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ICMPv4 Timestamp (Request) and Timestamp Reply Messages
(Page 2 of 3)
ICMPv4 Timestamp and Timestamp Reply Message Format
The ICMPv4 Timestamp and Timestamp
Reply messages have the same format. The originating device fills
in some of the fields, and the replying device others. The format is
as shown in Table 97
and Figure 147.
Table 97: ICMPv4 Timestamp and Timestamp Reply Message Format
Field
Name
|
Size (bytes)
|
Description
|
Type
|
1
|
Type: Identifies
the ICMP message type. For Timestamp messages the value is 13;
for Timestamp Reply messages the value is 14.
|
Code
|
1
|
Code:
Not used for Timestamp and Timestamp Reply messages; set
to 0.
|
Checksum
|
2
|
Checksum: 16-bit
checksum field for the ICMP header, as described in the
topic on the ICMP common message format.
|
Identifier
|
2
|
Identifier:
An identification field that can be used to help in matching Timestamp
and Timestamp Reply messages.
|
Sequence
Number
|
2
|
Sequence Number:
A sequence number to help in matching Timestamp and Timestamp
Reply messages.
|
Originate
Timestamp
|
4
|
Originate
Timestamp: A time value filled in by the originating device
just before sending the Timestamp message.
|
Receive
Timestamp
|
4
|
Receive Timestamp:
A time value filled in by the responding device just as it receives
the Timestamp message.
|
Transmit
Timestamp
|
4
|
Transmit
Timestamp: A time value filled in by the responding device just
before sending back the Timestamp Reply message.
|
Figure 147: ICMPv4 Timestamp and Timestamp Reply Message Format
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The Identifier and
Sequence Number fields are used to match Timestamp and
Timestamp Reply messages, exactly as they were used for Echo
and Echo Reply messages. The Identifier field is intended
as a higher-level label, like a session identifier, while the Sequence
Number is often used to identify individual messages within a series.
However, the use of these fields is up to the particular implementation.
All three timestamps are represented
as the number of milliseconds since midnight, Universal Time
(UT, also called Greenwich Mean Time or GMT). The
reason there are three timestamps instead of the two you might ordinarily
expect, is that a separate timestamp is recorded by the responding device
both when it receives the Timestamp message and when it generates
the Timestamp Reply. When the Reply is received back by
the originating device, it then has the times that both the Timestamp
and the Timestamp Reply were sent. This allows the originating
device to differentiate between the time required for transmitting datagrams
over the network, and the time for the other device to process the Timestamp
and turn it into a Timestamp Reply.
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Version 3.0 - Version Date: September 20, 2005
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