TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Because IP routers are not required to have synchronized clocks, they may
not be accurate.
Figure 3-26 gives an overview of the Internet time stamp option.
Figure 3-26 IP: Internet time stamp option
Where:
01000100 (Decimal 68) This field is the value of the option type for the internet
time stamp option.
Length This field contains the total length of this option,
including the type and length fields.
Pointer This field points to the next time stamp to be
processed (first free time stamp).
Oflw (overflow) This field contains the number of devices that cannot
register time stamps due to a lack of space in the data
field.
Flag This field is a 4-bit value that indicates how time
stamps are to be registered:
0 Time stamps only, stored in consecutive 32-bit
words.
108 TCP/IP Tutorial and Technical Overview
0 8 16 24 28
01000100 length pointer oflw flag
IP address
time stamp
...
1 Each time stamp is preceded by the IP address of
the registering device.
2 The IP address fields are prespecified; an IP
device only registers when it finds its own address
in the list.
Time stamp A 32-bit time stamp recorded in milliseconds since
midnight UT (GMT).
The originating host must compose this option with a sufficient data area to hold
all the time stamps. If the time stamp area becomes full, no further time stamps
are added.