On Fri, Jun 22, 2007 at 12:23:36PM +0100, Ian O'Connell wrote:
> On 22/06/07, Proinnsias Breathnach <proinnsias at linux.ie> wrote:
> >So, while ping _can_ tell you that there is a functioning network in
> >place, and indeed that end-to-end connectivity exists for ICMP packets,
> >and even how long the return path for ICMP packets is. It cannot be
> >considered a useful measure of latency on the link, as ICMP may (or
> >indeed may not) be subject to the same QoS restrictions as other
> >traffic.
>> How are latency/ping related to any specific protocol though? below or
> above the ip layer?
> its possible to do udp pings with the right software on either end or
> client side software to a standard service. latency is pretty much an
> abstract concept in packet switched networks but the only established
> way that i'm aware of to measure it would be a ping client of some
> sort no?
> (i'm not trying to be arguementative, i'm rather curious about this)
>As others have noted - QoS (quality of service) and packet
prioritisation etc can have a massive impact on "ping times". The ICMP
protocol will have very differnt settings to UDP (non-guaranteed
delivery) or TCP (guaranteed to keep trying if a failure occurs) in
network routing hardware.
Gamers are typically worried about "pingtimes" etc to servers - in the
hopes that a faster network will make the difference, however Lag can
occur at many levels in the OSI model, from basic electrical lag
(distance / speed-of-signal ), to lag caused by the host machine being
overworked and swapping to/from disk during packet receipt. There are
dozens of things can affect the results of a 'ping' test.
What it will tell you, certainly, is that there *is* connectivity - or
that connectivity (on that particular protocol/port) is not available if
you're using UDP or TCP 'pings'. However, it may not tell you much more
than that - and it may not even tell you that.
Pinging an average WinXP (SP2 and above) box will generally fail, as
ICMP requests are rejected - yet the host is certainly 'alive' on the
network.
P
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