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From: forster@cisco.com (Jim Forster)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: delay of an interface
Message-ID: <33643@boulder.Colorado.EDU>
Date: 26 Mar 91 04:32:16 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 53

Francis,

For the purposes of measuring the delay to be used in IGRP, the simplest
thing to do is run a bunch of 1500 bit ping packets, and use the average delay.
That's 1500 bits, not bytes.

Quoting from ftp.cisco.com:~ftp/igrp.doc

"In principle the composite delay, Dc, could be determined as follows:
  Dc = Ds + Dcir + Dt                            Eq. 2
  Where:
    Ds = switching delay;
    Dcir = circuit delay (propagation delay of 1 bit); and
    Dt = transmission delay (no-load delay for a 1500 bit message)."

Walt Prue's method of plotting the delay of various size packets is
interesting, as it yields Ds+Dcir.  The slope of the line is the bandwidth.
If you do this over a long path, or a line with some background traffic, you
can deduce the effective bandwidth of the path.
 
>> 10000*Bandwidth+Delay/10 with Bandwidth in kb/s and Delay in us

The Bandwidth must be inverted and scaled by 10E10.  Also quoting from
the same source:

"Bandwidth is inverse bandwidth in bits per sec scaled by a factor of
10e10.  The range is from a 1200 bps line to 10 Gbps....  Reliability is
given as a fraction of 255.  That is, 255 is 100%.  Load is given as a
fraction of 255....

Because of the somewhat wierd units used for bandwidth and delay, some examples
seem in order.  These are the default values used for several common media.

                            Delay           Bandwidth
            Satellite    200,000 (2 sec)           20 (500 Mbit)
            Ethernet         100 (1 ms)         1,000
            1.544 Mbit      2000 (20 ms)        6,476
            64 Kbit         2000              156,250
            56 Kbit         2000              178,571
            10 Kbit         2000            1,000,000
            1 Kbit          2000           10,000,000

6.3 Metric computations
Here is a description of the way the composite metric is actually  computed...

       metric = [K1*bandwidth + (K2*bandwidth)/(256 - load) + K3*delay] *
                                                [(reliability + K4)/K5]

       If K5 == 0, then there is no reliability term.
       The default version of IGRP has K1 == K3 == 1, K2 == K3 == K5 == 0"


  -- Jim