|IP option 4, Timestamp|
|Protocol type:||Connectionless network layer protocol.|
|Option length:||Variable length.|
|Links:||IANA: IP option numbers.|
RFC 791 page 22:
The originating host must compose this option with a large enough timestamp data area to hold all the timestamp information expected. The size of the option does not change due to adding timestamps. The intitial contents of the timestamp data area must be zero or internet address/zero pairs. If the timestamp data area is already full (the pointer exceeds the length) the datagram is forwarded without inserting the timestamp, but the overflow count is incremented by one. If there is some room but not enough room for a full timestamp to be inserted, or the overflow count itself overflows, the original datagram is considered to be in error and is discarded. In either case an ICMP parameter problem message may be sent to the source host.
The timestamp option is not copied upon fragmentation. It is carried in the first fragment. Appears at most once in a datagram.
|MAC header||IP header||IP option 4||Data :::|
IP Option 4:
Type. 8 bits. Set to 68.
C, Copy flag. 1 bit. Cleared to 0.
Indicates the option is not to be copied into all fragments.
Class. 2 bits. Set to 2.
The option is a debugging and measurement option.
Option. 5 bits. Set to 4.
The IP option number.
8 bits, unsigned. ? to 40.
Size of the option in bytes.
8 bits, unsigned. 5 to ?
The number of bytes from the beginning of this option to the end of Timestamp plus one (i.e., it points to the byte beginning the space for next timestamp). The timestamp area is full when Pointer is greater than Length.
4 bits, unsigned.
The number of IP modules that cannot register timestamps due to lack of space.
Flag. 4 bits.
|0||Timestamp contains only Time fields.|
|1||Timestamp contains IP address and Time fields.|
|3||The IP address fields are prespecified. An IP module only registers its timestamp if it matches its own address with the next specified internet address.|
Timestamp . Variable length.
One or more Timestamp structures.
Timestamp. 32 or 64 bits.
0 or 32 bits.
A right-justified timestamp in milliseconds since midnight UT. If the time is not available in milliseconds or cannot be provided with respect to midnight UT then any time may be inserted as a timestamp provided the high order bit of the timestamp field is set to one to indicate the use of a non-standard value.
[RFC 781] A SPECIFICATION OF THE INTERNET PROTOCOL (IP) TIMESTAMP OPTION.
[RFC 791] Internet Protocol.