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List:       user-mode-linux-user
Subject:    Re: [uml-user] Latency Issue
From:       Martin Maney <uml-list-sub () two14 ! net>
Date:       2003-11-29 17:10:36
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On Sat, Nov 29, 2003 at 04:49:17PM +0000, Rus Foster wrote:
> I'm seeing a weird error when trying to ping UML's remotly. The first ping
> is high and the rest are then normal latency

> PING rt.jvds.com (216.74.75.208): 56 data bytes
> 64 bytes from 216.74.75.208: icmp_seq=0 ttl=47 time=489.0 ms
> 64 bytes from 216.74.75.208: icmp_seq=1 ttl=47 time=175.1 ms
> 64 bytes from 216.74.75.208: icmp_seq=2 ttl=47 time=155.5 ms
> 64 bytes from 216.74.75.208: icmp_seq=3 ttl=47 time=156.7 ms
> 64 bytes from 216.74.75.208: icmp_seq=4 ttl=47 time=163.7 ms

Looks perfectly normal to me.  What does it look like if you kill it at
this point and re-invoke ping?  If it shows little or no "slow first
packet" then I think you can stop wondering - it's just the overhead of
learning by all the devices along the route.  Or is this a single hop
over a high-latency link?

> However I'm having trouble working out why. I'm using the command line

The RTT is calculated based on a timestamp placed in the packet when it
is "sent" - but that's actually just when it's being sent to the local
IP stack.  There can be quite a lot of stuff that takes longer for the
first packet that will be faster on subsequent ones because of caching
- IP routing and IP to MAC address mapping are both usully cached, for
example.  Perhaps there's a NAT along the route?  Etc.

-- 
Faced with the choice between changing one's mind and proving there is
no need to do so, almost everyone gets busy on the proof. -- JKG



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