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RFC 781                                                            Zaw-Sing Su                                                                           SRI                                                                      May 1981A SPECIFICATION OF THE INTERNET PROTOCOL (IP) TIMESTAMP OPTIONI. INTRODUCTION     Packet switching is store-and-forward by nature.  Network delay is atherefore a critical performance measure for packet-switching communications.A catenet is a system of packet-switched communication networks interconnectedvia gateways [Cerf 78].  The catenet "link" delays are thus variable.  Theirmeasurement, 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 headertimestamping.  Header timestamping allows monitoring of catenet delays foruser traffic, such as the case of Ft. Bragg users accessing ISID across thecatenet.  Packet header timestamping is also compatible with the use of testpackets for catenet delay measurement.  Another advantage of headertimestamping is that since it is an IP option, the gateway imposes littledifference in the treatment of such a packet.  In this note, a specificationof the timestamp option format for IP is presented.     Measurement of one-way delay, either end-to-end or across an individualnetwork, requires that device clocks be synchronized, using such facilities asWWVB clocks [Mills 81].  This specification assumes this capability in thegateways and involved network hosts.II.  FORMAT SPECIFICATION     As an IP option, the contents of the first two octets are dictated by theIP 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 inthe first fragment.REFERENCES[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.