As per Relevance of the word following, we have this rfc below:
RFC 778
DCNET Internet Clock
D.L. Mills, COMSAT
18 April 1981
Following is a description of the Internet
Service (ICS) provided by all DCNET hosts. The service
intended primarily for clock synchronization and one-
delay measurements with cooperating internet hosts,
provided using the Timestamp and Timestamp Reply messages
the proposed Internet Control Message Protocol (ICMP).
addition, in order to maintain compatability with
systems, this service will be provided for a limited
using the Echo and Echo Reply messages of
Gateway-Gateway Protocol (GGP).
It should be understood that ICMP and GGP datagrams
normally considered tightly bound to the Internet
(IP) itself and not directly accessable to the user on
TOPS-20 system, for example. These datagrams are
somewhat differently from user datagrams in gateways
DCNET hosts in that certain internal queueing mechanisms
bypassed. Thus, they can be a useful tool in providing
most accurate and stable time reference. The
motivation for this note is to promote the development
this service in other internet hosts and gateways so
the feasibility for its use thoughout the community can
assessed
ICS Datagrams and
At present, the ICS is provided using either ICMP
GGP datagrams. The only difference between these is
ICMP uses protocol number 1 and GGP uses protocol number 3.
In the following these will be referred to interchangably
ICS datagrams. ICS datagrams include an internet
followed by an ICS header in the following format
DCNET Internet Clock Service PAGE 2
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originate Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transmit Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ICS Datagram
The originator fills in all three timestamp fields
before the datagram is forwarded to the net. Each of
fields contain the local time at origination. Although
last two are redundant, they allow roundtrip
measurements to be made using remote hosts
timestamping facilities. The "Type" field can be either 8
(GGP Echo) or 13 (ICMP Timestamp). The "Code" field
be zero. The "Sequence" field can contain either zero or
optional sequence number provided by the user. The
of the datagram is thus 36 octets inclusive of the 20-
internet header and exclusive of the local-network leader
The host or gateway receiving an ICS datagram fills
the "Receive Timestamp" field just as the datagram
received from the net and the "Transmit Timestamp" just
it is forwarded back to the sender. It also sets the "Type
field to 0 (GGP Echo Reply), if the original value was 8,
14 (ICMP Timestamp Reply), if it was 13. The
fields are unchanged
The timestamp values are in milliseconds from
UT and are stored right-justified in the 32-bit fields
above. Ordinarily, all time calculations are
modulo-24 hours in milliseconds. This provides a
match to those operating systems which maintain a
clock in ticks past midnight. The specified timestamp
of milliseconds is consistent with the accuracy of
radio clocks and the errors expected in the
process itself
Delay
Delay measurements can be made with any DCNET host
simply sending an ICS datagram in the above format to it
processing the reply. Let t1, t2 and t3 represent the
timestamp fields of the reply in order and t4 the time
arrival at the original sender. Then the delays,
of internal processing within the DCNET host, are
(t2 - t1) to the DCNET host, (t4 - t3) for the return
DCNET Internet Clock Service PAGE 3
(t2 - t1) + (t4 - t3) for the roundtrip. Note that, in
case of the roundtrip, the clock offsets between the
host and DCNET host cancel
Although ICS datagrams are returned by all DCNET
regardless of other connections that may be in use by
host at any given time, the most useful host will
be the COMSAT-WWV virtual host at internet
[29,0,9,2], which is also the internet echo virtual
formerly called COMSAT-ECH. This virtual host is
in the COMSAT-GAT physical host at internet
[29,0,1,2], which is connected to the ARPANET via the
Gateway, Clarksburg SIMP and a 4800-bps line to IMP 71
BBN. The roundtrip delay via this path between
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-
virtual host is down (in this case possible only if
Spectracom radio clock is down or misbehaving) a "host
reachable" GGP datagram is returned. In
circumstances a "net not reachable" or "source quench"
datagram could be returned. Note that the references
"GGP" here will be read "ICMP" at some appropriate
time
Local Offset
All DCNET timestamps are referenced to a
virtual host called COMSAT-WWV (what else?) with
address [29,0,9,2]. This host is equipped with a
radio clock which normally provides WWVB time and date
within a millisecond. The clock synchronization
provides offset and drift corrections for other
relative to this host; however, offsets up to an
fraction of a second routinely occur due to the
of tracking with power-line clocks in some machines.
table of the current offsets can be obtained using
following procedure
1. Connect to COMSAT-GAT host at internet
[29,0,1,2] using TELNET and local echo
2. Send the command SET HOST HOST. A table with one
per DCNET host should be returned. Note the entry
the "Offset" column for the WWV host. This contains
offset in milliseconds that should be added to
timestamps generated by either the COMSAT-GAT
COMSAT-WWV hosts to yield the correct time as
by WWVB
3. Send the command SET WWV SHOW. A summary of
traffic is returned along with an entry labelled "
DCNET Internet Clock Service PAGE 4
time." The string following this is the last
received from the Spectracom unit in the format
DDD HH:MM:SS TZ=00
where is normally in case the WWVB signal
being received correctly or ? in case it is not.
DDD represents the day of the year and HH:MM:SS the
past UT midnight. The two digits following TZ
represent the time zone, here 00 for UT
4. Close the connection (please!).
[1]
Postel, J., "Internet Control Message Protocol", RFC 777,
USC/Information Sciences Institute, April 1981.
[2]
Strazisar, V., "How to Build a Gateway", IEN 109,
Beranek and Newman, August 1979.
DCNET Internet Clock Service PAGE 5
Following is a specification of the ICS header in PDP11
code
; GGP/ICMP
. = 0
GH.TYP: .BLKB 1 ;Message
GC.RPY = 0 ;Echo
GC.UPD = 1 ;Routing
GC.ACK = 2 ;Positive
GC.DNR = 3 ;Destination
GC.SQN = 4 ;Source
GC.RDR = 5 ;
GC.ECH = 10 ;
GC.STA = 11 ;Net interface
GC.NAK = 12 ;Negative
GC.TIM = 15 ;
GC.TRP = 16 ;Timestamp
GH.COD: .BLKB 1 ;Message
GH.SEQ: .BLKW 1 ;Sequence
GH.HDR = . ;Beginning of
;internet
GH.ORG: .BLKW 2 ;Originating
GH.REC: .BLKW 2 ;Received
GH.XMT: .BLKW 2 ;Transmitted
GH.LEN = . ;End of timestamp
Note that all PDP11 word fields (.BLKW above)
"byte-swapped," that is, the order of byte transmission
the high-order byte followed by the low-order byte of
PDP11 word
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
