RFC 778

DCNET Internet Clock Service                        PAGE   2

                       ICS Datagram Format



     The originator fills in all three timestamp fields just

before the datagram is forwarded to the net. Each of these fields contain the local time at origination. Although the last two are redundant, they allow roundtrip delay measurements to be made using remote hosts without timestamping facilities. The "Type" field can be either 8 (GGP Echo) or 13 (ICMP Timestamp). The "Code" field should be zero. The "Sequence" field can contain either zero or an optional sequence number provided by the user. The length of the datagram is thus 36 octets inclusive of the 20-octet internet header and exclusive of the local-network leader.



     The host or gateway receiving an ICS datagram fills  in

the "Receive Timestamp" field just as the datagram is received from the net and the "Transmit Timestamp" just as it is forwarded back to the sender. It also sets the "Type" field to 0 (GGP Echo Reply), if the original value was 8, or
  (ICMP  Timestamp  Reply),  if  it was 13.  The remaining
fields are unchanged.



     The timestamp values are in milliseconds from  midnight

UT and are stored right-justified in the 32-bit fields shown above. Ordinarily, all time calculations are performed modulo-24 hours in milliseconds. This provides a convenient match to those operating systems which maintain a system clock in ticks past midnight. The specified timestamp unit of milliseconds is consistent with the accuracy of existing radio clocks and the errors expected in the timestamping process itself.



Delay Measurements



     Delay measurements can be made with any DCNET  host  by

simply sending an ICS datagram in the above format to it and processing the reply. Let t1, t2 and t3 represent the three timestamp fields of the reply in order and t4 the time of arrival at the original sender. Then the delays, exclusive of internal processing within the DCNET host, are simply (t2 - t1) to the DCNET host, (t4 - t3) for the return and

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     Although ICS datagrams are returned by all DCNET  hosts

regardless of other connections that may be in use by that host at any given time, the most useful host will probably be the COMSAT-WWV virtual host at internet address [29,0,9,2], which is also the internet echo virtual host formerly called COMSAT-ECH. This virtual host is resident in the COMSAT-GAT physical host at internet address [29,0,1,2], which is connected to the ARPANET via the COMSAT Gateway, Clarksburg SIMP and a 4800-bps line to IMP 71 at BBN. The roundtrip delay via this path between the COMSAT-GAT host and the BBN Gateway is typically 550 milliseconds as the ICS datagram flies.



     As in the case of all DCNET hosts,  if  the  COMSAT-WWV

virtual host is down (in this case possible only if the Spectracom radio clock is down or misbehaving) a "host not reachable" GGP datagram is returned. In unusual circumstances a "net not reachable" or "source quench" GGP datagram could be returned. Note that the references to "GGP" here will be read "ICMP" at some appropriate future time.



Local Offset Corrections



     All DCNET timestamps are  referenced  to  a  designated

virtual host called COMSAT-WWV (what else?) with internet address [29,0,9,2]. This host is equipped with a Spectracom radio clock which normally provides WWVB time and date to within a millisecond. The clock synchronization mechanism provides offset and drift corrections for other hosts relative to this host; however, offsets up to an appreciable fraction of a second routinely occur due to the difficulty of tracking with power-line clocks in some machines. A table of the current offsets can be obtained using the following procedure.

.  Connect  to  COMSAT-GAT   host   at   internet   address
    [29,0,1,2] using TELNET and local echo.

.  Send the command SET HOST HOST.  A table with  one  line
per DCNET host should be returned. Note the entry under the "Offset" column for the WWV host. This contains the offset in milliseconds that should be added to all timestamps generated by either the COMSAT-GAT or COMSAT-WWV hosts to yield the correct time as broadcast by WWVB.

.  Send the command SET WWV SHOW.  A  summary  of  datagram
traffic is returned along with an entry labelled "NBS

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                  <code>  DDD HH:MM:SS  TZ=00



where <code> is normally <SP> in case the WWVB signal is being received correctly or ? in case it is not. The DDD represents the day of the year and HH:MM:SS the time past UT midnight. The two digits following TZ= represent the time zone, here 00 for UT.

.  Close the connection (please!).


REFERENCES



[1] ICMP



Postel, J., "Internet Control Message Protocol", RFC 777, USC/Information Sciences Institute, April 1981.



[]  GGP



Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and Newman, August 1979.

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; ; GGP/ICMP Header ;

 .       =       0

GH.TYP: .BLKB 1 ;Message type GC.RPY = 0 ;Echo reply GC.UPD = 1 ;Routing update GC.ACK = 2 ;Positive acknowledgment GC.DNR = 3 ;Destination unreachable GC.SQN = 4 ;Source quench GC.RDR = 5 ;Redirect GC.ECH = 10 ;Echo GC.STA = 11 ;Net interface status GC.NAK = 12 ;Negative acknowledgment GC.TIM = 15 ;Timestamp GC.TRP = 16 ;Timestamp Reply GH.COD: .BLKB 1 ;Message code GH.SEQ: .BLKW 1 ;Sequence number GH.HDR = . ;Beginning of original

                                 ;internet header

GH.ORG: .BLKW 2 ;Originating timestamp GH.REC: .BLKW 2 ;Received timestamp GH.XMT: .BLKW 2 ;Transmitted timestamp GH.LEN = . ;End of timestamp header



     Note that all  PDP11  word  fields  (.BLKW  above)  are

"byte-swapped," that is, the order of byte transmission is the high-order byte followed by the low-order byte of the PDP11 word.