RFC 781                                                            Zaw-Sing Su
                                                                      May 1981



     Packet switching is store-and-forward by nature.  Network delay is a
therefore a critical performance measure for packet-switching communications.
A catenet is a system of packet-switched communication networks interconnected
via gateways [Cerf 78].  The catenet "link" delays are thus variable.  Their
measurement, the measurement of delays across member networks of a catenet,
becomes important for catenet investigations.

     An effective way to measure catenet delays is by means of packet header
timestamping.  Header timestamping allows monitoring of catenet delays for
user traffic, such as the case of Ft. Bragg users accessing ISID across the
catenet.  Packet header timestamping is also compatible with the use of test
packets for catenet delay measurement.  Another advantage of header
timestamping is that since it is an IP option, the gateway imposes little
difference in the treatment of such a packet.  In this note, a specification
of the timestamp option format for IP is presented.

     Measurement of one-way delay, either end-to-end or across an individual
network, requires that device clocks be synchronized, using such facilities as
WWVB clocks [Mills 81].  This specification assumes this capability in the
gateways and involved network hosts.


     As an IP option, the contents of the first two octets are dictated by the
IP header format to be option type and option length in octets [Postel 80].
The next two octets are used to control this option.

     0               7               15              23              31
     |     type      |    length     |    offset     |overflw| flags |
     |                          internet ID                          |
     |                          time stamp                           |

     option type = 68 decimal (i.e., option class = 2 and option number = 4);

     option length = the number of octets with a maximum of 40 (limited by
                 IHL = 15);

     offset =    the number of octets from the beginning of this option to the
                 end of timestamps (i.e., the beginning of space for next
                 timestamp).  It is set to one, an odd number, when no more
                 space remains in the header for timestamps;

     overflow =  the number of IP modules that cannot register timestamps due
                 to lack of space;

     flag = 0 -- time stamps only
            1 -- each timestamp is preceded with internet ID of the
                 registering entity
            3 -- the internet ID fields are prespecified.  An IP module only
                 registers its timestamp if it matches its own ID with the
                 next specified internet ID;

     internet ID = ID for the timestamping device;

     timestamp = a right-justified, 32-bit timestamp in milliseconds modulo
                 24 hours from midnight UT.

     The timestamp option is not copied upon fragmentation.  It is carried in
the first fragment.


[Cerf 78]   Cerf, V., "The Catenet Model for Internetworking," Defense
            Advanced Research Projects Agency, Information Processing
            Techniques Office, IEN 48, July 1978.

[Mills 81]  Mills, D.L., "DCNET Internet Clock Service," RFC 778, COMSAT
            Laboratories, April 1981.

[Postel 80] Postel, J. (ed.), "DoD Standard Transport Internet Protocol,"
            Defense Advanced Reseach Projects Agency, Information Processing
            Techniques Office, RFC 760, IEN 128, January 1980.