[ntp:questions] requestor's time; transmit timestamp?
Nelson Minar
nelson at monkey.org
Sun Nov 27 00:24:58 UTC 2005
I run a server in pool.ntp.org. I'm doing some analysis on the
requests I get. I'd like to know what time my clients think it is.
In an NTP request which timestamp carries the requestor's notion of
what time it is?
Looking at my packet traces I think the requestor's notion of time
must be stored in what ethereal/tcpdump call "Transmit Time Stamp". In
the responses my server sends it copies this timestamp into the
"Originate Time Stamp" field in the response and puts its own current
time in the "Receive Time Stamp" (and very soon thereafter, the
Transmit Time Stamp).
Do I have this right? Some of the docs I've found online describe it
differently. I've been reading about and using NTP for 15 years and I
still get confused by this stuff! Oy!
Just to clarify, here's a tethereal request and response. From this I
infer that the requestor thinks it's Sep 16, 2005 07:07:55.1693 UTC,
and my server is telling it it's Sep 16, 2005 07:07:55.2240 UTC.
Factoring in delay, we more or less agree on what time it is.
Frame 1 (90 bytes on wire, 90 bytes captured)
Arrival Time: Sep 16, 2005 07:07:55.223935000
Time delta from previous packet: 0.000000000 seconds
Time since reference or first frame: 0.000000000 seconds
Frame Number: 1
Packet Length: 90 bytes
Capture Length: 90 bytes
Protocols in frame: eth:ip:udp:ntp
Ethernet II, Src: Cisco_f8:cf:ff (00:0d:bc:f8:cf:ff), Dst: Intel_66:96:35 (00:0e:0c:66:96:35)
Destination: Intel_66:96:35 (00:0e:0c:66:96:35)
Source: Cisco_f8:cf:ff (00:0d:bc:f8:cf:ff)
Type: IP (0x0800)
Internet Protocol, Src: 65.171.219.241 (65.171.219.241), Dst: 72.36.170.170 (72.36.170.170)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 76
Identification: 0x296e (10606)
Flags: 0x00
0... = Reserved bit: Not set
.0.. = Don't fragment: Not set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 118
Protocol: UDP (0x11)
Header checksum: 0x0ac8 [correct]
Source: 65.171.219.241 (65.171.219.241)
Destination: 72.36.170.170 (72.36.170.170)
User Datagram Protocol, Src Port: 51066 (51066), Dst Port: ntp (123)
Source port: 51066 (51066)
Destination port: ntp (123)
Length: 56
Checksum: 0x97f5 [correct]
Network Time Protocol
Flags: 0x19
00.. .... = Leap Indicator: no warning (0)
..01 1... = Version number: NTP Version 3 (3)
.... .001 = Mode: symmetric active (1)
Peer Clock Stratum: secondary reference (2)
Peer Polling Interval: 4 (16 sec)
Peer Clock Precision: 0.015625 sec
Root Delay: 0.0937 sec
Clock Dispersion: 0.5920 sec
Reference Clock ID: 200.23.51.205
Reference Clock Update Time: Sep 16, 2005 04:11:13.3765 UTC
Originate Time Stamp: Sep 15, 2005 07:45:02.2587 UTC
Receive Time Stamp: Sep 15, 2005 07:45:03.3921 UTC
Transmit Time Stamp: Sep 16, 2005 07:07:55.1693 UTC
Frame 2 (90 bytes on wire, 90 bytes captured)
Arrival Time: Sep 16, 2005 07:07:55.224010000
Time delta from previous packet: 0.000075000 seconds
Time since reference or first frame: 0.000075000 seconds
Frame Number: 2
Packet Length: 90 bytes
Capture Length: 90 bytes
Protocols in frame: eth:ip:udp:ntp
Ethernet II, Src: Intel_66:96:35 (00:0e:0c:66:96:35), Dst: Cisco_f8:cf:ff (00:0d:bc:f8:cf:ff)
Destination: Cisco_f8:cf:ff (00:0d:bc:f8:cf:ff)
Source: Intel_66:96:35 (00:0e:0c:66:96:35)
Type: IP (0x0800)
Internet Protocol, Src: 72.36.170.170 (72.36.170.170), Dst: 65.171.219.241 (65.171.219.241)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 76
Identification: 0x1179 (4473)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 64
Protocol: UDP (0x11)
Header checksum: 0x18ad [correct]
Source: 72.36.170.170 (72.36.170.170)
Destination: 65.171.219.241 (65.171.219.241)
User Datagram Protocol, Src Port: ntp (123), Dst Port: 51066 (51066)
Source port: ntp (123)
Destination port: 51066 (51066)
Length: 56
Checksum: 0x10b5 [incorrect, should be 0x87db]
Network Time Protocol
Flags: 0x1a
00.. .... = Leap Indicator: no warning (0)
..01 1... = Version number: NTP Version 3 (3)
.... .010 = Mode: symmetric passive (2)
Peer Clock Stratum: secondary reference (3)
Peer Polling Interval: 4 (16 sec)
Peer Clock Precision: 0.000001 sec
Root Delay: 0.0561 sec
Clock Dispersion: 0.0424 sec
Reference Clock ID: 67.18.58.210
Reference Clock Update Time: Sep 16, 2005 07:03:22.3182 UTC
Originate Time Stamp: Sep 16, 2005 07:07:55.1693 UTC
Receive Time Stamp: Sep 16, 2005 07:07:55.2240 UTC
Transmit Time Stamp: Sep 16, 2005 07:07:55.2240 UTC
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