Re: [fw-wiz] tests about latency

From: Mikael Olsson (mikael.olsson_at_clavister.com)
Date: 09/12/03

  • Next message: Jeff_Lopes_at_groove.net: "[fw-wiz] Sniffing DSL Connection" 
    To: Neale Banks <neale@lowendale.com.au>
Date: Fri, 12 Sep 2003 15:36:33 +0200

    Neale Banks wrote:
    >
    > Ob FW: Whilst obviously anything that's not simply routed (e.g. proxied
    > protocols) would be a completely different kettle of fish, to what extent
    > could one then reasonably generalise the results obtained from ping tests
    > (i.e. ICMP packets) to other protocols?

    Your question is already answered, but: one should also note that doing any
    kind of RTT tests (e.g. pinging) against routers is generally a Bad Idea.

    Example:
    - My default gateway: RTT ~1 ms
    - Hop outside my default gateway: RTT often 20-30 ms <-- NOTE!
    - Next hop after that: RTT ~5 ms
    - ... 10 hops away: RTT 15 ms

    How can this happen, you ask? Easy: forwarding and local processing
    is done in different processors in many routers. The forwarding
    processors can be just fine even though the "host" CPU can be totally
    overloaded by things like aggressive SNMP polling, large dynamic
    routing calculations (OSPF et al) and whatnot. 

    -- 
Mikael Olsson, Clavister AB
Storgatan 12, Box 393, SE-891 28 ÖRNSKÖLDSVIK, Sweden
Phone: +46 (0)660 29 92 00   Mobile: +46 (0)70 26 222 05
Fax: +46 (0)660 122 50       WWW: http://www.clavister.com
"Senex semper diu dormit"
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