuing). This is so because provided that a
sufficient number of test packets of each size are submitted, it may be
expected that at least one of these packets will pass through the network
without queuing and it will be this packet which records the minimum
round-trip time.
The FIG. 4 graph can also be used to obtain an estimate of the bandwidth
between the test station and the remote station 12 by considering the
minimum delay line 70 to be a straight line satisfying the general
equation:
Overall delay=(Pacet size/Bandwidth)+Propagation Delay
In this case, the bandwidth is the reciprocal of the slope of line 70. In
fact, of course, there will generally be a plurality of network hops
between the test station and remote station 12 each with its own bandwidth
and in these circumstances taking the reciprocal of the slope of the line
70 as the bandwidth is equivalent to saying that:
(1/Bandwidth)=(1/b1+1/b2+. . . 1/bn)
where b1, b2 . . . bn are the bandwidths of all the hops in the path. This
will only produce a bandwidth estimate close to accurate if there is one
hop in the path that is significantly slower than all the others. Where
this is not the case, the estimate may be somewhat inaccurate (the
inaccuracy arises because no account is taken of the fact that for a
multi-hop path, the intermediate nodes will delay onward transmission of a
packet until the whole packet has been received).
The graph of mean delay against packet size shown in FIG. 5 reveals further
characteristics of the transmission path between the stations 11 and 12.
This graph shows a fragmented line 71, 72 drawn between the mean delay
points on the graph; in addition the minimum delay line 70 of FIG. 4 is
also shown on the FIG. 5 graph. The difference between the mean and
minimum round-trip time delays for the various packet sizes provides an
indication of the average queuing delay for each size of packet. The
discontinuity in the mean delay line 71, 72 results from the fact that the
internal packet size of one or more of the network links has been exceeded
by the larger of the transmitted test packets, so that the test packets
have been segmented and subject to greater delay in queuing devices of the
network. Thus, from FIG. 5 it can be deduced that in the transmission path
followed by the test packets between the stations 11 and 12, one or more
of the network links has an internal packet size greater than 1000 bytes,
but less than 1500 bytes.
In addition to the above described network characteristics that can be
derived from the first test (that is bandwidth, propagation delay, average
queuing delay and internal network packet size) further characteristics
can be derived from the event list 51. These further characteristics
include packet lost rate and packet duplication rate.
Instead of, or additionally to the graphical displays illustrated in FIGS.
4 and 5, the sub-routine called by the main program 39 for the purpose of
analyzing the event list can be arranged to calculate the various network
characteristics described above as being derived from the display graphs.
Second Test Method
The second test implemented by the test apparatus embodying the present
invention utilizes a test packet sequence of the form illustrated in FIG.
6. In the FIG. 6 test sequence, a plurality of bursts of back-to-back test
packets are provided, each burst being isolated from the other bursts by
interval T, such that the effect of one burst on the network has subsided
before the next burst is sent. As illustrated, the test packets may all be
of the same duration, although for the purposes of the second test, this
is not necessary (indeed, the test sequence of FIG. 8 used for the third
test described below can also be used for the second test, though the
length of the test packets varies through each burst).
The second test is carried out by running the main test sequence program 30
in the manner described above for the first test, except that now the test
packet sequence list corresponding to FIG. 6 is accessed by the programme
and used to generate the appropriate test sequence. Furthermore, analysis
of the event list generated by the FIG. 6 test sequence is affected by a
sub-routine specific to the second test.
The purpose of the second test is to identify the minimum buffer length in
the transmission path between the test station and the selected remote
station 12; this is achieved by saturating the buffer such that packets
are lost. In order to achieve the desired saturation, the test sequence
includes bursts of packets, each burst effectively being one run through
of the second test.
In analyzing and displaying the results of the second test, the event list
is processed to determine the mean loss rate of packets for each packet
send position in each test sequence burst. The results of this processing
are then displayed on the display 22 in the form of a graph plotting
percentage of packets received against send position in burst, as
illustrated in the FIG. 7.
In FIG. 7, packets in the first three positions in a burst are shown as
being received at a near 100% rate; however, packets in the fourth and
fifth position in a burst are only rarely received. A graph of this form
provides a strong indication that there is a buffer in the transmission
path between the stations 11 and 12 which can only hold three packets
before over-flowing. It will be appreciated that not only can this
information be derived graphically from the results of analyzing the event
list, but it would also be possible to process the analyzed results to
derive such an indication.
Third Test Method
The third test implemented by the test apparatus according to the invention
is intended to ascertain whether any re-sequencing of packets takes place
in transmission between the stations 11 and 12. To this end a test
sequence of a form illustrated in FIG. 8 is used. As can be seen, the FIG.
8 test sequence comprises a plurality of test packet bursts isolated from
each other by a time interval T. Within a burst, the packets are of
decreasing size and are sent back-to-back. A test sequence of this form is
likely to identify any re-sequencing behaviour of the network because such
behaviour normally involves giving priority to shorter packets and these
packets are included towards the end of each burst; if re-sequencing
occurs, then it may be expected that the shorter packets will be received
earlier in each burst.
The third test is carried out by running the main test sequence program 30
in the manner described above for the first and second test, the primary
difference being the program now utilizes a test packet sequence list
corresponding to FIG. 8 to control the transmission of test packets.
Furthermore, the sub-routine for analyzing and displaying the results of
the event list generated by the third test is specific to the third test.
This analysis and display sub-routine is arranged to derive for each
receive position in a burst, the mean percentage of packets received by
their origin in the transmitted burst. The analysis results are displayed
on display 22 in a graph of the form illustrated in FIG. 9. Thus, it can
be seen that the first position in a burst received back by the test
station 11 is primarily occupied by the first test packet sent out in a
burst, but that in addition, the first receive position is also occupied
by a small percentage of second and third packets sent out in a burst. It
may be deduced from the FIG. 9 graph that re-sequencing of packets does
take place over the network path between stations 11 and 12 and that this
re-sequencing effects packets at least as large as 1500 bytes (the size of
the second test packet in each burst).
Variants
Various modifications and changes to the described test apparatus and
methods are, of course, possible. Thus, for example, instead of arranging
for each test packet to be looped back by the remote station, the same
information regarding network characteristics as derived above using
looped back test packets can be derived by one way transmission of the
test packet sequences with the remote station collecting each test packet
and noting its time of receipt. Such an arrangement is illustrated by the
collect and return station 13 in FIG. 1. Because analysis of the test
results requires, at least for some of the tests, a comparison of the time
of transmission and of receipt of test packets, the time stamping effected
by the collect and return station 13 must be co-ordinated with the
transmission time stamping effected by the test station 11. This can be
achieved in a number of ways, for example, by arranging for each station
to run the Network Time Protocol which serves to synchronize clocks at the
stations with a master clock system. After the collect and return station
13 has generated a receive event list similar to the event list 51 of FIG.
2, this list can be returned at the completion of the test sequence to the
test station 11 for analysis with the event list generated at the test
station at the time of packet transmission.
In fact, it is generally not necessary to have the test and target station
clocks closely synchronized with each other. The reason for this is that
much of the desired information can be derived by looking at variations in
trip time, rather than the actual value of trip time. Trip time variations
can be easily determined by knowing the time of transmission from the test
station 11 and by having a constant rate clock at the collect and return
station 13.
Because the provision of a loop back facility at a network station is very
common, carrying out the above described tests using packet loop back is
preferred over the collect and return approach embodied by station 13,
since in the latter case, special software will need to be provided at the
station it is wished to equip with this facility.
Certain network protocols such as the IP protocol with its associated ICMP
protocol, provide a loop facility with time stamping at the remote station
as well as at the originating station (for IP networks, this facility is
provided by ICMP Timestamp messages). In this case, it is possible to
derive information on each direction of transit between test and target
stations, This generally involves an estimate being made regarding clock
offset between the test and target station clocks, this offset being
estimated assuming a symmetrical path between the two machines; if this
assumption is not approximately correct, this will normally show up as
differing values for bandwidth in the two directions.
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