From: Guy Harris <guy@alum.mit.edu>
Date: Wed, 29 Jul 2015 16:17:30 +0000 (-0700)
Subject: Give more details about packet time stamps.
X-Git-Url: https://git.tcpdump.org/tcpdump/commitdiff_plain/d6390b26af400a5eab921a4fa8cdeb155fdcf86c

Give more details about packet time stamps.

Don't speak of "Ethernet" and "wire", as you might not be sniffing an
Ethernet or, indeed, any form of wired network.

Note that not only could there be a delay between the point at which the
interface is finished receiving the packet and when an interrupt is
delivered (whether due to bus delays, polling rather than immediate
interrupts being used, or delays in the CPU responding to the interrupt,
or more than one of those) but also a delay between the point at which
the kernel responds to the interrupt and the point at which it actually
applies a time stamp to the packet.
---

diff --git a/tcpdump.1.in b/tcpdump.1.in
index f9522cb8..ebf50ab6 100644
--- a/tcpdump.1.in
+++ b/tcpdump.1.in
@@ -1882,11 +1882,15 @@ is the current clock time in the form
 .fi
 .RE
 and is as accurate as the kernel's clock.
-The timestamp reflects the time the kernel first saw the packet.
-No attempt
-is made to account for the time lag between when the
-Ethernet interface removed the packet from the wire and when the kernel
-serviced the `new packet' interrupt.
+The timestamp reflects the time the kernel applied a time stamp to the packet.
+No attempt is made to account for the time lag between when the network
+interface finished receiving the packet from the network and when the
+kernel applied a time stamp to the packet; that time lag could include a
+delay between the time when the network interface finished receiving a
+packet from the network and the time when an interrupt was delivered to
+the kernel to get it to read the packet and a delay between the time
+when the kernel serviced the `new packet' interrupt and the time when it
+applied a time stamp to the packet.
 .SH "SEE ALSO"
 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
 pcap-filter(@MAN_MISC_INFO@), pcap-tstamp(@MAN_MISC_INFO@)